Half-cell corrosion potentials
The half-cell potential of the embedded steel strands was used in accordance with ASTM C876-09, “Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete,” to identify regions where corrosion was occurring. Figure 4 shows the half-cell corrosion potentials from all four sides of a pile. As stated in Section 5.3.2 of ACI 222.2-01,2 according to ASTM C876, a half-cell potential more negative than −350 mV indicates a 90% or greater probability of corrosion occurring. A half-cell potential less negative than −200 mV indicates a 10% or less probability of corrosion occurring. The results suggest that from 2 ft (0.6 m) above high tide and below, there is a high probability that corrosion is occurring. However, these potential measurements are only indicative of the propensity for corrosion to occur but provide no information regarding the potential rate of corrosion. The lack of oxygen in the submerged zones of piles explains why no corrosion-induced damage was seen, even though the half-cell potential suggests that corrosion is occurring. The tidal and splash zones of piles have adequate access to moisture and oxygen due to the wetting and drying cycles present.
The concentration of chlorides near the reinforcement surface is critical in causing the onset of corrosion; as a result, it is important to understand the migration of chlorides through the concrete and the interactions between those ions and the cementitious system. While other transport mechanisms, such as capillary action, can affect the transport of chlorides through the cement paste, Fick’s second law of diffusion is most commonly used to model the transport of chlorides (Cl–) through the concrete. The apparent diffusion coefficient of a concrete mixture exposed to chlorides can be determined experimentally and used for service life estimation in a given environment. 1
When the concentration of chlorides reaches a threshold value at the surface of the reinforcing steel, corrosion will initiate when the passive film is broken down. In practice, the total chloride content is used more frequently for threshold values, even though it is generally believed that the free chlorides are responsible for the initiation of corrosion.3 The chloride threshold level (CTL) is typically assumed to be between 0.4% to 1% mass of binder, or approximately 0.05% to 0.2% by mass of concrete, for total chloride content.1 Research by Moser et al.4 has also shown that crevice effects and surface imperfections on prestressing strand have an influence on the CTL for use with service life modeling.
For the purpose of investigating the chloride diffusion characteristics of concrete in the present study, total and free chloride concentrations were determined at various heights along the pile. The total chloride concentration was measured using the ASTM C1152/C1152M-04, “Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete,” procedure. Samples were obtained by taking 3 in. (76 mm) diameter cores through the depth of the cross section, and drilling at 1/2 in. (13 mm) increments using a 3/8 in. (10 mm) masonry bit to collect powder.
As seen in Fig. 5, the concentration of total chlorides at the level of the reinforcement was significantly higher than the typical CTL values given in the literature, with particularly high levels noted at the high tide level and below. The chloride profiles for high tide, -5 ft (1.5 m), and -12 ft (3.7 m) elevations were very similar, with the exception of the surface chloride content which varied widely between the elevations. The variation in surface chloride content may be due to the effects of leaching and capillary suction during wetting and drying cycles in the tidal zone compared to the fully saturated condition in the submerged region. At just 2 ft (0.6 m) above high tide, the chloride content was significantly less than in the submerged regions. Overall, the concentrations were in agreement with the measured half-cell corrosion potentials, suggesting that active corrosion was occurring due to chloride-induced depassivation of the steel.
The experimental data were fit by nonlinear regression analysis to Fick’s second law to determine the apparent diffusion coefficient and surface chloride concentration, and these were used for service life modeling. Additionally, the Life 365 Service Life Prediction Model, also based on Fick’s second law,5 was used to estimate the diffusion coefficient and expected service life of the piles. The Life 365 estimates were based on selection of a marine tidal zone exposure in Savannah, GA, for a 0.50 w/c concrete containing only portland cement. The experimentally determined data were compared to the Life 365 data for the estimated time to corrosion initiation based on a CTL of 0.05% by mass of concrete, which is the default value used by Life 365.6 Table 1 shows this comparison.
Overall, Life 365 gave reasonable predictions of diffusion coefficient, surface chloride concentration, and time to corrosion (Table 1). It should be noted that the predicted time to corrosion initiation represents only a portion of the usable service life of the structure, and the period of time for corrosion to propagate to a critical point can take years to occur after corrosion initiates. Life 365 underestimated the diffusion coefficient determined from curve fitting. However, the diffusion coefficient observed could be influenced by other damage mechanisms that were occurring in the piles. For example, the identified biological attack, sulfate attack, and potential cracking from construction practices could all lead to an increased permeability and diffusion coefficient which would not have been accounted for in the Life 365 estimate.
In addition to chlorides, the concrete piles were exposed to sulfate (SO42–) concentrations of up to 2000 ppm. Sulfate attack can damage concrete through the reaction with hydration products to form sulfate-containing phases, such as ettringite and gypsum (and in some cases brucite), which can contribute to expansion, cracking, softening, and/or loss of strength and stiffness.7 Visually, a whitish appearance of the cement paste in damaged areas, as well as greater incidence of cracking and spalling at corners and edges, are indications of potential sulfate attack. The potential for sulfate attack to contribute to the damage found in the bridge piles was assessed by visual inspection and surface hardness measurements. Additional techniques not discussed herein, including X-ray diffraction (XRD) and thermal analysis, were also employed; details on these findings are provided by Holland.8
Visual assessment of damage
In addition to the cracking in splash and tidal zones previously described, below the low tide line cracking was particularly noted near the corners of the piles, extending from the mudline up to the low tide elevation. The width of these cracks varied widely, with a maximum of 0.05 in. (1.3 mm), but most were approximately 0.025 in. (0.6 mm) wide; marine life was found growing along the cracks. Surface spalling and abrasion were also apparent in this region. Additionally, the paste fraction nearer to the surface had a whitish appearance (Fig. 6).
To assess if the changes in paste color observed through the concrete depth could be correlated with a change in properties, hardness of the concrete was measured on cross sections of the cover concrete. Vickers indentations were performed in accordance with ASTM C1327-08, “Standard Test Method for Vickers Indentation Hardness of Advanced Ceramics,” using 2.2 lb (1 kg) mass applied for 15 seconds. A minimum of five indentations were made at 1/4 in. (6.4 mm) increments into the section on sections polished with 1 micron alumina. Figure 7 shows the results of the measurements for 2 ft (0.6 m) above high tide and 12 ft (3.7 m) below in the submerged region. The reduced hardness in the outer 2 in. (51 mm) of the submerged region was low when compared to the same region at greater depth. The consistency of these values with those made at 2 ft (0.6 m) above high tide suggests that at depths of 2 in. (51 mm) or greater, the concrete is relatively unaffected by marine exposure. The depth associated with the lower hardness coincided with the location of the whitish color change in the matrix as well as compositional changes detected by XRD. These suggest sulfate attack as well as carbonation, leaching, or both, have contributed to softening in the near-surface paste.
Heavy marine growth was noted on the surface and within cracks in the splash and tidal zones. Visual inspection and microscopy were used to better understand the nature of the apparent biodeterioration.
Visual inspection of damage
After the marine growth was cleaned from the surface of the submerged region of the piles, large amounts of surface damage were made visible. The damage, as seen in Fig. 8, consisted of large pits on the pile surfaces, with the damage more pronounced along the corners. There, increased porosity (or perhaps “boreholes”) and some spalling were evident. Cores taken in the submerged region showed extensive damage to aggregate within 1 in. (25 mm) of the surface of the piles to depths over 1 in. (25 mm), as shown in Fig. 8(c). Interestingly, the damage was largely concentrated where Pleistocene limestone coarse aggregates had been present on or near the surface of the piles, with this pattern of damaged near-surface aggregate observed in over 70% of cores taken.
This damage pattern observed visually was consistent with reported descriptions of damage to limestone and coral by Cliona or boring sponges in brackish or seawater exposures. There have been reports of Cliona sponges at Gardiner’s Island, New York9; along the coast of Virginia10; Corpus Christi in Texas11; and off the coast of Jamaica.12 Studies on the erosion rate of the sponge show that the rate may exceed 0.04 in. (1 mm) per year of ingress in solid limestone.13 While the factors affecting rate of marine attack on concrete are largely unknown, the rate of biological degradation of the limestone aggregate in these piles, greater than 1 in. (25 mm) in 35 years, is consistent with the rate of attack measured by Neumann13 for Cliona on solid limestone.
It is believed the sponge’s acidic secretions penetrate calcium carbonate, forming boreholes.9, 14 The genus Cliona sponges leave silica-rich spicules near the surface of their borings. The length of the spicules varies by species but is typically between 200 to 400 μm.15 Micrographs (Fig. 9) of damaged aggregate in these piles show rod-like structures with one pointed and one rounded end, morphologically consistent with spicules, with energy-dispersive X-ray spectroscopy (EDS) spectra showing the structures are predominantly silica. Clearly, the loss of aggregate by boring sponge attrition will have a significant negative effect on concrete performance. In a marine environment, in particular, the localized reductions in cover depth are especially important, as corrosion initiation likely occurs at earlier than anticipated ages.
The forensic investigation of precast prestressed concrete piles taken from a coastal environment revealed extensive damage derived from multiple deterioration mechanisms. Chloride-induced corrosion of the prestressing strands in the splash and tidal zones of the piles led to cracking and delamination of the cover concrete as well as a loss of steel cross section. Measured patterns of chloride ingress suggested that the concrete quality was not adequate to provide a 100+ year service life in the marine environment. Additionally, severe deterioration of the concrete due to sulfate attack occurred in the submerged regions of the piles, with carbonation also evident there and in the tidal regions. Also, extensive damage to the Pleistocene limestone coarse aggregate in the submerged region was linked to biodegradation by Cliona boring sponges. It is worth noting that inspections of other bridges along the coast suggest that the causes of damage observed during the forensic investigation of piling from the Turtle River Bridge are generally representative of other regional coastal bridges.
The piles exhibited extensive damage that led to the discovery of unexpected threats to bridge substructures in marine environments, and the need for adequate protection from known environmental hazards. Furthermore, the results of the study emphasize the need to consider the presence of multiple modes of deterioration in concrete structures and their potential synergistic interaction. For example, deterioration of the cover concrete caused by sulfate attack along with the formation of boreholes in coarse aggregates very likely resulted in accelerated ingress of chlorides and subsequent chloride-induced corrosion of embedded reinforcing steel. As required service lives become longer for critical infrastructure, consideration of all potential modes of concrete deterioration becomes more and more important to ensure that adequate serviceability can be maintained.
The authors would like to acknowledge the financial assistance provided for this study by the Georgia Department of Transportation. Technical assistance provided by Jeremy Mitchell and colleagues in the Georgia Tech Structural Engineering and Materials Laboratory is also gratefully acknowledged. Permission to publish granted by Director, Geotechnical and Structures Laboratory, U.S. Army Engineer Research and Development Center.
1. Bertolini, L.; Elsener, B.; Pedeferri, P.; Redaelli, E.; and Polder, R.B., Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair, second edition, Wiley-VCH, Weinheim, Germany, 2014, 434 pp.
2. ACI Committee 222, “Corrosion of Prestressing Steels (ACI 222.2R-01) (Reapproved 2010),” American Concrete Institute, Farmington Hills, MI, 2001, 42 pp.
3. Mohammed, T.U., and Hamada, H., “Relationship between Free Chloride and Total Chloride Contents in Concrete,” Cement and Concrete Research, V. 33, No. 9, Sept. 2003, pp. 1487-1490.
4. Moser, R.D.; Singh, P.M.; Kahn, L.F.; and Kurtis, K.E., “Chloride-Induced Corrosion of Prestressing Steels Considering Crevice Effects and Surface Imperfections,” Corrosion, V. 67, No. 6, June 2011, pp. 065001-1-065001-14.
5. Ehlen, M.A., Life 365, version 2.0.1, 2009. (www.life-365.org)
6. Ehlen, M.A.; Thomas, M.D.A.; and Bentz, E.C., “Life-365 Service Life Prediction Model Version 2.0,” Concrete International, V. 31, No. 5, May 2009, pp. 41-46.
7. Marchand, J.; Odler, I.; and Skalny, J., Sulfate Attack on Concrete, CRC Press, New York, 2003, 232 pp.
8. Holland, R.B., “Durability of Precast Prestressed Concrete Piles in Marine Environments,” PhD dissertation, Georgia Institute of Technology, Atlanta, GA, 2012, 397 pp.
9. Nicol, W.L., and Reisman, H.M., “Ecology of the Boring Sponge (Cliona celata) at Gardiner’s Island, New York,” Chesapeake Science, V. 17, No. 1, Mar. 1976, pp. 1-7.