Holdfast Training Services Trial Blog

How risky is it to estimate the value of compressive strength of concrete using a rebound hammer test (RHT)?

The concrete testing failed when testing the concrete cylinders for one of our pile caps. One cyclinder failed early in shear and the other was a machine error.  This means we do not have the 56 day compressive test strength for the cylinders to indicate the strength of the pile cap.  We require 80MPa.  At 7 days we had 60MPa, at 28 days we had 69 MPa and we don’t have a value for it at 56 days.  The concrete company came out on site and conducted the RHT at 12 locations on the pile cap in question, and 12 locations on a similar pile cap as a comparison.  They estimate from this test the compressive strength is 86.2MPa so comfortably above the 80MPa required.  This doesn’t necessarily sit easy with me.  The RHT is a test to establish the surface hardness of the concrete structure so I don’t 100% understand how this can be compared to compressive strength unless through empirical data maybe.  The fact that we went from 60 at 7 days to 69 at 28 days doesn’t fill me with confidence that we will hit 80 at 56 on that current trend.

So what…..core it and test the core? or, does this pile cap actually need 80MPa therefore can we take the risk on it being borderline? – it is one of the mega piles which attaches into the core as part of the stability system though!!

I know the fact we have a bit of paper from the concrete manufacturers giving an estimate of 86.2MPa will probably suffice for BMC – someone has put their signature to it – but hardness vs strength doesn’t quite compute in my head.  Have we mitigated the risk appropriately or not?    Thoughts welcome.

Having sought advice….

The RHT (Schmidt) compares concrete thereby giving an indirect assessment of concrete strength.  In this scenario, the test can be conducted on the pile cap in question and another similar pile cap for which we have a positive crush strength, and the results compared to estimate the strength.

BS EN 12504-2 details the procedure for doing this (thanks John).  Having cast my eye over this short document there are a couple of points which are important to abide by to get reliable results:

1. The surface must be smooth to get an accurate result so any rough surfaces must be ground down first.
2. Test locations between comparative structures must be tested under similar conditions and they should be tested at similar positions to reduce any possible effects due to differences in rigidity and uniformity of concrete.
3. Moisture condition of the surface should be consistent throughout the testing.  Dry surfaces are preferred.
4. The coefficient of variation of individual readings within one test should be in the order of 10%.  It should decrease with an increase in strength.

You now know what you can do, just in case any of you end up in a similar position!!