I have to design a base plate with post installed anchors in a concrete column which has fc'=10 ksi. Profis warns that anchors are available up to 8 ksi of concrete strength. Are there any post installed anchors that can be used for this?
Hi Erika,
Additionally, the anchoring-to-concrete provisions found in ACI 318-14 Chapter 17 have a concrete compressive strength with an upper limit of 8,000 psi for post-installed anchors unless testing has been performed for higher compressive strengths. Tests performed for Hilti expansion anchors, screw anchors, and adhesive anchors in higher strength concrete have shown that loads do not drop below what is calculated at the ACI 318-14 maximum of 8,500 psi compressive strength. Having said that, if the product is installed in higher strength concrete, the load should be limited to what is calculated at 8,500 psi. Keep in mind, the ICC-ES report limits the compressive strength to 8,000 psi for calculation purposes.
For mechanical anchors, Hilti has not determined the upper limit at which you can install each anchor system successfully and where you might start to get a reduction in load performance. Unfortunately, at 12,000 psi for example, a screw anchor may have difficulty being installed in one concrete batch, but in another batch (with a different aggregate), it may work. Also, in very high strength concrete, drill bits typically wear down very fast making it difficult to install the anchors. Thus, the installer would need to make sure the bits are newer and monitor the behavior of the anchors during the setting process. If the screw anchors are not setting properly, it may be because the bit is worn down, and the hole is too small. For example, if expansion anchors need thirty hammer blows to install, this likely means the drill bit is worn down too much resulting in the wedge not having room to move over the mandrel when installed and loaded.
For adhesive anchors, Hilti has not tested the upper limits. Also, there is typically no concern related to installing the anchors with worn bits.
In summary, Hilti has limited testing showing that expansion anchors, screw anchors, and adhesive anchors will work in higher strength concrete applications, but the load should still be limited. Additionally, there will be a limit on mechanical anchors where they can no longer be set correctly due to drill bits typically wearing down very fast. If the anchoring application is safety related or the design utilization is high in the calculation, on-site evaluation, prescribed by the engineer on the project, or engineering judgment could be considered.
Regards,
Ashley