Aus dem Berichtsband zu Darmstadt Concrete 1998, Seite 131-141

THE INFLUENCE OF RECYCLED AGGREGATE ON THE COMPRESSIVE STRENGTH AND THE ELASTIC MODULUS OF CONCRETE

Andreas Bretschneider, Marcus Rühl

SUMMARY

Within the scope of this study, 21 different concrete mixtures were manufactured to determine the influence of aggregate derived from recycled mineral building material referring to elastic modulus and compression strength of concrete. These concrete mixtures consist of natura! dense aggregate, aggregate derived from concrete demolition material and brick demolition material with different grainsizes and varying quantities. All concrete mixtures contained the same cement-type, cement quality and water-cement-ratio.

1 INTRODUCTION

In order to determine the effect of recycled aggregate on the elastic modulus and compressive strength of concrete, 21 different concrete mixtures were manufactured during this study. The test samples only differ in the type and amount of the aggregate. As reference, a concrete sample containing 100% natural dense aggregates was manufactured. In subsequent samples, the natural aggregate were gradually changed by recycled aggregate. The recycled aggregate derived from concrete demolition material (BB) and recycled clay (ZI) possess a grainsize of 4-8 mm and 8-16 mm. In all concrete samples the fraction 0-4 mm consisted of 100% natural dense aggregate.
Further on, all concrete samples were led through compressive strength tests and elastic modulus-measurements. In addition to the measurements described in "DIN 1048 Teil 5" at the age of 28 days, 3 more measurement-dates were taken during the first week. Therefore it was possible, to figure out the time-dependent development of the elastic modulus and the compressive strength.

The compressive measurements of the samples at each measurement-date were just single rneasurements and not the average of three measurements. With the use of regression analysis including the 4 measurements it is possible, to calculate a fairly accurate estimation of the actual compresive strength.

2 DENOTATION OF THE TEST SAMPLES

The denotation of the samples derives from the material composition of the mixture. The samples are named with three 2 digit numbers seperated by a dash ( - ). The first number stands for NZ:= Volume percentage of natural dense material, the second stands for BB:= Volume percentage of aggregate made of concrete demolition rubbish and die third number stands for ZI:= Volume percentage of aggregate made of recycled clay.

Example: The designation of a test sample with 25 percent of natural dense aggragate in die grain size range of 4-16mm replaced by concrete demolition material is 75-25-00.

List of test samples used in this paper:

3. PROPERTIES OF TUE USED AGGREGATE

For the concrete mixture composition it is nessesary, to determine the speciflc gravity and the water absorption capacity during 10 minutes.

3.1 Specific gravity

Table 1 shows the specivic dry volume density of the used recycled aggregate in relation to the grain size.

Table 1: Dry volume density of the used aggregate

3.2 Water absorption during 10 minutes

Table 2 shows the measured water absorption capacity of the used recycled concrete aggregate. The water absorption capacity of this material cannot be neglected. Because of this effect, the core moisture of the aggregate has to be known for the dosage of aggregate and water. Therefore an "effective water-cement-ratio" is defined. For calculating, the ten minute water absorption is appropriated, because during this time the water absorption value reaches up to 90 % of the 24 hour water absorption value. If the core water content has approximately the magnitude of the 24 hours value of water absorption, no withdrawal of mixing water by the aggregate will take place during the handling of the concrete

Table 2: Water absorption canacitv of the used aggregate

4 CONCRETE COMPOSITION

For all manufactured concrete mixtures the following prerequisites had to be followed:

With die above shown prerequisites it can be supposed, that changes in the hardened concrete's properties only can cause by variation of the aggregate used in die mixture. The amount of aggregate needed for one cubic meter of concrete was determined with 705,77 dm³/m³ for all contrete mixtures. Presuming this value, die exact amount of each kind of aggregate can be calculated.

5 STORAGE

All the samples were poured into a form for 24 hours and then removed and placed in a water tank at 20^°C for storage. The test cubes remained in the water and were only removed when needed for testing the compressive strenght at the given time. The cylinders remained in the water and were only taken out to measure the elastic modulus. After the measurement they were stored under water again.

6 ELASTIC MODULUS

The test procedures to measure the elastic modulus of concrete are defined in "DIN 1048, Teil 5". Following this procedures, stress is applied in varying amounts with given upper and lower limits. The lower value is given with 0,5 N/mm². The upper limit depends on the age of the sample and has to be lower than 1/3 of die expected compressive strength.

Table 3: Measured elastic moduli

7 COMPRESSIVE STRENGTH

The test procedures to measure the compressive strength of concrete are defined in "DIN 1048, Teil 5". Table 4 shows the compressive strenght depending on the age of the test sample.

Table 4: Measured compressive strengths

8 DEVELOPMENT OF COMPRESSIVE STRENGTH AND ELASTIC MODULUS

The following figures show the time dependend development of the compressive strength and elastic modulus. Exemplarily only the courses of the test samples (100-00-00), (00-100-00) and (00-00-100) are shown.

9 THE INFLUENCE OF RECYCLED AGGREGATE ON TUE COMPRESSIVE STRENGTH AND TUE ELASTIC MODULUS OF CONCRETE

In the following two figures (4 and 5), die changes of compressive strength and elastic modulus in relation to the amount of substituted natural aggregate are shown. The measurement was taken 28 days after concrete production. The values, measured on concrete totally made of natural aggregate, are defined as 100%.

In figure 6, BB is replaced by ZI step by step. In this figure, the values measured an concrete made with aggregate derived from recycled concrete rubbish are defined as 100%.

10 RESULTS

The gained results show, that the elastic-modulus of the test samples decrease with rising amount of recycled-demolition-material. Aggregate derived from recycled clay, has the biggest influence. The elastic-modulus of concrete made of 100% recycled concrete rubble, is 12% lower than the elastic-modulus of the concrete made of 100% natural dense aggregate. By replacing 100% of the natural aggregate to recycled clay, the elastic modulus drops abaut 35%. When modifing particular size ranges the changes of measurements were lower.
Evaluating the compressive strength of test samples with amounts of concrete-demolition material, no definite changes to the sample with 100% natural dense aggregate is measured. According to the aggregate ratio, the measured compressive strengths of these samples scatter around the compressive strength value of concrete with 100% of natural dense aggregate. With the use of recycled clay, the compressive strength shows obvious changes. In comparison to the elastic-modulus, the addition of recycled clay causes a decreasing of compressive strength. This rising of compressive-strength can be explained by the fact, that the waterabsorption of recycled clay is higher than the absorption of natural dense aggregate and recycled concrete demolition rubble. Due to this fact, the clay ties parts of the mixing water in its capillaries. This leads to a decreasing of the water cement-ratio and therefore to an increasing of the compressive strength.

11 CONCLUSION

By using recycled concrete and recycled clay for concrete aggregate, a loss of elastic modulus has to be accepted. That means, that in constructions in which deformations have to be considered, the smaller elastic modulus, resulting from the use of recycled aggregate, has to be evaluated by tests. The study shows, that there is no decrease in the compressive strength, when aggregate derived from recycled concrete or clay is used. Building components made of concrete with recycled aggregate, can be designed with the same characteristic values as components of concrete made with natural aggregate.

Referenees