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Figure 1 is a photo that shows full depth leaking transverse cracks in a new HPC deck on US 123 over the Occoquan River constructed in 2007. 

Fig. 1. Full depth leaking transverse cracks in a new HPC deck on US 123 over the Occoquan River constructed in 2007.
            

Corrosion of Reinforcement in High Performance Concrete
Michael M. Sprinkel P.E., Virginia Center for Transportation Innovation and Research

Introduction

Controlling corrosion of the reinforcement continues to be the most controversial challenge for professionals responsible for reinforced concrete structures exposed to chlorides. Decades ago quality concrete had lower strength, higher permeability and fewer and tighter cracks than the concrete used today which is often called high performance concrete (HPC). Today’s HPC mixtures typically have higher strength and lower permeability, but may also lead to wider cracks (>0.2 mm) in the finished structure. Over the past 20 years, DOTs have reduced the cracking in HPC by using less cement, shrinkage reducing admixtures, and internal curing. However, cracking continues to be one of the most significant problems facing the concrete industry. The control of cracking induced by volume changes in plastic and hardened concrete has been identified as an Industry Critical Technology (ICT) by the Strategic Development Council (SDC) of the American Concrete Institute and a $1 million, 5-year project was initiated on 1-1-13 titled Reducing Volume Change-Induced Cracking of Concrete: Field Implementation and Evaluation of Crack-Reduction Technologies.(1)

The low permeability of HPC can be achieved by replacing a portion of the portland cement with fly ash and/or slag. Typical additions are 25 percent type F ash or 50 percent slag. Unfortunately, because the fly ash and slag contribute little to early strength, 25-50% of the portland cement is often not removed and the total cementitious materials content of HPC is higher than in the past, often more than 700 lb/yd3. The additional cement used to achieve the higher early strength to facilitate accelerated construction causes an increase in curing temperature, potentially leading to increased drying shrinkage and additional cracking. Prestressed and post-tensioned structures cast with HPC should last more than 100 years because of the high quality of the HPC and the lack of cracks because the HPC is compressed by the strands.

On the other hand, conventionally reinforced HPC structures may well have a shorter life than the structures of the past because of the wide cracks in the concrete that are typically caused by shrinkage and temperature differentials. Figure 1 shows full depth leaking transverse cracks in a new HPC deck on US 123 over the Occoquan River constructed in 2007.(2) Figure 2 shows full depth leaking transverse cracks in concrete placed in a deck on I81 near Marion in 2009. These decks are typical of the HPC decks constructed by VDOT over the past 20 years. The cracks typically align with the reinforcement that is parallel to the cracks and intersect with the reinforcement that is perpendicular to the cracks. The cracks allow chlorides and moisture direct access to the reinforcement. The liquid calcium applied today provides a more corrosive environment than the granular chloride and abrasives used in the past. Repeated cycles of wetting and drying and liquid chloride applications on bridge decks in particular provide a corrosive environment in wide cracks comparable to the splash zone in piers in a marine environment and aggressive accelerated lab tests. The steel surface corrodes rapidly in an environment with no protection provided by the concrete in the vicinity of the crack because the pH drops as the concrete carbonates. Water washes the corrosion deposits away allowing the steel surface to rapidly corrode and the cycle continues until the steel section fails because it is too weak to handle the shear, tensile and compressive stresses. The steel section that intersects a crack provides a small anode that is driven by a large cathode in the HPC. The section necks rapidly in the wet dry chloride contaminated low pH environment. A615 steel reinforcement has a short service life when exposed to a corrosive environment in a crack.

Figure 2 is a photo that shows full depth leaking transverse cracks in concrete placed in a deck on I81 near Marion, VA in 2009. 

Fig. 2. Full depth leaking transverse cracks in concrete placed in a deck on I81 near Marion in 2009.
            

Epoxy coated reinforcement (ECR) continues to be the preferred corrosion protection system of most DOTs.(3) Research conducted by the VDOT indicates that the initial corrosion protection provided by the coating depends on its condition and quality, but over time, the coating can delaminate allowing water and chlorides direct access to the steel surface.(4) The coating can trap moisture, preventing the water from evaporating and increasing the rate of corrosion. Figure 3 shows the corroded ECR in a section of deck that failed in shear in 2009 on I81 near Marion Virginia after 17 years in service.(4) The green coating has turned brown in the vicinity of the leaking construction joint that was approximately 0.5 mm wide, the typical width of cracks in decks constructed with HPC. For a number of reasons, including geometry, cracks may create a more corrosive environment than joints.

Figure 3 is a photo that shows corroded ECR in a section of deck that failed in shear in 2009 on I81 near Marion, Virginia after 17 years in service. 

Fig. 3. Corroded ECR in a section of deck that failed in shear in 2009 on I81 near Marion, Virginia after 17 years in service.
            

VDOT’s preferred low cost solution to the problem of corrosion of reinforcement in wide cracks is alloyed corrosion resistant reinforcement (CRR). CRR can last more than 100 years and like HPC contribute to a structure service life in excess of 100 years even when the structure is exposed to cycles of wetting and drying in a chloride environment. VDOT discontinued the use of ECR in 2010 and initiated the use of CRR. Other more expensive solutions include cathodic prevention that must be maintained, flexible polymer membranes and asphalt wearing surfaces that must be replaced periodically, routing and sealing of cracks with polymer crack filling materials that must be replaced periodically and epoxy injection of cracks. Use of A955 solid stainless reinforcement increases the installed cost approximately $1.24/lb compared to ECR and increases the cost of a deck approximately $74/yd2 based on VDOT bid tabulations for 2011 and 2012. Alternative treatments cost far more approaching or exceeding $150/yd2 either initially or over the life of the deck. VDOT has had no issues with the supply of solid stainless reinforcement and use of lower cost A1035 CRR which costs approximately the same as ECR can also be justified for many applications. VDOT uses solid stainless for the construction of bridges on Interstate and heavily travel primary routes. It uses A1035 CRR for rural and urban low volume roads.

For more information please contact Michael Sprinkel at Michael.Sprinkel@VDOT.Virginia.gov.

References

  1. David Darwin and JoAnn Browning, “Reducing Volume Change-Induced Cracking of Concrete: Field Implementation and Evaluation of Crack-Reduction Technologies” Research Project Title, University of Kansas Transportation Research Institute, Lawrence, KS, 1-1-2013.
  2. Stephen R. Sharp and Audrey K. Moruza “Field Comparison of the Installation and Cost of Placement of Epoxy-Coated and MMFX 2 Steel Deck Reinforcement: Establishing a Baseline for Future Deck Monitoring” FHWA/VTRC 09-R9, Charlottesville Virginia, 2009.
  3. Epoxy Interest Group, Anti-Corrosion Times, Volume 29 #1, Schaumburg, IL, 2-14.
  4. Sprinkel, Michael M. , Richard Weyers, Chris Blevins, Andrei Ramniceanu, Sean A Weyers, “Failure and Repair of Deck Closure Pour on Interstate 81” Transportation Research Board, DC 2010.