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 watercement
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 48 mm and 816 mm. In all concrete samples the fraction 04 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 416 mm replaced by 25 % of concrete demolition material
and 25 % of recycled bricks is 502525.
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/m^{3}
 Watercement ratio: 0,55
 Consistency range: KR
 Aggregate [04mm]: NZ
 Aggregate [416 mm]: NZ, BB, ZI
(Total
amount of aggregate 016 mm = 705 dm^{3}/m^{3})
 Particlesize 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.
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 watercement ratio.
Table
3: Water absorption capacity of the used aggregate
To calculate this value, the water soaked by the aggregate during ten minutes (m_{f10min}), is given additionly to the needed water.
2.1.4 Storage of the test samples
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/mm^{2}). 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 twophase 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 Poisonratio 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/mm^{2 }and 20,000 N/mm^{2 }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.
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