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Flowable Fill Concrete, Controlled low strength material (CLSM), is a self-compacting, flowable, low strength, cementitious material used primarily as backfill. Flowable Fill Concrete is primarily used as a replacement of compacted soil around pipes, bridge abutments, culverts and trenches; fill for embankments, bedding for slabs and pipes; insulating fill; fill for caissons and piles; and fill for abandoned storage tanks, shafts, and tunnels. It’s known that there are many construction projects which accompany a number of backfill activities. However, it’s common to here many complains due to improper backfilling and their poor performances due to narrow spaces left for backfill which do not allow heavy machines to get in for compaction. In addition, in some cases of back fill, it’s common to have places hard to reach which maybe left void and may result in poor performance. Thus using flowable fill concrete for back fill gives a solution for the above mentioned problems. The objective of this thesis is evaluating the mechanical property of the flowable fill made of fly ash, a by-product of coal combustion, and comparing the relative cost with the conventional granular backfill so that we can check its viability for using in the Ethiopian construction industry. In order to investigate the mechanical property of the material, testing program was conducted for the flowable fill concrete containing various combinations of Portland cement, fly ash, sand, and water. Mix designs were developed based on the ACI 229R_99 recommendations. These mix design has been used as a starting point, and minor adjustments have been made to manage the flowability and the respective compressive strength. Unconfined compressive strength, flow, setting time, plastic shrinkage, density, and Air content tests were conducted. Project goals included examining the effects of each mixture component on the overall performance of flowable fill and designing flowable fill containing fly ash that meets the ACI specifications. In the trial mixtures, water cement ratio has been adjusted to meet the flow requirements. Based on the achieved flowability, other tests are conducted. For high fly ash content mixtures, the unconfined compressive strength was found to be affected more strongly by the cement content. The strength has shown increment as cement content increased from 7% to 18%. Even though the 7% cement content mixture exhibited lower compressive strength, other mixtures has experienced much better strength v than the conventional backfill. The 10% cement content has showed satisfactory results in all the performed tests. Thus, it has been consider as better mix proportion and also selected for cost comparison scenario. A cost comparison of Flowable Fill Concrete and conventional granular backfill indicates that when trench dimensions are the same and only direct costs (labour, materials and equipment costs) are included, Flowable Fill Concrete costs approximately 128.58 ETB per unit volume more than conventional backfill. The other scenario considered for cost comparison is by differing the trench width. Conventional backfill normally requires additional trench width for compaction equipment and labours. A reduction of trench width on each side of a pipe from 2.3 meters down to 1.8 meter can be realized when using Flowable fill. The reduced trench width makes flowable fill cost less than the conventional backfill by approximately 36.40 ETB per meter of pipe for a 2.3m by 15m trench on a direct cost basis. In Addition to this, using flowable fill concrete highly saves time, requires less inspection, less testing, reduced liability concerns, and reduced or no future remedial work. And more importantly its environmental advantages by using industrial by products makes using Flowable Fill more viable to use it in the construction industry. |
