Bearing and deformation performance of concrete with recycled aggregates

Katharina Klemt
erschienen im Berichtsband zu "Darmstadt Concrete 97"

SUMMARY
Demolition material and crashed concrete can be preparated for their use as aggregates for concrete according to DIN 1045. The performance of hardened concrete is influenced by both matrix properties and aggregate properties. The inhomogenous composition of the combined aggregate, which can contain materials like concrete chippings, natural grains, brick chippings, plaster, glass and ceramics, requires due deliberations regarding bearing and deformation performance of concrete with recycled aggregates.

1. RECYCLED AGGREGATES

The technology of controlled demolition on the one hand and of preparating demolition materials on the other hand, allow production of secondary raw material with high quality, which can be used for the fabrication of concrete in accordance to DIN 1045.

The quality of the combined aggregate is highly depending on its composition. Recycled aggregates prepared from crashed concrete can be distinguished from recycled aggregates prepared from demolition materials.

The former results from demolishing pure concrete constructions and is characterized by its homogeneous composition. This combined aggregate is composed of natural aggregate grains (40 - 50 %), concrete chippings, i. e. natural aggregate grains with sticking rests of mortar (40 - 50 %). The amount of other materials, such as brick chippings, ceramics or glass is less than 10 %.

Preparated demolition material shows a very heterogeneous composition, consists of minerals like concrete chippings, ceramics, brick chippings, natural grains and mortar. The composition of demolition material shows great scatter depending on the specific construction work. This impedes the recycling of demolition material compared to the recycling of pure crashed concrete.

2 CONCRETE WITH RECYCLED AGGREGATES

2.1 Recycling of pure crashed concrete and recycling of demolition material

Recycling of pure crashed concrete is already established in projects in the field of road and street construction.

Test series are being carried out regarding the reutilization of demolition material as aggregates for concrete. It appeared that by taking care of some marginal conditions there is no objection to use prepared demolition material as aggregates for concrete in accordance to DIN 1045.

2.2 Qualities of fresh concrete

The amount of water for concrete production does not follow the regularity of normal concrete, while the recycled combined aggregate contains porous grains like brick chippings, mortar or concrete chippings which absorb water from the wet cement paste during the mixing operation. This problem is known from the technology of lightweight concrete. Solutions are elaborated, which deal with a water-absorptive activity as a function of the grains apparent specific gravity. The moisture content of the fresh concrete is being increased according to the absorbed water so that the desired consistency degree can be reached.

2.3 Mesostructure of concrete

Simplifying the mesostructure of normal concrete, it can be described as a system consisting of two phases: the matrix as continuous and the aggregate grains as disperse phase. In the following discussion the total content of mortar is treated as continuous phase.

In contrast to normal concrete where each phase separately is homogenous, concrete with recycled aggregates shows a heterogenous composition regarding the combined aggregate. Crashed concrete features a strength and an elastic modulus similar to the matrix properties. The interlocking between rough broken grain surface and matrix and possible hydration processes between grain surface and cement paste allow to regard concrete chippings embedded in the matrix as mortar. Thus assuming that properties of aggregate do not suffer under microcracks in the grains caused during the preparation process. The changes in the volume ratio is taken into account in the following discussion.

3 BEARING PERFORMANCE

The elastic modulus of a concrete is depending on the elastic moduli of its two phases matrix and aggregate. Discussing the influences of both phases to the elastic modulus regarding normal concrete and lightweight concrete allows to draw conclusions about the bearing performance of concrete with recycled aggregates.

Natural aggregates show a high elastic modulus in comparison with the matrix, so that the tensions are concentrated in the grains and transfered from grain to grain by mortar intermediate layers. Meanwhile the ratio of the elastic moduli of lightweight aggregates and matrix is just the opposite. The stress trajectories are formed around the grains.

With a decreasing content of mortar the elastic modulus is decreasing regarding concrete with a ratio E_A/E_M < 1.0 and it is increasing with a ratio E_A/E_M < 1.0. As far as concrete with recycled aggregates form pure crashed concrete is concerned, the volume ratio aggregate-matrix is changing in accordance to the content of concrete chippings in the combinded aggregate. Hence the elastic modulus of concrete is decreasing.

If the combined aggregate partly consists of grains with a lower elastic modulus than the matrix, the regularities of ligthtweigt concrete are valid. Researches indicate, that the elastic modulus is decreasing about 30 %, if concrete with recycled aggregate is compared with normal concrete showing the same compressive strength.

4 DEFORMATION PERFORMANCE

Hardened concrete made of prepared demolition material is distinguished from normal concrete by the deformation performance. Casting at the interactive behaviour of the phases matrix and aggregates shall illustrate the role of the latter for the deformation performance of the whole structure. In dependance on the qualities of matrix and combined aggregate five deformation states can be distinguished [1]:

• The matrix failure (I) is caused by exceeding the compressive strength of the matrix, while the strength of the aggregates is not fully used. The failure surface formed around the grains results from the relation of the two phases characterized by compared with the matrix high grain strength, but similar elastic moduli of matrix and aggregate (Fig. 1a)
• Cracking along the grain surfaces features the interlocking failure (II) (Fig. 1b). The bonding tensile strength in the transition zone between matrix and coarse aggregate is exceeded. The weakness of the transition zone is influenced by the surface structure of the aggregate grains.
• The failure surfaces of stability failure (III) and grain failure (IV) formate in the same way. Cracking begins in the matrix and concernes afterwards the grains, so that the failure surface is nearly plain (Fig. 1c). Grain failure occurs when the grain tensile strength is exceeded. if the whole system fails with reaching the maximum crack length it is stability failure. The possible crack length is defined as maximum distance between the grains, thus depending on the volume concentration of the aggregates and their distribution in the matrix.
• If the crack is caused in the grain by exceeding the grain tensile strength while the matrix does not show any cracks, the failure state is called limit compressive strength (IV), (Fig. 1c). This failure performance assumes compared with the aggregates a very high matrix strength. This case does not occur as long as cement based matrix are concerned. The crack figure can be observed on polymer concrete.
  

matrix failure interlocking Failure grain failure/ stability failure limit compressive strength Figure 1: Failure surfaces (schematic) a) Failure state I b) Failure state II c) Failure state III und IV d) Failure state V

Studying the failure surface of concrete with recycled aggregates, different failure mechanism are to identify. Figure 2 shows a schematic failure surface. The natural aggregate grain comports as known from normal concrete and fails by matrix failure. Aggregate grains with smooth surfaces like ceramics posses low strength transfer capabilities in the weak transition zone, hence interlocking failure is the reason for failure. Concrete chippings with strength similar to matrix strength and interlocking because of the rough broken grain surface are divided in two like stability failure or grain failure.

Figure 2: Failure surface of concrete with recycled aggregate, example.

The effects on concrete compressive strength can be simplified as follows. As far as gravel and natural grain is concerned, the grain strength is much higher than the matrix strength. Hence concrete compressive strength is equal to normal concrete. Concrete chippings embedded in the matrix can be seen as homogenous mortar system, because of similar strength qualities and good interlocking capabilities. Researches are going to point out the influence of the „old„ concrete quality on the „new„ concrete grade. Aggregate grains with low strength like plaster or brick chippings or with surfaces, which influence negatively the strength transfer matrix-aggregate, impairments of concrete strength have to be taken into account.

5. CONCLUSION

Determining bearing and deformation performances of concrete with recycled aggregates assumes the knowledge of several aggregate properties like grain tensile strength, elastic molulus and grain surface. The properties are to quantify in test series. Prepared crashed concrete features a quite constant composition, while the several component parts of prepared demolition material show great differences so that the determination of concrete performance is more complicate. Several test series varying each component part can lead to conclusions about properties of hardened concrete with aggregates of inhomgenous composition.

The substitution of natural aggregates by secondary raw material of high quality like prepared crashed concrete and prepared demolition material makes a valuable contribution regarding the circulation of building materials. On the one hand it is taken care of raw material resources, at the other hand waste and devaluation of material quality are avoided. Reutilizing demolition material on high quality in purpose to return the material into cycles has to become a matter of course of every construction project.

References