Concrete Basics

Concrete consists of three primary ingredients: aggregates, Portland cement, and water. Aggregates are categorized into coarse and fine varieties; coarse aggregate is typically stone/gravel, while fine aggregate is typically sand. Depending on the usage of the concrete, various mixtures of the ingredients will be used. There could be pea gravel, 1” or 2” stones used as coarse aggregate; varying amounts of sand, cement, and water will also be used. Considering cement is the paste that holds concrete together, this will be discussed in greater detail.

       Coarse Aggregate (stone)                        Fine Aggregate (Sand)

There are five types of Portland cement. Different types are required for different project uses. They are classified into Types I, II, III, IV, and V. In addition to these five, there are Types IA, IIA, and IIIA, meaning that air entrainment was added. Type I is a normal, everyday cement. Type II has moderate sulfate resistance; it is typically used when the concrete will be in regular contact with sulfate-rich water and soil, or when the concrete will be exposed to excessive heat. Type III Portland cement is used in applications when high early strength is necessary. Type IV is used in cases where heat generation must be kept to a minimum. Finally, Type V Portland cement is used in cases where high sulfate resistance is necessary. It is similar to Type II, but is used when the concrete is in contact with water and soil very high in sulfate content.

Portland Cement

Water-Cement Ratio

The water-cement ratio is calculated by dividing the pounds of water in one cubic yard of concrete by the pounds of cement in one cubic yard of concrete. For example, a concrete mix containing 200 pounds of water and 400 pounds of Portland cement per cubic yard has a water-cement ratio of 0.50 (200 ÷ 400 = 0.50). Typically, the lower the water-cement ratio is, the higher the strength of the concrete will be. For example, a mix with 180 pounds of water and 410 pounds of Portland cement per cubic yard (water-cement ratio of 0.44) will have a higher strength than the aforementioned mix with a water-cement ratio of 0.50.

Strength vs. Water-Cement Ratio

Fly Ash

Fly ash is a supplementary cementitious material used in concrete. It is produced in electrical power plants, resulting from the burning of coal. While combusting, the carbon, as well as other volatile matter, is burned off. Fly ash forms when the impurities (clay, feldspar, quartz, shale, etc.) in the coal fuse together during combustion. The result is very fine, glasslike particles that are so small they resemble Portland cement. Typically, fly ash particle sizes are less than 20 micrometers (0.00002 meters). Because of its small, glasslike nature, fly ash increases the workability of concrete, acting much like a lubricant. Because of this, fly ash is typically used when pumping concrete and for forms in which the concrete must fill many crevices and small openings.

Fly Ash

Slag

Slag is a byproduct of iron blast-furnaces. It is made from iron ore, coke (carbon-rich “fuel” resulting from the burning of coal), and a flux (limestone or dolomite). At the end of the smelting process, lime will have combined with components of the iron ore and coke, forming blast-furnace slag. There are several different ways by which slag can be cooled, the most common ways being air-cooled and water-cooled. When used as a cementitious material, the slag is known as ground granulated blast-furnace slag and is similar in consistency to sand. This method is typically done by water cooling. Slag can also be used as an aggregate in concrete. The air cooling method is typically used for this. As it sounds, the slag is cooled with ambient air. It is then crushed and used as an aggregate.

Ground Granulated Blast-Furnace Slag           Air-Cooled Coarse Aggregate Slag

Admixtures

There are many various concrete admixtures. The most common is air entrainment. Other commonly used admixtures include, but are not limited to: water reducers, superplasticizers, accelerators, retarders, corrosion inhibitors, and coloring pigments.

Air Entrainment: Air entrainment introduces tiny air bubbles into the concrete’s cement paste. The main goal is to reduce the cracking of concrete during freeze/thaw cycles. In addition, air entrainment increases workability and reduces segregation & bleeding in fresh concrete, and it reduces scaling caused by deicers in finished concrete. Air entrainment can be introduced into concrete in two ways. The first was mentioned above; some Portland cements come with air entrainment already added. If there is none already in the Portland cement, an air entrainment admixture is added to the concrete during batching or mixing.

Water Reducers: Water reducers will decrease the water-cement ratio, decrease cement content, decrease the water needed to produce a certain slump, or increase slump. Most water reducers decrease water content by 5% to 10%. Generally, the strength of the concrete increases when water reducers are added because the water-cement ratio decreases. One problem, however, that water reducers have is a tendency to increase drying shrinkage.

Superplasticizers: The purpose of a superplasticizer is to give concrete the ability to flow; that is, it greatly increases the slump while maintaining cohesion. Several areas where superplasticizers are best used include: thin-section placements, locations of densely laid steel reinforcement, underwater placements, pumped concrete, locations where consolidation is impossible or near impossible, and for decreased handling costs. In regard to workability, superplasticizers often increase flow for only about ½ hour to an hour.

Accelerators: The purpose of an accelerator is to speed both the rate of setting and the development of strength. The most commonly used accelerator is calcium chloride. The upside, as previously mentioned, is a faster development of strength. The downsides, however, are an increase in drying shrinkage, the possibility of reinforcement corrosion, discoloration of the concrete, and an increase in the possibility of scaling. Non-corrosive accelerators can be used to prevent the corrosion of reinforcement. They are essentially non-chloride accelerators, as the chloride is what causes the rusting of the reinforcing steel.

Retarders: Retarders are used to increase the setting time of concrete. The most common cause of rapid setting is high temperatures; therefore, retarders are most often used in high temperature applications. Retarders do not decrease concrete temperature; they increase the bleeding rate and capacity. In addition to hot weather, retarders are often used when large amounts of concrete are poured at one time, and when special finishes are to be applied to the concrete.

Corrosion Inhibitors: Just as the name implies, corrosion inhibitors slow rust formation on reinforcing steel. They are generally used in parking structures, marine structures, and bridges where the use of chloride salts is prevalent. The chloride reacts with the iron in the steel, and oxidation forms. Eventually, the steel reinforcement rusts and begins to fall apart. The corrosion inhibitors greatly slow this process, blocking the chlorides from reacting with the iron.

Coloring Pigments: Color pigmentation is a fairly straightforward admixture. It is added for aesthetic purposes and some safety applications. Pigments should not constitute more than 10% of the weight of the cement; if they are 6% or less of the weight of the cement, the color usually does not change the properties of the concrete.

Stamped Colored Concrete Walkway

Reinforcement

There are essentially three types of concrete reinforcement: steel reinforcing bars, steel mesh, and fiber reinforcement. Depending on the placement location of the concrete, different reinforcement will be used, if any.

Rebar: Steel reinforcing bars are probably the most commonly used reinforcement in concrete. Rebar is the most heavy-duty reinforcement against tension and flexure for concrete. Rebar, as well as steel mesh, however, is very susceptible to rusting. To counter corrosion, rebar is available in other forms besides common steel. Galvanized and stainless steel, as well as epoxy coated steel rebar are growing in popularity. Each has a higher initial cost than common steel, but the long-run savings typically outweigh this factor.

Common Rebar Sizes

Steel Mesh: Steel mesh is similar to rebar in that it utilizes steel rods running transversely through concrete. It is not, however, as heavy-duty. The main purpose of steel mesh is to control concrete cracking, not eliminate it altogether. When in tension or flexure, the concrete will want to crack, and without the reinforcement, it will essentially just crumble away. The steel mesh is easier to install than rebar because it is available in large sheets. Rather than laying bars one by one, a larger area can be done in less time.

Steel Mesh Reinforcement

Fibers: There are four different varieties of concrete reinforcing fibers; they are steel, glass, synthetic/plastic, and natural. Steel fibers add negligible compressive strength to concrete, but, like steel mesh and rebar, they add a large amount of tensile and flexural strength. The most common applications for steel fibers are runways, bridge decks, streets/highways, and industrial floors. Glass fibers generally add the most tensile strength of all the fibers. Common sense may not say that glass is stronger than steel, but the glass used is not everyday glass; the glass has fewer microscopic imperfections than steel, giving it greater strength. Synthetic, or man-made, fibers can be any one of acrylic, aramid, carbon, nylon, polyester, polyethylene, and polypropylene. The most widely used are polypropylene fibers. Natural fibers are most commonly wood cellulose, but coconut, bamboo, jute, elephant grass, and vegetable fibers can also be used. Wood cellulose fibers have comparable strengths to some synthetic, glass, and steel fibers, but the other natural fibers are somewhat lacking in strength.

Synthetic Fibers                             Steel Fibers

References

ConcreteNetwork.com. 2007. 16 July 2007. http://www.concretenetwork.com/concrete/whatis/

Concrete Thinker. 2007. Portland Cement Association. 18 July 2007. http://www.concretethinker.com/Papers.aspx?DocId=306

Headwaters Resources. 2005. Headwaters, Inc. 16 July 2007. http://www.flyash.com/

Kosmatka, Steven H., Beatrix Kerkhoff, and William C. Panarese. Design and Control of Concrete Mixtures, 14^th Ed. Portland Cement Association.

National Slag Association. 2007. National Slag Association. 18 July 2007. http://www.nationalslag.org/

Ontario Ministry of Agriculture, Food and Rural Affairs. 14 June 2007. Government of Ontario, Canada. 25 July 2007.

http://www.omafra.gov.on.ca/english/engineer/facts/06-023.htm

Portland Cement Association. 2007. 16 July 2007. http://www.cement.org/