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Durability

Durability is the ability to last a long time without significant deterioration. A durable material helpsthe environment by conserving resources and reducing wastes and the environmental impacts ofrepair and replacement. The production of replacement building materials depletes naturalresources and can produce air and water pollution.

Concrete resists weathering action, chemical attack, and abrasion while maintaining its desiredengineering properties. Different concretes require different degrees of durability depending onthe exposure environment and the properties desired. Concrete ingredients, their proportioning,interactions between them, placing and curing practices, and the service environment determinethe ultimate durability and life of the concrete.

The heavily traveled Wacker Drive replacement in downtown Chicago wasdesigned for a 75 to 100year life.

The design service life of most buildings is often 30 years, althoughbuildings often last 50 to 100 years or longer. Because of their

durability, most concrete and masonry buildings are demolished due to functional obsolescencerather than deterioration. However, a concrete shell or structure can be repurposed if a buildinguse or function changes or when a building interior is renovated. Concrete, as a structuralmaterial and as the building exterior skin, has the ability to withstand nature’s normaldeteriorating 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. Differentconcretes require different degrees of durability depending on the exposure environment andproperties desired. For example, concrete exposed to tidal seawater will have differentrequirements than an indoor concrete floor.

These 3by5foot concrete panels with decorative finishes weredisplayed outdoors in the relatively severe weather in the Skokie,Illinois, area (near Chicago). With only a few exceptions, theirappearance changed very little after more than 40 years ofexposure to bright sunlight, wind, snow, acid rain, freezing andthawing, hot summers, and cold winters

Factors Related to ConcreteDurability

High Humidity and Rain: With little to no organic content, concrete is resistant to deteriorationdue to rot or rusting by in hot,humid climates. Moisture can only enter a building through jointsbetween 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. Wallsneed 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 finishsystems (EIFS) or synthetic stucco systems that may have performance problems, includingmoisture damage and low impactresistance. Synthetic stucco is generally a fraction of thethickness of portland cement stucco, offering less impact resistance. Due to its composition, itdoes not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moistureeventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metalattachments. There have been fewer problems with EIFS used over solid bases such as concreteor 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. Usingcolored 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 softermaterials are inedible but still provide pathways for insects. Due to its hardness, vermin andinsects will not bore through concrete.

Moderate to Severe Exposure Conditions for Concrete: The following are important exposureconditions and deterioration mechanisms in concrete. Concrete can withstand these effects whenproperly designed. The Specifier’s Guide for Durable Concrete, EB221 and Design and Control ofConcrete Mixtures, EB001.15 are intended to provide sufficient information to allow thepractitioner to select materials and mix design parameters to achieve durable concrete in avariety of environments.

Resistance to Freezing and Thawing: The most potentially destructive weathering factor isfreezing 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, theaggregate particles, or both.

When it has a proper system of microscopic airbubbles, obtained through the addition of an airentraining admixture and thorough mixing, concrete ishighly resistant to freezing and thawing. Thesemicroscopic air bubbles within the concreteaccommodate the expansion of water into ice and thusrelieve the internal pressure generated. Concrete witha low watercementitious ratio (0.40 or lower) is moredurable than concrete with a high watercementitiousratio (0.50 or higher). Airentrained concrete with a lowwatercementitious ratio and an air content of 5 to 8percent of properly distributed air voids will withstand agreat number of cycles of freezing and thawing without distress.

Chemical Resistance: Concrete is resistant to mostnatural environments and many chemicals. Concreteis regularly used for the construction of waste watertransportation and treatment facilities because of itsability to resist corrosion caused by the highlyaggressive contaminants in the wastewater stream aswell as the chemicals added to treat these wasteproducts.

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 thecement paste and calciumbased aggregates. In addition to using concrete with a lowpermeability, surface treatments can be used to keep aggressive substances from coming incontact 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 oftreatments 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 aconcrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, andmagnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardenedcement paste. These reactions can induce sufficient pressure to slowly cause disintegration ofthe concrete.

Like natural rock such as limestone, porous concrete (generally with a high watercementitiousratio) is susceptible to weathering caused by salt crystallization. Examples of salts known tocause weathering of concrete include sodium carbonate and sodium sulfate.

Sulfate attack and salt crystallization are more severe at locations where the concrete is exposedto wetting and drying cycles, than continuously wet cycles. For the best defense against externalsulfate 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 usedalong with cements or cementitious material combinations specially formulated for sulfateenvironments

Confederate Bridge, spanning the Northumberland Strait between Prince Edward Islandand NewBrunswick, was specifically designed for high durability in a severe environmentand a 100year life. The bridge has to resist freezing and thawing, seawater exposure, andabrasion from floating ice.

Seawater Exposure: Concrete has been used in seawater exposures for decades with excellentperformance. However, special care in mix design and material selection is necessary for thesesevere environments. A structure exposed to seawater or seawater spray is most vulnerable inthe tidal or splash zone where there are repeated cycles of wetting and drying and/or freezingand thawing. Sulfates and chlorides in seawater require the use of low permeability concrete tominimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful.Proper concrete cover over reinforcing steel must be provided, and the watercementitious ratioshould not exceed 0.40.

Chloride Resistance and Steel Corrosion: Chlorides present in plain concrete (that which doesnot contain reinforcing steel) is generally not a durability concern. In reinforced, the paste protectsembedded steel from corrosion through its highly alkaline nature. The high pH environment inconcrete (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 thefilm. Once the chloride corrosion threshold is reached, an electrochemical current is formed alongthe 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 bridgedecks, it can be increased by using a low watercementitious ratio (about 0.40), at least sevendays of moist curing, and supplementary cementitious materials such as silica fume, to reducepermeability. Increasing the concrete cover over the steel also helps slow down the migration ofchlorides. Other methods of reducing steel corrosion include the use of corrosion inhibitingadmixtures, epoxycoated reinforcing steel, surface treatments, concrete overlays, and cathodicprotection.

Resistance to AlkaliSilica Reaction (ASR): AlkaliSilicaReaction (ASR) is an expansive reaction between certainforms of silica in aggregates and potassium and sodiumalkalis in cement paste. The reactivity is potentially harmfulonly when it produces significant expansion. Indications of thepresence of alkaliaggregate reactivity may be a network ofcracks, closed or spalling joints, or movement of portions of astructure. Alkalisilica reaction can be controlled throughproper aggregate selection and/or the use of supplementarycementitious materials (such as fly ash or slag cement) orblended 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. StandardGuide for Reducing the Risk of Deleteerious AlkaliAggregate Reaction in Concrete, ASTMC1778, provides thorough guidance.

Abrasion Resistance: Concrete is resistant to theabrasive affects of ordinary weather. Examples ofsevere abrasion and erosion are particles in rapidlymoving water, floating ice, or areas where steel studsare allowed on tires. Abrasion resistance is directlyrelated to the strength of the concrete. For areas withsevere abrasion, studies show that concrete withcompressive strengths of 12,000 to 19,000 pounds persquare inch (psi) work well.

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