Abstract:
Concrete is the most popularly used construction material in Bangladesh. It can be produced form locally available materials which make the product-cost lower than the others. Concrete mix design is a significant factor for determining concrete properties; however, very little research has been carried out in order to prepare a guideline of concrete mix design using locally available materials of the country. There are some prevailing concrete mix design methods, for example, ACI 211 and BS 812 which are used by Civil Engineers for carrying out the construction works, but subsequent aggregate gradations (i.e. ASTM C33 and BS 882) may not be achievable because of the possible variations of aggregate characteristics of this geographic region. Hence, Ashraf (2012) proposed one mix design guideline using aggregate gradation bands of local materials which considered compressive strength and workability as required parameters. This research has been carried out in order to produce a complete mix design guideline for Bangladesh incorporating strength, workability, durability and pumpability.
Cement used in this research has been CEM I. Since locally available materials have been used in this research, instead of conventional gradations (ASTM C33 and BS 882), two bands (5-10-14-18 and 5-10-18-22) of combined aggregate gradations, which have been developed by Ashraf (2012) have been used to produce the concrete mixes. Three water cement ratios (0.4, 0.5 and 0.6) have been used and cement content range has been kept between 350 to 500 kg/m3. For durability, sulfate attack, carbonation and Rapid Chloride permeability Tests (RCPT) have been performed. Of these three, RCPT gave distinct and reproducible results so this has been considered as a mix design parameter. Since durability has been considered as a parameter of the mix design, Rice Husk Ash (RHA) has been used as 10% and 20% replacement of cement. RHA has pozzolanic property that decreases permeability of hardened concrete thus increasing durability. It is readily available in a rice producing country like Bangladesh and its disposal has been a concern in recent time. Incorporation of RHA also results in strength increase. It has been found that 20% RHA replacement of cement produced concrete of compressive strength 10% higher than control concrete and permeability 59% lower in terms of RCPT values. RHA lowers slump values, for example, 20% RHA replacement gives upto 70% lower slump. Thus proper mix design can result in concrete of required properties.
By analyzing the results of the fifty four mixes of this work, a mix design process has been formed. Different charts and graphs have been produced. 28 day compressive strength, RCPT results as durability and slump have been used as the parameters of the mix design. A graph has been provided with the help of which pumpability of a mix can be checked using slump and pressure bleed test results. Finally four mixes have been prepared according to the newly proposed mix design guidelines and the results have been compared with the target values. All of the mixes showed greater strength and lower permeability values than the target values.
