In addition to the challenges of controlling day-to-day process operations, managers of coking units also face the equally critical issue of maintaining the structural integrity of the concrete support structures. It should come as no surprise that deterioration and damage to concrete support structures can have a severe and costly impact on coking operations, often leading to unforeseen and costly shutdowns. In fact, failure to monitor and remediate damage could lead to significant harm to the unit and its operators.

Concrete problems can stem from defects associated with design, construction, or materials. Additionally, there can be institutional factors that contribute to shortening the service life of even the best-intentioned concrete construction project. These barriers include:

• Inappropriate structural system selection;
  
  
• Inadequate design details;
  
  
• Focus on initial costs, not on life-cycle costs;
  
  
• Failure to implement design considerations during the construction phase;
  
  
• Inconsistent quality control during construction; and
  
  
• Lack of standardized educational tools, such as guidelines and specifications for repair of concrete problem areas.

Concrete can be damaged by manmade forces, such as impact, overloading, and physical attack, including the use of aggressive surface-applied chemicals. Concrete is also subject to degradation abrasion/erosion during process activities (i.e., repetitive tracking of hardened materials, high-pressure and/or -temperature process venting, and lack of timely cleanup of mildly aggressive chemical spills onto concrete surface.) Degradation in the form of corrosion of embedded metals is also prevalent when environmental conditions within the concrete are favorable for electrochemical processes.

Natural forces like erosion (due to wind, water flow, freeze-thaw cycling), earthquakes, floods, and fires can also affect the performance of concrete structures not adequately designed to resist these events.

The subject of concrete repair is complex. Concrete repair technology, as we know it, is less than 35 years old, although repairs associated with poor placement techniques have been around since the 19th century.1 To determine the appropriate repair approach, distressed concrete must be analyzed to identify the cause of the observed effect. Once that is done, the appropriate repair strategies should be carefully selected and tailored to meet the site-specific needs of the owner. Strategies can include surface repair, stabilization, strengthening, waterproofing, and protection. It's also possible these strategies can be combined on complex projects, such as the rehabilitation of large concrete structures.

For a concrete repair program to be considered successful, it must incorporate new repair materials with existing materials to form a composite structure able to withstand environmental conditions, process use, and extended service life requirements.

Repair Strategies

FIGURE 1: Typical overlay construction joint detail

Repair strategies can be classified into five main groups, each with several potential components.

• Surface repair: overlays, shotcrete, form and pump, hand applied, drypack, preplaced aggregate concrete, form and cast-in-place, and full member replacement;
  
  
• Stabilization: crack injection, void grouting, soil stabilization, post-tensioning cable/tendon repairs, and connection correction;
  
  
• Strengthening: stress relieving, enlargement, external post-tensioning, and composite bonding;
  
  
• Waterproofing: chemical grouting, expansion joints, waterstop repair, and sealant repair; and
  
  
• Protection: sealants, coatings, membranes, cathodic protection, and jacketing.

Most often, several of these strategies are implemented concurrently, as this maximizes value versus cost.

To further complicate the issue, each strategy and sub-strategy can require several materials and several different techniques/practices for installation. Matching the appropriate repairs to existing conditions requires the input of all parties involved. These accountabilities are described as follows:

• The engineer must understand the uncured (chemical) and cured (physical) properties of various repair materials if he is to specify the appropriate one.
  
  
• The repair material manufacturer must also understand the engineering aspects of how repair material will interact with an existing substrate under load-carrying conditions.
  
  
• The contractor must understand concrete deterioration mechanisms in order to understand surface preparation.
  
  
• The owner must be a generalist familiar with concrete problems and potential solutions.(1)

It is important to note that appropriate repair material selection, even when performed by seasoned repair designers, is an exercise in compromise. It's usually the case that no one material is likely to meet all needs, such as filling the repair cavity completely, exhibiting no shrinkage during curing, behaving in a manner similar to the existing substrate when subjected to loads, and responding in a similar manner to changes in temperature and moisture.

In light of this, repair designers must be prepared to answer the following questions: (1)

• What are the user performance requirements?
  
  
• What will be the service and exposure conditions?
  
  
• What are the load-carrying requirements?
  
  
• What will be the operating conditions during placement and cure?
  
  
• Has the original cause of deterioration been addressed?
  
  
• What placement technique is chosen?
  
  
• What characteristics are required for placement?
  
  
• What properties are required to meet the conditions and requirements?
  
  
• What materials or systems will provide the required properties?

Faced with this array of questions, it should be clear that the final selection of repair materials is made based on the relationship between cost, performance, and risk.

An example of the process of formulating and executing a repair strategy program to a technically sophisticated structure was done at a Midwest oil refinery in the United States. A conventional reinforced concrete coke drum support structure, built in 1970, was experiencing distress in the form of cracking, delamination, spalling, corrosion of embedded reinforcing steel, and surface scaling. A team was created to perform a complete tactile survey of accessible areas, in the process extracting concrete material specimens for analytical laboratory testing/examination. They went on to carry out a sophisticated three-dimensional finite element analysis, after which they conducted interviews with process engineers and technicians.

The information and data collected provided significant insight into the dynamics of the distressed structure, and offered guidance into addressing those problems. By addressing the distress mechanisms with a global focus and clear understanding of owner/process needs, a repair program was successfully designed and implemented without significant downtime to process operations.

Based on a clear understanding of the coke process cycle and the results of the structural evaluation and condition survey, the structure was definitely repairable while in service; this was an owner requirement. But to do so required extensive preplanning, significant owner buy-in and participation, and a knowledgeable contractor skilled in repair construction in an explosive environment and around and operating unit. (We believe that it is highly preferred - and should possibly be mandatory - that contractors working on an operating coking unit have specific experience in doing so.)

FIGURE 2: Typical octagonal slab opening repair

FIGURE 3: Typical corbel and switch deck soffit repairs

FIGURE 4: Typical formed deep repair detail for beams and columns

FIGURE 5: Typical switch deck crack repair detail

The daunting task faced by the contractor was to provide all labor, equipment, materials, and supervision for a modified lump-sum amount. The work was to be performed in accordance with project construction repair documents for a "one-of-a-kind" repair and guaranteed for a period of five years. The repair program has five main phases:

• Construction mock-ups of critical repair details;
  
  
• Performance of controlled production concrete demolition to remove distress concrete;
  
  
• Installation of rapid-setting concrete overlays, both horizontal and vertical (Figure 1, page 28);
  
  
• Repair of columns, beams, and octagonal slab edge by installing cast-in-place rapid-setting concrete repair sections (Figures 2-4, page 30 and above left);
  
  
• Repair of concrete cracks (routing and sealing techniques) using a flexible, chemical-resistant sealant; epoxy injection of cracks/joints where routing and sealing repairs are not appropriate2 (Figure 5).

The owner's responsibility was to maintain coking operations, perform liaison services between process and contractor personnel, and facilitate contractor requests for project document clarifications and scope-of-work changes.

The repair program was under way for approximately 18 months. During this period, the coker remained on is regular operating schedule. While the repair team was in charge of the work, the owner maintained a full-time presence on site in the form of a process technician located in the contractor's site trailer. Additionally, the owner opted for part-time monitoring by a structural consultant and a full-time quality control/quality assurance inspections carried out by both the consultant and contractor personnel during major repair material placements, both ready-mix and site-batched product installations.

Post-repair installation activities included direct axial tension bond testing of overlays and hammer sounding/chain dragging of repairs at 28 days after placement. Long-term monitoring of repair status during the five-year contractor guarantee period included a complete visual examination and hammer sounding/chain dragging of repaired areas on 12-month intervals. Defective areas, when encountered and defined, were removed and repaired under close scrutiny to determine the cause of the defect. Typically, these defects were debonding due to substrate contamination with coke oil residue. Over the course of the project, these defects totaled less than 1% of the area repaired and were confined to head deck repairs surrounding the coke chute penetrations.

In the end, the concrete repair team was able to provide durable repair designs that were used in ambient temperatures ranging from 0oF (-18oC) to 1400F (60oC); they were able to bond repair materials to vibrating concrete substrates; perform excavation/demolition of unsound concrete substrates in explosive environments, and complete the work on schedule. The success of this project came from understanding the owner's needs and requirements, close communication and coordination with all repair entities, and the desire by all parties to implement an enduring repair project.


References

1. Emmons, P.H., Concrete Repair and Maintenance Illustrated," Kinston, Mass.: R.S. Means Company Inc., 1993.

2. Kline, T.R., "Crack Repair: An Engineer's Perspective," Concrete International 13(6), June 1997, pp. 47-49.


About the Author

Tom Kline is manager of Structural Preservations Systems' (SPS; www.structural.net) Forensic Condition Survey and Evaluation Group. His team provides design-build capabilities and develops turnkey repair strategies supporting SPS's industrial repair construction operations. Kline has over 25 years' experience in repair construction. He is a member of the American Concrete Institute (ACI), ASTM, and the International Concrete Repair Institute (ICRI). He can be reached by phone (713) 433-2155 or e-mail tkline@structural.net.