Dowel bars are critical structural components in jointed concrete pavements. Metallic dowels have long been used to provide good load transfer, restraint from curl and warp movements and low bearing stresses (when properly designed). Existing corrosion protection schemes, however, have often failed prematurely (for example, paint, epoxy and plastic coating) or are very expensive (for example, stainless steel). Galvanic cathodic protection using a layer of zinc alloy offers a durable, economical approach to long-term corrosion resistance and satisfactory structural performance.
The durability and proper function of steel dowels depends mainly on their ability to resist corrosion. A thin film of iron oxide (Fe2O3), a corrosion product itself, normally helps to protect steel embedded in concrete from continued corrosion. This is sometimes referred to as “natural passive protection” of the steel. Unfortunately, this protective layer can be broken down easily in the presence of chloride ions (from deicing salts, for example) or by a reduction in the concrete pH through environmental exposure. Either mechanism will allow more destructive corrosion processes to take place.
Dowel corrosion can contribute to the development of pavement distresses in two ways: the loss of cross-sectional area in the vicinity of the joint, causing reduced joint stiffness and decreased load-transfer efficiency; and the accumulation of expansive corrosion products along the length of the dowel, causing the joint to lock and spall.
Weapons of mass protection
The most common approach to reducing corrosion of steel dowels has been the use of barriers, such as oil, grease, paint, epoxy, plastic and (most recently) stainless steel cladding and sleeves. The effectiveness of these techniques typically varies with the thickness and durability of the barrier, with thicker and harder layers being most resistant to damage during transport and construction. Stainless steel-clad or steel-sleeved dowels have generally offered good corrosion resistance at the joint, but can be very expensive.
Another approach is to produce dowels using only materials that are corrosion-resistant or noncorroding. Examples of this approach include solid and hollow bars made of stainless steel or fiber-reinforced polymer (FRP). The stainless steel products can be very expensive, while the FRP products are generally considered experimental because of concerns over their durability and behavior. In addition, the lower stiffness of FRP dowels has been associated with lower initial and long-term load-transfer efficiency, requiring the use of larger-diameter dowels or closer dowel spacings than for steel-based products.
Cathodic protection, which has been used successfully to prevent steel corrosion in bridge decks and piers, is now available for concrete paving applications. Instead of using an impressed current system (the type used in bridge decks), which requires the use of a regulated external power source, a galvanic system is used. Galvanic systems use the electrical properties of dissimilar metals to passively protect reinforcing steel. Galvanic cathodic protection is achieved when electrons from a sacrificial metal anode with a relatively high electrochemical potential (such as zinc) flow to the steel (the cathode) in the presence of a common electrolyte, such as chloride-saturated concrete. This form of cathodic protection is well-suited for pavement dowel applications because it is inexpensive and self-regulating (in other words, the current output automatically adjusts to different environmental conditions) and it requires no external power supply.
Zinc intake
A zinc-coated dowel has been developed and is produced by mechanically bonding a 50-mil-thick layer of solid zinc alloy strip to a standard carbon steel bar. The resulting product provides corrosion protection in two ways. First, the thick layer of zinc alloy acts as a surface barrier to the corrosion mechanism. Since this layer is five to seven times the thickness of most dowel corrosion barriers and is much harder than most epoxies and plastics, it is highly resistant to surface breaches caused by scratches, gouges and wear abrasion. In addition to being a barrier, the zinc alloy provides cathodic protection to any exposed steel because of the natural electrochemical differences between zinc and steel.
It should be emphasized that the zinc-coated dowel offers galvanic cathodic protection (as well as barrier protection), but is not a product of galvanization, a process that deposits a very thin coating of zinc (0.001-0.006 in.) on the steel through a high-temperature dipping process.
The corrosion resistance of the zinc-coated dowel was tested side by side with that of four other popular dowel products: an untreated carbon-steel dowel, a green epoxy-coated carbon-steel dowel, a 316L stainless steel-sleeved carbon-steel dowel and a microcomposite steel dowel. Four holes were drilled in each of the bars with barrier coatings (i.e., the zinc-coated dowel, the epoxy-coated dowel and the stainless steel-sleeved dowel) to simulate barrier breaches that might be caused by manufacturing flaws and handling damage. Holes were not drilled in the microcomposite and bare steel dowels because doing so would not expose a different material than was already present on the surface.
The samples were then immersed to identical depths with the exposed ends down in glass beakers containing 5% sodium chloride solution. The iron content of the test solution was measured prior to weekly replacement of the solution. The test was continued for 12 weeks. The presence of iron-corrosion products was observed in all beakers except those containing the zinc alloy-clad dowels.
Significant differences in the relative rates of corrosion were observed. The iron concentrations measured in the zinc alloy-sleeved dowel containers were comparable to what is commonly found in tap water, suggesting that the cathodic protection of the zinc alloy layer allowed very little, if any, corrosion of the underlying steel, in spite of the exposed end and drilled holes. The epoxy-coated dowels suffered corrosion rates that were 40 to 90 times higher than those of the zinc alloy-sleeved dowel, while the stainless steel-sleeved dowels corroded at even higher rates as the exposed carbon steel corroded preferentially to protect the stainless steel. The microcomposite steel dowels exhibited corrosion rates that were only slightly lower than those observed for the stainless steel-sleeved dowels, but had a much greater exposed surface area.
Additional corrosion testing of the zinc alloy-sleeved dowel was performed to determine the effectiveness of cathodic protection in preventing steel corrosion over large breaches at the center of the dowel (near the typical pavement joint location). Specimens were prepared with slots up to 1 in. wide and 0.25 in. deep cut through the zinc alloy layer about their circumference. These specimens were subjected to the same corrosion test protocol described previously. After 12 weeks of testing, there was no evidence of steel corrosion, and the iron ion concentrations in solution were similar to those observed in the drilled zinc alloy-clad dowels described previously. The cathodic protection provided by the zinc alloy sleeve was completely effective in bridging major breaches in the barrier layer and provides total corrosion protection of the underlying steel dowel.
The protection of the steel core of the zinc alloy-sleeved dowel is accomplished, in part, through sacrificial corrosion of the zinc alloy sleeve. Consumption rates of the zinc (as indicated by analyses of the ion concentrations in solution) suggest that it would take nearly 100 years of exposure to test conditions to completely deplete the zinc alloy layer. Actual consumption rates in field applications are expected to be much lower because field chloride exposure levels are unlikely to be as severe or continuous as those used in the lab testing.
The products of the corrosion of the zinc alloy are known to be dissipated into the concrete pore structure. Accelerated tests relating to similar bridge protection systems have been conducted by the Florida Department of Transportation at the Corrosion Research Laboratory in Gainesville. These tests suggest that zinc corrosion products diffuse into the concrete pores without causing detrimental effects on the zinc’s activity or the surrounding concrete.
Loads of promise
The National Association of Corrosion Engineers (NACE) has established that effective cathodic protection requires an electrochemical voltage shift of at least 100 mV between the structure surface and a stable reference electrode contacting the electrolyte. Measured half-cell potentials in the corrosion study described previously were -1,108 mV for the zinc alloy and -778 mV for the carbon steel, a difference of 330 mV—well beyond the minimum required to meet NACE standards of effective cathodic protection.
Dowels must be fabricated and installed in a manner that permits the joints to open and close with slab contraction and expansion. This is typically accomplished with a relatively smooth dowel surface and generally requires the application of a bond-breaker material, such as form oil or grease, prior to paving.
Dowel pullout testing was performed by S&ME Inc. of Louisville using the procedure described in AASHTO T-253 “Standard Method of Test for Coated Dowel Bars.” A total of six dowels were tested: two were treated with a heavy-duty form oil, two were treated with a grease and two were left untreated. All six exhibited pullout shear stresses that were significantly lower than 60 psi, a value commonly defined as the maximum allowable, and the untreated dowels averaged about 40 psi.
Three zinc alloy-sleeved dowels were installed in the outer wheel path of a full-scale concrete pavement test slab joint and were subjected to accelerated load testing in the University of Minnesota’s Minne-ALF test apparatus. This apparatus subjects the specimens to simulated passes of a 9,000-lb single wheel load while measuring slab deflections on each side of the joint and computing load-transfer efficiency.
The test program consisted of applying 10 million load cycles to the zinc alloy-sleeved dowels, with deflection and load-transfer measurement obtained periodically. Since every load in this test is a “critical load” that is applied directly over the dowel closest to the pavement edge, it represents significantly higher numbers of load passes in the field, where load placement varies. The test program was repeated using 1.5-in.-diam. epoxy-coated steel dowels.
