Concrete Compression test results 1

[mmarlow]

I am an EIT and the firm I work for is having issues with a job they had done 4 years ago. We designed an elevated slab for a paper mill using a high early mix because they had to shut down their process to do the pour, and they could only afford to be shut down for for a couple of weeks. This slab is subject to forklift traffic and was designed for a 135 psf storage LL. After some minor cracking, (2 yars ago, when I started) I was asked to rework the calcs in the area to determine if I could find any issues. Their were a few checks that were close, but nothing major stood out.

(FYI, one way shear was the check that was close, but none of the failures indicate shear issues.)

Now, 2 years later, the cracking and spalling is at a point where the owner is calling us daily to come check on it. Major cracks are developing over the carrier beams and areas of concrete mid span are spalling at the bottom and top of the slab. It looks as though the compression blocks are failing. Also, over 8' spans we are seeing close to 3/4" of deflection...FYI, one way shear was the check that was close, but none of the failures indicate shear issues.

Here's where the major confusion is coming from our engineers. Cylinder tests were all done when the slab was poured. They all reached the expected comprehensive strength (4000 psi) over an appropriate time frame, according to my coworkers, I haven't seen the submitals. We just had a company come in and test the slab by taking new cores, and testing 5' square grids with some sort of sonar device that can determine concrete strength. This company came back with compression strengths that range between 2000 and 2800psi....

How did the concrete loose compressive strength? None of the tests today were close to what were found years ago. Does anyone have experience with this?

Also, Id like to note that there are vibrations from machines impacting this floor daily (almost 24/7). I am not sure if the machines were shut down for the pour, and the curing. I presume they are, but I will look to figure that out.

Thank you!

-MMARLOW EIT

 

[Ron247]

Marlow,

I would not trust they only lift 1000 lbs. When work has to get done, they will do whatever they have to. That is most all facilities like that. Does the empty forklift control? I do some crane lift calcs sometimes and many loaders have the worst case when they are empty due to the rear counterweights.

This is what I call Speech #13. "Anytime you have more than one possibility of what happened, you also have ANY combination of those possibilities. Most any of us can find a single source cause but it is VERY difficult to accurately assess what happened when it is multiple sources. The more actual sources, the harder it gets to solve." You may have an unlucky Speech #13.

1. Forklift actually lifts 1550 lbs. One of the operators has a nephew in engineering that told him most things have a FOS of 1.67. So, 1000 lbs actually means 1,670 lbs.
2. Bar placed 1/2" from tolerance. You would be amazed how many people in construction read tape measures to the nearest 1/4". I was a frame carpenter years ago. We were told we had to frame within a 1/4". They meant a total of 1/4". But what happened was Person 1 measured to the nearest quarter. Person 2 marked what he called out within a quarter with that big carpenter pencil. The Person 2 placed the saw within 1/4" of the mark and cut it.
3. Curing was not effective.
4. Billy Joe vibrated the concrete but they fired him 2 weeks later when they found out he did not know what he was doing.
5. etc.
6. etc.

 

[OldBldgGuy]

Many years ago I designed a reinforced concrete slab supported on steel beams for a mezzanine level changeroom/lockerroom built inside an existing plant. I was young, thought I knew my stuff, knew the owners, all was good. In the course of a year the 20' span slab deflected >3", which with a concrete block shower room among other things on top, was obviously not acceptable (although the drain, from pure luck, was at the low point). Review of the design by myself and another engineer showed no deficiency, the design was good. I did some research and found that the elevated temperature of the plant (a heat-treating plant) was sufficient to induce unacceptable creep in the concrete despite acceptable strength and design. People work in the plant, so it's not like the temperatures are off the scale, but they are >30C pretty well all the time. We put steel beams under the slab like JAE, and it has been fine for 35+ years. Your slab is much more heavily loaded so is a different condition, but could temperature have been a problem?

 

[mmarlow]

OBG,

That is an interesting situation. I'll look into that for my general knolwedge. However, I dont feel that's the issue in this case. I could see that happening at other areas in the Mill, but not here.

I'm only 2 years into my career. Fortunately, I havent run into any major issues (I have good professionals above me who do thorough reviews). But, the more I read on Eng-tips, and the more I gain experience, the more I realize that there is so much to consider in design.. not just the stress/strength/servicability checks. A few months on the job, I checked this slab. Flexure, one way shear, two way shear, deflection... but never considered fatigue, impact loads, and the like.. I am seeing the importance of experience and good research techniques more and more every day.

Thank you for your response.

-MMARLOW EIT

 

[Ron247]

Marlow,
For a lot of us, experience is actually, "We survived our early mistakes". I will not speak for everyone, but I will speak for me. I was damn lucky. I am 60+. Went to school in the transition from slide rule to computer. Computers are fantastic, can compute the actual and allowable load on every column and beam. But the old geezers had to learn to "visualize" how forces moved through structures. We had to rapidly find the 1 or 2 most loaded columns or beams, because we had to hand calc them. If you were not good at that, you had to hand calc 4 or 20 of them. Point is, don't let computer usage inhibit your ability to visualize how the forces and movements occur in a structure. I heavily use computers but I more value my structural professor instilling in me the "sense" of how structures respond to forces. Engineers much older than me, very little computer ability. Engineers much younger than me, may not be as good at "visualizing" forces and movements. So, as I say, I am damn lucky, I got a little of both worlds because of where I fell in the "years".

Another old geezer trick, learn to look at an existing structure "in detail". Not easy to do. That is not 2 columns and a rafter, it is 1 pinned base column, one fixed-based column and a simple span rafter. Oh, the fixed based column is the lateral resisting element, the rafter is the RLL element, etc. We had to learn to do that as "survival" because failure to do so, led to more hand calcs work when we got back to the office and the structure did not check out. We had to go back, look again, and figure out why this had not already collapsed. Then we see what we missed the last time. Ability to "look in detail" is extremely valuable. Your forklift is an example. When you were there, what did you notice as it passed by? Manual versus computer can help give you that out of pure survival.

Now, your statement, " I have good professionals above me". Thank your lucky stars every night. Not all of us had that. Yes, there are mediocre to poor engineers we train under. I have always asked more than one person how they do something, assembled the answers and then picked out what made sense based on my education. 3 degrees and licensed in 20 states does not mean you are good at what you do. This is why I like this forum. You get a variety of responses from different experience levels. Geez, my early years, NO INTERNET. I had maybe 1-3 people to ask and that was it.

Keep an open mind, only consider comments that make sense and the hardest thing to do, realize I was wrong in the past on a subject, but not in the future.

 

[Ron]

I agree with Ron247...good advice.

I am from the same era...the transition between slide rule and calculator. The slide rule taught us order of magnitude and visualization....both important in engineering. Even with all the computer capability and modeling that we can do now, common sense still needs to be the order of the day!

You obviously have failure, even though your design checks out. That leads one to look further as you are doing. Were the design parameters correct? Is the end use more rigorous than considered in the design? Did the concrete get compromised during construction that the laboratory tests on the new concrete wouldn't show?

Compressive strength in concrete is easy to achieve and just as easily compromised by construction procedures. Since this is an elevated slab placed during a shutdown, it was likely done as expeditiously as possible. It was likely placed by pumping in constrained space, so likely a smaller concrete pump was used. Contractors like to place soupy concrete, and it sounds like your failures may be more related to durability of the concrete than design issues.

I don't know how your testing lab calibrated the ultrasonic testing, but hopefully they tested the cores in companion to actual compressive strength testing to carry that over to the in place measurements. If they did not, then the ultrasonic testing would be suspect. If you have microcracking in the concrete, that can throw off the ultrasonic measurements as well. As JAE noted, have a petrographic examination of the concrete done.

Keep in mind also,that live load and dynamic loading can be significantly different. Since you have vibrating equipment on the floor, I would also have some vibration monitoring done to see the vibration spectrum being imparted to the concrete. Low frequency vibrational fatigue in reinforced concrete is rarely an issue, but higher frequency and higher amplitude can damage concrete. Check out books on vibration by both Charles Dowding and Richart, Hall and Woods. Both are excellent.

 

[mmarlow]

Ron247 and Ron,

Thank you for your feedback. As I stated in the OP. This was not my design, but I had checked it. If i were asked to do this design (instead of checking it) I cant saissue. 100% confidence that I wouldnt habe done the same design (although I always prefer two mats of rebar). That's why I'm taking this opportunity to learn. Its very reasonable that I'll be doing more work like this in the coming years, and I'd like to avoid these issues.

-MMARLOW EIT

 

[Ron247]

Your Welcome Marlow,

Years ago when I found out the details of the Hyatt Regency collapse in Kansas City, I remember thinking "I could have easily missed how the proposed rod change would have changed the load path". That made me pay even more attention to load path and "thinking in detail". It occurred in 1981 and I graduated in the 80s. I believe most engineers stood a 50-50 chance of getting it right unless they literally sat down and re-did all calculations related to the suspended walkway before they made a decision to okay the change.