01-12-2012, 04:12 PM
MECHANICAL AND PHYSICAL PROPERTIES OF FLY ASH FOAMED CONCRETE
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ABSTRACT
Foamed concrete has become most commercial material in construction industry. Fly ash is receiving more attention now since their uses generally improve the properties of blended cement concrete, cost saving and reduction of negative environmental affects. The physical and mechanical properties of foamed concrete differ according to a different type of mixture and its composition. Therefore, this research investigates physical and mechanical properties of fly ash foamed concrete. Fly ash was used as fine aggregate. Six series of fly ash foamed concrete for target densities (1000, 1100,1200,1300,1400 and 1500 kg/m3) with constant cement to fly ash ratio (1:1.5) and cement to water ratio (1:0.65) by weight were prepared and tested. Tests were conducted to study physical properties (work ability, water absorption, drying shrinkage and carbonation) and mechanical strengths properties (compressive, splitting tensile and flexural strengths). Three types of specimens (cube, cylinder and prism) were used in different quantity and different purposes. The specimens of drying shrinkage test were opened after one day but, others specimens were de-moulded after three days and subjected to air curing under room temperature.
INTRODUCTION
Background
Concrete is one of the most widely used construction materials in the world today. It is made by mixing small pieces of natural stone (called aggregate) together with a mortar of sand, water, Portland cement and possibly other cementations materials.
Properly designed and constructed, concrete structures compare favourably with regard to economy, durability and functionality with structures made from other structural materials, such as steel and timber. One of the advantages of concrete is that it is readily moulded into virtually any required shape. Concrete is the preferred construction material for a wide range of buildings, bridges and civil engineering structures (Frank, 1989).
It is the second most widely consumed substance on earth, after water. Therefore, in concrete construction, self-weight represents a very large proportion of the total load on the structure, and there are clearly considerable advantages in
reducing the density of concrete. The chief of these are the use of smaller sections and the corresponding reduction in the size of foundations .Furthermore, with lighter concrete the form work need withstand a lower pressure than would be the case with ordinary concrete , and also the total weight of materials to be handled is reduced with a consequent increase in productivity, light weight concrete also gives better thermal insulation than ordinary concrete, the practical range of densities of lightweight concrete is between 300 and 1850 kg/m3, the weight reduction of a concrete structure would require less structural steel reinforcement.
Aerated concrete
Aerated concrete does not contain coarse aggregate, and can be regarded as an aerated mortar. Typically, aerated concrete is made by introducing air or other gas into a cement slurry and fine sand. In commercial practice, the sand is replaced by pulverized fuel ash or other siliceous material, and lime maybe used instead of cement.
Foamed concrete:
It is manufactured by entraining relatively large volumes of air into the cement paste by the use of a chemical foaming agent (Kearsley & Wainright, 2001). In other words, it is a mortar mix containing air voids that been produced by adding foaming agents which plays the role of creating pores within the concrete without chemically reacting to the cement. This is mainly used for in situ concrete suitable for insulation roof screeds or pipe lagging.
According to Aldridge (2005), the term foamed concrete is in itself misleading with the west majority of foamed concretes containing no large aggregates, only fine sand and with the extremely lightweight foamed materials only cement, water and foamed, so the product should be more accurately describe as a foamed mortar. As a rule of thumb a foamed concrete is describe as having an air content of more than 25% which distinguishes it from highly air entrained materials. In its basic from foamed concrete is blend of sand, cement, and water a pre-foamed foam, which in itself is a mixture of foaming agent (either synthetic or protein base), water and air.
Material of foamed concrete
Foaming Agent
Foaming agents is also defined as air entraining agent. Air entraining agents are
organic materials. When foaming agents added into the mix water it will produce
discrete bubbles cavities which become incorporated in the cement paste. The
properties of foamed concrete are critically dependent upon the quality of the foam.
There are two types of foaming agent:
i). Synthetic-suitable for densities of 1000 kg/m3 and above.
ii). Protein-suitable for densities from 400 kg/m3 to 1600 kg/m3.
Foams from protein-based have a weight of around 80 g/ litter. Protein-based
foaming agents come from animal proteins out of horn, blood, bones of cows, pigs
and other remainders of animal carcasses. This leads not only to occasional
variations in quality, due to the differing raw materials used in different batches, but
also to a very intense stench of such foaming agents.
Water
Water is once of the important material for the foamed concrete. The quality of the water must best on the BS3148. The criterion of portability of water is not absolute. Water with Ph 6 to 8 which not tested saline or brackish is suitable for use. Natural water that is slightly acidic is harmless, but water containing humic or other organic acids may adversely affect the hardening of concrete. The present of algae in the mixing water will result in air entrainment with consequent loss of strength. Hardness of water does not affect the efficiency of air-entraining admixtures. The use of the sea water as mixing water must be considered. Sea water has, typically, a total salinity of about 3.5 per cent. It can cause the long term-strength of the concrete become low. Sea water also contain a lot of chlorides it can cause corrosion for the reinforcement concrete. So the mixing water shall be clear and apparently clean (Neville & Brooks, 2001).