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Re Core tests

Re Core tests

Re Core tests

(OP)
Hi
We are erecting a new steel building. The building is 60’ by 40 ‘. The steel supports are placed at 20’ intervals. Largest supporting steelwork is respectively north to south placed at 20’and 40’. The building faces north with a width of 40’ the length is 60’ north to south. We were asked to pour a 3’ foundation, no spec was given for width of foundation so we poured at 16” wide. Where the steel supports are situated we made an area of 3’ square. Because of the site lay we poured part of the foundations to get us to ground level, leaving approx. 18” still to pour. In the foundations were also places rebar 1\2” and incased in wire mesh.
A problem arose so the client decided to have the foundations tested for strength. An engineer came out and used a rebound hammer to test the concrete. His instrument read 2000 psi in all locations.
His recommendation was then to have core samples taken and tested.
Three locations were taken and the following results were given.

CR    Unit weight    Compressive Strength psi
1    133.8            960
2    133.9            2030
3    120.1            510

All areas tested were poured at the same time using the same mix. We mix our own concrete on site.
Also the test was done at about the 20th day after the pour. Core samples were soaked in a lime/water solution for a min of 24 hours prior to the tests. (Is the normal?).

From these results the engineer has recommended that all the concrete be removed and we start pouring again from scratch following his guidelines.

Can any one please help shed some light on this matter?
As I see the foundations are just not required in the first place.  

RE: Re Core tests

Dear Alex,
Sorry for being late but i had a discontinuity in the network. Regarding your problem I have several comments:
1- No specified concrete strength given to evaluate the stress
2- Rebound Hammer is not an accurate test
3- Cores results are very wide, I beleive you have problem in extraction, dimension of the core and/or compression test
4- Removing of concrete should be attained when a specified strength is mentioned and that core strength are not doubtful.
My advice is to extract three other cores and tested as per ASTM C42-00 and core strength to be evaluated according to ACI 318.
Should you need any further information please contact me at abedkayrouz@yahoo.com

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