EVALUATION OF MODULUS OF ELASTICITY OF CONCRETE WITH RECYCLED AGGREGATE

Peter Grübl, Marcus Rühl, Mark Bührer



SUMMARY
Within the scope of this study, different concrete mixtures were produced to determine the influence of aggregate derived from recycled mineral building material referring to the Modulus of Elasticity. These concrete mixtures consist of natural dense aggregate, aggregate derived from concrete demolition material and brick demolition material with different grain sizes and quantities. All concrete mixtures contained the same cement content, cement type and water-cement ratio.

 

1. INTRODUCTION

In order to determine the effect of recycled aggregate on Modulus of Elasticity of concrete, different concrete mixtures were produced. The test samples only differ in the type and amount of the aggregate. As reference, a concrete sample containing 100% natural dense aggregate was used. In different aggregate mixtures, the natural dense aggregate was partly replaced by recycled aggregate. The recycled aggregate derived from concrete demolition material (BB) and recycled bricks (ZI) possesed a grain size range of 4-8 mm and 8-16 mm. In all concrete samples the fraction 0-4 mm consisted of 100% natural dense sand. The experience shows, that in general the Modulus of Elasticity from concrete made of recycled aggregate is lower than the modulus of elasticity of concrete made with natural dense aggregate. There are a lot of models for calculating the Modulus of Elasticity for concrete made with natural dense aggregate. The task of this study is to investigate, if these relations are suitable for calculating the Modulus of Elasticity of concrete made with recycled aggregate.


2. PERFORMANCE OF TESTS

2.1 Composition of concrete

2.1.1 Aggregate

The marking of the samples is related to the material composition of the mixture and is listed in Table 1. Therein NZ means natural dense aggregate, BB means aggregate derived from recycled concrete and ZI means aggregate derived from recycled bricks.
Example: The test sample with 50 percent of natural dense aggregate in the grain size range of 4-16 mm replaced by 25 % of concrete demolition material and 25 % of recycled bricks is 50-25-25.

Table 1: Tested samples

2.1.2 Mix proportions

All concrete mixtures had the following constituents and mix proportions:
- Cement type: CEM 132,5 R
- Cement content: 320 kg/m3
- Water-cement ratio: 0,55
- Consistency range: KR
- Aggregate [0-4mm]: NZ
- Aggregate [4-16 mm]: NZ, BB, ZI
                       (Total amount of aggregate 0-16 mm = 705 dm3/m3)
- Particle-size distribution: AB 16

As the mixproportion of all investigated concretes are constant, a change in Modulus of Elasticity is only caused by the different aggregate types.

2.1.3 Specific data of the used aggregate


In the Tables 2 and 3 the measured dry volume density and the water absorption capacity of the recycled aggregate used are shown.

Table 2: Dry volume density of the aggregate used

The water absorption capacity of the recycled material cannot be neglected. Because of this effect, it has to be known for the dosage of aggregate and water (Table 3), as well as to evaluate the effective water-cement ratio.

Table 3: Water absorption capacity of the used aggregate

To calculate this value, the water soaked by the aggregate during ten minutes (mf10min), is given additionly to the needed water.


2.1.4 Storage of the test samples

After casting, the samples (cylinders, diameter = 15 cm, height = 30 cm) were led in the moules for 24 hours, stripped and cured in water until the age of seven days. Then, they were stored at 20 °C and 65 % rel. humidity for 21 days.

2.1.5 Testing the Modulus of Elasticity

The tests to measure the Modulus of Elasticity of concrete are carried out in accordance to the German Standard DIN 1048. In accordance to that German Standard, the measurement takes place at the samples age of 28 days. The Modulus of Elasticity can be calculated by the difference of the measured stresses and strains on an upper level (i.e. 1/3 of the value of compressive strength) and a lower level (i.e. 0.5 N/mm2). The Modulus of Elasticity then is given by the equiation

 

3. RESULTS

The results of the measurement are shown in Table 4.

Table 4: Measured Modulus of Elasticity.

In Figure 1 are given the measured Modulus of Elasticity.

Fig. 1: Amount of recycled aggregate versus Modulus of Elasticity

Figure 2 shows the test results of Modulus of Elasticity.

Fig. 2: Boundaries for Modulus of Elasticity given by German Standards and entering of the test results

 

4. EVALUATION BY MEANS OF EXISTING EQUATIONS

The proposed models to calculate the Modulus of Elasticity can be devided into two different classes of approach.
In general we can regard concrete as a two-phase material, one phase is the aggregate, the other phase is the cementstone and fine sand. Each phase can be characterised by its Modulus of Elasticity, its Poison-ratio and value. Most of the equations to calculate the Modulus of Elasticity of concrete describe the elastic deformations by means of springs with different spring constants. One group uses a serial arrangement of springs (type 1), the other group works with parallel elements (type 2). Most of the models use a combination of parallel and serial connected springs (type 3).
Every model needs material constants and calculable values to ascertain the modulus of elasticity. Necessary are the following constants, shown in Table 5.

Table 5: Used constants.


In Table 6 are listed the models as well as the measured values for selected aggregate com binations.

Table 6: Comparing of calculated and measured Modulus of Elasticity for concrete with recycled aggregate

 

Table 6 shows, that the models are mostly unable to calculate the Modulus of Elasticity of concrete with recycled aggregate. The lower the Modulus of Elasticity of the recycled aggregate used, the higher the deviation between the measured and the calculated value. The Modulus of Elasticity of the dense aggregate and the cementpaste was chosen to 40,000 N/mm2 and 20,000 N/mm2 respectively to get an efficient coincidence between measured and calculated values for concrete with dense aggregate.

 

5 Conclusion

Several models to describe the Modulus of Elasticity of concrete with dense aggregate were used to describe the Modulus of Elasticity of concrete made with recycled aggregate. The results show, the lower the Modulus of Elasticity of different types of aggregate is, the higher the difference between the measured and calculated Modulus of Elasticity of concrete with recycled aggregate is. There is a need to find an adequat procedure to describe the Modulus of Elasticity of concrete made of recycled aggregate.


References

[1]
Manns, W.: Über den Einfluß der elastischen Eigenschaften von Zementstein und Zuschlag auf die elastischen Eigenschaften von Mörtel und Beton; Dissertation RWTH Aachen, 1969.

[2]
Rühl, M.: Der Einfluß von Recyclingzuschlägen aus Bauschutt auf die Frisch- und Festbetoneigenschaften und die Bewertung hinsichtlich der Eignung für Baustellen und Transportbeton; Forschungsthema E/03, Bericht im Rahmen des Forschungsvorhabens Baustoffkreislauf im Massivbau - BiM, TU Darmstadt 1999.

[3]
Bührer, M.: Elastizitätsmodul von Beton - Theoretische Beschreibung und Anwendung auf Beton mit rezykliertem Zuschlag, TU Darmstadt Institut für Massivbau; Studienarbeit, Vt 090/98, Darmstadt 1998.