Early-age deformations and cracking in high-performance concrete: measurements, modelling and mitigation
Event details
| Date | 22.03.2012 |
| Hour | 12:15 › 13:15 |
| Speaker | Prof. Pietro Lura |
| Location | |
| Category | Conferences - Seminars |
The matrix of a high-performance concrete typically has low porosity, typically obtained by keeping a low water-to-cement ratio and by silica fume addition. High-performance concrete possesses advantageous properties compared with traditional concrete, e.g. high strength from early ages, low permeability and improved durability. However, its practical application of faces some difficulties, in particular due to sensitivity to early-age cracking. These cracks constitute a serious problem with regard to strength and especially to durability. A main cause of early-age cracking is restrained autogenous deformation, a direct consequence of cement hydration in a low water-to-cement mixture. Another cause of cracking is represented by thermal stresses that are a consequence of the exothermic nature of cement hydration.
As cement hydration proceeds in low water-to-binder ratio concrete, the water becomes bound into hydration products and adsorbed on their surfaces. The capillary pores are emptied and the air-water menisci become progressively smaller, the internal relative humidity (RH) drops (self-desiccation) and an increasing capillary tension is produced in the pore fluid. The capillary stress exerts a compressive stress on the whole porous body, resulting in bulk shrinkage.
Self-desiccation occurring in low w/c cement paste leads not only to autogenous shrinkage, but also causes a substantial increase in the coefficient of thermal expansion in the cement paste. The latter effect is due to the RH changing with temperature which causes additional expansion acting in the same direction as the pure thermal dilation of a partially saturated medium.
Internal curing by means of superabsorbent polymers (SAP) is a method for promoting hydration of cement and limiting self-desiccation and further self-desiccation shrinkage. SAP are introduced dry into the mixture and form stable, water filled inclusions by absorbing pore solution. This water is released to the cement paste during hydration, as clearly shown by neutron tomography experiments. SAP are effective in both eliminating autogenous shrinkage and decreasing the thermal expansion coefficient of the cement paste, thereby reducing the risk of early-age cracking.
In this presentation, our work on early-age deformation in high-performance concrete will be presented, including experiments, poromechanical modelling and mitigation techniques.
Bio: Pietro Lura is Head of the Concrete and Construction Chemistry Laboratory since 2008 and professor at ETH Zurich, Institute of Building Materials, since 2011. He received his MS in 1998 from the University of Brescia, Italy, and his PhD in 2003 from the Delft University of Technology, The Netherlands. He has been assistant professor at the Technical University of Denmark (2003-6), visiting researcher at the National Institute for Standard and Technology (2002) and at Purdue University (2005), and patent examiner at the European Patent Office in Munich, Germany (2006-8).
His research interests include hydration and early-age properties of concrete, in particular microstructure development, shrinkage, setting, early-age cracking and internal curing. In these fields he has done important contributions to both understanding of the fundamental mechanisms and to advancement in the state of the art. His work on measuring techniques of autogenous deformation earned him two awards from the Transportation Research Board, the Bryant Mather Award in 2006 and the Fred Burggraf Award in 2007. For his work on internal curing he received the American Concrete Institute Wason Medal for Materials Research in 2007. He received for the second time the ACI Wason Medal in 2009 for his contribution to the understanding of the mechanisms of plastic shrinkage cracking. In 2009 he also received RILEM L'Hermite Medal in recognition of his outstanding contribution to the study of the early-age behavior and volume instability of cement-based materials.
In addition to heading the Concrete and Construction Chemistry Laboratory at Empa, Pietro Lura is teaching at ETH Zurich since 2009. He is also active in teaching and organizing summer courses targeted both at students and practitioners.
As cement hydration proceeds in low water-to-binder ratio concrete, the water becomes bound into hydration products and adsorbed on their surfaces. The capillary pores are emptied and the air-water menisci become progressively smaller, the internal relative humidity (RH) drops (self-desiccation) and an increasing capillary tension is produced in the pore fluid. The capillary stress exerts a compressive stress on the whole porous body, resulting in bulk shrinkage.
Self-desiccation occurring in low w/c cement paste leads not only to autogenous shrinkage, but also causes a substantial increase in the coefficient of thermal expansion in the cement paste. The latter effect is due to the RH changing with temperature which causes additional expansion acting in the same direction as the pure thermal dilation of a partially saturated medium.
Internal curing by means of superabsorbent polymers (SAP) is a method for promoting hydration of cement and limiting self-desiccation and further self-desiccation shrinkage. SAP are introduced dry into the mixture and form stable, water filled inclusions by absorbing pore solution. This water is released to the cement paste during hydration, as clearly shown by neutron tomography experiments. SAP are effective in both eliminating autogenous shrinkage and decreasing the thermal expansion coefficient of the cement paste, thereby reducing the risk of early-age cracking.
In this presentation, our work on early-age deformation in high-performance concrete will be presented, including experiments, poromechanical modelling and mitigation techniques.
Bio: Pietro Lura is Head of the Concrete and Construction Chemistry Laboratory since 2008 and professor at ETH Zurich, Institute of Building Materials, since 2011. He received his MS in 1998 from the University of Brescia, Italy, and his PhD in 2003 from the Delft University of Technology, The Netherlands. He has been assistant professor at the Technical University of Denmark (2003-6), visiting researcher at the National Institute for Standard and Technology (2002) and at Purdue University (2005), and patent examiner at the European Patent Office in Munich, Germany (2006-8).
His research interests include hydration and early-age properties of concrete, in particular microstructure development, shrinkage, setting, early-age cracking and internal curing. In these fields he has done important contributions to both understanding of the fundamental mechanisms and to advancement in the state of the art. His work on measuring techniques of autogenous deformation earned him two awards from the Transportation Research Board, the Bryant Mather Award in 2006 and the Fred Burggraf Award in 2007. For his work on internal curing he received the American Concrete Institute Wason Medal for Materials Research in 2007. He received for the second time the ACI Wason Medal in 2009 for his contribution to the understanding of the mechanisms of plastic shrinkage cracking. In 2009 he also received RILEM L'Hermite Medal in recognition of his outstanding contribution to the study of the early-age behavior and volume instability of cement-based materials.
In addition to heading the Concrete and Construction Chemistry Laboratory at Empa, Pietro Lura is teaching at ETH Zurich since 2009. He is also active in teaching and organizing summer courses targeted both at students and practitioners.
Practical information
- General public
- Free
Organizer
- Prof. Nikolas Geroliminis and Prof. Katrin Beyer
Contact
- Prof. Jean-François Molinari