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Ali Shokati Sayyad, MS Student Of Islamic Azad University Chaloos Branch Kooros
Nekoofar Asisatant Professor Of Islamic Azad University Chaloos Branch
With increasing concern over the excessive exploitation of natural aggregates, synthetic lightweight aggregate produced from environmental waste is a viable new source of structural aggregate material. The uses of structural grade lightweight concrete reduce considerably the self-load of a structure and permit larger precast units to be handled. In this paper, the mechanical properties of a structural grade lightweight aggregate made with fly ash and clay will be presented. The findings indicated that water absorption of the green aggregate is large but the crushing strength of the resulting concrete can be high. The 28-day cube compressive strength of the resulting lightweight aggregate concrete with density of 1590 kg/m3 and respective strength of 34 MPa. Experience of utilizing the green lightweight aggregate concrete in prefabrication of concrete elements is also discussed.
Most of normal weight aggregate of normal weight concrete is natural stone such as limestone and granite. With the amount of concrete used keeps increasing, natural environment and resources are excessively exploited. Synthetic lightweight aggregate produced from environmental waste, like fly ash, is a viable new source of structural aggregate material. The use of lightweight concrete permits greater design flexibility and substantial cost savings, reducing dead load, improved cyclic loading structural response, longer spans, better fire ratings, thinner sections, smaller size structural members, less reinforcing steel, and lower foundation costs [1-3]. Weight of lightweight concrete is typically 25% to 35% lighter but its strengths is comparable to normal weight concrete.
2. Experiments and Results
2.1. Characteristics of the aggregate
The quality of the green aggregate [in terms of crushing strength] was specified by a crushing strength test based on GB2842-81 (China Standard). The strength as measured by compressing the aggregate in a steel cylinder through a prescribed distance of 20 mm is 3.8 MPa. Results of the sieve analysis and water absorption of the aggregate at different time are given in Table 1.
Fig. 1 examines the topography of the aggregate specimen using an optical microscope with 200X magnification. We can see that there is a thick shell rich in PFA at the outside surface of the aggregate. The compact external shell of the aggregate contributes to the higher strength resistance than the traditional lightweight aggregate without coating. Moreover, it is critical in controlling the water absorption of aggregate during concrete mixing, reducing the slump loss of concrete with time .
3. Prefabrication using Lightweight Aggregate Concrete
The structural lightweight aggregate was used to develop precast concrete elements for green construction. The mix proportion used is given in Table 2.
Table 2: Mix proportion of the green lightweight concrete (kg/m3)
|Cement||Sand||Water||AC agg. (pre-wetted)||Admixture|
Fig. 2 displays that a good workable fresh concrete for concrete casting. The slump of lightweight concrete measured 30 minutes after batching was 50 mm.
Fig. 3 shows the protocol of finished lightweight concrete precast façade.
Comparison of the design requirements with the concrete quality of the prefabricated façade are given in Table 3 below. It is seen that the gross weight of the lightweight concrete façade achieved only 70% of the density of normal weight concrete with the same compressive strength. Fig. 4 also indicated the bonding between reinforcing steel and lightweight concrete is good.
Table 3: Comparison of design requirement with actual concrete produce
|Unit weight||2275 kg (normal concrete)||1590 kg|
|1-day strength||15 MPa||14.5 MPa|
|28 days||30 MPa||34 MPa|
|Slump||75 mm||50 mm|
|Density||2400 kg/m3||1750 kg/m3|
The following advantages are concluded for using lightweight concrete in prefabrication in building:
• Reduce the dead weight of a façade from 5 tons to about 3.5 tons
• Reduce craneage load, allow handling, lifting flexibility with lighter weight
• Good thermal and fire resistance, sound insulation than the traditional granite rock
• Allow design and construction flexibility for larger prefabrication modules
• Allow maintenance flexibility with replaceable modules
• Factory production of module enhances quality of product
• Enhance speed of construction, shorten overall construction period
• Enhance green building construction, minimize wet trade on site
• Improve damping resistance of building
• Utilization of PFA in aggregate production resolves the waste disposal problems of ash and reduce the production cost of concrete
1. Short and W. Kimniburgh. Lightweight Concrete, 3rd ed., Applied Science Publishers, London, 1978.
2. FIP Manual of Lightweight Aggregate Concrete, 2nd ed., Surry University Press, Glasgow and London, 1983.
3. Satish Chandra and Leif Berntsson. Lightweight Aggregate Concrete, Noyes Publications, New York, USA, 2002.
4. Lo,Y., Cui, H.Z., and Li, Z.G. “Influence of Aggregate Prewetting and Fly Ash on Mechanical Properties of Lightweight Concrete.” Journal of Waste Management. (in press).