10 Aesthetic Defects; 10 Structural Defects; and Repair Techniques & Procedures
Excerpt from NPCA's 2013 downloadable guide.
This guide is not meant to be all-inclusive, but rather is a collection of best practices commonly used to repair precast concrete. The guide explains procedures and time-proven techniques used to make a multitude of precast concrete repairs. Precast concrete product repairs can be related to engineering and design, production, handling, shipping, erection, other trades (typically on the job site), job site conditions and environment. While it would be impossible to address every possibility where a repair may be needed, this guide will address some of the most common situations. It covers the basics and common methodologies of repairs. Unique situations will require you to develop repair techniques based on the appropriate methodology.
This guide is not a replacement for good quality concreting practices, which will reduce the amount of production-related repairs. For more information, see the NPCA Quality Control Manual for Precast and Prestressed Concrete Plants.
Aesthetic defects are considered minor defects. They are usually production-related and can be fixed quickly at the plant. Some examples include bugholes, small chips, crazing cracks or others described in this guide. Aesthetic defects do NOT impact the structural integrity or intended service life of the product.
Surface voids can be a common surface blemish on precast concrete. These are usually small voids found in clusters and commonly referred to as bugholes. While these do not compromise the structural integrity of the product, they can be considered unsightly, especially with architectural finishes. The common causes of bugholes include entrapped air, water pockets or the improper application of form release agent.
When a release agent is applied too heavily to the surface of a form, it can pool at the base of the mold or form droplets. When the concrete is placed into the form, the pools or droplets prevent the concrete from occupying that space so that when the form is removed a bughole is left behind where the droplet or pooling occurred.
Fine cracks occur at the surface and are very small, with a width typically less than 0.01 inches.
Shrinkage cracks occur when water is removed too quickly from fresh concrete. The loss of water causes a volume change in the concrete, and since the concrete is still fresh, the tensile strength is not adequate to resist the volume-changing force. Shrinkage cracks can be avoided by placing concrete in a controlled environment where relative humidity, concrete temperature and wind velocity are favorable for concrete curing. When necessary, shrinkage cracks can be repaired using epoxy injection methods.
Crazing cracks usually occur very soon after the concrete has been placed. The cracks are shallow and typically do not cause wear resistance or durability issues. Crazing cracks are often attributed to a lack of hydration on the surface of the concrete during the curing process. Crazing can be avoided by using curing compounds, covering the product during curing, refraining from "over-finishing" the surface of the concrete, and not finishing the product while bleed water is still present on the surface. Crazing cracks are typically not repaired because they are not structural and they are so small that it would be nearly impossible to fill them with any material.
Chips are relatively small sections of products that have been removed, typically as a result of impact. Chips may be as large as 8 in. in diameter by 1 in. deep and are usually of irregular shape. As chips become larger, they require a different approach to repair. This may include adding reinforcement (also known as pinning) and using a build-up application technique. This will be discussed under the section on spalls. Most chips can be repaired in one application with the appropriate patching material.
Efflorescence occurs when soluble salts come to the surface of concrete. All concrete and mortars will experience some level of efflorescence. This natural phenomenon is most prevalent in moist environments and low- temperature conditions. Efflorescence will typically appear as a white substance, so it will be more noticeable on dark-colored concrete. Efflorescence can be removed by pressure washing before it reacts chemically to form calcium carbonate. Once the calcium carbonate reaction occurs, the use of a mild acid solution is often required to remove efflorescence. After application of the mild acid, it is important to rinse all acid and remaining calcium carbonate from the concrete to prevent discoloration of concrete or a relapse of the efflorescence cycle.
Missing Architectural Details
Missing architectural details such as false joints, quirks and miters occasionally can occur in the manufacturing of architectural precast concrete. The use of a thorough quality control program and a highly skilled design and detailing firm should greatly limit those occurrences, however.
It's obvious, but should be stated: architectural precast must look good and meet the intended aesthetic purpose. Aesthetics can be subjective in nature, however, so this is always a potential area for dispute.
Precast products are cast over many days using several batches of concrete. The best way to minimize batch-to-batch variations is to follow good concreting procedures: Purchase all materials needed for a project from the same lot or run. Blend materials when more than one lot is used. Do not change from approved sources midstream in project. Maintain the specified water/cement ratio and mix design, control variations. Maintain proper placement and consolidation techniques. Follow consistent and proper curing procedures.
Discoloration can be caused by a multitude of factors. These factors include changing cement lots, varying aggregate properties, inconsistent mixing, inconsistent finishing or a change in curing conditions. Virtually any change in the concreting process can lead to a change in coloration.
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