University of West of Scotland and Robeslee Concrete

Self-compacting concrete for use in an existing precast concrete plant – a feasibility study

Executive summary

Concrete is the most widely consumed material in the world after water.  Existing technology requires slow, heavy, noisy, expensive, energy-consuming and often dangerous mechanical vibration to compact freshly placed conventional concrete in order to achieve the required strength and durability. Vibration is often difficult to apply homogeneously and both over- and under-vibration can result in defects in the concrete elements, which may have to be repaired or replaced, causing delays and increases in costs.  Modern technology - Self-compacting concrete (SCC) is now available after many years of R&D. With high flowability, passing ability and resistance to segregation, SCC flows freely into the pre-formed moulds/formwork and achieves full compaction under its own weight, thus eliminates the need for mechanical vibration. This saves time, reduces labour needs, cuts energy demands and extends the life of moulds - and particularly removes the health/safety risks associated with vibration (e.g. deafness, “white fingers”, and noise-led stress).  It is widely recognised that SCC can revolutionise concrete construction.  To date, however, though steadily increasing, the use of SCC has been very limited in the UK, mainly for high profile projects and in large precast concrete plants where the appropriate expertise and resources are available. The full benefits of SCC are far from being exploited.  This is mainly because for SCC the materials cost is often higher and its production/quality control more technically demanding, compared to conventional concrete of equivalent properties.  Furthermore, different applications often require specific mix properties.

In December 2007, a KTTBE award was received by the School of Engineering and Science at the University of the West of Scotland in partnership with Robeslee Concrete Co, a Scottish based SME.  This award was to study the potential and feasibility of producing practical SCC mixes using local materials, and particularly to achieve improved surface quality for concrete elements produced by the SME.  This study was carried out in different stages, including:

• A small desk study to review the general development and main characteristics of SCC, particularly its application in precast concrete plants.
• A lab-based experimental study to develop practical SCC mixes of suitable fresh properties and surface quality required using local materials available.
• Semi-production scale trials at the plant to test the feasibility and potential of producing and using SCC in practice and to identify possible problems and areas where further improvements are required.

The study concluded that:

• SCC mixes with a reasonable increase in materials costs (15 – 20%) could be produced using existing materials and suitable selection of a superplasticiser and a viscosity modifying admixture. 
• With good quality control/monitoring measures provided, and a modification made to the mixing batch size (i.e. increase from 0.25 m3 to 0.5 m3 per batch), SCCs with satisfactory fresh properties were successfully produced and used in the existing concrete plant.  It has also proved that concrete elements produced using the SCC mixes could achieve excellent surface quality and sufficient strength required for the current production cycle.
• The use of SCC on the production line can eliminate the vibrating compaction work and reduce the associated noise greatly.  The benefits of SCC for health and safety have been clearly demonstrated and greatly appreciated by the workers.
• It is estimated that by using SCC, about 90 minutes per day could be gained in the production process as that is the amount of time spent vibrating the moulds.  Furthermore, since the steel moulds are no longer subjected to heavy vibration these moulds would last longer and require less maintenance.
• Producing and using SCC mixes demand much higher level of technical support for quality control and monitoring than that generally required for using conventional concrete as small variations of the materials and admixtures could have a significant influence on the fresh properties and surface quality for the SCC mixes.  Further technical training of workers and additional equipment for producing and monitoring/testing SCC is clearly required at the existing precast concrete plant.
• To take the full advantages of SCC’s fast speed of placing, it is essential that the production capacity of the plant, placing equipment and capability, organisation arrangements of the transportation and production process, as well as the operators at the plant are all considered and optimised.  To achieve this goal, further work is clearly necessary for the development of an efficient manufacturing process centred on SCC.