WATER ABSORPTION CAPACITY OF RECYCLED DEMOLITION RUBBISH

von: Marcus Rühl
aus: Darmstadt Concrete 97, Ausgabe 12

SUMMARY

Aggregates have a huge influence on the freshly mixed and hardened concrete. Aggregates made of demolition rubbish are different in their capabilities compared with the capabilities of natural aggregates and aggregates made of concrete failure. Therefore it is not possible to compare theire influence when they shall be used for concrete aggregates, transferring knowledge from studies on concrete made of concrete failure.

1 Introduction

Researches, made at this Institute, shall give an answer to the question, if aggregates made of demolition rubbish can be used as aggregates for concrete. Therefor it is necessary to get information about the apparent specific gravity and its water absorption capacity. The information of the amount of water absorption from recycled aggregates are necessary to reach the right w/z-ratio in the planned concrete composition to get the wanted compressive strength. To create a concrete with a good workability, the water absorption values of the recycled aggregates must be known. Specially to the ready mixed concrete industry the water absorption problem is very important to solve, because the concrete consistency changes dramatically by using recycled demolition material for aggregates in concrete.

2 Ingridients of demolition material

Aggregates made of recycled demolition material consist a huge amount of bricks, concrete failure and finish (Figure 1).

These are components with a great porosity. This porosity is the reason, why aggregates made of demolition material have huge values of water absorption capacities. This high porosity is the reason, why a big amount of the concrete mixing water is sucked away when this material is used in concrete. This absorpt water has to be replaced by increasing the mixing water value to reach a good consistence of the freshly mixed concrete. The range of the adding water is a function of the used materials porosity and its dry density.

3 Density classes

To get an overview of these dry densities of recycled aggregates, three density classes where defined.

Figure 2 shows the category content expressed in percentage of the whole material that was tested. About 75% of the tested material has got a dry density bigger than 2.0 kg/dm³, that means, it is usable as aggregates for concrete and the use will lead to good compressive strenghts of the made concrete.

4 Water absorption performance

To clarify the water absorption performance, the tested material was dried to constant weight and sieved into the grain size ranges 4/8, 8/16 and 16/32 mm. After the dry density determination the dry weight of the material was measured and it was stored 10 minutes under water. After this 10 minutes the weight was measured again to calculate the value of water absorption after 10 minutes of water immersion. The same procedure was done to get the value for water absorption after 24 hours of water immersion. This value represents the value for water saturation.

In the figures 3 and 4 the relation between the grain size range, the dry volume density and the water absorption of the recycled demolition material is shown (10 minutes and 24 hours of water immersion).

These figures show, the higher the grain size, the lower the water absorption of the aggregates. Besides the water absrption sinks with increasing dry volume weight..

5 Results

Figure 5 shows the water absorption of the material 4/32 in general. It is interesting, that about 90% of the water saturation values are reached after 10 minutes of water immersion. Now it is possible, by knowing the dry volume weight of the recycled aggregates, to determinate the water which has to be added to the mixing water to reach the wanted consistency.

This diagram supplies necessary information for concrete-mixtures when recycled demolition rubbish shall be used as aggregates for concrete.