Ifsttar PhD subject

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Title : Enhancement of recycled concrete aggregates by bioprecipitation

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality Génie civil

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Marne-la-Vallée

Doctoral affiliation ECOLE CENTRALE NANTES

PhD school SCIENCES POUR L'INGENIEUR, GEOSCIENCES, ARCHITECTURE (SPIGA)

Planned PhD supervisor SEDRAN Thierry  -    -

Planned financing Contrat doctoral  - Ifsttar

Abstract

: Recycled concrete aggregates (RCA) contain, due to their origin, residual cement paste which gives them high porosity and moderate performance. The porosity leads to a strong water absorption. This is a major difficulty on the industrial level because itcomplicates the adjustment of water in concrete batches, which allows to control their workability in production. The RCA manufacturing process results in having more paste in the finer particles and therefore more absorption. As a result, while the industry today recycles coarse RCA into concrete relatively well, it uses small amounts of RCA sand because of their greater porosity. Yet, during the manufacture of RCA, about 50% sand and 50% coarse aggregates are obtained. Consequently, the porosity of RCA sand hinders the circular economy of concrete.
A number of techniques have been proposed for removing or improving the residual cement paste, but they are expensive. The natural carbonation of RCA by atmospheric CO2 helps with decreasing their water absorption by obstructing their porosity, but this is a several month reaction. Research is ongoing to makeaccelerated carbonation (by concentrating CO2, for example) on an industrial scale.
The present work explores an alternative idea, which consists in forming in a few days, using biocalcifying bacteria, a matrix of CaCO3 around the RCA and especially the sand part, in order to limit the access of water to their porosity. First, candidate non-pathogenic bacteria were identified, selected, adapted to the alkaline medium of RCA, then we checked their ability to produce CaCO3. In a second step, we detemined the conditions, which favor uniform bacterial colonization and production of CaCO3 on the surface of model agar media. Homogeneity is indeed mandatory to obtain good water tightness. We thus confirmed the value of selectingbacteria capable of producing biofilm. Finally, themethods developed were applied to model mortar disks facilitating visual observations. Preliminary results confirm that it is possible to significantly lower the absorption of these mortars within one month.
Further work is needed to confirm these encouragingresults on sand part of RCA

Keywords : concrete, recycling, recycled concrete sand, biocarbonation, water absorption

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	Génie civil
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Marne-la-Vallée
Doctoral affiliation	ECOLE CENTRALE NANTES
PhD school	SCIENCES POUR L'INGENIEUR, GEOSCIENCES, ARCHITECTURE (SPIGA)
Planned PhD supervisor	SEDRAN Thierry - -
Planned financing	Contrat doctoral - Ifsttar
Abstract : Recycled concrete aggregates (RCA) contain, due to their origin, residual cement paste which gives them high porosity and moderate performance. The porosity leads to a strong water absorption. This is a major difficulty on the industrial level because itcomplicates the adjustment of water in concrete batches, which allows to control their workability in production. The RCA manufacturing process results in having more paste in the finer particles and therefore more absorption. As a result, while the industry today recycles coarse RCA into concrete relatively well, it uses small amounts of RCA sand because of their greater porosity. Yet, during the manufacture of RCA, about 50% sand and 50% coarse aggregates are obtained. Consequently, the porosity of RCA sand hinders the circular economy of concrete. A number of techniques have been proposed for removing or improving the residual cement paste, but they are expensive. The natural carbonation of RCA by atmospheric CO2 helps with decreasing their water absorption by obstructing their porosity, but this is a several month reaction. Research is ongoing to makeaccelerated carbonation (by concentrating CO2, for example) on an industrial scale. The present work explores an alternative idea, which consists in forming in a few days, using biocalcifying bacteria, a matrix of CaCO3 around the RCA and especially the sand part, in order to limit the access of water to their porosity. First, candidate non-pathogenic bacteria were identified, selected, adapted to the alkaline medium of RCA, then we checked their ability to produce CaCO3. In a second step, we detemined the conditions, which favor uniform bacterial colonization and production of CaCO3 on the surface of model agar media. Homogeneity is indeed mandatory to obtain good water tightness. We thus confirmed the value of selectingbacteria capable of producing biofilm. Finally, themethods developed were applied to model mortar disks facilitating visual observations. Preliminary results confirm that it is possible to significantly lower the absorption of these mortars within one month. Further work is needed to confirm these encouragingresults on sand part of RCA
Keywords :	concrete, recycling, recycled concrete sand, biocarbonation, water absorption

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