How to Find a Good Patio Contractor
Have you decided you want a flagstone, limestone or concrete patio for your CALIFORNIA home. Wondering how to get it done and who you should use? Well, I’m going to tell you how. I’m going to give you a step by step process stating the 3 red flags that signal a bad or uneducated [tag] contractor. These 3 things will ensure you get it done the right way, by the right small concrete contractors contractor in CALIFORNIA .
Steps To Finding The Right Concrete Contractor in CALIFORNIA
First step, first things first. Every Stone Installation Needs A Concrete Footing which might also be called a concrete foundation . This is not opinion but fact. If you want it done right and want it to withstand the test of time, this is the way it’s done, no exceptions. A red flag should go up when any contractor is offering bypassing a concrete foundation as a cost saving option. Unfortunately 85% of the small concrete contractors contractors out there will tell you they can lay the stone right on top of compacted granite or base material. This type of contractor is exactly who you want to avoid. While what they are recommending to you gives the appearance of a patio, it won’t withstand the climate, erosion and soil movement beneath the patio in CALIFORNIA .
BEWARE OF CONCRETE CONTRACTOR SCAMS
You might get a few crackless years at best, until it begins to fall apart. This method is the most recommended scam or shortcut of trying to lower pricing and still get the job. Notice the price came down because the concrete footing was removed from the job cost. This option of ” no concrete necessary” is most often offered to those with strict financial limitations. The contractors offering this solution don’t care about your patio or home, they just want your money.With no concrete foundation you get a type of patio, but it’s really just a veneer laid on the ground.
How much concrete do i need?
A concrete patio or sidewalk slab or footing should be a minimum of 4 inches thick. This is sometimes reduced to around 3 1/2 inches due to preexisting structural limitations for small concrete contractors in CALIFORNIA . If you encounter space limitations, you really do need a good masonry contractor to resolve the issues with other options. this leads us to our next step.
The 2nd step is hire a knowledgeable masonry contractor in CALIFORNIA . Do a little research before you start getting pricing. Most good masonry contractors will be able to talk concrete chemistry and technology with you. Yes, I said technology. In the last 10 years there have been some really cool advances in concrete additives and mixes. These advances have provided solutions for the problems and limitations of old. A true patio or masonry professional will be able to talk shop with you, it’s his livelihood. A contractor who can’t do this or doesn’t seem comfortable is more than likely a novice at best. This should be the other red flag that goes up. Basically, Concrete slabs and mortar are like a cake mix. Correctly mixing the ingredients and correctly letting the slab or masonry product cure, are the factors that determine a good solid installation. Again, Talk with your contractor and Make sure he’s knowledgeable about the chemistry, additives and curing process of concrete & masonry. If he is this will ensure the likelihood of a great patio. 3rd step and last but definitely not least, Check references, check reviews and ask to see work they’ve done. Really check this stuff out and try to look at at least 1 job they have done. Ask point blank if the previous client is related to the contractor. Any resistance or confusion in this process would be red flag number
3.So, in order from 1 to 3. All masonry installations need a concrete footing. Make sure your patio contractor is knowledgeable about masonry chemistry, additives and enhancing solutions. Check references, reviews and stay away from the dirt cheap deals of a lifetime from any concrete contractor in CALIFORNIA .If you get even 1 red flag, get another contractor. There are enough guys out there that do know what they’re doing.
Not to be confused with cement or mortar (masonry). Exterior of the Roman Pantheon, finished 128 AD, still the largest unreinforced concrete dome in the world. Interior of the Pantheon dome, seen from beneath. The concrete for the coffered dome was laid on moulds, probably mounted on temporary scaffolding. Opus caementicium
exposed in a characteristic Roman arch. In contrast to modern concrete structures, the concrete used in Roman buildings was usually covered with brick or stone.
Concrete is a composite material composed of coarse aggregate bonded together with a fluid cement that hardens over time. Most concretes used are lime-based concretes such as Portland cement concrete or concretes made with other hydraulic cements, such as ciment fondu. However, asphalt concrete, which is frequently used for road surfaces, is also a type of concrete, where the cement material is bitumen, and polymer concretes are sometimes used where the cementing material is a polymer.
When aggregate is mixed together with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses. Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete.
Famous concrete structures include the Hoover Dam, the Panama Canal, and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome. Today, large concrete structures (for example, dams and multi-storey car parks) are usually made with reinforced concrete.
After the Roman Empire collapsed, use of concrete became rare until the technology was redeveloped in the mid-18th century. Today, concrete is the most widely used man-made material (measured by tonnage).
The word concrete comes from the Latin word "concretus" (meaning compact or condensed), the perfect passive participle of "concrescere", from "con-" (together) and "crescere" (to grow).
Perhaps the earliest known occurrence of cement was twelve million years ago. A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes. These ancient deposits were investigated in the 1960s and 1970s.
On a human timescale, small usages of concrete go back for thousands of years. Concrete-like materials were used since 6500 BC by the Nabataea traders or Bedouins who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan. They discovered the advantages of hydraulic lime, with some self-cementing properties, by 700 BC. They built kilns to supply mortar for the construction of rubble-wall houses, concrete floors, and underground waterproof cisterns. The cisterns were kept secret and were one of the reasons the Nabataea were able to thrive in the desert. Some of these structures survive to this day.
In the Ancient Egyptian and later Roman eras, it was re-discovered that adding volcanic ash to the mix allowed it to set underwater.
German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400–1200 BC. Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of waterproof concrete. Concrete was used for construction in many ancient structures.
The Romans used concrete extensively from 300 BC to 476 AD, a span of more than seven hundred years. During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Roman Architectural Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs in terms of both structural complexity and dimension.
Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.
Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete (ca. 200 kg/cm2 [20 MPa; 2,800 psi]). However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application was also different:
Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
The long term durability of Roman concrete structures has been found to be due to its use of pyroclastic (volcanic) rock and ash, whereby crystallization of strätlingite and the coalescence of calcium–aluminum-silicate–hydrate cementing binder helped give the concrete a greater degree of fracture resistance even in seismically active environments. Roman concrete is significantly more resistant to erosion by seawater than modern concrete which is also due to the use of pyroclastic materials which react with seawater to form Al-tobermorite crystals over time.
The widespread use of concrete in many Roman structures ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges such as the magnificent Pont du Gard have masonry cladding on a concrete core, as does the dome of the Pantheon.
After the Roman Empire, the use of burned lime and pozzolana was greatly reduced until the technique was all but forgotten between 500 and the 14th century. From the 14th century to the mid-18th century, the use of cement gradually returned. The Canal du Midi was built using concrete in 1670.
Perhaps the greatest driver behind the modern use of concrete was Smeaton's Tower, the third Eddystone Lighthouse in Devon, England. To create this structure, between 1756 and 1759, British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate.
Developed in England in the 19th century, a method for producing Portland cement was patented by Joseph Aspdin in 1824. Aspdin named it due to its similarity to Portland stone which was quarried on the Isle of Portland in Dorset, England. His son William Aspdin is regarded as the inventor of "modern" Portland cement due to his developments in the 1840s.
Reinforced concrete was invented in 1849 by Joseph Monier. In 1889 the first concrete reinforced bridge was built, and the first large concrete dams were built in 1936, Hoover Dam and Grand Coulee Dam.
Many types of concrete are available, distinguished by the proportions of the main ingredients below. In this way or by substitution for the cementitious and aggregate phases, the finished product can be tailored to its application. Strength, density, as well chemical and thermal resistance are variables.
Aggregate consists of large chunks of material in a concrete mix, generally a coarse gravel or crushed rocks such as limestone, or granite, along with finer materials such as sand.
Cement, most commonly Portland cement, is associated with the general term "concrete." A range of other materials can be used as the cement in concrete too. One of the most familiar of these alternative cements is asphalt concrete. Other cementitious materials such as fly ash and slag cement, are sometimes added as mineral admixtures (see below) - either pre-blended with the cement or directly as a concrete component - and become a part of the binder for the aggregate.
To produce concrete from most cements (excluding asphalt), water is mixed with the dry powder and aggregate, which produces a semi-liquid slurry that can be shaped, typically by pouring it into a form. The concrete solidifies and hardens through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, creating a robust stone-like material.
Chemical admixtures are added to achieve varied properties. These ingredients may accelerate or slow down the rate at which the concrete hardens, and impart many other useful properties including increased tensile strength, entrainment of air and water resistance.
Reinforcement is often included in concrete. Concrete can be formulated with high compressive strength, but always has lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension, typically steel rebar.
Mineral admixtures are becoming more popular in recent decades. The use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation, and the discovery that such materials often have complementary and valuable properties. The most conspicuous of these are fly ash, a by-product of coal-fired power plants, ground granulated blast furnace slag, a byproduct of steelmaking, and silica fume, a byproduct of industrial electric arc furnaces. The use of these materials in concrete reduces the amount of resources required, as the mineral admixtures act as a partial cement replacement. This displaces some cement production, an energetically expensive and environmentally problematic process, while reducing the amount of industrial waste that must be disposed of. Mineral admixtures can be pre-blended with the cement during its production for sale and use as a blended cement, or mixed directly with other components when the concrete is produced.
The mix design depends on the type of structure being built, how the concrete is mixed and delivered, and how it is placed to form the structure.
Main article: Cement A few tons of bagged cement. This amount represents about two minutes of output from a 10,000 ton per day cement kiln.
Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and many plasters. British masonry worker Joseph Aspdin patented Portland cement in 1824. It was named because of the similarity of its color to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of calcium silicates (alite, belite), aluminates and ferrites - compounds which combine calcium, silicon, aluminium and iron in forms which will react with water. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay or shale (a source of silicon, aluminium and iron) and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).
In modern cement kilns many advanced features are used to lower the fuel consumption per ton of clinker produced. Cement kilns are extremely large, complex, and inherently dusty industrial installations, and have emissions which must be controlled. Of the various ingredients used to produce a given quantity of concrete, the cement is the most energetically expensive. Even complex and efficient kilns require 3.3 to 3.6 gigajoules of energy to produce a ton of clinker and then grind it into cement. Many kilns can be fueled with difficult-to-dispose-of wastes, the most common being used tires. The extremely high temperatures and long periods of time at those temperatures allows cement kilns to efficiently and completely burn even difficult-to-use fuels.
Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely.
As stated by Abrams' law, a lower water-to-cement ratio yields a stronger, more durable concrete, whereas more water gives a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.
Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete to form a solid mass.
Cement chemist notation: C3S + H → C-S-H + CH Standard notation: Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2 Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2 (approximately; the exact ratios of the CaO, SiO2 and H2O in C-S-H can vary) Crushed stone aggregate Main article: Construction aggregate
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
The size distribution of the aggregate determines how much binder is required. Aggregate with a very even size distribution has the biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as pasting the surfaces of the aggregate together, and is typically the most expensive component. Thus variation in sizes of the aggregate reduces the cost of concrete. The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.
Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients.
Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.
In addition to being decorative, exposed aggregate may add robustness to a concrete.
Constructing a rebar cage. This cage will be permanently embedded in poured concrete to create a reinforced concrete structure. Main article: Reinforced concrete
Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete adds either steel reinforcing bars, steel fibers, glass fibers, or plastic fibers to carry tensile loads.
Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing. (See the section on Concrete Production, below.)The common types of admixtures are as follows:
Inorganic materials that have pozzolanic or latent hydraulic properties, these very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements). Products which incorporate limestone, fly ash, blast furnace slag, and other useful materials with pozzolanic properties into the mix, are being tested and used. This development is due to cement production being one of the largest producers (at about 5 to 10%) of global greenhouse gas emissions, as well as lowering costs, improving concrete properties, and recycling wastes.
Concrete plant facility showing a Concrete mixer being filled from the ingredient silos.
Concrete production is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete production takes place in a large type of industrial facility called a concrete plant, or often a batch plant.
In general usage, concrete plants come in two main types, ready mix plants and central mix plants. A ready mix plant mixes all the ingredients except water, while a central mix plant mixes all the ingredients including water. A central mix plant offers more accurate control of the concrete quality through better measurements of the amount of water added, but must be placed closer to the work site where the concrete will be used, since hydration begins at the plant.
A concrete plant consists of large storage hoppers for various reactive ingredients like cement, storage for bulk ingredients like aggregate and water, mechanisms for the addition of various additives and amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities to dispense the mixed concrete, often to a concrete mixer truck.
Modern concrete is usually prepared as a viscous fluid, so that it may be poured into forms, which are containers erected in the field to give the concrete its desired shape. Concrete formwork can be prepared in several ways, such as Slip forming and Steel plate construction. Alternatively, concrete can be mixed into dryer, non-fluid forms and used in factory settings to manufacture Precast concrete products.
A wide variety of equipment is used for processing concrete, from hand tools to heavy industrial machinery. Whichever equipment builders use, however, the objective is to produce the desired building material; ingredients must be properly mixed, placed, shaped, and retained within time constraints. Any interruption in pouring the concrete can cause the initially placed material to begin to set before the next batch is added on top. This creates a horizontal plane of weakness called a cold joint between the two batches. Once the mix is where it should be, the curing process must be controlled to ensure that the concrete attains the desired attributes. During concrete preparation, various technical details may affect the quality and nature of the product.
When initially mixed, Portland cement and water rapidly form a gel of tangled chains of interlocking crystals, and components of the gel continue to react over time. Initially the gel is fluid, which improves workability and aids in placement of the material, but as the concrete sets, the chains of crystals join into a rigid structure, counteracting the fluidity of the gel and fixing the particles of aggregate in place. During curing, the cement continues to react with the residual water in a process of hydration. In properly formulated concrete, once this curing process has terminated the product has the desired physical and chemical properties. Among the qualities typically desired, are mechanical strength, low moisture permeability, and chemical and volumetric stability.
See also: Volumetric concrete mixer and Concrete mixer
Thorough mixing is essential for the production of uniform, high-quality concrete. For this reason equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work.
Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete. The paste is generally mixed in a high-speed, shear-type mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, superplasticizers, pigments, or silica fume. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.
Decorative plate made of Nano concrete with High-Energy Mixing (HEM) Pouring and smoothing out concrete at Palisades Park in Washington DC. Main article: Concrete slump test
Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability. Excessive water leads to increased bleeding or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate blend with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot readily be made more workable by addition of reasonable amounts of water. An undesirable gradation can mean using a large aggregate that is too large for the size of the formwork, or which has too few smaller aggregate grades to serve to fill the gaps between the larger grades, or using too little or too much sand for the same reason, or using too little water, or too much cement, or even using jagged crushed stone instead of smoother round aggregate such as pebbles. Any combination of these factors and others may result in a mix which is too harsh, i.e., which does not flow or spread out smoothly, is difficult to get into the formwork, and which is difficult to surface finish.
Workability can be measured by the concrete slump test, a simple measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod to consolidate the layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches (25 or 50 mm) out of one foot (305 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by the flow table test.
Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio. Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.
High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.
After mixing, concrete is a fluid and can be pumped to the location where needed.
A concrete slab ponded while curing.
A common misconception is that concrete dries as it sets, but the opposite is true - damp concrete sets better than dry concrete. In other words, "hydraulic cement" needs water to become strong. Too much water is counterproductive, but too little water is deleterious. Curing allows concrete to achieve optimal strength and hardness. Curing is the hydration process that occurs after the concrete has been placed. In chemical terms, curing allows calcium-silicate hydrate (C-S-H) to form. To gain strength and harden fully, concrete curing requires time. In around 4 weeks, typically over 90% of the final strength is reached, although strengthening may continue for decades. The conversion of calcium hydroxide in the concrete into calcium carbonate from absorption of CO2 over several decades further strengthens the concrete and makes it more resistant to damage. This carbonation reaction, however, lowers the pH of the cement pore solution and can corrode the reinforcement bars.
Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation. Addition of short-cut polymer fibers can improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression strength.
Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can cause scaling, reduced strength, poor abrasion resistance and cracking.
During the curing period, concrete is ideally maintained at controlled temperature and humidity. To ensure full hydration during curing, concrete slabs are often sprayed with "curing compounds" that create a water-retaining film over the concrete. Typical films are made of wax or related hydrophobic compounds. After the concrete is sufficiently cured, the film is allowed to abrade from the concrete through normal use.
Traditional conditions for curing involve by spraying or ponding the concrete surface with water. The picture to the right shows one of many ways to achieve this, ponding – submerging setting concrete in water and wrapping in plastic to prevent dehydration. Additional common curing methods include wet burlap and plastic sheeting covering the fresh concrete.
For higher-strength applications, accelerated curing techniques may be applied to the concrete. One common technique involves heating the poured concrete with steam, which serves to both keep it damp and raise the temperature, so that the hydration process proceeds more quickly and more thoroughly.
Main article: Pervious concrete
Pervious concrete is a mix of specially graded coarse aggregate, cement, water and little-to-no fine aggregates. This concrete is also known as "no-fines" or porous concrete. Mixing the ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened concrete contains interconnected air voids totalling approximately 15 to 25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze–thaw climates to minimize the possibility of frost damage.
Two-layered pavers, top layer made of pigmented HEM Nanoconcrete.
Nanoconcrete is created by high-energy mixing (HEM) of cement, sand and water. To ensure the mixing is thorough enough to create nano-concrete, the mixer must apply a total mixing power to the mixture of 30 - 600 watts per kilogram of the mix. This mixing must continue long enough to yield a net specific energy expended upon the mix of at least 5000 joules per kilogram of the mix. A plasticizer or a superplasticizer is then added to the activated mixture which can later be mixed with aggregates in a conventional concrete mixer. In the HEM process, the intense mixing of cement and water with sand provides dissipation of energy and increases shear stresses on the surface of cement particles. This intense mixing serves to divide the cement particles into extremely fine nanometer scale sizes, which provides for extremely thorough mixing. This results in the increased volume of water interacting with cement and acceleration of Calcium Silicate Hydrate (C-S-H) colloid creation.
The initial natural process of cement hydration with formation of colloidal globules about 5 nm in diameter spreads into the entire volume of cement – water matrix as the energy expended upon the mix approaches and exceeds 5000 joules per kilogram.
The liquid activated high-energy mixture can be used by itself for casting small architectural details and decorative items, or foamed (expanded) for lightweight concrete. HEM Nanoconcrete hardens in low and subzero temperature conditions and possesses an increased volume of gel, which reduces capillarity in solid and porous materials.
Bacteria such as Bacillus pasteurii, Bacillus pseudofirmus, Bacillus cohnii, Sporosarcina pasteuri, and Arthrobacter crystallopoietes increase the compression strength of concrete through their biomass. Not all bacteria increase the strength of concrete significantly with their biomass.:143 Bacillus sp. CT-5. can reduce corrosion of reinforcement in reinforced concrete by up to four times. Sporosarcina pasteurii reduces water and chloride permeability. B. pasteurii increases resistance to acid.:146Bacillus pasteurii and B. sphaericuscan induce calcium carbonate precipitation in the surface of cracks, adding compression strength.:147
Main article: Polymer concrete
Polymer concretes are mixtures of aggregate and any of various polymers and may be reinforced. The cement is more costly than lime-based cements, but polymer concretes nevertheless have advantages, they have significant tensile strength even without reinforcement, and they are largely impervious to water. They are frequently used for repair and construction of other applications such as drains.
Concrete, when ground, can result in the creation of hazardous dust. The National Institute for Occupational Safety and Health in the United States recommends attaching local exhaust ventilation shrouds to electric concrete grinders to control the spread of this dust.
Main article: Properties of concrete
Concrete has relatively high compressive strength, but much lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.
Tests can be performed to ensure that the properties of concrete correspond to specifications for the application.
Compression testing of a concrete cylinder
Different mixes of concrete ingredients produce different strengths. Concrete strength values are usually specified as the compressive strength of either a cylindrical or cubic specimen, where these values usually differ by around 20% for the same concrete mix.
Different strengths of concrete are used for different purposes. Very low-strength - 14 MPa (2,000 psi) or less - concrete may be used when the concrete must be lightweight. Lightweight concrete is often achieved by adding air, foams, or lightweight aggregates, with the side effect that the strength is reduced. For most routine uses, 20 MPa (2,900 psi) to 32 MPa (4,600 psi) concrete is often used. 40 MPa (5,800 psi) concrete is readily commercially available as a more durable, although more expensive, option. Higher-strength concrete is often used for larger civil projects. Strengths above 40 MPa (5,800 psi) are often used for specific building elements. For example, the lower floor columns of high-rise concrete buildings may use concrete of 80 MPa (11,600 psi) or more, to keep the size of the columns small. Bridges may use long beams of high-strength concrete to lower the number of spans required. Occasionally, other structural needs may require high-strength concrete. If a structure must be very rigid, concrete of very high strength may be specified, even much stronger than is required to bear the service loads. Strengths as high as 130 MPa (18,900 psi) have been used commercially for these reasons.
The Buffalo City Court Building in Buffalo, NY.
Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time. Structures made of concrete can have a long service life. Concrete is used more than any other manmade material in the world. As of 2006, about 7.5 billion cubic meters of concrete are made each year, more than one cubic meter for every person on Earth.
Main article: Mass concrete Aerial photo of reconstruction at Taum Sauk (Missouri) pumped storage facility in late November, 2009. After the original reservoir failed, the new reservoir was made of roller-compacted concrete.
Large concrete structures such as dams, navigation locks, large mat foundations, and large breakwaters generate excessive heat during cement hydration and associated expansion. To mitigate these effects post-cooling is commonly applied during construction. An early example at Hoover Dam, installed a network of pipes between vertical concrete placements to circulate cooling water during the curing process to avoid damaging overheating. Similar systems are still used; depending on volume of the pour, the concrete mix used, and ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.
Another approach to mass concrete structures that is becoming more widespread is the use of roller-compacted concrete, which uses much lower amounts of cement and water than conventional concrete mixtures and is generally not poured into place. Instead it is placed in thick layers as a semi-dry material and compacted into a dense, strong mass with rolling compactors. Because it uses less cementitious material, roller-compacted concrete has a much lower cooling requirement than conventional concrete.
Main article: Decorative concrete Black basalt polished concrete floor
Raw concrete surfaces tend to be porous, and have a relatively uninteresting appearance. Many different finishes can be applied to improve the appearance and preserve the surface against staining, water penetration, and freezing.
Examples of improved appearance include stamped concrete where the wet concrete has a pattern impressed on the surface, to give a paved, cobbled or brick-like effect, and may be accompanied with coloration. Another popular effect for flooring and table tops is polished concrete where the concrete is polished optically flat with diamond abrasives and sealed with polymers or other sealants.
Other finishes can be achieved with chiselling, or more conventional techniques such as painting or covering it with other materials.
The proper treatment of the surface of concrete, and therefore its characteristics, is an important stage in the construction and renovation of architectural structures.
40-foot cacti decorate a sound/retaining wall in Scottsdale, Arizona Main article: Prestressed concrete
Prestressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose those experienced in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement. For example, a horizontal beam tends to sag. Prestressed reinforcement along the bottom of the beam counteracts this. In pre-tensioned concrete, the prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.
More than 55,000 miles (89,000 km) of highways in the United States are paved with this material. Reinforced concrete, prestressed concrete and precast concrete are the most widely used types of concrete functional extensions in modern days. See Brutalism.
Extreme weather conditions (extreme heat or cold; windy condition, and humidity variations) can significantly alter the quality of concrete. In cold weather concreting, many precautions are observed. Low temperatures significantly slow the chemical reactions involved in hydration of cement, thus affecting the strength development. Preventing freezing is the most important precaution, as formation of ice crystals can cause damage to the crystalline structure of the hydrated cement paste. If the surface of the concrete pour is insulated from the outside temperatures, the heat of hydration will prevent freezing.
The American Concrete Institute (ACI) definition of cold weather concreting, ACI 306, is:
In Canada, where temperatures tend to be much lower during the cold season, the following criteria is used by CSA A23.1:
The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.
Concrete roads are more fuel efficient to drive on, more reflective and last significantly longer than other paving surfaces, yet have a much smaller market share than other paving solutions. Modern-paving methods and design practices have changed the economics of concrete paving, so that a well-designed and placed concrete pavement will be less expensive on initial costs and significantly less expensive over the life cycle. Another major benefit is that pervious concrete can be used, which eliminates the need to place storm drains near the road, and reducing the need for slightly sloped roadway to help rainwater to run off. No longer requiring discarding rainwater through use of drains also means that less electricity is needed (more pumping is otherwise needed in the water-distribution system), and no rainwater gets polluted as it no longer mixes with polluted water. Rather, it is immediately absorbed by the ground.
Energy requirements for transportation of concrete are low because it is produced locally from local resources, typically manufactured within 100 kilometers of the job site. Similarly, relatively little energy is used in producing and combining the raw materials (although large amounts of CO2 are produced by the chemical reactions in cement manufacture). The overall embodied energy of concrete at roughly 1 to 1.5 megajoules per kilogram is therefore lower than for most structural and construction materials.
Once in place, concrete offers great energy efficiency over the lifetime of a building. Concrete walls leak air far less than those made of wood frames. Air leakage accounts for a large percentage of energy loss from a home. The thermal mass properties of concrete increase the efficiency of both residential and commercial buildings. By storing and releasing the energy needed for heating or cooling, concrete's thermal mass delivers year-round benefits by reducing temperature swings inside and minimizing heating and cooling costs. While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both external insulation and thermal mass to create an energy-efficient building. Insulating concrete forms (ICFs) are hollow blocks or panels made of either insulating foam or rastra that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.
A modern building: Boston City Hall (completed 1968) is constructed largely of concrete, both precast and poured in place. Of Brutalist architecture, it was voted "The World's Ugliest Building" in 2008.
Concrete buildings are more resistant to fire than those constructed using steel frames, since concrete has lower heat conductivity than steel and can thus last longer under the same fire conditions. Concrete is sometimes used as a fire protection for steel frames, for the same effect as above. Concrete as a fire shield, for example Fondu fyre, can also be used in extreme environments like a missile launch pad.
Options for non-combustible construction include floors, ceilings and roofs made of cast-in-place and hollow-core precast concrete. For walls, concrete masonry technology and Insulating Concrete Forms (ICFs) are additional options. ICFs are hollow blocks or panels made of fireproof insulating foam that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.
Concrete also provides good resistance against externally applied forces such as high winds, hurricanes, and tornadoes owing to its lateral stiffness, which results in minimal horizontal movement. However this stiffness can work against certain types of concrete structures, particularly where a relatively higher flexing structure is required to resist more extreme forces.
As discussed above, concrete is very strong in compression, but weak in tension. Larger earthquakes can generate very large shear loads on structures. These shear loads subject the structure to both tensile and compressional loads. Concrete structures without reinforcement, like other unreinforced masonry structures, can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest earthquake risks globally. These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. school buildings in Istanbul, Turkey).
Concrete spalling caused by the corrosion of rebar Main article: Concrete degradation
Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel reinforcement bars, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water). The micro fungi Aspergillus Alternaria and Cladosporium were able to grow on samples of concrete used as a radioactive waste barrier in the Chernobyl reactor; leaching aluminium, iron, calcium and silicon.
The Tunkhannock Viaduct was begun in 1912 and is still in regular service as of 2017.
Concrete can be viewed as a form of artificial sedimentary rock. As a type of mineral, the compounds of which it is composed are extremely stable. Many concrete structures are built with an expected lifetime of approximately 100 years, but researchers have suggested that adding silica fume could extend the useful life of bridges and other concrete uses to as long as 16,000 years. Coatings are also available to protect concrete from damage, and extend the useful life. Epoxy coatings may be applied only to interior surfaces, though, as they would otherwise trap moisture in the concrete.
A self-healing concrete has been developed that can also last longer than conventional concrete. Another option is to use hydrophobic concrete.
Concrete mixing plant in Birmingham, Alabama in 1936
Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, highways, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats. Concrete is used in large quantities almost everywhere mankind has a need for infrastructure. Concrete is one of the most frequently used building materials in animal houses and for manure and silage storage structures in agriculture.
The amount of concrete used worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminum combined. Concrete's use in the modern world is exceeded only by that of naturally occurring water.
Concrete is also the basis of a large commercial industry. Globally, the ready-mix concrete industry, the largest segment of the concrete market, is projected to exceed $100 billion in revenue by 2015. In the United States alone, concrete production is a $30-billion-per-year industry, considering only the value of the ready-mixed concrete sold each year. Given the size of the concrete industry, and the fundamental way concrete is used to shape the infrastructure of the modern world, it is difficult to overstate the role this material plays today.
Main article: Environmental impact of concrete
The manufacture and use of concrete produce a wide range of environmental and social consequences. Some are harmful, some welcome, and some both, depending on circumstances.
A major component of concrete is cement, which similarly exerts environmental and social effects.:142 The cement industry is one of the three primary producers of carbon dioxide, a major greenhouse gas (the other two being the energy production and transportation industries). As of 2001, the production of Portland cement contributed 7% to global anthropogenic CO2 emissions, largely due to the sintering of limestone and clay at 1,500 °C (2,730 °F).
Concrete is used to create hard surfaces that contribute to surface runoff, which can cause heavy soil erosion, water pollution, and flooding, but conversely can be used to divert, dam, and control flooding.
Concrete is a contributor to the urban heat island effect, though less so than asphalt.
Workers who cut, grind or polish concrete are at risk of inhaling airborne silica, which can lead to silicosis. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution.
The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and radioactivity. Fresh concrete (before curing is complete) is highly alkaline and must be handled with proper protective equipment.
Recycled crushed concrete, to be reused as granular fill, is loaded into a semi-dump truck. Main article: Concrete recycling
Concrete recycling is an increasingly common method for disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits.
Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crushing machine, often along with asphalt, bricks and rocks.
Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On 3 March 1983, a government-funded research team (the VIRL research.codep) estimated that almost 17% of worldwide landfill was by-products of concrete based waste.
The world record for the largest concrete pour in a single project is the Three Gorges Dam in Hubei Province, China by the Three Gorges Corporation. The amount of concrete used in the construction of the dam is estimated at 16 million cubic meters over 17 years. The previous record was 12.3 million cubic meters held by Itaipu hydropower station in Brazil.
The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was pumped through a vertical height of 715 m (2,346 ft).
The world record for the largest continuously poured concrete raft was achieved in August 2007 in Abu Dhabi by contracting firm Al Habtoor-CCC Joint Venture and the concrete supplier is Unibeton Ready Mix. The pour (a part of the foundation for the Abu Dhabi's Landmark Tower) was 16,000 cubic meters of concrete poured within a two-day period. The previous record, 13,200 cubic meters poured in 54 hours despite a severe tropical storm requiring the site to be covered with tarpaulins to allow work to continue, was achieved in 1992 by joint Japanese and South Korean consortiums Hazama Corporation and the Samsung C&T Corporation for the construction of the Petronas Towers in Kuala Lumpur, Malaysia.
The world record for largest continuously poured concrete floor was completed 8 November 1997, in Louisville, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of 225,000 square feet (20,900 m2) of concrete placed within a 30-hour period, finished to a flatness tolerance of FF 54.60 and a levelness tolerance of FL 43.83. This surpassed the previous record by 50% in total volume and 7.5% in total area.
The record for the largest continuously placed underwater concrete pour was completed 18 October 2010, in New Orleans, Louisiana by contractor C. J. Mahan Construction Company, LLC of Grove City, Ohio. The placement consisted of 10,251 cubic yards of concrete placed in a 58.5 hour period using two concrete pumps and two dedicated concrete batch plants. Upon curing, this placement allows the 50,180-square-foot (4,662 m2) cofferdam to be dewatered approximately 26 feet (7.9 m) below sea level to allow the construction of the Inner Harbor Navigation Canal Sill & Monolith Project to be completed in the dry.
"Boulder Dam" redirects here. For other uses, see Boulder Dam (disambiguation).
Hoover Dam, originally known as Boulder Dam from 1933 to 1947, when it was officially renamed Hoover Dam by a joint resolution of Congress, is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the U.S. states of Nevada and Arizona. It was constructed between 1931 and 1936 during the Great Depression and was dedicated on September 30, 1935, by President Franklin D. Roosevelt. Its construction was the result of a massive effort involving thousands of workers, and cost over one hundred lives. The dam was named after President Herbert Hoover.
Since about 1900, the Black Canyon and nearby Boulder Canyon had been investigated for their potential to support a dam that would control floods, provide irrigation water and produce hydroelectric power. In 1928, Congress authorized the project. The winning bid to build the dam was submitted by a consortium called Six Companies, Inc., which began construction on the dam in early 1931. Such a large concrete structure had never been built before, and some of the techniques were unproven. The torrid summer weather and lack of facilities near the site also presented difficulties. Nevertheless, Six Companies turned over the dam to the federal government on March 1, 1936, more than two years ahead of schedule.
Hoover Dam impounds Lake Mead, the largest reservoir in the United States by volume (when it is full). The dam is located near Boulder City, Nevada, a municipality originally constructed for workers on the construction project, about 30 mi (48 km) southeast of Las Vegas, Nevada. The dam's generators provide power for public and private utilities in Nevada, Arizona, and California. Hoover Dam is a major tourist attraction; nearly a million people tour the dam each year. The heavily traveled U.S. Route 93 (US 93) ran along the dam's crest until October 2010, when the Hoover Dam Bypass opened.
As the United States developed the Southwest, the Colorado River was seen as a potential source of irrigation water. An initial attempt at diverting the river for irrigation purposes occurred in the late 1890s, when land speculator William Beatty built the Alamo Canal just north of the Mexican border; the canal dipped into Mexico before running to a desolate area Beatty named the Imperial Valley. Though water from the Imperial Canal allowed for the widespread settlement of the valley, the canal proved expensive to maintain. After a catastrophic breach that caused the Colorado River to fill the Salton Sea, the Southern Pacific Railroad spent $3 million in 1906–07 to stabilize the waterway, an amount it hoped vainly would be reimbursed by the Federal Government. Even after the waterway was stabilized, it proved unsatisfactory because of constant disputes with landowners on the Mexican side of the border.
River view of the future site of Hoover Dam, circa 1904
As the technology of electric power transmission improved, the Lower Colorado was considered for its hydroelectric-power potential. In 1902, the Edison Electric Company of Los Angeles surveyed the river in the hope of building a 40-foot (12 m) rock dam which could generate 10,000 horsepower (7,500 kW). However, at the time, the limit of transmission of electric power was 80 miles (130 km), and there were few customers (mostly mines) within that limit. Edison allowed land options it held on the river to lapse—including an option for what became the site of Hoover Dam.
In the following years, the Bureau of Reclamation (BOR), known as the Reclamation Service at the time, also considered the Lower Colorado as the site for a dam. Service chief Arthur Powell Davis proposed using dynamite to collapse the walls of Boulder Canyon, 20 miles (32 km) north of the eventual dam site, into the river. The river would carry off the smaller pieces of debris, and a dam would be built incorporating the remaining rubble. In 1922, after considering it for several years, the Reclamation Service finally rejected the proposal, citing doubts about the unproven technique and questions as to whether it would in fact save money.
Sketch of proposed Boulder Canyon dam site and reservoir, circa 1921
In 1922, the Reclamation Service presented a report calling for the development of a dam on the Colorado River for flood control and electric power generation. The report was principally authored by Davis, and was called the Fall-Davis report after Interior Secretary Albert Fall. The Fall-Davis report cited use of the Colorado River as a federal concern, because the river's basin covered several states, and the river eventually entered Mexico. Though the Fall-Davis report called for a dam "at or near Boulder Canyon", the Reclamation Service (which was renamed the Bureau of Reclamation the following year) found that canyon unsuitable. One potential site at Boulder Canyon was bisected by a geologic fault; two others were so narrow there was no space for a construction camp at the bottom of the canyon or for a spillway. The Service investigated Black Canyon and found it ideal; a railway could be laid from the railhead in Las Vegas to the top of the dam site. Despite the site change, the dam project was referred to as the "Boulder Canyon Project".
With little guidance on water allocation from the Supreme Court, proponents of the dam feared endless litigation. A Colorado attorney proposed that the seven states which fell within the river's basin (California, Nevada, Arizona, Utah, New Mexico, Colorado and Wyoming) form an interstate compact, with the approval of Congress. Such compacts were authorized by Article I of the United States Constitution but had never been concluded among more than two states. In 1922, representatives of seven states met with then-Secretary of Commerce Herbert Hoover. Initial talks produced no result, but when the Supreme Court handed down the Wyoming v. Colorado decision undermining the claims of the upstream states, they became anxious to reach an agreement. The resulting Colorado River Compact was signed on November 24, 1922.
Legislation to authorize the dam was introduced repeatedly by Representative Phil Swing (R-Calif.) and Senator Hiram Johnson (R-Calif.), but representatives from other parts of the country considered the project as hugely expensive and one that would mostly benefit California. The 1927 Mississippi flood made Midwestern and Southern congressmen and senators more sympathetic toward the dam project. On March 12, 1928, the failure of the St. Francis Dam, constructed by the city of Los Angeles, caused a disastrous flood that killed up to 600 people. As that dam was a curved-gravity type, similar in design to the arch-gravity as was proposed for the Black Canyon dam, opponents claimed that the Black Canyon dam's safety could not be guaranteed. Congress authorized a board of engineers to review plans for the proposed dam. The Colorado River Board found the project feasible, but warned that should the dam fail, every downstream Colorado River community would be destroyed, and that the river might change course and empty into the Salton Sea. The Board cautioned: "To avoid such possibilities, the proposed dam should be constructed on conservative if not ultra-conservative lines."
On December 21, 1928 President Coolidge signed the bill authorizing the dam. The Boulder Canyon Project Act appropriated $165 million for the Hoover Dam along with the downstream Imperial Dam and All-American Canal, a replacement for Beatty's canal entirely on the U.S. side of the border. It also permitted the compact to go into effect when at least six of the seven states approved it. This occurred on March 6, 1929 with Utah's ratification; Arizona did not approve it until 1944.
Hoover Dam architectural plans
Even before Congress approved the Boulder Canyon Project, the Bureau of Reclamation was considering what kind of dam should be used. Officials eventually decided on a massive concrete arch-gravity dam, the design of which was overseen by the Bureau's chief design engineer John L. Savage. The monolithic dam would be thick at the bottom and thin near the top, and would present a convex face towards the water above the dam. The curving arch of the dam would transmit the water's force into the abutments, in this case the rock walls of the canyon. The wedge-shaped dam would be 660 ft (200 m) thick at the bottom, narrowing to 45 ft (14 m) at the top, leaving room for a highway connecting Nevada and Arizona.
On January 10, 1931, the Bureau made the bid documents available to interested parties, at five dollars a copy. The government was to provide the materials; but the contractor was to prepare the site and build the dam. The dam was described in minute detail, covering 100 pages of text and 76 drawings. A $2 million bid bond was to accompany each bid; the winner would have to post a $5 million performance bond. The contractor had seven years to build the dam, or penalties would ensue.
The Wattis Brothers, heads of the Utah Construction Company, were interested in bidding on the project, but lacked the money for the performance bond. They lacked sufficient resources even in combination with their longtime partners, Morrison-Knudsen, which employed the nation's leading dam builder, Frank Crowe. They formed a joint venture to bid for the project with Pacific Bridge Company of Portland, Oregon; Henry J. Kaiser & W. A. Bechtel Company of San Francisco; MacDonald & Kahn Ltd. of Los Angeles; and the J.F. Shea Company of Portland, Oregon. The joint venture was called Six Companies, Inc. as Bechtel and Kaiser were considered one company for purposes of 6 in the name. The name was descriptive and was an inside joke among the San Franciscans in the bid, where "Six Companies" was also a Chinese benevolent association in the city. There were three valid bids, and Six Companies' bid of $48,890,955 was the lowest, within $24,000 of the confidential government estimate of what the dam would cost to build, and five million dollars less than the next-lowest bid.
The city of Las Vegas had lobbied hard to be the headquarters for the dam construction, closing its many speakeasies when the decision maker, Secretary of the Interior Ray Wilbur came to town. Instead, Wilbur announced in early 1930 that a model city was to be built in the desert near the dam site. This town became known as Boulder City, Nevada. Construction of a rail line joining Las Vegas and the dam site began in September 1930.
Workers on a "Jumbo Rig"; used for drilling Hoover Dam's tunnels "Apache Indians employed as high-scalers on the construction of Hoover Dam." – NARA
Soon after the dam was authorized, increasing numbers of unemployed people converged on southern Nevada. Las Vegas, then a small city of some 5,000, saw between 10,000 and 20,000 unemployed descend on it. A government camp was established for surveyors and other personnel near the dam site; this soon became surrounded by a squatters' camp. Known as McKeeversville, the camp was home to men hoping for work on the project, together with their families. Another camp, on the flats along the Colorado River, was officially called Williamsville, but was known to its inhabitants as "Ragtown". When construction began, Six Companies hired large numbers of workers, with more than 3,000 on the payroll by 1932 and with employment peaking at 5,251 in July 1934. "Mongolian" (Chinese) labor was prevented by the construction contract, while the number of blacks employed by Six Companies never exceeded thirty, mostly lowest-pay-scale laborers in a segregated crew, who were issued separate water buckets.
As part of the contract, Six Companies, Inc. was to build Boulder City to house the workers. The original timetable called for Boulder City to be built before the dam project began, but President Hoover ordered work on the dam to begin in March 1931 rather than in October. The company built bunkhouses, attached to the canyon wall, to house 480 single men at what became known as River Camp. Workers with families were left to provide their own accommodations until Boulder City could be completed, and many lived in Ragtown. The site of Hoover Dam endures extremely hot weather, and the summer of 1931 was especially torrid, with the daytime high averaging 119.9 °F (48.8 °C). Sixteen workers and other riverbank residents died of heat prostration between June 25 and July 26, 1931.
General Superintendent Frank Crowe (right) with Bureau of Reclamation engineer Walker Young in 1935
The Industrial Workers of the World (IWW or "Wobblies"), though much-reduced from their heyday as militant labor organizers in the early years of the century, hoped to unionize the Six Companies workers by capitalizing on their discontent. They sent eleven organizers, several of whom were arrested by Las Vegas police. On August 7, 1931, the company cut wages for all tunnel workers. Although the workers sent away the organizers, not wanting to be associated with the "Wobblies", they formed a committee to represent them with the company. The committee drew up a list of demands that evening and presented them to Crowe the following morning. He was noncommittal. The workers hoped that Crowe, the general superintendent of the job, would be sympathetic; instead he gave a scathing interview to a newspaper, describing the workers as "malcontents".
On the morning of the 9th, Crowe met with the committee and told them that management refused their demands, was stopping all work, and was laying off the entire work force, except for a few office workers and carpenters. The workers were given until 5 p.m. to vacate the premises. Concerned that a violent confrontation was imminent, most workers took their paychecks and left for Las Vegas to await developments. Two days later, the remainder were talked into leaving by law enforcement. On August 13, the company began hiring workers again, and two days later, the strike was called off. While the workers received none of their demands, the company guaranteed there would be no further reductions in wages. Living conditions began to improve as the first residents moved into Boulder City in late 1931.
A second labor action took place in July 1935, as construction on the dam wound down. When a Six Companies manager altered working times to force workers to take lunch on their own time, workers responded with a strike. Emboldened by Crowe's reversal of the lunch decree, workers raised their demands to include a $1-per-day raise. The company agreed to ask the Federal government to supplement the pay, but no money was forthcoming from Washington. The strike ended.
Overview of dam mechanisms; diversion tunnels shown
Before the dam could be built, the Colorado River needed to be diverted away from the construction site. To accomplish this, four diversion tunnels were driven through the canyon walls, two on the Nevada side and two on the Arizona side. These tunnels were 56 ft (17 m) in diameter. Their combined length was nearly 16,000 ft, or more than 3 miles (5 km). The contract required these tunnels to be completed by October 1, 1933, with a $3,000-per-day fine to be assessed for any delay. To meet the deadline, Six Companies had to complete work by early 1933, since only in late fall and winter was the water level in the river low enough to safely divert.
Tunneling began at the lower portals of the Nevada tunnels in May 1931. Shortly afterward, work began on two similar tunnels in the Arizona canyon wall. In March 1932, work began on lining the tunnels with concrete. First the base, or invert, was poured. Gantry cranes, running on rails through the entire length of each tunnel were used to place the concrete. The sidewalls were poured next. Movable sections of steel forms were used for the sidewalls. Finally, using pneumatic guns, the overheads were filled in. The concrete lining is 3 feet (1 m) thick, reducing the finished tunnel diameter to 50 ft (15 m). The river was diverted into the two Arizona tunnels on November 13, 1932; the Nevada tunnels were kept in reserve for high water. This was done by exploding a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble into the river until its natural course was blocked.
Following the completion of the dam, the entrances to the two outer diversion tunnels were sealed at the opening and halfway through the tunnels with large concrete plugs. The downstream halves of the tunnels following the inner plugs are now the main bodies of the spillway tunnels. The inner diversion tunnels were plugged at approximately one-third of their length, beyond which they now carry steel pipes connecting the intake towers to the power plant and outlet works. The inner tunnels' outlets are equipped with gates that can be closed to drain the tunnels for maintenance.
To protect the construction site from the Colorado River and to facilitate the river's diversion, two cofferdams were constructed. Work on the upper cofferdam began in September 1932, even though the river had not yet been diverted. The cofferdams were designed to protect against the possibility of the river flooding a site at which two thousand men might be at work, and their specifications were covered in the bid documents in nearly as much detail as the dam itself. The upper cofferdam was 96 ft (29 m) high, and 750 feet (230 m) thick at its base, thicker than the dam itself. It contained 650,000 cubic yards (500,000 m3) of material.
Looking down at "high scalers" above the Colorado River
When the cofferdams were in place and the construction site was drained of water, excavation for the dam foundation began. For the dam to rest on solid rock, it was necessary to remove accumulated erosion soils and other loose materials in the riverbed until sound bedrock was reached. Work on the foundation excavations was completed in June 1933. During this excavation, approximately 1,500,000 cu yd (1,100,000 m3) of material was removed. Since the dam was an arch-gravity type, the side-walls of the canyon would bear the force of the impounded lake. Therefore, the side-walls were excavated too, to reach virgin rock as weathered rock might provide pathways for water seepage.
The men who removed this rock were called "high scalers". While suspended from the top of the canyon with ropes, the high-scalers climbed down the canyon walls and removed the loose rock with jackhammers and dynamite. Falling objects were the most common cause of death on the dam site; the high scalers' work thus helped ensure worker safety. One high scaler was able to save life in a more direct manner: when a government inspector lost his grip on a safety line and began tumbling down a slope towards almost certain death, a high scaler was able to intercept him and pull him into the air. The construction site had, even then, become a magnet for tourists; the high scalers were prime attractions and showed off for the watchers. The high scalers received considerable media attention, with one worker dubbed the "Human Pendulum" for swinging co-workers (and, at other times, cases of dynamite) across the canyon. To protect themselves against falling objects, some high scalers took cloth hats and dipped them in tar, allowing them to harden. When workers wearing such headgear were struck hard enough to inflict broken jaws, they sustained no skull damage, Six Companies ordered thousands of what initially were called "hard boiled hats" (later "hard hats") and strongly encouraged their use.
The cleared, underlying rock foundation of the dam site was reinforced with grout, called a grout curtain. Holes were driven into the walls and base of the canyon, as deep as 150 feet (46 m) into the rock, and any cavities encountered were to be filled with grout. This was done to stabilize the rock, to prevent water from seeping past the dam through the canyon rock, and to limit "uplift"—upward pressure from water seeping under the dam. The workers were under severe time constraints due to the beginning of the concrete pour, and when they encountered hot springs or cavities too large to readily fill, they moved on without resolving the problem. A total of 58 of the 393 holes were incompletely filled. After the dam was completed and the lake began to fill, large numbers of significant leaks into the dam caused the Bureau of Reclamation to look into the situation. It found that the work had been incompletely done, and was based on less than a full understanding of the canyon's geology. New holes were drilled from inspection galleries inside the dam into the surrounding bedrock. It took nine years (1938–47) under relative secrecy to complete the supplemental grout curtain.
Columns of Hoover Dam being filled with concrete, February 1934 (looking upstream from the Nevada rim)
The first concrete was poured into the dam on June 6, 1933, 18 months ahead of schedule. Since concrete heats and contracts as it cures, the potential for uneven cooling and contraction of the concrete posed a serious problem. Bureau of Reclamation engineers calculated that if the dam was built in a single continuous pour, the concrete would take 125 years to cool, and the resulting stresses would cause the dam to crack and crumble. Instead, the ground where the dam would rise was marked with rectangles, and concrete blocks in columns were poured, some as large as 50 ft square (15 m) and 5 feet (1.5 m) high. Each five-foot form contained a series of 1-inch (25 mm) steel pipes; cool riverwater would be poured through the pipes, followed by ice-cold water from a refrigeration plant. When an individual block had cured and had stopped contracting, the pipes were filled with grout. Grout was also used to fill the hairline spaces between columns, which were grooved to increase the strength of the joins.
The concrete was delivered in huge steel buckets 7 feet high (2.1 m) and almost 7 feet in diameter; Crowe was awarded two patents for their design. These buckets, which weighed 20 short tons (18 t) when full, were filled at two massive concrete plants on the Nevada side, and were delivered to the site in special railcars. The buckets were then suspended from aerial cableways, which were used to deliver the bucket to a specific column. As the required grade of aggregate in the concrete differed depending on placement in the dam (from pea-sized gravel to 9-inch or 23 cm stones), it was vital that the bucket be maneuvered to the proper column. When the bottom of the bucket opened up, disgorging 8 cu yd (6.1 m3) of concrete, a team of men worked it throughout the form. Although there are myths that men were caught in the pour and are entombed in the dam to this day, each bucket only deepened the concrete in a form by an inch, and Six Companies engineers would not have permitted a flaw caused by the presence of a human body.
A total of 3,250,000 cubic yards (2,480,000 m3) of concrete was used in the dam before concrete pouring ceased on May 29, 1935. In addition, 1,110,000 cu yd (850,000 m3) were used in the power plant and other works. More than 582 miles (937 km) of cooling pipes were placed within the concrete. Overall, there is enough concrete in the dam to pave a two-lane highway from San Francisco to New York. Concrete cores were removed from the dam for testing in 1995; they showed that "Hoover Dam's concrete has continued to slowly gain strength" and the dam is composed of a "durable concrete having a compressive strength exceeding the range typically found in normal mass concrete". Hoover Dam concrete is not subject to alkali–silica reaction (ASR) as the Hoover Dam builders happened to use nonreactive aggregate, unlike that at downstream Parker Dam, where ASR has caused measurable deterioration.
The upstream face of Hoover Dam slowly disappears as Lake Mead fills, May 1935 (looking downstream from the Arizona rim)
With most work finished on the dam itself (the powerhouse remained uncompleted), a formal dedication ceremony was arranged for September 30, 1935, to coincide with a western tour being made by President Franklin D. Roosevelt. The morning of the dedication, it was moved forward three hours from 2 p.m. Pacific time to 11 a.m.; this was done because Secretary of the Interior Harold L. Ickes had reserved a radio slot for the President for 2 p.m. but officials did not realize until the day of the ceremony that the slot was for 2 p.m. Eastern Time. Despite the change in the ceremony time, and temperatures of 102 °F (39 °C), 10,000 people were present for the President's speech, in which he avoided mentioning the name of former President Hoover, who was not invited to the ceremony. To mark the occasion, a three-cent stamp was issued by the United States Post Office Department—bearing the name "Boulder Dam", the official name of the dam between 1933 and 1947. After the ceremony, Roosevelt made the first visit by any American president to Las Vegas.
Most work had been completed by the dedication, and Six Companies negotiated with the government through late 1935, and early 1936, to settle all claims and arrange for the formal transfer of the dam to the Federal Government. The parties came to an agreement and on March 1, 1936, Secretary Ickes formally accepted the dam on behalf of the government. Six Companies was not required to complete work on one item, a concrete plug for one of the bypass tunnels, as the tunnel had to be used to take in irrigation water until the powerhouse went into operation.
Oskar J. W. Hansen's memorial at the dam which reads in part "They died to make the desert bloom."
There were 112 deaths associated with the construction of the dam. The first was J. G. Tierney, a surveyor who drowned on December 20, 1922, while looking for an ideal spot for the dam. The last death on the project's official fatality list occurred on December 20, 1935, when an "electrician's helper", Patrick Tierney, the son of J. G. Tierney, fell from an intake tower. Included in the fatality list are three workers, one in 1932 and two in 1933, who committed suicide onsite. Ninety-six of the deaths occurred during construction at the site. Of the 112 fatalities, 91 were Six Companies employees, three were BOR employees, and one was a visitor to the site, with the remainder employees of various contractors not part of Six Companies.
Not included in the official number of fatalities were deaths that were recorded as pneumonia. Workers alleged that this diagnosis was a cover for death from carbon monoxide poisoning (brought on by the use of gasoline-fueled vehicles in the diversion tunnels), and a classification used by Six Companies to avoid paying compensation claims. The site's diversion tunnels frequently reached 140 °F (60 °C), enveloped in thick plumes of vehicle exhaust gases. A total of 42 workers were recorded as having died from pneumonia; none were listed as having died from carbon monoxide poisoning. No deaths of non-workers from pneumonia were recorded in Boulder City during the construction period.
Hansen's bas-relief on the Nevada elevator
The initial plans for the facade of the dam, the power plant, the outlet tunnels and ornaments clashed with the modern look of an arch dam. The Bureau of Reclamation, more concerned with the dam's functionality, adorned it with a Gothic-inspired balustrade and eagle statues. This initial design was criticized by many as being too plain and unremarkable for a project of such immense scale, so Los Angeles-based architect Gordon B. Kaufmann, then the supervising architect to the Bureau of Reclamation, was brought in to redesign the exteriors. Kaufmann greatly streamlined the design, and applied an elegant Art Deco style to the entire project. He designed sculptured turrets rising seamlessly from the dam face and clock faces on the intake towers set for the time in Nevada and Arizona — both states are in different time zones, but since Arizona does not observe Daylight Saving Time, the clocks display the same time for more than half the year.
At Kaufmann's request, Denver artist Allen Tupper True was hired to handle the design and decoration of the walls and floors of the new dam. True's design scheme incorporated motifs of the Navajo and Pueblo tribes of the region. Although some initially were opposed to these designs, True was given the go-ahead and was officially appointed consulting artist. With the assistance of the National Laboratory of Anthropology, True researched authentic decorative motifs from Indian sand paintings, textiles, baskets and ceramics. The images and colors are based on Native American visions of rain, lightning, water, clouds, and local animals — lizards, serpents, birds — and on the Southwestern landscape of stepped mesas. In these works, which are integrated into the walkways and interior halls of the dam, True also reflected on the machinery of the operation, making the symbolic patterns appear both ancient and modern.
Tile floor designed by Allen Tupper True
With the agreement of Kaufmann and the engineers, True also devised an innovative color-coding for the pipes and machinery, which was implemented throughout all BOR projects. True's consulting artist job lasted through 1942; it was extended so he could complete design work for the Parker, Shasta and Grand Coulee dams and power plants. True's work on the Hoover Dam was humorously referred to in a poem published in The New Yorker, part of which read, "lose the spark, and justify the dream; but also worthy of remark will be the color scheme".
Complementing Kaufmann and True's work, the Norwegian-born, naturalized American sculptor Oskar J.W. Hansen designed many of the sculptures on and around the dam. His works include the monument of dedication plaza, a plaque to memorialize the workers killed and the bas-reliefs on the elevator towers. In his words, Hansen wanted his work to express "the immutable calm of intellectual resolution, and the enormous power of trained physical strength, equally enthroned in placid triumph of scientific accomplishment", because "[t]he building of Hoover Dam belongs to the sagas of the daring." Hansen's dedication plaza, on the Nevada abutment, contains a sculpture of two winged figures flanking a flagpole.
Hoover Dam memorial star map floor, center area
Surrounding the base of the monument is a terrazzo floor embedded with a "star map". The map depicts the Northern Hemisphere sky at the moment of President Roosevelt's dedication of the dam. This is intended to help future astronomers, if necessary, calculate the exact date of dedication. The 30-foot-high (9.1 m) bronze figures, dubbed "Winged Figures of the Republic", were each formed in a continuous pour. To put such large bronzes into place without marring the highly polished bronze surface, they were placed on ice and guided into position as the ice melted. Hansen's bas-relief on the Nevada elevator tower depicts the benefits of the dam: flood control, navigation, irrigation, water storage, and power. The bas-relief on the Arizona elevator depicts, in his words, "the visages of those Indian tribes who have inhabited mountains and plains from ages distant."
Hoover Dam releasing water from the jet-flow gates in 1998
Excavation for the powerhouse was carried out simultaneously with the excavation for the dam foundation and abutments. A U-shaped structure located at the downstream toe of the dam, its excavation was completed in late 1933 with the first concrete placed in November 1933. Filling of Lake Mead began February 1, 1935, even before the last of the concrete was poured that May. The powerhouse was one of the projects uncompleted at the time of the formal dedication on September 30, 1935—a crew of 500 men remained to finish it and other structures. To make the powerhouse roof bombproof, it was constructed of layers of concrete, rock, and steel with a total thickness of about 3.5 feet (1.1 m), topped with layers of sand and tar.
In the latter half of 1936, water levels in Lake Mead were high enough to permit power generation, and the first three Allis Chalmers built Francis turbine-generators, all on the Nevada side, began operating. In March 1937, one more Nevada generator went online and the first Arizona generator by August. By September 1939, four more generators were operating, and the dam's power plant became the largest hydroelectricity facility in the world. The final generator was not placed in service until 1961, bringing the maximum generating capacity to 1,345 megawatts at the time. Original plans called for 16 large generators, eight on each side of the river, but two smaller generators were installed instead of one large one on the Arizona side for a total of 17. The smaller generators were used to serve smaller communities at a time when the output of each generator was dedicated to a single municipality, before the dam's total power output was placed on the grid and made arbitrarily distributable. The present contracts for the sale of electricity expire in 2017.
Before water from Lake Mead reaches the turbines, it enters the intake towers and then four gradually narrowing penstocks which funnel the water down towards the powerhouse. The intakes provide a maximum hydraulic head (water pressure) of 590 ft (180 m) as the water reaches a speed of about 85 mph (140 km/h). The entire flow of the Colorado River passes through the turbines. The spillways and outlet works (jet-flow gates) are rarely used. The jet-flow gates, located in concrete structures 180 feet (55 m) above the river, and also at the outlets of the inner diversion tunnels at river level, may be used to divert water around the dam in emergency or flood conditions, but have never done so, and in practice are only used to drain water from the penstocks for maintenance. Following an uprating project from 1986 to 1993, the total gross power rating for the plant, including two 2.4 megawatt Pelton turbine-generators that power Hoover Dam's own operations is a maximum capacity of 2080 megawatts. The annual generation of Hoover Dam varies. The maximum net generation was 10.348 TWh in 1984, and the minimum since 1940 was 2.648 TWh in 1956. The average power generated was 4.2 TWh/year for 1947–2008. In 2015, the dam generated 3.6 TWh.
The amount of electricity generated by Hoover Dam has been decreasing along with the falling water level in Lake Mead due to the prolonged drought in the 2010s and high demand for the Colorado River's water. Lake Mead fell to a new record low elevation of 1,071.61 feet (326.63 m) on July 1, 2016 before beginning to rebound slowly. Under its original design, the dam will no longer be able to generate power once the water level falls below 1,050 feet (320 m), which could occur as early as 2017. To lower the minimum power pool elevation from 1,050 to 950 feet (320 to 290 m), five wide-head turbines, designed to work efficiently with less flow, are being installed and will be fully online by 2017. Due to the low water levels, by 2014 it was providing power only during periods of peak demand.
Control of water was the primary concern in the building of the dam. Power generation has allowed the dam project to be self-sustaining: proceeds from the sale of power repaid the 50-year construction loan, and those revenues also finance the multimillion-dollar yearly maintenance budget. Power is generated in step with and only with the release of water in response to downstream water demands.
Lake Mead and downstream releases from the dam also provide water for both municipal and irrigation uses. Water released from the Hoover Dam eventually reaches several canals. The Colorado River Aqueduct and Central Arizona Project branch off Lake Havasu while the All-American Canal is supplied by the Imperial Dam. In total, water from the Lake Mead serves 18 million people in Arizona, Nevada and California and supplies the irrigation of over 1,000,000 acres (400,000 ha) of land.
Electricity from the dam's powerhouse was originally sold pursuant to a fifty-year contract, authorized by Congress in 1934, which ran from 1937 to 1987. In 1984, Congress passed a new statute which set power allocations from the dam from 1987 to 2017. The powerhouse was run under the original authorization by the Los Angeles Department of Water and Power and Southern California Edison; in 1987, the Bureau of Reclamation assumed control. In 2011, Congress enacted legislation extending the current contracts until 2067, after setting aside 5% of Hoover Dam's power for sale to Native American tribes, electric cooperatives, and other entities. The new arrangement will begin on October 1, 2017. The Bureau of Reclamation reports that the energy generated under the contracts ending in 2017 is allocated as follows:
Water enters the Arizona spillway (left) during the 1983 floods. Lake Mead water level was 1,225.6 ft (373.6 m)
The dam is protected against over-topping by two spillways. The spillway entrances are located behind each dam abutment, running roughly parallel to the canyon walls. The spillway entrance arrangement forms a classic side-flow weir with each spillway containing four 100-foot-long (30 m) and 16-foot-wide (4.9 m) steel-drum gates. Each gate weighs 5,000,000 pounds (2,300,000 kg) and can be operated manually or automatically. Gates are raised and lowered depending on water levels in the reservoir and flood conditions. The gates could not entirely prevent water from entering the spillways but could maintain an extra 16 ft (4.9 m) of lake level. Water flowing over the spillways falls dramatically into 600-foot-long (180 m), 50-foot-wide (15 m) spillway tunnels before connecting to the outer diversion tunnels, and reentering the main river channel below the dam. This complex spillway entrance arrangement combined with the approximate 700-foot (210 m) elevation drop from the top of the reservoir to the river below was a difficult engineering problem and posed numerous design challenges. Each spillway's capacity of 200,000 cu ft/s (5,700 m3/s) was empirically verified in post-construction tests in 1941.
The large spillway tunnels have only been used twice, for testing in 1941 and because of flooding in 1983. During both times, when inspecting the tunnels after the spillways were used, engineers found major damage to the concrete linings and underlying rock. The 1941 damage was attributed to a slight misalignment of the tunnel invert (or base), which caused cavitation, a phenomenon in fast-flowing liquids in which vapor bubbles collapse with explosive force. In response to this finding, the tunnels were patched with special heavy-duty concrete and the surface of the concrete was polished mirror-smooth. The spillways were modified in 1947 by adding flip buckets, which both slow the water and decrease the spillway's effective capacity, in an attempt to eliminate conditions thought to have contributed to the 1941 damage. The 1983 damage, also due to cavitation, led to the installation of aerators in the spillways. Tests at Grand Coulee Dam showed that the technique worked, in principle.
The bypass in front of the dam
There are two lanes for automobile traffic across the top of the dam, which formerly served as the Colorado River crossing for U.S. Route 93. In the wake of the September 11, 2001 terrorist attacks, authorities expressed security concerns and the Hoover Dam Bypass project was expedited. Pending the completion of the bypass, restricted traffic was permitted over Hoover Dam. Some types of vehicles were inspected prior to crossing the dam while semi-trailer trucks, buses carrying luggage, and enclosed-box trucks over 40 ft (12 m) long were not allowed on the dam at all, and were diverted to U.S. Route 95 or Nevada State Routes 163/68. The four-lane Hoover Dam Bypass opened on October 19, 2010. It includes a composite steel and concrete arch bridge, the Mike O'Callaghan–Pat Tillman Memorial Bridge, 1,500 ft (460 m) downstream from the dam. With the opening of the bypass, through traffic is no longer allowed across Hoover Dam; dam visitors are allowed to use the existing roadway to approach from the Nevada side and cross to parking lots and other facilities on the Arizona side.
Hoover Dam opened for tours in 1937 after its completion, but following Japan's attack on Pearl Harbor on December 7, 1941, it was closed to the public when the United States entered World War II, during which only authorized traffic, in convoys, was permitted. After the war, it reopened September 2, 1945, and by 1953, annual attendance had risen to 448,081. The dam closed on November 25, 1963 and March 31, 1969, days of mourning in remembrance of Presidents Kennedy and Eisenhower. In 1995, a new visitors' center was built, and the following year, visits exceeded one million for the first time. The dam closed again to the public on September 11, 2001; modified tours were resumed in December and a new "Discovery Tour" was added the following year. Today, nearly a million people per year take the tours of the dam offered by the Bureau of Reclamation. Increased security concerns by the government have led to most of the interior structure being inaccessible to tourists. As a result, few of True's decorations can now be seen by visitors.
View upstream from Hoover Dam, Sept. 2009. Water elevation on this date was 1093.77 ft.
The changes in water flow and use caused by Hoover Dam's construction and operation have had a large impact on the Colorado River Delta. The construction of the dam has been implicated in causing the decline of this estuarine ecosystem. For six years after the construction of the dam, while Lake Mead filled, virtually no water reached the mouth of the river. The delta's estuary, which once had a freshwater-saltwater mixing zone stretching 40 miles (64 km) south of the river's mouth, was turned into an inverse estuary where the level of salinity was higher close to the river's mouth.
The Colorado River had experienced natural flooding before the construction of the Hoover Dam. The dam eliminated the natural flooding, which threatened many species adapted to the flooding, including both plants and animals. The construction of the dam devastated the populations of native fish in the river downstream from the dam. Four species of fish native to the Colorado River, the Bonytail chub, Colorado pikeminnow, Humpback chub, and Razorback sucker, are listed as endangered.
Los Angeles Times political cartoon commenting on the attempts of Ickes to keep 'Hoover' off of the dam.
During the years of lobbying leading up to the passage of legislation authorizing the dam in 1928, the press generally referred to the dam as "Boulder Dam" or as "Boulder Canyon Dam", even though the proposed site had shifted to Black Canyon. The Boulder Canyon Project Act of 1928 (BCPA) never mentioned a proposed name or title for the dam. The BCPA merely allows the government to "construct, operate, and maintain a dam and incidental works in the main stream of the Colorado River at Black Canyon or Boulder Canyon".
When Secretary Wilbur spoke at the ceremony starting the building of the railway between Las Vegas and the dam site on September 17, 1930, he named the dam "Hoover Dam", citing a tradition of naming dams after Presidents, though none had been so honored during their terms of office. Wilbur justified his choice on the ground that Hoover was "the great engineer whose vision and persistence ... has done so much to make [the dam] possible". One writer complained in response that "the Great Engineer had quickly drained, ditched, and dammed the country."
After Hoover's election defeat in 1932 and the accession of the Roosevelt administration, Secretary Ickes ordered on May 13, 1933, that the dam be referred to as "Boulder Dam". Ickes stated that Wilbur had been imprudent in naming the dam after a sitting president, that Congress had never ratified his choice, and that it had long been referred to as Boulder Dam. Unknown to the general public, Attorney General Homer Cummings informed Ickes that Congress had indeed used the name "Hoover Dam" in five different bills appropriating money for construction of the dam. The official status this conferred to the name "Hoover Dam" had been noted on the floor of the House of Representatives by Congressman Edward T. Taylor of Colorado on December 12, 1930, but was likewise ignored by Ickes.
When Ickes spoke at the dedication ceremony on September 30, 1935, he was determined, as he recorded in his diary, "to try to nail down for good and all the name Boulder Dam." At one point in the speech, he spoke the words "Boulder Dam" five times within thirty seconds. Further, he suggested that if the dam were to be named after any one person, it should be for California Senator Hiram Johnson, a lead sponsor of the authorizing legislation. Roosevelt also referred to the dam as Boulder Dam, and the Republican-leaning Los Angeles Times, which at the time of Ickes' name change had run an editorial cartoon showing Ickes ineffectively chipping away at an enormous sign "HOOVER DAM," reran it showing Roosevelt reinforcing Ickes, but having no greater success.
In the following years, the name "Boulder Dam" failed to fully take hold, with many Americans using both names interchangeably and mapmakers divided as to which name should be printed. Memories of the Great Depression faded, and Hoover to some extent rehabilitated himself through good works during and after World War II. In 1947, a bill passed both Houses of Congress unanimously restoring the name "Hoover Dam." Ickes, who was by then a private citizen, opposed the change, stating, "I didn't know Hoover was that small a man to take credit for something he had nothing to do with."
Hoover Dam was recognized as a National Civil Engineering Landmark in 1984. It was listed on the National Register of Historic Places in 1981, and was designated a National Historic Landmark in 1985, cited for its engineering innovations.
Hoover Dam 2011 panoramic view from the Arizona side showing the penstock towers, the Nevada-side spillway entrance and the Mike O'Callaghan – Pat Tillman Memorial Bridge, also known as the Hoover Dam Bypass