Adhesive Anchors in Concrete Under Sustained Loading Conditions (2009)

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66 The objective of this research project was to develop a draft AASHTO standard test method to determine the ability of adhesive anchors to resist sustained tension load. The draft AASHTO test standard developed for this project is based on developing a stress versus time-to-failure interaction diagram for individual adhesive products. The interaction diagram (or simply a table) can be used to determine the percent of an adhesive’s short-term design strength that is acceptable for use over the life of the structure. The project was divided into several phases: literature review, development of a testing pro- cedure, demonstration and evaluation of the testing procedure, and conclusions and recommendations. The final product of this research is presented as Appendix A and entitled “Standard Method of Test for Evaluation of Adhesive Anchors in Concrete Under Sustained Loading Conditions.” Recommendations It was determined that the stress versus time-to-failure method is a viable method to determine an adhesive anchor’s ability to resist sustained tensile loads. It is a relatively simple test that builds upon established ASTM and ICC-ES test methods. The procedures included in the proposed test method use exist- ing test procedures and apparatus. In other words, any labora- tory that is equipped to conduct adhesive anchor tests under ICC-ES AC308 (3) will generally be able to conduct this test. The stress versus time-to-failure method is different than the pass/fail method specified in ASTM E1512-01 (29) and ICC-ES AC308, but it is an established analysis method found in other ASTM standards. Listed below are specific changes from existing test methods (i.e., ASTM E488-96 [28], ASTM E1512-01 [29], and ICC-ES AC308 [3]) as well as elements that are not specified in the exist- ing test methods that have been incorporated into the draft AASHTO test method. Also included are suggestions developed during the validation of the test method that can improve the results of the tests. All of the following recommenda- tions and suggestions have been incorporated into the draft AASHTO test method: • The required sampling rate of 30 s for the static load test is faster than the 1-s sampling rate required for ASTM E488-96 for better data resolution. • Sampling for the sustained load (creep) test is more fre- quent than it is in ASTM E1512-01 and ICC-ES AC308. The least frequent level of sampling in the AASHTO method is once per hour, and this level of sampling frequency begins 10 hours after loading. • The incremental load rate, as mentioned in ASTM E488-96, is not used in the determination of mean static load. It was noted that load rates of longer duration, such as the incre- mental load rate, could produce lower static load strengths in an anchor. • All baseline static and sustained load tests are restrained (confined) tests. • All baseline test series of static and sustained load tests consist of a minimum of five specimens. • The confining sheet thickness of 0.020 ±0.004 in. specified in ICC-ES AC308 was considered to be too thin and was increased to a maximum of 0.06 in. thick. • In the sustained load (creep) test, the initial load is to be applied for the same duration as it is applied in the static load test. • The analysis of sustained load is not based on pass/fail cri- teria, as specified in ICC-ES AC308, but rather generates a stress versus time-to-failure relationship. • The concrete strength requirement was changed from what was specified in ASTM E1512-01 to what is specified in ICC-ES AC308. • The recommended anchor diameter was increased to 5⁄8 in. from the 1⁄2 in. specified in ASTM E1512-01 in order to avoid steel failure. • A minimum embedment depth was specified, as found in ICC-ES AC308, in order to avoid steel failure. ASTM E1512-01 specifies a 4.5-in. embedment depth. C H A P T E R 4 Conclusion and Recommendations

• The static load tests are to be conducted at an elevated temperature of 110°F. • All tests are to be conducted at a controlled humidity below 40%. • The sustained load (creep) tests have at least two load level ranges within which the anchors are to be tested. It is not necessary that all specimens in a series be loaded at the same percent load; instead, they should be loaded within a specified load level range. It is suggested that the load lev- els be set below 80% of mean static load (1) to avoid early failures and (2) to lie outside a reasonable statistical varia- tion of the mean static load. Additionally, to avoid long test durations, load levels should not be set too low. It is sug- gested that the two load level ranges should be 80% to 70% of mean static load and 70% to 60% of mean static load, with the average of each series separated by 10%. • The load should be smoothly transferred during the sus- tained load (creep) test. • The finish on the concrete member should be as smooth as possible to preclude surface spalling of the concrete during testing. Benefits of the Stress versus Time-to-Failure Test Method As discussed in Chapter 1 of this report, some of the benefits of the stress versus time-to-failure method are the following: • Test results in the form of a stress versus time-to-failure graph or a table of stress values for given lifetimes of struc- tures provide more useful design data for the practicing engineer than the pass/fail criteria in ICC-ES AC308 (3). • The reduction factor generated from the stress versus time- to-failure approach can easily be incorporated into an LRFD approach, which is in agreement with current AASHTO design philosophy. • Existing data from ICC-ES AC308 can be incorporated into stress versus time-to-failure graphs, which builds upon the database of current test results. • The stress versus time-to-failure method allows manufactur- ers to qualify a product above the minimum pass/fail stan- dard established by ICC-ES AC308 in order to distinguish their product among the products of their competitors. For example, both Adhesive A and B passed ICC-ES AC308 cri- teria, but Adhesive A performed significantly better in stress versus time-to-failure evaluation than Adhesive B. • The stress versus time-to-failure method removes the uncer- tainty associated with the mathematical projection of displacement in the ICC-ES AC308 approach. • The stress versus time-to-failure method removes the uncer- tainty associated with establishing a limit on projected dis- placement in the ICC-ES AC308 approach. • The stress versus time-to-failure method provides a rational method of incorporating test results conducted at other temperatures. • This test method creates a conservative stress versus time- to-failure relationship because it assumes that the anchor is exposed to an elevated temperature of 110°F for its lifetime. Implementing the Test Method It is recommended that this test method be provided to two or three independent laboratories qualified to conduct adhesive anchor tests in order to validate the approach and to test for precision and bias. Further Research Further research is needed to investigate this method and other factors that affect adhesive anchors. The following is a list of research needs: • It is recommended that this test procedure be conducted on more specimens in order to see if the scatter in the results can be reduced and trendlines with higher R2 values can be obtained. • Due to project budget and timeline, the effect of in-service moisture was not investigated in this project. As discussed in Chapter 1, there are many factors that affect adhesive anchors. Currently, studies and tests have been conducted only to evaluate the effects on static load strength. Research is needed to investigate these varied effects on sustained load strength. • The incremental load rate showed promise as a possible method to determine an adhesive anchor’s sensitivity to creep at high load levels. Research should be conducted to investigate whether a method using an incremental load rate can be developed to evaluate an adhesive anchor’s per- formance under sustained load. Alternatively, this could serve as a method to compare adhesives. • Design standards for adhesive anchors appropriate for AASHTO applications and load conditions should be developed and incorporated into AASHTO’s LRFD Design Specifications. • Tests of the adhesive alone show promise as methods to prescreen or fingerprint adhesive anchor systems. Research could be conducted to see if a correlation can be made to actual sustained load test results. • Stress versus time-to-failure tests should be conducted at other temperatures to investigate the change in slope of the time-to-failure curves at lower temperatures and the con- servativeness of offsetting parallel lines. 67