Cracking in fiber-reinforced overlay systems

A model of the symmetric portion of a three-phase overlay system (Bolander and Berton, 2004; Bolander, 2004) is shown in Fig. 1. A cement composite overlay is bonded, through an interfacial region, to a mature concrete substrate. This problem is motivated by previous finite element analyses done by Martinola and Wittmann (1995). Each component of the system is assigned appropriate diffusion properties (i.e. humidity dependant diffusion coefficient). At time t = 0, the system is saturated down to a depth of 60 mm from the top surface, whereas all material below has relative humidity H = 0.9. Through convective boundary conditions, the top surface is exposed to the atmosphere with H = 0.5. A fiber-reinforced composite overlay is shown on the right side of the figure.

Figure 1: Computational grid for simulating drying in an overlay system

With increasing t, moisture flux occurs between the specimen surface and the atmosphere. Figure 2 shows humidity profiles through the model depth at various times after initial exposure to the drying environment. Based on the same model geometry, diffusions properties, boundary conditions, etc., the commercial finite element package FEMLAB provides essentially the same results.

Figure 2: Relative humidity profiles for various durations of drying

Humidity gradients provided by the moisture diffusion analysis lead to stress development in the coupled Rigid-Body-Spring Network model of the overlay system. The overlay, substrate, and interfacial zone tend to be fracture sensitive and an adequate description of crack propagation in these components requires the use of nonlinear fracture mechanics within the irregular lattice model. Figure 3 shows the traction versus crack opening displacement relations assigned to each component within the RBSN analyses.

Figure 3: Traction versus crack opening displacement relations for system components

Prior to 20 days exposure to drying, fracture zones develop rather uniformly along the top surface of the overlay. These fine cracks are not evident in the following animation sequence. Fracture later localizes into the system of cracks seen below, which resembles the cracking pattern obtained by Martinola and Wittmann (1995) for this same case. After 110 days of drying, the largest crack opening is 0.294 mm. No debonding occurs for the strong, tough interface considered here, but rather fracture advances into the substrate. The lateral branching of shrinkage cracks (seen here) has been discussed by Bisschop (2002).

Figure 4: Crack development in unreinforced overlay during 110 days of drying

 

When polypropylene fibers (l = 12 mm; diameter = 0.039 mm; Ef = 42.8 MPa) are added to the overlay material, cracking does occur through the overlay, but the fibers act to keep crack widths small so that they are not evident in the following animation. Due to the lack of stress relief in the overlay, a large crack develops at the end of the model and then runs parallel to the overlay. After 110 days exposure to drying, this crack has a mouth opening of 0.255 mm.

Figure 5: Crack development in fiber-reinforced overlay during 110 days of drying

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References

  1. Bolander, J.E. and Berton, S., "Simulation of shrinkage induced cracking in cement composite overlays." Cement & Concrete Composites, 26, 2004, 861-871.
  2. Bolander, J.E., "Numerical modeling of fiber reinforced cement composites: Linking material scales." In Proceedings of the Sixth International RILEM Symposium on Fibre-Reinforced Concretes - BEFIB 2004, eds. M. di Prisco, R. Felicetti and G.A. Plizzari, RILEM, 2004, pp. 45-60.
  3. Martinola, G. and Wittmann, F.H., "Application of fracture mechanics to optimize repair mortar systems." In Fracture Mechanics of Concrete Structures, ed. F.H. Wittmann, AEDIFICATIO Publishers, Freiburg, 1995, pp. 1481-1486.
  4. FEMLAB v2.3, COMSOL AB, Stockholm, Sweden, 2002.
  5. Bisschop, J., "Drying shrinkage microcracking in cement-based materials." PhD Thesis, Delft University of Technology, Delft, 2002.