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transport). The transport rates for the stone and sand were calculated by taking the difference between delivered (to Pinetown station) and undelivered costs from the suppliers shown in Table 2-18 on page 133. Cement had a fixed delivery charge from the supplier.
The transport cost for the HDPE, BA and SCBF were based on the costing methodology by Carter & Troyano-Cuturi (2009) and was calculated taking the rate to hire a 10 m3 tipper truck or bakkie, depending on quantity of the materials required. The volume of each material was calculated based on the scenario quantities and this indicated how many loads would be required using the load capacity of the tipper or bakkie. The rates for hire were given in R/hour, excluded fuel cost (dry rates) and required a minimum of 9 hours hire. The fuel required was calculated by multiplying the quotient of the distance to the mill and recycling plants over 100, by the average fuel consumption (l/100km) of the 10m3 tipper or bakkie. The fuel cost was then calculated by multiplying the fuel required by the current fuel cost per litre.
Evaluation
The economic analysis was assessed in terms of the difference in cost for each of the scenarios using the selected waste concrete mixes versus conventional concrete.
Comments were firstly made on the difference in concrete material cost between the waste mixes and the conventional concrete.
The scenario analysis was then used to comment on the percentage that each aspect of the construction (material, labour, plant, transport) contributed to the total cost to construct the scenario. Transport costs were especially significant, considering the distances that needed to be travelled with regard to the waste materials.
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Assumptions that had to be made for the specific heat analysis were based on laws of thermal behaviour due to the lack of costly specialized machinery and software.
For greater accuracy, thermal measuring machinery such as heat flow meters and a Differential Scanning Calorimeter with specific heat and thermal conductivity software should be utilised, neither of which were available for this study.
It is understood that toughness is a significant property investigated with regards to fibre reinforcement, as fibres act as crack resistors generally increasing the energy absorption capacity (toughness) of the mix. However this study looked to compare waste modified mixes with a conventional concrete using the main properties generally associated with conventional concrete specification and not just fibre reinforced concrete, such as strength, durability, workability and cost. Toughness is recommended as a topic for future research in conjunction with research into crack behaviour.
The equipment used for SEM had a limit on sample size and hence imaging could only be done on small areas that were of interest as opposed to a large area reflecting all the concrete constituents.
NPC original black 42.5N was used based on the NPC products available to the university. It is known that this does contain extenders, however a blend with the least extender content from products available was selected and this was used as a control (no waste material added) to evaluate the effects of adding waste materials, which in essence is the aim of this research.
The study did not evaluate the change in W/C ratio and it was kept as a fixed variable. This was because the study focused on evaluating the change in properties due to waste material and not the effect of varying W/C ratios. The method of keeping W/C constant and investigating changes in slump, etc. was also used by Gul, et al. (2014). Tests related to varying the W/C ratio would be used more for developing a specification for the material or develop an empirical strength W/C relationship, which could be a topic for future research.
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The elastic modulus test was carried out to provide general additional information on the waste concrete mixes. It did not have a bearing on the primary aim of this study which was to access the viability of the waste mixes in terms of strength, durability, workability and cost. The minimum single sample was used for the elastic modulus test only, however because it was a non-destructive test the specimens for each mix where still tested under three loading and unloading cycles per test. This was confirmed as adequate by Contest Concrete Technology Services Durban and BS 1881: part 121:1983 (BSI, 1983). Four strain gauges were used as opposed to two, to factor in possible errors due to bonding or malfunctioning gauges. The approximate factor of 0.8 was used to convert from cube to cylinder strengths and hence is listed as a limitation to the study. This was however compared to a formula from BS 8110 (BSI, 1983), to evaluate the variance in load points
The water demand was not changed, as the moisture effects of adding the waste materials was assessed by investigating the change in concrete material properties, and any variation in water demand could be implied from these results. This methodology was used by Rao (2010). The effect of moisture absorption and water content was further evaluated by measuring the slump and compressive cube strength of samples using oven-dry waste (maximum absorption potential) and comparing the results to the results of samples using waste at natural moisture state.
In terms of sustainability, the embodied CO2 was not considered in this study. For it to provide meaningful results a full life cycle analysis would be required and this validates an entire research topic on its own.
An in-depth investigation of shrinkage, cracking and creep behaviour was not part of this study. Due to their complex nature, an investigation of these properties would constitute a separate research topic at MSc level, as carried out by Gaylard (2011).
Elastic modulus was however investigated in this study, and the relationship of elastic modulus to shrinkage and cracking was discussed where possible.
The durability tests on the SCBF mixes did not factor in the effect or rate of potential natural fibre deterioration which may affect long-term concrete durability.
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The cost analysis was just the production cost for the scenarios and not the total projects cost. The duration analysed was just the construction duration and not the duration up until handover which would include curing time. The cost analysis did not take into account the project life costs as this detail would constitute a separate study.