We are working on a project for Heathrow, and they are pushing to get the project moved on. We are installing a demountable modular building on a new concrete slab. The end client is pushing for the anchor plates to be installed (they haven't completed the pour yet) but are asking when we can drill and fix the anchor plates. We have always worked on 28days but does anyone have any information as to whether this can be reduced and at what cost to the overall strength of the anchors. They are pouring C32/40 concrete.
Thanks for your question. Tests were performed with Hilti chemical anchors and the reductions to be applied have been identified even to very early age concrete. The conclusion from these tests is that the drop in the resistance is much coincident to the reductions factor identified by EC2 for early age concrete. Note that the cement used in the concrete has a significant influence before the 28 days and as such the reductions to be observed must have this extra variable in consideration.
As practical rules please considerer the following:
1. Before the concrete reach 3 days of curing it is not advisable to install the anchors since the scatter for the concrete resistance is to high.
2. From day 3 to day 20 a reduction should be applied considering the values presented according to the
EC2 expressions for early age concrete. Please consider the diagram below that identifies the reductions to each concrete age based on the cement class.
3. After day 20 the reductions are minimal regardless of the cement class and as such no special reductions need
to be applied.
