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Introduction to Concrete Repair

Repairing concrete is probably one of the toughest jobs a contractor can undertake. What looks good when you leave a job often has a short service life. The Bureau of Land Management (BLM), the federal agency that monitors famous structures such as the Hoover Dam, studied the effectiveness of their concrete repairs. Their researchers found that less than 25% of the repairs their contractors had performed lasted more than five years without additional work. 

Fortunately, there is a lot of good practical information, effective repair products, and helpful tools you can use to address a concrete concern that will satisfy your customer’s problem. Turn to your White Cap expert for the support you need.

How to Begin a Successful Concrete Repair Project

Contractors often encounter projects on which they must update worn surfaces, repair cracks, and replace deteriorated concrete on slabs, sidewalks, floors, bridge decks, foundations and other elements. Since each repair or update is unique, you need to know which procedure will bring the best results. These procedures could include recaulking, sealing, surface and structural repair, and epoxy or urethane injections. 

Cracks in concrete can also be a sign of a more significant issue. The expression “all concrete cracks” has become the common answer to practically all questions or concerns when an owner or inspector points out a problem on a project to a contractor. 
Introduction to Concrete Repair
While the saying has an element of truth, it’s probably not good to rely on such a general answer when asked to examine a crack or defect in a slab, wall, or driveway. 

And if you believe this mantra, try repairing concrete the old way – you are just asking for call-backs and even more problems down the road. So, let’s look at a list of questions you might ask when approaching the task of repairing concrete.

 

WHEN: Establish the Timing
The first set of questions you should ask will help you understand the timing of the problem. These questions include:

“How old is the concrete? 

“When was the concrete placed?” 

“When did you first notice the crack or deformation?”


The answers to these questions will help you identify the cracking mechanism. Contractors need to distinguish between cracks that appear two weeks after placement from those that are noticed a year later. 

 

WHERE: Mapping the “Problem”
The best way to start a repair is with a thorough visual inspection of the floor or wall. By answering the question “Where are the problems?”, you’ll have another tool for your investigation. 

You can get further guidance from ACI 201.1R-08, “The Guide for Conducting a Visual Inspection of Concrete in Service”. It contains a comprehensive approach. You should also document any observations related to environmental and loading conditions. This approach also allows you to include any supplementary information from nondestructive tests, destructive tests, and other investigations. 

Also look for any environmental exposures that are on the project. These can include poor drainage, wall movement, and even poor landscaping.

 

Determining the Type of Problem

Before beginning any repair work, you should determine whether the crack in question is dormant or active. Active cracks are indentified as growing or moving, and unless repaired, will cause greater damage. Dormant cracks are stable and unmoving.  

To determine activity, measure the crack in two or three places at regular intervals. Say once a day or once a week. The simplest way is to place a mark with tape and marking pen on either side of the crack and look for the mark to change. You can use a comparator, a business card sized gauge that helps you determine the width of the openings. 

Most slabs don’t move for weeks, months or years. But when a crack that has been dormant for many years suddenly shows movement pay close attention. The action suggests that there’s a new live load. The source can be from frost action, unanticipated weight additions, or problems somewhere else in the structure. Unless the reason for the added force is identified, and the movement stabilized, the condition will only worsen. 

A static crack is a thin (hairline) surface crack that usually propagates at a very slow rate. As such, the crack may appear to be unchanging (static). Unlike dynamic cracks, which are active and require urgent repair, static cracks may need little to no structural repair because they are mainly an aesthetic issue. 

Static cracking can also be called “pattern cracking” or “craze cracking”. The cracking is usually shallow (less than 1/8-inch deep) and not a structural issue.
 

Static Concrete Crack Repair

Should You Repair a Static Crack?

Not all static cracks require attention. Before undertaking the project, refer the American Society of Concrete Contractor’s Position Statement #33 (PS-33-cracks-in-structural-concrete.pdf [ascconline.org]). This document outlines the considerations for contractors when asked to repair a visible crack. ACI 224R-01, “Control of Cracking in Concrete Structures,” indicates 0.016 in. as a reasonable crack width for reinforced concrete under service loads for a dry air exposure.
 

Should You Repair a Dynamic Crack?  
When you encounter a dynamic crack, it’s probably a good time to reassess your team’s capability and skill level. On many projects, you’ll consult with a structural engineer to determine the cause. Water damage, corroded reinforcement, or changes in the structure’s load can cause distress, and a cosmetic repair would only mask the problem. 

If you opt to undertake the repair, the Guideline for Selecting Application Methods for the Repair of Concrete Surfaces, published by the International Concrete Repair Institute (ICRI) offers contractors the most current information on best practices. Written by contractors, for contractors, the guide illustrates and describes the application methods commonly used for placement of concrete repair materials, along with material requirements, the best applications, and cautions and limitations for each. In addition, it covers engineering considerations, surface preparation, constructability, environmental factors, quality assurance/control, and safety.


Determine the Type of Crack

ACI categorizes cracks as occurring either in plastic concrete or hardened concrete. Cracks that occur in plastic concrete are usually determined to be static. While cracks that occur in hardened concrete can be either static or active.
Plastic shrinkage cracking is usually associated with the rapid loss of moisture caused by a combination of factors that include high air and concrete temperatures, low relative humidity, and high wind velocity at the surface of the concrete. These cracks are most often found in any corner that forms an angle of 180˚ or less (re-entrant corner) or around circular objects in the slab. These cracks are narrow and extend through the depth of the slab.
Concrete Settlement in Concrete Repair
Plastic settlement cracking occurs when the concrete is in its final stage of set. The material often continues to consolidate after initial placement due to nearby vibration or finishing. The cracks occur when the fresh concrete is locally restrained by reinforcing steel, a previous concrete placement, or formwork causing a tension crack.
Drying Shrinkage in Concrete Repair
Drying shrinkage is caused by the loss of moisture from the cement paste. This action can cause the slab to shrink by as much as 1%. Fine cracks called surface crazing, that look similar to a spider web, are one type of premature drying crack. These are not a cause for concern except in terms of aesthetics.
Thermal Cracking in Concrete Repair
Thermal cracking occurs when there’s a significant temperature difference within a concrete placement. This situation could be caused by portions of the structure losing heat or hydration at different rates or by the weather conditions cooling or heating one portion of the structure to a different degree or at a different rate than another portion of the structure. These temperature differences result in differential volume changes.
Environmental Factors - Freeze Thaw and Heat Cracks in Concrete Repair
Cracks can be caused by environmental factors such as freeze-thaw or even extreme heat. These cracks occur when the soils around the concrete freezes, and as a result it expands. This action can lift even the heaviest slab, only to drop back to its original elevation. Under extreme heat, the concrete expands pushing against the soil. In each case, since concrete is not flexible, it may crack. Expansion joints address this problem by creating a flexible separation between adjacent surfaces
Cracks can also be caused by design error. The ACI cites poor design examples that include:

  • Not designing the slab or floor for the current load
  • Poor detailing on re-entrant corners
  • Lack of adequate contraction joints
  • Improper design of foundations 

These errors can result in unanticipated differential movement within the structure. These conditions must always be reviewed by a structural engineer prior to repair.

 

Support from the Experts

Most repairs on cracks and other types of surface defects can be easily handled by experienced professionals. Our White Cap experts have the experience and know-how to aid in your approach to each repair. They can recommend the right type of equipment, repair material and safety supplies that can help make each job a successful one.  
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