Experimental study of mixing parameters on graphene-oxide reinforced concrete compressive strength and its applicability to sub-Saharan Africa

This study is about improving the compressive strength of Graphene oxide-reinforced concrete systems (GORCS). It started with a pilot investigation that promoted Carbon-based nanomaterials reinforced concrete (Cb-NRC) in addressing sub-Saharan Africa (SSA) housing construction sustainability, followed by experimental lab testing of GORCS based on the outcome from the pilot study. Finally, a feedback assessments were conducted to review the experimental lab outcomes with a few of the stakeholders.

The pilot study was conducted through a survey questionnaire that involved 25 Housing construction Small and Medium Enterprises (Hc-SMEs) in SSA with outcomes that prioritise innovations using different formworks, reducing constructing time and building a good balance between compressive strength and cost together for adoption of Cb-NRC through GORCS.

While the compressive strength of ordinary concrete can be easily predicted using various mix designs and water-cement (w/c) ratios, the compressive strength of GORCS is a unique challenge. Its dependency on the mixing speed and the mixing duration, especially for the optimum weight percentages of Graphene Oxide (GO), has not yet been comprehensively investigated. The unpredictability of GORCS early age strength is a unique aspect in knowing ahead of its standard compressive strength to ensure the success of concrete projects that require using GO as an additive.

The experiments were conducted on 900 specimens to study the impact of mixing speed and duration on compressive strength. This lab testing is the largest conducted on this specific subject. It includes tests on the compressive strength of GORCS and ordinary concrete with 0.2 and 0.35 w/c ratios at 2, 7 and 28 curing days. 0.2 and 0.35w/c ratios are workable GORCS optimal ratios, which still fall in the range of 0.2 to 0.5w/c ratio, widely used in housing construction. The experimental tests were conducted using a range of mixing speeds between 800 and 3405 rotations per minute (rpm) and a mixing duration between 9 and 38 minutes. Each test was repeated three times according to the standard. The data analysis was done using the Pearson correlation coefficient (PCC) test to assess the relationship between variables, curve fittings to find the parameters from established mixing-compressive strength models applied and simple regression to verify linearity and validate the confidence level of the useful equations.

Unlike 140 rpm applied in 3 to 5 minutes, popularly recommended in ordinary concrete-making standards, three non-standardised mixing intensities generally offer between 45-84% improvement in compressive strength for both GORCS 0.2 and 0.35 w/c ratios while the best outcome at 104% improvement was from 1065rpm applied in 9 minutes for 0.2 w/c ratio at 61.03MPa. In general, both the mixing duration significantly influences the compressive strength of GORCS with 0.2 w/c while mixing speed influences the GORCS of higher w/c ratio of 0.35.

Out of the eight models tested, only three closely predict the experiment with calculated fitting parameters. The GORCS model mix with a 0.35 w/c ratio showed greater replicability than the mix with a 0.2 w/c ratio with a ± 2 to 5Mpa differences. Two of these applicable prediction models are ideal for HSMEs application in SSA due to the situation, mixing intensity need and delivery method if they produce 20-50% improvement in compressive strength.