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**

**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.

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/m^{3}

- 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 dm^{3}/m^{3 }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/mm^{2} , Elastic modulus from recycled clay
2.000-25.000 N/mm^{2}). 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/mm^{2}. The value of the modulus of elasticity from

concrete made of recycled clay is about 18.000 N/mm^{2}. Therefore the
area underneath 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 |