Concrete Strength 2

[Spiteman]

Would like to make a reasonably precise determination of the expected performance variations of a slab with design strength specified as 3000 psi or 4000 psi mix design. Floor to be reinforced concrete, SOG, with 3k versus 4k being the only variable. Floor is approx 30 ksqft and intended for warehouse operations with heavy traffic of hard rubber tired skid and pallet loaders. Design slab thickness is 8 inches. Thanks.

 

[Ron]

In this application, the compressive strength is needed moreso for durability implications than actual load resistance. For industrial floor applications such as this, the minimum compressive strength should be 4000 psi.

 

[jheidt2543]

What kind of floor loading do you expect that you need a 4,000psi, reinforced slab 8" thick or is the subgrade poor soil?

 

[gtrainor]

Explain more of what you mean by "the expected performance variations." Do you mean from the 28 day compression strength results? It isn't so much that the results vary from one another, but that all of the core samples meet or exceed your design strength. Make sure to have your load transfer dowels in place.

 

[Spiteman]

Thanks all, Just a follow up to the question... By expected performance variations I mean resistance to surface cracking (spalling) and failure through the slab. Presuming a satisfactory subgrade of modified stone at nominally 95% I would expect failures to be by crushing and more spall related than a thru-slab crack. My question stems from a durability perspective than anything else because it would seem 3k or 4k have little difference with adequate support. Comments? Thanks, Spite

 

[Ron]

Spiteman...the reason to go to 4000 psi as I stated earlier is for durability of the application. You don't likely need the strength for load resistance. The difference in paste integrity between 3000 psi and 4000 psi can be significant from a durability perspective.

 

[gtrainor]

Higher strength will give you more durability, but you should also look at the type of aggregate (non reactive), cement type, air entrainment (even inside, a little air will help) w/c ratio, and admixtures. But the highest cause of bad performance is placement and finishing.

 

[Polecat]

I agree with Ron.

Just one more bag of cement per CY of concrete is fairly inexpensive and will add many years of durability to the floor.

Another thing to pay special attention to is the water cement ratio. The higher the slump, the more likely it is to get surface cracking, bleeding, etc. Finishers like it wet, engineers like it only plastic enough to trowel.

As we used to joke about it, don't let them place the floor with a 20/30 slump. In pouring house footers, that would be 20 feet one way and 30 feet around the corner!

In reality, a 5 to 7" slump would be more likely fit the job.

http://www.spiraleng.com

 

[Spiteman]

Hi Kids and thanks so much for the input. What we have here is a slab, specified to be 4k and we have reason to believe 3k was poured. If the plant was a bit generous, we might be looking at an average of 3200 psi when the after-the-fact testing is completed. The contractor was kind enough to run a cherry picker over the 5-day floor with obvious results. He has a few sections to replace. My concern is for the balance of the floor and the down-the-road repairs that could be necessary. Short of forcing a complete replacement, I am inclined to insist on a bond, paid for by the GC, that would provide for repairs for 10 years hence. Most likely, we will never have the fancy floor envisioned but it will be functional and because its palletized storage under roof, the load (wear) bearing portion will be less than 50% of the net area. Ron, if as you say the real benefit of 4k versus 3k is hardness and wear resistance then the repairs could be expected to be surface cracking and spalling that lend themselves to epoxy repairs if addressed promptly. I appreciate everyone's help with this matter. Any other thoughts would be appreciated. I'll try to post the outcome FYI. Thanks, Jay

 

[Polecat]

Spiteman:

Woah! Let's look at this again.
If a cherry picker ran over that floor in 5 days, all else being equal, you only had about 60% of your 28 day strength developed (assuming that a regular Type I cement was used). Hence, it may not be a surprise that some flukey things might have occurred.

If you do take the bond route, remember that field epoxy repairs are very expensive, not only from the standpoint of labor and materials, but someone will have to be paid to move stuff around ---- not to mention the inconvience and loss of business in the interim.

I'd be inclined to fix it now and not have to rely on a bonding company that may not even be in business 10 years hence (even the big boys are bellying up like polluted fish these days.

http://www.spiraleng.com

 

[Spiteman]

Polecat,
Let me elaborate just a bit more. You're right! At 5 days the full strength had not developed. That's why the floor cracked and is being replaced in those sections.

Ah, bonding company or GC...which will fail first. The floor is not cracked right now but with 3k versus 4k, Ron thinks it will fail at the surface. I tend to agree. So, how do we settle this matter and get an extended warranty for the owner at minimum cost? We don't know the extent of the failures we will experience due to the 3k/4k thing and if there are none in 5 or 10 years, they can have the money back. BUT if there is one or six or sixty, I want them fixed. Neither of our crystal balls can answer that one. I just wanted a feeling of what we could reasonably expect, hence my original question.

You sound like you know something about bonding companies. A truly nasty bunch but sometimes necessary. I'm in a related biz so be careful what you say... Thanks, Jay

 

[jheidt2543]

Another Whoa! There are millions of square feet of floor slabs in the U.S. that have been poured with 3,000 psi concrete without any problems. Before you go and take up your floor slab I think you need to look into the following:

1. What was actually specified. From your note you say 4,000 psi concrete. How was that specified? Are you asking for 4,000 psi or did you ask for a 5, 5.5 or 6 bag mix design?

2. What strength do you NEED based on the loads to be imposed on the floor? What do you have for a subgrade?

3. What kind of finish did you specify, both in flatness and in smoothness?

4. In the vast majority of concrete test reports I have seen in our Midwest area of the U.S. the 7 day test cylinder brakes have been braking at or above the 28 day strengths. So, a 5 bag, 3,000 psi mix design has been breaking in the neighborhood of 3,000 psi in 7 days. I think you need to find out what mix design was actually poured and see the test reports before doing anything.

5. The difference in cost to the contractor between a 5 bag and a 6 bag mix concrete is between $3.50 and $7.00 per cubic yard. So, while there may be some incentive to cheat, it is in no way proportional to the chance of getting caught and having to replace the slab. Have you determined if this was a honest mistake or done on purpose? If you feel cheated, then cut your deal now with either a credit on the reduced cement content or add a liquid hardener to the surface. A liquid hardener like "Lapidolith" can be put on after the floor is 14-28 days old and increases the surface strength considerably. The floor should be power washed with an industrial scrubber and two coats of the Lapidolith put down. If you use the water soluble version there is no odor and it can be used in an enclosed space and walked on in 2-3 hours. The treated slab is slightly darker and will develop a dull sheen over time. I don't know what you meant by "fancy floor envisioned" for a warehouse, but for my two cents this is the solution.

6. It is true that the added cement content provides added strength, but the REAL reason for more cement is to have a thicker, richer cement past to finish at the top of the slab. But, added cement content means added shrinkage, that's why you don't often seen 7 and 8 bag mixes. I still go back to my original statement, there are plenty of sound, well finished floor slabs made of 5 bag, 3,000 psi concrete. The main reason is they are economical.

That's my two cents, best of luck!

 

[kenvlach]

I agree with Ron & Polecat re 4000 psi minimum for surface durability.

I also like the premise of jheidt2345’s item 5. Get a deal done and fix it now (forget about bonds, lawyers, accountants, expert witnesses, subpeonas, worries about moving stuff around during future repairs, etc.). However, if you didn’t get what you contracted for, don’t settle for a silicate hardener. Have the contractor pay for floor prep. and application of 2 coats of a clear, penetrating, high-solids epoxy concrete sealer*. It will wick into and strengthen cracks and pores and bring the top ~1/8” up to ~10,000 psi. Abrasion-resistant and reduces friction to further minimize spalling and scrapes from forklifts. Preparation was a 3% HCl etch, it got rid of residual alkalinity and sheen, and etched the cement matrix down a tad for a stronger polymer concrete surface. Final rinse was DI water, after which dried well (hot air blowers).

*I used ‘Starglaze 2000’ from Carboline; $70/gal in 1993 but worth it (needed acid resistance, etc.). Starglaze 2000 is the chemical and abrasion resistant (trucks, forklifts and 55-gal drums) concrete sealer chosen by the DOE in 1994 for hazardous waste storage areas at the Paducah Gaseous Diffusion (uranium enrichment) Plant. (

http://www.globalsecurity.org/wmd/library/report/enviro/ea0937.html)

Carboline sold off the product line to a Houston-area company ca. 1996—can’t recall the name. In 1997 found a generic (amino-phenolic epoxy) supplier in So. California. MSDSs are available to compare components but ignore the pigments; I used the clear version for better penetration. (

http://www.msdsxchange.com/xchange_search.cfm)

For a fancy floor, apply a compatible topcoat within a certain period (2 days?) after which it is cured too hard and need to sand for adhesion.

 

[KRSServices]

I would like to add a wee bit regarding the bond. I suggest scrapping the bond idea because after five or ten years, it will not be worth the paper it is written on. I've been there, done that. If you want some sort of a warranty security, go with a secured letter of credit. Good luck getting a contractor to secure it for more than 18 or 24 months past the construction completion date though. The shear hassle and legal wrangling of trying to prove the cause of failures after 10 years simply will not be worth the fight to your client, the lawyers maybe, but not your client.

Instead, go with whatever the contract specified. If the specification was 4,000 psi, then it will be up to you to either accept, accept with a penalty (if contractor agrees, but will be a good bargaining chip) or reject. Your client will surely want to have input into the matter as well, but at least with a penalty, in theory, future repairs could be partially taken care of.

There will be so many mitigating factors in future failures, that the bonding company will have a field day in providing excuses and justification for blocking release of the funds. Once you sign off on acceptance, the contractor is paid and good luck recovering any cost without litigation either.

Nope, I would utilize the wording in the contract (with the assumption the contract has some teeth) and resolve it now, versus later.

KRS Services

http://www.krs-services.com

 

[RDK]

What idiot contractor would load a 5 day old concrete pour?
What idiot allowed this to happen?

I hope I’m not offending anyone here but loading green concrete to failure is always a stupid idea.

Sometimes in construction mistakes happen. Sometimes a 4,000 mix is specified and a 3,000 mix is supplied. As professionals it is our duty to address these problems and come up with solutions.

There are ways to overcome this problem, especially since the main reason for the higher strength is durability and not a structural strength consideration (generally only a problem in structural slabs, beams and columns where the size of the member is designed to be as small as possible and not in slab on grade where the strength is not the real issue.)

The concern is the long term durability of the concrete. If the problem was discovered in 5 days special curing methods could have been employed that would have brought up the durability of the slab and not have caused any long term problems. Previous posts outline some of these.

It sounds like the contractor simply did not have any idea what he was doing. Even if the structural strength was the real issue then loading a green slab to design conditions would have been the worst thing to do. As mentioned the strength would be only 60% of 28 day strength and failure is a certainty under these conditions.

I generally do not allow any significant loads for 7 days on concrete. (foot travel to get started for the follow on work for the first couple of days, keep shoring in place for up to 28 days).

I would forget the bond idea. It does not give you what you want which is a trouble free floor. In my opinion special bonds are not worth the paper that they are printed on since there are a lot of reasons to fight them 10 years down the line (Was proper maintenance done? Was the loading as specified? Was the equipment changed? Are you using the same tires, inflated to the same air pressures? Etc. Can you prove this? Sounds like a pot of gold for some law firm.)

I would also be wary of letting the GC apply the fix. If he could not understand the curing curve of the concrete, how can you trust him to apply specialty coatings properly. I’d talk to the local reputable coating suppliers and get them to provide you with a coating, installed with a 10 year warranty and back charge the contractor . Remember to include something for your time and trouble as well. Add something in for consultant services from eng-tips and send it to Dave to support the site.

If you had a consultant supervising the construction and he allowed the load testing to failure of a green slab, I would talk to the local professional association and suggest that the contractor talk to the consultants errors and omissions insurance company as well. There are better ways to determine the in situ strength of concrete than loading to failure. Schmidt hammers and core samples are the more usual methods. The load testing of the green concrete is simple professional incompetence
Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[kenvlach]

Spiteman,
First, what are the 28-day results on the concrete?

You've gotten good legal and practical advice for proceeding if it tests out less than 4000 psi. Forget bonding, fix it now, etc. Re fix, as there is no way to increase the strength of the full 8" slab by 1000 psi, use this as a bargaining chip for giving a disproportionate increase to the surface layer, as with a penetrating epoxy sealer.

All: Does anyone know another method of doing so, i.e., bringing the surface layer to ~10,000 psi?
(or better epoxy than amino-phenolic? I have more floor)

Spiteman: Please keep us appraised of the outcome.

 

[dicksewerrat]

Look into a thin overlay. Some of the Midwest DOT's have been using this method of repair on interstates. I think Wisconsin and Iowa both did this on some bridge decks. Check ACI reports from 1998-2000. It seems to me they were using some very fine cementitious additives to the mix to get very high strengths.

 

[kenvlach]

Spiteman,
What are 28 day results?
If < 4000 psi, what strategy?

 

[Spiteman]

Thanks everyone, By way of update we are awaiting test results from cores. In typical fashion, contractor is non-responsive. We are probably looking at 500k for full replacement. One other &quot;...oh, by the way...&quot; The ding-dong did not put in the required expansion joints nor did he properly space control joints. I got him, its just a question of how bad. We're going to work through this and most likely will not replace the entire slab but with the leverage we have, its sort of like naming his poision. There is no vertical movement in the slab so the ideal solution might be to spend a month saw cutting and installing expansion joints and control joints section by section. When done with that I'm thinking we chase the cracks and epoxy fill them with something like Sekadur 55 ... but I will have to look that up. Thanks to you all, Jay

 

[RDK]

Spiteman

Did you have site inspection/supervision in place or did you simply let the contractor be responsible for everything?

Even if you did not have full time on site inspection, the designer is, at least in Canada under the National Building Code, legal precedent and professional ethics, responsible to inspect the work to ensure conformity with design intention.

I often find that owners are hesitant to put suitable inspection in place In that case you get contractors who will not do everything, like expansion joints etc. and hope that they can get away with it for the one year warranty. After that any workmanship problems are yours and he has your money.

Even if the contractor is honest and reliable, having a second look at a job will help ensure that nothing is missed. After all nothing firms up a construction project like a couple hundred cubic meters of concrete.
Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com