Large Slab Poured During Hard Rain

[avl58]

I am having a large ICF home built. It is an on grade slab with 36" footings and a 4" slab. The foundation was engineered.

Shortly after the pouring started rain came that lasted for nearly 6 hours, basically during the entire pour. Rain varied from heavy to light. Parts of the slab were covered but a lot of it was not. At one point one of the cement trucks got stuck in mud, so they brought in a pump to pump in the concrete from 120 feet from the road. So there was about a 45 minute gap between pours and also a slight change in concrete mixtures.

A lot of the surface did not look good so they added dry Portland cement and worked it in the with power trowler. Some of the corners are washed away and you can see rough stone. There is also a lot of color variation with some areas being completely white and others gray.

The contractor and concrete company are saying it is just cosmetic and are going to do a PSI check in 28 days.

What should we do? Is it enough to do the PSI test? Or, does the surface need to be removed and a few inches added onto it?

 

[CELinOttawa]

You need the support of a local Structural Engineer. You do not know construction, and are playing for keeps against people who's job it is to tell you that everything is fine. It may very well be fine, but the chances of this are very low given what you have outlined above.

IN GENERAL: A lot depends on your contract, location, the acceptable standards of practice for your area, and you ability to win what will be a protracted fight with the builder, but effectively most of this is unacceptable.

Wording varies, but within different language and other tweaks, all concrete placement specifications require:

- Formwork sufficiently stiff to not shift or change shape during pour.
- Protection of concrete from direct concrete with ground, unless this has been considered in the design.
- The temperature at pour is to be within a permissible range and maintained therein for a set period.
- The pour is to be protected from additional water.
- No additional cementitious powder is to be added to the placed concrete.
- No water is to be added to the concrete once it leaves the batch plant.
- Surfaces shall not be overworked.
- Wet curing shall be undertaken as soon as the surface has set, or alternatively the slab shall be cured with a purpose-made compound in strict compliance with manufacturer's written directions.

Frankly now that the slab is poured, you're going to have one hell of a fight. The contractor is going to do everything they can do deny replacement of the slab, but what you are describing is a recipe for poor performance and lack of durability.

Note: If the cores are not CUT CORES removed from the slab, they are meaningless. Cores are often poured at the start and end of the batch placement right from the truck into cardboard tubes. They are protected from the rain and cured on site to show that the curing conditions, temperatures, etc, were acceptable. Worse yet, some contractors and batch plants actually take the cores back to a curing room; This would then mean that the values will only ever be able to show that the mix design was correct, but tell you nothing about the contractor's preparations, placement, handling, weather protection, etc.

You need LOCAL HELP, NOW. Stop posting to the Internet. Find a local engineer, or suffer the consequences.

 

[darthsoilsguy2]

Not taking anything away from what CEL said, even if they cut cores... it is a song and dance. The strength is tested in compression and apart from not being able to get a good sample due to size restrictions due to limited thickness.... the weakest concrete will be on the surface and may not affect the failure mechanism of that test much because it may be cushioned by the capping method for the core. i might not be making sense to you on this... but ... basically ask yourself if you are worried about the concrete in the middle depth of the 4" slab or the overall original strength of the concrete, if you are worried about that testing cores makes sense...... but if you are concerned about the surface and think the center of the slab is probably fine, then testing cores doesn't work. seeing how they volunteered to see what the strength is at 28, it is probably because they have cast cylinders that were never rained on.

If it were my slab, and and assuming they have cylinders cast, i would get those samples before they break them so that you can have a control set for your engineer or some other BS... but if nothing else it let's them know you don't give a crap about their 28-day cylinder stength. The cylinders were cast from your concrete, they belong to you. While at it, i would get the batch tickets for the pour too to look into your concern on the cold joint delay among other things your engineer may wish to review.

6-hr rainstorms don't sneak up on people. your concrete guys and whoever got them to pour that day do not give a crap. The concrete manufacturer is probably in the clear on this one as they have no control over placed material protection or means/methods so don't let them in the discussion unless they are throwing the GC under the bus. i hope you win your fight, not many do.

 

[Ron]

CEL and DSG2 have given you good advice.

Strength and durability are two different animals in concrete....you can achieve strength without achieving durability. A compressive strength core will be taken and its top and bottom surfaces will be trimmed away to create a relatively flat and planar surface for testing. They will likely remove the offending durability issue, so you are left with a test that is misleading and supportive of the contractor, particularly from a contractual standpoint....your contract probably only says that the contractor will supply 3000 psi concrete....I doubt that it says that it will be batched, mixed, delivered, placed and finished in accordance with any particular standards. Therein lies the problem.

Your contractor has deviated from accepted industry practices and might have violated building codes, depending on where you are located and the applicable standards referenced by your code.

DO NOT LET THE CONTRACTOR OFF THE HOOK FOR THIS!!! He will use poor methods to make the slab look better ("they added dry Portland cement and worked it in the with power trawler"....this is one example...absolutely unacceptable practice).

If you want good evidence of the contractor's breaches of accepted practice and to show the real issues with the concrete (varying water-cement ratio with depth, poorly bonded cement paste, water gain, air content, microcracking, poor finishing, etc) you will have to get a competent petrographic examination of the concrete. This is done from cores extracted from the slab; cut and polished for microscopic examination, and then examined for various anomalies expected from this type of abusive construction practice. Each core subjected to petrography will cost about $1000 US.

Do not accept an extended warranty from the contractor......he will offer. If this were my slab or owned by one of my clients, the decision would be simple...tear it out and start over. As DSG2 noted and I agree, I doubt that a 6-hour rain was a surprise event. The contractor took the risk to work that day and do something grossly detrimental to the long-term performance of your slab (you will experience inordinate shrinkage, more cracking, scaling, poor tile adhesion, excessive vapor migration, re-emulsification of latex/acrylic adhesives for floor coverings...the list goes on). Good luck...as CEL noted, you'll have a hell of a fight on your hands, but stick to your guns and get appropriate technical backup...not a compressive strength test. While that might give you information (and if it fails, it will be great info), it does not show the whole picture which, unfortunately, YOU must show.

 

[CELinOttawa]

I think we've covered it now... Excellent expansions by darthsoilsguy and Ron... Next steps are NOT on the Internet! You need a good local Structural Engineer to help you out.

Please repay our kindness (ie: of not simply telling you to pound sand because this site is not meant for free advice) by letting us know how it all turns out. You'll probably find that we're all nice enough and sufficiently interested to keep giving you bits of advice as this progresses as well, should you choose to keep us up to date.

Good luck!

 

[avl58]

Thank you all for taking the time to post your advice. We went to the building site with a local Structural Engineer this morning and he confirmed much of the same. We also pointed out how the rebar to tie in the ICF walls is loose, some of it is too small, and many of them are in the wrong spots for the ICF blocks. The contractor wants to take some core samples from the actual slab. The Structural Engineer said to keep the slab you would have to take a couple hundred core samples to test, and most if not all of the rebar for the ICF wall ties would have to be replaced. Basically it would be pointless to attempt to keep and better to start fresh.

We have a meeting with the Structural Engineer and our contractor we hired later this week where the findings from the Engineer will be presented.

 

[BigH]

PLEASE look up Neville's article on "Core Tests: Easy to Perform, Not Easy to Interpret" - it is in Concrete International, November 2001. This is a very enlightening and valuable addition to your library.

Neville has good advise on Planning a Testing Plan - you did mention coring . . .
"British Standard BS 6089-1981, while giving no guidance on the selection of locations of cores, proffers important advice in clause 4.2: 'Before any programme [of testing] is commenced, it is desirable that there is complete agreement between the interested parties on the validity of the proposed testing procedure, the criteria for acceptance . . .' Following this wise advise obviates subsequent hassle over the interpretation of the core test results. The "agreement" referred to above should not be simply a compromise between the engineer and the contractor; it must take into account the questions that the engineer sihes to have answered. European Standard BS EN 12504-1 states explicitly in clause 5.1: 'It is essential that full consi8detration is given to the aims of the testing and the interpretation of the data, before deciding to drill the cores.' The word BEFORE should be noted."