5.2 CONCRETE INGREDIENTS
The essential ingredients of concrete are cement, aggregate, and water. A mixture of only cement and water is called cement paste. In large quantities, however, cement paste is prohibitively expensive for most construction purposes.
Most cement used today is portland cement. This is a carefully proportioned and specially processed combination of lime, silica, iron oxide, and alumina. It is usually manufactured from limestone mixed with shale, clay, or marl. Properly proportioned raw materials are pulverized and fed into kilns where they are heated to a temperature of 2,700°F and maintained at that temperature for a specific time. The heat produces chemical changes in the mixture and transforms it into clinkera hard mass of fused clay and limestone. The clinker is then ground to a fineness that will pass through a sieve containing 40,000 openings per square inch.
Types of Cement
There are five types of Portland cement covered under "Standard Specifications for Portland Cement." These specifications are governed by the American Society for Testing and Material (ASTM) types. Separate specifications, such as those required for air-entraining portland cements, are found under a separate ASTM. The type of construction, chemical composition of the soil, economy, and requirements for use of the finished concrete are factors that influence the selection of the kind of cement to be used.
Air-entrained portland cement is a special cement that can be used with good results for a variety of conditions. It has been developed to produce concrete that is resistant to freeze-thaw action, and to scaling caused by chemicals applied for severe frost and ice removal. In this cement, very small quantities of air-entraining materials are added as the clinker is being ground during manufacturing. Concrete made with this cement contains tiny, well-distributed and completely separated air bubbles. The bubbles are so small that there may be millions of them in a cubic foot of concrete. The air bubbles provide space for freezing water to expand without damaging the concrete. Air-entrained concrete has been used in pavements in the northern states for about 25 years with excellent results. Air-entrained concrete also reduces both the amount of water loss and the capillary/water-channel structure.
An air-entrained admixture may also be added to types I, II, and III portland cement. The manufacturer specifies the percentage of air entrainment that can be expected in the concrete. An advantage of using air-entrained cement is that it can be used and batched like normal cement. The air-entrained admixture comes in a liquid form or mixed in the cement. To obtain the proper mix, you should add the admixture at the batch plant.
The material combined with cement and water to make concrete is called aggregate. Aggregate makes up 60 to 80 percent of concrete volume. It increases the strength of concrete, reduces the shrinking tendencies of the cement, and is used as an economical filler.
Aggregates are divided into fine (usually consisting of sand) and coarse categories. For most building concrete, the coarse aggregate consists of gravel or crushed stone up to 1 1/2 inches in size. However, in massive structures, such as dams, the coarse aggregate may include natural stones or rocks ranging up to 6 inches or more in size.
Purpose of Aggregates
The large, solid coarse aggregate particles form the basic structural members of the concrete. The voids between the larger coarse aggregate particles are filled by smaller particles. The voids between the smaller particles are filled by still smaller particles. Finally, the voids between the smallest coarse aggregate particles are filled by the largest fine aggregate particles. In turn, the voids between the largest fine aggregate particles are filled by smaller fine aggregate particles, the voids between the smaller fine aggregate particles by still smaller particles, and soon. Finally, the voids between the finest grains are filled with cement. You can see from this that the better the aggregate is graded (that is, the better the distribution of particles sizes), the more solidly all voids will be filled, and the denser and stronger will be the concrete.
The cement and water form a paste that binds the aggregate particles solidly together when it hardens. In a well-graded, well-designed, and well-mixed batch, each aggregate particle is thoroughly coated with the cement-water paste. Each particle is solidly bound to adjacent particles when the cement-water paste hardens.
PARTICLE DISTRIBUTION. Experience and experiments show that for ordinary building concrete, certain particle distributions consistently seem to produce the best results. For tine aggregate, the recommended distribution of particle sizes from No. 4 to No. 100 is shown in table 6-1.
The distribution of particle sizes in aggregate is determined by extracting a representative sample of the material, screening the sample through a series of sieves ranging in size from coarse to fine, and determining the percentage of the sample retained on each sieve. This procedure is called making a sieve analysis. For example, suppose the total sample weighs 1 pound. Place this on the No. 4 sieve, and shake the sieve until nothing more goes through. If what is left on the sieve weighs 0.05 pound, then 5 percent of the total sample is retained on the No. 4 sieve. Place what passes through on the No. 8 sieve and shake it. Suppose you find that what stays on this sieve weighs 0.1 pound. Since 0.1 pound is 10 percent of 1 pound, 10 percent of the total sample was retained on the No. 8 sieve. The cumulative retained weight is 0.15 pound. By dividing 0.15 by 1.0 pound, you will find that the total retained weight is 15 percent.
The size of coarse aggregate is usually specified as a range between a minimum and a maximum size; for example, 2 inches to No. 4, 1 inch to No. 4, 2 inches to 1 inch, and so on. The recommended particle size distributions vary with maximum and minimum nominal size limits, as shown in table 6-2.
Table 6-1.Recommended Distribution of Particle Sizes
Table 6-2.Recommended Maximum and Minimum Particle Sizes
A blank space in table 6-2 indicates a sieve that is not required in the analysis. For example, for the 2 inch to No. 4 nominal size, there are no values listed under the 4-inch, the 3 1/2-inch, the 3-inch, and the 2 1/2-inch sieves. Since 100 percent of this material should pass through a 2 1/2-inch sieve, there is no need to use a sieve coarser than that size. For the same size designation (that is, 2 inch size aggregate), there are no values listed under the 1 1/2-inch, the 3/4-inch, and the 3/8-inch sieves. Experience has shown that it is not necessary to use these sieves in making this particular analysis.
Since 66 to 78 percent of the volume of the finished concrete consists of aggregate, it is imperative that the aggregate meet certain minimum quality standards. It should consist of clean, hard, strong, durable particles free of chemicals that might interfere with hydration. The aggregate should also be free of any superfine material, which might prevent a bond between the aggregate and the cement-water paste. The undesirable substances most frequently found in aggregate are dirt, silt, clay, coal, mica, salts, and organic matter. Most of these can be removed by washing. Aggregate can be field-tested for an excess of silt, clay, and the like, using the following procedure:
Figure 6-2.Quart jar method of determining silt content of sand.
An easily constructed rig for washing a small amount of aggregate is shown in figure 6-3.
Figure 6-3.-Field-constructed rig for washing aggregate.
Weak, friable (easily pulverized), or laminated (layered) aggregate particles are undesirable. Especially avoid shale, stones laminated with shale, and most varieties of chart (impure flint-like rock). For most ordinary concrete work, visual inspection is enough to reveal any weaknesses in the coarse aggregate. For work in which aggregate strength and durability are of vital importance, such as paving concrete, aggregate must be laboratory tested.
Handling and Storage
A mass of aggregate containing particles of different sizes has a natural tendency toward segregation. "Segregation" refers to particles of the same size tending to gather together when the material is being loaded, transported, or otherwise disturbed. Aggregate should always be handled and stored by a method that minimizes segregation.
Stockpiles should not be built up in cone shapes, formed by dropping successive loads at the same spot. This procedure causes segregation. A pile should be built up in layers of uniform thickness, each made by dumping successive loads alongside each other.
If aggregate is dropped from a clamshell, bucket, or conveyor, some of the fine material may be blown aside, causing a segregation of fines on the lee side (that is, the side away from the wind) of the pile. Conveyors, clamshells, and buckets should be discharged in contact with the pile.
When a bin is being charged (filled), the material should be dropped from a point directly over the outlet. Material chuted in at an angle or material discharged against the side of a bin will segregate. Since a long drop will cause both segregation and the breakage of aggregate particles, the length of a drop into a bin should be minimized by keeping the bin as full as possible at all times. The bottom of a storage bin should always slope at least 50° toward the central outlet. If the slope is less than 50°, segregation will occur as material is discharged out of the bin.
The two principal functions of water in a concrete mix are to effect hydration and improve workability. For example, a mix to be poured in forms must contain more water than is required for complete hydration of the cement. Too much water, however, causes a loss of strength by upsetting the wqtercement ratio. It also causes "water-gain" on the surface-a condition that leaves a surface layer of weak material, called laitance. As previously mentioned, an excess of water also impairs the watertightness of the concrete.
Water used in mixing concrete must be clean and free from acids, alkalis, oils, and organic materials. Most specifications recommend that the water used in mixing concrete be suitable for drinking, should such water be available.
Seawater can be used for mixing unreinforced concrete if there is a limited supply of fresh water. Tests show that the compressive strength of concrete made with seawater is 10 to 30 percent less than that obtained using fresh water. Seawater is not suitable for use in making steel-reinforced concrete because of the risk of corrosion of the reinforcement, particularly in warm and humid environments.
Admixtures include all materials added to a mix other than portland cement, water, and aggregates.
Admixtures are sometimes used in concrete mixtures to improve certain qualities, such as workability, strength, durability, watertightness, and wear resistance. They may also be added to reduce segregation, reduce the heat of hydration, entrain air, and accelerate or retard setting and hardening.
We should note that the same results can often be obtained by changing the mix proportions or by selecting other suitable materials without resorting to the use of admixtures (except air-entraining admixtures when necessary). Whenever possible, comparison should be made between these alternatives to determine which is more economical or convenient. Any admixture should be added according to current specifications and under the direction of the crew leader.
Materials, such as hydrated lime and bentonite, are used to improve workability. These materials increase the fines in a concrete mix when an aggregate is tested deficient in fines (that is, lacks sufficient fine material).
The deliberate adding of millions of minute disconnected air bubbles to cement paste, if evenly diffused, changes the basic concrete mix and increases durability, workability, and strength. The acceptable amount of entrained air in a concrete mix, by volume, is 3 to 7 percent. Air-entraining agents, used with types I, II, or III cement, are derivatives of natural wood resins, animal or vegetable fats, oils, alkali salts of sulfated organic compounds, and water-soluble soaps. Most air-entraining agents are in liquid form for use in the mixing water.
The only accepted accelerator for general concrete work is calcium chloride with not more than 2 percent by weight of the cement being used. This accelerator is added as a solution to the mix water and is used to speed up the strength gain. Although the final strength is not affected, the strength gain for the first 7 days is greatly affected. The strength gain for the first 7 days can be as high as 1,000 pounds per square inch (psi) over that of normal concrete mixes.
The accepted use for retarders is to reduce the rate of hydration. This permits the placement and consolidation of concrete before initial set. Agents normally used are fatty acids, sugar, and starches.
Portland cement is packed in cloth or paper sacks, each weighing 94 pounds. A 94-pound sack of cement amounts to about 1 cubic foot by loose volume.
Cement will retain its quality indefinitely if it does not come in contact with moisture. If allowed to absorb appreciable moisture in storage, however, it sets more slowly and strength is reduced. Sacked cement should be stored in warehouses or sheds made as watertight and airtight as possible. All cracks in roofs and walls should be closed, and there should be no openings between walls and roof. The floor should be above ground to protect the cement against dampness. All doors and windows should be kept closed.
Sacks should be stacked against each other to prevent circulation of air between them, but they should not be stacked against outside walls. If stacks are to stand undisturbed for long intervals, they should be covered with tarpaulins.
When shed or warehouse storage cannot be provided, sacks that must be stored in the open should be stacked on raised platforms and covered with waterproof tarps. The tarps should extend beyond the edges of the platform to deflect water away from the platform and the cement.
Cement sacks stacked in storage for long periods sometimes acquire a hardness called warehouse pack. This can usually be loosened by rolling the sack around. However, cement that has lumps or is not free flowing should not be used.