Long-Term Performance of Epoxy Adhesive Anchor Systems (2013)

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114 The objective of this project was to develop recommended standard test methods and specifications, design guidelines and specifications, and quality assurance guidelines and construction specifications for the use of adhesive anchor systems in transportation structures. Development of these tests, specifications, and guidelines was founded on the results of a program of experiments to determine, predict, and verify the sustained load performance of these systems in their dif- ferent applications and environments. Conclusions Anchor Testing The results from the unconfined short-term tests suggests that the 0.75 ratio of unconfined bond strength to confined bond strength in ACI 355.4-11 to determine the unconfined bond strength from a series of confined tests might be a sig- nificant overestimate of unconfined bond strength. Tests on the three high bond strength adhesives in this research project produced factors from 0.37 to 0.53. Sustained Load Sensitivity A stress versus time-to-failure approach was used to evaluate the sustained load performance of three adhe- sive anchor systems in concrete. SvTTF relationships were developed for the baseline (control) and for multiple param- eters. An aST-baseline relationship was developed for each parameter, which assumed that the reduction in strength at any point in time was the same as the reduction in strength experienced in short-term testing. An influence ratio was determined to evaluate sensitivity to sustained loading. For the parameters tested in this project, only elevated service temperature [>120°F (>49°C)] and manufacturer’s cure time were shown to have an influence on the sustained load performance. Sustained Load Testing The short-term tests (which failed at 100% MSL and 2 min- utes) plotted well above the SvTTF relationship generated from sustained load tests alone. The reduced expected failure stress level for short-duration loads appears to result from a dual requirement placed on the polymer. The magnitude of the load causes the polymer to undergo plastic deformation as it redistributes the load down the anchor, and the sustained nature of the load causes the polymers to migrate within the adhesive. These two actions occurring simultaneously reduce the capacity. Displacements at failure from sustained load tests were 1.3 to 2.2 times larger than peak failure displacements from short-term tests and 1.4 to 2.9 times the limiting displacement at loss of adhesion. The current ACI 355.4 projection method for sustained load displacement projects to a limiting failure displacement from short-term tests. This ACI 355.4 approach was determined to be reasonable as the limiting displacement was essentially a lower bound (5% fractile) of sustained load failure displacements observed in the test program. If an SvTTF approach is adopted, sufficient testing should be required to minimize the influence of test scatter of time to failure for given stress levels. Adhesive-Alone DSR Creep to Dogbone Testing Correlation The DSR creep tests and dogbone testing for the three adhe- sives did not provide consistent correlation. Time–temperature superposition did not work for Adhesives A and B due to nonlinear viscoelastic behavior. Time–stress superposition worked for Adhesive B, but only with a shift factor that was dependent on stress level. There was no good relationship for Adhesive A, which is probably due to a large amount of fillers in the product as determined in the thermogravimetric analysis. However, time–temperature superposition did work very well for Adhesive C due to its linear viscoelastic behavior. C H A P T E R 5 Conclusions and Suggested Research

115 showed that this value can be in the range of 0.35 to 0.55 and is significantly less than the value of 0.75 currently assumed in ACI 355.4-11. It is proposed that AASHTO require testing to verify this value for each adhesive. Sustained Load Sensitivity If it can be shown that an anchor would be expected to be at or above 120°F (49°C) for significant portions of its service life, it is proposed that AASHTO require the adhesive anchor system to be tested and evaluated for Temperature Category B at a temperature equal to or greater than its highest service temperature. Additionally it is suggested that adhesive anchors that will be used for sustained load applications be allowed to cure an additional 24 hours beyond the manufacturer’s minimum cure time prior to loading or torquing. Modifications to AASHTO TP 84-10 Due to scatter in the time to failure at a given stress level, it is proposed that AASHTO TP 84-10 include sustained load tests at three stress levels as opposed to two stress levels. Furthermore, as discussed above, the short-term test results should not be included in the SVTTF curve developed in AASHTO TP 84-10. Suggested Research The following topics are suggested for further research. • Complete reliability study of AASHTO load and resistance factors pertaining to adhesives anchor applications. • Additional sustained load testing at high stress levels could be performed to better identify the stress versus time to failure relationship within the time to failures of a few hours. • Adhesive anchor sustained load tests at room tempera- ture at various adhesive cure times could be performed to evaluate the influence of cure time on the sustained load performance. Adhesive-Alone to Anchor Pullout Testing Correlation No consistent correlation between adhesive-alone (dogbone or DSR creep) and anchor creep tests was discovered. While the dogbone tests did a very poor job predicting the SvTTF results for Adhesives B and C, they did a better job for Adhesive A. This is due to the difference in loading of adhesive anchors and dogbone specimens. The adhesive anchors are confined speci- mens in a hole loaded under shear. The dogbones are uncon- fined specimens loaded in pure tension. This is possibly also due to the poor adhesion of Adhesive A. Adhesive anchor systems with better adhesion can develop more friction along the sides of the hole prior to failure as the adhesive/anchor “plug” will have pieces of concrete attached to it. Dogbone specimens do not have this additional frictional resistance. In summary, dogbone tensile specimens are poor predictors of long-term and short-term performance, and are not recom- mended for qualification testing for adhesives for anchors. Early-Age Evaluation The effect of early-age concrete on the short-term bond strength for the three adhesives was investigated. ACI 355.4 specifies a 21-day minimum on concrete age for adhesive anchor installations. It was shown that for the three adhesives tested the bond strength did not increase significantly after 14 days (Adhesive A) and 7 days (Adhesives B and C). It is believed that the high level of internal moisture existent in early-age concrete was the leading contributor to lower bond strengths in the earlier-age concrete tests. Suggestions Anchor Testing The alpha-setup factor for the relationship between un confined to confined bond strength in ACI 355.4-11 should be adjusted or a test series added to determine this relation- ship for individual products. The results of this research