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The impact strength, as well as the tensile strength, of normal concrete is low and this can be improved by the introduction of randomly orientated fibers into the concrete. Steel, polypropylene, asbestos glass, carbon and even wood fibers have all been used with some success in precast products and in-situ concretes, including pipes, building panels and piles.
Concrete requires little maintenance and has good fire resistance. Concrete has other properties which may on occasions be considered less desirable, for example, the time-dependent deformations associated with drying shrinkage and other related phenomena.
1.2 Preparation of Normal Concrete
We can learn how a structural concrete is prepared from Fig. 3.3. Curing after the placing of concrete is very important for obtaining good properties.
Making good concrete is difficult without addition of plasticizers or superplasticizers.
Good concrete has to be satisfactory in its hardened state (i.e. needed strength and good durability), and also in its fresh state while being transported from mixers and placed in formworks (sometimes through pumping).
1.3 Constituents in Concrete and Their Basic Knowledge
Concrete is composed mainly of three materials, namely, cement, water and aggregate, and an additional material, known as a chemical admixture, is sometimes added to modify certain of its properties. Fly ash, ground blast-furnace slag powder and silica fume are often used to replace partial cement to modify the properties of concrete and reduce the cost of concrete.
Cement is a chemically active constituent but its reactivity is only brought into effect with mixing water. The aggregate plays no part in chemical reactions but its usefulness arises because it is an economical filler material with good resistance to volume changes which take place within the concrete after mixing, and it improves the durability of concrete.
A typical structure of hardened concrete and the proportions of the constituent materials encountered in most concrete mixes are shown in Fig3.4. In a properly proportioned and compacted concrete the voids are usually less than 2%. The properties of concrete in its fresh and hardened state can show large variations depending on the type, quality and proportions of the constituents.
1.Cement
The different cements used for making concrete are finely ground powders and all have the important property that when mixed with water a chemical reaction (hydration) takes place. This in time produces a very hard and strong binding medium for aggregate particles. In the early stages of hydration, cement mortar gives fresh concrete its cohesive properties.
2. Aggregate
Aggregate in concrete can be classified as coarse aggregate and fine aggregate according to its diameter. The diameter of the coarse and fine aggregate is larger than 5mm and smaller than 5mm respectively. The coarse aggregate used in concrete is often cobblestone and crushed gravel and the fine aggregate is river sand and mountain sand.
Aggregate is much cheaper than cement and maximum economy is obtained by using as much aggregate as possible in concrete. Its use also considerably improves both the volume stability and the durability of the resulting concrete. The physical characteristics and in some cases the chemical composition of aggregate affecting to a varying degree on the properties of concrete in both plastic and hardened state.
Sand is generally considered to have a lower size limit of about 0.08mm and a little less. Materials which size is smaller than 0.08mm is classified as clay which does harm to the mechanic properties and durability of concrete.
Sea sand is not permitted to be used for making structural concrete before decreasing its salt content to less than 0.6% by washing.
3. Water
In general water fit for drinking, such as tap water, is acceptable for mixing concrete. The impurities that are likely to have an adverse effect when present in appreciable quantities include silt, clay, acids, alkali, and other salts, organic matter and sewage.