Maintain concrete properly to preserve its strength
In today's lexicon, the word "concrete" has come to symbolize strength, stability, and the image of being set in stone. Yet, concrete in plants and industrial facilities comes under attack from both natural and manmade forces almost from the time it is first poured and formed. The relative rate of degradation resulting from these assaults depends on a wide variety of factors of which only some ...
In today's lexicon, the word "concrete" has come to symbolize strength, stability, and the image of being set in stone. Yet, concrete in plants and industrial facilities comes under attack from both natural and manmade forces almost from the time it is first poured and formed. The relative rate of degradation resulting from these assaults depends on a wide variety of factors of which only some are controllable.
A basic understanding of what concrete is, the lessons learned throughout its centuries of use, and how to strengthen and repair a concrete building is critical for maintaining safety and durability. This fundamental knowledge will provide the foundation for recognizing when your facility is in need of repair.
Over the course of the next few years, millions of dollars will be spent on plant and facility infrastructure and repairs that generate little to no revenue return. While integration of new equipment or an expansion project may add to the bottom-line, maintenance and repair of concrete structures is often overlooked, since it "produces no revenue." However, such benign neglect is dangerous, because concrete requires preventive maintenance.
Although the process of deterioration or damage often takes years to manifest itself, the damage accelerates as time progresses, developing into more expensive and extensive repairs. (See sidebar below, "Deterioration phases and maintenance costs.")
The response to the progressive levels of degradation, which is framed mostly within the context of repair and economic 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, are 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, but all too common, cycle of repairing the repairs.
To understand how to avoid becoming trapped in this vicious cycle, it is important to review what concrete actually is. This review helps determine the root causes of concrete degradation.
Concrete: a primer
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.
Portland cement, the most widely used type of cement was first patented in England in 1824 by Joseph Aspdin. He named it "portland" because of its resemblance to stone found in a quarry on the Isle of Portland. Today, portland cement is described by the Portland Cement Association (PCA) as a calcium silicate cement. There are eight types of portland cement, the most common of which is Type 1, a general-purpose cement used in concrete for buildings, bridges and pavements. Other kinds of cement are manufactured for specialized applications such as dams, blast furnaces and sulfur pits.
In its basic composition, concrete has been around for almost 200 years. Typically, a mix is about 10% to 15% cement, 60% to 75% aggregate, and 15% to 20% water. Entrained air in many concrete mixes may also take up another 5% to 8%. A common "standard" composition is 11% portland cement, 41% gravel or crushed stone, 26% sand, 16% water, and 6% air.
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. 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.
At some point as the concrete hardens and sets, the material will have hardened to a stage where hydration transitions to a process called "curing." In its simplest terms, curing is the rate at which the concrete gives up its moisture content and, thus, hydrates. Proper curing is absolutely critical to the integrity and strength of concrete. According to the PCA, "Curing has a strong influence on the properties of hardened concrete such as its durability, strength, water-tightness, abrasion resistance, volume stability, and resistance to freezing and thawing."
Most, if not all, of these factors play critical, interrelated roles in the degradation of concrete. Obviously, it is impossible for today's engineering team to know how the concrete of an infrastructure was mixed, poured, and cured. However, the knowledge of the basic properties of concrete will aid in determining the best repair strategies.
Natural and manmade challenges
In addition to the institutional mistakes contributing to concrete degradation, a range of natural and manmade factors come into play. Although structural engineers can include design elements that would help prevent damage from natural forces, often these elements are beyond control. For example, freeze and thaw cycles are at Mother Nature's whim. Some years will be worse than others, and an unforeseen period of consecutive bad years can hasten degradation. Earthquakes, floods, high winds, or extended abnormal temperature extremes are all examples of natural disasters that are difficult or impossible to predict.
Conversely, man-made forces affecting degradation are often controllable and fall into two general categories: those that cause deterioration because the people were less knowledgeable about concrete 40 yr ago; and those that cause degradation due to obvious design deficiencies or subsequent neglect. If acid were allowed to leak into the soil around a supporting pier, for example, it could degrade the concrete.
Similarly, 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 existing problem. High-pressure or high-temperature venting and physical forces such as flexing, overloading, or repeated impact are other potential contributors to the deterioration process.
Foremost among all causes of concrete degradation is the internal damage caused by the corrosion of embedded reinforcing steel (See Fig B). Besides deteriorating the steel itself, the corrosion also affects the concrete surrounding it, which results in cracking, spalling, and delamination. This is a significant issue since virtually all of the concrete found in structures is steel-reinforced. The science surrounding this phenomenon is still evolving. However, it is critical to learn from previous misconceptions to avoid these same pitfalls in the repair process.
Unfortunately, the methods 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 first small crack invites intrusion by moisture or corrosive agents. Inevitably, the outward symptoms of scaling, cracking, and spalling gradually begin to appear.
Concrete degradation always exists to some degree. The only variables are how and when they will manifest themselves, and whether they will have reached the stage at which they jeopardize ongoing operations and have an impact on profitability.
Choosing the right ingredients
Defined in the simplest 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. Further, a successful repair undertaking is also one in which normal operations and processes are allowed to continue while the repairs are taking place.
Once the decision is made to undertake a concrete repair project, the next step involves examining repair strategies and selecting the best course of action. Today's available technologies are advanced far beyond the simple concrete patch and a range of solutions can be utilized to implement an effective concrete repair program. A basic understanding of these options - surface repair, protection, stabilization, strengthening and waterproofing - will allow selection of the best program for your facility. A concrete repair specialist can help determine both the underlying cause of the problem and the optimal solution.
With so many possible strategies available to the repair designer, a typical repair strategy may implement solutions from more than one category in order to achieve the best results. Determining the most appropriate repair is best accomplished by involving all parties associated with the repair, including the engineer, contractor, processing manager and the facility's owner.
Factors such as ongoing facility operation and service life require consideration. Deciding on the most appropriate concrete repair materials is often an exercise in compromise. Even the most seasoned repair technicians find it difficult to select one product that meets all of a project's needs. At a minimum, the materials chosen should fill the repair cavity completely, avoid shrinking during the curing cycle, and behave in a similar manner as the existing substrate when subjected to loads, temperature fluctuations, and/or changes in moisture content (See Fig C).
Putting it all together
Facility maintenance budgets are typically well-funded for repair of equipment and process systems, but structural repair budgets are given a much lower priority - until a problematic structural condition deteriorates to the point where the plant process is jeopardized. (See Fig D)
As in any decision-making process, knowledge is power. Effective concrete repair solutions include a commitment to learning from past lessons and addressing problems with the goal of ensuring the long-term integrity and service life of a facility's infrastructure. "Value-added" services relating to concrete repair should be no different from those provided by equipment suppliers. Education of personnel concerning concrete material basics, deterioration mechanisms, repair strategies, and construction techniques will enable repair teams to make better repair strategy decisions and prevent costly repairs.
Failure to take preventive maintenance steps or properly address signs of deterioration can have a dramatic impact on the operations of a facility. Degrading concrete is a lurking threat that facility owners and management teams cannot afford to ignore.
Tom Kline is a graduate construction engineer with 23 years of experience in concrete distress and failure investigations. He is a member of ACI, ASTM, and ICRI. He serves on the ICRI Technical Activities Committee and is chairman of an ICRI task group on Monolithic Bonding of Concrete Repairs. Article edited by Richard L. Dunn, executive editor, 815-236-2196, or firstname.lastname@example.org .
Deterioration phases and maintenance costs
All concrete deteriorates, although the speed of deterioration can vary widely. The natural evolution of the concrete deterioration process and the influence of maintenance on this process can be divided into three phases, with associated costs increasing with each phase:
Preventive Maintenance Phase: In this phase, the owner may spend a fixed annual maintenance cost to install systems such as protective coatings to slow down the deterioration process. Money spent in this phase will delay the ingress of aggressive materials, thus delaying the start of active deterioration (Repair Phase).
Repair Phase: In this phase, the concrete deterioration has begun, and the repair cost curve increases exponentially over time. The reason for the rapid increase in cost is that once aggressive materials that cause deterioration have sufficiently permeated into the concrete (a process that may take 20 to 30 years), the deterioration rate is rapid and irreversible.
Replacement Phase: In this phase, a "wholesale" deterioration occurs throughout the structure at such a rapid rate that repair costs may exceed the costs of replacing the entire structure. However, total replacement of the structure may not be an option because of interruption to the function of the structure.
Prevalent causes of concrete problems
Modern specialists in the field of concrete repair point to a range of possible shortcomings that contribute to increasing urgency for repairs to concrete facilities 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
Change of structure use from the original design intent.
Keeping these factors in mind can be useful in avoiding the same mistakes in future concrete construction and repair strategies.
Questions to ask when planning repairs
During the material selection process, the repair designers should consider each of the following questions:
What are the performance requirements?
What are 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 the deterioration been addressed?
What placement techniques have been selected, and what characteristics are required for placement?
What properties are necessary to meet the conditions and requirements?
What materials or systems will provide the required properties?
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