HARDENING. Select a small sample of the material and mix it with enough water to make a plastic paste of a consistency similar to that generally used in cement mortars. Then mold it into a pat about 3 inches in diameter and 3/4 inch thick. Observe the paste several times an hour to determine whether or not the paste is setting (hardening). The cement has attained a final set when the surface is hard enough to be unmarked when a pencil point or a fingernail is pressed against it with moderate force. If it sets within 1 to 10 hours, the material is probably a cement.

COLOR. If it has been fairly well established that the material in question is a cement, color may serve as a means of further classification. If the material is gray, it is likely to be a portland cement; if brownish gray, it may be a natural cement; if black, an aluminous cement; and if white, it probably is hydraulic lime, plaster, or possibly white Portland cement,

AIR-ENTRAINED CEMENT. In the test to determine whether or not a given material contains an air-entraining agent, place a sample of the material in a glass cylinder to a depth of about 1 inch. Add water to a depth of about 6 inches and shake the cylinder and its contents vigorously. If a considerable volume of stable, persistent foam forms on the surface, the cement probably contains an air-entraining agent.

HIGH-EARLY-STRENGTH CEMENT. A way to recognize high-early-strength cement (Type III) is to make a batch of concrete using the unknown material and at the same time a similar batch using a known cement. Concrete that contains high-early-strength cement will usually harden in less time than concrete containing regular portland cement. High-early- strength concrete, if molded into standard concrete beams and tested after 3 days for flexural strength, should have a modulus of rupture more than 150 pounds per square inch higher than similar specimens containing regular portland cement concrete. A discussion of flexural strength testing will follow later in this chapter.

Water plays an important part in the concrete mix. Its principal uses are to make the mix workable and to start hydration. Any material in the water that retards or changes the hydration process is detrimental. A good rule of thumb is "if its good enough to drink, it may be used for concrete."

ORDINARY WATER. The materials found in some types of water include organic compounds, oil, alkali, or acid. Each has its effect on the hydration process. Organic material and oil tend to coat the aggregate and cement particles and to prevent the full chemical reaction and adherence. The organic material may also react with the chemicals in the cement and create a weakened cementing action, thus contributing to deterioration and structural failure of the concrete. Alkalis, acids, and sulfates in the water tend to react with the chemicals in the cement. The result is inadequate cementing and weakened concrete. Water must be free of these chemicals to be used in concrete mixing.

SEAWATER. The salts in seawater are normally thought of as being corrosive; however, seawater is used sometimes in concrete mixing with satisfactory results. A loss of 10 to 20 percent in compressive strength can be expected when the same amount of seawater as fresh water is used. That can be compensated somewhat by reducing the water-cement ratio.

The aggregates normally used for concrete are natural deposits of sand and gravel, where available. In some localities, the deposits are hard to obtain and large rocks must be crushed to form the aggregate. Crushed aggregate usually costs more to produce and will require more cement paste because of its shape. More care must be used in handling crushed aggregate to prevent poor mixtures and improper dispersion of the sizes through the finished concrete. At times, artificial aggregates, such as blast-furnace slag or specially burned clay, are used.

TYPES OF AGGREGATE. Aggregates are divided into two types as follows:

. FINE AGGREGATE. "Fine aggregate" is defined as material that will pass a No. 4 sieve and will, for the most part, be retained on a No. 200 sieve. For increased workability and for economy as reflected by use of less cement, the fine aggregate should have a rounded shape. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.

When properly proportioned and mixed with cement, these two groups yield an almost voidless stone that is strong and durable. In strength and durability, aggregate must be equal to or better than the hardened cement to withstand the designed loads and the effects of weathering.

It can be readily seen that the coarser the aggregate, the more economical the mix. Larger pieces offer less surface area of the particles than an equivalent volume of small pieces. Use of the largest permissible maximum size of coarse aggregate permits a reduction in cement and water requirements.

 One restriction usually assigned to coarse aggregate is its maximum size. Larger pieces can interlock and form arches or obstructions within a concrete form. That allows the area below to become a void, or at best, to become filled with finer particles of sand and cement only. That results in either a weakened area or a cement-sand concentration that does not leave the proper proportion to coat the rest of the aggregate. The maximum size of coarse aggregate must be no larger than the sizes given in table 13-1. The capacity of the mixing equipment may also limit the maximum aggregate size.

GRADATION. Gradation of aggregate refers to the amount of each size of particle used in the mix. Too large a proportion of coarse aggregate leaves voids that require more cement paste to fill. That affects the

Table 13-1.Maximum Recommended Size of Coarse Aggregate

economy of the mix. Too much fine aggregate, besides preventing a good bonding, also increases the surface area that must be coated with cement paste. That weakens the concrete. Good gradation results in a dense mass of concrete with a minimum volume of voids, an economical mix, and a strong structure. Optimum strength, water tightness, and durability in the hardened concrete require careful control of aggregate gradation.

DURABILITY. Durability is the ability to resist the elements of weathering and the load pressures. Weak or easily crushed rock or other mineral particles that break down under the applied loads will cause changes in the internal stresses and a breakdown of the concrete.

Rocks or mineral particles that are absorptive or susceptible to swelling when saturated will disintegrate when acted upon by different weather conditions. Freezing moisture causes expansion stresses that can easily rupture absorptive rocks. Radiant heat from the sun causes rocks to swell. If the heat is then followed by sudden cooling because of a shower and temperature drop, shrinkage and a breakdown of some rocks frequently occur. The aggregate must be chosen to withstand these forces of nature.

DETERIORATION. Deterioration of concrete, in many cases, can be traced to the aggregate. An excessive amount of organic material in or on the aggregate prevents the cement paste from forming an adequate bond with the aggregate particles. A large percentage of clay or fine silts adhering to the aggregate may prevent the cement paste from reaching the particles. That results in a structurally weak concrete that also is susceptible to breakdown by weathering. Washing the aggregate to remove the silts, clays, and organic material prevents this problem.

CHEMICAL COMPOSITION. Chemical composition of the aggregate is also important. Any chemical reaction between aggregate and cement in the presence of water reduces the hardening and cementing process. Any reduction in the amount of water-cement paste caused by a chemical reaction reduces the amount available to bond to the aggregate. This result is similar to one caused by an insufficient amount of cement.