The effectiveness of a percussion drill method for making concrete cube samples to assess the characteristics of precast zero-slump concrete

Du Plessis, Pieter

The effectiveness of a percussion drill method for making concrete cube samples to assess the characteristics of precast zero-slump concrete

Thesis / Dissertation

2023

Abstract

The precast industry in South Africa consumes about 28% of the total cement produced, and a large proportion of its concrete is prepared with zero-slump concrete mixes. Precast finished products are load tested to confirm compliance, but the zero-slump concrete is often not tested by cube or cylinder samples, as is the case with ready-mix concrete. There is an industry practice of using the percussive action of a rotary percussion drill to compact control samples when necessary. However, variation in the compaction of the specimens results in variation in the density and compressive strength results. Therefore, there is a need for a simple, standardised quality control method that considers the compaction achieved by precast machines. A suitable procedure was pursued with the following specific objectives; i) to formulate a practical and economical procedure for quality control based on the concrete density achieved by precast machines, and ii) to apply this method (i.e., percussion drill method) to mix optimisation with the specific objective of partial replacement of the cement in a specific factory mix with a suitable fine filler in order to reduce cost and the carbon footprint of a specific precast facility. The study methodology involved a literature review on available quality control methods, and a laboratory investigation that combined the percussion drill method with a target density method. The laboratory investigation produced representative samples of an industry mix based on density tests on core samples of the same mix produced by different precast machines. The method was then used to test different proportions of cement replacements by volume with a fine filler. The purpose of the partial cement replacement by fine filler mixes was to assess the effectiveness of using the percussion drill method in mix optimisation. The experimental investigation involved determining the density of concrete produced by three different precast machines using the same industry mix. The density was measured for samples in different moisture conditions, i.e., the as-received state, oven-dried state, saturated state and in an environmentally controlled room. Based on the results, it was recommended to use the as-received density for quality control and mix optimisation at precast production facilities when measuring target density using the percussion drill method. The specific factory mix used in producing the precast elements was employed in the laboratory to produce cube samples using the percussion drill method. The specific factory mix had a water-to-binder ratio of 0.25, a very stiff and dry mix. Therefore, the percussive action of a percussion drill was used for compaction to achieve a target density. The aim was to achieve a compressive strength of 30 MPa at 18 hours and 50 MPa at 28 days. The compressive strength achieved was 1.9 MPa at green state, 40.7 MPa at 18 hours, and 66.5 MPa at 28 days, higher than the target strengths. The green state strength was required to assess the ability of a mix to retain its shape without formwork after extrusion. Thus, the specific factory mix provided a benchmark value. In achieving the second objective, quartz flour was selected as a fine filler and replaced cement in the concrete at replacement rates of 20%, 30%, and 40% by volume of cement. This selection was based on its lower cost and carbon footprint compared to Portland cement and the fact that the particle size and texture are similar to Portland cement. CEM I 42.5 R was used in this objective instead of CEM II / A-L 42.5 N used in the first objective due to its potential to provide sufficient early age strength. The results indicate that the 20% replacement rate outperforms the specific factory mix and other replacement rates in strength at all three critical time intervals. This was either due to the quartz filling effect, which improved the concrete compaction, or a higher early hydration rate of the CEM I 42.5 R than the CEM II / A-L 42.5 N, or a combination of two. The lower strength performance of 30% and 40% replacement rate was related to the fact that quartz flour is inert, which reduces compressive strength with further cement reduction. However, the green state, 18-hour, and 28 days compressive strengths of the 40% replacement mix were still above the minimum requirements. Therefore, the 40% replacement mix was appropriate for industry application. However, it must be noted that these results were obtained in a controlled laboratory environment, which is not replicated at the specific precast facility. Therefore, the recommended trial replacement volume of cement by quartz flour is 33% for an industry trial to anticipate less ideal curing conditions at the production facility, resulting in a net CO2 emission reduction of about 113.1 kg /m3 of compacted concrete. Generally, the results indicate that the percussion drill method can effectively provide adequate quality control measures, and mix optimisation if compaction achieves a predetermined density.

Keywords

Civil Engineering

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