The light weight concrete can be produced by various methods. All depends on either the presence of air voids in aggregates or in the matrix, or by committing fine aggregates. We can classify light weight concrete as follows:
Figure Showing Light weight concrete
Lightweight aggregate concrete is produced using light weight aggregates such as pumice, expanded slag, clinker, etc. Aerated concrete is produced using foaming agents such as aluminum powder that produce low unit weight through generation of gas while the concrete is still plastic. No fines concrete made with gravel aggregates is not strictly light weight concrete though its weight is about 70% of normal concrete.
Lightweight concrete is now an established building material. It is used not only on account of its light weight but also because of high thermal insulation compared with normal concrete. Lighter the concrete larger is the insulation properties in general. An earliest application of light weight concrete was in building industry and for producing insulating screeds. The principal structural use of light weight concrete is in construction of under bed of floors and bed slab, where substantial economy can be achieved by decreasing the dead load. Indirect saving in steel and decreasing size in foundation are advantages.
The disadvantages are greater cost, need for care in placing, greater porosity and greater drying shrinkage.
Structural light weight aggregate concrete can be produced with the strength in excess of 310 MPa and even strengths have been attained in certain cases, although at the expense of increased densities. The aggregates used include sintered pulverized fuel ash, expanded of these aggregates absorb considerably quantities of water (up to 80% by volume), the effect on workability within a few minutes of mixing is such that a wet mix can become too dry. It is therefore necessary to wet, but not saturate, the aggregate before mixing. A good portion of the mixing water is also best added before introducing the cement. Rich mixes containing 350 kg/m3 cement or more are usually required to give a satisfactory strength.
Figure Showing Light weight concrete
The cover to reinforcement when using light weight concrete should be 10 mm more than that used for normal dense concrete. The increased cover is necessary because, besides being more permeable, light weight concrete carbonate more quickly than dense concrete and the protection affordable to the steel by the alkaline lime is cost.
A density of kg/m3 may be considered as the upper limit for a true light weight concrete although this value is sometimes exceeded. Lower densities than those stated will be obtained in cases where the concrete is only partially compacted.
In general, the lower densities can only be achieved at the expense of lower strengths. The range of strength in each case are typical of what may be achieved in practice, but it should be noted that rather rich mixes are required for higher strength values in case of the light weight fines to achieve these higher strengths. The tensile and shear strengths of light weight aggregate concrete are less than that for natural aggregate concrete of the same compressive strengths. The reduction in the case of tensile strength may be as much as 30%.
The modulus of elasticity of light weight concrete is about 0.5 to 0.75 times the value of natural aggregate concrete of the same compressive strength varying from 7 to 21 kN/mm2. Values of elastic deformation, shrinkage and creep are greater for light weight concrete. Extra reinforcement is also necessary.
Clinker and Breeze: clinker and breeze aggregates have been in use for many years in the production of blocks and slabs for internal partitions and other interior walls. These aggregates are cheap and plentiful, and provide a very useful product after they have been crushed and graded.
In general clinker is regarded as a well burnt fused or sintered mass containing little combustible material; whereas breeze is a more lightly sintered and less well burnt residue and therefore contains more combustible matter.
Increasing quantities of combustible content is determined approximately be igniting a small sample of the clinker as specified temperatures and finding the percentage loss in weight.
Pumice Concrete (Natural aggregate): pumice is most widely used natural light weight aggregate in common use. Provided it is free from fine volcanic dust and materials not of volcanic origin such as clay, pumice produces a satisfactory light weight concrete with a density of between 720 kg/m3 to 1440 kg/m3. Pumice provides better thermal insulation than other type of light weight concrete.
Foamed Slag: Foamed slag is made by rapidly quenching blast furnace slag produced in the manufacture of pig iron. Its texture and strength are dependent on the chemical composition and the method of treatment but in general the structure is similar to natural pumice.
Expanded Minerals: Naturally occurring clays, shales and slates may be used to produce light weight porous materials of a cellular texture by suitable treatment and heating up to temperature of about 1000°C to 1200°C. Materials with similar characteristics may also be obtained from pulverized fuel ash or fly ash. After crushing and screening to the desired size, these processed materials form good light weight aggregates.
Aerated cement mortars (often referred to simply as aerated concrete)to produce light weight porous materials of a cellular texture by suitable treatment and he are made by introducing air, or specially foamed gas, into a cement slurry so that, after setting, a hardened mortar with a cellular structure is formed. The slurry usually consists of a mixture of cement and siliceous material such as sand or pulverized fuel ash.
Coarse natural or light weight aggregate can also be used in conjunction with aerated cement mortars. Natural aggregates increase the density and to some extent reduce the moisture movements. Light weight aggregates do not cause the same increase in density, but the moisture movements are more nearly those of aerated mortars.
No-Fines concrete is composed of cement and coarse aggregate only, the fine aggregate being omitted in order to leave uniformly distributed voids throughout the mass. The aggregates may be gravel or crushed stone, blast furnace slag, crushed brick or one of the light weight aggregates. No fines concrete presents some difficulty in the fixing of various fittings and it is necessary to embed nailing blocks of timber, saw dust cement or foamed slag. No fines concrete has little resistance to the penetration of water, but there is also very little capillary action. Thus there is no tendency for the water to be drawn into the wall; the provision of a rendering, with a care in arranging and fixing flashing at various openings, is sufficient to waterproof the structure satisfactorily.