1LIGHT WEIGHT CONCRETE ABSTRACT The developments in the field of construction raise the need for concrete with less weight. This is beneficial for different applications starting from the less load applied to foundations and soil till the reduction of carriage capacity required for lifting precast units. In this paper, the production of light weight concrete from light local weight aggregate is investigated. Light weight concrete proved also to be more impact and fire resistant. However, as expected, it needs separate calibration curves for non-destructive evaluation. Brief History: Romans appeared to have used Light weight Concrete in 2 nd century, in the construction of 44m dia dome of Pantheon. But due to rarity and non-availability of porous light weight aggregates, the search for artificial aggregates began in the beginning of 20th century. In 1907, Ottoman [Hungarian] developed a method of converting Blast furnace slag in to foamed structure, which is the first milestone of Light Weight Concrete. But, the first commercial scale production of light weight aggregate manufactured in Kanas city, in 1917, named as “HAYDITE”. Subsequently, other expanded clay aggregates such as “GRUELITE” “TERLITE” “ROCKLITE” were marketed by various American firms. In 1930, Germany developed hollow globular burnt clay particles known as globulite. Industrial waste ‘cinder’ is converted into light weight aggregate by secondary burning and marketed in the name – “AGLOPYRITE”. The mail historical point is during 2 nd world war, 105 light weight ships were built using Light weight concrete. 2 In India, Pune based company, B.G.Shirke Construction Company Ltd is manufacturing steam cured, light weight, autoclaved and strong floor slabs, roof slabs, panels, lintels etc. with the brand name “SIPOREX”. What is Light Weight Concrete? Light weight concrete can be simply defined as follows, “These are the concretes which are much lighter than the conventional concrete, due to different constituents and methods used in manufacturing process”. Density of conventional concrete varies from 2400-2500 Kg/m 3.But the density of light weight concrete is in between 300-1900 Kg/m 3. How great difference !!!. As per RILEM, the light weight concrete committee “The LWC is a concrete capable 0f hardening, to a mass having oven dry density not more than 1800 Kg/m3 ”. As per ACI, “Concretes have a 28 days compressive strength in excess of 175 Kg/m 3 and 28 day air dried unit weight not exceeding 1850 Kg/m 3”. Advantages over Conventional concrete : Reduction of Dead load due to less density. Increases progress of work due to less weight. Lowers Haulage and Handling charges. In extreme climatic conditions, use of LWC is advisable because of high thermal insulation (due to porousness). More sound and heat insulation. Its volume per unit weight is more than that of conventional concrete, because of low density, results in economical construction. 3 Due to low density, it reduces the size and the cost of structural members such as beams, columns, foundations etc. This is advantageous in the case of tall structures which are to be constructed on soils of low bearing capacity. It is eco-friendly because it gives an outlet for industrial waste such as clinker, fly ash, slag etc. which otherwise create pollution. How to make concrete LIGHT ?? Weight of concrete Wc is a function of weights of aggregates, binder and volume of voids. Wc = f [ Wca, Wfa, Wb, Wv ] Therefore, we can reduce the weight Wc by Reducing the weight of coarse and fine aggregates, i.e. use of light weight aggregates which are occurred either Naturally (Pumice) or Artificially ( Globulite, Terlite). Increasing the volume of voids by one or both of following, • • Introducing air or gas into the concrete Remove certain fractions of fine aggregate in the concrete so that volume of void can be increased. Classification of L.W.C. : Classification based on RILEM • • • Fully compacted concrete Partially compacted concrete No finer concrete 4 • • Aerated concrete produced by chemical process (gas concrete) Aerated concrete produced by physical process (foam concrete) Classification based on density and strength • • • L.W.C of Low strength and good thermal insulation having density ranges from 250- 800 Kg/m3. L.W.C. of medium strength and adequate thermal insulation having density ranges from 800 – 1400 Kg/m3. L.W.C. of structural strength and limited thermal insulation having density ranges from 1400 – 2100 Kg/m3. Methods : To realize Light Weight Concrete, increase the pore volume of the concrete by, Omitting fine sized aggregates while grading Concrete. Using cellular porous aggregates Concrete. Introducing air or gas in to concrete i.e. Aerated Concrete. i.e. Light Weight Aggregate i.e. No Fines No Fines Concrete : 5 19mm – 9mm aggregates IN this type of L.W.C., fine aggregate fraction is omitted i.e. only cement and coarse aggregates are used (19mm – 9mm). Therefore it contains many uniformly distributed voids. Its density varies from 2/3 rd to3/4th of conventional concrete. It’s strength mainly depends on W/C ratio and A/C ratio. W/C ratio must be in between 0.38 to 0.52 and A/C ratio must be in between 6:1 to 10:1. In case of No fines concrete, W/C ratio must be selected very carefully. Because, If W/C ratio selected is too low, then the paste will be very dry and aggregates do not get bonded. This results in insufficient adhesion between particles, hence reduction in strength. If W/C ratio is too high, then the cement paste flows to bottom. Hence, segregation and bleeding takes place particularly when vibrated. This also leads to reduction in strength. The density of No fines concrete varies from 1600-1900 kg/m3. but it can be reduced up to 360 kg/m3 by using LWA. And it’s compressive strength varies from 14 – 140 kg/cm2 But it has some disadvantages, possesses poor workability low compressive strength low flexural strength high permeability therefore nowadays it is not widely used as L.W.C. Light weight Aggregate concrete : 6 It is one of the oldest and common method of producing L.W.C. is the use of Light weight aggregate, the light weight aggregates can be classified as Natural light weight aggregates Artificial light weight aggregates Natural Light weight aggregates: These are naturally available aggregates. Some of the examples are, • • • • • • Pumice Dolomite Scoria Volcanic cinders Saw dust Rice dust Of the above, the Pumice and scoria are the rocks of volcanic origin. By the time of origin, the cavities formed due to escape of gases. The dome of “Pantheon” is also constructed with pumice only. The ‘Dolomite’ is amorphous hydrated silica, derived from microscopic plants ’Dialomos’. Processed Natural L.W. Aggregates : • Expanded clay and shales – When certain clays and shales are heated in their semi-plastic state, up to point of incipient fusion, they bloat to as much as 7 times of original volume. But they retain their cellular structure even after cooling. A good example for this is the well known ‘Haydite’. • Expanded slate – The slate is a laminar flaky structured rock. When it is heated rapidly to higher temperature so as not allow gases to escape, slate looses it’s laminar structure and expands to several times of their volume and it converts into L.W.A. Processed Industrial byproduct L.W.A. : 7 • Foamed blast furnace slag – when Iron ore reduced to metallic iron by means of coke and lime in blast furnace, silica and Al. contents combine with lime to form molten slag at 1400 – 1600degree temperature, which collects on top of iron. Cooling this with controlled amount of water, the steam is tapped inside the mass results in L.W.A. known as foamed slag and expanded slag. • Pulverized fuel ash – It is a grey powder left as residue on the burning of coal in boilers of thermal power stations. This fly ash is mixed with limited amount of water and made into pellets and then introduced to a temperature of 1000-1200 degree c, from which it is converted into aggregate. It is most commonly used aggregate because of it’s high strength/ density ratio and less shrinkage value. Aerated concrete (Cellular concrete): The general composition of aerated concrete is mortar, suitably aerated. Thus concrete can be regarded as a more or less homogeneous, fine grained, silicon structure enclosing non-communicable air cells & also called as foam concrete, cellular concrete, gas cone etc. Aerated concrete is made by introducing air or gas into slurry composed of Portland cement or lime and finely crushed silicon material so that when mix sets and hardens, a uniformly cellular structure is formed. Filler material could be finely grounded sand, burnt shale, slag or pulverized fuel ash. In India, the factories manufacturing aerated concrete are Siporex, vayuthan, celcrete . Methods of aeration: 1. by the formation of gas by chemical reaction within the mass during liquid or plastic state : in this method finely powdered Al or Zn is added to the slurry which in turn reads with ca(OH)2 generated during hydration 8 of cement. H2 gas liberated by such reaction will lead to cellular concrete. It is used normally in precast concrete factories 2. By mixing preformed stable foam with slurry : stable foam is mixed GROUP DRY DENSITY Kg / m3 Class A Class B Class C Class D Class E 851 to 1000 751 to 850 651 to750 551 to 650 451 to 550 Min. Compressive Strength ( Kg / cm2) 70 60 50 35 20 with slurry thus causing cellular structure when the slurry sets and hardened. It is suitable for small scale productions and in situ works. Classification of Aerated concrete : As per IS 6072 – 1971 and IS 6073 – 1971, the aerated concretes are classified into five groups based on their density, as follows 9 Advantages: Low density i.e. about 300 – 800 kg/ m3 Good thermal insulation. Medium density grades are used for insulation purpose Higher density grades are used in the manufacture of pre fabricated structural members in conjunction with reinforcement. Implementation of L.W.C : • • • • Since the strength of L.W.C. is low, it is used in the construction of roof slabs, small houses with load bearing walls etc. It is also used in the construction of stairs, windows, garden walls, etc. In large buildings also, this is used in the construction of partition walls. These are moulded in the form of slabs and used as thermal insulators inside the building. Conclusion : Since, the Light weight concrete is of low compressive strength; this can not be used for the construction of structural members such as beams, columns, etc. But this can be effectively used in the construction of partition walls, windows, floor slabs, etc. The implementation of light weight concrete more in the structure leads to more economy. Therefore we can conclude that, “To achieve the low cost construction, first we have to implement Light weight Concrete”. That’s why, Attempts are made in the PAST, Attempts are continuing at PRESENT, 10 Attempts will continue in the FUTURE, TO MAKE CONCRETE LIGHTER AND LIGHTER!! Bibliography: 1. Light Weight Concretes by Mr. M.G. Srinivasan 2. Concrete Technology by A.M. Neville & J.J. Brooks 3. Concrete Technology by D.F.Orchard 4. Excerpts from Internet Downloading.
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