A Complete System Philosophy for Concrete Repair

Significant progress has been made in understanding the durability of concrete repair, yet it still remains the foremost problem facing the industry today. We have only to look at our newly repaired bridges, parking structures, and buildings to see that we do not yet have adequate answers. Spalling, cracking, rust staining, and corrosion of reinforcing steel are all visible problems we face today.

Many, if not most, durability problems arise from lapses in establishing realistic design objectives, or a lack of quality in the design or construction. In other words, they are reliability-related problems. The corollary of this is that solutions can be achieved through higher-quality design engineering, better supervision and quality control, and better-educated and better-trained staff, in addition to the development of more advanced materials.

At its heart, concrete repair is a complex process, presenting unique challenges that differ from those associated with new construction. The repair process must successfully integrate new materials with old, forming a composite system capable of enduring exposure to service loads, the environment, and time. The durability of this composite system must be ensured on a systematic basis that includes research, condition evaluation, design objectives, detailed design, material selection, construction practices, and quality control.

Yet, despite the large and expanding restoration and rehabilitation market, little has been done to establish a sound methodology for design and construction of durable repairs that will last well into the next 20 years.
Concrete repair is not a "band-aid" to a structure in trouble; it is a complex engineering task. And only a system approach in design and implementation of the repair project will allow us to achieve the desired quality. Without such rational analytical tools, durable concrete repair will remain more of an art than a reliable technology.

Total system concept

A repaired concrete structure is a composite system of interacting materials exposed to the interior and exterior environments of a structure. The system is represented structurally by three basic subsystems: the existing substrate, the repair material, and the transition zone between them; see Figure 1. Similarly, the process of implementing a concrete repair project is also a system which encompasses the following important subsystems:

• assessing the condition of an existing structure;
  
  
• assessing the cause(s) of deterioration/distress;
  
  
• establishing the nature and severity of the interior environment in the existing structure;
  
  
• ascertaining the probable service life of the repaired structure;
  
  
• establishing realistic design objectives;
  
  
• selecting an appropriate repair system;
  
  
• developing repair details and specifications; and
  
  
• implementing the repairs as specified.

A total-system approach to repair projects will ensure that no part of the system is overlooked, and will take into account the concurrent interaction of a variety of factors. Unfortunately, the way we often approach repair/rehabilitation projects may be described by two extremes: unorganized complexity or organized simplification. In both cases, the unfortunate results are well-known.

With a system approach, however, the effective interaction of all subsystems in a repair project is somewhat similar to a jigsaw puzzle; see Figure 2. To put the puzzle together successfully, it must be workable. In other words, if not enough attention is paid to each element composing the concrete repair system, the task is not solved and the project will be a failure.

Condition evaluation

The ideal concrete repair project should start with a comprehensive condition evaluation of the existing structure, the establishment (together with the owner) of realistic project objectives, and a definition of the performance criteria related to the existing and expected interior environmental conditions.

To guarantee the best future performance of a repaired concrete structure, events that presently threaten the structure's durability or may threaten it in the future must be identified. The design of durable repaired structures with realistic performance objectives will then have to be concentrated on two parallel activities: ensuring adequate resistance to the predicted internal and external environmental effects, and providing adequate structural capacity and safety under the expected loading.

Because the concept of durability of a repaired structure can be difficult to quantify and to use in practical design, very rarely is a repair-project objective related to prevention of further deterioration. A more realistic objective is to ensure sufficiently slow deterioration, or to prolong the next remedial action for as long as possible. But how can we adequately predict the future service life of the repair? Ideally, this estimate should be based on rational engineering analysis, knowledge from observations, and through trial and error.

Compatibility of materials

To understand how various physical factors affect the performance of a composite repair system, we must refer to the term "compatibility." The meaning of compatibility in concrete repair systems relates to a balance of physical, chemical, and electrochemical properties, as well as deformations between the repair and substrate (as shown in Figure 3), all of which guarantee that the system as a whole can withstand stresses induced by restrained volume changes and chemical and electrochemical effects without premature deterioration or distress over a designed period of time.

One of the most important components is the dimensional compatibility of repair materials, with the existing substrate including the following factors:

• drying shrinkage of the repair material;
  
  
• thermal expansion or contraction differences between repair and substrate materials;
  
  
• differences in modulus of elasticity (could cause unequal load-sharing and strains, resulting in interface stresses);
  
  
• creep properties; and
  
  
• relative fatigue performance of the components in the composite repair structure.

Incompatible materials may result in initial tensile stresses that either crack the repair material or cause de-bonding at the repair-substrate interface. The menu of materials for particular applications is becoming increasingly extensive as new technologies develop. But even the latest wonder materials (like silica fume and high-range water reducers), while very useful in some applications, can have a negative impact on others. For instance, a "high-performance" repair material that delivers high compressive strength may have a negative effect on durability.

The worst sin in an engineering material is not lack of strength or stiffness, but lack of resistance to initiation and propagation of cracks. One can allow for lack of strength or stiffness in design, but it is much more difficult to allow for cracks, which are life-threatening wounds to concrete structures.

Another critical problem is the often unjustifiable use of high-early strength materials. It may be necessary to use such materials for some special applications, but not for "conventional" repairs where it creates a great potential for higher shrinkage and cracking.

Field practices

If we are going to see improved durability, we not only need to design and specify repairs correctly, we need to ensure that the requirements written into the specifications concerning materials, surface preparation, and application and curing really are observed. We need to learn to supervise and control the repair operations. The use of adequate design and repair materials is of critical necessity, but they are not a substitute for getting the basics of field execution and quality control right.

Many manuals and instructions for inspectors have been prepared, and there is a wider effort underway in training for this critically important work. But this is an area of the concrete repair system we must continue to improve. Adequate quality control is a foremost "must" if we are going to get what we are paying for.

Science, research, and education in concrete and concrete repair technology also are critical elements of the concrete repair system. An important step in improving research and education is to establish a more penetrating dialogue between the concrete repair industry and the universities. In addition to the fundamental research performed by the universities, applied research and development needs more attention. Education of future engineers needs stronger input and participation from the industry in order to push the sector forward.

Conclusion

A total-system concept in regard to the design and implementation of concrete repair projects?with required durability under conditions defined in advance?has become a task of first magnitude. Every means of rendering concrete repair technology more effective has an enormous economic and engineering significance, considering the present-day volume of concrete structures in need of repair.

About the Authors

Peter Emmons is CEO of Structural Group, the Baltimore-based company he founded in 1976. He has more than 30 years of experience in structural concrete repair and is considered to be among the industry's foremost authorities. He can be reached at pemmons@structural.net.

Alexander Vaysburd, Ph.D., is the president of Vaycon Consulting in Hanover, Md., specializing in consulting and technical management services in concrete repair technology. He can be reached at avaysburd@structural.net.