Simple tests and methodology for corrosion resistant rebar coating / Fatma Noka & Nikoll Ndrejaj

Abstract This report summarizes the findings and recommendations on the use of epoxy and red-oxide coating in reinforced concrete structures both for bond strength and corrosion resistance of steel rebar. Laboratory tests were conducted to characterize the properties of one and two-layer coating. Different tests were performed to characterize mechanical properties of steel rebar in lap splice and anchorage areas. For corrosion properties , tests were conducted to evaluate the corrosion resistance and performance of epoxy-coated steel and rebar. Observations and experimental procedures from various tests are documented. The resistance of red-oxide and epoxy coating were compared. Based on limited laboratory tests, a single and a two-layer coating has been shown to be practical and effective for both corrosion resistance and mechanical strength. Performance Features: Blocks Corrosion, Maintains Flexibility, Lubricate, Self Healing , Seals surface, Multi-functional, High resistance , prevention of corrosion caused by dissimilar metals. CEM-OX rust preventatives are formulated with a proprietary blend of waxes, metal wetting agents and corrosion rust inhibitors which penetrate into the pores of the metal preventing under film corrosion . No taste or odor is imparted to the water supply. With CEM-OX, because of reduced labor costs due to less intensive surface preparation, single coat application and minimal downtime of your water system , it has low cost. Surface Preparation - is generally done by hand or power driven brush to remove loose rust scale. CEM - OX and its rust inhibitor system penetrate into the metal surface therefore this steel coating is more resistant to under film corrosion. • Application: This corrosion product can be cold applied with a stiff bristle brush, roller or applied hot using airless spray equipment. The total thickness of two coats must be at least 2mm. To ensure that CEM-OX has the possibility of effectively developing its anticorrosive properties, it is essential that the iron, before treatment, is completely freed from rust. Keywords: This section should be written in italic and include a maximum of 6 keywords related with the study. 1. Coated rebars 2. Corrosion resistance 3. Rust preventative 4. Bond strength 5. Anchorage length

Fatma Noka

OBJECTIVES

The main research tasks includes:

1) Characterize the bond strength between and mechanical properties of coated rebars.

2) Characterize the corrosion resistant properties of a coated rebar compared with an uncoated one.

3) Study the behavior of simple methods to prevent corrosion.

• Introduction

A significant problem for asset managers is that for the majority of existing reinforced concrete structures it is currently not economically feasible to provide an ‘early warning’ of the risk of corrosion to the steel during the ingress of aggressive species through the cover concrete. This results in a form of “reactive” corrosion monitoring that basically relies on the physical degradation of the steel reinforcement itself to indicate that the structure is exhibiting signs of distress. The reality is that some level of repair to the concrete is an inevitable element of the life history of a significant proportion of reinforced concrete structures.

THE TESTS

Corrosion is a complex electrochemical process. Its onset and severity cannot be detected accurately with one specific test or evaluation method.

In this study, 15 tests were conducted to

understand and quantify the process of corrosion in cem-ox and epoxy-coated steel bars and uncoated rebar.

THE ANTI-CORROSION PRODUCTS CEM-ox / EPOXY

Performance Features of corrosion inhibitor products:

• Blocks Corrosion,

• Maintains Flexibility, Lubricate, Self-Healing ,

• Seals surface,

• Multi-functional,

• High resistance ,

• prevention of corrosion caused by dissimilar metals.

• CEM-OX rust preventatives are formulated with a proprietary blend of waxes, metal wetting agents and corrosion rust inhibitors which penetrate into the pores of the metal preventing under film corrosion . No taste or odor is imparted to the water supply.

ADVANTAGES AND DISADVANTAGES

Epoxy Resins in the primer systems are used, where the corrosion is severe. EP could also be Red-Oxide or Zinc Chrome based.

Two coats of primer are applied followed by two coats of Epoxy Finish. It is to be remembered that Epoxy Resins are tough and are abrasion resistant.However, the epoxy coatings are not resistant to UV Radiation. They tend to chalk in one or two years. They have however excellent corrosion resistance to most of the chemicals and solvents.

This powder improves the adhesion of the coating on its surface. This is attributed to the porous, but a strong hard coat of the iron complex resulting in increased surface area and more mechanical keys to hold on to.

• Safety preacautions need to be taken like use of gloves, protective glasses and clothing.

Steel corrosion is by far the biggest durability issue for reinforced concrete structures, although other deterioration mechanisms will lead to attack of the concrete itself, he notes, such as freeze-thaw scaling, moisture, acid or sulfate attack, thermal cracking, shrinkage from drying, impact, erosion, and wear. For reinforcing steel to corrode, the passive oxide layer on the steel must fail. If the concrete cover that protects the reinforcing steel is damaged and the bond between the concrete and steel reinforcing bar is broken, the steel’ s passive layer will break down and active corrosion of the steel will start. There are two mechanisms, however, that will damage the steel’ s passive layer even if the concrete cover is not damaged: chloride ions (Cl-) and carbonation. Reinforcing steel corrosion in concrete can happen under oxygen-starved conditions as well. In these situations, with oxygen only at the cathodic areas of steel, the corrosion products stay in solution at the anode and the steel dissolves without forming a solid product at the steel/concrete interface. The time period from corrosion initiation, when the passive layer is damaged, to the first signs of corrosion damage is known as the active corrosion time. Tests have been carried out by the construction institute in support of implementation of SSH ISO 17025 (The realization of tests is assisted by Phd.Altin Bidaj).

The corrosion protection method is also recognized as a means of prevention of corrosion damage in new structures, where for a small percentage of the capital cost the design life expectations can be met without repeated and expensive repairs. For prevention of corrosion of existing RCC structures migration of corrosion inhibitor of sodium monoflurophosphate is most suitable method. It creates high vapour pressure at the surface thus forcing these inhibitor molecules to traverse inside through the pores of the concrete for forming a protective layer around the rebar. Where ever carbonation is the major cause of corrosion, in such structures anti-carbonation coating of acrylic based is most suitable. It can also be used in marine and industrial environments. There are mainly two major causes of steel corrosion in concrete – chloride attack and carbonation. The reinforcing steel is well protected in Portland cement concrete by a passive oxide layer film because of high pH of concrete. Once this layer is broken, either due to carbonation or ingress of chloride ions, the steel reinforcing bars are depassivated and corrosion occurs. Corrosion accelerates due to presence of oxygen and moisture, along with low resistivity of concrete and lower concrete cover. However, the intrusion of chloride ions is the most important factor in the corrosion of steel reinforcing bars embedded in concrete. Possible sources of chlorides include: aggregates, mix water, admixtures (accelerators) and seawater. The resulting corrosion products occupy volumes several times that of the volume of steel. The increased volume induces tensile stress in the concrete that results in cracking, delamination and spalling . As a result, the reinforcements are exposed to direct environmental attack and the corrosion is accelerated. Corrosion Prevention Methods • Seal the surface of the concrete to prevent ingress of chlorides, carbon dioxide, moisture etc. • Modify the concrete to reduce its permeability, thus increasing the time it takes for the chlorides to reach the reinforcing steel. • Protect the reinforcing bars to reduce the effects of chlorides when they do reach the steel. Epoxy-coated Reinforcing Bars Precleaned reinforcing bars are protected with a coating of powdered epoxy that’s fusion-bonded to the steel in an assembly line process. Typically, manufacturers have the capability to coat straight bars, but only a few can coat bent bars. The coating physically blocks chloride ions. Performance: Poor to excellent Possible Problems: Unless the bars are coated after bending, there’s a potential for cracking and chipping of the epoxy coating during bending. Damage to the epoxy coating also may occur during field-handling of the bars. Relative cost: Medium. •Results Conclusions of the case study versus conclusions of our simple tests . In comparison with the uncoated specimens, the epoxy-coated specimens had a bond strength that was greater by 5% and the red oxide-coated specimens had a bond strength that was reduced by 10%. This can be explained based on the failure mechanism of them. The strength between rebar and concrete is generally a combination of adhesion, friction, and concrete bearing. Corrosion is a complex electrochemical process. It cannot be detected accurately with one specific test or evaluation method. In this study, 15 tests were conducted to understand and quantify the process of corrosion in epoxy-coated steel bars and uncoated rebars. •References •“Effective Cost Analysis for Repairing of Corrosion Damaged Reinforced Concrete Structures,” Department of Trade and Industry, http://projects.bre.co.uk/rebarcorrosioncost(September 3, 2015). •G. Tilly, “Past Performance of Concrete Repairs,” Concrete Solutions: Proceedings of the 2nd International Conference (St. Malo, France: BRE Press, 2006). •NACE Standard SP0390-2009 (formerly RP0390), “Maintenance and Rehabilitation Considerations for Corrosion Control of Atmospherically Exposed Existing Steel-Reinforced Concrete Structures” (Houston, TX: NACE International, 2009). •[Excerpts from http://concreteconstruction.net/Images/ Corrosion % 20 Protection % 20 for % 20 Reinforced% 20 Concrete_tcm45-356632.pdf]