Painting of bridge bolt joints
High strength bolts have been used in structural steel assemblies for over 60 years, supported by the Research Committee on Rivet and Bolted Structural Joints (RCRBSJ – Research Committee on Rivet and Bolted Structural Joints). RCRBSJ was founded in 1947 and is now known as the Research Council on Structural Connections (RCSC – Research Council on Structural Connections). Despite the long history of use of these structural connections, there are still many difficulties and confusions when painting joint surfaces, bolt holes and fasteners. The content described in this paper will eliminate confusion in some areas (professional paint booths, paint lines and exhaust gas treatment equipment manufacturers: Fan 13141458653) and provide considerations to be taken into account when designing and painting bolted connections.
Before interpreting the results of the coating tests used for friction-type connection nodes, it is helpful to understand the relevant test procedures. The testing and certification of this special coating has been detailed in the RCSC’s Specification for Structural Joints Using High Strength Bolts. The revised version in effect at the time of this writing is August 1, 2014 (April 2015 errata), although proposed revisions are currently under discussion. Certification requires that both the slip coefficient test and the tensile creep test meet the criteria, as described in Appendix A of the specification.
Prior to the tensile creep test, the specimen must first pass the slip coefficient test. class A certification requires a minimum slip coefficient of 0.30 and class B certification requires a minimum slip coefficient of 0.50. although not discussed herein, class C certification is also available and requires a minimum slip coefficient of 0.35. class C is applicable to rough hot-dipped galvanized surfaces.
The slip coefficient test is used to determine the average slip coefficient of a coating under short-term static loading. Test plates for the slip coefficient test are made of 15.8 mm (5/8 in.) thick flat carbon steel (without raised edges, protruding defects or bends) with a minimum yield strength between 36 and 50 ksi. The steel sample plate measured 15.8 mm x 101.6 mm x 101.6 mm (5/8 in. × 4 in. × 4 in.), with a 25.4 mm (1 in.) diameter hole drilled 38 mm (1 ½ in.) from the edge and one side (5/8 in. × 4 in.) machined smooth. Although, the specification does not specify whether to use hot-rolled or cold-rolled steel, the surface must be kept as flat as possible. The surface of cold-rolled steel is usually flatter than that of hot-rolled steel, providing greater flatness, so the test will usually use cold-rolled steel.
The steel plate is sandblasted and cleaned and coated with the coating material to be evaluated, such as inorganic zinc, organic zinc-rich coating, or thermal sprayed metal layer (TSC). The standard requires that the test coating material be applied to both sides of each sample plate at a thickness 50 microns (2 mils) greater than the maximum dry film thickness that will be applied to the structure (typically, 2 mils greater than the manufacturer’s recommended maximum dry film thickness), but the certification report thickness does not include the additional 2 mils.
Test Plate Slip Coefficient Test
The average slip coefficient of the coating is determined by testing five replicate specimens (three test panels per specimen). The test setup has two main loading components, one for applying clamping forces to the specimen plates and the other for applying a compression load to the center plate, thereby transferring the load to the joint surface by friction (Figure 2). The threaded rod is inserted into the drilled holes of the three plates instead of on the bolts. The nut on the rod end holds the plate in place and a clamping force of 49 ± 0.5 kips is applied and maintained throughout the test using a hydraulic cylinder to represent the minimum clamping force of the A490 bolt. A vertical load was then applied to the center plate at a rate of no more than 25 kips/minute until slippage occurred between the plates. The test took approximately seven minutes to complete for each replicate sample.
Tensile creep is the tendency of a coating to deform under continuous use loads, including the effect of loss of clamping force due to significant compression or creep deformation of the coating. The tensile creep test plate consists of a 5/8-inch thick flat carbon steel (again without raised edges, protruding defects or bends) measuring 101.5 mm x 177.8 mm (4 x 7 inches) with two 1-inch holes drilled at each end 1½ inches. The surface finish and application of the test material was identical to that of the plates prepared for the slip coefficient test. Each specimen consisted of three plates with the top half coated with the tested coating material. Figure 3 shows the two plates used for the tensile creep test.
The three replicate specimens (three plates each) were joined together in a single chain-like arrangement (Figures 4 and 5). Clamping force was achieved by connecting the painted portions of the plates with 7/8-inch diameter A490 bolts, which were secured with the corresponding nuts. The unpainted part was attached using only loose pin bolts and was not part of the test. A load was applied to the chain under tension and held for 1000 hours (approximately 42 days). At the end of 1000 hours, the tension is increased to the final load within a few minutes. 1000 hours of locking tension and the final tension applied to the specimen are derived from formulas based on the classification of the specimen slip coefficient and the average clamping force. While the actual clamping force depends on the bolt type or mounting method, for Class B, the minimum locking tension is 32.7 kips and the minimum final tension is 49 kips. Since the bolts are used to hold the specimen in place, the results may be affected if any material (e.g., paint, TSC, or galvanizing on the joint surface or under the bolt head or washer/nut and steel plate) is severely compressed, which may affect the clamping force.
Interpretation of slip coefficient and tensile creep results INTERPRETATION OF SLIP COEFFICIENT AND TENSION CREEP RESULTS
According to the RCSC specification, the average slip coefficient of the coating can be classified as Class A or Class B. As mentioned earlier, the minimum slip coefficient rating is 0.30 for Class A and 0.50 for Class B.
Abrasive blast cleaned bare steel substrates and most inorganic zinc-rich primers can meet Class B requirements. Some organic zinc-rich primers can meet the requirements of Class B, but some other organic zinc-rich primers can meet the requirements of Class A only. For TSC metal coatings, the FHWA study (Slip and Creep of Thermal Spray Coatings, Publication No. FHWA-HRT-14-083) tested sealed and unsealed metal layers of 100% zinc and 85/15 (zinc/aluminum) alloys. Both unconfined metal coating systems had slip coefficients greater than 0.75, which very easily exceeded the requirements for Class B. The slip coefficients for the closed coating systems were 0.414 for 100% zinc and 0.439 for the 85/15 alloy, meeting the requirements for Class A only. For tensile creep, both unsealed metal layer systems meet the Class B standard. When closed, both coating systems failed, even though the tensile creep parameters were based on the less stringent Class A requirements. When used in joints, the TSC metal coating should not be closed.
There are several factors to keep in mind when applying slip coefficient and tensile creep test results in shop and field work. It should be realized that the tests are performed using the same materials on the mating surfaces. For example, the same brand X primer is applied to all surfaces of the test panel and then a certificate is issued for the specific product tested. While it is possible to apply brand X inorganic zinc to one side of a joint and brand Y inorganic zinc to the other side of a joint, the certification does not involve the use of two different brands in a single joint unless specifically tested. Similarly, the certification does not involve the use of brand X inorganic zinc on one side and brand Z organic zinc-rich on the other, even if both products are made by the same manufacturer, unless specifically tested.
The minimum primer cure time used during qualification testing must be strictly adhered to prior to assembly of the joint; otherwise, the uncured paint at the joint may act as a lubricant, and the RCSC specification states that studies have shown that “at the time of joint assembly, all curing reactions have effectively ceased and the incompletely cured paint acts as a lubricant. If the curing time at the joint is shorter than the curing time used in the qualification test, then the slip coefficient of the assembled joint will be severely reduced. Therefore, the curing time before the joint surfaces are matched is an important parameter that needs to be strictly specified and controlled during construction.” The qualification test determines the minimum time required to achieve adequate curing of the product so as not to affect the slip resistance of the bonding surface.
The maximum thickness and thinner type shown on the certification must not be violated for the certification to be valid. This is why the thickness of the primer specified for the joint is usually different (lower) than the thickness specified for the rest on the steel.
Test panels are usually coated with the same material in the same thickness on the back side (under the bolts, washers and nuts) and on the joint surface. That is, if an inorganic zinc primer of 177.8 microns (7 mils) is applied to the joint surface, the same inorganic zinc primer of 177.8 mm (7 mils) needs to be applied to the backside of the test plate. The paint on the back of the specimen has no effect on the slip coefficient test results because (professional paint booth, paint line and exhaust gas treatment equipment manufacturers:
