Important environmental factors that affect culvert durability include the acidity (pH) of the effluent and the soil, the electrical resistivity of the effluent and the soil, and the concentration of sulfates and chlorides. Data on these factors should be obtained at each pipe location, unless a random sampling plan is justified by establishing that the samples are uniform throughout a given length of the project. Water samples should be taken only during times of typical flows. If corrosive conditions are found to be present in the soil but not in the water samples, consideration should be given to using a better backfill material.
Concrete Pipe. Environmental factors that can affect the deterioration of concrete culverts include freeze-thaw, acids, sulfates, and chlorides. Freeze-thaw damage can occur if water penetrates the concrete interstices and then freezes and expands, causing cracking. Such damage would occur only at exposed ends of culverts, and low water — cement ratios or air entrainment can increase resistance. Continuous exposure to severe acidity is detrimental to concrete pipe; a pH below 5.0 is considered aggressive and below 4.0 highly aggressive. Improved resistance to acid attack can be attained by selecting aggregate that increases the total alkalinity of the concrete, increasing concrete cover over reinforcement, or adding barrier linings (e. g., epoxy coatings). Sulfates in the soil, groundwater, or effluent can be aggressive to concrete. Such problems, which are generally limited to arid regions with alkali soils, may be addressed with special cements and mix design. Chloride attack can potentially result from use of deicing salts and subsequent runoff.
Metal Pipe. Environmental factors that affect the corrosion of metal culverts include the acidity (pH) and the resistivity of the soil and water, and the moisture content, soluble salt content, oxygen content, and bacterial activity of the soil. These corrosion processes all involve the flow of current from one location to another. The current flows from an anodic area to a cathodic area through moist soil acting as an electrolyte, and this system is known as a corrosion cell. Thus, durability increases with increasing resistivity. Acid soils, those with low pH, tend to be more corrosive. Also, soils with high moisture content, such as loams and clays, tend to be more corrosive. High levels of chlorides and sulfates increase corrosion, as do increasing levels of dissolved oxygen and carbon dioxide. Numerous field studies have shown that the culvert invert is the portion most susceptible to corrosion, because it is generally exposed to water for a greater length of time. Thus, design charts are usually based on service life observed in the invert.
Plastic Pipe. PE and PVC pipe are not affected by acid conditions, or by sulfates or other alkalis. These materials can become embrittled from ultraviolet radiation as a result of prolonged exposure to direct sunlight, such as at culvert ends, but inhibitors are added to the composition of the material to substantially reduce this effect. If problems are encountered, ends can be shaded, covered with a coupling, or painted.
