Cast-in-Place vs. Precast Concrete: Parking Structure Maintenance Considerations

PROJECT OVERVIEW

Every garage, no matter how it’s built, is susceptible to deterioration. But the type, location and extent of this deterioration can vary greatly, depending on how the garage was constructed. It’s important to understand the design differences between precast concrete and cast-in-place garages — as well as how these differences affect the potential for damage — so you can develop a maintenance approach that best serves your structure.

 

Cast-in-place vs. precast concrete parking garages

A precast parking garage is essentially built like a Lego® set. Individual components, like beams, walls and columns, are built off-site, typically in a climate-controlled manufacturing facility, and then shipped and assembled on-site. Once constructed, the connections are then welded, bolted and sealed.

Because precast parking structures are built in a controlled plant environment, it’s easier for the precast producer to monitor the concrete quality and other design criteria, such as accuracy of reinforcing placement. This type of construction can also reduce the costs associated with building a new parking garage because the job is often completed more quickly.

A cast-in-place parking garage is constructed on-site by pouring concrete into forms that will later be removed. This type of garage is traditionally reinforced with mild steel or post-tensioning, which increases its strength.

Cast-in-place structures are highly durable, with fewer joints and mechanical connections than a precast garage. Fewer joints means a smoother ride for visitors and less opportunity for deterioration caused by leakage. With this type of garage, owners also get more design flexibility, customization options and reduced building movement.

 

Maintenance considerations by parking structure type

Now that we understand the difference between cast-in-place and precast parking structures, it’s time to talk about how these differences affect maintenance and repair needs. From an engineering perspective, the way a concrete garage was built will influence how it performs and how it needs to be inspected. Likewise, the main problems that occur — structural or otherwise — often stem from how the garage was originally constructed.

Areas of concern in precast parking garages

Top Concerns

  • Waterproofing methods at the joints
  • Failure points at mechanical connections
  • Exposure of load-bearing structural elements

The areas that need the most attention in a precast parking structure are often the joints. Usually the joints, the most vulnerable parts of the structure, pose the most deterioration risk. Joint sealants can be used to waterproof these areas, but without continued upkeep, it’s easy for water to seep in at these connection points, especially as the joint sealants break down over time due to exposure to the elements. Thus, regularly inspecting and replacing waterproofing methods should be on everyone’s preventive maintenance list for precast parking structures.

When considering structural issues, the joints are a common culprit again. The interfaces and connections between the segments can be subject to wear and deterioration. Thermal expansion and contraction, dynamic loading from excessive vehicle speed and exposure from failed sealants can cause significant damage to embedded connections. These problems can be identified during a routine inspection. For structural repairs, engineers and contractors must work closely together to identify the root problem and then design and implement an engineering solution.

Areas of concern in cast-in-place parking garages

Top Concerns

  • Corrosion of the steel framework
  • Shallow concrete cover
  • Internal concrete failure

Cast-in-place structures have fewer joints, so the areas of concern for these garages tend to be the internal steel reinforcement and the concrete itself. Here, water plays a different role in deterioration than it does in precast structures. Because the internal reinforcing is susceptible to rust and corrosion, waterproofing is necessary to ensure the reinforcing remains protected. Although you can’t always observe this damage externally, there can be a lot going on beneath the surface that routine inspections and testing will catch. Engineers can use technology like ground-penetrating radar (GPR) and acoustic sounding that can help them identify damage under the surface. In any case, waterproofing systems, like sealants and coatings, should always be replaced once they’ve reached their recommended service life.

The concrete used for a cast-in-place structure must also be monitored regularly because it tends to crack easier, allowing water to penetrate its surface and cause corrosion. Lack of needed repairs, overloading and chemical damage can all exacerbate this issue. Another design flaw that may be present in cast-in-place garages is shallow concrete cover — the result of reinforcing steel being placed too close to the concrete surface. This allows water to penetrate through the shallow concrete cover and reach the steel reinforcing, where it can lead to corrosion.

It can be difficult to recognize the signs of deterioration in your garage, especially when many issues begin below the surface. But routine inspections and assessments by an experienced parking structure engineering team often catch these problems before major repairs are necessary. Whether you have a cast-in-place or precast concrete parking structure, you’ll want to continue preventive maintenance activities by monitoring for cracks, leaks, rust and other structural issues throughout your garage’s service life.