Sustainable Development of Concrete Technology in Bangladesh – Part 3
Md. Rakibul Hasan, Asraful Angur, Fatema Ahmed, and Asif Sayeed
The Department of Civil Engineering of The University of Asia Pacific (UAP) has taken an initiative for the sustainable development of concrete technology in Bangladesh. The department has planned to complete this study in four parts, such as Part 1, Part 2, Part 3 and Part 4 by a two-year period. Part 3 was carried out in Spring 2006, which covered the following investigations:
Quality of Various Cement Brands in
Bangladesh – Part 2
The demand for cement in Bangladesh is increasing rapidly. In the meantime, to meet the demand of cement in the local market, more than sixty cement companies were established. Among the most commonly used cement brands, twenty cement brands were investigated. Normal consistency, initial setting time, final setting time, fineness (#100 and #200 sieves), bleeding, and compressive strength for 3, 7, 14, and 28 days were tested as per ASTM. Some cement show low compressive strength, high amount of bleeding, and long setting time. To avoid the problems, related to the poor quality of cements in the market, it is necessary to evaluate the quality of cement before use in construction.
Recycling of Demolished Concrete
as Coarse Aggregate
Recently, many concrete structures in Bangladesh are demolished due to the aging of the structure, earlier deterioration, and replacement of low-rise buildings by relatively high-rise buildings due to the booming in real estate business. Recycling of the demolished concrete as coarse aggregate for new construction has several advantages, such as solving the disposal problems of demolished concrete, saving natural resources by reducing aggregate consumption, etc. Generally, the old buildings in Bangladesh are made of brick aggregates. Literatures related to recycling of brick made aggregates can be rarely found. Therefore, a detailed investigation on recycling of demolished brick concrete as coarse aggregates for new construction works was carried out. Aggregates from five different demolished buildings of ages 15 to 50 years were collected. One-year old cylinder specimens tested for compressive strength of concrete are also broken into pieces as coarse aggregates for investigations. The properties of aggregates, such as absorption capacity, unit weight specific gravity, and abrasion value were evaluated. Standard grading of coarse and fine aggregates was used for all cases. Cylinder concrete specimens (15 cm by 30 cm, and 10 cm by 20 cm) were made with W/C 0.55, 0.5, 0.45, and 0.4 for 7, 14, and 28-day strength and stress-strain behavior evaluation. Also workability of fresh concrete was evaluated by slump test. Cement contents in concrete were 340 and 400 kg/m3 of concrete. For low W/C ratio, a naphthalene based high range water reducing chemical admixture was used. To reduce the water absorption and aggregate abrasion of recycle aggregates, cement paste coating was applied over the recycled aggregate and the mechanical properties of concrete made with these aggregates were also evaluated.
The workability of concrete made with recycled aggregates is lower compare to the same made with virgin aggregates but it can be improved by introducing a cement paste coating around the aggregates before using for new construction. The strength and Young’s modulus of concrete made with recycled aggregates is lower than the same made with virgin aggregates but if W/C ratio of the concrete made with recycled aggregates is reduced to a lower value (say from 0.55 to 0.5 or 0.45), a higher strength and a higher Young’s modulus can be obtained compare to the concrete made with virgin aggregates with relatively high W/C.