*
We recently poured a 3,200 sf slab on grade and the concrete compression tests have indicated there is a problem. We took 8 test cylinders, four each from opposite coeners of the building. At 7 days two broke at 3,200 psi (3,000 was spec) and two broke at 1,600. The gc delayed additional work on the building until the second break. We did the 28 day break this morning. Two cylinders came in at 4,000 psi and two at 2,250.
We know pretty close where in the slab the weak samples were taken because the test lab noted “20 ft from east wall and 5 ft from south wall” or something like that. The gc has not called yet with any proposed solutions. I would have expected that they would immediately run out to the site and take half a dozen core samples to see if the cylinders were possibly mis-handled. Assuming the core samples also test low, I see two options: remove the entire slab or determine where the line of demarcation is and remove and replace that portion.
Any suggestions from been-there-done-that guys? How successful would it be to remove only a portion of the slab?
Replies
*
Has the gc spoken with the concrete supplier....what are there proposed solutions.....did the finishers add too much water.....i would check the load tickets...in our area this is well documented.....also if 3000 is spec will 2250 support the loads imposed....if so i wouldnt worry a whole bunch......i would rather it be alittle under spec in a place or two than break the integrity of the slab......if the pour was not under a bearing wall and out in the open with now 2250# load on it what will it hurt....then again if you paid for 3000 psi i think the concrete company should eat it....or at least reimburse you for something
*Since this is fine home building(not meant to be an insult), I have to ask what is the usage for this slab?Since you state slab on grade, I assume that no soil problems exist, or that the final usage makes soil problems neglible.You also have to ask what methods of storage, transportation, curing, etc., each set of cylinders went thru.Elaborate further, please!
*It is for a small (3,200 sf ) single story office building, wood frame construction, essentially a house being used for commercial purposes. We spent a lot of money on the foundation work: removed about 24" of soil, replaced with 36" of properly compacted select fill, all monitored by a testing lab. The gc has calculated where the suspect concrete is, and they think it is one truck load of 11 cy in the corner of the building, so it includes part of the front wall (supporting the roof trusses) and part of the side wall (supporting stone veneer). It was possibly the last truck of the day, the official high temp that day was 100*, and we suspect that they either overwaterd the mix or it dried too quickly. We assume that the cylinders were handled properly by the testing lab; they were taken to their shop and stored in the 100% humidity curing room, etc. It is possibly that they could have been kicked over on site, but what's the chances? All four cylinders in area 1 tested well above spec, and all four cylinders in area 2 tested well below spec. The odds are that all eight cylinders received the same handling.Yes, 2250 psi may be acceptable, but not for us. We spec'd 3,000 and we're willing to grant the ACI alowance of 500 psi, but that means we're still 250 psi low.No offense guys, but you're avoiding the question. yes, it sure would be good to know why it happened, but the question now is - what do we do? Can a good gc successfully remove and replace part of a slab?
*I am not a gc. I've never poured concrete. All I know is what I've read on this board.Having said that, I am curious about the answer to "Can a good gc successfully remove and replace part of a slab?"Isn't it "simply" a matter of cutting where the bad ends, demoing the bad section, drilling and epoxying rebar into the good slab, and pouring? This has got to be something that has been done a "few" times before.Rich Beckman
*Hi Ed,Here's the bottom line.You bought a product with a specified strenght. As long as the GC didn't do anything to change the strenght (add water)it's up to the supplier to prove that his product will meet the requirements demanded. If he can't, then he has to be given the opportunity to remedy the situation ASAP.Depending on the standards for concrete in your jurisdiction, 70% average can be sufficient to meet the requirements of your slab on grade.It's up to you, the owner, to demand of your GC accountability for this product and then it's his responsibility to demand accountability from the supplier, unless you paid the supplier directly.Yes, you may be able to remedy the structural intergrity of your slab. Your engineer will give instructions as to the work required. I can't see all your test results nor was I present to see if the cylinders were properly handled prior to breaking so it's hard to comment. The answer to your problem may be to underpin or excavate the perimeter or wall side in question and add to the concrete.Gabe
*Ok, here's what we hope to be the final chapter in the saga. After the 28 day cylinders failed, the gc hired a different lab to take core samples of the suspect area, and this morning they were tested. All of the cores came in between 4,000 and 4,600 psi, which is roughly where the set of 'passing' cylinders tested. The new lab is a larger outfit than the first, and has a slightly better reputation. The best we can determine, the first lab switched our cylinders with a different project.
*It's highly unlikely that there was a switch at the lab, it's more likely that the technician at your site, improperly handled your test cylinders and you may have had excessive air in the cylinder. What was the density of the samples that failed in comparison to the ones that passed?Gabe
*The first lab did not include that info on their test result report. I'll see if they recorded it and get back to you.
*I have worked the past seventeen years has a field technician with a indepentent testing lab that deals with concrete, soil, asphalt. It is more likely that a bad set of clyinders was taken. Either on the tail end of the truck or an area that was not completely mix. If the clyinders was mishandle, then one or two out of that set would of been good. It is not likely that your cylinder was switch, it is easier to pencil in a results that switch concrete.2300 psi is not really bad concrete. altough concrete is design for 28 days a heavy dose retarder or excess water will slow the cure and proper psi might not show till 56 days. we hold all bad breaks for 56 days before causing a scare.Correct procedure: Break 28 - bad break other 28- bad hold 56 - bad core slab break cores- bad notify engineer- is slab acceptable- tear slab Majority of bad concrete is not the producer but the finish crew adding water, delay in placement, over vibration If mix was bad, over air the slab be chaulky. over plas it would never set. over water, the area be unlevel due to shrink and be slicker than others. Also the owner NEVER allows the GC hire the testing lab, The GC knows if cores are bad he will have to buy a slab, he will make sure that the cores pass. Either by coring in a good area or money under the table. It happens every day. The testing lab only have obligation to the one who hire them. It is the good ol boys network. The tech can pass a test without the lab general manger knowing. I can pass a test while you are watching and talking to me and you never know.
*JB, you had me going until your last group of paragraphs. Bullshit, baby.Regardless of WHO hires the lab, the key word is "independant" lab.NO independant lab will risk modifying lab results for ANY contractor or owner.A proper test report will contain truck sequence; load number, truck number, time it left the plant, time it arrived on the site, time it started to unload, time it finished unloading. It will identify the number of cu. yds. or meters on board, if water was added, slump, air, temperature of atmos. and of the concrete and the number of cylinders cast, by who, date of pour, exact location on site and stated concrete type, strenght and name of supplier.A proper test report will contain all of the above with the test results, including, when the cylinders arrived at the lab. type of mold, age of test, date of test, density, the specified 28 day strenght, the results of the compressive strenghts at various days. The vast majority of failed individual cylinders are as a result of improper field procedures by the technicians. The vast majority of failed groups of cylinders are as a result of improper field procedures by the contractors.The vast majority of failed entire batches are as a result of errors at the plant.General contractors depend heavily on testing labs for protection from liability. Most times it's the owners that don't want to pay for the services.Gabe
*JB, I don't know you, but you sure came across wrong. 17 years as a field tech and your talking that crap? Maybe it was you that...oh, never mind.Gabe, you're right, and in this case the owner (me/us) did contract with the testing lab, and they are an independent that has been inbusiness for several years, and they have an acceptable repoutation. We don't believe that they intentionally did anything wrong or underhanded. Mistakes happen, and we were simply trying to find 1: the correct psi and 2: what happened. #2 is still a mystery, but it's not a big deal since we're confident that we have a good slab. Al the data that you mentioned was included in the field and lab reports, except the density of the samples. it was our idea to include the testing as part of the project, since the cost compared to the vaule and the overall project cost was insignificant.JB, you're right that 2600 psi concrete is acceptable, but that's not what we purchased for this project. Maybe you're used to getting less than you pay for, but we're not. And I think any reputable contractor or sub expects to give a fair days work for a fair pay - if you quote a price for a job, then you should be willing to meet your promise. If the owner changes the specs after signing the contract, then of course you should have the opportunity to revise your price.
*i did not mean to misguide you in any way and I only mentions the truth because i have no attachment in this project.I also mention every possibilty, no BS was implided. the main purpose of the testing lab is to make money, we are not a government agency and the only power we have is to the one that hires us. This is real life, not the way it ought to be. If a major contractor that gives us million of dollars worth of work need a slab to pass that is marginable compare to a home owner that will never see again, who you think going to win. I have had this happen before. Excess water added by finish crew, slab break bad, core are taken but not in the right area. I have been told by mangers to keep quiet. I need to put food on the table too, so I just log my personal field book and keep going. Here it is very hard to get a bad batch of concrete due to the many supplier that we have. All 100% are computer controlled with testing being done by state highway dept. An avg 10 yard load can hold extra 40 gallons of water with no problem. I know thats a lot but the cement content for 3000 is 7 sack. Due to hot weather the mixer must be able to handle the water. every lab has there own clienttel that we see almost daily. the avg field test is an minimum wage boy that can only make clyinders and thats all. This is how labs make money. I wish it was that the pay was good, and that some power in the private section was like the public sector has. I wish everybody was qualified to the hilt and that ethics was the name of the game. I get told daily not to upset any contractors. It is a fine tooth comb out there. I can hold the book on a project and next thing I'm making clyinders on curb and gutter. I wasn't BS when I said bad breaks dissappear in the paperwork trail. Most likely they say the clyinders was hit by a truck and thrown out. It is always the testing lab field technician fault.In reality what happen was crew added too much water, truck was delayed in placement, more water added, strength goes down 80 lbs per gallon added over mix.everything in life does not goes the way it ought too.
*Johnny boy, you're not the tech. I would want on my site doing testing with an attitude like that. If you haven't the balls to do your job then find another job.Gabe
*I,m not saying I,m a bad tech. I,m saying this town is ran by big old family contruction companies and I get over ridden often on daily projects. It is not just me but all the other techs at all the othe labs in town. I'm not doing anything wrong. It just the way the upper mangement does business. It is the same in nearly all towns. There is just no power with the indepentents. As long as everything goes good we are the godsend but once something goes bad no matter who resposenible , that lab doesn't know anything. It is relly a game out there to see what the contractor will try to get away with. Not one contractor in town will build to specs. No way they can make money with each other cut throating each other. I really try to build by book but most time, I get overridden. The buddy-buddy system.My job is real easy to describe. I am a data collector. write and log. I do not go out of my way to pass a bad test but I alway hear my nuke is off or a broke the clyinder too fast.I not happy with the way it works. that why I bump up my PDs so they compact to 97% instead of the 95 required. I got yelled at by super today about arguing with finish crew about water. If every contractor built to spec everytime it would put the testing lab out of business. I be honest you take a good mix, make a normal clyinder, leave it on the job for three days. Then haul in a truck rolling side to side, under the toolbox, the clyinder will still break good. that 2300 clyinder was bad concret from the beginning, not a bad clyinder. Then it broke 4000 on cores by another lab hired by the contractor. I wonder why. It was either cored in the wrong area or it was penciled. the clyinder was not mishandled because the others broke good but explain how the cores broke good. I,m just explaining how life works in this area. I do not have to like it but I got to but food on the table.
*Just curious - what city/state are you operating in?