ECONOMICAL DESIGN OF ENGINEERED CEMENTITIOUS COMPOSITE

Abstract

Like most countries, major infrastructure construction in Pakistan is done using concrete as a building material due to its various positive aspects such as high compressive strength, durability and stability. To improve tensile strength of concrete, extensive reinforcement is provided. A better proposed solution for enhancing structural performance compared to conventional reinforcing techniques is Engineered Cementitious Composite (ECC). ECC is a composite of fibers, cement, fine sand and water. Other admixtures such as super plasticizer and Supplementary Cementitious Materials (SCMs) can also be used. Due to absence of coarse aggregate and presence of randomly oriented fibers, ECC shows ductile behavior. Hence, ECC can be used to improve structural performance. Polyvinyl-Alcohol fibers are most commonly used in the production of ECC. Due to the high cost of these PVA fibers, ECC is an uneconomical for use on an industrial scale. Furthermore, ECC requires more cement per unit volume due to absence of aggregate, which further increases the cost. In order to reduce the cost, the materials used in ECC were replaced with locally available cheaper materials. Instead of PVA fibers, a low modulus locally available fiber was used, and cement was partly replaced with a supplementary cementitious material(SCM) that was cheaper than cement. For this purpose, six mix variations were cast. Two out of these six variations were of mortar, containing no fibers and varying fly ash content at 0% and 60%. The remaining four were ECC variations, all having fiber content at 2% by volume and varying amount of fly ash content at 0%, 60%, 90% and 120%. In order to study the mechanical properties of these variations, cubes measuring 100mm x 100mm x 100mm, cylinders measuring 100mm x 200mm (diameter x height) and prisms measuring 250mm x 100mm x 25mm (length x width x depth) were cast and cured underwater for 7, 14 and 28 days. For testing, these samples were taken out, air dried and used for conducting compressive strength, tensile strength and flexural strength tests. PVA-ECC has compressive, tensile and flexural strengths in range of 30-90 MPa, 4-12 MPa and 10-15 MPa, whereas Nylon-66 ECC gave these properties in the range of 43.9 - 50.1 MPa, 3.63 - 4.28 MPa and 4.32 - 11.52 MPa respectively. 7 This clearly shows that ECC produced with nylon 66 fiber having 12mm length and 150µm diameter shows comparable mechanical properties to that of PVA-ECC. Furthermore, the prism samples tested under the three-point bend test showed strain hardening behavior which was absent in the mortar samples. Nylon 66 ECC was found to be 65.5% cheaper than PVA-ECC without the addition of fly ash which reduces cost by 1.73% for each 10% increment but also takes a toll on the mechanical properties i.e. 60% fly ash increment causes a 20% decrease in flexural strength, 8% decrease in tensile strength and 3% decrease in compressive strength.