Low concrete test for retaining wall footing

[McSEpllc]

Hi Everyone,
A concrete cylinder break was tested at 1,140 psi at 7 days for the footing of a retaining wall. This is below the roughly 1,575 psi compressive strength it should be. The design and specification are based on 3,000 psi. Swiss hammer tests confirmed the low concrete strength. The shear capacity of the heel of the footing is over-stressed for this condition, the toe is fine. The retaining wall is 19’-6” high and is supporting a pool on the upper level.

I am thinking about adding additional concrete on top of the footing and tying it with dowels for shear transfer and monolithical behavior. The GC already set the reinforcing and form for 60-ft of the wall in place.
My questions are:

- Has anyone done a repair like this before?

- Can the wall be placed before the footing extension? In theory only the heel would need strengthening, which could be doweled to the face of the wall. This goes against my gut-feel though.

- I presume I can find the amount of shear resisted by EI ratio of the existing and added footing. Is this how you would analyze this?

- A retarder had been added to the concrete mix as this was a pump mix. Could a retarder delay strength to this degree after 7 days? Does anyone have past experience on what percentage the strength would be at 7 days vs. 28 days using retarder?

Thanks!
Eric




Eric McDonald, PE
McDonald Structural Engineering, PLLC

 

[Ron]

Eric...don't do anything until you get the 28-day strength results. There are characteristics of the mix design that can cause delayed strength gain. Retarders don't usually affect the mix for that long, but that can also depend on several other mix characteristics.

I assume from your estimate of 1575 psi at 7 days, you are using a mix that contains either fly ash or ground, granulated blast furnace slag cement....both cause delayed strength gain. If used together, it can be very pronounced.

Also don't rely on the Swiss hammer for anything related to strength. It is not good for that. You can use it to check the consistency of the low strength issue by comparing similar rebound numbers, but it might or might not be indicative of the actual compressive strength.

If the strength does not come up at 28 days, core the footing and check the in-situ strength. If still low, then consider repair alternatives.

 

[McSEpllc]

Hi Ron, Thanks for your feedback. Yes, I do not put much trust into the swiss hammer test myself even though it seemed to be in the right range testing other concrete. The testing agency is core-drilling three 4" diameter samples to confirm strength. The concrete provider is known to have concrete high in cement, but I have not seen the concrete mix design as of yet. The time constraints on this project are such, that the GC will rather do a repair then wait 3 weeks.

Eric McDonald, PE
McDonald Structural Engineering, PLLC