Construction, demolition and excavation waste (CDEW) makes up more than half of the national total waste in most countries of the world. For this reason, pressures for more sustainable development have led to a significant amount of research and development focusing on the development of technologies for recycling these wastes, primarily back into construction as recycled aggregates.

The concrete industry consumes approximately 40% of the total worldwide construction aggregate production. However, at present its use of recycled aggregates is marginal, with possibly as few as 3% of all aggregates used being from recycled sources. One of the main reasons for this is a misconception
that they are inferior aggregates. This article addresses these misconceptions and shows that appropriated selected, recycled aggregates can be used
across the whole spectrum of concrete activities. The paper draws on the experiences of the author through research carried out at the Universities of
Dundee and Bath, UK.

Recycling of CDEW can take place either at the site from which the material is sourced using mobile crushers, or as is increasingly the case in Europe, the
material may be transported to a central recycling facility (in the UK known as washing plants) where large stockpiles may be accumulated. Recycled aggregates from these central recycling facilities undergo a number of processes to ensure higher quality. This may include: magnets, picking stations
(although these are increasingly rare), trash screens, screens, log washers, water pumps and sludge tanks.

In contrast, recycled aggregates produced on-site using mobile crushers are rarely sophisticated enough to remove all impurities. Most users of recycled aggregates generally distinguish between:

• Recycled aggregates containing mainly crushed concrete, which they refer to as recycled concrete aggregate (RCA) and
• Recycled aggregate containing significant proportions of other waste materials.

For example, BS 8500-2, which defines the constituent materials that may be used in concrete in the UK, defines RCA as material predominately composed of concrete (at least 83.5% by mass) with no more than 5% masonry. Any recycled aggregate not meeting this classification is known as RA.

However, because of the low proportion of masonry permitted, it has proven difficult for most European producers to produce RCA and consequently since 2008, EN 12620, the European standard for aggregates has included a new classification for recycled aggregates in an attempt to promote the use of material containing less crushed concrete. This is based on the proportions of unbound stone (Ru), crushed concrete (Rc), crushed masonry (Rb), bituminous material (Ra), glass (Rg), floating material (FL) and other constituents (X) that are present. Table 1 shows the typical composition of RA from ten washing plants in the UK, demonstrating the large proportion of unbound natural aggregate present in these materials.

Knowledge

There has been considerable research carried out on the use of recycled aggregates in concrete over the past 20 years, and this has grown considerably over the past five years as industry and Government have recognised the need for greater sustainability in construction.

Research has shown that coarse recycled aggregates can be used in concrete up to a compressive strength of 80 N/mm; although there is a loss in strength when recycled aggregates are used as a direct replacement of natural aggregate at the same water/cement (w/c) ratio. However, most researchers report that a certain proportion of coarse recycled aggregates (usually in the range 20-30% by mass of coarse aggregate) can be added to natural aggregate without affecting performance. The reason for the loss in strength is usually associated with: (i) the weaker interfacial transition zone between aggregate and mortar, due to recycled aggregates having a coat of weak mortar already attached which raises the porosity of the concrete and (ii) the inclusion of weak and porous aggregate particles, for example low strength bricks, glass and plaster. In general, the flexural strength and modulus of elasticity of recycled ggregate concrete have been reported to be proportional to the loss in compressive strength [2].

Whilst research has shown that fine RCA could be used in very low quantities it tends to cause difficulties with the stability of the mix and the strength of the resulting concrete [3]. For these reasons the British standard for constituents of concrete (BS 8500-2), does not permit use of fine RCA in concrete. However, given that fine RA contains significantly lower quantities of 'cement dust', its use as fine aggregate is of considerably greater potential.

Recycled aggregate concrete tends to have higher levels of drying shrinkage than natural aggregate concrete for the same strength. This is mainly because
recycled aggregates provide less restraint to movement (lower elastic modulus), and because of the higher cement contents that are often used in these
concretes to achieve the required strength. However, up about 20% by mass of coarse aggregate, the shrinkage of recycled and natural aggregate concrete
can be assumed to be comparable [4].

Since recycled aggregate contains particles with higher porosity and higher water absorption properties (e.g. old mortar, brick, masonry) than most natural
aggregates, recycled aggregate concrete tends to be less resistant to most permeation-based exposure conditions (e.g. carbonation and chloride ingress) than equivalent natural aggregate concrete at the same w/c ratio. However,there is some evidence that when these properties are compared in concretes of the same strength (made with the same cements), performance differences between recycled aggregate and natural aggregate concrete are minimal (Table 2). All the same, the most practical way to ensure resistance of recycled aggregate concrete to permeation-based deterioration is to use an appropriate blended cement to reduce the porosity of the paste and to bind aggressive agents [5].

Research to identify whether recycled aggregate concrete is more susceptible to damaging alkali-silica reactions (ASR) has established that the use of recycled aggregates is likely to be low risk and that there is no correlation between the alkali release content of the recycled aggregates and ASR expansion.

Therefore in general terms, research suggests that equivalent performance between recycled aggregate and natural aggregate concrete can be obtained
provided the concretes are designed to achieve the same compressive strength. One very efficient method for carrying this out is to reduce the w/c ratio in relation to the amount of recycled aggregate used as a replacement of natural aggregate: recognising that there is no need to reduce w/c ratios when the recycled aggregate content is below a certain limit (e.g. 30% by mass of coarse aggregate). An example is given in Figure 1 which shows the w/c ratio for a natural aggregate and the adjustment necessary to the w/c ratio for various RCA contents to achieve a given strength.

Based on the mix design described above, a number of demonstrations have been carried out to establish case studies suitable for long-term monitoring
of in-situ performance of concrete designed to achieve appropriate design strengths. An example is shown in Figure 2, where a RCA concrete pavement was constructed using material crushed on-site to produce new concrete of strength 35 N/mm2 and designed to withstand an aggressive freeze/ thaw  environment [7]. The pavement consisted of a 150mm deep RCA concrete laid on top of a 300mm deep recycled aggregate sub-base. In addition, igmentation and imprinting were required since the client requested that the finished surface resemble an existing road from the first phase of development where red coloured block, paving had been used. Specimens taken from selective mixes for investigation in the laboratories have shown excellent performance.

Practice

Worldwide there are currently few standards and specifications in place for use of recycled aggregates, and where these aggregates are allowed it is usually
standard practice to subject them to the same rules as natural aggregates. This tends to be an obstacle to their greater use in high value applications like concrete.

In Europe, the standard for aggregates for concrete, EN 12620:2008 permits the use of recycled aggregates, and has a list of categories of recycled aggregate based on composition. However, there is no data relating these categories of recycled aggregate to their performance of concrete. As a result they have little impact in increasing recycled aggregate use, and in practice the performance of concrete is not necessarily dependant on the composition of the recycled aggregate, but the quality of the individual components.

Therefore, a more logical approach is to perhaps relate limits on allowable recycled aggregate to performance-related characteristics (as is partially
done in Japan). As a result recycled aggregate quality could be related to concrete performance across the whole range of potential RA quality, independent of composition and source. The overall concept being that good quality recycled aggregate (conforming to,say, class A) would be suitable for high erformance
and severe exposure applications, meeting the relevant standards and specifications, whilst lower classes will be more appropriate for lower performance
applications (Figure 3).

A major research project to investigate the effect of the individual characteristics of recycled aggregate on the performance of concrete, has shown that this
methodology is possible [4]. The research has produced general guidance supporting the wider use of recycled aggregates in concrete as well as grouping
aggregate particle composition into three classes of recycled aggregate suitable for different applications:

• Class A - recycled aggregates for use in a wide range of concrete including marine environments;
• Class B - covering most combinations of natural and recycled aggregate and suitable for most 'moderate' exposure conditions; and
• Class C - those aggregates suitable for only the 'mildest' exposure conditions. Consequently, the technical detail required to increase the use of recycled aggregate in concrete, and  give industry greater confidence and informing future revisions to the concrete standard is now available.

Concluding Remarks

This article has shown how research and practice is making a positive contribution towards greater use of recycled aggregates in concrete. However, there are Figure 3. Performance-related approach to use of recycled aggregates in concrete still numerous barriers to overcome before recycled aggregates become a common concrete constituent. Even so, it is important to ensure that the manner in which these aggregates are used is the most sustainable.
For example, it is not sufficient to claim that a process is sustainable simply on the grounds that it is recycling, as recycling in itself involves transport and
processing. Therefore, in achieving sustainability it is important that users are aware of, and have quantified, the impact of a particular approach, and that
they are making the most effective and appropriate use of sustainable materials [8]. No-one is advocating the use of recycled aggregates in prestige concrete, high-strength concrete, or in concrete subject to the most aggressive environments. However, these concretes probably account for less than 10% of the world's annual output of 5 billion m3 of concrete.

Acknowledgements

Professor Ravindra Dhir (Applying Concrete Knowledge Ltd.) is thanked for discussions leading to the preparation of this article.

References

• Dhir, R K, Paine, K A, Halliday, J E. Facilitating the wider use of coarse and fine recycled aggregates from washing plants. WRAP Report. AGG 105-003. 2008, 45pp.
• Paine, K A, Collery, D C, Dhir R K. Strength and deformation characteristics of concrete containing coarse recycled and manufactured aggregates. 11th International Conference on Non-conventional Materials and Technologies (NOCMAT 2009), September 2009, Bath, UK
• Dhir, R K, Limbachiya, M C, Leelawat, T. Suitability of recycled concrete aggregate for use in BS5328 Designated mixes. Proceedings of the Institution of Civil Engineers: Structures and Buildings, 134, (3), 1999, pp 257-274.
• Dhir, R K, Paine, K A. Performance related approach to use of recycled aggregates. WRAP Final Report. Project Report. Waste and Resources Action Programme. 2007.
• Ann, K Y, Moon, H Y, Kim, Y B, Ryou, J. Durability of recycled aggregate concrete using pozzolanic materials. Waste Management, 28 (2008), pp993-999
• Paine, K A, Dhir, R K, Caliskan, S. Use of recycled concrete aggregate in concrete. Technology Application Document 3. Final Report to Department of Trade and Industry, UK. 2005, 33pp
• Dhir, R K, Paine, K A, O'Leary, S. Use of recycled concrete aggregate in concrete pavement construction: A case study. Proceedings of the Conference on Sustainable Waste Management,Thomas Telford Publishing, London, 2003, pp.
• Dhir, R K, Dyer, T D, Paine, K A. Appropriate use of sustainable construction materials. Concrete, 40 (9), 2006, pp. 20-23

Dr Kevin A Paine,Senior Lecturer,BRE Centre for Innovative Construction Materials,University of Bath, UK