Freeze/thaw protection of concrete with optimum rubber crumb content
Introduction
According to Dovi et al. (2009), “Current and future developments in National and World economies are closely connected to sustainable efficient and safe usage of raw materials and upon energy based on cleaner production concepts and approaches that are ecologically and economically appropriate for the short and for the long term future of society”. Worldwide generation of waste tyres amounts to 5 million tones per year, representing 2% of total annual solid waste (Singh et al., 2009).
The UK Government is driving a sustainability culture (WRAP, 2011b) and any additive that creates a more durable concrete product of enhanced environmental credentials is worthy of investigation. Long et al. (2001:65) state that, “It has been estimated that the value of the infrastructure and built environment represents 50% of the national wealth within most European countries, because of the degree and rate of degradation of the built environment in Europe, it is of enormous economic and technical importance to provide a low maintenance environment”. This view is also taken by Mulheron (2001:1) who states that, “The need to improve our ability to both understand the mechanisms by which deterioration occurs, and the impact that methods of preventing deterioration have on subsequent material performance, is driven by the high cost of maintaining an ageing infrastructure”.
If rubber crumb is found to be effective in providing enhanced durability and is adopted as a concrete additive, the reduction in maintenance and remedial work to concrete may be significant. The size of the UK concrete repair sector is estimated to exceed 3% of the entire construction industry output (Waterman, 2006), which currently stands at £106 million GBP (Office of National Statistics, 2011). In addition, “with the world pouring around 5 billion tonnes of concrete a year – nearly one tonne per person per year – concrete is probably the most common material in modern construction” (Kernan, 2003). If this concrete can have low life cycle costs due to enhanced durability, this will have a lower environmental impact upon our world, with subsequent benefits of reduced carbon emissions and careful use of natural finite resources.
“The UK produces 487,000 tonnes of used tyres each year that have to be reused or disposed of…”(Environment Agency, 2010). The Landfill Directive has banned the land filling of whole used tyres since 2003 and shredded tyres since 2006 (Defra, 2009). At present, end of life tyres have varied uses such as; carpet underlay, and as tyre derived fuel (Singh et al., 2009) used in cement kilns (WRAP, 2011a and Strazza et al., 2011) or in asphalt (Milanez and Bührs, 2009) but this does not fully utilise the volume of tyre waste being produced. The economic benefits of utilising rubber crumb to provide a durable concrete provide a significant long term benefit to society, due to lower life cycle costs. Adhikara et al. (2000) suggest that, “Among various methods of disposal of scrap/waste rubber products, recycling or reclaiming of rubber is the most positive approach, because it not only saves our limited resource fossil feedstock but also maintains our environmental quality”. However the use of rubber crumb as an additive to coal causes air pollution that arises from the combustion process (Fang et al., 2001). Rubber crumb contains volatile components that need to be re-burnt to met the requirements of the Large Combustion Plant Directive (LCPD, 2001/80/EC) (Singh et al., 2009) and this requires a complex infrastructure to deal with the problems of corrosive elements and particulate control.Addressing the potential use of rubber crumb as a concrete additive, avoids the environmental concerns attributed to its disposal. Thus this paper addresses the potential of rubber crumb as an air entrainment agent.
Introducing an air entrainment agent into concrete is known to reduce the compressive strength. For every 1% of additional air entrained, the concrete strength will fall by typically 5–6% (Cement Admixtures Association, 2006) and this effect is also exhibited on the introduction of rubber crumb. Ganjian et al. (2009:1832) found that, on adding 5% by volume of powder rubber as a sand replacement, the compressive strength was reduced by approximately 5%. Savas et al., 1996, Benazzouk et al., 2006, Paine and Dhir, 2010 all carried out work on rubber crumb in concrete with regard to the freeze-thaw resistance of concrete. Each researcher showed a noticeable increase in the durability factor of the concrete samples containing rubber but no research has considered the optimum quantity of rubber crumb required to maximise the compressive strength and freeze/thaw protection of the concrete produced.
Freeze-thaw damage can occur in concrete at any stage, between pouring and achieving full cure. To provide freeze-thaw protection, current practices use air entraining admixtures which induce pockets of air into the concrete. These air pockets act as expansion chambers during the expansion and contraction of the concrete when subject to freeze-thaw cycles. This study aims to discover the possibility of achieving an optimum rubberised concrete mix that provides maximum freeze-thaw protection, whilst minimising the compressive strength loss.
Section snippets
Methodology
A variety of current freeze/thaw standards were adapted to carry out this research. The British Standard CEN/TR 15177:2006 defined the duration of the freeze-thaw cycles, while the American Society for Testing and Materials (ASTM C 666) informed the dry freeze and wet thaw procedure as defined within ASTM 666 as Procedure B. The calculations of the durability factor are common to both standards, in that it considers the percentage change from the original value to the final value.
The main
Optimum air content/compressive strength determination (pilot study 1)
A pilot test was required to determine the optimum percentage of rubber crumb addition that displayed the highest compressive strength and highest percentage of air entrainment. The pilot study was informed by the work of; Ganjian et al., 2009, Khatib and Bayomy, 1999, Topçu, 1995 and Biel and Lee (1994) who all suggest that 5% rubber addition (by volume) provides noticeable air entrainment while concrete strength is not affected.
Six batches of six concrete cubes were produced, five of these
Batching concrete
A slump test was used to determine consistency with batches A and B achieving a 50 mm true slump and batches C and D a 70 mm true slump. The results of this test indicate that the rubberised concrete achieved a reduced slump and the results are in keeping with the findings of Eldin and Senouci (1993), and Khatib and Bayomy (1999).
Compressive strength and density
The density of the batched concrete was determined and recorded as follows: washed rubber crumb concrete (A) 2129.6 kg m−3, unwashed rubber crumb (B) 2162.8 kg m−3,
Conclusion
The use of 0.6% rubber crumb by weight provided significant freeze-thaw protection in the concrete test specimens used for this study. The plain concrete samples failed before the completion of the freeze-thaw test programme whilst the rubberised samples had minimal surface scaling or internal damage. In addition to this, both rubberised concrete batches displayed a reduced overall density. This would further indicate the presence of internal air voids. Analysis of the rubber sample under a
Further work
The results show that a waste material can be incorporated into the supply chain and can support sustainable construction practices (Pelisser et al., 2011). The challenge as described by Kürzinger (2004) is to build in the capacity of profitable environmental management. Small to medium sized businesses (SME) would be a good starting point as multinational companies have a much greater inertia to change. Paine and Dhir (2010) reiterate the need for the construction industry to develop rubber
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