WRAP (The Waste & Resources Action Programme)

Aggregates Research: Engineering properties of concrete containing recycled aggregates

The objective of this project (AGG0079-002) was to identify whether the use of recycled concrete aggregate (RCA) affects the applicability of engineering property relations for concrete based on compressive strength given in the structural design Eurocode EN1992 [1] which is referred to here as EC2. It was also to determine whether the ultimate strength of concrete structural members containing RCA can be predicted through compressive strength, as defined in EC2.

This report provides the details of the test programme, background to the development of EC2 relations and the test variables used in the test programme. It includes results, interpretation and discussion of the following:

  • The 28 day tests on hardened concrete
  • The creep and drying shrinkage tests
  • The pull-out tests
  • The full-scale flexural and shear tests on beams.
  • Variability of compressive strength and its effects when recycled concrete aggregates are present in structural concrete.

The results of the tests gave rise to the following conclusions with respect to the range of concrete mixes used:

  • The EC2 relations on material properties, which usually express other properties via the mean compressive strength or characteristic compressive strength, can be used in the determination of other mechanical properties with the exception of secant modulus of elasticity.
  • The free shrinkage behaviour of RCA concretes was comparable to, and generally less than, the behaviour of corresponding Non-RCA concrete.
  • The creep strains of RCA concrete specimens were seen to be larger than that of Non-RCA concrete by a
    factor of about 2 and, thus, relatively poor.
  • Although the elastic modulus of concrete cylinders was found to be much lower than that of Non-RCA concrete, corresponding larger deflections were not measured in the structural beams where the reinforcement bars contribute significantly to stiffness.
  • The flexural performance of RCA-concrete beams was similar to that of Non-RCA concrete beams.
  • RCA-concrete beams had shear strengths similar to that of the Non-RCA beam. However, the failure modes for the two types of concrete were different, indicating a need to better understand the behaviour of RCA concrete beams in shear.
  • The steel reinforcement-concrete interface bond tests indicated that the bond strength of RCA concretes and the corresponding Non-RCA concrete are similar for the type of deformed bars used in this project.
  • The variability of the considered samples of RCA-concretes is sufficiently low to allow the use of the EC2 formula to determine the target mean strength for a given strength class and, knowing the mean strength from tests, to determine a conservative value of the characteristic strength.
  • Provided the previously mentioned matters related to stiffness, creep and shear strength are addressed and good quality control is maintained on site, currently there is no other reason to doubt the adequacy of structural safety and reliability of beams made of RCA concrete of the sort considered in this project.

Based on the above conclusions, it is recommended that designers are encouraged to use EC2 design formulae and material property relations for the design of RCA-concrete structural components from materials of the type considered here, but subject to the following conditions:

  • Types of RCA, mix proportions and strength classes to be within the range considered inn this project.
  • Calculated short term deformations to be increased by 15%.
  • Engineering properties of concrete containing recycled aggregates 2

Use is restricted to situations where creep deformation is not important or can be accounted for sufficiently. (This restriction can be taken away once suggested creep tests on beams are conducted.)  Care to be used when designing in shear critical situations. (This can be resolved once further tests are undertaken as suggested.)

Within the conclusions of the report, test and analysis programmes that can resolve the outstanding issues related to elastic modulus, creep behaviour and shear strength have been suggested.

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