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Recycled Aggregates in New Concrete ABSTRACT Concrete is the world’s second most consumed material after water, and its widespread use is the basis for urban development. It is estimated that 25 billion tonnes of concrete are manufactured each year. Twice as much concrete is used in construction around the world when compared to the total of all other building materials combined. Recycling or recovering concrete materials has two main advantages it conserves the use of natural aggregate and the associated environmental costs of exploitation and transportation, and it preserves the use of landfill for materials which cannot be recycled. Whilst crushed concrete can be used as a sub-base material for pavements and civil engineering projects2, this Best Practice Guide outlines its use as a higher grade resource - as aggregate in new concrete. However, recycled concrete aggregate that is significantly contaminated may not be economical to decontaminate for use as concrete aggregate, whereas it may be suitable ‘as is’ for use as sub-base material. Green building schemes such as Green Star New Zealand3 recognise C&DW reuse and provide credits for the use of recycled materials including recycled concrete aggregate. Because waste minimisation and reducing the burden on landfills is a global issue, extensive research has been carried out worldwide on the use of recycled aggregate in concrete. This includes substantial research reports by WRAP4, NRMCA5 and RILEM6. Globally the concrete construction industry has taken a responsible attitude to ensuring its natural resources are not overexploited. In some cases the preservation of dwindling natural aggregate sources is a significant issue driving the use of recycled aggregates. Reduction in the impact of aggregate cartage on cost and environmental issues is also a factor where material processed from the demolition phase of a project, using a portable aggregate processing plant, can be reused in concrete for the construction phase of the project. This is a better option than transporting natural aggregates from quarries which through urbanisation are located at an ever increasing distance from city areas. In some cases in Auckland for instance, quarried aggregate is transported over 100 km from source to construction project. Introduction The key to local materials recovery and the recycling industry sector is to achieve a balance between economic pressures and ecologically sound practices. This balance is critical not only to ensure a sustainable future for the industry, but also to secure essential quality improvements and development of markets for value-added products, which are required to make recycled materials more attractive and economicalThe need to develop and adopt performance requirements specifically for secondary and recycled products will not only promote secondary and recycled materials specification, but will also ensure that externally verified quality certification and compliance systems covering both materials and recycling plants are adopted. This will inevitably create further market opportunities for using recycled materials as aggregate in concrete, which constitutes a step forward in providing contractors and clients with confidence when specifying recycled products. Nevertheless primary materials will continue to meet the bulk of the demand for construction materials. The Use of Recycled Aggregate in Concrete The use of crushed aggregate from either demolition concrete or from hardened leftover concrete can be regarded as an alternative coarse aggregate, typically blended with natural coarse aggregate for use in new concrete. The use of 100% recycled coarse aggregate in concrete, unless carefully managed and controlled, is likely to have a negative influence on most concrete properties – compressive strength, modulus of elasticity, shrinkage and creep, particularly for higher strength concrete. Also the use of fine recycled aggregate below 2 mm is uncommon in recycled aggregate concrete because of the high water demand of the fine material smaller than 150 µm, which lowers the strength and increases the concrete shrinkage significantly. Many overseas guidelines or specifications limit the percentage replacement of natural aggregate by recycled aggregate. In general leftover concrete aggregate can be used at higher replacement rates than demolition concrete aggregate. With leftover concrete aggregate, information will generally be known about the parent concrete – strength range and aggregate source etc. for demolition concrete very little information may be known about the parent concrete, and the resulting aggregate may be contaminated with chlorides or sulphates and contain small quantities of brick, masonry or timber which may adversely affect the recycled aggregate concrete. Often the sources of material from which a recycled aggregate came (and there could be more than one source), are unknown and the variability and strength of the recycled aggregate concrete could be adversely affected in comparison with a recycled aggregate concrete where the recycled aggregate came from one source with a known history of use and known strength. Logistics of Recycled Aggregate Production The location of recycled aggregate production will normally be dictated by the availability of a suitable aggregate crushing plant in the vicinity of the concrete plant making recycled aggregate. For use of leftover concrete aggregate, the ideal situation is where the concrete plant and aggregate plant are on the same site thus minimising the cartage of the leftover concrete to the crushing plant. It may be feasible to return leftover concrete from other plants nearby for processing; however the additional cartage distance of the returning trucks needs to be a consideration. The proximity of a source of demolition concrete to a crushing plant and onto the concrete plant is also a consideration Characteristics of Recycled Aggregates Physical Properties The variability in the physical properties of recycled aggregate will depend on the variability of the parent concrete. Recycled concrete aggregate which has been sourced from a number of demolition concretes will have greater variability than recycled concrete aggregate from one demolition concrete source, and this is likely to have an effect on the uniformity of the recycled aggregate concrete. The crushing resistance of recycled concrete aggregate, i.e. the ten percent fines value, when tested in accordance with NZS 3111:198629, will generally be lower than the value of the parent aggregate. The amount of adhered mortar present when dislodged under test load will influence the test result. However, recycled aggregate will still generally pass internationally industry accepted requirements. Chemical Properties Establishing chemical properties of recycled concrete aggregate is important because the history of the demolition concrete is unlikely to be known. For leftover concrete, because the properties of the parent concrete correspond to the properties of natural aggregate processed by a particular ready mixed concrete plant, there is less uncertainty about the chlorides, sulphates and alkali present than for recycled aggregate concrete. Therefore contaminants are not the issue for leftover concrete aggregate that they are for recycled concrete aggregate. Alkalis and Alkali Reactivity When the use of recycled aggregate in concrete was first introduced, the potential for alkali leaching from recycled aggregate concrete was considered to be an issue. However comparative tests with standard concrete have shown that there is no additional leaching from recycled aggregate concrete when compared to concrete using natural aggregates. (a) The alkali contributed from the adhered mortar. (b) Expansion of the existing ASR gel in the recycled concrete aggregate when exposed to higher levels of moisture in the recycled aggregate concrete. (c) Fresh exposed faces of the original aggregate broken during crushing of the parent concrete. (d) Swelling of the adhered mortar on the recycled concrete aggregate is believed to compound the ASR expansion in the recycled aggregate concrete. Manufacture of Recycled Aggregate Concrete Concrete Mix Proportions Mix proportioning of recycled aggregate concrete will depend on the percentage replacement of coarse aggregate by recycled aggregate. Up to 20% replacement level, substitution of coarse aggregate with adjustment only for the specific gravity of the recycled aggregate may be all that is necessary. At higher aggregate replacement levels, adjustments may need to be made to the aggregate proportions to account for the grading, shape and texture of the recycled aggregate. Admixture quantities may also need adjustment. At higher levels of coarse aggregate replacement, the water demand of the recycled aggregate concrete will increase for a given workability and either a small increase in cement content may be necessary, or alternatively the use of water reducing or superplasticising admixtures to maintain target strength requirements. The increase in water demand will be dependent on the recycled aggregate source and properties. The compressive strength of the parent concrete will also influence the strength potential of the recycled aggregate concrete, particularly at higher target strengths. Crushed recycled fines are not often used in recycled aggregate concrete. However one investigation into uses for recycled fines has found it viable to use 10% to 30% of recycled fines as a percentage of the total fine aggregate38. Use of fine recycled concrete aggregate generally results in comparatively higher strength gain beyond 28 days owing to ongoing cementing action of the fines. Recycled Aggregate Concrete Production Variability In general, coefficients of variation for recycled aggregate concrete compressive strength up to 100% replacement level can be similar to those for natural aggregate concrete. If demolition aggregates are drawn from a variety of sources, however, then the coefficient of variation will increase. ACI 555R-0148 gives the variation in compressive strength of 12 recycled aggregate concretes with 100% coarse aggregate replacement based on a single mix design but using recycled aggregate crushed from various 15 year old demolition concrete sources with strengths ranging from 39 to 85 MPa. The target water/cement ratio of the recycled aggregate concrete was 0.57 and the resulting compressive strength ranged from 30 to 49 MPa (mean 39.7 MPa). Model Specification for Recycled Aggregate and Recycled Aggregate Concrete This section provides suggested model specification clauses for the supply and use of recycled aggregates and recycled aggregate concrete in New Zealand. It is designed to be used in combination with NZS 3121:198615, NZS 3104:20038 and NZS 3109:199718 in the absence of appropriate clauses in these standards to accommodate recycled materials. However the specification does not carry any regulatory status. Feedback from users of the specification will enable it to be refined and eventually developed into revisions of or amendments to the relevant New Zealand standards. The model specification aims to maintain the quality of concrete currently provided by the industry. The combination of the incentives provided by Green Star and the low risk approach of the model specification should encourage the uptake of the technology.