By Jack Robinson, Stewart Hooks, John Lawson PE, SE

Introduction/Cracking Pattern

Despite the multitude of benefits tilt-up construction has to offer in low rise commercial buildings, there is still room for improvements in the performance of tilt-up buildings. One opportunity for improvement is to minimize a particular, undesirable cracking pattern often seen in concrete tilt-up panels (See Figure 1), which in some cases requires significant cosmetic patching (See Figure 2). This concrete cracking can first appear as non-problematic and non-structural, but even small cracks can become weathered and lead to water intrusion and rebar corrosion. At the very least, these cracks are cosmetic concerns for the owner or tenant potentially resulting in significant paint/patch costs.

Figure 1

Figure 2

As part of an undergraduate research project, the authors sought to determine the cause of this cracking pattern in the panel’s lower half and to have a convincing argument for where the problem originates. Work was conducted at the California Polytechnic State University in San Luis Obispo (Cal Poly) within the Architectural Engineering Department.

Historically, there have been many suggested causes for this cracking pattern; including, setting panels too hard, excessive out-of-plane bending stress, in-plane shear stress, poor bond-breaker performance and lifting stresses, or other construction-related issues. However, to an engineer, a handful of indicators such as the cracking location, direction, amplitude, and cracking entirely through the panel, suggest that these cracks are due to the shrinkage restraint at the base of the panel. Despite the regularly spaced panel joints in tilt-up, natural concrete shrinkage can still be restrained by ties to adjacent panels, to the adjacent floor slab, and to the foundation, preventing free movement.

 

Case Study – San Luis Obispo Tilt-Up Retail Building

Figure 3: Case Study Measured Cracking Pattern

As part of this research project, a local tilt-up building in San Luis Obispo, California, was identified as experiencing this cracking pattern, and was used in our analyses and computer modeling to verify our hypothesis of the cracking cause. Nearly every visible panel showed signs of the cracking pattern being investigated. The panel showing the most distinctive cracking was measured with four predominate cracks recorded using dimensions in the horizontal and vertical axis as shown in Figure 3. Next, a finite-element computer model (SAP2000) was created to independently predict if similar cracks would form in a panel given various restraint conditions. If a crack was predicted to initiate, the model would further help indicate where the first crack was likely to appear as well as its direction of travel through the panel. Additionally, it was critical to predict when crack formation would occur, and thus it was important to accurately model the behavior in which concrete shrinks over time. This was accomplished using ACI 209R for time-dependent shrinkage predictions. Because material shrinkage behavior is similar to thermal contraction behavior, the finite element computer model was calibrated using a pseudo temperature change.

It was reported by the case study building’s contractor that cementitious grout setting pads were used beneath the panel joints instead of plastic shim packs, potentially causing excessive restraint forces. Grout setting pads are often used in some parts of the United States. It was decided to introduce into the computer model a frictional resistance to the shrinkage movement (restraint) at the lower panel corners. Because concrete cracking occurs due to internal tension stresses, of primary interest are the locations where the computer model predicts high principal tension stresses evaluated in each finite element grid space. Figure 4 illustrates the high principal tension stresses in blue as distributed in the modeled panel.

Figure 4: Principal Stresses in Panel

Evaluating the Computer Results

The results from the computer model were nearly an exact match of the observed field cracking in the lower half of the case study tilt-up panel. When excessive restraint occurs due to setting pad frictional resistance and/or foundation tie and slab tie connections, the internal tension stresses exceed the modulus of rupture near the bottom of the panel, thus cracking is predicted. In tilt-up concrete panels, the modulus of rupture (or cracking point) is taken as (7.5× 2/3 ×√(f’c)) or (5×√(f’c)) due to the internal restraint caused by the reinforcing (See Tilt-Up Today, Vol. 15 No. 2, pp. 12-17). With cracking predicted, it was important to see if the computer model could predict the crack’s direction of travel accurately. By initiating a crack at the same location in the computer model as in the field and orienting the crack’s travel direction perpendicular to the principal stress arrows, the model’s predicted cracks are remarkably similar to the actual field observations as seen in Figures 5, 6, 7 and 8. This provides strong evidence that the cracking in this panel and similar cracking found in other tilt-up panels are due to panel shrinkage restraints at the base. In Figures 5, 6, 7 and 8, the blue portion of the crack is predicted to occur given crack propagation behavior of concrete. Crack propagation is the behavior in which a material continues to crack under significantly less stress than needed to initiate the crack. The portion in red is theorized as a crack extension that occurs after the initial stresses redistribute as further cracking occurs.

Figure 5a: Model Predicted Left Crack  /  Figure 5b: Actual Left Crack

Figure 6a: Model Predicted Middle Crack  /  Figure 6b: Actual Middle Crack

Figure 7a: Model Predicted Right Crack 1  /  Figure 7b: Actual Right Crack 1

Figure 8a: Model Predicted Right Crack 2  /  Figure 8b: Actual Right Crack 2

Conclusions

There is sufficient evidence to claim that this cracking pattern under investigation is due to the panel’s concrete shrinkage restraint. The cracking pattern determined from the computer model provides a close match to the actual cracks measured in the field. While reducing the shrinkage potential of the concrete mix may help alleviate the resulting cracking, it may be more significant to reduce the sources of external restraint to the panel, allowing more freedom of shrinkage movement. Where possible, for example, the use of plastic shim packs instead of grout setting pads at the ends of the panels will significantly reduce the frictional restraint. Additionally, connections between adjacent panels and between the foundation and slab should be thought through in consideration of this research.

About the Authors

Jack Robinson and Stewart Hooks each recently graduated with a BS in Architectural Engineering from California Polytechnic State University in San Luis Obispo. Their research advisor, John Lawson, is a licensed Structural Engineering in California and Arizona, and Associate Professor in Architectural Engineering California Polytechnic State University in San Luis Obispo.