The following is a summary of a Joint Meeting of The Concrete Society And The Institution of Civil Engineers Held At The Engineering Centre, Glasgow on 3rd of February 2000

 

Would you use Recycled Concrete Aggregate?

 

Speaker :  John Lay, Technical Services Manager, RMC Aggregates (UK) Ltd.

This presentation gave an overview of opportunities and barriers for the use of recycled materials in construction.  Recycled material may include general construction and demolition waste or may be from a more controlled source where it consists entirely of crushed concrete or previously unbound aggregates.  The main benefit of recycling material is the reduction in materials sent to landfill sites, hence it is more sustainable.  In 1998, the total aggregate market in the UK was 255Mte, of which 13Mte was recycled construction and demolition waste (ca. 5%).  This accounted for about half of the construction and demolition waste generated, with the remainder deposited in landfill sites.  A third of the total aggregate used was for concrete but most of the construction and demolition waste was used in low grade applications such as general fill, hardcore and sub-base material.

Reasons for the predominance of low-grade applications include:

·         Processing requirements for use in concrete are more stringent and more costly, particularly on a small scale; therefore minimum processing results in fill material.

·         The source of construction and demolition waste is disparate and contractors may do minimum processing themselves on site and reuse it as fill material rather than transporting it off site.

·         Users and specifiers have concerns over long-term durability despite substantial testing.  Their main concern is about potential contaminants in the aggregate.

·         There are few recognised standards and specifications for concrete aggregate from recycled sources, although this is changing.

·         There is no demand for higher quality recycled aggregate therefore there is no supply and vice versa.

·         The economics of recycled construction and demolition waste is variable.  It is more economically viable in the south east of England where there is a high demand for aggregate but limits on its extraction from primary sources.  However, in Scotland, the plentiful supply of primary aggregate means that high quality recycled material is not economically viable.

 

The table below shows the composition and properties of a typical construction and demolition waste aggregate.  This particular aggregate is not suitable for use in concrete but it gives an indication of the type of materials which may be encountered.

 

Table 1. Typical composition and properties of recycled construction and demolition waste aggregate

Composition

 

Properties

 

Concrete

34%

10% fines

135kN

Brick

23%

Absorption

7%

Mortar & Plaster

10%

Saturated surface dry density

2.39

Gravel

23%

Bulk density

1.28

Miscellaneous

(breezeblock, 2%; slate, 1%; clay tiles, 2%; asphalt, 5%; other, 0.05%.)

10%

Grading

29%<5mm

 

The Quality Scheme for Ready Mixed Concrete (QSRMC) has developed regulations for the use of recycled material in concrete.  They have considered four types of recycled aggregate:

·         Recycled concrete

·         General construction and demolition waste

·         Reclaimed aggregate - cementitious component washed out from residual concrete in truck mixers

·         Unbound aggregate - aggregate which was previously used unbound

The last two are treated more like primary aggregate and are not greatly processed.  The first two are crushed and graded prior to use and should meet the specifications set out in Table 2 below. Pulverised Fuel Ash is generally used to replace some cement to mitigate the risk of alkali-silica reaction as recycled concrete aggregate must be considered as a high risk aggregate.

 

Table 2. QSRMC Specification for recycled aggregate.

Limiting criteria

Recycled Concrete

General Construction and Demolition Waste

Maximum concrete grade

50Mpa

20Mpa

Maximum fines content (<63mm)

5%

3%

Maximum brick content

5%

100%

Maximum light weight material

 (1000-2000kg/m3)

1%

5%

Maximum ultra-light weight material

 (<1000kg/m3)

0.5%

1%

Maximum asphalt content

5%

10%

Foreign material

1%

1%

 

A number of demonstration projects have been undertaken using recycled concrete aggregate:

·         BRE, Garston - 100% of coarse aggregate replaced foor grade C25 and C35 concrete.  The concrete reached the specified strength although it was lower strength than for a similar mix made with good quality primary aggregate.

·         BRE, Cardington Large Building test facility - 20% of course aggregate replaced with recycled concrete aggregate.  There was no noticeable difference in strength between this concrete and one made with primary aggregate.

·         Wessex Water, new headquarters in Bath - 40% of course aggregate replaced with recycled concrete aggregate with no strength reduction.  Local quarry waste was to be used but this proved too friable therefore recycled concrete aggregate produced from old railway sleepers was used.

To summarise, recycled concrete aggregate can be treated as a normal concrete aggregate and can be used in structural elements.  However, due to economics and quality control, it is likely to be used mostly in low grade unbound applications while primary aggregates will remain the preferred option for concrete.

The potential for use of recycled aggregates and glass cullet in asphalt was also mentioned.

 

                                                              Report by Chris Hoy