Effects of Substances on Concrete and Guide to Protective Treatments

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Portland cement concrete is durable in most natural environments; however ..... provides a protective oxide film on the steel that is passive and non- corrosive.
CONCRETE TECHNOLOGY

Effects of Substances on Concrete and Guide to Protective Treatments by Beatrix Kerkhoff

Introduction Portland cement concrete is durable in most natural environments; however, concrete is sometimes used in areas where it is exposed to substances that can attack and deteriorate it. This publication discusses the effects of many substances on concrete and provides guidelines to protective treatments. The first line of defense against chemical attack is to use quality concrete with maximum chemical resistance. This is enhanced by the application of protective treatments in severe environments to keep corrosive substances from contacting the concrete or to improve the chemical resistance of the concrete surface. Protective surface treatments are not infallible, as they can deteriorate or be damaged during or after construction, leaving the durability of the concrete element up to the chemical resistance of the concrete itself.

Proper maintenance—including regularly scheduled cleaning or sweeping, and immediate removal of spilled materials—is a simple way to maximize the useful service life of both coated and uncoated concrete surfaces.

Improving the Chemical Resistance of Concrete Quality concrete must be assumed in any discussion on how various substances affect concrete. In general, achievement of adequate strength and sufficiently low permeability to withstand many exposures requires proper proportioning, placing, and curing. Fundamental principles and special techniques that improve the chemical resistance of concrete follow. Refer to Design and Control of Concrete Mixtures (Kosmatka et al. 2002) for further information.

Fig. 1. Aggressive substances can compromise the durability of concrete. Shown are concrete beams exposed to high-concentration sulfate soils/solutions. (PCA/CALTRANS test plot, Sacramento, California) (Stark 2002) (IMG12296)

Low water-cement ratio (w/c)—the water-cement ratio or the water-cementitious materials ratio (where applicable) should not exceed 0.45 by weight (0.40 for corrosion protection of embedded metal in reinforced concrete). Water-cement ratios for severe chemical exposures often range from 0.25 to 0.40 to maximize chemical resistance.

Chemical admixtures (optional)—dosage varies to achieve desired reduction in permeability and to improve chemical resistance. Water reducers (ASTM C494) and superplasticizers (ASTM C1017) can be used to reduce the water-cement ratio, resulting in reduced permeability and less absorption of corrosive chemicals. Polymer admixtures, such as styrene-butadiene latex, used in the production of polymer-modified concrete, greatly reduce the permeability of concrete to many corrosive chemicals. A typical dosage of latex admixture would be about 15% latex solids by weight of cement. Certain integral water-repelling admixtures, also called hydrophobic pore-blocking or dampproofing admixtures, can slightly improve the chemical resistance of concrete to certain chemicals such as formic acid (Aldred 1988). However, many integral water-repellents offer little to no improvement; therefore tests should be performed to determine the effectiveness of particular admixtures. (See “Evaluating the Effectiveness of Concrete Surface Protection by Testing.”) Admixtures containing chloride should not be used for reinforced concrete. Corrosion inhibitors (ASTM C1582) reduce chloride-induced steel corrosion. (See “Corrosion of Reinforcement” under “Design Considerations.”) Alkali-silica reactivity inhibitors, such as lithium nitrate, can be considered when potentially reactive aggregate is used and when alkali solutions will be in contact with concrete. Shrinkage reducing admixtures can reduce the formation of shrinkage cracks through which aggressive chemicals can penetrate the concrete.

Cement content—at least 335 kg/m3 (564 lb/yd3) of cementitious material should be used for concrete exposed to severe freeze-thaw, deicer, and sulfate environments. Suitable cement type—cement should be suited to the exposure, such as sulfate-resistant cement to help prevent sulfate attack (Table 1). Sulfate-resistant cements, however, like other portland or blended hydraulic cements, are not resistant to most acids or other highly corrosive substances. Suitable aggregate—quality aggregate is not prone to freezethaw deterioration or chemical attack. If an aggregate is shown by field performance (history) or by testing to be susceptible to alkaliaggregate reaction (AAR), appropriate measures should be taken to design a concrete mixture to minimize its susceptibility to AAR. (See Farny and Kerkhoff 2007 and PCA 2007 for further guidance.) Some aggregates may be more suitable than others for certain chemical exposures. (See “Acids” under “Design Considerations.”) Suitable water—mixing water should not contain impurities that can impair basic concrete properties or reduce chemical resistance. Steinour (1960), and Abrams (1920 and 1924) discuss the effects of impure mixing water.

Supplementary cementitious materials (optional)—dosage varies to improve chemical resistance. Supplementary cementitious materials (SCMs) such as fly ash and metakaolin (ASTM C618), slag

Table 1. Requirements for Concrete Exposed to Sulfate-Containing Soils and Solutions Cement type*

Maximum watercementitious material ratio, by mass

Sulfate exposure

Sulfate (SO4) in soil, % by mass

Sulfate (SO4) in water, ppm

Negligible

Less than 0.10

Less than 150

Moderate**

0.10 to 0.20

150 to 1500

II

IP(MS), IS(