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Figure 1: Concrete Repair Process

For all its seeming permanence, concrete comes under attack from both natural and man-made forces almost from the time it is first formed and placed. The relative rate of progress of the degradation resulting from these assaults depends on a wide variety of factors... some controllable, some not.

In a refinery setting, the response to the progressive levels of degradation, framed mostly within the context of repair and economics issues, poses a set of challenges that are vastly different (and more complex) than those encountered in new construction. Owing to an array of physical forces -- many of which are only recently being studied and therefore not widely understood -- the underlying danger in undertaking repair efforts is that the repairs themselves, rather than producing a solution, may become part of a costly cycle. In effect, continual repair of previous repairs.

Defined in the simplest possible terms, successful concrete repair integrates new materials with existing materials to form a composite structure that can withstand environmental conditions and operational processes, while at the same time providing extended service life. Taking this definition one step further, for refinery applications a successful repair undertaking is also one in which normal operations and processes are allowed to continue while the repairs are taking place.

Complicating this even more is the fact that most of the engineering and maintenance experience and assets in a refinery are focused, on those elements and processes that produce profits. Equipment and process systems dominate maintenance budgets. The structural components that house or support those systems are fairly far down on the list of everyday priorities, particularly when considering that whatever structural problems exist may not be readily visible or apparent. Indeed, among those who specialize in the business of recognizing, understanding, interpreting, and diagnosing such anomalies, a phrase sometimes used is "a solution in search of a problem."

Without exception, however, concrete degradation problems do exist to some degree. The only variables are how and when they will manifest themselves, and whether they will have reached the stage where they jeopardize ongoing operations and have an impact on refinery profitability.

The notion that the degradation of concrete, a subsidiary component of the infrastructure of a refinery, might somehow lead to interrupted operations should serve as a wake-up call for facilities managers and engineering staff. Their level of awareness leads them to question how and why concrete degrades, and how to address problems in the most cost-effective and least intrusive manner.

The answers to those questions are still evolving. They have been the subjects of numerous comprehensive and wide-ranging technical abstracts. Here is a helpful review of what concrete actually is... and how, where and why concrete degradation typically begins.

Concrete... A Primer

Figure 2: What is reinforced concrete?

Despite it being a material that has mighty potential to cause expensive headaches, concrete is a relatively ordinary substance. Its basic composition has been around for almost 200 years. According to the Portland Cement Association, portland cement, which comprises 11 percent of the composition of concrete, was first patented in England in 1824. Joseph Aspdin, its inventor, named it "portland" due to its resemblance to stone that was found in a quarry on the Isle of Portland. Today, portland cement is described by PCA as a calcium silicate cement.

There are eight types of cement, the most common of which is Type 1, a general-purpose cement used in concrete for buildings, bridges, and pavements (see fig. 2). Other kinds of cement are manufactured for specialized applications such as dams, blast furnaces and sulfur pits.

In addition to the 11% portland cement component, the remaining ingredients that make up concrete are 41% gravel or crushed stone, 26% sand, 16% water, and 6% air. Concrete is formed with a mixture of paste and fine and coarse aggregates. The paste, a blend of cement and water, adheres to the surfaces of the coarse and fine aggregates - the sand and gravel. A process called "hydration" then begins as the paste hardens and the whole mixture eventually assumes the final form of concrete.

There are any number of variables that can affect the strength and integrity of concrete. For example, water with too many impurities or with chemical compounds beyond certain thresholds will affect its durability. (Water quality also contributes to the corrosion of reinforcing steel, a major factor in degradation; more on that to follow.) In addition, the relative size and coarseness of the aggregates plays a role in the size and thickness of the structural components in which they are to be used.

Figure 3: Drying Shrinkage

Figure 4: Excessive Loads

Figure 5: Cause: Carbonation

As the concrete sets and hardens, it is undergoing hydration. At some point, the material will have hardened to a stage where hydration transitions to a process called "curing." (see fig. 3) Curing is actually a continuation of hydration, and proper curing is absolutely critical to the integrity and strength of concrete. In its simplest terms, curing is the rate at which the concrete gives up its moisture content and thus hydrates. According to the PCA: "curing has a strong influence on the properties of hardened concrete such as its durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing..."

History Lessons

Most if not all of the factors listed just above play critical, interrelated roles in the degradation of concrete. Obviously, an engineering staff faced with a problem today cannot go back and rewrite plant construction history - how the concrete in the infrastructure of a given refinery was mixed, poured or cured. The problems exist here and now, and hindsight will be of little consequence, except to the extent that knowledge of the properties of concrete will have a bearing on determining the best repair strategies.

Such quality control issues might be only one part of an inherent myopia that may have been in place at the time of the original construction, and is manifesting itself today in the form of shortened service life in infrastructure components.

Modern specialists in the field of concrete repair look back and point to a range of possible shortcomings that contribute to the epidemic necessity for repairs to refineries today:

• Inappropriate structural system selection.
  
  
• Inadequate design details.
  
  
• Lack of clear direction during design implementation in the construction process.
  
  
• Lack of consistent quality control during construction.
  
  
• Lack of standardized educational tools, such as guidelines for repair of concrete problem areas.

Unfortunately, only hindsight is 20/20, but it can be useful in avoiding the same mistakes in future concrete construction and repair strategies.

Physics and Chemistry Lessons

In addition to the institutional mistakes contributing to concrete degradation, a range of other factors comes into play. Some of these are man-made and controllable. Others are tied to the forces of nature and are therefore largely beyond anyone's control -- except to the extent that a structural engineer might have taken those forces into account, and included design elements to mitigate them when they occur.

Freeze and thaw cycles, are vagaries of unpredictable weather patterns. Some years will be worse than others, and an unforeseen period of consecutive bad years would certainly hasten degradation. Earthquakes and floods, are other examples of natural disasters that may have been impossible to predict and take into account. Likewise, high winds or extended abnormal extremes of heat or cold are difficult to predict with any degree of certainty.

Man-made forces affecting degradation fall into two general categories: those that cause deterioration largely because less knowledge about the subject was available 40 years ago; and those that cause degradation due to obvious design deficiencies, or simply because of neglect. For instance, if acid were allowed to leak into the soil around a supporting pier, it could degrade the concrete. If chemicals or some other mildly aggressive agents spilled onto a concrete surface and were not properly cleaned up in a timely manner, they could cause degradation, or exacerbate an already existing problem. High-pressure or high-temperature venting, plus such physical forces as flexing, overloading or repeated impact, are other potential contributors to the deterioration process.

First and foremost among all the causes of concrete degradation, however, is the corrosion of the embedded reinforcing steel. In addition to deterioration of the steel itself, the corrosion also affects the concrete surrounding it, resulting in cracking, spalling and delamination. Since virtually all of the concrete found in refineries and other industrial structures is steel-reinforced, it is safe to assume that this is a widespread problem.

Corrosion of reinforcing steel, and the related damage to the surrounding concrete, falls into the man-made categories. Some degree of the deterioration may simply be inevitable, while some of it may have been avoidable with more knowledgeable or more farsighted design specifications. Even so, the science surrounding the phenomenon is still evolving, which makes it hard to fault architects and engineers whose knowledge was limited by the scanty data available at the time. And, as always, finding fault and pointing fingers is essentially counterproductive when engineers today are confronted with serious repair problems. The key is to learn from previous misconceptions or lack of knowledge, and to avoid the same pitfalls during the repair process.

What you See is Not Necessarily What you Get

Unfortunately, the ways in which concrete deterioration manifests itself typically do not indicate the true depth, complexity or severity of a problem. Virtually from day one, concrete comes under attack from environmental factors, and the deterioration process is both insidious and continuous. The very first small crack in its protective film invites intrusion by moisture or corrosive agents, and eventually, almost inevitably, the outward symptoms of scaling, cracking and spalling gradually begin to appear.

The question is not necessarily if but more likely when the degradation will approach a stage where either the damage itself, or the drastic steps needed to repair it, will jeopardize the ongoing operational business of the refinery, thus becoming an even more expensive repair scenario.

As managers and engineers examine repair strategies, will they choose wisely? Will they look at a repair strategy only as a way to sidestep process interruption? Or will the repair strategy include a commitment to learn from the lessons of the past, and address problems with the goal of ensuring the long-term integrity and service life of the refinery's infrastructure?

Neither the questions nor the answers are simple. What is clear, though, is that degrading concrete is a lurking threat that refinery owners and management teams cannot afford to ignore.

Part II: Choosing a Solid Concrete Repair Strategy
Concrete Degradation in Refinery Operations, Part II