Use of Recycled Aggregates

Figure 1 - Recycling Plantat Tuen Mun Recycled aggregates (RA) are hard inert materials mostly originated from construction and demolition (C&D) wastes. In broad terms, the C&D wastes can be anything ranging from broken concrete and bricks from demolition sites, excavated materials from foundation work sites to broken up road surfaces arising from the road maintenance works. In Hong Kong construction activities generate about 14 million tonnes of C&D waste materials each year. In the past, these materials were treated as waste debris and they were dumped normally in the government landfill sites.

In recent years, in a drive for better environmental sustainability, better ways to manage C&D wastes were explored. The Hong Kong SAR Government had promulgated a number of measures, including the implementation of Waste Management Plans and Pay for Environmental Scheme in construction projects. These schemes have provided incentives to the construction industry to reduce the generation of C&D materials. A recycling plant was set up in mid-2002 to recycle the inert portions of C&D waste into useful aggregates. Currently, the RA produced by recycling of the C&D waste materials have already been used in different areas. For the slightly lower grade RA, they are used as road sub-base and drainage bedding layers. For the higher quality RA, they are used for asphalt production and in minor concrete structures.

A trial project had also been started in June 2002 to test out the long-term performance of paving blocks made with RA.

Figure 2 - Typical paving blocks

Figure 3 - Laying of paving blocks in trial project

With more confidence gained in the quality of the RA, the Housing Authority (HA) further looks into the possibility of extending the use of RA in structural concrete as a sustainable building material.

Figure 4 - Aggregate crusher C&D wastes can broadly be classified into soft materials and hard inert materials. In Hong Kong, approximately 59 percent of C&D wastes are soft materials, made up of soil, earth and slurry. These materials, if properly separated, are suitable for filling and reclamation works. The other 25 percent of the C&D waste materials are hard and inert materials. They are normally materials derived from rocks and broken concrete. These hard inert materials are suitable for reclamation work, and with proper sorting and tests, can be further used as aggregates in concrete production. The remaining 16 percent of the C&D wastes are actually debris. They can be metal, plastic, wood and so on. Some of these materials, if properly cleaned and carefully sorted, can be reused by the industry. Before the C&D waste materials can be recycled and reused in the 'new' works, they have to undergo sorting and processing. A common processing method is to crush the materials in recycling plants. In the process, the materials are broken into different sizes, sorted and selected for their target uses. The crushing process has also the added advantage of improving the general strength of the materials. This is because in the crushing process, the soft materials adhering to the materials will break off and only the 'hard' materials is retained in the batch. In this way, the materials have effectively undergone a screening process; hence, the quality of the remaining materials is to a certain extent assured.

 

Since the boom in construction industry in the 90's, there was always a consistently high demand for the basic construction material --- concrete, and hence aggregates. In fact, HA alone consumed annually over 1 million cubic meters of structural concrete and over 600,000 tonnes of aggregates. Such high consumption is definitely a good potential outlet for the use of RA. Though HA's flat production rate is expected to be adjusted downwards, HA probably will still be one of the largest consumer of concrete, and hence aggregates. HA therefore continued her proactive role in studying the possible use of RA in HA's building structures.

 

Before RAC can be generally used, a number of technical and non-technical issues have to be resolved. Technical issues include the need to ascertain the suitability of using RAC in building structures and further study to acquire sufficient knowledge of the of RAC's properties. Non-technical issues that need to be studied include the cost and liabilities arising from the use of RAC.

Local experience in the use of RAC is limited, particularly in building works Literature search carried out by HA indicated that published work on the application of RAC in overseas multi-storey building works was rare (ref. 1 - 28). Thus, a local RAC research was considered necessary to verify the properties of local RA and performance of RAC for use in building works.

The study conducted by HA focused on the effects of replacing 20% of coarse natural aggregates by RA on the various properties of the concrete. In the study, twenty concrete mixes of grades 35 MPa and 45 MPa were designed and produced by two concrete suppliers. The design parameters were also suitably chosen to match the parameters of normal concrete mixes currently used in HA projects.

RA used in the trial was supplied by the recycling plant at Tuen Mun. Over 2,300 samples have been produced and tested to correlate the quality, strength and durability of RAC with normal aggregate concrete (NAC). The testing regime is shown in the following table:-

Test		Test Age (Days)		Number
Petrographical Examination of RA		NA		17
Original Mortar Content		NA		3
Ten Percent Fines		NA		6
Compresive Cube Strength		7 28 90 105		240 720 300 120 (TOTAL:1380)
Slump Text		NA		240
Modulus of Elasticity		28		120
Bond Strength		28		108
Drying Shrinkage		56		120
Chloride Permeability		56		120
Water Permeability		94		120

 Test findings

1.

Quality of RA :- As RA at the recycling plant was produced from construction wastes from different sources, samples of RA were randomly taken over a period of four months to study the consistency of RA. In terms of strength measured by '10% fine' tests, the variation was relatively little. However, the volume of original mortar varied significantly among samples. From observations on sections, the area of old mortar could be as much as 80%.

Figure 5 - Microscopic inspection
Figure 6a - Samples showing interface of NA & RA	Figure 6b - Samples showing interface of NA & RA

	2.	Mechanical properties of RAC :- The series of tests did not show any significant difference in the strength, strength development, workability, Young's modulus and bond strength between concrete made with 20% RA and without RA.
	3.	Durability :- Shrinkage of RAC was found up to 7% higher than that of control NAC whilst the chloride permeability of RAC was found up to 10% higher than its counterpart. However, from the variation of results between different mixes, it can be considered that with appropriate adjustment in the concrete mix design, these seemingly adverse effects could be minimized.

Figure 7 - Sample preparation for chloride test	Figure 8 - Sample preparation for shrinkage test

Summary of Findings
In summary, the performance of RAC, at a 20% replacement of natural aggregate, are largely similar to those of NAC. However, there is slightly higher shrinkage and chloride permeability, up to 7% and 10% respectively.

Technical engineering issues and the need for building up of local experience are not the only factors which affect the general use of RAC in building construction. Considerations for acceptance of RAC are different for end-users, contractors, engineers and developers. Some of these factors are discussed below.  
 
Public perception
In the eyes of the lay public, RA was derived from C&D waste and they are of inferior quality. The use of RAC could hardly be a selling point to home buyers who have no confidence on the product. A dilemma exists here in that insufficient confidence deters the public from accepting RAC whereas on the other hand, confidence could only be built up from proven part records of using RAC.

HA is a unique developer in that HA is not building flats for sale. With the study and possibly future pilot trial, HA will be able to provide hard data to demonstrate that the performance of RAC can be engineered to be comparable to ordinary concrete. From there, performance record can be built up gradually to combat the negative perception of RAC.

Costs
From the government's perspective, the cost of setting up and operating facilities to produce RA as a means to disposing C&D wastes compares favorably with the cost for providing and maintaining the traditional landfill facilities for disposing the wastes. Take for example, the three strategic landfills1 in Hong Kong, occupying a total of 270 hectares of land, require approximately HK$6 billion to construct and HK$400 million each year to operate. Furthermore, the disposal is free of charge. The situation will be worsen in the near future as these strategic landfills will be filled up in 10 to 15 years and no new suitable sites can be found at present. Without these outlets, the cost for the government to absorb the C&D wastes is very expensive and will definitely propel the government to look for alternate outlet for C&D waste disposal, including the use of RA and RAC.

To a developer or construction contractor, the envisaged higher cost for RAC and current unsteady supply of RA supply deter the use of RAC. Despite the fact that supply of RA is free of charge, additional costs are incurred for the storage, transportation, processing and quality control.

The situation, however, will be improved in near future. Levy of the 'landfill charging' scheme for handling C&D materials is being effected by the SAR Government. With the implementation of the scheme, it will provide incentive for developers and construction contractors to reduce C&D waste generation and to carry out sorting to facilitate reuse and recycling. With better cost incentive, C&D waste can be reduced and RAC usage can gain popularity in the long run.

---

1 They are located at Nim Wan, Tseung Kwan O and Ta Kwu Ling. 
 
Liabilities
Another issue, which is closely related to cost in the using of RA and RAC, is liability. The contractual liability to use normal aggregates from a particular quarry clearly lies with the contractor. This division of responsibility, however, becomes blurred in the use of RA since the government is currently the sole supplier of RA. Under such circumstance, contractors and concrete suppliers consider that they have exposed to higher risk than normal because they cannot select the source and control the quality of RA and performance of RAC.

However, the problems may be resolved when RA production is fully commercialized. By then, RA and RAC will become one of the construction materials available in market that can be chosen by contractors at competitive prices and liabilities would fully rest with the contractors.

The Hong Kong SAR Government has promulgated a number of measures to better manage the C&D wastes with a view to maintaining the environmental sustainability and promoting the use of RA and RAC is one of these measures.

HA, as one of the largest developer in Hong Kong, had conducted a study on the properties and performance of RAC for the use in building construction. The study concluded that RAC could generally be used in building works as a sustainable construction material.

Besides technical consideration, there are other considerations affecting the general acceptance of its wider use, which include public confidence, costs and liability.

Whilst the HA study forms a good basis to promote the wider use of RA in the construction works, the following issues may have to be addressed to ensure good advancement in its use :-

	1.	Exploring possibility of further trial on use of RAC in HA projects in order to provide good opportunities to obtain longer term performance data on RAC;
	2.	Exploring ways to encouraging the development of RA and RAC market to bring in more choices and competition so as to enhance the supply; and
	3.	Enhancing traceability on sources of RA so as to provide quality RA with known history.

	1.	Ahmad S.H., Fisher D. and Sackett K., Mechanical Properties of Concretes with North Carolina Recycled Aggregate, Integrated Design & Environmental Issues in Concrete Technology, 1996, pp 251-261
	2.	Building Research Establishment, Recycled Aggregates, BRE Digest 433, 1998
	3.	Frondistou-Yannas S., Waste Concrete as Aggregate for New Concrete, ACI Journal, August 1977, pp 373-376
	4.	Fujii T., Strength and Drying Shrinkage Behavior of Concrete Using Concrete Crushed Aggregate, Proc. of 2nd Int. RILEM Symposium, 1988, pp 660-669
	5.	Gomez-Soberon J.M.V., Porosity of Recycled Concrete with Substitution of Recycled Concrete Aggregate An Experimental Study, Cement and Concrete Research 32, 2002, pp 1301-1311
	6.	Hansen T.C. (ed.), Recycling of Demolished Concrete and Masonry, RILEM Report 6, 1998
	7.	Hansen T.C. and Narud H., Strength of Recycled Concrete Made from Crushed Concrete Coarse Aggregate, ACI 5, January 1983, pp 79-83
	8.	Hong Kong SAR Government, Buildings Department, Use of Recycled Aggregates in Concrete, Practice Notes for Authorised Person and Registered Structural Engineer No. 275, 2003
	9.	Hong Kong Housing Authority, Particular Specification for Designed Mix Concrete with Recycled Coarse Aggregates, 2003
	10.	How J.C., Yen T and Kuan H.C., Use of Building Rubbles as Recycled Aggregates, Cement and Concrete Research 33, 2003, pp 125-132
	11.	Ikeda T., Yamane S. and Sakamoto A., Strengths of Concrete Containing Recycled Concrete Aggregate, Proc. of 2nd Int. RILEM Symposium, 1988, pp 585-594
	12.	Institution of Structural Engineers, Using Recycled Aggregates: Wessex Water New Operation Centre, Bath, The Structural Engineer Journal, Volume79 / No.12, 2001
	13.	Kaga H., Kasai Y., Takeda K. and Kemi T., Properties of Recycled Aggregate from Concrete, Proc. of 2nd Int. RILEM Symposium, 1988, pp 690-698
	14.	Kasai Y., Guidelines and the Present State of the Reuse of Demolished Concrete in Japan, Proc. of 3rd Int. RILEM Symposium, 1994, pp 93-104
	15.	Kasai Y., Hisaka M. and Yanagi K., Durability of Concrete Using Recycled Coarse Aggregate, Proc. of 2nd Int. RILEM Symposium, 1988
	16.	Kikuchi M., Mukai T. and Koizumi H., Properties of Concrete Products Containing Recycled Aggregate, Proc. of 2nd Int. RILEM Symposium, 1988, pp 595-604
	17.	Kim K.W., Lee B.H., Park J.S. and Doh Y.S., Performance of Crushed Waste Concrete an Aggregate in Structural Concrete, Utilization of Waste Materials in Civil Engineering Construction, pp 332-343
	18.	Kobayashi S. and Kawano H., Properties and Usage of Recycled Aggregate Concrete, Proc. of 2nd Int. RILEM Symposium, 1988, pp 547-556
	19.	Morel A., Gallias J.L., Bauchard M., Mana F. and Rousseau E., Practical Guidelines for the Use of Recycled Aggregate in Concrete in France and Spain, Proc. of 3rd Int. RILEM Symposium, 1994, pp 71-81
	20.	Mulheron M. and O'Mahony M., The Durability of Recycled Aggregates and Recycled Aggregate Concrete, Proc. of 2nd Int. RILEM Symposium, 1988, pp 633-642
	21.	Nishibayash S. and Yamura K., Mechanical Properties and Durability of Concrete from Recycled Coarse Aggregate Prepared by Crushing Concrete, Proc. of 2nd Int. RILEM Symposium, 1988, pp 652-659
	22.	Olorunsogo F.T. and Padayachee N., Performance of Recycled Aggregate Concrete Monitored by Durability Indexes, Cement and Concrete Research 32, 2002, pp 179-185
	23.	Ryu, J.S., An Experimental Study on the Effect of Recycled Aggregate on Concrete Properties, Magazine of Concrete Research, 2002, 54, No. 1, February 2002, pp 7-12
	24.	Sagoe-Crentsil K.K., Brown T. and Taylor A.H., Performance of Concrete Made with Commercially Produced Coarse Recycled Concrete Aggregate, Cement and Concrete Research 31, 2001, pp 707-712
	25.	Topcu I.B., Physical and Mechanical Properties of Concretes Produced with Waste Concrete, Cement and Concrete Research 27, No. 12, 1997, pp 1817-1823
	26.	Yagishita F., Sano M. and Yamada M., Behaviour of Reinforced Concrete Beams Containing Recycled Coarse Aggregate, Proc. of 3rd Int. RILEM Symposium, 1994, pp 331-342
	27.	Yanagi K., Hisaka M. and Kasai Y., Physical Properties of Recycled Concrete Using Recycled Coarse Aggregate Made of Concrete with Finishing Materials, Proc. of 3rd Int. RILEM Symposium, 1994, pp 379-390
	28.	Zaharieva R., Buyle-Bodin F., Skoczylas F. and Wirquin E., Assessment of the Surface Permeation Properties of Recycled Aggregate Concrete, Cement and Concrete Composites 25, 2003, pp 223-232

 

2011© All rights reserved. No part of this paper may be reproduced, distributed, published, or transmitted without the prior permission of the copyright owner.