Concrete & Formwork
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CHAPTER 1
CONCRETE AS A BUILDING MATERIAL
 

Portland cement is universally considered to be the most important masonry material used in modern construction. Its numerous advantages make it one of the most economical, versatile, and universally used construction materials available. It is commonly used for buildings, bridges, sewers, culverts, foundations, footings, piers, abutments, retaining walls, and pavements. A concrete structure, either plain or reinforced, is almost unique among the many systems of modern construction. In its plastic state concrete can be readily handled and placed in forms and cast into any desired shape. Quality concrete work produces structures which are lasting, pleasing in appearance, and require comparatively little maintenance.

Limitations. Recognition of the limitations of concrete construction in the design phase will eliminate some of the structural weaknesses that detract from the appearance and serviceability of concrete structures. Some of the principal limitations and disadvantages are:

(1) Low tensile strength. Concrete members which are subjected to tensile stress must be reinforced with steel bars, high-strength steel wire, or mesh.

(2) Drying shrinkage and moisture movements. Concrete, like all construction materials, contracts and expands under various conditions of moisture and/or temperature. This normal movement should be anticipated and provided for in the design, placement, and curing. Otherwise, damaging cracks may result.

(3) Permeability. Even the best concrete is not entirely impervious to moisture. It contains soluble compounds which may be leached out to varying degrees by water. Impermeability is particularly important in reinforced concrete where reliance is placed on the concrete cover to prevent rusting of the steel, and where the structure is exposed to freezing and thawing.

CONCRETE COMPONENTS

Chemical Process. The essential ingredients of concrete are cement and water which react chemically in a process called hydration to form another material having useful strength. Hardening of concrete is not the result of the drying of the mix, as can be seen from the fact that fresh concrete placed under water will harden despite its completely submerged state. The mixture of cement and water is called cement paste, but such a mixture, in large quantities, is prohibitively expensive for practical construction purposes and undergoes excessive shrinkage upon hardening.

Aggregates. Inert filler materials in the form of sand, stone, and gravel are added to cement and water in prescribed amounts to increase the volume of the mixture. When concrete is properly mixed each particle of aggregate is completely surrounded by paste and all spaces between aggregate particles are completely filled. The paste is the cementing medium that binds the aggregate particles into a solid mass.

Grout. Grout is a mixture of portland cement, lime, fine aggregate, and water in such proportions that the mixture is fluid. Exact proportions and the maximum size of the aggregate are dictated by the intended purpose.

Mortar. Mortar is a mixture of portland cement, lime, fine aggregate, and water in such proportions that the mixture is plastic. Exact proportions and the maximum size of the aggregate are determined by the intended purpose.

PROPERTIES OF CONCRETE

A plastic concrete is a concrete mix that is readily molded, yet changes its shape slowly if the mold is immediately removed. The degree of plasticity influences the quality and character of the finished product. Control of the ingredients in the mix limits the variables to the proportions of the ingredients. Significant changes in the mix proportions are indicated by the slump. The desirable qualities of plastic concrete are:

Workability. This property indicates the relative ease or difficulty of placing and consolidating concrete in the form. The consistency of the mixture is measured by the slump test (Appendix A) and is maintained as necessary to obtain the required workability for the specific conditions and method of placement. A very stiff mix would have little slump and would be very difficult to place in heavily reinforced sections. It is a good mix to place in a slab where reinforcing is not used. A more fluid mix can be placed where reinforcing steel is present. Workability is controlled largely by the amounts of and proportion of fine to course aggregate used with a given quantity of paste.

Nonsegregation. A plastic concrete should be handled so that there will be a minimum of segregation and the mix will remain homogeneous. For example, to prevent segregation, plastic concrete should not be allowed to drop (free fall) more than 3 to 5 feet. Care must also be taken in handling to prevent bleeding.

Uniformity. For uniformity every batch should be accurately proportioned according to the specifications. Uniform quality of the hardened concrete is desirable from both economic and strength considerations.

HARDENED CONCRETE

Hardened concrete in finished form is the actual basis of any concrete design. The essential qualities which must be considered are:

Strength. Strength is the ability of the concrete to resist a load in compression, flexure, or shear. The principal influencing factor on strength is the ratio of water to cement. About 2-1/2.) gallons of water are required for hydration (chemical reaction with water) of a sack of cement. Additional water is used to thin the paste, thus allowing it to coat more particles. This increases the yield obtainable from each sack of cement, thereby producing a more economical mix. However, excessive water-cement ratios should be avoided because thin paste is weak and a reduction in strength of the hardened concrete occurs due to the dilution of the paste. The minimum and maximum amounts of water generally used for economical mixes range from 4 to 8 gallons per sack.

Durability. Durability in concrete is the ability of the hardened mass to resist the effects of the elements, such as the action of wind, frost, snow, and ice, the chemical reaction of soils, or the effects of salts and abrasion. Durability is affected by climate and exposure. As the water-cement ratio is increased, the durability will decrease correspondingly. Air-entrained cements produce concretes with improved durability.

Water-Tightness. Water-tightness is an essential requirement of concrete. Tests show that the water-tightness of the paste is dependent on the amount of mixing water that is used and the extent to which the chemical reactions between cement and water have progressed. Frequently specifications for water-tightness limit the amount of water used in concrete mixes to 6 gallons per sack of cement. Water-tightness of air-entrained concrete is superior to that of non-air-entrained concrete.

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