Date of Award
Master of Science
SUFIYANU MOMOH, for the Masters of Science degree in Civil Engineering, presented on April 12, 2017, at Southern Illinois University Carbondale. TITLE: Utilization of Recycled Coarse Aggregates to Develop High Strength Concrete. MAJOR PROFESSOR: Dr. Prabir K. Kolay The use of recycled coarse concrete aggregates (RCA) for the construction of road and other non-structural concrete has become a common practice these days. With the increasing demand of concrete, there has been an increase in the demand of natural coarse aggregate (NCA), which has sparked the research on the use of recycled coarse aggregate in structural concrete as an alternative sustainable solution. The objective of this study was to produce high strength concrete using recycled coarse aggregates. ACI 211.4R-5 (1998) suggests that concrete yielding compressive strength of over 6000 psi at 28 days of curing can be considered as high strength concrete. Therefore, various concrete mixes containing different amounts of RCA were tested to determine the optimum replacement of NCA with RCA to obtain the targeted compressive strength of 6000 psi at 28 days of curing. The study was divided into two phases. During the first phase, the effect of using recycled coarse aggregate (RCA) and pozzolan such as micro silica fume (MSF) to make high strength concrete was investigated. Due to inconsistent results from this phase of the study, the second phase of the study consisted of reinvestigating the effect of using recycled coarse aggregate (RCA) to make high strength concrete without micro silica fume by narrowly following the recommendations of ACI 211.4R-5 (1998). In addition, Part 2 of this phase consisted of evaluating the effect of the quantity of cement on the compressive strength of RCA concrete compared to the concrete mixes prepared using 100% NCA. This goal of this part of the study was to determine if there is an optimum amount of cement beyond which the strength gain is minimal. In the first phase of this study, five concrete mixes consisting of 0%, 25%, 50%, 75%, and 100% replacement (by volume) of natural coarse aggregates with recycled concrete coarse aggregates were made using a targeted water to cement (w/c) ratio of 0.4. These mixes were tested after curing periods of 7, 14, and 28 days for compressive strength and after a curing period of 28 days for splitting tensile strength. Also, five additional mixes were prepared and tested using the same mix proportions but using micro silica fume to replace 25% of the portland cement. The results presented show that the addition of micro silica fume (MSF) in all the mixes increases the compressive and tensile strength of the concrete mixes. However, trends of the results from this phase of the study were inconsistent with those available in published literature.. Further scrutiny of the results revealed that the quantity of cement and water used in these mixes were not sufficient for the quantity of aggregates used in the mix designs, thereby, resulting in dry concrete mixes with low slumps causing ambiguities in the results. Therefore, the second phase of the study was initiated which also consisted of two parts. The first part (Part 1) of this phase involved studying the effects of recycled coarse aggregates in concrete, as was done in Phase 1, but the mixes were designed based on the ACI 211.4R-5 (1998) recommendation. Published literature and results from this study show that the recycled coarse aggregates have substantially higher loss of material when subjected to wearing conditions, probably due to weaker cement paste layer and sand particles loosely sticking at the surface. Therefore, during Part 2 of Phase 2 of the study, various quantities of cement i.e., 650 lbs./yd3, 675 lbs./yd3, 700 lbs./yd3 and 725 lbs./yd3 were used in concrete mixes consisting of 100% RCA and 100% NCA in an effort to determine the optimum amount of cement beyond which there is minimal increase in the compressive strength of concrete made with 100% RCA.. The results presented show that as the quantity of cement increases, the strength of the concrete made out of both, the natural and the recycled coarse aggregates increases. It is also observed that while the improvement in strength of the concrete made with 100% natural coarse aggregate increases consistently at about 6% for every additional 25 lbs/yd3 of cement, the rate of increase in the compressive strength of concrete made out of recycled coarse aggregates gradually decreases.
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