Concrete Mix Design Procedure Using Locally Available Materials

Abstract

Concrete is the most popular and widely 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 works as a major determining factor for the properties of concrete, but at present there is no concrete mix design guideline for Bangladesh using locally available materials. There are already some conventional methods of concrete mix design, such as, ACI 211and BS 812. But subsequent aggregate gradations (i.e. ASTM C33, BS 882) may not be achievable through these standard methodologies due to possible variation in aggregate characteristics of this particular geographic region. Thus, in this research a comprehensive methodology has been developed for the suitable gradation of locally available aggregates as well as for concrete mix design. In order to determine the most suitable aggregate gradation method, different existing methodologies have been compared via laboratory experiments in terms of fresh concrete workability (slump) and28 days concrete compressive strength. The test results signify that concrete compressive strength and slump can vary up to 21 MPa and 65 mm respectively, at particular mix content because of the differences in aggregate gradations. Effectiveness of . different parameters of aggregate gradation namely fineness modulus (FM) of fine aggregate (fa), FM of total aggregate (ta), coarseness factor, workability factor etc. have also been analyzed along with the test results. It has been found that contemporary 'band gradations' of aggregates result in somewhat better properties of concrete when compared with conventional gradations (ASTM C33, BS 882). Accordingly, two band aggregate gradations- '5-10-14-18' and '5-10-18-22' have been developed; for which the ranges of materials retaining on individual sieve sizes are specified in a way that any aggregate gradation falling within the range will also have their FM and falta within a range. In the next step, compatibility and other features of the '5-10-14-18' and '5-10-18-22' aggregate gradation bands have been examined via another set of laboratory experiments. Variations of concrete pioperties( compressive strength and slump) due to various possible aggregate gradations within the particular band have been found to follow some specific patterns. To incorporate these variations of concrete compressive strength and slump into mix design, two FM vs. falta charts have been introduced where percentage of variations in these concrete properties are given for various FM and falta values. Finally, these '5-10-14-18' and '5-10-18-22' aggregate gradation bands are applied to prepare a wide array of concrete mixes which are then used to prepare a concrete mix proportioning guideline. Two most available cement types in Bangladesh, CEM I and CEM II/B-M have been used in this concrete mix design method. The proposed concrete mix design method has been found to be advantageous and user-friendly since it considers the effects of cement-water paste volume on concrete properties; and also, for a target strength and slump requirements it provides multiple mix proportions as alternative options to the user. This method can be used to prepare mixes with compressive strength form 2.5 MPa to 55 MPa. Finally, to examine the performance of proposed concrete mix design method three sets of concrete mixes have been prepared with different target 28 days compressive strengths; 25 MPa, 35 MPa and 45 MPa. It has been found that in case of 28 days compressive strength the mix design method shows an appreciable performance with very low standard deviation (7%). But for concrete workability the mix design results in somewhat greater slump value than the target slump with 44% standard deviation. Apparently, concrete mixes prepared with this proposed method do not result in lower concrete properties (i.e. slump, compressive strength) than the target values.