You ask, we answer! As part of our limited series, we give you the chance to ask our forensic experts questions about the evaluation of damage to buildings, equipment, and other components of the built environment. Those submitting questions may have their question anonymously posted and answered in our monthly web blog, as below.
Q. Can you determine what caused cracks in a concrete pool or pool deck?
Yes. This is a common issue that forensic engineers are asked to evaluate. Cracking in concrete pools is typically caused by one or more of the following: concrete shrinkage in concert with inadequate reinforcement, differential movement of the pool structure, or material deficiencies.
Concrete shrinkage is a common cause of cracking in concrete elements and structures. As concrete cures (hardens), it undergoes a reduction in volume, called “shrinkage.” When this shrinkage results in tensile stress in the solid concrete greater than that which the concrete material alone can resist, cracking will occur. Proper reinforcing of the concrete and measures taken to cure the concrete after it has been placed are both important for the control of shrinkage-related cracking. Curing for pool shells is often accomplished by repeated and regular wetting, and evaporation control, of the concrete prior to the application of the plaster for the pool (Figure 1).
Figure 1. Concrete pool prior to plastering.
Reinforcement is used within the concrete matrix to control cracking; however, the relative amount of reinforcing often utilized will not eliminate cracks from occurring. Cracks related to shrinkage are typically limited in width and can often be identified based on the crack pattern and location. Shrinkage cracking is common at pool deck slabs (Figure 2).
Figure 2. Shrinkage crack at a concrete pool deck.
Differential movement of a pool structure can occur due to post-construction volumetric change in the soils supporting the pool. Common causes of differential pool movement include soil settlement, interaction with expansive soils, and the effect of hydrostatic pressures on the pool (e.g., buoyant pressures on a pool after it has been drained). Evaluation for movement includes measuring the pool for levelness using surveying tools and other measurement devices, and mapping of visible cracking. If the pool remains filled with water, the waterline can be used as a measurement reference, since water is inherently self-leveling. The pattern of cracking can be compared to the results of the levelness data to determine if there is a relationship. When movement has occurred, cracking and vertical offsets will often be noted near/at the interface of the pool shell with the pool deck. For example, cracking at a pool deck that is parallel to the pool shell and is located in close proximity to the shell may be indicative of differential movement (Figure 3). It is relatively common for movement to occur at pool deck slabs, while significant movement of the pool structure itself is less common.
Figure 3. Movement-related cracking at a pool deck.
Cracking related to material deficiencies can occur if the concrete is of poor quality or if there is an internal mechanism within the concrete that is causing the cracks, such as Alkali-Silica Reaction (ASR). Such cracking often appears after a period of time has passed, can be relatively severe, and even cause leaks in the pool shell (causing loss of pool water when trying to keep the pool full and operational). As with the other causes mentioned, the pattern of cracking resulting from deficient materials can be telling. For example, ASR often causes “pattern” cracking at the concrete surface and can also manifest in relatively large fractures at geometric and material transitions. (Figure 4). The confirmation of material deficiencies in the concrete, and deleterious chemical reactions within, often requires coring of the concrete and evaluation of the cores by a petrographer.
Figure 4. ASR induced cracking in a swimming pool structure.
