The quality of the hardened concrete is largely dependent on the quality of the plastic concrete. The curing process also contributes to attainment of the specified design criteria. Because the consistency of the mix may vary, careful adherence to design criteria is extremely important. When the ingredients are tested for suitability the only variable factor in the plastic mix may be in the proportioning, which must be accurate. Plastic concrete should be workable, free from segregation of the coarse aggregate, and of uniform quality.
Workability of the plastic mix is the relative ease or difficulty of placing and consolidating concrete in the form. The consistency of the mixture shall be maintained as necessary to obtain the workability required for the specific conditions and method of placement. The slump test which is used to measure the consistency of the mix is a measure of workability as well. To perform this test, the mixed concrete is placed in a standard cone in three respective layers, approximately equal volume, and each layer is rodded 25 times. The last layer is placed, rodded, and struck off. Then the cone is lifted vertically away from the plastic concrete, which is allowed to slump, or settle. The measurement of settlement below the top of the cone is recorded as a measure of workability of the mix. The maximum or minimum slump will be in accordance with specifications for the particular job. If constant quantities of materials are used, the slump should remain constant. Any marked variation in slump or consistency will immediately indicate a deviation from the mix design. An investigation should be made immediately to determine the cause.
3. No segregation
Plastic concrete should, be handled and placed so that it is free from segregation of the coarse aggregate and completely homogeneous. To prevent segregation, the plastic concrete should not be dropped, or allowed to fall free, for a distance greater than 3 to 5 feet.
For uniformity of the mix, every batch should be accurately proportioned in accordance with design criteria. Uniformity increases the economy of the mix and improves the quality of the hardened concrete.
Properties of Concrete in Hardened State
1. Essential Qualities
Essential qualities of hardened concrete in pavement are uniform flexural strength and durability.
Concrete pavements must have sufficient flexural strength to carry the required load. To attain the desired strength, the most important factor is the ratio of water to cement. In general, not more than 7 gallons and not less than 4 gallons of water are required for each sack of cement. Although a high water-cement ratio increases the yield of each sack of cement, a thin paste is inherently weak and evaporation of excess water during the curing process leaves voids that will reduce the strength of the hardened concrete.
Durability of finished concrete is the ability of the artificial stone to resist the effects of the weather, such as the action of the wind, frost, snow, ice, and the combined effects of wetting-drying and freezing-thawing cycles; the chemical reaction of soils or the effects of salts; and abrasion. Durability is affected by climate, thickness of pavement, and exposure. As the water-cement ratio is increased above the minimum of 4 gallons per sack, the durability is likely to decrease correspondingly.