SECTION II. DESIRABLE CONCRETE PROPERTIES
1-17. Plastic Concrete is a concrete in a relatively fluid state that is readily molded by hand, like a lump of modeling clay. A plastic mix keeps all the grains of sand and the pieces of gravel or stone encased and held in place (homogeneous). The degree of plasticity influences the quality and character of the finished product. Significant changes in the mix proportions affect plasticity. Desirable properties of plastic concrete are listed below.
1-18. Workability is the relative ease or difficulty of placing and consolidating concrete in the form. It is largely determined by the proportions of FAs and CAs added to a given quantity of paste. One characteristic of workability is consistency, which is measured by the slump test (see Appendix B). A specific amount of slump is necessary to obtain the workability required by the intended conditions and method of placement. A very stiff mix has a low slump and is desirable for many uses, although difficult to place in heavily reinforced sections. A more fluid mix is necessary when placing concrete around reinforcing steel.
1-19. Plastic concrete must be homogeneous and carefully handled to keep segregation to a minimum. For example, plastic concrete should not drop (free fall) more than 3 to 5 feet nor be transported over long distances without proper agitation.
1-20. The uniformity of plastic concrete affects both its economy and strength. It is determined by how accurate the ingredients are proportioned and mixed according to specifications. Each separate batch of concrete must be proportioned and mixed exactly the same to ensure that the total structural mass has uniform structural properties.
1-21. Hardened Concrete is the end product of any concrete design. The essential properties that it must have are strength, durability, and watertightness.
1-22. The concrete's ability to resist a load in compression, flexure, or shear is a measure of its strength. Concrete strength is largely determined by the ratio of water to cement in the mixture (pounds of water to pounds of cement). A sack of cement requires about 2 1/2 gallons of water for hydration. Additional water allows for workability, but too much water (a high water:cement [W/C] ratio) reduces the concrete's strength. The amount of water in economical concrete mixes ranges from 4 gallons minimum to 7 gallons maximum per sack.
1-23. Climate and weather exposure affect durability. Thus, the concrete's ability to resist the effects of wind, frost, snow, ice, abrasion, and the chemical reaction of soils or salts are a measure of its durability. As the W/C ratio increases, durability decreases correspondingly. Durability should be a strong consideration for concrete structures expected to last longer than five years. Air-entrained concrete has improved freeze-thaw durability.
1-24. Tests show that the watertightness of a cement paste depends on the W/C ratio and the extent of the chemical-reaction process between the cement and water. The Corps of Engineers specifications for watertightness limit the maximum amount of water in concrete mixtures to 5.5 gallons per sack of cement (W/C = 0.48) for concrete exposed to fresh water and 5 gallons per sack (W/C = 0.44) for concrete exposed to saltwater. The watertightness of air-entrained concrete is superior to that of nonair-entrained concrete.
|David L. Heiserman, Editor||
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Revised: June 06, 2015