The design of repairs to concrete structures presents special concerns that are not generally associated with new construction. These include:

Success in this business is very dependent on experience, but, since many concrete repair projects contain unique problems and unique constraints, directly applicable past experience needs to be supplemented by on-the-project mockups during design and prior to production.

On most non-routine concrete repair projects, it is prudent to include a mockup phase during design development to construct trial repairs as a test of the viability of the concepts prior to final design. In addition, during construction it is wise to require the contractor to construct prototypes of the repairs prior to beginning full-scale production to allow a final study of the repair parameters and to set an agreed standard for comparison to production repairs. These mockup phases are directed at removing the "repair" environment unknowns listed above.

Many of the difficulties encountered on concrete repair projects are associated with the basic condition that the concrete is unsound. Often it becomes difficult to determine the difference between concrete requiring removal and that which can remain in place.

Many of the efforts of the restoration engineer are directed at determining the competency of and the problems with the insitu concrete. Various non-destructive tests are conducted and sometimes core samples are taken from the concrete for visual and microscopic examination.

However, when it comes to the time for repair, there are many important characteristics of the substrate concrete that are not fully revealed by the standard condition appraisal techniques including:

1) the ease of concrete removal and
2) the extent of reinforcing steel deterioration.

During mockup of conceptual repairs, a subjective measure of the difficulty to remove concrete can be made. Typically, the distressed concrete is easy to remove.

However, it is typical to require additional concrete removal from the repair cavity to completely expose the reinforcing steel. This sometimes requires excavation of non-distressed concrete around the back of the reinforcing steel. This is typically hard work, and a mockup repair should help avoid future claims from the contractor that the concrete is "harder" than he assumed in his bid. The concept that the good concrete is "too hard" is unlikely to gain the sympathy of the engineer and the owner. The mockup demolition also exposes large areas of reinforcing steel so that the damage to the steel can be assessed and proper details and planning made for its supplement.

The ability to achieve controlled localized removal of deteriorated and debonded concrete toppings is influenced greatly by the degree of bond at undamaged areas of the topping. If the bond is marginal, removal of the damaged areas may cause debonding of the topping around the repair area thereby causing repair quantities to grow. If the topping is well bonded to the substrate, removal at the perimeter of damaged areas may be difficult to accomplish without damaging the substrate. This is especially true for thin precast concrete substrates, like hollow-core plank. Special removal techniques, equipment, and sequencing may be required to control repair quantities or avoid unnecessary substrate damage. Mockup demolition will determine the best techniques, equipment, and sequencing necessary for a successful project.

In older structures (+50 years) the demolition during mockup sometimes reveals very soft but competent concrete with large rounded aggregates and lack of compaction that is that requires special preparation to receive and hold a patch. As with bonded toppings, such concrete may require special removal techniques, equipment, and sequencing to control repair quantities and avoid unnecessary substrate damage. Some older concretes may be shown during mockups to greatly benefit from the use of bonding agents.

The business of concrete repair is a young business that is experiencing rapid growth. Only recently have industry organizations begun to address the development of repair standards. ICRI is leading the way, but much more work is necessary.

The literature abounds with representations of minimum bond strength recommendations for concrete patches and for attachment of protective coatings to concrete. However, like many quantitative data in this young industry, the tests bases for the data differ greatly and are usually so poorly defined as to preclude their use in contract performance specifications.

Currently, the best way to measure whether a combination of substrate preparation, material properties, and material installation will result in adequate attachment of the patch or coating is to actually install the repair in a mockup and to conduct destructive tests. If the failure occurs in the substrate this indicates that the bond of the repair material is at least equal to the internal strength of the substrate.

During predesign mockups and preproduction mockups and throughout the time of full-scale production, pull-off test devices are useful for measuring bond performance. The device shown in Photo 1 utilizes a core cut through the patch and into the substrate that is attached via an epoxy-fastened lug to a screw jack with a load cell to measure the tensile force required to pull the patch from the substrate. The most important feature of this test is the observation of where the failure occurs - at the bond line or in the substrate. The quantitative results obtained from the load cell can also be compared to similar testing information.

Some published bond test results and recommendations for bond tests indicate that a minimum bond strength of 150 psi should result in acceptable performance; however, in our experience running bond tests on over two dozen projects, we often see pull-off test results less than 150 psi even though many of these tests have resulted in failure within the substrate. Based on our experience with large-scale concrete overlays, especially our investigation of large failures, we strongly recommend that all contractors insist on a surface preparation mockup with significant bond testing of the overlay to secure pre-approval from the engineer and owner.

Photo 2 shows a smaller pull-off testing device specially designed to test the adhesion of coatings to a concrete substrate. This device also allows a quantitative measurement of the bond strength, but more importantly it allows retrieval of a sample for examination to determine if the failure occurred at the interface between the coating and the concrete or within the concrete under the coating.

Numerous articles have been written about preparation techniques for concrete repair. These techniques range from simple chipping hammers to complex water demolition systems. The availability of these techniques however is limited by the experience of the local repair contractor market; most repair contractors only possess equipment and experience with one or two preparation techniques. The concrete repair designer will best serve his client if he can incorporate the available preparation techniques into his design rather than exotic preparation equipment that must be imported, unless the job is especially large or demanding. During the predesign mockup various local preparation techniques can be observed and their performance measured. Prior to the start of full-scale repair production, the installation contractor can be given the opportunity to present his most productive preparation techniques for review and acceptance.

Mockups during design and prior to full-scale production allow the engineer to examine the expertise of the local contractors with different placement techniques. In general, concrete repair placement can be broken down into three categories: hand-applied, formed, and spray-applied. Our experience indicates that the formed repairs are generally very reliable, but hand-applied and spray-applied (both low pressure and shotcrete) repairs are very dependent on the installers’ skill. This skill can be demonstrated with mockups.

The need for mockups is especially great with the spray-applied systems. In our geographic area, we have found only a few contractors capable of constructing good patches with spray-applied techniques. Generally, the contractors ability to successfully install with spray techniques can be proved by on-site observations and testing of mockups. Recently developed spray-applied materials and equipment, especially low pressure systems, should be carefully scrutinized during the mockup. Multi-layer applications should be sounded and cored to confirm good interlayer bond.

On large hand-applied repairs, the repair are should be carefully scrutinized for surface cracking to determine if appropriate curing is occurring with the mockup techniques. Curing problems are especially prevalent on exterior facade repairs exposed to wind, and several curing techniques should be tried during the mockup.

Regardless of the installation technique used, the mockup should be disassembled to determine the adequacy of bond and the degree of compaction achieved in the repair material.

There are many competing products in the repair market both for concrete patches and concrete coatings. Each product usually possesses unique performance characteristics, and there are usually several products that are adequate for any project. Mockups are useful to allow evaluation of the material performance. On many occasions, the contractor will propose alternate materials which he can obtain at much less cost than the specified materials. Since the industry does not have well developed standards to allow direct comparison of competing materials measurements of the performance in a mockup should be used gather information helpful in evaluating the proposed substitution.

Concrete repair projects almost always involve occupied buildings, and the building occupants are generally very sensitive to the disruption of construction. Typically demolition noise and vibration is not a problem on residential building projects since the occupants are away during the day, but in office structures and in hotels we have found that demolition disturbance can be a significant problem. During mockups, several different demolition techniques can be tried to measure the noise and vibration impact. This may allow the specification of the least disruptive method. Of equal importance, the mockup is useful in preparing the building users for the level of disturbance that will occur during the repair.

An even larger problem than demolition disturbance is the sensitivity of building occupants to odors associated with many products used in concrete repair. Many of the solvent-based concrete coatings used in the past have resulted in building evacuations and project shut-downs when fumes were not adequately controlled. The relatively recent movement to the use of water-based and solventless coatings in confined repair areas has eliminated the concern about solvents; however, even though such materials are less dangerous, addressing complaints by occupants sensitive to the odors of such materials can disrupt a project. Mockups can be used to demonstrate the odors that will occur during construction and allow comparison of the odor problems with different products.

During the mockup, it may also be possible to try different ventilation techniques to reduce the odor problems. Care should be taken during the mockup phase to determine if the building under repair has a positive or a negative ventilation system; we once repaired an exterior wall on a research building that had a negative pressurization that directed the fumes from a single tube of epoxy directly into the occupants’ work area resulting in a two-day suspension of the work.

Most concrete repair projects are complex. They involve optimization of concrete removal and replacement, and, since removal and replacement of concrete is so expensive, numerous compromises are usually considered to control repair cost. Sometimes it becomes necessary to prove the adequacy of a marginal repair concept through testing. For instance, recently we have been involved in several projects requiring the repair of spandrel beams above the window heads with access only at the exterior of the building. This has necessitated complex formwork and unusual concrete placement techniques as well as unusual coordination with window attachments and window sealants. The mockup is useful to allow development of the necessary unique formwork, and experimentation with various placement techniques. This work is unusually labor intensive and; therefore, it is difficult to predict cost of the work in the absence of a mockup.

Recently, we have been involved with projects with new repair materials like fiberglass wrapping, carbon fiber reinforcement, and topping rebonding with gravity fed resins. The use of materials and techniques without long histories is a risk that is sometimes justified. The use of mockups during development of repairs with new materials and techniques or materials and techniques with which the engineer is not experienced can allow proper measurement of the risk and allow development of details to mitigate the risk.

Because many concrete projects are unique or involve techniques and materials for which there is little experience, mockups serve as useful indicators of the ultimate repair cost. The ultimate goal of the concrete repair designer is to achieve a fair price for the repair work for his client. When projects involve unique operations and or unique materials, there is a tendency for contractors to pad their prices to account for the risk of the unknown.

Mockups allow a period where the contractor can experiment with the unknown at little-to-no risk. The information gained during the mockup will allow both the contractor and the engineer to develop more realistic budgets for consideration of the owner. There is always the risk that the mockup will demonstrate to the contractor that the repair is more expensive than he would have imagined without the mockup, but if the ultimate goal is fairness, a better construction project will result from the added knowledge gained during the mockup.

Perhaps the most important feature of a mockup program is it allows the engineer and the repair contractor an opportunity to demonstrate to the building owner examples of the repair to confirm that the building owners’ expectations for the repair project, both in terms of level of disruption and aesthetics, can be met.

Most concrete repairs do not match well the original concrete. Patches and crack repairs are typically very unsightly. The need to camouflage repairs coupled with the need to develop greater impermeability to the environment has resulted in the increased use of coatings on exposed-to-view concrete. Thin concrete coatings and vehicular traffic bearing waterproofing will not hide and often accentuate substrate surface irregularities. As such, the finished coating will not have the perfect appearance of the manufacturer’s sales samples. Achieving such an appearance, if even possible, would require widespread and expensive filling and grinding of surface irregularities. Different coating material and installation techniques can be used, but one of the most important decisions about the coating is its final appearance. Large and predominant areas of the building should be treated in a mockup to allow the owner to choose color and texture, and to serve as the acceptance standard for final coating appearance.

Each mockup must be tailored to the needs of the individual project needs, but the following should be considered as a checklist when planning for one: