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The use of seawater does not appear to have any adverse effect on the strength and durability of Portland cement concrete but it may cause surface dampness, efflorescence and staining and should be avoided where concrete with good appearance is required. Seawater also increases the risk of corrosion of steel and its use in reinforced concrete is not permitted.
1. Chemical admixtures
Chemical admixtures are chemical substances introduced into a batch of concrete, during or immediately before its mixing, in order to alter or improve the properties of the fresh or hardened concrete or both.
Nowadays, there are more than 15 types of chemical admixtures produced and they benefit concrete structures in many ways. Since chemical admixtures may also have detrimental effects, their suitability for a particular concrete should be carefully evaluated before use, based on a knowledge of their main active ingredients, on available performance data and on trial mixes.
2. Mineral admixtures
Fly ash, slag powder and silica fume can used to replace a part of cement to reduce concrete cost and enhance concrete properties, especially its durability. Nowadays, chemicals admixture and mineral admixture have become the two necessary raw materials for preparation of High-performance Concrete, the fluidity and durability of which is both much better than that of normal concrete.
1.4 Concrete Mix Design
While designing a concrete mix proportion, the following factors must be carefully considered.
1）Workability of the fresh concrete;
2）Strength of the hardened concrete;
3）Durability of the concrete;
4）Cost of the concrete.
Because of its complication, it should be noted that the concrete mix proportion designed should be tested and modified before being used in practice.
2. Mechanics of reinforced concrete
Concrete in strong in compression but weak in tension. As a result, cracks develop whenever loads, of restrained shrinkage of temperature changes, give rise to tensile stresses in excess of the concrete. In the plain concrete beam, the moments due to applied loads are resisted by an internal tension-compression couple involving tension in the concrete. Such a beam fails very suddenly and completely when the first crack forms. In a reinforced concrete beam, steel bars are embedded in the concrete in such a way that the tension forces needed for moment equilibrium after the concrete cracks can be develop in the bars.
The construction of a reinforced concrete member involves building a form or mold in the shape of the member being built. The form must be strong enough to support the weight and hydrostatic pressure of the wet concrete, and any forces applied to it by workers, concrete buggies, wind, and so on. The reinforcement is placed in this form and held In place during the concreting operation. After the concrete has hardened, the forms are removed.
The choice of weather a structure should be built of concrete, steel, masonry or timber depends on the availability of materials and on a number of value decisions.
3. Factors affecting choice of concrete for a structure
3.1 Economy
Frequently, the foremost consideration is the overall cost of the structure. This is, of course, a function of the costs of the materials and the labor necessary to erect them. Frequently, however, the overall cost is affected as much or more by the overall construction time since the contractor and owner must allocate money to carry out the construction and will not receive a return on this investment until the building is ready for occupancy. As a result, financial savings due to rapid construction may more than offset increased material costs. Any measures designer can take to standardize the design and forming will generally pay off in reduced overall costs.
In many case the long-term economy of the structure may be more important than the first cost. As a result, maintenance and durability are important considerations.