Empa Activities 09/10 Civil and Mechanical Engineering
Cracking in cement paste induced by autogenous shrinkage High-performance concrete is prone to microcracking due to shrinkage occurring in the first days after casting. Available techniques to detect the microcracks lack the required resolution or may produce additional cracks. A novel technique based on gallium-intrusion solves these problems.
Pietro Lura, in collaboration with Ole Mejlhede Jensen, DTU (DK), Jason Weiss, Purdue University (USA)
The cement paste of high-performance concrete (HPC) shows low porosity and a discontinuous capillary pore structure, obtained by keeping a low water to cement ratio and by adding silica fume. While HPC can possess good workability, high early strength, low permeability, and improved durability, they also exhibit autogenous shrinkage, i.e., volume reduction in sealed conditions at constant temperature. Restraint of the shrinkage by aggregates or other structures may result in the formation of micro and macro cracks that impair strength, durability and aesthetics. 10mm
12mm
Stainless steel rod
Fig.1: Cement paste samples with rods of 1.5, 3 or 6 mm diameter.
A recently-developed technique allows the identification of microcracks while avoiding artifacts induced by unwanted restraint, drying, or temperature variations during sample preparation. Small cylindrical cement paste samples are cast with steel rods of different diameters in their centre (Fig.1). The rods represent aggregates within the HPC. As the aggregates, they restrain the shrinkage of the paste and may cause crack formation. The crack pattern is identified by impregnation with gallium and analyzed by optical and scanning electron microscopy (Fig. 2). A non-linear numerical analysis of the samples is performed, using as inputs autogenous strain, elastic modulus, fracture energy, and creep as a function of hydration time. Samples with larger steel rods show the highest probability of developing microcracks both in the experiments and in the numerical analysis (Fig. 2). Both the pattern and the width of the observed microcracks show good agreement with the simulation results. The numerical simulations confirm that, in specimens with small rods, cracks form at the interface with the steel bar but stop at a short distance from the interface. On the contrary, once a crack forms in specimens with large rods, it propagates to the outer edge of the specimen. The results of this project represent a step towards predicting whether internal microcracks will form in high performance concrete as a function of the aggregate size. The final goal is to reduce amount and width of cracks in concrete structures, thereby increasing their durability.
200 Âľm
Fig. 2: Scanning electron microscope image of a crack running through a sample with a 6-mm steel rod.
Links: www.empa.ch/abt135
Contact: pietro.lura@empa.ch Reference: P. Lura, O.M. Jensen, J.Weiss, Materials and Structures, 42, 8, 1089 –1099 (2009)
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