Q.: A structure has been designed to be built on 12-inch-diameter piles on a 20-foot grid on a low-lying island about 20 miles upstream from the mouth of a river. Tests have shown the soil to be quite acidic within the range of about 3.8 to 4.6. We're concerned about acid attack on the piles. Is there a concrete mix that would be durable in such an environment? If not, what kind of protection would you advise for members that can't be reached for recoating?
A.: There's evidence that ordinary concrete of good quality would be durable with no coating or membrane. In a letter to Concrete magazine (British), February 1986, page 23, T. P. Lees of the Cement and Concrete Association reports findings that concrete that has been in soil at pH 3.5 to 4.5 for 20 years showed "no sign of any sort of attack." He also quotes another investigator, Marjory Eglinton, who has said "there is no evidence of structural weakening of mass concrete exposed to flowing water containing [carbon dioxide] and humic acid and no evidence of damage to concrete placed in acidic ground conditions when free water is absent." T. P. Lees believes that where the pH isn't caused by industrial wastes but by native acids, where the pH is 3.5 or more, and where there is no flow of water across the concrete, there isn't any cause for concern. There may be cause if the pH is less than 3.5. Natural acidity rarely falls lower than this, so a lower pH usually indicates an outside source of acid.
A subsequent letter to the same magazine (April 1986, page 52) from Guy Brennan, of The Concrete Pipe Association in England, supports the views of Lees and reports good performance by precast concrete sewer pipe that had been subjected to untreated effluent from a plating plant where the pH alternated between 1 and 12 in three-hour cycles for some unspecified length of time. When examined, the concrete surface had been well etched but the body of the concrete was sound.
In another instance concrete specimens had been obtained from test borings that penetrated some old foundation beams in the soil on the site of an old gas plant. From the context of his discussion the soil must have been acidic. These stained and dirty concrete fragments were opportune test pieces because they already had a history of exposure to soil acids. Examination showed the bodies of the test pieces to be unaffected and their surfaces to be sound and firm. Brennan recommended that there should be no problems in specifying the use of concrete structures in this area. Unfortunately, his published letter doesn't report the pH of the soil. He acknowledges that extreme cases undoubtedly exist where "some of the more virulent moorland waters [with] just the right type of acid at exactly the wrong concentration might affect some concrete." However, he is convinced that "well constructed [concrete] products that satisfy today's demanding specification should have no difficulty in surviving such an environment."
According to The Concrete Pipe Association, those precast concrete pipes and other drainage products that meet the specification contained in British standard BS 5911 are resistant to most moorland waters. Many of these waters contain humic or carbonic acids. Concrete of the quality required can be characterized as having high cement contents and low water-cement ratios, being highly compacted, and having low porosity and low absorption.