Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. The production of replacement building materials depletes natural resources and can produce air and water pollution.
Concrete resists weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and the properties desired. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of the concrete.
The heavily traveled Wacker Drive replacement in downtown Chicago was designed for a 75- to 100-year life.
The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Because of their durability, most concrete and masonry buildings are demolished due to functional obsolescence rather than deterioration. However, a concrete shell or structure can be repurposed if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters.
Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor.
These 3-by-5-foot concrete panels with decorative finishes were displayed outdoors in the relatively severe weather in the Skokie, Illinois, area (near Chicago). With only a few exceptions, their appearance changed very little after more than 40 years of exposure to bright sunlight, wind, snow, acid rain, freezing and thawing, hot summers, and cold winters
Factors Related to Concrete Durability
High Humidity and Rain: With little to no organic content, concrete is resistant to deterioration due to rot or rusting by in hot,humid climates. Moisture can only enter a building through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Walls need to breathe or concrete will dry out if not covered by impermeable membranes.
Portland cement plaster (stucco) should not be confused with exterior insulation and finish systems (EIFS) or synthetic stucco systems that may have performance problems, including moisture damage and low impact-resistance. Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.
Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm concrete. Using colored pigments in concrete retains the color in aeshetic elements (walls or floors, for example) long after paints have faded due to the sun’s effects.
Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, vermin and insects will not bore through concrete.
Moderate to Severe Exposure Conditions for Concrete: The following are important exposure conditions and deterioration mechanisms in concrete. Concrete can withstand these effects when properly designed. The Specifier’s Guide for Durable Concrete, EB221 and Design and Control of Concrete Mixtures, EB001.15 are intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments.
Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both.
When it has a proper system of microscopic air bubbles, obtained through the addition of an air entraining admixture and thorough mixing, concrete is highly resistant to freezing and thawing. These microscopic air bubbles within the concrete accommodate the expansion of water into ice and thus relieve the internal pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8 percent of properly distributed air voids will withstand a great number of cycles of freezing and thawing without distress.
Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is regularly used for the construction of waste water transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products.
However, concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is especially prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids attack concrete by dissolving the cement paste and calcium-based aggregates. In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments, IS001, discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack. Read more on acid resistance.
Resistance to Sulfate Attack: High amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to slowly cause disintegration of the concrete.
Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate.
Sulfate attack and salt crystallization are more severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, concrete with a low water to cementitious material ratio (w/cm) (less than 0.45 for moderate sulfate environments and less than 0.40 for more severe environments) should be used along with cements or cementitious material combinations specially formulated for sulfate environments
Confederate Bridge, spanning the Northumberland Strait between Prince Edward Island and NewBrunswick, was specifically designed for high durability in a severe environment and a 100-year life. The bridge has to resist freezing and thawing, seawater exposure, and abrasion from floating ice.
Seawater Exposure: Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over reinforcing steel must be provided, and the water-cementitious ratio should not exceed 0.40.
Chloride Resistance and Steel Corrosion: Chlorides present in plain concrete (that which does not contain reinforcing steel) is generally not a durability concern. In reinforced, the paste protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually (greater than 12.5) causes a passive protective oxide film to form on steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion threshold is reached, an electrochemical current is formed along the steel or between steel bars and the process of corrosion begins.
The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increased by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection.
Resistance to Alkali-Silica Reaction (ASR): Alkali-Silica Reaction (ASR) is an expansive reaction between certain forms of silica in aggregates and potassium and sodium alkalis in cement paste. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. Alkali-silica reaction can be controlled through proper aggregate selection and/or the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction. With some reactive aggregates, controlling the concrete alkali level has been successful. Lithium-based admixtures have also been shown to prevent deleterious expansion due to ASR. Standard Guide for Reducing the Risk of Deleteerious Alkali-Aggregate Reaction in Concrete, ASTM C1778, provides thorough guidance.
Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 pounds per square inch (psi) work well.