By Eric Carleton, P.E.
Editor’s Note: This article provides information about repairing precast concrete drainage and buried infrastructure products. Though some information pertains to architectural and prestress products, these products’ unique aesthetic and structural properties are beyond the scope of this article. For additional information, please review the Precast Concrete Architectural Repair Guide found on precast.org or read PCI’s Manual for the Evaluation and Repair of Precast Prestressed Concrete Bridge Products and the NCHRP Report 654, “Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web.”
Precast concrete producers strive to cast the perfect product every time. However, even with highly trained employees and controlled production conditions in a precast plant, some cast structures may not exhibit the perfection anticipated when stripped from the form. Minor surface imperfections could include honeycombing, a small void in a corner or slight slumping of the concrete near a joint header. In those situations, durable concrete repair is necessary before the product is sent to the job site.
Most concrete repair literature from the U.S. Bureau of Reclamation, U.S. Army Corps of Engineers and American Concrete Institute is directed toward installed concrete structures that have experienced a problem due to environmental factors, corrosion or extreme service conditions. However, the processes and procedures described can also be applied as best practices for precast concrete repair.
Step 1: Evaluate the product
The first step is to carefully review the product needing repair and determine the cause and extent of the damage. Graham Bettis, P.E., of Texas Department of Transportation Bridge Division’s construction and maintenance branch, described it during a webinar presentation as evaluating the damage and diagnosing the problem. He further broke down the evaluation and cause as three components: defect, damage and deterioration.
It is important for the repair crew to understand the specific issue to be corrected to ensure the correct repair process and products are used. Additionally, this information is vital for management to know for making necessary corrections to eliminate future repairs, whether that is updating production practices, repairing or replacing formwork, material movement or handling practices and/or stored materials inspection.
The first evaluation is to determine whether the product should be repaired. Most ASTM standards address this issue. Therefore, an important part of the evaluation is not simply can it be repaired, but can it be repaired to meet the specification’s finished product requirements. In some cases, this includes the approval of the owner. This may not be a concern if the repair is a small honeycombed area along the vertical surface of a buried inlet box, but may be of concern if the repair is a broken spigot for a gasketed joint system requiring tight or low-degree of tolerance.
To assist field inspectors on department of transportation projects for this varying subject of acceptability of repaired precast drainage products, an AASHTO task group of engineers, construction groups and industry representatives published R73-16, “Standard Practice for Evaluation of Precast Concrete Drainage Products.” This document can provide guidance for evaluating repairs.
Categorize and classify
When consensus supports the precast section should be repaired, the analysis then focuses on classifying the type of repair as structural, performance or cosmetic.
A large majority of precast concrete repair is to correct a defect. Typically, product defects are caused during the manufacturing process. These issues characteristically show up as non-structural surface anomalies such as honeycombing, bugholes, small voids or shrinkage cracks. In addition, some production defects could be a minor corner spalling or small surface slump cracks, which can occur during stripping and moving dry-cast products prior to a hardened set. These defects are often first observed early in the form-removal process while the concrete is still plastic in the case of dry-cast concrete, or within 24 hours of initial set in the case of wet cast or self-consolidating concrete products. This can provide advantageous cementitious repair options to be discussed later.
Many of these surface and minor spalling defects are not structural in nature and in most cases require cosmetic repair. However, depending on the depth of the surface defect and placement of the reinforcement, these issues may also be performance related and potentially could reduce the steel’s durability and structure’s service life. Even buried drainage products that are out of sight may have special aesthetic applications, such as a precast box culvert placed for a pedestrian underpass, a wet well or lift station with the top 2 feet exposed for worker access or a projecting culvert-flared end section. When determining if a cosmetic repair is necessary, consider these two items:
- Even if the precast concrete product is buried, a precast product’s appearance can be your best salesman or your worst.
- A precaster’s reputation by the receiving contractor and inspector can be improved by the product’s appearance.
Some production defects would fall under the structural classification. Examples are an incorrect form setup or a form core shift that modifies a product wall thickness beyond accepted tolerance or steel placement without adequate cover. Even if these defects may have viable repair remedies, this type of defect would typically require the approval of the owner or jurisdiction having authority.
Damage to precast products is typically due to handling the hardened concrete in the plant, or in the yard when storing for inventory or shipment. This type of damage is often characterized by spalling. Depending on the spall size and placement, the repair classification could be cosmetic, performance or structural. However, it is also important for the repair evaluation group to carefully inspect damaged sections for cracking. If impact cracks are found, further analysis will be needed to determine if the crack is repairable and the structure is sound.
Since we are talking about plant repair of precast concrete products, deterioration evaluation would not typically be considered part of the process. However, there could be a situation when a long-inventoried product is pulled from storage and used on a project. The storage may have initiated some deterioration, which will require repair before it would be in compliance with the intended standard. Though not specifically a precast concrete issue, other attached products could have been damaged, corroded or faded, such as ultraviolet-resilient rubber boot connectors or polymer steps, steel lift inserts or product identification markings. Concrete deterioration – if more than surface deep – can require extensive removal and rework, which could prove more expensive than simply manufacturing a new product.
Step 2: Choosing proper repair methods
The next step is to choose the proper method and product for the repair. This task might seem overwhelming based on the numerous repair products and options currently available on the market. However, keeping simplicity and timing in mind can help with your decision.
The Bureau of Reclamation’s Guide to Concrete Repair, Second Edition, states, “Whenever possible, repairs made on new or old concrete should be made as soon as possible after the need for repair is realized. Especially for new concrete work, repairs that will develop the best bond and are most likely to be durable and permanent are repairs that are made immediately after stripping forms, while the concrete is still green. For this reason, repairs to newly constructed concrete should be completed within 24 hours after the forms have been removed and no more than 72 hours after the concrete is placed.”
Newly cast concrete often provides an ideal surface on which to affix the repair mix. It is uncontaminated by dirt, dust and other debris. The repair area also still has a damp, likely surface-saturated dry, active cementitious surface ready to receive the repair mix. For these conditions, it is not only beneficial but economical to use the same or similar concrete mix that was used to cast the original structure. Depending on the size or depth of the repair, coarse aggregate may be added. If coarse aggregate will hinder the placement of the repair concrete, it can be removed by sieving. If that is impracticable, a cement mortar can be made using the plant cement mixed with fine aggregate and clean water. Cement mortar mix use should be accomplished by trial batches to evaluate and optimize curing properties, such as shrinkage and color match.
Upon completion of the concrete repair, the newly placed concrete needs to cure in accordance with good practices. Should normal production procedure dictate that the hardened product be taken to the yard, accommodations need to be made to keep those repaired sections within the curing area until the repaired concrete has set and bonded with the precast section. It should be noted the repaired area is still fragile and will require special treatment to keep it away from thermal shock or direct sunlight. It should also be kept in a moist atmosphere.
If the repaired concrete is to be placed on hardened concrete from a section that has been in the yard or job site, then using a bag-repair mix might be the best option. Bagged mixes are available as cementitious, cementitious with polymer additives, cementitious with epoxy additives or simply epoxy material. The large variety of bag-repair mixes available can also make choosing the right product for the repair confusing. One method to pare down the list of available products is to determine if the municipality, DOT or contractor has developed a list of preapproved materials. In addition, the dimensions of the repair and application of the repaired product help dictate the type of repair material to use.
Generally, cementitious materials work well for most repairs on precast products. If the repair is thin – less than 1/2 inch – then a polymer or epoxy mix may be the best option. However, it is important to understand the correct application procedures when using concrete mixes with polymers or epoxy-type repair materials. Often the surface prep is not surface-saturated dry like typical cementitious mixes, but perfectly clean and dry. In addition, a special bonding agent needs to be applied prior to placing the repair material. The number one reason specialty repair bag mixes fail is a failure by the technician to follow the proper mixing and curing directions.
When reviewing the repair mix options, consult with a trusted professional for recommendations. When comparing product data, verify they have been tested to the protocols outlined within American Concrete Institute 364.3R, “Guide for Cementitious Repair Material Data Sheet,” or the International Concrete Repair Institute 320.3R, “Guideline for Inorganic Repair Material Data Sheet Protocol.”
An important factor when choosing the correct repair material is to ensure there is compatibility between the original precast concrete and the repair material. Many incorrectly believe a stronger bag mix means a faster cure and better repair. However, unless those specific attributes are required, the opposite is often the better solution. The best repair product is one with mechanical properties that closely match the original concrete. Therefore, if the precast product has compressive strength required to exceed 4,000 psi and the actual testing shows strength of 5,000-to-6,000 psi, the repair material needs to exceed 4,000 psi to ensure compliance. ACI 546R-14, “Guide to Concrete Repair,” states the problem of product compatibility in regard to mechanical attributes in strong mixes – such as modulus of elasticity, permeability and thermal expansion – may not be in harmony with the original concrete base. This may lead to premature delamination of the repaired section when the product is put into service. Unless repair and shipping constraints require a rapid curing product, a slower setting – along with extended curing time for the repaired product – will provide the best results. When evaluation and product comparisons have been completed, it is time to choose the correct product and repair procedure.
Step 3: Write it down
The third step is to prepare and develop repair procedure documents for design and construction. Because these repairs are to be accomplished at the plant, some precasters may think it’s OK to skip this process and go directly to the repair activity. However, it’s a best practice to have a specific and detailed written procedure developed for each company or plant’s specific type of repair. This means separate procedures need to be included if the repair is a surface defect, a spall, slab off, crack, lift insert, replaced manhole step or any other unique repair.
Document items to include can be:
- The minimum training and qualifications of the personnel doing the repair work
- A list of repair products
- Mixing requirements
- Descriptions of surface preparations
- Material applications procedure
- Curing requirements
- Quality control inspection and approval process
The development and application of this document will provide better assurance of repair uniformity, quality, function and durability. This is the primary reason it is a required element of Section 4.7 of the NPCA Quality Control Manual for Precast Concrete Plants. A well-written repair procedure document ensures that repair best practices and lessons learned are shared with everyone. It also serves as a historical record when implementing small but continuous improvements to repair processes and products used.
The International Concrete Repair Institute, in conjunction with ACI Committee E706 Concrete Repair Education, developed a detailed repair procedure document series called, “Repair Application Procedure.” This can be used for reference or as a template when writing your document. Similarly, many DOTs have developed detailed repair procedures that can be implemented into a plant-specific document. In addition, technical information can be provided by the repair mix manufacturer.
Most owners or contractors that receive a precast product that needed a repair want to know it was accomplished correctly, professionally and that it will have no effect on the product’s function or service life. Those owners and contractors may request a repair procedure document and a thorough, complete document will ensure the greatest success for acceptance.
Step 4: Complete the repair
The final step is to perform the repair work and implement the action plan and repair procedures developed within the third step. A critical action is to ensure the repair crew has been adequately trained on the materials and processes. A concrete repair craftsman is often as much an artist as a technician. It takes a special skill set, pride in the work and patience to repair concrete structures to new condition. However, even the best repair employee who has only worked with plant mix mortar repair will need training if the next repair requires the use of a bag-mixed material or one with a polymer additive. That training should also include appropriate safety information and familiarity with the product’s material safety data sheet. Some companies provide a special repair training program and issue certificates to newly qualified employees who have successfully completed training. Only those plant-qualified employees can do product repairs.
Upon completion of the repair, it is important for the work to be inspected by another party other than the repair crew. This is typically done by the plant’s QC technician who can use a variety of simple tests – in addition to visual inspection – to verify a solid repair has been accomplished correctly. An excellent practice found in the NPCA QC Manual states, “After repairs are completed and inspected, a mark shall be made on the product indicating that it is acceptable, or that it is rejected.”
This practice not only provides accountability to the repair, but offers some added insurance to the contractor and owner of the precast product that the repair was done well.
As humans, we are not perfect. Consequently, sometimes what we make is not perfect. Fortunately for precast concrete, many of those imperfections can be corrected with well-planned and executed concrete repair best practices.
Eric Carleton, P.E., is NPCA’s director of codes and standards.
ACI 364.13T-15, “Repairs for Reinforcement with Shallow Cover“
ACI 364.3R, “Guide for Cementitious Repair Material Data Sheet”
ACI 546R-14, “Guide to Concrete Repair“
ACI 546.3R-14, “Guide to Materials Selection for Concrete Repair”
ICRI 320.3R, “Guideline for Inorganic Repair Material Data Sheet Protocol”
ICRI RAP-10, “Leveling and Reprofiling of Vertical and Overhead Surfaces”
NPCA Quality Control Manual for Precast Concrete Plants, 12th EDITION.
Webinar, TXDOT Proper Design and Implementation of Concrete Repairs. Graham Bettis, P.E.
U.S. Army Corps Engineers, “Evaluation and Repair of Concrete Structures,” 1995.
U.S. Bureau of Reclamation, “Guide to Concrete Repair,” Second Edition.