Corrosion Protection and Repair of Reinforced Concrete

CORROSION PROTECTION AND REPAIR OF REINFORCED CONCRETE

By: Alex Van Der Werf

WHY CONCRETE SPALLS
Concrete spalling is usually caused by corrosion of the steel reinforcement bar embedded in the concrete, but can be caused by other ferrous elements either fully or partially embedded in the structure. Corrosion is the cause of spalling and splitting in older concrete structures.

Corrosion is typically caused by carbonation (which lowers the pH in the concrete) and chloride ions from salt-laden air combined with water, moisture and oxygen which creates a corrosive environment.

Bar Penetration

Chloride induced corrosion is more common around ocean front structures (show in pink). Carbonation-induced corrosion tends to develop later and proceeds at slower rates than chloride-induced rust. In this process carbon dioxide penetrates the concrete through pores, cracks, and imperfections in the concrete. In addition by the time visible corrosion damage is noticed, structural integrity is already compromised.

The net result of the corrosion of the rebar is the growth of ferrous oxide scale (rust) on the rebar. The ferrous oxide scale can expand 5-10x in volume and exerts enough force on the surrounding material that it splits the concrete. This splitting is what we refer to as spalling.

Cracking and Spalling

Once the corrosion process begins, the cumulative damage becomes exponential. The successful repair and protection of concrete structures which have been damaged or which have deteriorated requires professional review, then design, supervision and execution of a technically correct strategy. The assessment of the condition of a damaged or deteriorated reinforced concrete structure should only be made by qualified and experienced people.

CONDITION SURVERYS
A detailed corrosion or condition analysis is vital in order to discover the exact cause and extent of deterioration before repair options are considered. There are a number of sophisticated test procedures to determine the extent of deterioration though some of the most basic and effective procedures include visual and delamination surveys. The Truesdell Corporation has developed a rating system as seen in Figure 1.1 on Page 4 to help determine what stage of corrosion has taken place.

VISUAL ASSESMENT
Corrosion damage may be identified and defined using a logical visual survey. Classification of visual evidence of deterioration must be done objectively, following clear guidelines that define damage in terms of appearance, location and cause.

Defects may be defined in terms of cracks (caused by corrosion, temperature, shrinkage, or fatigue), joint deficiencies (joint spalls, upward movement, lateral movement, seal damage) surface damage (abrasion, rust stains, delaminations, pop outs, spalls), changes in member shape (curling, deflection, settlement, deformation) and textural features (blow holes, honeycombing, sand pockets, segregation).

Spalls

Visual assessment of deterioration can provide useful information when done in a logical, systematic manner but the data may come too late for cost-effective repairs.
Rebar corrosion damage is often only fully manifested at the surface after significant deterioration has occurred. Early evidence of distress can sometimes be detected by an experienced engineer before major distress takes place.

 

DELAMINATION SURVEY
A hammer survey or chain drag is a simple method of locating areas of delamination in concrete. Hollow sounding areas can be marked up on the concrete. Delamination surveys often under estimate the full extent of internal cracking and should not be considered as definitive. Radar and ultrasonic instruments may provide a more sophisticated approach to locating areas of delamination, particularly at greater depths.

Chain Deck

REPAIR STRATEGIES
Currently there are a number of repair options that are available and the Truesdell Corporation continues to work on the forefront of new technologies to make an impact in the field of concrete repairs. There is a proactive and reactive approach when dealing with concrete repair. The suitability and cost effectiveness of repairs depends on what level of deterioration has taken place and specific conditions of the existing structure.

PROACTIVE REPAIR STRATEGIES
A variety of coating and penetrate systems are available that create a barrier system to seal the surface thereby stifling corrosion by restricting oxygen flow to the reinforcing steel. Proactive repair methods fall under Stage 1 in figure 1.1.

Application of coatings (sometimes referred to as penetrating pore-liners) may be used to reduce the moisture content of concrete and thereby suppress the corrosion reaction. The drying action works on the fact that surfaces lined with coatings repels water molecules during wetting but allows water vapor movement out of the concrete to facilitate drying. Coatings generally are most effective on uncontaminated concrete, free from cracks and surface defects.

 

Corrosion Inhibitors

Penetrating systems with migrating corrosion inhibitors use a chemical substance that reduces the corrosion of metals without a reduction in the concentration of corrosive agents. It is designed to soak through the concrete and protect the reinforcing steel. Corrosion inhibitors work by reducing the rate of the anodic and/or cathodic reactions thereby suppressing the overall corrosion rate in the reinforcing steel.

Migrating corrosion inhibitors are generally organic-based materials that move though unsaturated concrete by vapor diffusion. The effectiveness of migrating corrosion inhibitors appear to be enhanced when used with coatings to reduce moisture levels in the concrete. This has been noted in both laboratory trials and field monitoring of repairs.

REACTIVE REPAIR STRATEGIES
Once concrete begins to show signs of defects and/or surface damage, the structural integrity is already compromised. Reactive repairs are shown as Stages 2-4 on Figure 1.1. A coating or corrosion inhibitor would be insufficient until the concrete is repaired. Depending on what stage of deterioration has taken place will directly coincide with what repair method will be required.

PATCH REPAIRS
Patch repairs all follow the same basic activities. The goal is to clean the reinforcing steel to prevent future damage.
To gain access to the reinforcing steel, mechanical means are used to remove the concrete cover which can include pneumatic hammers, hydro jetting or milling. All of the cracked and delaminated concrete must be removed to fully expose the corroded reinforcement.

The corroded steel is then cleaned by sandblasting or wire brush and an application of a protective anti-corrosion coating to the steel is applied. At areas where the reinforcing bar has corroded beyond 20% of the original size, additional bar shall be spliced in to guarantee structural integrity.

Parts of a Patch

Once all of the reinforcing steel has been cleaned and protected an application of repair mortar is applied to replace the damaged concrete.

CRACK INJECTION
In some instances corroding reinforcing steel may not be the reason concrete cracks. Some typical causes for cracking can include dry shrinkage, settlement, lack of appropriate control joints, or overload conditions. One of the potentially effective repair procedures is to inject epoxy under pressure into the cracks.
The injection procedure will vary, subject to the application and location of the crack(s), with horizontal, vertical, and overhead cracks requiring somewhat different approaches. Depending on the specific requirements of the job, crack repair by epoxy injection can restore structural integrity and reduce moisture penetration through concrete cracks 0.002 in. in width and greater.

CONCLUSION
The idea that reinforced concrete structures require little to no maintenance during their service life is greatly being dispelled. Corrosion is like a cancer and its damage is on an exponential scale. Once corrosion starts the only way to stop it is to go directly to the source (the reinforcing steel). Ignoring corrosion calculates into much more expensive repairs at a later stage.

High quality repairs require an investigation into the cause of the deterioration, an appropriate repair procedure and execution by a qualified and competent repair contractor. Defects can be categorized into low, medium, and high priority. It is good practice to establish a repair program that addresses each level of deterioration and how to correct them over time.

Areas that are classified as high priority should be addressed and repaired immediately. By the time visible corrosion damage is noticed, structural integrity is already compromised. A proper repair program is designed to attend to what is structurally obsolete and focus on preventing low priority items from becoming unsafe and costly.

References

  1. Mackechnie, JR, & Alexander, MG. “Repair principles for corrosion-damaged reinforced concrete structures.” Research Monograph No. 5. University of Cape Town: Department of Civil Engineering, 2001
  2. Bavarian, Behzad, & Reiner, Lisa. “Corrosion Protection of Steel Rebar in Concrete using Migrating Corrosion Inhibitors, MCI 2021 & 2022.” California State University, Northridge: Dept. of Manufacturing Systems Engineering & Management, 2002
  3. Verma, Neerav, & Balasubramaniam, R. “Corrosion of Steel Reinforcements in Concrete.” Indian Institute of Technology, Kanpur: Department of Materials and Metallurgical Engineering.
  4. Cathodic Protection Technology. “Understanding Corrosion.” Stop Concrete corrosion by understanding the process. CPT, Web. 1 Sept. 2013.
 


Home | Affiliations | Mission Statement | Products | Careers | Contact Us | Plan Room | List of Services | Employee Mail | Site Map

Our Licences: Arizona: ROC208273-B04;ROC147478-KA;ROC137832-K5;ROC163145-B1 - Califorina: 615058 A,C61,C33,D06
Idaho:00703-AA-1-2-3-AA - Iowa: C10293 - Nevada: 68575-A - New Mexico: 91865-GB98
North Dakota: 42461-Class A - Utah: 9609940-5501-B100, E100

1310 W. 23rd Street - Tempe, Arizona 85282 - Phone (602) 437-1711 - Fax (602) 437-1821
Copyright © 2013 Truesdell Corporation. Designed by Alex Van Der Werf.