Sulfate Attack on Stabilized Pavements: US Air Force Case Study, Slides of Construction

Download Sulfate Attack on Stabilized Pavements: US Air Force Case Study and more Slides Construction in PDF only on Docsity! Sulfate Attack on Stabilized and Recycled Materials in Pavements R. Rollings US Army Corps of Engineers Engineering Research and Development Center-CRREL, Hanover, NH, USA M. Rollings Consulting Engineer, Lyme, NH, USA ABSTRACT: There have been several instances of sulfate attack on portland-cement- and lime-stabilized materials and on recycled portland-cement concrete used in pavements at US Air Force (USAF) airfields. While the chemical reactions are similar to those found in conventional sulfate attack on portland-cement concrete, the nature of the attack in a pavement setting makes conventional methods for protecting portland-cement concrete from sulfate attack ineffective for these stabilized and recycled materials. This paper will review the nature of the failures encountered on these USAF facilities. These examples show that even use of Type V sulfate-resistant cements proved inadequate to prevent attack. The paper will conclude with a discussion of our current shortfalls in knowledge and guidance in this area. KEY WORDS: Sulfate attack, recycled concrete, cement stabilization, lime stabilization 1. INTRODUCTION Sulfate attack is a well recognized phenomenon for portland-cement concrete, and defenses against such are well established and broadly published (e.g., American Concrete Institute 1972). The potential for sulfate attack on both lime- and cement-stabilized materials was clearly established in the laboratory in the 1950s and 1960s (Sherwood1962), but it was not until Professor Mitchell’s 1986 Terzaghi lecture (Mitchell 1986) brought attention to field problems of sulfate attack on lime-stabilized materials that the issue received much wide- spread recognition. The military, like most organizations, largely ignored the earlier work from the 1950s and 1960s that showed sulfate attack was a potential danger for stabilized materials in military pavements. However, a series of pavement failures and increased recognition of the problem in the technical literature forced the military to revamp its stabilization guidance. Warnings about sulfate attack are now included in the relevant technical manuals and educational classes. This paper briefly reviews the mechanism of sulfate attack and then examines specific cases of sulfate attack on recycled concrete and stabilized materials at U.S. Air Force facilities. Even when materials were made with Type V sulfate-resistant cements, they remained vulnerable to sulfate attack. SULFATE ATTACK MECHANISMS Classically, sulfate attack occurs when sulfates combine with calcium aluminate hydrate produced by hydration of the calcium aluminate phase of the portland cement to form ettringite, a potentially expansive calcium-sulfoaluminate mineral. Sulfate attack by calcium sulfate (gypsum) is described by (Taylor 1997, DePuy 1994): 3 2 4 19 17 3 3 32 4 2 2 3 2 2 2 3 4 2 ( )CaSo H O CaO Al O H O H O CaO Al O CaSO H O ⋅ + ⋅ ⋅ + → ⋅ ⋅ ⋅ or gypsum plus calcium aluminate hydrate plus water produces ettringite. Sodium sulfate is more soluble and hence is potentially more destructive than calcium sulfate. Magnesium sulfate is particularly destructive as the magnesium ion can also participate in reactions with the cement paste. The volume of ettringite is over 200 percent that of the original constituents (disregarding the water) which results in massive swelling and cracking when sufficient ettringite forms by sulfation of alumina. Ettringite forms in either of two ways: (a) primary ettringite generally formed by the sulfate ion in solution acting on chemically active alumina to produce topochemical ettringite, in situ and (b) secondary ettringite formed when primary ettringite is dissolved and redeposited from solution in cracks and voids. Ettringite is only expansive when it is formed by chemical reaction; once formed it is no longer expansive. The formation of secondary ettringite is not expansive since there is no chemical reaction, only precipitation from solution, and its formation plays no part in the damage caused by sulfate attack. Similarly, primary ettringite that forms in materials that are plastic or easily deformed does not cause problems. In fact, gypsum (calcium sulfate) is usually interground with portland cement clinker to prevent premature stiffening, and such ettringite formed in the early life of the concrete is not detrimental. Once the concrete is hardened, however, formation of primary ettringite may be highly destructive. Thaumasite (Ca3[Si(OH)6·12H20]SO4)(CO3) is a chemical analog of ettringite with carbonate and silica substitution for alumina. The thaumasite crystal is similar to ettringite, and generally it requires X-ray diffraction to differentiate the two. Thaumasite seems to require ettringite formation (perhaps as a nucleating agent or by transformation of one to the other by substitution) and temperatures of 5 to 10oC (40 - 50oF) are favorable for its formation (Taylor 1997, DePuy 1994, American Concrete Institute 1992). Protection against sulfate attack in portland-cement concrete is provided by sulfate resistant cements (U.S. Types II or V) that combat the reaction by limiting the alumina content of the portland cement phases. The maximum tricalcium aluminate (C3A) content in Type II cement is 8 percent for moderate resistance, and for Type V is 5 percent for high sulfate resistance. Some pozzolans, slags, and silica fume have also proven effective for providing sulfate resistance for Type I (ordinary) cement and in enhancing sulfate resistance of Type II and V cements. Good mixture proportioning and construction practices that maintain low permeability in the concrete also aid sulfate resistance. Sulfate attack mitigation more permeable to both water and sulfate salts in the local soil and groundwater. Consequently, the reduced alumina present in the Type V cement now had ready access to these critical contributors to sulfate attack. Dense impermeable concrete has always been considered as an important part of the protection against sulfate attack, and in a permeable crushed form, the recycled concrete is simply more vulnerable to attack than when it exists as conventional concrete. Under the paved surfaces, there is abundant moisture and sulfate available in the recycled base, and simply relying on Type V sulfate-resistant cement to combat sulfate attack is inadequate. This failure led some to conclude that recycled concrete was a poor construction material. The proper conclusion is that recycled concrete, even when made with sulfate- resistant cements, is vulnerable to sulfate attack. Hence, recycled concrete should not be used in applications where it will be exposed to sulfates. STABILIZATION ISSUES Lime or portland cement may be used to stabilize soil and aggregate materials used in pavement subgrades, subbases, and bases. This stabilization may be undertaken for a variety of reasons including strengthening of the pavement section, improving marginal materials to allow their use in the pavement, modifying adverse material characteristics, or providing an all-weather construction platform. Generally, strength and durability to freezing and thawing are the primary characteristics engineers consider when dealing with pavement stabilization. However, the dangers of sulfate attack must also be weighed. During construction of an auxiliary runway, taxiway, and ramp complex for Laughlin AFB, Texas, heaving from sulfate attack formed in the lime-stabilized subgrade. The sulfate attack damage appeared as transverse and longitudinal ridges up to 50 mm (2 in.) high, 300 to 600 mm (1 to 2 ft) wide, and 1.8 to 6.1 m (6 to 20 ft) long. During lime stabilization, the pH is raised to approximately 12. Above 9, solubility of silica and alumina rises exponentially, and this frees chemically active alumina from the soil’s clay minerals. This alumina can now participate in the desirable pozzolonic reactions of conventional lime stabilization. However, if sulfate is present, it is also now free to combine with calcium from the lime and water to form ettringite. Consequently, when sulfate is present, lime-stabilized materials are vulnerable to the same chemical reactions that cause sulfate attack in conventional concrete. Portland-cement stabilization provides the same chemical ingredients as conventional portland-cement concrete to support formation of ettringite and sulfate attack. Use of Type V sulfate-resistant cement would appear to provide the same protection to cement-stabilized materials as it does conventional concrete. This proves untrue unfortunately. As with lime stabilization, portland-cement stabilization raises the system pH thereby freeing chemically active alumina from the soil minerals to participate in the undesired formation of ettringite. This negates the usefulness of the low-alumina Type V cement. Figure 3 shows an example of sulfate attack on a taxiway’s stabilized base made with Type V sulfate resistant cement. Figure 3: Heaving of a taxiway surface caused by sulfate attack on a cement-stabilized base made with Type V sulfate-resistant cement, Holloman AFB, New Mexico. ANALYSIS Sulfate attack has been a recognized durability issue with conventional portland-cement concrete for over fifty years. Exposure limits and guidance for making sulfate-resistant concrete are widely published and incorporated into practice. However, when we use recycled concrete within the pavement structure, we are significantly changing exposure conditions, and guidance based on conventional concrete practice is no longer adequate. Currently, the U.S. military does not recommend using recycled concrete if there are sulfate contaminants present in the soil or groundwater that may be in contact with the recycled material. Even if this recycled concrete was manufactured with Type V cement, it may still be vulnerable to sulfate attack. Lime and portland-cement stabilization both raise the pH of the stabilized system so that chemically active alumina is freed from clay minerals in the soil. This is the source of a needed ingredient for the formation of ettringite and ensuing sulfate attack effects. For this reason, Type V cement will not provide any protection against sulfate attack of cement- stabilized materials. Sulfate attack of stabilized materials has received more research since Professor Mitchell’s Terzahi lecture in 1986 (Mitchell 1986). However, consensus on protective measures and allowable exposure limits has not been reached by the various investigators. Table 1 provides a summary of various reported sulfate attack problems on stabilized materials. Significant sulfate attack has been reported at relatively low sulfate contents and low percents of fines. Some work on countermeasures for sulfate attack on lime- stabilized materials has been conducted but results appear mixed to date. Consequently, the U.S. military currently discourages lime or cement stabilization if sulfates are present. Table 1: Comparison of sulfate attack on stabilized materials as reported by different investigators Project Reference Stabilizer Swelling Sulfate Content PPM Clay- Size Fraction, % Clay Minerals Las Vegas Streets Hunter 1989 Lime Moderate to Severe 10,000 10 - 55 Minor 20,500 <10 Halloysite with some smectite and kaolinite Joe Pool Lake Parks Perrin 1992 Lime Loyd Park Severe 2,000 – 9,000 3 - 18 Cedar Hill Park Severe 21,200 high Smectite and kaolinitic- montmorillonitic clays Laughlin AFB Runway Perrin 1992 Lime Moderate 14,000 - 25,000 34 – 63 Smectite WES Lab Study Lime Low to Moderate 500 – 5,000 Smectite McCallister and Tidwell 1997 Moderate to Serious 5,000 – 12,000 Very Serious > 12,000 Lab Study Lime Slight to Severe 0.3 – 6.2% 30 Kaolinite Mitchell and Dermatas 1992 None to Moderate 0.3 – 6.2% 30 Montmorillonite Georgia Road Rollings et al 1999 Portland Cement Moderate 6 – 13 Halloysite with some smectite and kaolinite Holloman AFB Taxiway Portland Cement Severe High 33 - 56 Kaolinite and Chlorite Notes: Information as reported by authors. Consistent definitions and sulfate measurement techniques not used between different investigators. PPM is parts per million. Clay-sized fraction is percent < 0.002 mm. Holloman AFB reporting percent passing the No. 200 sieve. More research is needed to provide improved guidance on at what threshold sulfate attack becomes a threat to stabilized materials, what characteristics of the soil are involved (clay mineralogy and clay content), and what countermeasures are effective in mitigating this sulfate attack of stabilized materials. Sulfates are most often associated with arid and semiarid regions, and the authors have commonly dealt with sulfate contaminant problems in the America Southwest and the Middle