Investigation on Mechanical Strength of Concrete Using Steel Bottle Caps as Fiber

Metals used as cap for containers preserve liquids in the bottles, but the dumping of caps particularly soft drink bottle caps are harmful for the environment. On the other hand, concrete has low tensile strength and to offset this problem some fiber like material can be added to concrete to increase its tensile strength. Hence, an effort was made in the present investigation to study the influence of addition of waste materials like soft drink bottle caps at a dosage of 0.5%, 1% and 1.5%, of total weight of concrete as fiber. In this study, caps were cut into strips of size of 3 mm width and 10 mm length. Experimental investigation was carried out adding bottle caps in concrete and tests were carried out as per recommended procedures by relevant codes. The experimental values such as compressive strength and split tensile strength were found to be increased. The most improved compressive strength and split tensile strength were found for 1% bottle cap fiber. The strengths were increased by maximum 22.98% and 23.14% respectively.


Introduction
Concrete being a brittle material which has low tensile strength and low strain capacity, as a result, the mechanical behaviour of concrete is critically influenced by crack propagation (Behera et al. 2015). Introduction of fibers in discrete form in plain or reinforced concrete may provide a better solution (Romualdi et al. 1963). Experimental studies proved that fibers improve the mechanical properties of concrete such as flexural strength, compressive strength, tensile strength, creep behavior, impact resistance and toughness (Shukla, 2011 This study intends to study the impact of steel bottle cap fibers of 0.5%, 1% and 1.5% with a constant size of 3 mm x 10 mm on improvement of concrete compressive and split tensile strength. The bottle caps that were used in this study to prepare fibers were collected from local shop in Khulshi, Chittagong, Bangladesh as waste materials (Fig. 1).

Materials
Ordinary Portland cement was used for casting of concrete. Specific gravity of cement was tested. Locally available sand collected from Sylhet, Bangladesh was used as fine aggregate (FA). Crushed stones of 19 mm 2 Investigation on Mechanical Strength of Concrete Using Steel Bottle Caps as Fiber downgrade were used as coarse aggregates (CA). All the tests conducted on the materials are mentioned in Table 1 with the results.

Mix Design
From the results of materials tests, ACI mix design was conducted targeting 3000 psi compressive strength and the ratio among cement (C), FA and CA and water-cement ratio (W/C) were found as given in Table 2. The slump value was found as 28 mm.

Casting and Curing
The fibers were mixed with concrete as shown in Fig. 2. Total 12 cubes and 12 cylinders were casted (Fig. 3) for compressive strength and split tensile strength tests respectively. The amounts of steel bottle cap fibers were maintained as 0.5%, 1% and 1.5% of weight of concrete. The sizes of fibers were also maintained as 3 mm width and 10 mm length.

Compressive and Split Tensile Strength Test
The cubes were tested for compressive strength (ASTM C39-C39M-17) and the cylinders were tested for split tensile strength (ASTM C496-C496M-11) with respective ages of curing as 7, 14 and 28 days. Universal Testing Machine (UTM) of capacity of 1000 KN was used for both compressive strength and split tensile strength tests as shown in Fig. 4 and Fig. 5.

Result
The results of the tests on concrete samples are given below.
• For 0.5% bottle cap fiber, the compressive strength was improved by 5.4% to 21.8% and the split tensile strength was improved by 0.25% to 9.83% compared to plain concrete in all curing periods. • For 1% bottle cap fiber, the improvement was in between 12.25% to 22.98% for compressive strength and 6.62% to 23.14% for split tensile strength. • Also for 1.5% bottle cap fiber, the improvement was reduced than 1% fiber and the improvement was in between 4.56% to 7.18% for compressive strength and 0.35% to 6.45% for split tensile strength compared to plain concrete in all curing periods.

Discussion
From Fig. 6 and 7, it can be seen that, both the compressive and split tensile strength were improved in all curing periods for all percentages of steel fibers. The improvement was gradually increasing from 0.5% fibrous concrete to 1% fibrous concrete, but again the improvement on 1.5% fibrous concrete was reduced compared to 1% fibrous concrete. Overall, the most improved compressive and split tensile strength were found for 1% fibrous concrete.

Conclusions
A simple experimental study was conducted on concrete with addition of steel bottle cap fibers. The strengths were improved by adding the fibers. But surplus mixing of steel fibers reduced the strength. It happened due to over mixing of fiber with respect to other materials in concrete. So, steel fibers if used in adequate quantity can improve the mechanical strength of concrete.