CEMENT AND CONCRETE

Cement and concrete are among the most important building materials. Cement is a fine, gray powder. It is mixed with water and materials such as sand, gravel, and crushed stone to make concrete. Cement and water form a paste that binds the other materials together as the concrete hardens. People often misuse the words cement and concrete. A person may speak of "a cement sidewalk." But the sidewalk actually is made of concrete.

Concrete is highly fire-resistant, watertight, and comparatively cheap and easy to make. When first mixed, concrete can be molded into almost any shape. It quickly hardens into an extremely strong material that lasts a long time and requires little care.

Nearly all the cement used today is port/and cement, which is a hydraulic cement, or one that hardens under water. This cement was named portland because it has the same color as stone quarried on the Isle of Portland, a peninsula on the south coast of Great Britain.

Uses of cement and concrete

Nearly all skyscrapers and factories and many homes stand on concrete foundations. These buildings may also have concrete frames, walls, floors, and roofs. Concrete is used to build dams to store water and bridges to span rivers. Cars and trucks travel on concrete highways, and airplanes land on concrete runways.

Concrete tunnels run through mountains and under rivers. Concrete pipe distributes water, carries away sewage, drains farmland, and protects underground telephone wires and electric power lines.

Portland cement is used chiefly to make concrete. But it can also be mixed with soil and water to form soil-cement, which is used in road paving and dam construction and for lining reservoirs.

Kinds of concrete

There are special ways of strengthening concrete or of making concrete building materials. These include (1) reinforced concrete, (2) prestressed concrete, (3) precast concrete, and (4) concrete masonry. Engineers have also developed special kinds of concrete for certain uses. These include (1) air-entrained concrete, (2) high-early-strength concrete, and (3) lightweight concrete.

Reinforced concrete is made by casting concrete around steel rods or bars. The steel strengthens the concrete. Almost all large structures, including skyscrapers and bridges, require this extra-strong type of concrete.

Prestressed concrete usually is made by casting concrete around steel cables stretched by hydraulic jacks. After the concrete hardens, the jacks are released and the cables compress the concrete. Concrete is strongest when it is compressed. Steel is strong when it is stretched, or in tension. In this way, builders combine the two strongest qualities of the two materials. The steel cables can also be bent into an arc, so that they exert a force in any desired direction, such as upward in a bridge. This force helps counteract the weight of the bridge. Prestressed concrete beams, roofs, floors, and bridges are often cheaper for some uses than those made of reinforced concrete.

Precast concrete is cast and hardened before being used for construction. Precasting firms make concrete sewer pipes, floor and roof units, wall panels, beams, and girders, and ship them to the building site. Sometimes builders make such pieces at the building site and hoist them into place after they harden. Precasting makes possible the mass production of concrete building materials. Nearly all prestressed concrete is precast.

Concrete masonry includes many shapes and sizes of precast block. It is used to make about two-thirds of all the masonry walls built each year in the United States. Some concrete masonry is decorative or resembles brick.

Air-entrained concrete contains tiny air bubbles. These bubbles are formed by adding soaplike resinous or fatty materials to the cement, or to the concrete when it is mixed (see RESN). The bubbles give the water in concrete enough room to expand as it freezes. The bubbles also protect the surface of the concrete from chemicals used to melt ice. Such qualities make air-entrained concrete a good material for roads and airport runways.

High-early-strength concrete is chiefly used in cold weather. This concrete is made with high-early-strength portland cement, and hardens much more quickly than ordinary concrete. It costs more than ordinary concrete. But it is often cheaper to use, because it cuts the amount of time the concrete must be protected in cold weather.

Lightweight concrete weighs less than other kinds of concrete. Builders make it in two ways. They may use lightweight shales, clays, pumice, or other materials instead of sand, gravel, and crushed rock. Or they may add chemicals that foam and produce air spaces in the concrete as it hardens. These air spaces are much larger than the air spaces in air-entrained concrete.

How concrete is made

Materials. Concrete is a mixture of portland cement, water, and aggregates. Aggregates are materials such as sand, gravel, crushed rock, and blast furnace slag (waste). The cement and water form a paste that binds the aggregates into a rocklike mass as the paste hardens. Builders generally use both a fine aggregate such as sand, and a coarse aggregate such as crushed rock, to make concrete. The aggregates must be free from silt, mud, clay, dust, and other materials that might weaken the concrete. The water used to make concrete should also be free from dirt and other impurities.

Builders may add materials called admixtures to concrete to give it special properties. Very fine materials such as fly ash, a product of coal-burning power plants, make fresh concrete more plastic (easily shaped). Other admixtures include various fats, sugars, and minerals. These are used to speed up or slow down the hardening of the concrete or to give it color or increased durability and weather resistance.

Mixing. Before concrete is mixed, workers measure the proper amounts of the materials. The strength and durability of concrete depend chiefly on the amount of water used. If too much water is added, the cement paste will be too weak to hold the aggregates together firmly when it hardens. The less water used, within reasonable limits, the stronger the concrete will be.

Concrete can be mixed either by hand or by machine. Machine mixing makes more uniform batches. Proper mixing coats every particle of aggregate and fills all the spaces between them with cement paste. For most home repairs, concrete can be hand mixed.

The methods for mixing concrete by machines vary. The concrete may be mixed by machines at the place where the concrete will be used. Ready-mix companies make huge batches of concrete at mixing plants, and haul it to the work site in trucks. Some firms use mixing machines mounted on trucks. These machines mix the concrete as the truck carries it to the building site.

Homeowners can buy prepared mixtures of cement and aggregates for small repair jobs. Only water has to be added to such mixtures.

Placing. Workers place the freshly mixed, wet concrete into forms made of wood, plywood, or steel. The forms hold the concrete in shape until it hardens. The concrete may be dumped directly into the forms, or poured down chutes. Workers use wheelbarrows, two-wheeled carts called buggies, small rail cars, trucks, or buckets lifted by cranes. The concrete may also be pumped through steel pipes.

After the concrete is placed, it must be worked into the corners and sides of the forms with wooden spades and puddling sticks. The concrete should also be tamped, or packed down, to prevent open spaces called honeycombs. Sometimes workers stick vibrators into the concrete or fasten them to the forms in order to help settle the concrete.

Concrete placed for floors, sidewalks, and driveways should be leveled off with a straight-edged board. Next, it should stand until the film of moisture on its surface has disappeared. Then, the concrete should be smoothed off with a wooden trowel called a wood float. The float produces a rough surface that prevents slipping or skidding after the concrete hardens. A smoother surface can be made by using a steel trowel after the wood float. Motorized rotary steel floats are often used.

Curing makes concrete harden properly. After the concrete becomes firm enough to resist marring, it should be sprinkled with water, then covered with wet canvas, wet burlap, or wet sand. This cover keeps the concrete from drying too rapidly. A chemical reaction between portland cement and water makes concrete harden. For this reason, the longer concrete remains moist, the stronger it becomes. In hot weather, concrete should be kept moist at least three days. Cold weather slows the rate at which concrete hardens. Hardening concrete must be protected by canvas or straw when the temperature drops near freezing.

Concrete shrinks as it hardens. This results from the loss of moisture as the concrete dries, or from the cooling of the concrete. The chemical reaction of water and portland cement produces heat. When large amounts of concrete are used, as in dams, this heat must be drained away to make the concrete harden properly. This is usually done by running cold water through pipes stuck into the concrete. Cement companies have developed a special portland cement that produces less heat than other cements.

Raw materials. Portland cement contains about 60 per cent lime, 25 per cent silica, and 5 per cent alumina. Iron oxide and gypsum make up the rest of the materials. The gypsum regulates the setting, or hardening, time of cement. The lime comes from materials such as limestone, oyster shells, and a type of clay called marl. Shale, clay, silica sand, slate, and blast-furnace slag provide the silica and alumina. Iron oxide is supplied by iron ore, pyrite, and other materials.

Most cement plants are located near limestone quarries. They may also be near deposits of clay and other raw materials. Ships, trains, trucks, and conveyer belts haul the limestone and other raw materials to the plants. In the plants, the materials go through a chemical process that consists of three basic steps: (1) crushing and grinding, (2) burning, and (3) finish grinding.

Crushing and grinding. The quarried limestone is dumped into primary crushers that can handle pieces as large as an upright piano. This first crushing smashes the rock into pieces about the size of a softball. Secondary crushers, or hammer mills, then break the rock into pieces about 3/4 inch (19 millimeters) wide.

Next, the crushed rock and other raw materials are mixed in the right proportions to make portland cement. This mixture is then ground in rotating ball mills and tube mills. These mills contain thousands of steel balls that grind the mixture into fine particles. The materials can be ground by either a wet or dry method. In the wet process, water is added during the grinding until a soupy mixture called a slurry forms.

Burning. After the raw materials have been ground, they are fed into a kiln, a huge cylindrical furnace made of steel and lined with firebricks. A cement kiln rotates about one turn a minute, and is the largest piece of moving machinery used in any industry. It may be over 25 feet (8 meters) in diameter and 750 feet (229 meters) in length. The kiln is mounted with one end higher than the other. The ground, raw materials are fed into the higher end and slide slowly toward the lower end as the kiln revolves. It takes about four hours for the materials to travel through the kiln. Oil, gas, or powdered coal is burned at the lower end. This produces a flame that heats the materials to 2600 to 3000 ⁰F. (1430 to 1600 ⁰C). The heat changes the materials into a substance called clinker, in pieces about the size of marbles.

Finish grinding. Large fans cool the clinker after it leaves the kiln. The clinker may be stockpiled for future use, or it may be reground at once in ball or tube mills. A small amount of gypsum is added to the clinker before the regrinding. This final grinding produces powdery portland cement that is finer than flour. The cement is stored in silos until it is shipped.

Shipping. Cement plants ship cement either in bulk (unpackaged) or packed in strong paper sacks. Unpackaged cement is shipped by railroad, truck, or barge. Packaged cement is shipped in sacks containing 94 pounds (43 kilograms), or 1 cubic foot (0.03 cubic meter), of cement to the sack.

History

The ancient Romans developed cement and concrete similar to the kinds used today. Their cement had such great durability that some of their buildings, roads, and bridges still exist. To make cement, the Romans mixed slaked lime (lime to which water has been added) with a volcanic ash called pozzuolana. The ash produced a hydraulic cement that hardened under water. People lost the art of making cement after the fall of the Roman Empire in the A.D. 400’s. In 1756, John Smeaton, a British engineer, again found how to make cement.

Construction of the Erie Canal created the first big demand for cement in the United States. In 1818, Canvass White, an American engineer, discovered rock in Madison County, New York, that made natural hydraulic cement with little processing. Cement made from this rock was used in building the canal.

Portland cement. Joseph Aspdin, a British bricklayer, invented portland cement in 1824 and gave the cement its name. Aspdin made a cement that was superior to natural cement by mixing, grinding, burning, and regrinding amounts of limestone and clay. David 0. Saylor probably established the first portland cement plant in the United States at Coplay, Pennsylvania, in 1871.

Joseph Aspdin, a British bricklayer, invented portland cement in 1824 and gave the cement its name. Aspdin made a cement that was superior to natural cement by mixing, grinding, burning, and regrinding amounts of limestone and clay. David 0. Saylor probably established the first portland cement plant in the United States at Coplay, Pennsylvania, in 1871.

At first, portland cement manufacturers developed their own formulas. In 1898, manufacturers used 91 different formulas. In 1917, the National Bureau of Standards (now the National Bureau of Standards and Technology) and the American Society for Testing Materials established a standard formula for portland cement. The Portland Cement Association was formed in Chicago in 1916. Its research laboratories perfected air-entrained concrete in the early 1940’s.

Joseph Monier, a French gardener, developed reinforced concrete about 1850. In 1927, Eugene Freyssinet, a French engineer, developed prestressed concrete.

The cement and concrete industry. The United States produces about 156 billion pounds (71 billion kilograms) of portland cement a year, which is about 7 percent of the world’s total. Other major cement producers include China, Germany, Japan, Korea, and Russia. The leading states are California, Michigan, Pennsylvania, and Texas.

The production of ready-mixed concrete ranks as the biggest single concrete industry in North America. About 4,000 U.S. firms and about 1,200 Canadian firms produce ready-mixed concrete. More than 60 percent of the cement produced in the United States is sold to ready-mix producers. The second largest branch of the concrete industry is the manufacture of precast concrete for construction.

Contributor: John A. Neal, Ph.D., Associate Professor, Civil Engineering, State

Univ. of New York, Buffalo.

 

From World Book C 2000 World Book, Inc., 233 N. Michigan, Chicago, IL 60601. All rights reserved.

 

Concrete: What color do you want?

Concrete is ideal for driveways, walks, patios, floors, walls and structural applications. Available everywhere, concrete is so common and used for so many things it's just taken for granted it only comes in one color. Davis Colors™ mix right in to transform ordinary concrete into structures that stand out or pavement that blends in. They're strong, durable and last as long as the concrete. Installation is cleaner and easier than toppings, stains or coatings and requires less labor. There are premium colors which are bold and intense, standard colors that add less than a dollar per square foot, and subtle shades for any budget. As the leading producer of colors for concrete since 1952, we offer the widest spectrum available. The hardest thing  about colored concrete may be deciding what color you want.

FIBERMESH POLYPROPYLENE FIBER

FIBERMESH fiber is an engineered fiber for concrete manufactured to an optimum gradation from 100% virgin polypropylene which provides protection against non -structural cracks in concrete, Increases abrasion- impact and shatter resistance while reducing permeability, imparts toughness to hardened concrete and is an alternate system to welded wire fabric when used or crack control in non-structural concrete.

FIBERMESH applications include, but are not limited to, slabs on grade, elevated slabs, precast concrete products, pavements, bridge decks, overlays, toppings, barrier walls, concrete tanks, pools, ditches, slope walls, stucco, shotcrete and gunite applications.

· FIBERMESH fibers work without affecting the chemical hydration of the cement. Their action is purely mechanical and is compatible with all concrete mixes and admixtures.

· FIBERMESH fibers cannot rust or stain; they are non-corrosive and alkali/proof.

· FIBERMESH fibers are light and easy to handle, packaged in pre-measured 1.5 lb. bags, the recommended application rate for one cubic yard of concrete.

 

FIBERMESH TEST REPORTS

· Use of FIBERMESH fibers to inhibit concrete cracking rind to provided abrasion, impact and shatter resistance while lowering permeability and imparting toughness, is supported by extensive laboratory studies- The following reports represent the most comprehensive research conducted with polypropylene fiber concrete - FIBERMESH concrete.

 

FIBERMESH Effect on Concrete Shrinkage Cracking:

Tests run at San Jose State University and the University of California, Berkeley, show without exception that FIBERMESH concrete typically inhibited cracking in the range of 90% to 100% compared to the non-fiber control specimen.

Relative Effect of FIBERMESH fibers on Early Age Plastic Shrinkage Cracking of Plastic Concrete:

Tests by Webster Engineering and Associates. Inc. have shown that the addition of FIBERMESH fibers to plastic concrete substantially increases the resistance of the concrete to early age plastic shrinkage cracking and cracking In response to vibration at early ages.

Addition of 1.5 lbs. per cu. yd. of FIBERMESH polypropylene fibers increases the strain capacity (ability to resist strain without developing visible cracking) of the immature concrete.

 

Water Migration/PermeabiIity of FIBERMESH Concrete:

The Von Test method was used to make this comparison at San Jose State University.

· Migration of water rates indicated reduction in concrete permeability of 33-44% at I lb. of FIBERMESH fibers per cubic yard as high as 79% at 2 lbs. per cubic yard.

 

Toughness of FIBERMESH Reinforced Hardened Concrete:

· Toughness is the measure of fibrous concrete's ability to sustain load after first crack. Toughness indices can be used as a measure of the reinforcing fibers ability to hold cracks together under load.

To meet ASTM 0 1116 requirements for fiber reinforcement, a fiber should preferably be made from polypropylene and have been tested to score 3.0 or better on an ASTM C-1018 "Standard Test Method for Flexural Toughness and First-Crack Strength of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading).

FIBERMESH fibrillated, graded fiber, when used at a ratio or 5 pounds per cubic yard (0.1% by volume), meets both the material recommendations and performance requirements of ASTM C-Ill 6 and C-1018Toughness Index I_5.

 

 

Load Test of FIBERMESH fibers versus Welded Wire Fabric in Composite Deck Systems:

Load tests which used the FIBERMESH fiber reinforced concrete were equal or better than the welded wire fabric reinforced concrete tests indicating an equivalency for structural performance in composite slabs as well as the diaphragm behavior. As a follow-up to the load tests, a test program was conducted on the strengths of headed stud shear connectors in concrete comparing the use of FIBERMESH micro-reinforcing systems to the use of welded wire fabric.

 

The results of the Pushout Tests show that the strength and ductility of shear connectors in steel mesh and FIBER-MESH reinforced concrete are comparable. The use of FIBERMESH fibers also eliminates the construction problems experienced with welded wire fabric in metal decking and provides a constant solution to the problems concerning the amount and location of the reinforcement.

 

CODE CERTIFICATON

FIBERMESH fiber complies with the BOCA Basic National Building Code and Supplement, the Standard Building Code, and the Uniform building Code, as reported by the Council of American Building Officials, National Evaluation Service Committee--Report No. NES-284. FIBERMESH fiber is accepted by various State Departments of Transportation.

SHORT FORM SPECIFICATION

Synthetic fibrous reinforcing material shall be 100% virgin polypropylene fibrillated fibers containing no reprocessed olefin materials and specifically manufactured to use as concrete secondary reinforcement.

Volume of fibrous reinforcing material per cubic yard shall equal a minimum of 0.1% (1.5 pounds) (.9 kg per cubic meter). Fibers are for the control of cracking due to drying shrinkage and thermal expansion/contraction, reduction of permeability, increased impact capacity, shatter resistance, abrasion resistance and added post crack toughness.

Fiber manufacturer must document evidence of 5 year satisfactory performance history, compliance with applicable building codes and ASTM C-1116 Type III 4. I. 3 and ASTM C-1116 Performance Level 1 I₅ (Ref: ASTM C1018) outlined in Section 21, Note 17.

Fibrous concrete reinforcement shall be manufactured by FIBERMESH, 4019 Industry Drive, Chattanooga,Tennesse, USA, 37416. (423) 892-7243. Complete Specification in CSI Manu-Spec format available both as hardcopy and IBM diskette (ASCII file) from your FIBERMESH representative.