Temperature Can Affect Upper Floors of Precast Garages
Precast concrete double-tees and joists are a common feature on many parking decks. While the use of these materials is proven for parking deck construction, several owners have noticed that the pre-stressed framing members on the top level would begin to camber. While the temperature remains fairly constant for the lower portions of the structure, the joists on the top level are directly exposed to the sun and therefore experience large temperature swings.
This process can create stress on the structure, causing cracking and spalling. In response to this scenario, two common repair strategies have emerged to combat this challenge: the use of carbon fiber reinforced polymers (CFRP) to strengthen the structure and the installation of supplemental joist hangers.
Understanding the Issue
In parking garage construction, pre-stressed framing members are set in place and cast into concrete beams before placement of the concrete deck. Generally, pre-stressed members have an upward camber when installed, so that the member will not deflect below the level point when loads are added to the structure. When exposed to direct sunlight, the deck can become extremely hot, which causes expansion.
The joists are secured at both ends so this expansion manifests itself as a similar upward camber in the joists. When the joists heat up, and expand, the stress can result in a tearing of the connection with the beam. When the concrete at the top expands and the concrete on the bottom is unchanged, a knuckle-effect of cambering and bowing creates small surface spalls on the beam.
As the joist bows, the concrete that it is resting on becomes damaged. If the spall is severe enough, the joist could fail because of the downward pressure. Where the joist is cast into the concrete and the concrete is stronger than the joist, the stem could crack at the interface of the embedded portion. Primarily a challenge in warmer climates, the condition needs to be remedied to ensure that failure does not occur.
Solution 1: Cazaly Hangers
The first solution is the use of joist hangers, commonly referred to as Cazaly hangers, when the garage is constructed. A Cazaly hanger is a metal hanger that is cast into the joist and then cast into the beam in a slip. The installation of the hanger accommodates the movement that results from the sun camber; therefore, it prevents spalling.
Solution 2: CFRP
Typically, the damage of the spalling beam is caused because of the additional unanticipated downward force on the beam. In this case, it is often advantageous to strengthen the beam with carbon fiber reinforced polymers (CFRP). These systems are paper-thin fabric sheets bonded to concrete members with an adhesive to significantly increase their load-carrying capacity. Installation typically is achieved by applying an adhesive to the prepared surface, installing the FRP fabric into the adhesive and then applying a second layer of the adhesive to encapsulate the fiber. After curing, the CFRP composite will add considerable capacity to the element despite the fact that it is a very thin laminate. This is because the CFRP has tensile strengths up to 10 times that of steel.
Solution 3: CFRP and Supplemental Joist Hangers
In some projects, the best solution is to incorporate two repair strategies. As previously mentioned, in some cases, the sun camber results in the joist stem cracking at the interface with the beam. Strengthening of the beam is not necessary in these cases since it is already outperforming the joists. The crack in the joist has compromised the structural integrity of the joist within the beam so the bearing point needs to be moved outside the beam. These supplemental joists hangers are mechanically attached to the beam on both sides of the joists and the joists rest on the hangers inboard of the damaged area. The damaged area can be injected with epoxy resin for durability issues and the hangers address the structural concerns.
The key to success when tackling a repair project resulting from a sun camber is hiring a specialty repair contractor that is experienced and diversified enough to implement the repair strategy that is best for the project. Understanding what causes sun camber and what to look for will help identify problems early on and result in the correct repair of the cause, not just treatment of the effect. In many cases spalled concrete is simply repaired and eventually spalls again because the camber issue was not addressed in the repair.
If you have a garage constructed with a pre-stressed joist system and observe any concrete deterioration in the area where the joists are connected to the beam, contact a structural engineer or competent repair contractor and ask for their opinion. The sooner the problem is addressed, the less damage will occur, and it will not become a safety issue.
Matt Shane is Business Development Manager for the Florida West branch of Structural Preservation Systems (SPS). Stephen Johnson is a Project Engineer with SPS. For more information, visit www.structural.net.