placement. A very stiff mix has a low slump and, although difficult to place in heavily reinforced sections, is
desirable for many uses. A more fluid mix is necessary when placing concrete around reinforcing steel.
Nonsegregation. 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
Uniformity. The uniformity of plastic concrete affects both its economy and strength. Uniformity is determined
by how accurately 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
This is the end product of any concrete design. The essential properties it must have are strength, durability, and
Strength. The ability of concrete 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 and
pounds of cement). A sack of cement requires about 2 1/2 gallons of water for hydration. More water is added to
allow for workability, but too much water (a high water and cement (w/c) ratio) reduces concrete strength. The
amount of water in economical concrete mixes ranges from 4 gallons minimum to 7 gallons maximum per sack.
Durability. Climate and weather exposure affect durability. Concrete's ability to resist the effects of wind, frost,
snow, ice, abrasion, and the chemical reaction of soils or salts is 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.
Watertightness. Tests show that the watertightness of a cement paste depends on the w/c ratio and the extent of
the chemical reaction progress between the cement and water. 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 5.0 gallons per sack (w/c = 0.44) for concrete exposed to salt water.
The watertightness of air-entrained concrete is superior to that of non-air-entrained concrete. (See Lesson 2,
Table 2-1 on page 2-3.)
PART B - DESIRABLE CONCRETE COMPONENTS
Portland cements contain lime and day minerals (such as limestone, oyster shells, coquina shells, marl, clay, and
shale), silica, sand, iron ore, and aluminum.
The raw materials are finely ground carefully proportioned, and then heated (calcined) to the fusion
temperature (2,600 to 3,000 Fahrenheit (F)) to form hard pellets called clinkers. The clinkers are
ground to a fine powder.
The cement powder is so fine that nearly all of it will pass