THE INFLUENCE OF RECYCLED AGGREGATE CORE MOISTURE ON FRESHLY MIXED AND HARDENED CONCRETE PROPERTIES

Andrew Nealen, Sven Schenk

SUMMARY
The following results show the influence of the porosity of aggregate from demolition material on freshly mixed and hardened concrete. Before manufacturing the concrete, the aggregate's core moisture is varied in 1,0 mass-% increments. The other components of the concrete mixture are constant. So the differences in the evaluation of the experiments are connected to the respective state of the porosity and absorptive capacity for water of the used aggregate. First the demolition material is tested isolated (water absorption), and then in combination with freshly mixed and hardened concrete in accordance with German codes and guidelines. For a more extensive analysis, the series of experiments contains concrete which is made of regular aggregate for comparison.

1 COMPOSITION OF THE DEMOLITION MATERIAL

The recycled demolition material that was used for the following experiments can be described as high-quality aggregate. About 97 % of the components of the demolition material consists of concrete, gravel, ceramic, brick and tue. lt is important to take the composition of the material into consideration, because the aggregate can be made up of many different materials, which depend on the recycling process and the structure of the wrecked buildings.
The porosity is the main reason, why aggregate made from demolition material has a larger water absorption than natural aggregate.

2 WATER ABSORPTION

Figure 1 describes the proportion of the water absorption after 10 minutes and 24 hours. The water absorption capacity after 10 minutes represents a good value for practical conditions in the production of ready-mixed concrete using aggregate made from demolition material. The recycled aggregate's water absorption of the fraction 2/8 mm and 8/16 mm averaged at 3,6 % by mass after 10 minutes and 4,5 % by mass after 24 hours.

The effect of the water absorption in connection with the workability of the produced concrete must be considered in a sufficient manner.

3 COMPOSITION OF THE CONCRETE MIX FOR THE TESTS

The composition is based on the following criterions:

• grading curve AB 16 (according to DIN 1045)
• strength dass ofconcrete: B 35 (equivalent to C30/37 according to EN 206)
• flow table value of the freshly mixed concrete always within measureable range (35- 60 cm)

4 CONSISTENCY DEGREE

The effect of the water absorption is obvious, when evaluating the development of rigidity between 4 and 90 minutes after initial mixing, as seen in Figure 2. The boundaries are the completely dry aggregate (core moisture 0 % by mass) and the completely water saturated aggregate (core moisture 4,5 % by mass). The other diagrams are naturally within these boundaries.

The boundaries can be described as nearly linear (max. core moisture), and logarithmic (core moisture 0 % by mass).
In the case of 0 mass-% core moisture, the development of rigidity is related to both the water absorption of the recycled aggregate and the stiffening of the cement paste. When using water saturated aggregate, only die rigidity of the cement paste is responsible for the reduction in flow table value, as die recycled aggregate absorbs no more water.
Figure 3 shows the development of flow table value of the reference concrete mix with natural aggregate in comparison to the concrete mix with water saturated recycled aggregate. The diagrams run parallel, but die natural aggregate mix shows a slightly higher flow table value than the recycled aggregate mix. This is mostly related to the rough surface of the recycled aggregate [1], and to the fact, that recycled aggregate tends to break when mixing. In bodi cases, this leads to a higher specific surface of the aggregate and therefore to a slightly lower flow table value.

5 COMPRESSIVE STRENGTH

A direct comparison in compressive strength between concrete made from natural aggregate and concrete made from recycled aggregate only makes sense, when die amount of free water in the cement paste is identical, therefore leading to a comparable mortar strength. This should be the case with the mixtures made with water saturated recycled aggregate. Test results of the compressive strength after 28 days show an average compressive strength of the concrete made from water saturated recycled aggregate of 41,13 N/mm² (see Fig. 4). This is a value, which is slightly above the average compressive strength of the reference cement mix with natural aggregate, which achieved 40,67 N/mm².
This comparison shows, that the use of a heterogenic recycled aggregate with it's hybrid bearing behaviour [2] does not lead to a reduction in compressive strength.

The loss of corn strength is mostly compensated for by the rough surface of the recycled aggregate and therefore improved interlock between aggregate and mortar, leading to a similar compressive strength, as when using natural aggregate.

Figure 5 shows the calculatory effective water/cement ratio, based on the amount of free water in the cement mortar and the achieved compressive strength.

6 CONCLUSIONS

The quality of the used recycled aggregate was high, due to the greater amount of concrete rubble in the composition with a high dry density. Practical application of concrete made from recycled building material of this quality involves no risk towards the achieveable compressive strength. A class B 35 concrete can be produced without problem.

The stronger development of rigidity and the lower initial consistency of concrete made with completely dry recycled aggregate must be considered in the production procedure. The best solution would be to keep the recycled aggregate in water saturated state at all times by wetting it at the concrete mixing plant, therefore reducing the loss of workability to a value known from concrete made of natural aggregate.

References