Effects of cotton waste fibers on the strength development and flexural toughness of densified high-strength concrete.

Cotton Waste Fibers (CWF), a non-synthetic industrial by-product, form a type of micro-diameter concrete reinforcing fibrous material. Fiber dispersion and fiber balling reduce the application of CWF in structural concrete due to discontinuous interfacial transition zones (ITZ) in the concrete matrix. In this paper, compressive strength development and flexural toughness of concrete with 0%–1.5% 0.25 mm diameter and 50 mm length CWF fibres, designed using the densified mix design algorithm (DMDA) method, were studied. Two coarse aggregate (CA) grades: S1(10–12.5) mm and S2(5–10) mm; an intermediate aggregate (IA), S3(2.36–5) mm; and Natural River Sand (NRS) of 5 mm maximum aggregate size (MAS) fine aggregate (FA), and CEM II were adopted. An intermediate and coarse aggregate blend ratio of 1:1.33:5.34 in S3:S2:S1, enough to saturate the aggregate-to-aggregate pores, and an overall concrete mix proportion of 1:1.3:2.6, were achieved. A 0.38 water-cement (w/c) ratio and a maximum 0.0703 water-solid (w/s) ratio for 1.5% CWF dosage were used. A total of 84 cubes, 42 cylindrical, and 42 unnotched prisms, and 21 notched beam specimens were cast and cured at ambient temperature to determine the compressive strength, tensile strength, flexural strength, and toughness properties of CWF-HSC, respectively. A 62.71 MPa 28-day compressive strength was achieved in the control specimens, while a 0.37%, 0.88%, and 3.05% improvements in 0.25%, 0.50%, and 0,75% CWF was followed by a strength decline in 1%, 1.25%, and 1.50%, respectively. All CWF-HSC specimens achieved a higher early strength gain from 0.02 MPa/hr. to 0.13 MPa/Hr. in 0.25% and 1.5% CWF dosages, and a 0.08 MPa/Hr. gain rate at optimal 0.75% fiber dosage, compared to the control specimens. At all CWF dosages, an increase in tensile strength from 3.703 MPa up to 26.36% at 0.75% fiber content; while the flexural strength improved from 7.201 MPa in control specimens by 8.75%, 14.05%, and 16.44% in 0.25%, 0.50%, and 0.75% CWF content, followed by a slight decline up to 1.5% dosages. The peak and limit flexural toughness, I5 and I10, in specimens with CWF improved from 1 KJ/m2 to 1.73 KJ/m2 and 4.21 KJ/m2 at 0% and 0.75% CWF dosages, respectively. These experimental results confirm the feasibility of using sorted industrial cotton waste fibers in structural concrete within a DMDA framework for better fiber dispersion to enhance both strength development and flexural toughness properties.