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Test Introduction Concrete is a man-made composite the major constituent of which is natural aggregate (such as gravel and sand) and binding medium (such as cement paste, bitumen and polymers). The binding medium is used to bind the aggregate particles together to form a hard composite material.22877
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In its hardened state concrete is a rock-like material with a high compressive strength. By virtue of the ease with which fresh concrete in its plastic state may be mounded into any shape it may be used for decorative purposes.
Normal concrete has a comparatively low tensile strength and for structural applications it is normal practice either to incorporate steel bars to resist any tensile forces (steel reinforced concrete) or to apply compressive forces to the concrete to counteract these tensile forces (pre-stressed concrete or post-stressed concrete). Concrete is also used in conjunction with other materials, for example, it may form the compression flange of a box section and the remainder of which is steel (composite construction). Concrete is used structurally in buildings, shell structures, bridges, sewage-treatment works, railway sleepers, roads, cooling towers, dams, chimneys, harbors, off-shore, coastal protection works and so on. It is used also for a wide range of precast concrete products which include concrete blocks, cladding panels, pipes and lamp standards.
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-sich 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.
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.
The tensile properties of concrete can be enhanced substantially by incorporating high strength and small diameter short steel fibers．which leads to the steel fiber reinforced concrete(SFRC)．In conventional SFRC，the steel fiber content is usually within the range of 0．2％—2％ by volume．At such a low 6her content．the tensile response of SFRC would assume a non hardening type．which is characterized by the widening of a single crack，similar to an unreinforced concrete ．The contribution of fibers is apparent in the post—cracking response, represented by an increase in post—cracking ductility due to the work associated with pullout of fibers bridging a failure crack. However，improvements in some other properties are insignificant ．Moreover，the softening segment of the stress—strain curve of SFRC with such a low fiber content under uniaxial tension is difficult to be got with normal experimental methods．Many works have been done to find a suitable and relatively easy way to analyze the tensile characteristics ．And it was reported that the whole curve could be got on a normal testing machine with stiffening components added.