REINFORCED CONCRETE PROTECTION USING THERMAL SPRAYED COATINGS. Thomas D. Gibbons, P.E. Greenman-Pedersen, Inc. Babylon, New York, USA

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1 REINFORCED CONCRETE PROTECTION USING THERMAL SPRAYED COATINGS Thomas D. Gibbons, P.E. Greenman-Pedersen, Inc. Babylon, New York, USA REINFORCED CONCRETE CORROSION Concrete is made up of various mixtures of materials: cement, fine and coarse aggregates and water are the basic materials. The cement is made of several different chemical mixtures of calcium compounds. Concrete is a very non-homogeneous mixture of many materials. This leads to disparity in the actions of the materials and their environments. The intrusion of liquids can completely upset the balance of the designed mix. The aggregates may have small of large breaks in the cement matrix, and may contain aggressive ions. Water is used to make the mix, and some is left over after the initial hydration is completed. The leftover water creates voids, called pores. If too much water is leftover the concrete will be very porous and weak. Additionally the water may contain aggressive ions. Admixtures are added to assist in the placement and curing of the concrete. Every time something is added that is not cement, water and aggregate, something in the concrete is reduced, i.e., strength, durability, hardness, et al. Some of the admixtures that are added are: Mineral admixtures o Pozzolans Liquid Admixtures o Air entraining agents o Water reducers o Plasticizers o Crack reducers o Set accelerators o Set retarders o Corrosion inhibitors o Waterproofing agents 1 of 11

2 The most important material in the mixture is the glue that holds it all together, which contains the seeds to its own destruction under the right environmental conditions. The most common type of cement used today is Portland cement. To the cement is added mineral mixtures called pozzolans. The pozzolans include: Fly ash, slag and condensed silica (silica fume). The hardened cement paste (HCP) is a very complex binder. It has a pore structure that transports the reactants of corrosion of reinforcement. According to ACI i the following chart lists the components of the cement itself. Component Chemical Formula Shorthand Notation a Wt. % Remarks Tricalcium Silicate Ca 3 SiO 5 C 3 S 55 Also called alite Dicalcium Silicate Ca 2 SiO 4 C 2 S 8 Also called belite Tricalcium Aluminate Ca 3 Al 2 O 6 C 3 A 9 Tetra calcium Alumino Ferrite Type II & V have lower amounts Ca 4 Al 2 Fe 2 O 10 C 4 AF 8 Vary A F molar ratio Gypsum CaSO 4 2H 2 O CSH 2 6 Added during grinding Alkali sulfates <1 Water-soluble potassium sulfates a Standard cement chemists notation: C=CaO, S=SiO 2, A=Al 2 O 3, F=Fe 2 O 3, S=SO 3, H=H 2 O REACTION LEADING TO DETERIORATION The complex reaction of the cements, aggregates and the water that takes place is summarized as follows: Hydration Process Calcium Silicates (C 3 S or C 2 S) C-S-H + Calcium Hydroxide (CH) Calcium Aluminates (C-A of C-AF) + Gypsum (CSH 2 ) Ettringite (AFt) Ettringite (AFt) Calcium monosulfoaluminate hydrate (AFm) 2 of 11

3 Calcium Silicates yield C-S-H and Calcium Hydroxide. As this is happening, the Calcium Aluminates, including Tetra-Calcium AluminoFerrite, react with the Gypsum to form the Ettringite. The Ettringite then reacts to form Calcium monosulfoaluminate hydrate. If the water cement ratio in the mixture is approximately 0.5, the following components of the mixture results: Component Components of Mature Matrix (w/c = 0.5) Approx. Vol. Fraction (%) Length Scale C-S-H nm Calcium Hydroxide Principal Characteristics Colloidal material with intrinsic porosity 15 10nm 1mm Solubility = 1g/L AFm 10 10mm Can react with Chlorides Unreacted cement Capillary porosity 5 10mm Remnants of larger particles 15 3nm-1mm Remnants of water-filled space. Volume depends on w/c. The composition of the cement frequently leads to the degradation of the concrete. The cement is the binder and may play a role regarding the through following factors: ph, sulfate content and porosity of the concrete. Degradation can be systemic internal in source. It can be caused by high ph + reactive aggregate, for example, alkali-silica reaction, or by excess sulfate not consumed in early set, causing delayed ettringite formation. Degradation can come from external sources, such as, since the aggregate is generally not porous, then the movement of an external agent through cement. Examples are: Carbon Dioxide (CO 2 ) as a gas or in rainwater causing carbonation equaling loss of strength and rebar corrosion, or inclusion of chloride ions, causing rebar corrosion. Cement Void Structure The cement pore structure plays the major role in this movement of agents through the concrete. Reactants move through the voids, but there are different processes in the reactant transport. The voids vary in size greatly, and their size influences the rate of movement through the cement. Large voids are due to three major reasons: design, placement or shrinkage of concrete. 3 of 11

4 The design voids are the joints, such as: expansion joints, repair systems, etc. The placement voids include: honeycomb and rock pockets. The shrinkage voids are due to cracks from shrinkage of the concrete during curing or the delamination caused by corrosion or shrinkage. The small voids are due to two additional reasons and one of the same for the large voids: mixing, cement hydration and concrete placement. The mixing operations can cause the inclusion of voids via intentional or inadvertent reasons; entrained air or entrapped air, respectively. The hydration of the cement can cause gel pores or capillary pores. The construction process may cause voids along the construction joints. The size range of the pores is shown in the following table. Void Sizes From To Concrete Mixing Entrained Air 0.05 mm 1.25 mm Entrapped Air > 1.25 mm Cement Hydration Gel Pores <10 nm Capillary Pores 10 nm 10 µm Concrete Placement Construction joints <10 nm Hexagonal Crystals Ca(OH) 2 or mono sulfate Entrapped air voids Interparticle Spacing CSH Entrained air bubbles Capillary Voids 0.001μm 1nm 0.01μm 10nm 0.1μm 10 2 nm 1μm 10 3 nm 10μm 10 4 nm Seven Magnitudes of Sizes 100μm 10 5 nm 1mm 10 6 nm 10mm 10 7 nm Gel pores are associated with the formation of the CSH gel. They consist of intrinsic nanosized pores, small capillary pores and are trapped in CSH volume. Water in these pores does not act the same as bulk water. The water will not freeze until it gets below - 40 C and transport through these pores is via molecular diffusion rather than flow. As such, these pores are not involved in corrosion. Capillary pores contain the remnants of water filled space. They are irregular in shape, location and size. These are the voids where most of the transport takes place. Several factors affect the transport in these voids. One is the extent of connectivity. Connectivity is interfered 4 of 11

5 with because of the aggregate which tends to block transport by sealing off the pores. Water Cement ratio also has a large part to play in the size of the capillary pores, with the lower w/c ratio constricting the diameter of the pores and reducing the flow through them. The pore water in the capillary pores has a great effect on the transport and potential degradation of the concrete and the steel. Liquid in the capillary pores is not pure water. It is saturated with most soluble hydrate CH, small quantities of soluble alkali ions (Na, K). This presence suppresses Calcium ions [Ca 2+ ] and increases the Hydroxyl ions [OH - ]. This increases ph from 12.3 to High alkali cements ph can exceed All of this leads to further protection of the reinforcing steel. While the aggregate in concrete retard the flow of pore water through the concrete matrix, the space at the face of the aggregate and the cement matrix is highly porous and increases the porosity in the concrete. This space is called the Interfacial Zone (ITZ). It reduces the density of small particles near large particles. ITZ around aggregates can form a percolating network of porosity. Excessive heating and, or drying can increase this network through differential shrinkage and bond cracking. Additions of fly ash and quenched slag reduce the amount of hydroxyl ions, and therefore reduce the connectivity of the pores. Fly ash is particularly good at doing this. EFFECTS OF AND DURING CURING Curing is a particularly critical time regarding the formation of the transport pores in the hardened cement paste. Heating during curing can increase the rate of strength gain, but it also alters the microstructure, causing coarser capillary pores, with larger threshold diameters. It also reduces the amount of aluminate hydration. Both of these phenomenon increase susceptibility to corrosion. Drying after curing removes water, causes drying shrinkage and can produce cracks. It also increases electrical resistivity. CREVICE CORROSION IN CONCRETE Having concluded our introductory lesson in concrete chemistry and the factors that lead to its strengths and weaknesses it is time to move on to the particular type of corrosion that we are endeavoring to mitigate or prevent. That type of corrosion is crevice corrosion in the concrete structure itself. This type of corrosion is particularly insidious in the chloride laden environments either in coastal or marine areas and those areas where the ingenuity of man has transported the ocean to the middle of the country in the snow-belt areas particularly on the backs of the saltspreader trucks. This is not the only areas as those processes that produce chlorides and either let or accidentally escape from the process are prone to this type of reinforced concrete degradation. This particular type of corrosion that we are addressing is not the typical rust-the-rebar and popout-the-concrete or delaminate it, although that may occur along side this particular type of corrosion. The corrosion effect that we are considering here is the one where the entire reinforcing bar is consumed and there are no visible effects of the deterioration of the rebar on the outside of the structure, until it collapses. This is not limited to just highway bridges, but can occur to any load 5 of 11

6 bearing reinforced concrete structure in a chloride environment, whether the chlorides are environmentally induced or induced through the efforts of humans. We have all seen the deterioration of concrete where the concrete has fallen off the structure and the rebars are exposed to the atmosphere. Anyone who has visited a coastal park or a harbor has seen this many times. This type of corrosion is due to many things: Carbonation changing the ph in the concrete so it no longer protects the steel, Chlorides invading the concrete and transporting to the steel and causing expansion of the rust and popping off of the concrete cover for the bars. So what is the difference? In this type of corrosion there are no rust stains to indicate that the corrosion is taking place. Normally rust stains on a concrete surface are indicative of the corrosion of the interior reinforcing steel bars. In the case of the crevice corrosion of the bars due to chlorides in the crevices and the change of the ph of the water and the concrete at the apex of the crevice, there are no iron ions migrating to the surface of the concrete, and therefore no rust stains. Following is an illustration of concrete crevice corrosion. It can result in substantial section loss, may occur without spalling or delamination, and while the sounding Sounds good, but the member has failed. So what happened to the iron (Fe) ions? Earlier on we discussed the chemistry of the cement. In that section of this presentation, we mentioned that the cement was comprised of various calcium compounds. One of those compounds is tetra-calcium alumino-ferrite (Ca 4 Al 2 Fe 2 O 10 ) or C 4 AF for short. The combination of the C 4 AF plus the very acidic condition at the apex of the crevice (ph as low as 1) causes the Fe ions to be absorbed by the cement and the cement goes from its normal alkaline state to the acidic state. It not only consumes the Fe ions, but also destroys the binding capability of the cement, which loses some or all of its strength in the process. Where are these crevices? Ask any structural engineer who designs reinforced concrete structures and he or she will tell you that they exist on the tension side of every load bearing structure. For the reinforcing steel to go into tension and permit the concrete to resist bending, the concrete from the face of the concrete surface to the reinforcing bar must crack. These cracks are very narrow and are crevices in and along the entire section in tension. It only occurs in every reinforced concrete structure that carries a tension load. What are the other conditions? Basically the macro and micro environments are the other conditions that are required to provide the necessary ingredients for the corrosion to occur. Influences on corrosion initiation and rate include: concrete electrical resistivity combined with moisture content, temperature, pore water, initial alkalinity, carbonation, and presence of aggressive ions, e.g. Cl. The various combinations and levels of each affect the corrosion in the concrete. The item that is the most critical cause of the corrosion is the transport process in the concrete. Without the transport process the corrosion reactants could not reach the depth sufficient to affect the steel and the concrete. 6 of 11

7 The transport process is comprised of several factors. The corrosion reactants are: ph, chlorides, carbonation and oxygen. To corrode the reinforcing bar they must reach the surface of the bar. So we must examine the ways they move through the concrete. The transport process may include any or all of the following: permeability, capillary suction, diffusion and wicking. The process that dominates depends on the depth of the rebar in the concrete, and the orientation and exposure of the members. Capillary suction dominates at the concrete surface, while ion diffusion dominates below the surface. Permeability is the bulk flow of water under pressure and is not a major factor in bridges and other above ground structures, except tanks. Capillary suction is the spontaneous filling of the small concrete pores and is intensified by alternate wetting and drying. It pulls in the CO 2 and O 2 in the atmosphere into the pores and washes the surface chlorides into the pores. It also plays a role in wicking. Ionic Diffusion is a spontaneous process involving the net movement of molecules from the high concentration areas to the lower concentration areas. It is the major transport process. Ionic Diffusion Wicking is a special case of capillary suction which requires a connected capillary network. It is a major problem in concrete piles in seawater. Other factors that affect the transport process are: Adsorption or deliquescence which binds the chlorides to the surface of the aluminates in the cement. Ionic mobility which is the ease with which some ions move more readily than others Factors that reduce connectivity, such as the admixtures pozzolans, latex, hydrophobic agents Pore structure which either more easily conducts the corrodents or retards their transport, based upon size, connectivity and tortuosity (amount of twisting and turning of the routes). 7 of 11

8 Cracks. In the scheme of transport ease, cracks are freeways and pores are narrow garden paths. This means that the cracks make the transport very easy. Crack width is very important, and the opening at the surface is not the size of the crack further in the concrete. Weathering will open the mouth of the crack, while the width at the steel bar is quite a bit narrower. There is disagreement upon the critical crack width, which if it is below a certain width, the critical width, no corrosion can occur. The proposed critical widths vary from 0.01 mm to 0.3 mm (4 mils to 120 mils). That aside, all agree: CRACKS ARE PROBLEMS! INSPECTION AND IDENTIFICATION The first step in the protection of the existing concrete structure is the inspection and identification of the contaminants on and in the concrete. For those structures subject to normal atmosphere and located near bodies of salt water both carbonation and chloride contamination of the concrete should be expected and examined. Various tests and procedures exist for the examination of concrete for the presence of both of these well-known contaminants. This discussion will not go into the particular tests, more than to say they should be used where either or both of the contaminants are expected or known to exist. Inspection of several structures has indicated the complete loss of the reinforcing steel and deterioration of the concrete due to the actions of both the CO 2 in the air and the chlorides in the marine and coastal environments. The loss of the steel reinforcement was not evident by visual inspection, as the Fe ions were absorbed into the cement matrix by the C 4 AF matrix. This prevented any indication of staining on the surface of the concrete, and resulted in a structural failure. This crevice corrosion in concrete followed the similar reactions that occur in metal couplings. Cracks and, or cold joints provided preferential paths for the ingress of chlorides and CO 2. In time, hydrolysis in the openings accelerates the corrosion. The presence of the chlorides decreased the ph down to levels as low as a ph of 1. The Fe 2+ ions migrated from the rebars as chloride complexes, and therefore, hoop stresses do not develop. No hoop stress; no cracking around the rebar. No cracking around the rebars; no rust stains on the surface of the concrete. In one example reported ii, a marine structure with apparently structurally sound concrete did not exhibit and corrosion problem when tested using acoustic sounding techniques. However when rebar potential mapping was accomplished at a cold joint a significant issue was indicated by a dramatic change in corrosion potential. Much of the rebar no longer existed. Over 50% of the section was missing. In a second example, a marine viaduct had differential settlement and pier cap sustained a crack of less than 20 mils (< 0.5 mm). Salt spray wetted the beam and the crack. Here to, the chloride ions preferentially followed the crack. Again the sounding of the concrete did not indicate a problem; however, corrosion potentials showed a dramatic shift. The problem was identified by potential mapping. 8 of 11

9 The third example involved a pile settlement. The crack ran across the beam face and the only indication was from the potential mapping. Every stirrup showed a section loss of over 50%. The indication here is that the way to identify and inspect each of the areas of potential impending failures is to perform a grid of close interval electrical potential testing for corrosion. A recommended distance between the actual locations each of the reading is on the order of 5 to 6 inches (125 to 150 mm). This will provide a very good map with sufficient detail to pinpoint the actual locations of the steel rebar failures from corrosion. It also limits the need to excavate large areas of concrete to be sure that you have identified it all. REMOVAL AND REPAIR Removal and repair of the contaminants to protect both the concrete and the steel reinforcement is necessary. The procedures necessary to protect the structure from further deterioration from the effects of both involves the removal of the obviously deteriorated concrete and replacement of the rebars. Additionally, since this type of repair constitutes the inclusion of materials that are different from the original, and therefore, provides and additional dissimilar metal and additional pathway through the cold joints, something else must be done. That something is the use of cathodic protection of the repair and the remaining structure. Repairs can easily include the installation of relatively small anodes that will corrode preferentially to the new steel that is place in the repair. This will protect the repair long beyond the normal 3 to 5 year cycle that these repairs normally remain before needing additional repair. The new repair deteriorates because of the difference between the existing steel and the repair steel and the newly created pathway created by the cold joint around the outside of the repair, itself. Without further protection the repair will rapidly deteriorate because of it. Beyond the repair the remainder of the structure is still filled with CO 2 and chlorides in the pores of the concrete. Now we must address the remainder of the structure. There are several ways to provide protection of the structure from further ingress of the CO 2 and the chlorides, but beyond that we need to remove or mitigate and further damage from these existing contaminants. OPTIONS Our options must address the mitigation of the existing contaminants and the prevention of further entry of the contaminants into the structure. This limits us to a few options. The options may include: a. Total removal and replacement of the structure b. Removal of a portion of the structure and patching the area c. Repair and use of both a cathodic protection (CP) system to mitigate the current concrete contamination and prevent further contamination and damage. 9 of 11

10 Option a is the most costly of the options and without the CP would eventually suffer the same fate as the original structure. Option b is less expensive but without CP would begin to deteriorate in approximately 3 to 5 years. Option c is the most efficacious method to provide the desired repair, mitigation and protection over the long period. To protect and mitigate the contaminants in existing structures, the most useful method in the author s opinion, would be to thermal spray iii a zinc alloy (Al-Zn-In) iv coating on the surface of the structure v. This zinc coating would accomplish both mitigation and protection by drawing the chlorides away from the steel reinforcement preferentially to the zinc; and combining with the CO 2 in the air and in the concrete to provide an impermeable layer of zinc carbonate, which is insoluble in water. Other forms of cathodic protection could be used including impressed current with an electronic net and, or anodes attached to the structure vi. These however would not be as uniform in their ability to basically provide the same draw for the chlorides away from the steel, nor would they provide the overall zinc carbonate protection from the atmospheric borne contaminants. Due to the nature of the concrete and the pores the thermal sprayed zinc coating would have good adhesion and mechanical attachment to the concrete. It is electrically conductive and if additional draw is required, impressed current cathodic protection (ICCP) could be applied directly to the zinc. Thermal sprayed coatings are virtually as easy to apply as other sprayed coatings and require similar surface preparation of the concrete. They can be performed without much other preparation for sound structures in the chloride environments. They can provide much more protection for the structures than regular coatings in the proper environments at essentially the same initial cost and reduced annual coasts. Like other coatings they can be renewed as needed. The use of this method should be reviewed for appropriateness at the particular location and a review of the Federal Highway Administration (FHWA), Office of Research, Development and Technology s document, Corrosion Protection: Concrete Bridges, available on their website: CONCLUSION The basis for the analysis and the information that is provided here is due to the application of basic reinforced concrete design principles to all structures that are located in chloride laden atmospheres. The CO 2 exists in the atmosphere naturally and is available to you for the protective coating of the zinc free of charge. 10 of 11

11 This process is not applicable only to bridges and piers where the original examples were provided, but in every reinforced concrete structure that carries a load or a force, which of course, is all of them. The greater the load - the higher is the concern for the chlorides infiltrating the cracks and joints on their way to the steel. Only every reinforced concrete structure that exists in the indicated environments is affected. To protect against the corrosion of the steel rebars and the deterioration of the concrete from acidification must be addressed. The use of thermally sprayed zinc is an economical way of providing protection from both of these deteriorating processes in a reasonable and sustainable manner. i American Concrete Institute, Country Club Dr., Farmington Hills, MI ii Tinnea, J. and Howell, K.M., P.E., Corrosion Assessment of Bridges, NACE International, Houston TX, 2007 iii Kerkhoff, B., Effects of Substances on Concrete and Guide to Protective Treatments, Portland Cement Association, Skokie, Illinois , 2001 iv Daily, S. F. and Ault, P. J., PE, Thermal spray for Cathodic Protection of Concrete, JPCL, Feb. 2003, pp , Pittsburgh, PA v Protecting NASA Signature Building from Corrosion, Matcor Corrosion News, Fall 2006, Matcor Inc., 301 Airport Blvd., Doylestown, PA vi Daily, S. F., Understanding Corrosion and Cathodic Protection of Reinforced Concrete Structures, CP-48, Corrpro Companies Inc., P.O. Box 1179, Medina, OH of 11