WC Branch: Monthly Technical Meeting – September 2018
The September technical meeting of the branch was held at the University of Cape Town and featured two presentations. Branch Chairman, Mfundo Taliwe, welcomed everyone to the event and thanked visitors and students alike for their attendance. He introduced and thanked each speaker for their contribution to the Western Cape Branch of the Concrete Society.
The first presentation entitled, “AAR in Western Cape concrete: influence of reactive crushed sand together with reactive coarse aggregate”, was given jointly by Alice Titus Bakera and Prof Mark Alexander. Alice Bakera is a PhD student in Civil Engineering at the University of Cape Town and Mark Alexander is Emeritus Professor of Civil Engineering, and a Senior Research Scholar, at the University of Cape Town. He is also, of course, the current National President of the Concrete Society.
Greywacke crusher sand is now a common component of concrete in Cape Town. It has been used for decades, and is highly alkali-reactive, requiring mitigation measures such as use of cement extenders. With the incorporation of Greywacke crushed sand in concrete mixes, the question arose as to whether this was likely to exacerbate the ASR potential for local mixes.
This presentation considered the ASR performance of combined coarse and fine aggregate concretes.
Based on the principles of the Accelerated Mortar Bar Test (AMBT), a test was devised to represent ‘field concrete’, using micro-concrete. It was found that the concurrent use of reactive fine and coarse aggregate in a concrete mix impacts ASR expansion, with a limited effect observed when varying the amount of reactive fine aggregate in the sand blend. This raised questions regarding the effectiveness of previously prescribed amounts of cement extenders.
This was investigated, and it was found that 20% class F fly ash or 40% ground-granulated corex slag were sufficient in reducing the expansion to acceptable values. However, it is important that long-term performance tests also be carried out.
The second presentation was entitled “Behaviour of bi-component polymer fibres under creep and in aggressive environments regarding durability performance” and was given by Mario Manser, CEO of Contec Fiber AG from Switzerland. He completed his civil engineering degree at Brugg-Windisch (Switzerland) and his Executive MBA at University St. Gallen (Switzerland).
In underground applications spraying of concrete is a well-established and an economical alternative to conventional casting techniques. Further time and cost savings are achieved when fibre-reinforced concrete is applied. Steel fibres are mainly used at this time. Corrosion risk, damage of water impermeable films by the stiff steel fibres and a relatively high fibre rebound, are some of the drawbacks of steel fibres.
A suitable alternative may be found in polymer-based fibres. Such fibres were thought to incur other problems regarding the long-term application. Their performance under permanent load (creeping effect), and also their behaviour in aggressive environments, has been questioned.
A recently developed polyolefin based bi-component fibre was investigated regarding its long-term behaviour. Pre-cracked, fibre-reinforced square panels were exposed to permanent load and different aqueous solutions (sodium chloride, sodium sulphate, sulphuric acid). Long-term deformation and residual load capacity were determined.
In these laboratory tests, made by EMPA (Swiss federal laboratories for materials testing and research), it could be demonstrated that for this type of structured synthetic fibre, no longer are creep or the exposure to aggressive underground environments, limiting factors.