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INTRODUCTION AND SCOPE

Concrete is one of the most widely used construction materials in the world. This fact attests to concrete's performance as a versatile building material. Durability represents one of the key characteristics of concrete that has led to its widespread use. Durability of hydraulic-cement concrete is determined by its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration. Durable concrete will retain its original form, quality, and serviceability when exposed to its environment. Properly designed, proportioned, placed, finished, tested, inspected, and cured concrete is capable of providing decades of service with little or no maintenance. Certain conditions or environments exist that will lead to concrete deterioration. Attacking mechanisms can be chemical, physical, or mechanical in nature, and originate from external or internal sources. Chemical and physical attacking mechanisms often work synergistically. Depending on the nature of attack, distress may be concentrated in the paste, aggregate, or reinforcing components of the concrete (or a combination thereof).

The various factors influencing durability and the particular mechanism of deterioration should be considered in the context of the environmental conditions to which the concrete would be subjected. In addition, consideration should be given to the microclimate to which the specific structural element is exposed. Deterioration, or the severity of deterioration, of a given structure may be affected by its orientation to wind, precipitation, or temperature. For instance, exterior girders in a bridge structure may be exposed to a different and more aggressive environment than interior girders.

The concept of service life is increasingly used for the design of new structures. To provide durable concrete, the specific demands on the concrete in its intended use should be given careful consideration. Required service life, design requirements, and expected exposure environments (macro and micro) should be determined before defining the appropriate materials and mixture proportions necessary to produce concrete suitable for a particular application. The use of good materials and proper mixture proportioning will not necessarily ensure durable concrete. Appropriate measures of quality control, testing, inspection, placement practices, and workmanship are essential to the production of durable concrete. Properly designed testing and inspection programs that use trained and certified personnel are also important to ensure that durable concrete is produced. ACI has a number of certification programs that are applicable. This guide discusses the more important causes of concrete deterioration and gives recommendations on how to prevent such damage. Chapters on fresh concrete, freezing and thawing, alkali-aggregate reaction (AAR), aggressive chemical exposure, corrosion of metals, and abrasion are included. Fire resistance of concrete and cracking are not addressed in detail, because they are covered in ACI 216.1, 224R, and 224.1R, respectively.

Fresh or unhardened concrete can be consolidated and molded to the desired shape to serve its intended purpose. During this stage, a number of properties significantly influencing the durability of the hardened concrete are established. Pore structure development, air-void system formation, material mixing, placement and consolidation, curing, and minimizing or eliminating cracking of plastic concrete are all important to the ultimate durability of concrete.

Deterioration of concrete exposed to freezing conditions can occur when there is sufficient internal moisture present that can freeze at the given exposure conditions. Freezingand- thawing damage is a serious problem, and is greatly accelerated by the use of deicing salts. Fortunately, concrete made with high-quality aggregates, a low water-cementitious material ratio (w/cm), a proper air-void system, and that is allowed to mature before being exposed to freezing and thawing, is highly resistant to freezing-and-thawing damage.

Although aggregate is commonly considered to be inert filler, this is not always the case in a concrete environment. Certain aggregates can react with alkali hydroxides from cement and other materials, causing expansion and deterioration

Potential issues with AARs can often be identified by reviewing the historical performance of candidate materials under similar exposure conditions and by evaluation based on the appropriate laboratory testing techniques. Methods for mitigating AARs are: use of low-alkali cement, avoidance of reactive aggregate, use of sufficient levels of suitable supplementary cementitious materials (SCMs), use of sufficient levels of appropriate chemical admixtures, or a combination of these.

Sulfates and other salts in soil, groundwater, or seawater may have a deleterious effect on hardened concrete, but can be resisted by using suitable cementitious materials and proper quality control and curing of a properly proportioned concrete mixture.

Because the topic of delayed ettringite formation (DEF) remains a controversial issue and is the subject of various ongoing research projects, no definitive guidance on DEF is provided in this document. It is expected that future versions of this document will address DEF in significant detail.

Quality concrete can resist occasional exposure to mild acids, but no portland-cement concrete offers good resistance to attack by strong acids or compounds that convert to acids; special protection is necessary in these cases. Substances that are initially inert may convert to acids over time, and can also present durability issues. Corrosion of embedded steel reinforcement produces reaction products that occupy additional volume. This can lead to internal stress within reinforced concrete and subsequent distress. The spalling of concrete due to corrosion is a particularly serious problem.

Corrosion can also weaken reinforcing steel and thus reduce the structural capacity of concrete. One of the principal causes of reinforcing steel corrosion is the ingress of chloride made available from sources such as deicing salts. Ample cover over the steel and use of a low-permeability, air-entrained concrete will ensure durability in the majority of cases. More positive protection, such as epoxy-coated reinforcing steel, cathodic protection, noncorrosive reinforcement, or chemical corrosion inhibitors, is needed for severe exposures.

Abrasion can be a problem in industrial floors from traffic or process conditions. In hydraulic structures, particles of sand or gravel in flowing water can erode surfaces. The use of high-quality concrete and, in extreme cases, abrasionresistant aggregate, will usually result in adequate durability under these exposures. The use of studded tires on vehicles has caused serious wear in concrete pavements; conventional concrete will not withstand this damage. Impact caused by operating equipment, such as fork lifts and heavy equipment, will pulverize concrete of inadequate strength, and may require the selection of special aggregates and mixture proportions.

In summary, this document addresses durability by dividing specific modes of attack into separate chapters. This approach was taken for organizational purposes and for convenience to the reader. In practice, one will find that damage to concrete is often due to multiple distress mechanisms