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

 

[StrucDesignPE]

There are quite a few factors that can affect the strength gain of concrete during or after placement that may not necessarily show up in the results from cylinder breaks. What was the weather like during placement and initial curing? Were hot/cold weather procedures followed? Did the water content of the concrete placed differ from that reported or used to make the cylinders? Were the cylinders field cured or lab cured? Was excessive vibration used to cause the aggregate to settle to the bottom of the slab formwork? What about placement of reinforcing steel in the slab? Was it placed correctly? Is it the correct size? Can this be verified through inspection reports?

It may not have "lost" compressive strength, but possibly never had it. Other testing may help verify the strength of the concrete such as windsor probes or even core samples.

 

[JAE]

You can also core sample the concrete and have petrographic analyses performed on them to see what the actual "recipe" was for the concrete.

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[structSU10]

Those test values likely don't mean much - anything that relies on reflected sound waves or the like would be affected by cracks, spalls, rebar, etc. that I would not trust it. Plus you say other machines are running - they may screw up the reading of that instrument as well by causing interference.

To test the concrete strength in this environment, you are likely better off taking cores and testing them.

Something to consider is the fatigue of repeated fork truck traffic, if these are hard wheeled fork trucks, and if the vibrating machines are close enough to also affect the floor in question.

 

[hokie66]

A paper mill is an aggressive environment. Spalling sounds like corrosion of reinforcement. 4000 psi concrete is submarginal for an agressive environment, and it sounds like you didn't get even that.

 

[oldestguy]

A common problem on concrete slabs is over working the surface too early in the setting process. Generally the workers do not want to wait for a harder surface, generally when a trowel rings. That early over working brings up water so the near surface w/c ratio is far above what the cylinders were possessing, etc. I'd go back to the original concrete contractor for any claim.

 

[JAE]

Sounds like a thin concrete slab-on-steel beams that I once had to replace in a postal facility truck dock area with a lot of forklift traffic.

Similar deterioration - cracks in all directions - all due to fatigue of repeated loading. Once the cracking starts, the harder wheels of the forklifts start hammering on the cracks, adding vibration and "working" the cracks further - a vicious cycle.

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[IDS]

The OP says they took cores as well as using a sonar device.

I would suggest:
Analysis of the concrete for alkali aggregate reaction, early carbonation and/or any other deleterious chemical change.
Check the design for fatigue effects as well as maximum stresses and ultimate strength.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[mmarlow]

Thank you for the responses everyone.

StructSU10-
The sampled cores have not been tested yet. I can see what you mean about the reflected sound waves and the voids.

StructdesignEIT-
The pours were done on a 65 degree day. I will look into the inspection reports

JAE-
Yes, the fork trucks are hard wheeled. I can see what you mean. This slab is realtivley thin. 5.5" thick

IdS-
We will look into fatigue


-MMARLOW EIT

 

[JAE]

That's exactly what I had - a 5 1/2" thick slab with, believe it or not, top and bottom mats of rebar.

We ended up adding support beams (the span was just too much for the slab depth) and replacing the slab with similar reinforced concrete - but added a combo- synthetic plus metallic fiber reinforcement to help minimize cracking.

We also modeled the slab (FEM) with the new beams to check for maximum stress in the concrete under various forklift wheel loads - trying to keep the moments below Mcr.



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[mmarlow]

Jae,

keeping the moments below MCR must have been challenging. In this situation we're dealing with 10kip axle loads. We've limited them to 1000lb lifts, so the rear axle unloaded is controlling. I havent used FEM to run any checks on this project. Im used to fork truck load spreading per SDI, and I just run multiple checks to get a worst case.

Long-term I feel that we may have to add some intermediate steel framing if we do thee redesign.

Thank you for your comments.

I have done some research on fatigue loads in concrete slabs/beams. Its apparent that the design yield strength of the concrete and rebar should be reduced from the original design.

-MMARLOW EIT

 

[MotorCity]

A few things to consider.....

1. Fatigue in concrete is rarely an issue.

2. The results of lab tested cylinder breaks provide an upper bound of the in place strength. They are cast, cured, and tested in ideal (lab) conditions where as there are a number of things that can affect the concrete strength based on how its placed, finished, and cured. Breaking cylinders taken from the slab will give you a true picture of the in-situ strength

3. If you are seeing cracks over the beams, it is likely that the negative slab reinforcement is either non-existent or inadequate. Perhaps reinforcing over the beams that was intended for crack control is being forced to resist the negative bending moment over the beam.

4. Was composite metal deck used? If so, the deck may have de-bonded from the slab, thus negating its contribution to the slab strength. SDI strongly cautions against using it where forktrucks are present

5. Any chance settlement of the slab supports is an issue (i.e. deflection of the beams supporting the slab)?

 

[mmarlow]

MotorCity,

Fatigue may not be the issue, but the failures indicate its possible. This is a high trafficked area almost 24/7

I understand what you are saying about getting cylinders off from the slab now. Unfortunately, I dont have that data yet.

The reinforcement was designed to support negative flexible over the beams.

There is not deck. IT is a flat slab that was formed

I will give more thought to the deflection of the supporting beams.

Thank you for your input





-MMARLOW EIT

 

[Robbiee]

For the numbers you mentioned, I am not sure how one-way shear is close!
I think if the one-way shear is that close, then the slab is very thin for bending and deflection.

 

[mmarlow]

The rebar mat was placed at the center of the slab. So the 5000lb +/- (unfactored for LRFD) point loads are close to ØVn. The d is approximately 2.75".

Checking as a 1' strip..
Øvn= ØVc.
ØVc=2*λ*d*b*sqrt(f'c)*Ø
ØVc=2*2.75*12*1*sqrt(4000psi)*.75= 3,130.65lbs/ft

Va:
point load spreading per SDI = 1500 lbs/ft (with a 2' spread)
Slab DL, and misc. pipe loads = 4'*100=400 lbs/ft. (Heavy pipe loads from below)

1.2(400)+1.6(1500)= roughly 2880lb/ft (factored for LRFD). This does not in impact loads.

These are not my exact calcs. But very close. Please let me know if there is something I'm missing here.





-MMARLOW EIT

 

[structSU10]

center of the slab rebar - no wonder there are issues - you have a large cover, and thus a large crack that will form to get the rebar to be effective. it likely gets big enough that for truck tire impact it, making the problem worse. When the fork truck hits a crack, it is jostled a bit, which could make a bit of an impact force possible as well.

For repeated loading as you have, you also generally want to keep stress ratio's down - similar to how pavements are designed for vehicle passes (see ACI 360 / PCA fatigue curve for some data developed for highway pavements). According to that research, if you have a stress ratio of 0.9 on a pavement, it can handle 7 passes as allowable loads. If you keep the stress ratio at .6, about 30,000 load cycles, and 0.45 or less, unlimited (since it is remaining elastic).

 

[mmarlow]

structSU10

Thank you for the input. Yes, two mats is more common in my limited experience. The impact on the large cracks completely makes sense to me. Im hopeful that we can get cores sampled to confirm compression strength of concrete.

That is good information on the stress ratio's. I will keep this in mind as I continue my career.

-MMARLOW EIT

 

[hokie66]

Elevated slab, thin, one layer of reinforcement mid-depth, 135 psf superimposed, forklift traffic, paper mill...the design was woefully inadequate. The concrete strength is largely irrelevant.

 

[MIStructE_IRE]

With the “tension steel” located on what should be the neutral axis I’m not surprised you have issues.

The slab section has had to shift into a new state of equilibrium, raising the height of the neutral axis thus cracking the lower section (what should have been the tension zone) in order for the bars to take up the slack.

As a result, your slab is now circa half the thickness it should have been, and paper storage is not light. I would get this propped immediately and consider permanent steel beams underneath.

 

[Ron247]

Test samples taken during construction only reflect the "delivered concrete" strength. The installed concrete would almost have to be cored to get a good idea of its strength. As far as "non-destructive" test methods, I am very leery of their results. I cannot count the times that I found errors in the equipment, the logic or the use of the equipment. 90% of the time, the ability of the equipment was oversold or there was "User Error" on purpose. Sometimes, the User is just too conservative. Confirm how and what they used with your own investigation. I was told once of a 100% moisture content of wood on an upper kitchen cabinet via a non-destructive tester. I think I am correct that 100% means 1 lb of water for every 1 pound of wood. You would see the water dripping from the wood under those circumstances. I pin tested it and it was 8%.

I would absolutely confirm the forklift information and whether they changed the forklift over the years. Moving vehicles are very hard to model, especially multiple ones. 135 psf sounds "light" to me if you are using forklifts to move the items. 100 psf is a church or restaurant. Also hard tire versus inflated tires makes a huge difference.

I would also investigate how well the concrete was actually poured and the rebar placed.