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2nd International RILEM Conference on Progress of Recycling in the Built Environment 2-4 December 2009, São Paulo, Brazil

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STRUCTURAL CONCRETES MADE FROM RECYCLED PRECAST ELEMENTS

Bassan, M. (1), Quattrone, M. (1) and Basilico, V. (2) (1) Technical University of Milan, Italy (2) Independent Engineer, Italy

ABSTRACT The techniques to recycle rubble from building dismantling are replacing landfill

techniques.

From a quantitative point of view, it is now established (at least in most European Countries) that debris recycling regards above all ceramics, brick and concrete rubble.

The use of these recycled materials is not particularly problematic for low-performance applications, on the contrary, their use to make recycled aggregate concrete (RAC) for high performance structural elements requires more attention.

In this case, second materials, with their specific characteristics, require a deep knowledge of potentially achievable performances and of suitable procedures for their maximization.

A number of ongoing experimental tests, carried out by the “Recycled Building Materials” Group of the Politecnico di Milano’s Building Environment Science and Technology Department, have allowed to identify both specific characteristics of recycled materials and procedures for their correct use to manufacture RAC.

In particular, working in collaboration with precast industries, their production waste has been recycled to produce the recycled aggregates to put into production cycle again. In this way the waste’s quantity to dispose of has been reduced enough.

In this paper a first series of results, concerning test on specimens and on full scale prestressed structural elements, are presented. These results, when conveniently completed, will be used to provide a set of recommendations for the structural use of RAC. The final goal is to draft a set of guidelines for designers and building contractors.

Keywords: C&D Waste, Recycling, Recycled Aggregate, Recycled Aggregate Concrete (RAC), Prestressed RAC Structural Elements.


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1. INTRODUCTION

The problem of wastes from demolition works and their disposal, is today very important; many industrialized countries direct their policies toward the reduction of wastes to landfill, favouring their reuse/recycling.

From this perspective, the so-called “recycled aggregates”, made by mechanical treatment of generic or concrete rubble, represent an effective opportunity to achieve the goals described above.

In different European countries, for many years, the use of recycled aggregate concrete to build structures is a frequent practice, as proved by their technical rules about structural concrete.

In Italy this opportunity has been given, for the first time, with the enacting of the new Construction Regulations (first version 2005) and now with the enacting of the last updated version.

Since 2000 the Building Environment Science and Technology (BEST) Department of Politecnico di Milano has carried out studies and experimental campaigns on recycled aggregate concrete in particular using recycled aggregates from precast industries’ production waste.

During the first tests, consequently the analysis of the collecting data, cubic specimens had been tested. The good achieved results have allowed to carry on the research; in particular 8 beams, 8 meter long, have been made and tested. The used recycled aggregate concretes have a percentage of recycled aggregate from concrete rubble equal to 0%, 20% and 30%. The bending test results shown no differences between the ordinary concrete and the recycled one. After the tests, some cylindrical specimens have been cored from the beams to determine the E modulus and the tensile strength. The achieved values were be a part of the ordinary interval of experimental variation.

As a consequence of these good results, the experimental activity has been developed on one side with tests on specimens with a 100% of recycled aggregate from concrete rubble and on the other side with tests on full scale prestressed structural elements and on their production processes.

2. PREPARATION OF THE AGGREGATE

All the tested mixes were constituted by cement, natural sand and coarse recycled concrete aggregates. In particular, the recycled aggregate came from production waste’s crushing of the factory that produced and tested the elements.

The production waste (concrete rubble) has been treated through the following steps: crushing by grab, primary crushing by jaw crusher, magnetic separation, grain sorting by vibrating screen. Figure 1 and figure 2 show the above-mentioned steps.


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The production waste’s homogeneity has been the warranty of the good mechanical, chemical and physical characteristics of the produced recycled aggregate.

The recycled aggregates’ characteristics were comparable with those of natural ones and suitable to make a recycled aggregate concrete with compression strength very similar to the ordinary one.

The particular production modalities used by precast industries (controlled series production), involving continuous checks on the mixes to assure constant mechanical characteristics of concretes, have represented an additional advantage.

Among the produced four granulometry classes, 0/5 mm, 5/15 mm, 15/40 mm and >40 mm, only the granulometry 5/15 mm, for the experimental campaign, has been used. This class was suitable for the production of the tested hollow core slabs.

The characteristics of recycled aggregates have been determined according to EN 1097-6 “Tests for mechanical and physical properties of aggregates - Determination of particle density and water absorption”

The main properties of the recycled aggregate are listed in table 1.

Figure 1. Production waste and crushing phase by grab

Figure 2. Primary crushing by jaw crusher and sorting by vibrating screen


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Table 1. Properties of the recycled concrete aggregate

Humidity (%) 2.84 Water absorption (%) 5.56 Bulk density (g/cm3) 2.58

Saturated density with dry surface (g/cm3) 2.34 Figure 3 shows the grain distribution of the recycled aggregate. In particular the grain

distribution curves have been determined for samples crushed inside the precast plant and then for the same samples crushed again using a laboratory crusher.

3. MAKING OF SPECIMENS AND STRUCTURAL ELEMENTS The experimental campaign has been characterized by tests on specimens and on full scale

structural elements. The tests have been carried out to develop the right mix composition, to evaluate the performances of RAC with 100% of coarse recycled aggregates, to observe and evaluate possible differences in behaviour between ordinary concrete structural elements and recycled ones.

Regarding the specimens, two groups (four series per group) have been made. The group G1 was constituted by six cubic specimens per series made with cement 32.5R; the group G2 was constituted by five cubic specimens and six cylindrical ones per series. Every series had a different water/cement ratio in a range from 0.45 to 0.6. Table 2 and figure 4 show the mixes’ quantities and some specimens before the tests. Regarding the G2 group’s series R100 with W/C = 0.45, it has been necessary to increase the superplasticizer’s quantity, because during the casting of the concrete inside the formwork some workability’s difficulties are appeared.

Figure 3. Grain distribution of recycled aggregate’s samples.


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Table 2. Mix design of the specimens – quantities [kg] for 1 m3 of mix Materials Groups

G1 G2 R100 R100 R100 R100 R100 R100 R100 R100

Cement CEM 32.5 R 380 380 380 380 - - - - Cement CEM 52.5 R - - - - 380 380 380 380 W/C ratio 0.45 0.50 0.55 0.60 0.45 0.50 0.55 0.60 Natural sand (0/4 mm) 600 600 600 600 600 600 600 600 Recycled aggregate (5/15 mm) 1270 1270 1270 1270 1270 1270 1270 1270 Superplasticizer 2.28 2.28 2.28 2.28 5.70 2.28 2.28 2.28

Regarding the experimental campaign on full scale structural elements, the choice to

produce five prestressed hollow core slabs has been motivated by practical reasons; in this way, it has been possible to make all the elements in one day with the same environmental conditions and without stopping the ordinary production cycle. All the slabs were 10.80 meters long and 30 centimetres tall.

To verify the mechanical behaviour of the elements, 3 mixes have been made:

the mix usually used to produce this kind of element, named NAT;

the mix with a percentage of coarse recycled aggregates equal to 20%, named R20;

the mix with a percentage of coarse recycled aggregates equal to 30%, named R30.

About the five prestressed hollow core slabs, one of them (NAT) has been made as pattern, two with the mix R20 and the others with the mix R30. It has been chosen to make just one element as pattern because the factory has a deep knowledge of the behaviour under load conditions of this kind of elements. The mix design has been kept equal to the ordinary production one; the quantities of water, cement, additive and sand are constant while the only difference regards the substitution of coarse natural aggregates with the 20% and 30% of the recycled ones, as shown in table 3.

Figure 4. Specimens. Group G1 (on the left) and Group G2 (on the right)


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Table 3. Mix design of the slabs– quantities [kg] for 1 m3 of mix

Materials Mix NAT Mix R20 Mix R30 Cement CEM 52,5 R 360 360 360

Water 135,5 135,5 135,5 W/C Ratio 0,38 0,38 0,38

Sand (0/5 mm) 740 740 740 Natural Aggregate (3/6 mm) 395 296 259 Natural Aggregate (5/15 mm) 765 632 553

Recycled Aggregate (5/15 mm) - 232 348 Additive 1,581 1,581 1,581

The prestressed elements have been made by using the slip-form technique; the only

reinforcement is represented by the prestressed cables.

After 20÷24 hours of heat curing, the single elements have been cut from the single casting. Contemporary with the casting phase, three groups of specimens, one for each mix, have been made. Figure 5 shows some production phases.

4. TESTS ON SPECIMENS AND RESULTS

The cubic specimens of group G1 have been tested to determine the compression strength after 28 days of curing. The achieved results have been statistically analyzed to evaluate their reliability. Figure 6 shows the results.

Figure 5. Industrial production. Modelling of the hollow core slabs (on the left) and casting after the vibration (on the right)


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As group G1, the group G2 specimens have been tested. For this group, cubic specimens have been tested to determine the compression strength while the cylindrical specimens have been tested to determine the elastic modulus in compression. All tests have been carried out according with European Standards (EN).

Hereafter the results have been presented. The achieved compression strength values have been statistically analysed to evaluate their reliability.

Regarding the elastic modulus, experimental and theoretical values have been determined and compared as figure 8 shows.

Figure 6. Group G1. Single values of compression strength (on the left) and confidence interval of the mean value (on the right)

Figure 7. Group G2. Single values of compression strength (on the left) and confidence interval of the mean value (on the right)

Figure 8. Group G2. Elastic modulus (on the left) and variation in percentage between theoretical and experimental values (on the right)


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5. TESTS ON FULL SCALE STRUCTURAL ELEMENTS AND RESULTS The slip-form technique has been used to make the structural elements; during the casting

some specimens have been made to evaluate the compression strength of the mixes after 1, 7, 14, 28 days of curing. The slip-form technique needs a concrete with a very low slump. It has been possible to observe that the substitution of coarse natural aggregates with the recycled ones does not affect the production process on the contrary it favours the reduction of the slump’s value keeping it under 10 mm.

As mentioned, the hollow core slabs were 10.80 meters long and their cross section was 30 centimetres tall. Bending and shear tests have been carried out on the elements. Figure 9 shows the static scheme used for the bending and the shear tests; on the left side of the figure the location of the instruments to measure the vertical displacements is shown.

To preserve the measure instrumentation, the displacements have been recorded up to load’s values equal to 2/3 of the estimated breaking load.

To collect many data during the shear test, the original elements have been cut at the middle point and both supports have been tested.

Figure 11 shows the specimens’ compression strength results for every mix. The values of the recycled aggregate concrete (R20 and R30) and those of the ordinary concrete are very similar.

Figure 10. Elements during the bending test (on the left) and during the shear test (on the right)

Figure 9. Static schemes used for the bending and shear test and location of deflectometer


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Regarding the bending tests, figure 12 shows the mean values of the breaking load and the graph load vs displacement at middle point.

The shear tests highlight that the behaviour of the RAC elements is the same of the ordinary concrete ones. Every shear test has been carried out following these phases: a first load step up to 10000 daN and then the unloaded. Then the element has been loaded up to the breaking. Figure 13 shows every single recorded value and the breaking load’s mean value.

Figure 11. Compression strength vs curing time

Figure 12. Bending test. Breaking load’s mean values (on the left) and load vs displacement at midpoint (on the right)

Figure 13. Shear test. breaking load Single values (on the left) and mean value (on the right)


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6. CONCLUSIONS The experimental campaign on specimens has shown that the use of recycled concrete

aggregates allows to make structural recycled aggregate concrete with high performance and useful for the ordinary concrete structural applications. The inverse correlation between the compression strength and the water cement ratio shown by group G2 is linked to the use of the cement CEM 52.5 R. The high fineness of this one has generated some variation in W/C ratio, but this problem could be solved with a slight increase of superplasticizer’s quantity. Concerning the elastic modulus reduction, it seems correct to considerer a reduction coefficient equal to 0.8 for RAC with 100% of coarse recycled aggregates.

Concerning the tests on full scale structural elements, according to the observations and to the results shown above, it is possible to state the following argumentations.

First, the use of recycled concrete aggregates with good characteristics and with a water absorption of about 5-6% and with a limit of substitution up to 30%, does not affect the particular production process used to make the prestressed hollow core slabs.

Experimental results on specimens have shown that the compression strength of concretes with recycled aggregates is subject to small variations (included in a range of 5 MPa) after 7 and 14 days of curing. After one day of curing, when the single elements must be cut, the compression strength is very similar for all concretes.

Bending tests have highlighted that the behaviour of RAC elements has been better than the ordinary elements. The breaking load for the RAC has been higher than the recorded load for the pattern element. The displacements at middle point reflect this behaviour. The deformation of RAC elements has been lower than that of the ordinary element, however these variations are very small and are part of a range ordinarily shown by traditional elements. These variations are strongly influenced by the stockpiling of the elements during the curing time.

The shear tests have shown a substantial equivalence among the structural elements with recycled aggregates and those with natural ones.

Results show that with known source’s recycled aggregates, as the precast production wastes are, it is possible to make structural elements, reinforced or prestressed, with a percentage of substitution bigger than that recommended by the Italian constructions rules in force.

REFERENCES [1] Go, S., Yoo, D., Lee, H., Lee, G. and Park, Y., ‘A study on the physical properties of hard concrete mixing various type of recycled aggregate’, in ‘Sustainable Construction’, Proceedings of the SB’07 International Congress, Lisboa, Portugal, September 2007.

[2] Gomes, M. and de Brito, J., ‘Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates’, in ‘Sustainable Construction’, Proceedings of the SB’07 International Congress, Lisboa, Portugal, September 2007.

[3] Bassan, M., Basilico, V. and Quattrone, M., ‘Use of recycled aggregates for structural elements made with high-performance concrete’, in ‘R’07 Recovery of Materials and Energy


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for Resource Efficiency’, Proceedings of the 8th International Congress, Davos, Switzerland, Empa, St.Gallen, September 2007

[4] Bassan, M. Menegotto, M. and Moriconi G., ‘Making RAC accepted as a structural Material’, in ‘Construction Materials’, Proceedings of the International Congress ConMat ’05, Vancouver, BC, Canada, August 2005

[5] Vázquez, E. and Gonçalves, A., ‘Recycled aggregate in concrete’, in ‘Use of Recycled Materials’, Final Report of RILEM TC 198-URM, Edited by Ch. F. Hendriks, G.M.T. Janssen and E. Vázquez, RILEM Publications S.A.R.L. 2005

[6] Bassan, M., Menegotto, M., Moriconi, G. and Basilico, V., ‘The Use of Recycled Concrete for Structural Applications’, in ‘R’05 Recovery, Recycling and Re-integration’, Proceedings of the International Congress R’05, Beijing, China, September 2005

[7] Sogo, M., Sogabe, T., Maruyama, I., Sato, R. and Kawai K., ‘Flexural Properties of Reinforced Recycled Concrete Beams’, in ‘Use of Recycled Materials in Buildings and Structures’, Proceedings of theInternational RILEM Conference on the, RILEM Publications S.A.R.L. 2004

[8] Sánchez de Juan, M. and Alaejos Gutiérrez, P., ‘Influence of Attached Mortar Content on the Properties of Recycled Concrete Aggregate’ in ‘Use of Recycled Materials in Buildings and Structures’, Proceedings of the International RILEM Conference on the, RILEM Publications S.A.R.L. 2004

[9] Sogo, M., Sogabe, T., Maruyama, I., Sato, R. and Kawai, K., ‘Shear Behaviour of Reinforced Recycled Concrete Beams’, in ‘Use of Recycled Materials in Buildings and Structures’, Proceedings of the International RILEM Conference on the, RILEM Publications S.A.R.L. 2004

[10] de Oliveira, M.J., de Assis, C.S. and Terni A., ‘Study on compressed stress, water absorption and modulus of elasticity of produced concrete made by recycled aggregate’, in ‘Use of Recycled Materials in Buildings and Structures’, Proceedings of the International RILEM Conference on the, RILEM Publications S.A.R.L. 2004

[11] Etxeberria, M., Vázquez, E., Marí, A., Hendriks Ch. F. and van Maasakkers M.H.J., ‘The role and influence of recycled aggregate, in “recycled aggregate concrete” ’, in ‘Use of Recycled Materials in Buildings and Structures’, Proceedings of the International RILEM Conference on the, RILEM Publications S.A.R.L. 2004

[12] Quattrone, M. and Basilico, V., ‘Materie prime seconde’, Modulo (326/2006)

[13] Quattrone, M. and Basilico, V., ‘Mobili o Fissi. Impianti per il riciclaggio delle macerie’, Recycling (07/2005)

[14] Bassan, M. and Basilico, V., ‘Produzione di elementi strutturali in calcestruzzo riciclato’, Industrie della Prefabbricazione (n.7/2005)

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STRUCTURAL CONCRETES MADE FROM …demo.webdefy.com/rilem-new/wp-content/uploads/2016/10/ff3c0c50ecbd...STRUCTURAL CONCRETES MADE FROM RECYCLED PRECAST ... In Italy this opportunity