Self-compacting concrete (SCC) is a relatively new product that sees the addition of superplasticiser and often a VMA stabiliser (viscosity modifying admixture) to a concrete to significantly increase the ease and rate of flow. John Eustace, Product Manager for Sika Concrete reports
Self-compacting concrete is becoming a more widely used product by the UK construction industry. Specialist admixtures play an integral part in providing a robust concrete. The formulation of modern admixtures allows a range of properties to be imparted into the same mix. Good mix design practice is still of course paramount, taking into account the properties of the locally available aggregates and cementitious materials.
By its very nature, SCC does not require vibration. It achieves compaction and placement into every part of the mould or formwork simply by means of its own weight without any segregation of the coarse aggregate. SCC has been produced successfully with both normal-weight and lightweight aggregates.
Developed in Japan and continental Europe, SCC is now being increasingly used in the UK where, apart from health and safety benefits, it offers faster construction times, increased consistence and ease of flow around heavy, congested reinforcement. With no need for vibrating equipment, workers are spared from exposure to vibration and construction sites are quieter.
The fluidity of SCC ensures a high level of consistence and durability, while the rapid rate of placement provides an enhanced surface finish. SCC is certainly the way forward for both in-situ and precast concrete construction.
Concrete admixtures are a fundamental part of the mix design and the properties of SCC that are now required are imparted by the most modern PCE materials (polycarboxylate ethers).
PCE superplasticisers are nowadays widely used in concrete admixtures to improve the consistence during placing and the durability after hardening of concrete. In addition to this, PCEs can be used to reduce the environmental impact of concrete, mainly by enabling mix designs to be optimised, reducing amounts of cement and/or increased use of secondary cementitious materials like GGBS, fly ash or limestone, although this is by no means a primary consideration in the production of SCC.
PCEs can be described as polymers similar to a comb in shape. By varying the polymer structure, the properties of the resulting superplasticiser can be influenced. The main parameters that are thereby varied are the length and nature of the polycarboxylic acid backbone and the length and number of side chains used. These essential properties can be adjusted for each application.
The major characteristic of polycarboxylate ether-based superplasticiser technology is its targeted polymer design to achieve specific concrete properties.
Characteristics that can be influenced are:
- Adsorption speed
- Water reduction with high initial liquefaction/consistence
- Slump retention without retardation and subsequent fast strength development