Architectural Engineering Department
BS in Architectural Engineering
The purpose of this project is to investigate a particular, undesirable cracking pattern in concrete tilt-up panels that, until now, did not have a known definite cause. The cause of this cracking pattern is hypothesized to be due to shrinkage restraint of the concrete panels. The cracking under investigation occurs at the bottom corners of the Tilt-Up panels, suggesting that the base of the panel is restrained from shrinkage. This project models various components of Tilt-Up Construction that have potential for restraining the panels from shrinking. This project consists of the following main components.
The first aspect of this project was to investigate and become familiar with the means and methods of Tilt-Up Construction. To determine the potential shrinkage restraints on the panels, the connections and details associated with Tilt-Up must be thoroughly understood. This involved reviewing typical details of connections as well as contacting engineers and contractors in the field to determine the typical means and methods of Tilt-Up construction and construction sequencing.
Once typical construction practices were understood, the first shrinkage restraint investigated was the friction developed by the panel setting pads. Once the panel is ready to lift, it is set on grout pads or plastic shims, typically located at the ends of the panel. To determine the amount of restraint caused by friction, an experiment was conducted to determine the coefficient of static friction. Tests were run to find the coefficient of friction for concrete against grout, and concrete against plastic shims.
The third aspect of this project was to develop an effective computer model of stresses in Tilt-Up panels induced by shrinkage restraint. The goal of this model was to be able to run various scenarios, to determine the effects of panel concrete mix design, panel geometry, and construction sequencing.
The last aspect of this project was to collect enough data from the computer model to determine whether or not shrinkage restraint induces enough stress in the panel to initiate cracking, determine when the cracking would occur given construction sequencing, as well as determine if the cracking pattern matches the pattern seen out in the field. Conclusions will have to be made on a case by case basis, but the panel specifications in this analysis were chosen from a Home Depot building in San Luis Obispo, CA, an as-built Tilt-Up project. After running about 70 different cases, it was discovered that the grout pads by themselves did not provide enough shrinkage restraint to initiate cracking in the panel. This led to further investigation of panel connections, specifically the panel to slab connection at the pour strip.
This paper concludes that when combining the shrinkage restraint from grout pad friction and pour strip reinforcement tension, there is potential for cracking in the panel. Even further, the cracking pattern determined from the computer model provides nearly an exact match to the actual cracks under investigation and measured in the field. Although this report provides evidence for potential cracking in Tilt-Up panels due to shrinkage restraint, recommendations for limiting the potential of cracking in panels will need to be made on a case by case basis.