THE INFLUENCE OF RECYCLED AGGREGATE ON THE STRESS-STRAIN RELATION OF CONCRETE

Marcus Rühl, Gordon Atkinson


SUMMARY
Within the scope of this study, different concrete mixtures were manufactured to determine the influence of aggregate derived from recycled mineral building material referring to stress-strain relation of concrete. These concrete mixtures consist of natural 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 stress-strain relation of concrete, different concrete mixtures were manufactured during this study. The test samples only differ in the type and amount of 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 stress-strain tests. The measurements were realized as described in the "Test Methods for Technological Analyses", resigned in the research project "Recycling of Mineral Building Materials" (Baustoffkreislauf im Massivbau, BiM). To get the full stress-strain curve, the deformation of the test-samples was held constant to e' = 0,30 %o min-1

 

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 in the grain size range 4-16 mm, the second stands for BB:= Volume percentage of aggregate made of concrete demolition rubbish in the grain size range 4-16 mm and the third number stands for ZI:= Volume percentage of aggregate made of recycled clay. In all concrete mixtures' natural dense aggregate was used in the grain size range 0-4 mm.

Example: The designation of a test sample with 50 percent of natural dense aggragate in the grain size range of 4-16mm replaced by 50 percent of recycled concrete aggregate is: 50-50-00.

Table 1: List of test samples used in this paper:

 

3 PROPERTIES OF THE USED AGGREGATE

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


3.1 Speeific gravity

Table 2 shows the specivic dry volume density of the used recycled aggregate in relation to the grain size.

Table 2: 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 the water absorption capacity, 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 ab sorption, no withdrawal of mixing water by the aggregate will take place during the mixing and handling of the concrete.

Table 3: Water absorption capacity of the used aggregate

 

4 CONCRETE COMPOSITION

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

- Cement type: CEM I 32,5 R
- Cement content: 320 kg/m3

- Water-Cement-Ratio: 0,55
- Consistency range: KR
- Aggregate [0-4mm]: exclusively NZ
- Aggregate [4-16mm]: NZ, BB, Zi
- Particle-size distribution: AB 16


With the above shown prerequisites it can be supposed, that changes in the concretes stress-strain relation only can cause by variation of the aggregate used in the mixture. The amount of aggregate needed for one cubic meter of concrete was determined with 705,77 dm3/m3 for all concrete mixtures. Presuming this value, the 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 untill the seventh day and then were stored by 20°C and 65% rel. moistness until the age of 28 days.

6 STRESS- STRAIN RELATION

For one-dimensional acting force Hookes law a = E * e can be used to describe the relation between force and deformation. E describes the elastic modulus, e is standing for the deformation caused by the stress a. Concrete only follows this coherence unless to 40% of its compressive strength. Beyond this mark, the deformations increasing disproportionate to the raised stress.

This means, that there is an increasing nonreversible part of the deformation. This part in creases by increasing stress. A tremendous loss of strength can be observed by transgession of the maximal sustainable stress. Exceeding the maximal sustainable stress level, the deformation increases by decreasing stress. A complete stress-strain curve including the downswinging part can only be measured, when the deformation speed is let constant during the whole attempt.

6.1 Failure behaviour

The design of concrete structures bases on the stress-strain curve unless the short-term stress. Standard however is the stored energy unless the complete demolition of the concrete structure. The stored energy gives an evidence regarding the ductility of concrete structures. The strain-stress curve allows an evaluation of the collapsing behaviour of different types of concrete. The toughness sinks with increasing compressive strength.

 

7 TEST RESULTS

7.1 Compressive strength

The relized inquests give an answer to the question, if the used aggregat causes some negative effects on the compressive strength. Figure 1 shows the ascertained results of the realized inquests.

Figure 1: Maximum stress of concrete with different combinations of aggregate

Figure 1 shows, that there is no significant relation between the used aggregate and the value of the measured compresive strength. This can be explained with the substantial composition of the used materials. The recycled concrete aggregate derived from material, that consisted of 100% concrete. The amount of pollution and material with very low strength was less than 1.1 %. The recycled clay derived from 100% of former bricks. The amount of material with very low strength was under 2%. There were no materials like bitumen or asphalt in the aggregate mix. According to that, the compressive strength of the concrete was only influenced by the hardened cement paste.


7.2 Deformation

Following the test procedures, the deformation was applied with constant speed. In Figure 2 the deformation under maximum stress in dependence of the used concrete mixture is shown. Figure 3 shows the aggregates influence of the deformation under maximum stress. Picture 1 shows the checking facility.

Figure 2: Deformation under maximum stress.

Picture 1: Checking facility

Figure 3: Deformation under maximum stress in reliance of the used recycled aggregate

Notedly can be seen, that the value of the deformation increases by increasing the volume of the recycled aggregate. By changing 100% of natural dense aggregate through recycled concrete aggregate in the grain size 4-16 mm, the deformation increases about 20%. Changing the dense aggregate through recycled clay in the grain size 4-16 mm the deformation increases about 27%. This escalation is the result of the usage of material with a lower modulus of elasticity than the natural dense aggregate has. (Elastic modulus of recycled concrete aggregate 20.000 N/mm2 , Elastic modulus from recycled clay 2.000-25.000 N/mm2). Consequence therefor is, that the toughness of the concrete raises. This also can be seen in Figure 1, in which the complete stress-strain curves of concrete made of natural dense aggregate, 100% recycled concrete aggregate in the grain size range 4-16 mm and concrete made of 100% recycled clay in the grain size range 4-16 mm are shown.

Figure 4: Stress-strain relation curves of concrete made with diferent aggregates

By standardizatiori of the present stress to the maximum stress, the three concrete behaviours can be compared. Notedly the increasing area underneath the stress strain curve shows the ability of the concrete, to signalize the approaching failure. Concrete made with aggregate deriving from recycled concrete possesses an modulus of elasticity of ~25.000 N/mm2. The value of the modulus of elasticity from
concrete made of recycled clay is about 18.000 N/mm2. Therefore the area under
neath the stress-strain curve increases with sinking modulus of elasticity.

 

8 RESULTS

The gained results show, that the deformations of the test samples increasing with rising amount of recycled-demolition-material. Aggregate derived from recycled clay, has the biggest influence. The deformation under maximum stress of concrete made of 100% recycled concrete rubble, is 20% higher than the deformation of the concrete made of 100% natural dense aggregate. By replacing 100% of the natural aggregate to recycled clay, the deformation rises about 30%. Evaluating the compressive strength of test samples with amounts of recycled-demolition material, no definite changes to the sample with 100% natural dense aggregate were 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.


9 CONCLUSION

By using recycled concrete and recycled clay for concrete aggregate, a gain of deformation 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 noticed. 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.


References

1. GRÜBL. P. Die Erstellung von Bauwerken unter Verwendung von industriell gefertigten Betons mit rezykliertem Zuschlag (Creation of Buildings with Industrial made Concrete Containing Recycled Aggregate); 18. Darmstädter Massivbau Seminar, Volume 18, 1997

2.

RÜHL, MARCUS. Water Absorption Capacity of recycled demolition Rubbish; Darmstadt Concrete Volume 12, 1997, Darmstadt

3.

"Prüfverfahren für technologische Untersuchungen", "Test Methods for Technological Analyses", resigned in the research project "Recycling of Mineral Building Materials" (Baustoffkreislauf im Massivbau, BiM), 1996