CORRECTION OF CONSISTENCY OF CONCRETE MADE WITH AGGREGATE DERIVED FROM CONCRETE RUBBLE
Christoph Lemmer, Marcus Rühl, Andrew Nealen
SUMMARY
Concrete, made with aggregates from recycled concrete-crush, get
stiff quickly due to the waterabsorbtion of the porous
aggregates. A possibility to counteract this stiffening is the
addition of superplasticizer. The investigations reported in this
text show, that there is nearly no influence from the type of
cement, auditioned fly ash, environment-temperatures, time of
superplasticizer-addition or investigated kind of
superplasticizer to the amount of superplasticizer that is
needed, to correct the consistency of a stiffened concrete by a
certain value. The influence of the type of cement, fly ash,
environmental-temperatures, time of the addition of
superplasticizer and kind of superplasticizer is discussed.
1
Introduction
The
reuse of recycled demolition-material as aggregates for concrete
is a theme of research at the Institut of Massivbau at the TU
Darmstadt since 1996. First successes, particularly with reusing
recycled concrete-crush, as shown at the building
"Vilbeler-Weg" in Darmstadt make clear, that in the
future a huge amount of the existing demolition-material in
Germany does not need to be deposited and can be reused useful if
it is recycled properly.
lt is known, that recycled demolition-materials absorb water from
the fresh-concrete due to their porosity. The quantity of the
waterabsorbtion depends on the apparent specific gravity of the
material [1]. The absorbed water is neither no longer at the
disposal for hydration nor the fresh concrete to reach a better
consistency. The question of the amount of water, which is
absorbed by the aggregates from concrete-crush in the fresh
concrete, is not answered exactly yet. Because of that, the
production of concrete with aggregates of concrete-crush with
exactly dosed quantity of water has not come into practice. A
practicable solution, to correct the consistency of stiffened
concrete to a certain value, is the addition of superplasticizer.
In this report it is examined, which quantity of superplasticizer
is needed to correct the consistency of a stiffened concrete with
aggregates of recycled concrete-crush to a certain value. The
influence of the cement-type, environmental-temperature, time of
superplasticizer-addition and an addition of fly ash on the
needed quantity of superplasticizer-addition is discussed.
2
Investigations
The decisive value
for mixing the concrete-patterns was only the consistency after 4
minutes. lt was targeted to have a consistency of a = 52 cm (Flow-Table-Test). lt was not tried, to keep
a defined water-cement-ratio in each case. lt was added as much
water to the concrete as needed to get a consistency of a = 52 cm
after 4 minutes. The waterabsorbtion of the aggregates of
recycled concrete-crush took place predominantly in die first
minutes after the addition of water, and then very quickly.
Because of that, short delays (½ minute) during the measurements, addition of water
or emptying the mixer caused noticeable differences in the
results. For this reason the quantity of additional water varied
insignificantly from pattern to pattern, because the only fixed
value was the consistency after 4 minutes.
| Fig. 1: | Schematic Overview |
All examined
concrete-mixtures were first mixed without cement for 60 sec. and
then mixed for another 60 sec. with the cement. After that, there
was added as much water as needed to reach the consistency of
approximately a = 52 cm after 4 minutes. The consistency of the
mixed concrete was measured 4, 15, 30, 45, 60 and 90 minutes
after adding die water with the Flow-Table-Test and die Degree of
Compactibility. As soon as die measured consistency no longer
laid within the KR-Area according to DIN 1045 (a < 42 cm or v
> 1,07), it was added as much superplasticizer as needed to
reach back the consistency of approximately a = 52 cm. Fig. 1
gives a schematic overview of die procedure.
2.1 Aggregates
The used
aggregates consisted of wet recycled concrete-crush. To reach the
same moist conditions of the aggregates at the beginning, the
fractions 2/8, 8/16 and 16/32 mm bad been dried for 24 hours at
105°C. Two different grading-curves type AB had been examined.
One with 16 mm and the other with 32 mm largest-coarse. For die
fraction 0/2 mm a governmentally authorised recycled sand was
used, which had a moist of constantly 4 M.- %. This moist was
calculated to the normal added water.
2.2 Cement and Fly
Ash
The used cements were ordinary portland cements CEM I 32,5 R and
CEM I 42,5 R. Each concrete bad a cement content of 310 kg/m3.
In the Fly ash concretes 40 kg/m3 of fly ash was
additionally added to the cement.
2.3
Superplasticizer
Two types of superplasticizer were used. FM 26 based on
Naphthalinsulfonat and the other one FM 29 based on Melaminharz.
2.4
Environmental-Temperatures
Right after mixing the concretes had been exposed to the
different environment temperatures of 5°C, 20°C and 40°C. The
concretes were remixed every 10 minutes. The temperatures of the
fresh concrete varied from 17 - 20°C.
3
Results
3.1
Development of Rigidity
All investigated concrete patterns showed quick development of
rigidity in die first minutes. This result was expected after the
investigation in [2]. The largest decrease of consistency took
place in the first 10 to 15 minutes after adding the water. (Fig.
2)
| Fig. 2: | Development ofRigidity of investigated Concrete Pattems (Average-Value) |
The relation between flow-table-value and degree of compactibility of the investigated concrete patterns is shown in Fig. 3. lt is obvious that there is a relation between die flow table-value and the degree of compactibility. This relation does not agree with the values in DIN 1045 for consistency-classes. The reason for that is the broken contour of die aggregates. The biggest deviation is viewable in the flowable consistency-area. In this area is observed that for raising flow-table-values the degree of compactibility almost remains the same. This is to be attributed to that with the flow-table-test die inner friction of the aggregates is better recognised than with the degree of compactibility. So the broken form of the aggregates and a liquid cement paste is considered better with the flow-table test than with the degree of compactibility.
| Fig. 3: | Relation between Flow-table-test-value and degree of compactibility for broken aggregates |
The influence of
the varied parameters was as expected. The flow-table-value of
concrete with CEM 1 42,5 R was approximately 4 cm larger after 15
minutes than with CEM I 32,5 R. However, this better consistency
had shrunk after 90 minutes to 1 cm.
Concrete patterns with fly ash needed little more water than
those with none to reach the consistency of a = 52 cm at the
beginning. But than consistency of a = 2 cm remains better over
the whole investigated time.
The development of rigidity under die different
environmental-temperatures of 5°C, 20°C and 40°C in die first
30 minutes after adding the water were almost the same. After 30
minutes the effect of warm and cold conditions was recognisable.
The rigidity of concrete patterns with environmental-temperatures
of 5°C developed more slowly than the rigidity of patterns
within 20°C. The rigidity of the patterns with
environmental-temperatures of 40°C developed very quickly after
30 minutes.
3.2 Additional
Amount of Superplasticizer
The criterion
for the effectiveness of an addition of superplasticizer was die
gained inprovement of consistency a (Flow-Table-Test) and v
(Degree of compactibility) due to a certain added quantity of
superplasticizer. In this report only the charts with die gained
improvement of consistency investigated with the Flow-Table-Test
are shown. All charts describing the consistency with die degree
of compactibility show the same described facts.
All examined patterns showed a linear context between die amount
of added superplasticizer and the gained improvement of
consistency. This result was not influenced from any examined
Parameter. (Fig.4)
None of the examined parameters pointed significant influence to
the effectiveness of an addition of superplasticizer. Only the
cement influenced the effectiveness slightly. A Portlandcement
CEM I 42,5 R reached an improvement of consistency that was
approximately 1 to 2 cm higher than the improvement with a CEM I
32,5 R with the same amount of added superplasticizer. All other
parameters like environmental-temperature, time of addition, kind
of examined superplasticizer or fly ash-addition showed no
significant influence. (Fig 5 to 9).
| Fig. 4: | Improvement of consistency due to the addition of superplasticizer |
| Fig. 5: | Improvement of consistency due to an addition of superplasticizer for different types of cement |
| Fig. 6: | Improvement of consistency due to an addition of superplasticizer with added fly ash |
| Fig. 7: | Improvement of consistency due to an addition of superplasticizer at different environment-temperatures |
| Fig. 8: | Improvement of consistency due to an addition of superplasticizer with different kinds of superplasticizer |
| Fig. 9: | Improvement of consistency due to an addition of superplasticizer at different addition-times |
The reasons for the loss of linearity at high dosages were difficulties with the exact determination of the flow-table-value with stiff consistencies (a < 33 cm). The destinated consistency was always a = 52 cm (Flow-Table-Test) so very stiff consistencies needed high dosages of superplasticizer. From a flow-table-value a = 33 cm on a correct determination of the flow-table-value is very uncertain, and so the value is overestimated in most cases. Because of this, the improvement a seems smaller in many patterns as it is supposed to be. You can see this by regarding the degree of compactibility (not given here). There the effect ofthe loss of linearity at high dosages is not viewable.
| Fig. 10: | Improvement of consistency due to an addition of superplasticizer |
That straight, shown in the chart of Figure 10 is supposed to be a good approximation to determine the improvement of consistency caused by an addition of superplasticizer for concrete with aggregates of recycled concrete-crush, independent from the examined parameters.
Restrictively it has to be said, that the straight, shown in Fig. 10, only can be used for cement-contents of 310 kg/m3 and with the used combination of components, due to the strong material-depending reaction cement - superplasticizer. The effect to superplasticise concrete with superplasticizer based on the dispergation of the single cement-agglomerats. lt is to be expected that with higher cement-contents die same amount of superplasticizer related to the cement-weight cause better improvement of consistency.
4 Conclusions
References:
| [1] | Grübl. P.: Baustoffkreislauf im Massivbau, Bauingenieur 72 (1997), 5. 425-430. |
| [2] | Grübl. P.; Rühl. M.: noch nicht veröffentlicht. |