Light Weight Concrete

LIGHT WEIGHT CONCRETE It may be defined as the concrete of substantially lower unit weight than that made from gravel or crushed stone. Ordinary concrete is quite heavy and its density is 2300 kg/m3. It is not suitable for use in floor filling as filler in general. If used, it adds considerably to the dead weight of the structure. By using suitable aggregates the density of concrete can be reduced. This light weight concrete not only results in reducing dead weights on structure, but also has a better insulation against heat and sound. The strength of such concrete is however low. But it is of no consequence as this concrete is not expected to bear any loads. Because of the light weight, this concrete is very suitable for earthquake proof structures. Lightweight concrete mixes are commonly used in the construction industry where weight savings is an important factor. One of the most common uses for lightweight concrete is with floor, roof or bridge decks; others include pavement systems, masonry blocks and offshore oil structures. Lightweight concrete is made by replacing some or all of the normal weight aggregate with lightweight aggregate. Often the coarse fraction is replaced with lightweight aggregate and the fines are normal weight sand.

APPLICATION OF LIGHTWEIGHT CONCRETE Light weight concrete finds its use generally in situations like: . Decks of long span bridges. . Fire and corrosion protection. . Covering for architectural purposes. . Heat insulation on roofs. . Insulation of water pipes. . Filling for floor and roof slabs. . Construction of partition walls and panel walls in framed structures. . Production precast building blocks and low cost housing.

WHAT ARE THE AGGREGATES USED TO PRODUCE LIGHTWEIGHT CONCRETE? The following light weight aggregates are generally used to produce lightweight concrete:

BREEZE OR CLINKER These aggregates are obtained from coal and are very light. Some types of coal produce clinker which expands on coming in contact with water and is injurious to concrete causing it to crack. The more the combustible matter remaining in the clinker, the more moisture movement is there. The other injurious matter is sulpher which may be found in these aggregates. Its percentage should be kept low. To minimize the failure of concrete prepared with this aggregate, the fine aggregate is almost omitted from the concrete. In general clinker should be used for such partition walls etc. which are not exposed to weather directly. It should not be used in reinforced concrete where the steel is liable to get corroded. The crushing strength of this concrete is about 15 to 30 kg/cm2 and its density varies from 800 to 1000 kg/cm3. PUMICE STONE It is also a suitable material to produce light weight concrete provided it is free from volcanic dust or clay etc. It gives concrete having density 800 to 1100 kg/cm3 and has a high thermal insulation. Its strength is about 15 to 40 kg/cm2.

FOAMED SLAG Foamed slag is also used for preparing light weight concrete. It is obtained by rapidly quenching the blast furnace slag in water. It should be free from combustible materials like coke and sulphur. It also shows expansion on getting moist. It produces concrete having a density of about 1300 to 1450 kg/cm2. It has a high crushing strength.

EXPANDED VERMICULITE SLATE This aggregate is obtained by heating some natural products like vermiculite (a mica product) to suitable temperatures. By heating its volume increases several times and forms a light weight aggregate. The weight of this concrete may be as 600 kg/cm3.

DESIGN OF LIGHTWEIGHT AGGREGATE CONCRETE Mix design methods applying to normal weight concrete are generally difficult to use with light weight aggregate concrete. The lack of accurate value of absorption, specific gravity, and the free moisture content in the aggregate make it difficult to apply the water/cement ratio accurately for mix proportioning. Light-weight concrete mix design is usually established by trial mixes. The proportions of fine to coarse aggregate and the cement and water requirement are estimated based on the previous experiences with particular aggregate. Various degree of water absorption by different lightweight aggregates is one of the serious difficulties in the design of mix proportions. A reliable information of saturated, surface-dry bulk specific gravity becomes difficult. Sometimes the aggregate is saturated before mixing so that it does not take up the water used for mixing. The quality of concrete does not get altered on account of absorption by aggregate. It has been seen that the strength of the resulting concrete is about 5 to 10 per cent lower than when dry aggregate is used for the same content and workability. This is due to the fact that in the latter case some of the mixing water is absorbed prior to setting. This water having contributed to the workability at the time of placing gets absorbed later, thus reducing the bad effect of excess of water. Moreover, the density of concrete made with saturated aggregate is higher and the durability of such concrete, especially its resistance to frost is lower. On the other hand, when aggregate with high absorption is used, it is difficult to obtain a sufficiently workable and yet cohesive mix, and generally aggregates with absorption of over 10 per cent are presoaked.

MIXING PROCEDURE OF LIGHTWEIGHT AGGREGATE CONCRETE Mixing procedure for light-weight concretes may vary with different types of aggregates. The general practice for structural light-weight concrete is to mix the aggregate and about 2/3 of the mixing water for a period upto one minute prior to the addition of cement and the balance mixing water. Mixing is done continuously as required for homogeneity. Usually 2 or more minutes are required to get uniform mixing. In case of some insulating concrete, the aggregate is added at the end of mixing to minimize degradation. Mix design data have been prepared for several, proprietary light-weight aggregates available in the United Kingdom. The parameters obtained from these charts cannot be taken as final answers. However, they may give information for first trial.