Cracks, chips and spalls happen in concrete. When damage such as this occurs to a structure, it is important to understand the severity in order to use proper repair procedures.
In this article recently published by Locke Solutions, they talk about the differences in these defects, what some of the common causes are and what to look for when assessing how to repair a precast concrete structure.
Chipping often is the result of impact on concrete, typically occurring on the edges or corners. Chips tend to be smaller areas of breakage.
Chips are a cosmetic issue and normally do not impact the structural integrity of the structure. During the precast manufacturing process, chips can occur when structures are removed roughly from molds or are improperly handled or stored. On the jobsite, chips typically occur from improper handling, such wrapping chains around a structure and dragging. Chips also can be the result of unintentional collisions with other structures during the installation process.
Spalls are similar to chips but typically occur on the edge of a concrete surface. Spalls also typically are cosmetic in nature but have a wide range of size variety.
Similar to chips, spalls usually occur from impact or pressure. Spalls that expose structural reinforcement are considered more severe, and repairing these structures correctly is critical for the product’s long-term life.
The most common cause of spalled edges occurs during handling and storage. Dunnage typically is used to support precast structures and allows forklift access underneath. Imperfections along the edges of a concrete surface can lead to a “point load” effect where the concrete is placed on dunnage or when forks are picking up the structure.
A great way to reduce spalling in these situations is by creating a rounded or chamfered edge where the forks and dunnage support the structure.
Cracks can vary in size and depth. They also require more experience to evaluate. Catastrophic cracks requiring major repairs to maintain a product’s structural integrity.
Minor, surface cracks sometimes are found on concrete structure tops. Running across an unformed surface of a structure and typically occurring when the curing process is poor, cracks sometimes are called “temperature” or “shrinkage” cracks. These types of cracks can occur when the surface dries quickly because of heat or wind and no controls are used to cure the surface at a slower rate to maintain moisture while the cement hydrates. These types of cracks also may present themselves if there is a high water to cement ratio in concrete mix design.
A ”re-entrant corner” crack due to stress on corners, sometimes occurring with structures that have squared-off edges. The cracks normally present themselves once the structure has cured. One way to minimize this problem is to create structures with rounded edges. Rounder edges provide more structural strength and can greatly help to reduce these types of cracks.
More severe cracking can occur on structures not properly handled or stored. For prefabricated concrete structures, they should rest on level surfaces and on dunnage that has been properly placed based on the design of the structure. Concrete is heavy and instances with the weight not properly distributed across the dunnage can lead to excessive stress on the structure. If it has not been designed for those stresses, there is a potential for it to develop stress cracks. Storage of larger precast sections should be evaluated and designed by an engineer.
Addressing The Cause and Preventative Measures
Before repair begins, it is important to understand the root damage and what the final product use is. For structures with major damage, an engineer assessment that includes evaluation of structural integrity must come first before repair.
In addition, industry best practice includes preparing a damage assessment report to find out why the damage occurred and how to prevent it in the future.
One of the most common ways for precast products to get damaged is improper handling. Products should always be carefully lifted and stripped from casting molds after reaching the design stripping compressive strength.
Normally, precast structures are not at full 28-day design strength when being demolded, but there should be a stripping strength designated, and the concrete should be tested to ensure accuracy.
Molds should be checked regularly to ensure that they are working properly and not causing any damage to structures. If molds are not properly coated with form release agents, it may create problems during the stripping process and allow binding when the product is being removed.
Cosmetic Concrete Repairs
“Patching” is the term for performing cosmetic repairs to concrete structures. Patch mixtures can be cement-based or have additional mineral additives. They also can be comprised of an epoxy mortar, epoxy cement or a polymer mixture.
When repairing a damaged structure, first understand the severity of the damage. Qualified and competent personnel should assess the damage and, if necessary, a qualified engineer should be involved for any potential structural damage.
In handling cosmetic repairs, the structure surface should first be prepared properly before the repair material application. The surface should be clean and free of any debris. Weather conditions also should be taken into consideration, as colder temperature will slow the patch material from curing.
Once the surface the has been prepped and the patch mixture is ready, water or a bonding agent may be applied to the site with a brush. This adds moisture to the area. Once this takes place, a trowel can be used to apply the patch mixture. It is important that the mixture is applied evenly across the surface of the structure.
Once the mixture has been evenly distributed, it can be smoothed over with sweeping motions.
Once the patch mixture has been evenly distributed, use a brush to smooth over the surface.
Repairs can be costly, especially when there is significant damage. Structures that receive extensive structural damage may even be required to be replaced.
That is why it is critical to thoroughly inspect and assess when damage occurs to ensure that costly repairs are minimized.