POSSIBILITIES AND LIMITATIONS OF ON-SITE ANCHOR TESTING
HOW MANY ANCHORS / POST-INSTALLED REBARS SHOULD BE TESTED?
There is no universal rule regarding the percentage of anchors or rebars that should be tested, nor is there any existing statistical basis for the percentages usually specified. Therefore, Hilti investigated the existing national and European standards to provide a proposal for the number of tests that should be done. However, the numbers as given in Fig. 3 should be seen as an indication only. This is because the requirements for the proof load program may vary significantly from case to case. Clearly, the number of anchors to be proof loaded is dictated by structural safety as well as practical considerations and the reasons for testing. For example, while it is typical on a large job to require that anywhere from 2.5 to 20 percent of the installed anchors of a given type and size be proof loaded, this requirement must be adjusted where, say, only four large anchors in a baseplate are to be verified. In such a case, it is not unreasonable to require that all four anchors be proof loaded, particularly if the consequences of failure are significant. For highly redundant applications and less critical applications such as rebar doweling for shotcrete applications or slab on grade doweling, proof loading of a minimum random sampling of 5% of the anchors may suffice. Ultimately, the Engineer of Record should determine the sampling rates.
Anchor and post-installed rebar systems carrying an approval, installed according to the manufacturer’s installation instructions and in base materials within the scope of the approval do not require on-site testing for performance verification. Generally speaking, you will encounter just three situations when on-site testing should be performed: 1. To determine the design resistance in a similar but not identical material as given in the related approval (destructive or non-destructive tests) 2. To validate the quality of installation of anchors used on the job site (non-destructive tests) 3. Combination of (a) with additional geometrical requirements like edge distances, rebar/rod diameter etc. that are different to the values reported in the related approval Non-destructive loading (proof loading) is done by applying tension loads. The selected load level is sufficiently high to provide assurance of correct installation or to determine targeted design resistance values, but not so high as to result in damage (e. g. in the form of yielding or permanent slip) to a correctly installed anchor. Proof loads should be maintained long enough to enable a determination of no anchor or rebar movement. Given this objective, it should be clear that proof loads are set as a percentage of the tested tension capacity of the anchor or rebar, not the design tension load. Note that, depending on the embedment to diameter ratio and the steel grade, this load might or might not subject the anchor to yield level stresses. Where lower yield steels are used, it should be verified that the proof loads do not exceed 80% of the nominal yield stress of the steel anchor components. If a proof load is used to verify proper installation, proof loading equipment may have load reactions close to the anchor but with sufficient clearance so any movement would be visible. If a proof load is used to determine design resistance values, proof loading equipment may have load reactions far away from the anchor to determine the base material strength. Note that Hilti provides a complete on-site testing service with the latest equipment, including a detailed test report and evaluation report. Destructive loading is also done by applying tension loads. The load level is selected sufficiently high to result in damage (e.g., in the form of yielding or base material failure). However, on-site testing with one or multiple products independent of the reason can never: 1. Serve as a substitution of the approval testing for assessing the suitability of an anchor 2. Serve as a means to conclude which is a “better” product by comparing loads from on-site testing of product A vs product B Although no universal standard exists in Europe for conducting on-site testing, this type of assessment has been in use as an adjunct to anchor installation quality control and for the determination of design resistance for many decades. Therefore, Hilti investigated the existing national and European standards to provide a consistent and global on-site testing service that is state of the art.