Properties of adhesives and CPVC materials proposed for steel tank lining

Transcription

1 Properties of adhesives and CPVC materials proposed for steel tank lining Tarek K. Hassan a, John D. Vickery b and Sami H. Rizkalla c,* a Assistant Professor, Ain Shams University, Faculty of Engineering, Structural Eng. Dept., Abbasia, Cairo, Egypt b Graduate Student, North Carolina State University, Constructed Facilities Laboratory, 2414 Campus Shore Dr., Raleigh, NC, , USA c Distinguished Professor, North Carolina State University, Constructed Facilities Laboratory, 2414 Campus Shore Dr., Raleigh, NC, , USA 1

2 Abstract This paper focuses on the behavior of structural adhesives used in tank lining applications. The experimental program addresses the fundamental material properties and bond characteristics of adhesives when subjected to normal and severe environmental conditions. A total of 132 adhesive and CPVC tension coupons were examined under severe environmental exposure to determine the influence of these conditions on the tensile strength. Double-lap shear tests were conducted to evaluate the shear strength of the adhesive using fifty CPVC-to-steel specimens. The overall composite behavior of the proposed lining for steel tanks was investigated using three small-scale test specimens to simulate the conditions of a typical tank lining. The specimens were subjected to extreme temperature changes to examine the thermal gradient distribution and the composite interaction of the CPVC liner to the steel wall. Research findings provide better understanding of the performance of adhesives when subjected to severe environmental conditions and different sustained load levels. Keywords: Adhesives, Bond, Environmental conditions, Lining, Steel, Tanks 2

3 1. Introduction and background Environmental conditions and corrosive environments can significantly reduce the service life of steel tanks. Tank linings and coatings have been used for decades to extend the service life of steel tanks and also to avoid replacement of damaged tanks. In the past few years, thermoplastic sheet linings have garnered attention as tank lining materials. Excellent corrosive characteristics and low installation cost make the material ideal for several applications. Structural adhesives are commonly used as the bonding agent of this lining material to tank walls. Nevertheless, the susceptibility of adhesive joints to aggressive environments has not been supported by fundamental research. In typical tank lining applications, the adhesives act not only as a connection, but also as a buffer between the two surfaces. Therefore, the demands required from the adhesives are usually extreme for most service conditions. When subjected to cyclical heating and cooling conditions, the demands become much more complex due to the differences in coefficients of thermal expansion of various materials To date, limited research is available on the bond characteristics of adhesives under severe environmental conditions. One of the earliest studies on structural adhesives by Kreiger [1] demonstrated a consistent degradation in bond strength of adhesives under hostile environment. It was observed that the degradation was progressive, beginning at the edges and proceeding inwards until total failure of the joint took place. Fay and Maddison [2] investigated the effect of surface preparation on the durability of adhesively-bonded steel specimens subjected to salt spray conditions. Major improvements in durability were achieved using pretreatments applied to the dry steel surface. However, it was proposed that the pretreatment material should be applied to very clean surfaces, which is difficult to control in field applications. A study by Karbhari and Shulley [3] using wedge-test specimens showed that hot water was the most aggressive environment, causing severe breakdown of the bond between steel and FRP adherends. The influence of thermal stresses on the bond behavior of adhesives was studied by Tsamasphyros et al. [4] using finite element analysis. The study showed that the thermal residual effect could lead to 25% reduction in the effectiveness of the bonded joint. Hahn and Ewerszumrode [5] highlighted the effect of heat evolved during curing of the adhesive on the behavior of bonded joints. Experimental results showed that the adhesive bond strength and slip capabilities decreased with 3

4 the increase in the relative displacements induced during preparation. Such a phenomenon was highly pronounced when thicker adhesive layers were used. The durability of lap-shear joints (steel-frp) was investigated by Ramani et al. [6]. As a result of environmental exposure, a significant drop in the lap-shear strength by approximately 25% was observed. The strength of all joints was controlled by the adhesive-frp interface. A recent study by Smith et al. [7] highlighted the effect of temperature and ph level of the immersing solution on the bond behavior of different adhesives. The study showed that the surface preparation of the steel is quite significant to the performance of adhesives under severe exposure conditions. The proposed thermoplastic tank lining material used for the current study is chlorinated polyvinyl chloride (CPVC). The material has excellent chemical resistance and has been shown to endure o thermal conditions up to 90 C [8]. The ability to bend, shape and weld the sheets made from CPVC enable its use in a wide variety of applications including bleach tanks, scrubbers, and ventilation systems. A typical bleach tank under construction is shown in Fig. 1. This paper provides in-depth understanding of the fundamental behavior of adhesives used in lining typical bleach tanks. Extensive analyses for some of the challenges facing the adhesive/cvpc tank liner are presented. General guidelines of the behavior of the adhesive used in the current study under severe environmental conditions are also proposed. 2. Experimental program The experimental program consisted of three phases. The first phase of the experimental program focused on evaluating the tensile strength of both the CPVC and the adhesive under various environmental conditions including temperature, sustained stress level, soaking solution, and time of exposure. The second phase was conducted to examine the bond characteristics of the adhesive in steel-to-cpvc double-lap shear specimens. Primary considerations in this phase were focused on determining the effect of environmental conditions such as temperature and sustained stress level on the shear strength. The third phase of the experimental program consisted of an examination of a cross-section of a steel tank having an adhesively-bonded 4

5 CPVC lining. The thermal gradient distribution and the composite interaction among different materials as affected by temperature changes were investigated. 2.1 Materials Methacrylate adhesive was selected throughout this study. The adhesive is produced by one of the private companies in USA and is currently used by industry for bonding composites, metals, and other plastics. The working time of the adhesive is sufficiently high and ranges from 38 to 55 minutes. CPVC sheets of a total thickness of 3 mm were used in the current study. The sheets are produced in USA and are commercially available in thicknesses ranging from 3 to 75 mm. A total of five tension coupons of the CPVC and the adhesive were tested under room temperature (control specimens) to establish the basis of comparison for other specimens tested under severe environmental conditions. The specimens were prepared and tested in accordance to ASTM D638 [9]. Based on test results, the average tensile strength of both the CPVC and adhesive were 55 and 14.5 MPa, respectively. Steel adherends with a total thickness of 1.6 mm were used in the current study. The steel has a yield strength and modulus of elasticity of 400 MPa, and 200 GPa, respectively. 2.2 Phase I: Influence of environmental conditions on the tensile strength Test specimens A total of 66 specimens for each of the CPVC and adhesive were tested under various environmental conditions. The main variables included temperature, sustained stress level, type of immersing solution and exposure period as summarized in Table 1. The sustained stress level was determined as a percentage of the tensile strength of the control specimens tested under normal environmental conditions. The specimens were divided into two main groups, namely A and B. Eighteen test specimens of Group A were immersed in water under varying sustained stress levels and temperatures for a constant exposure period of 30 days. To investigate the effect of the exposure period and the type of the immersing solution, the specimens of Group B were immersed in either tap or salt water for different exposure periods. The specimens were subjected to various sustained stress levels and temperatures. Specimens survived the exposure periods, were tested in tension to evaluate the environmental effect on the material 5

6 characteristics. The dimensions of both the CPVC and adhesive specimens are shown in Fig. 2. Sixteen-mm-diameter anchorage holes were drilled at each end of the specimen to fasten the specimen to the test setup as will be discussed later. Two aluminum reinforcing tabs were bonded at each end of the specimen to strengthen the specimen at the holes and to ensure failure within the gage length of the specimen. The tabs were bonded with the same adhesive used in this investigation Environmental exposure test setup In order to execute the environmental exposure tests, the load was applied to the test specimens according to a specially designed test setup. Each specimen was placed in a CPVC cylindrical container filled with either tap or salt water. The cylindrical containers were fabricated by securing and sealing J-bolts through a CPVC cap as shown in Fig. 3a. The bolts and caps were secured to wooden beams resting on the floor. The CPVC tubes were sealed to the anchored caps, as shown in Fig. 3b. Immersion heaters wired into thermostats were used to control the temperature of the solution inside the cylindrical containers as shown in Fig. 3c. Acrylic tops with holes cut out were used to hold the heaters and thermostats in place as well as to minimize evaporation of the water. The loading mechanism utilizes a lever system to apply various levels of sustained tensile stress to the specimens as shown in Fig. 4a. One end of each specimen was hooked onto the J- bolt in one of the CPVC tubes. The other end of the specimen was hooked onto another J-bolt that was secured to a 1600 mm long hollow structural steel beam. The steel beam pivots about a roller as shown in Fig. 4b. At the other end of the HSS member, another J-bolt was secured, and 10 kg steel plates were hung from that bolt to add weight to the system when necessary. The roller position was varied to achieve the tension force required for a given sustained stress level. Test results of Groups A and B for the CPVC and adhesive are given in Table Results and discussion Group A CPVC specimens The average tensile strength of the CPVC specimens after surviving the exposure period of 30 days is shown in Fig. 5a at different sustained stress levels and temperatures. At any given level of sustained stress, temperatures up to 60 o C did not have any 6

7 detrimental effect on the tensile strength of the CPVC. The combined effect of high sustained stress level and elevated temperatures started to be pronounced at temperatures greater than 60 o C, causing significant reduction of the tensile strength of the material. Results indicated that the CPVC material could maintain at least 75 percent of its tensile strength quite well throughout all the conditions with the exception of specimens subjected to a temperature of 80 o C and a sustained stress level of 30 percent. Test results showed that the post-exposure tensile strengths of the specimens exposed to zero sustained stress level and tested at 60 o C and 80 o C were even slightly higher than those measured for the specimens tested at 23 o C at any given stress level. This behavior demonstrates the detrimental effect of sustained stress level in comparison to temperature. Brittle failure was observed for all specimens subjected to sustained stresses as shown in Fig. 5b. Adhesive specimens The average tensile strength of the adhesive specimens after 30 days of exposure is shown in Fig. 6a. Test results showed that sustained stress levels up to 30 percent did not affect the tensile strength of the adhesive tested at room temperature. For specimens exposed to zero sustained stress, temperature has a minor effect on the tensile strength of the adhesive. Slight reduction in the post-exposure tensile strength was measured for the adhesive specimens exposed to zero sustained stress and tested at 60 o C and 80 o C compared to those tested at room temperature. Specimens subjected to the most extreme conditions of 30 percent sustained stress and 80 o C did not survive more than few minutes. Similarly, specimens subjected to 15 percent sustained stress and 80 o C as well as those subjected to 30 percent sustained stress and 60 o C survived less than one day. Fig. 6b depicts few samples of the adhesive specimens after failure. It was observed that heated water changed the color of the adhesive from black to a yellowish-brown when subjected to 80 o C. Although this color change would apparently indicate a significant change in the material properties, the resulting decrease in the tensile strength due to temperature was within 15 to 20 percent. 7

8 Group B Based on the results of Group A specimens, the maximum sustained stress level was reduced to 10 percent of the tensile strength of the control specimens to simulate typical field conditions. Test results for Group B specimens are given in Table 1. CPVC specimens In general, no noticeable decrease in the post-exposure tensile strength was observed for the CPVC specimens under sustained stress level of 10 percent at any given temperature or soaking period as shown in Table 1. Furthermore, the use of 5 percent NaCl showed no detrimental effect on the post-exposure tensile strength of the material at different temperatures. Adhesive specimens The adhesive s performance was significantly affected by the temperature and the sustained stress level. Test results indicated that the post-exposure tensile strength of the adhesive exposed to a sustained stress level of 10 percent and a temperature of 60 o C was 25 percent less than that of the control specimen tested at normal environmental conditions. The results clearly demonstrated the poor performance of the adhesive when subjected to the combined effect of high temperature of 80 o C and any sustained stress level. The soaking period has a minimal effect on the tensile strength of the adhesive. Results suggest that the maximum temperature exposure should be less than 60 o C with a sustained stress level in the range of 10 percent of the tensile strength of the material tested under normal environmental conditions. 2.3 Phase II: Double-lap shear tests Test specimens A total of 50 CPVC-to-steel, double-lap shear specimens were tested to evaluate the shear strength of the adhesive under various environmental conditions. The specimens were prepared according to ASTM D3528 [10]. However, the dimensions and layout of the specimens were slightly altered to account for the inferior strength of the CPVC compared to steel and also to fit the specimens in the environmental test setup. The specimens were divided into two main groups, A and B. In Group A, an overlap bond length of 12.5 mm was provided between the CPVC and steel. The specimens were subjected to varying temperatures and sustained stress 8

9 levels for 30 days. An overlap bond length of 25 mm was used for Group B specimens. The specimens were soaked in either tap or salt water for 30 days and subjected to various sustained stress levels and temperatures. At least two specimens were tested at the same environmental condition. Specimens that survived the 30 days exposure period were tested in tension up to failure to evaluate the influence of the environmental conditions on the bond strength. The dimensions of the specimens of Groups A and B are shown in Figs. 7a and 7b, respectively. For both groups, the thickness of the CPVC tab at the end was reduced in order to fit the grips of the tension testing machine. Prior to bonding, the steel tabs were grinded, sandblasted, and wiped with acetone. The bonding surface of the CPVC was visually inspected and wiped with a dry paper towel to remove any debris. During bonding, 0.8 mm glass beads were placed in the adhesive layer to ensure consistent bond thickness. Approximately 14 kg steel plates were applied over each set of five specimens during the curing process of Group A specimens. For Group B, the specimens were subjected to a temperature of 80 o C for one hour as recommended by the adhesive s manufacture to increase its bond characteristics. Five control specimens were tested from each group under normal environmental conditions to establish the basis of comparison for other specimens tested under severe environmental conditions. Based on test results, the average shear strength for specimens of Groups A and B was 9.4 and 9.6 MPa, respectively. The test matrix as well the results for the double-lap shear specimens are given in Table Test results Group A A substantial loss of the adhesive s shear strength was observed when exposed to high temperatures in water. The only specimens survived the 30 days exposure period were the specimens exposed to zero sustained stress and those subjected to a sustained stress level of 15 percent at a temperature of 23 o C. The remaining specimens failed within the first two days. The specimens survived the exposure period were tested in tension as discussed previously. Test results showed that the temperature could significantly affect the bond characteristics and failure mode of bonded CPVC-to-steel joints. Typical cohesive failure was observed for the specimens tested at a room temperature of 23 o C. The failure occurred in the adhesive layer as shown in Fig. 9

10 8a. The adhesive fracture surfaces had numerous ragged edges indicating good interfacial adhesion. Conversely, adhesive failure was observed for specimens subjected to 60 o C and 80 o C as shown in Fig. 8b. The failure surface of the steel was smooth and clean indicating poor adhesion. This behavior could be attributed to the ingress of hot water throughout the joint, which degraded the interfacial zone. The shear strength of the adhesive at 60 o C was 20 percent less than that of the specimens tested at room temperature. Increasing the temperature to 80 o C decreased the shear strength of the adhesive by an additional 36 percent Group B Environmental conditions were less severe for Group B specimens than for Group A. The maximum sustained tensile stress level was reduced to 15 percent instead of the 30 percent used for Group A specimens. The maximum exposure temperature was also reduced to 60 o C and additional specimens were tested at a temperature of 50 o C. The number of the specimens tested at each environmental condition is given in Table 2. Test results showed that the shear strength of the adhesive could not survive combined sustained stresses of 15 percent at temperatures of 50 o C or 60 o C. Specimens loaded with 15 percent sustained stress and heated to 50 o C survived an average of 11 days, while specimens heated to 60 o C survived an average of 4 days. The results clearly indicated that the adhesive could sustain a stress of 15 percent at room temperature without any sign of degradation. At a sustained stress level of 8 percent, the maximum temperature that can be used without significant reduction of the shear strength is 50 o C. Therefore, field use of the adhesive at temperatures greater than 50 o C could potentially be hazardous. All the test specimens soaked in salt water at room temperature survived the 30 days exposure period without a significant sign of deterioration. Typical cohesive failure was observed for all specimens tested at room temperature. Increasing the temperature to 50 o C or 60 o C weakens the interfacial zone, leading to adhesive failure (failure at the interface) of the bonded joint. 10

11 2.4 Phase III: Thermal gradient tests Test specimens This phase of the experimental program was conducted to evaluate the thermal gradient distribution and the composite interaction of the layers as affected by temperature changes. Small-scale specimens were prepared to simulate the tank, including the steel, adhesive, and CPVC lining. Each specimen consisted of a steel plate, 100 x 200 x 6 mm, bonded to a CPVC plate of dimensions 110 x 220 x 3 mm. Both plates were bonded together with the same adhesive used in this investigation. Three specimens were constructed with adhesive thicknesses of 1.1, 1.3 and 2 mm, respectively. Glass beads were used as spacers to ensure uniform thickness of the adhesive bondline. The test setup was designed to determine the thermal gradient from the liner to the steel wall and to evaluate the induced strains at the interface layers at different temperatures. For thermal expansion measurements, both plates were instrumented by Vishay Micro-Measurement 350-Ohm electric resistant strain gauges attached on both sides of each plate in the short (X) and long (Y) directions. For temperature measurements, Omega 36-gage wire, calibration type-k thermocouples were attached on each of the outer surfaces of the steel and CPVC plates as well as on the adhesive layer for a total of three thermocouples to measure the temperature distribution within the composite specimen. A square stainless steel box (300x300 mm) with a depth of 250 mm was used as the testing container. The box was filled with water and was heated to expose the material surfaces to a temperature range of 23 o C to 80 o C. To maintain a fairly constant temperature, 50 mm thick Styrofoam Scoreboard insulation was taped around the sides and the bottom of the box. Another piece of insulation was used as the top cover. A rectangular opening, slightly smaller than the specimens, was cut out of the cover. During testing, the specimens were rested on top of this opening so that they were only supported in the vertical direction at their outer edges. A Lindberg/Blue immersion heater with a dial thermostat was placed inside at the bottom surface as shown in Fig. 9. An insulating cover was used to protect the specimen and to subject the bottom surfaces of the steel and CPVC to the desired temperature as shown in Fig. 9. The first phase of the thermal strain gradient tests was to experimentally characterize the thermal expansion behavior of individual materials used in the current study. Three tests were conducted 11

12 on each of the steel and CPVC plates to measure their coefficients of thermal expansion (CTE) in both the long and short directions. Each test was repeated three times and the CTE was calculated based on the average of all tests. The steel plates were then bonded to the CPVC plates using different adhesive thicknesses. The specimens were subjected to a temperature range of 23 o C to 80 o C while strains and temperatures were continuously recorded using a data acquisition system. Once the bottom surface of the specimen reached 80 o C, the heat was maintained for several minutes to allow stabilization of the data. The insulating cover was then detached and the hot water in the bath was replaced with cooler water. The complete heating and cool down cycle lasted around four to five hours Coefficients of thermal expansion The measured coefficients of thermal expansion for the steel and CPVC are shown in Fig.10. In general, the measured CTE is influenced by the rate of temperature increase. Increasing the rate of temperature increase increases the measured CTE. Based on test results, the average CTE for the steel in both short and long directions of the plate was 12.1 ppm/ o C. Results clearly demonstrated the anisotropic nature of the CPVC material. The measured CTE for the CPVC in the short (Y) direction was 98 ppm/ o C, which is 26 percent higher than that in the long (X) direction. Divergence of the measured CTE values at higher temperatures was observed for the CPVC specimens. Such a behavior could be attributed to different material characteristics of the samples used in testing. Despite the specimens being cut from the same sheet, these differences are likely attributed to the inconsistent thermal characteristics of the CPVC, which is common for many plastic materials. Similar tests conducted on an adhesive sample showed that the average CTE for the adhesive used in the current study was 68 ppm/ o C Test results of bonded specimens Three bath tests were conducted on specimen 1 with a nominal adhesive thickness of 2 mm. As expected, the maximum temperature occurred on the bottom of the CPVC, which was directly exposed to the heat and moisture generated by the water bath. The strain behavior in both the long (X) and short (Y) directions of specimen 1 is shown in Figs. 11a and 11b, respectively. Similar behavior was observed for specimens 2 and 3 with nominal adhesive thicknesses of

13 and 1.3 mm, respectively. The figures clearly show the interaction between the steel and CPVC. Results indicated an immediate distinction in the dissimilar behavior of CPVC in the short and long directions. Steel, however, remained linear and behaved as expected with the bottom surface expanding more than the top surface. A non-linear behavior was observed in the straintemperature relationship of the CPVC. The non-linearity was highly pronounced in the short direction of the specimen. This behavior could be attributed to the anisotropic behavior of the CPVC with the CTE value in the short direction is 26 percent higher than that in the long direction. Furthermore, the selected rectangularity of the specimen (l long /l short = 2.0) could increase the measured strains in the short direction. Finite element analysis confirmed the above observation, as will be discussed later in this paper. Results clearly indicate that as temperature increases the bond between the steel and CPVC deteriorates significantly. Such a phenomenon was evidenced by the highly non-linear behavior of the CPVC material. Consequently, the CPVC is allowed to expand more freely at these high temperatures, preventing the full composite action. Nevertheless, the maximum measured strains in the CPVC were significantly less than those measured in the individual tests conducted on an unbonded CPVC plate. This behavior suggests that the adhesive, though much less effective at high temperatures, still has a restraining effect. Cross-sectional snapshots of the strain profile in the short direction are shown in Fig. 12 for specimens with 1.3- and 2-mm thick adhesive layers. At relatively low temperatures, around 40 o C, minimal strains existed in both the X and Y directions and linear strain distribution over the thickness was observed. A considerable increase in the induced strains was observed at temperatures greater than 70 o C. The tensile strains at the top and bottom of the steel plate were almost identical in the X and Y directions. However, the measured strains in the CPVC were much higher in the Y direction compared to the X direction. The induced shear strain within the adhesive layer is strongly influenced by the strain difference between the top of the CPVC and the bottom of the steel plates, Δε, and the thickness of the adhesive, t a. It should be highlighted that the strains are not uniform across the width of the specimen due to the restraints provided along 13

14 the perimeter of the specimen. The shear strain, γ, within the adhesive layer is directly proportional to Δε / t a as expressed by Eq. (1) γ Δε (1) t a Test results of different specimens showed that increasing the thickness of the adhesive layer increases the shear strain within the adhesive layer and weakens the interfacial zone considerably. At a temperature of 60 o C, a 61 percent increase in the shear strain was observed by increasing the thickness of the adhesive from 1.3 to 2 mm. Test results showed that such an increase in shear strain is attenuated at high temperatures. 3. Development of a finite element model To gain better understanding of the test results, a linear elastic finite element model was developed using Strand7, Version [11]. The model was created using 10,368 8-node brick elements. Each node has 3 translational degrees of freedom. Three layers were used within the thickness of the steel, adhesive and CPVC materials as shown in Fig. 13. The mesh dimensions were selected to provide adequate aspect ratio for all elements. The CPVC was modeled as an orthotropic material with different properties in both the long (X) and short (Y) directions. Material properties used in the analysis are summarized in Table 3. Vertical restraints were assigned to all the nodes along the perimeter of the bottom surface of the CPVC plate, thus allowing free expansion in the horizontal plane. The maximum temperature gradient measured for specimen 1 with an adhesive thickness of 2 mm, was selected to verify the analytical model. All the nodes at the interface as well as those at the top and bottom surfaces were subjected to varying temperatures. The analysis showed that the CPVC material expands at a higher rate than the steel, resulting in warping of the composite section. Higher shear strains were predicted within the adhesive layer in the Y direction than the X direction, which matched the observed behavior during testing. The predicted strains in the steel and CPVC plates compared to the measured values are plotted in Figs. 14a and 14b. The predicted strains compared well with the measured values for the steel plate at the top and bottom surfaces. However, due to non-linear behavior of the CPVC material, less agreement was observed at temperatures greater than 50 o C. 14

15 3.1 Influence of end restraints To study the influence of the vertical restraints used in the original model on the strain distribution within the specimen, the restraints were removed from the perimeter of the bottom surface of the CPVC plate. Therefore, the model was allowed to expand freely with no effect from the boundary conditions. All other design considerations remained the same as the original model. The analysis indicated that the strains at the top and bottom surfaces of the CPVC plate in the long (X) direction were increased by 31 and 42 percent, respectively. As a result, a corresponding decrease in the strains in the short (Y) direction was observed. At the steel surfaces, the changes in the tensile strains were less pronounced. 3.2 Influence of specimen s geometry To study the influence of the rectangularity of the test specimen, the model was cut in half. The resulting square model has overall dimensions of 100 x 100 mm and was modeled with 5, node brick elements. A comparison among different models shows significant changes only in the CPVC material. The large difference for CPVC strains in the short and long directions was almost diminished. The differences in the strains are only attributed to the orthotropic behavior of the CPVC. 4. Conclusions Based on the results of this investigation, the following conclusions can be drawn: 1. The combination of high temperature and high sustained stress level has a significant effect on the tensile strength of the tank lining materials used in the current study. 2. Temperatures up to 60 o C did not have any detrimental effect on the tensile strength of the CPVC provided that the sustained stress level is less than 30 percent of the tensile strength of the material tested under normal environmental conditions. The combined effect of high sustained stress level and temperatures started to be pronounced at temperatures greater than 60 o C, causing significant reduction of the tensile strength of the CPVC material. 15

16 3. The maximum temperature exposure for the adhesive considered in the current study should be less than 60 o C with a sustained tensile stress level in the range of 10 percent of the tensile strength of the material tested under normal environmental conditions. 4. Temperature significantly affects the bond characteristics and failure mode of bonded CPVC-to-steel joints. Typical cohesive failure was observed for the specimens tested at a room temperature of 23 o C. Increasing the applied temperature deteriorates the interfacial zone and results in an adhesive-type failure due to the ingress of hot water at the interface. 5. The adhesive could sustain a stress level of 15 percent at room temperature without any sign of degradation in the bond strength. At a sustained stress level of 8 percent, the maximum temperature that can be used without significant reduction of the bond strength of the CPVC-to-steel joints is 50 o C. 6. The adhesive thickness affects the performance of the tank lining materials. Increasing the adhesive thickness results in a corresponding increase in the induced shear strains within the adhesive and accelerates debonding-type failure. Such an increase in the shear strains is highly dependent on the surrounding temperature. 7. Detailed design of tank lining is mandatory for different tanks. The performance of the lining material is highly dependent on the boundary conditions, temperatures, adhesive thickness and geometry of the tank. Acknowledgments This project was conducted while Dr. Hassan was a visiting scholar at North Carolina State University. The authors would like to acknowledge the support provided by the National Science Foundation (NSF) Industry/University Cooperative Research Center (I/UCRC) for the Repair of Buildings and Bridges with Composites (RB2C) and the support provided by IPS Corporation, CA, USA. References [1] Krieger, R. B. Evaluating structural adhesives under sustained load in hostile environment. 5 th International Sampe Technical Conference 1973:

17 [2] Fay, P. A. and Maddison, A. Proceedings of the structural adhesives in engineering II. Butterworths Scientific 1989: [3] Karbhari, V. M. and Shulley, S. B. Use of composites for rehabilitation of steel structuresdetermination of bond durability. Journal of Materials in Civil Engineering 1995;7(4): [4] Tsamasphyros, G. J., Kanderakis, G. N., and Marioli-Riga, Z. P. Thermal analysis by numerical methods of debonding effects near the crack tip under composite repairs. Applied Composite Materials 2003;10: [5] Hahn, O. and Ewerszumrode, A. P. Influence of the setting conditions on the property profile of adhesive-bonded joints between adherends with different coefficients of expansion. Welding and Cutting 1998;50(3): [6] Ramani, K., Verhoff, J., Kumar, G. and Blank, N. Environmental durability of moisturecured urethane adhesive joints. International Journal of Adhesion and Adhesives 2000;20: [7] Smith, G., Hassan, T. and Rizaklla, S. Bond characteristics and qualifications of adhesives for marine applications and steel pipe repair. Proceedings of the third International Conference on Construction Materials 2005: CD-ROM. [8] Corzan Industrial Systems. Thermal conductivity of Corzan CPVC. Design Manual [9] ASTM D638-03, Standard test method for tensile properties of plastics. American Society for Testing Materials, West Conshohocken, PA, USA; [10] ASTM D , Standard test method for strength properties of double lap shear adhesive joints by tension loading. American Society for Testing Materials, West Conshohocken, PA, USA; [11] Strand 7 Finite Element Software version Theoretical manual, Australia;

18 List of Figures Fig. 1 Typical bleach tank during construction Fig. 2a CPVC test specimen Fig. 2b Adhesive test specimen Fig. 3a. Bolts and caps secured in place Fig. 3b. Tubes sealed onto caps Fig. 3c. Heater and thermostat Fig. 4a. Environmental exposure test set-up Fig. 4b. Schematic layout of the environmental exposure test set-up Fig. 5a. Average tensile strength of Group A CPVC specimens Fig. 5b. Typical failure of CPVC specimens exposed to zero sustained stress level Fig. 6a. Average tensile strength of Group A adhesive specimens Fig. 6b. Typical Failure of adhesive specimens Fig. 7a Dimensions of double-lap shear specimen for Group A Fig. 7b Dimensions of double-lap shear specimen for Group B Fig. 8a Typical cohesive failure of double-lap shear specimens at 23 o C Fig. 8b Typical adhesive failure of double-lap shear specimens at 60 o C and 80 o C Fig. 9 Test setup for thermal gradient tests Fig. 10 Coefficients of thermal expansion of different materials Fig. 11a Typical strain behavior in long (X) direction Fig. 11b Typical strain behavior in short (Y) direction Fig. 12 Typical strain profile at different temperatures in the short (Y) direction Fig. 13 Linear elastic finite element model Fig. 14a Predicted and measured strains in the steel in the short direction Fig. 14b Predicted and measured strains in the CPVC in the short direction 18

19 Fig. 1 Typical bleach tank during construction 19

20 Fig. 2a CPVC test specimen R= All dimensions are in mm R= Fig. 2b Adhesive test specimen 20

21 Fig. 3a. Bolts and caps secured in place Fig. 3b. Tubes sealed onto caps Fig. 3c. Heater and thermostat 21

22 Fig. 4a. Environmental exposure test set-up Hollow structural steel member J-bolts Steel roller Test specimen placed inside a CPVC cylindrical container Movable wooden support Additional weights are added to reach the designed sustained load level Strong floor Fig. 4b. Schematic layout of the environmental exposure test set-up 22

23 80 70 CPVC Temperature = 23 degrees Temperature = 60 degrees 60 Temperature = 80 degrees Tensile strength (MPa) o C 60 o C 80 o C 0 0% 15% 30% 0% 15% 30% 0% 15% 30% Sustained stress level (%) Fig. 5a. Average tensile strength of Group A CPVC specimens Fig. 5b. Typical failure of CPVC specimens exposed to zero sustained stress 23

24 25 20 Adhesive Temperature = 23 degrees Temperature = 60 degrees Temperature = 80 degrees Tensile strength (MPa) Specimens failed during the exposure period at 15% and 30% sustained stress levels 0% 15% 30% 0% 15% 30% 0% 15% 30% Sustained stress level (%) Specimens failed during the exposure period at 15% and 30% sustained stress levels Fig. 6a. Average tensile strength of Group A adhesive specimens Increasing temperature Fig. 6b. Typical failure of adhesive specimens 24

25 R= CPVC 12.5 Steel 1.6 Fig. 7a Dimensions of double-lap shear specimen for Group A R= CPVC 25 Steel All dimensions are in mm Fig. 7b Dimensions of double-lap shear specimen for Group B 25

26 Fig. 8a Typical cohesive failure of double-lap shear specimens at 23 o C Fig. 8b Typical adhesive failure of double-lap shear specimens at 60 o C and 80 o C 26

27 Fig. 9 Test setup for thermal gradient tests 27

28 70 Difference in temperature ( o C) Coefficient of Thermal Expansion (PPM/ o F) CPVC in short (Y) direction CPVC in long (X) direction Steel (in both short and long directions) Difference in temperature ( o F) ` Steel CPVC 105 Fig. 10 Coefficients of thermal expansion of different materials X Y X Y Coefficient of Thermal Expansion (PPM/ o C) 28

29 190 CPVC Bottom Steel Top CPVC Top 77 Temperature ( o F) Steel Bottom Specimen 1 Adhesive thickness = 2 mm Temperature ( o C) Tensile strain x 10 6 Fig. 11a Typical strain behavior in long (X) direction 190 CPVC Bottom Steel Top Steel Bottom 77 Temperature ( o F) CPVC Top Temperature ( o C) 90 Specimen 1 Adhesive thickness = 2 mm Tensile strain x 10 6 Fig. 11b Typical strain behavior in short (Y) direction 29

30 mm 2 mm Temperature ( o C) Steel Adhesive CPVC Tensile strain x mm 2 mm Temperature ( o C) Steel Adhesive CPVC Tensile strain x mm 2 mm Temperature ( o C) Steel Adhesive CPVC Tensile strain x 10 6 Fig. 12 Typical strain profile at different temperatures in the short (Y) direction 30

31 Steel Adhesive CPVC Fig. 13 Linear elastic finite element model 31

32 Steel Top Steel Bottom (EXP) Experimental FEA Temperature ( o F) Adhesive thickness = 2 mm Temperature ( o C) Tensile strain x 10 6 Fig. 14a Predicted and measured strains in the steel in the short direction Temperature ( o F) Initial slope (EXP) CPVC Top (FEA) CPVC Bottom (FEA) CPVC Bottom (EXP) CPVC Top (EXP) Adhesive thickness = 2 mm Temperature ( o C) Tensile strain x 10 6 Fig. 14b Predicted and measured strains in the CPVC in the short direction 32

33 List of Tables Table 1. Tension test results of the CPVC and adhesive materials Table 2. Test results of double-lap shear specimens Table 3. Material properties used in FEA 33

34 Table 1. Tension test results of the CPVC and adhesive materials Material CPVC Adhesive CPVC Adhesive Solution Water Water Water 5% NaCl Water 5% NaCl Sustained stress Tensile Strength (MPa) * Soak Time (days) level (%) T=23ºC T=60ºC T=80ºC Group A 0% % % % % NA** NA 30% NA NA Group B % % % % % NA 10% NA NA NA % NA 10% NA NA NA * The tensile strength is based on the average of two specimens tested at the same environmental conditions. The difference in the measured tensile strength of any two specimens ranged from 6 to 15%. ** The specimens failed during the exposure period. 34

35 Table 2. Test results of double-lap shear specimens Temp (ºC) Test conditions Average shear Sustained stress level Solution strength (MPa) (%) Group A # of specimens Days survived 0% ºC Water 15% % NA** % ºC Water 15% NA % NA % ºC Water 15% NA % NA Group B 0% ºC Water 8% % % ºC Water 8% % NA % ºC Water 8% % NA 4 4 0% ºC 5% NaCl 15% ** The specimens failed during the exposure period. 30%

36 Table 3. Material properties used in FEA Property Steel CPVC X-direction CPVC Y-direction Adhesive Elastic modulus (MPa) 200, Poisson's ratio Shear modulus (MPa) 77, CTE (ppm/ o C)