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In some occasions no fines concrete is used in houses because of its good thermal insulation properties. Basically no fines concrete consists of coarse aggregates and cement without any fine aggregates. It is essential that no fines concrete should be designed with a certain amount of voids to enhance thermal insulation.
Measurement of constituents for concrete is normally carried out by weight because of the following reasons :
(i) Air is trapped inside cement while water may be present in aggregates. As such measurement by volume requires the consideration of the bulking effect by air and water.
In designing protective coating on concrete structures, stoppage of water ingress through the coating is normally required. Since chloride ions often diffuse into concrete in solution and cause deterioration of concrete structures, the prevention of water transmission into the coating certainly helps to protect the concrete structure. However, if water gets behind the coating from some means and becomes trapped, its effect may not be desirable. Firstly, vapour pressure would be developed behind the surface treatment and this leads to the loss of adhesion and the eventual peeling off of the coating. Moreover, the water creates a suitable environment for mould growth on concrete surface.
Pulverized fly ash is a type of pozzolans. It is a siliceous or aluminous material which possesses no binding ability by itself. When it is in finely divided form, they can react with calcium hydroxide in the presence of moisture to form compounds with cementing properties.
(i) Similarities
Both GGBS and PFA are by-products of industry and the use of them is nvironmentally friendly. Most importantly, with GBS and PFA adopted as partial replacement of cement, the demand for cement will be drastically reduced. As the manufacture of one tonne of cement generates about 1 tonne of carbon dioxide, the environment could be conserved by using less cement through partial replacement of PFA and GGBS.
On the other hand, the use of GGBS and PFA as partial replacement of cement enhances the long-term durability of concrete in terms of resistance to chloride attack, sulphate attack and alkali-silica reaction. It follows that the structure would remain to be serviceable for longer period, leading to substantial cost saving. Apart from the consideration of long-term durability, the use of PFA and GGBS results in the reduction of heat of hydration so that the problem of thermal cracking is greatly reduced. The enhanced control of thermal movement also contributes to better and long-term performance of concrete.
From structural point of view, GGBS replacement enhances lower heat of hydration, higher durability and higher resistance to sulphate and chloride attack when compared with normal ordinary concrete. On the other hand, it also contributes to environmental protection because it minimizes the use of cement during the production of concrete.
To increase the strength of concrete, say from 40MPa to 80MPa, it definitely helps in improving the structural performance of the structure by producing a denser, more durable and higher load capacity concrete. The size of concrete members can be significantly reduced resulting in substantial cost savings. However, an increase of concrete strength is also accompanied by the occurrence of thermal cracking. With an increase in concrete strength, the cement content is increased and this leads to higher thermal strains. Consequently, additional reinforcement has to be introduced to control these additional cracks caused by the increase in concrete strength. Moreover, the ductility of concrete decreases with an increase in concrete strength. Attention should be paid during the design of high strength concrete to increase the ductility of concrete. In addition, fire resistance of high strength concrete is found to be less than normal strength concrete as suggested by Odd E. Gjorv (1994).
Normal Portland cement is usually adopted in water retaining structures.
Where sulphates or chemical agents are anticipated in groundwater, sulphate-resisting cement may be used to guard against sulphate and chemical attack.
There is common confusion about the terms “High strength concrete” and “High performance concrete” and it appears that they refer to the same thing. In fact, “High performance concrete” refers to the concrete which has been specially designed to achieve a certain particular characteristics such as high abrasion assistance and compaction without segregation.
Foam concrete is mainly composed of cement, water and air pores with filler (such as PFA, sand etc.) without any course aggregates. The air pores are formed by agitating air with a foaming agent. The typical size of air bubbles is around 0.3-0.4mm in diameter. For cement grout, it mainly consists of cement and water.
