Large Deflections in Concrete Slab Without Cracking 6

[Rdar43]

I have an unusual issue on a project where there is a concrete slab that is deflecting 2.5" in the worst location only under construction loading. The two-way slab was designed using RAM concept and from the results, there should only be 0.6" of deflection occurring under construction. The area in question is a 33' span, the slab is 10" thick with an f'c of 5,000psi.

I've gone to the site and confirmed the deflections but strange part is there was no visible cracking on the top or bottom of the slab. The slab was poured all in 1 day and there were no visible construction joints in the slab either. I also noticed that there were larger than expected deflections throughout the slab, not just at the worst case condition. At 1st we thought that the contractor may have pulled the forms off to early as they pulled them 15 days and didn't reshore but the concrete break tests indicate roughly 4,000 psi at just 7 days (High-range ware reducers were used in the mix). It's also worth noting, the slab was poured 6 months ago and then there was a pause on the job over the summer so the 2nd story slab has yet to be constructed. The contractor stated that there were no materials that were stored on the slab over the pause and they noticed the deflections when water would begin to pool when it rained. Based on the inspection reports everything seemed to be correct regarding the rebar and pour. We have asked the contractor to reshore the entire slab until the source of the problem can be found.

Due to the lack of cracking my current theory is that there may have been an issue with the formwork while the slab was being poured that resulted in the slab deflecting before it ever started to cure. The issues is this doesn't fully explain why the deflection can be seen on the top and bottom of the slab or why the larger deflections are consistent throughout. If we can't determine the source our next step is to try and have testing done on the slab to see if there is a discrepancy between the slab and the concrete core results. Does anyone have any suggestions on what else could be the cause?

 

[Brad805]

Tom, yes, that is true. This is the point in the discussion where the poster usually goes silent since they did not get the answer they were hoping for. I suppose I could start another thread to discuss.

 

[Rdar43]

I apologize for not responding, been on vacation for the past week and return tomorrow. I'll try and find some time to go through the comments and address them

 

[Rdar43]

To answer the question I can right now:

1) This is the 1st time I've considered temperature during construction as an effect on the deflection of a slab so there is a good chance the load steps I used were inaccurate to account for the difference between 2.6" and 0.8". I agree that there wouldn't be much of an effect for a continuous slab but, my initial thought process was the larger the slab, the more thermal expansion would occur and thus would potentially produce more cracking and stresses. For those who ran a model, if you could provide the exact load steps you used to get a larger deflection for me to confirm with my model that would be greatly appreciated.

2) The building is located in New Jersey, without a location a temperature range from -125 to 125 makes sense but over the period of the slab construction (March - September) I've estimated a temperature between 40 to 125 would be accurate.

3) The 0.8" mentioned previously was for a 6 month load step with 3ft mesh. Long term deflection (5000 days) the slab was right around 2" deflection with moderate ECR in concept, changing it to severe would likely bring it to the 2.25 range. I'll run it at 1ft mesh when I can. I imagine this will increase the deflection but would be naively surprised if reducing the mesh to 1ft would cause such a large increase in deflection. The lack of validity for concept model less than 90 days is an interesting consideration as I imagine this slab has been through the worst case conditions.

4) Ownership has gone with a similar route mentions by bookowski. The strength of the slab still works, pin the deflection on the pause and learn from the span/depth ration error that occurred. Regarding haynewp's comment, I don't think there has been an issue yet. The plan is to pour a topping to make any partitions above level and below is intended to be an open area.

5) For the span/depth ratio concerns, my interpretation (and those of my coworkers) is that there is no indication of loading for these ratios. So the slab could be loaded with a significant amount of load, let say 35SDL/125LL, and the same span to depth ratio would be used for this project 15SDL/40LL. Should the same span/depth ratio's be followed or should there be an increase due to the reduced loading?

6) Brad805, we haven't had them pressured wash it but we had them do a survey to estimate the additional concrete topping required so it should be fully cleared for another indeth inspection. The large deflections and lack of cracking was what led us to believe that there was a construction issue rather than a design issue to begin with but after reading these comments perhaps that isn't the case. The increased topping will increase the demand but if it causes strength failure we are planning on adding additional supports/reinforcement.

 

[Celt83]

Here is the load history I used:
All Dead = self weight of the slab only, I did not apply any superimposed dead loads
CL = L = 20 psf on the entire slab for Construction Live Load
Tp = Thermal load of +120 top +40 bottom assumes sun exposure on the top surface
Tm = Thermal load of -40 top and -40 bottom assumed uniform cooling

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Tomfh]

Temperature and live load differences won’t explain the difference. OP gets 0.8”, and other people (and the slab itself) get 2”+.

 

[Celt83]

If op ran a single load history step of 180 days, that would result in a different result as the creep and shrinkage computations would be based on 180 day value and the slab would start uncracked. I've found you really need to break it down into the "load history" over those first several months and then a longer sustained period, this tends to result in much more early life curvature that then propagates through the later histories.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Brad805]

I have been noodling this problem for my own continuing professional development off and on for a bit. I thought I would share a few thoughts.

I used loads similar to Celt. Mapping the time steps in the software I use is a tad bit more onerous, so I created a spreadsheet of the functions needed.

The first analysis is below. The deflection in this case was much lower than reported. This analysis assumed the concrete age, t0, was 28d when the load steps started. That was not the case here.

I re-ran the analysis using a t0=5d. Here I did not include the temperature load case. The deflections are closer to what is being reported.

Now you may say, but we used a high early admixture. I think you have created a design that is very sensitive to creep and shrinkage. Do you know how the curing methods or admixtures affect the hydration process? You need to look at that carefully. Below are some excerpts from the CEB fib Model code 2010. In this you can see how the age of loading affects your creep parameters. Shrinkage can be affected similarly. I am sure Celt adjusted his parameters correctly, but I am not sure you have nor considered this in detail.

To show the impact of the age of loading on the basic creep coefficient I created a simple calculation of the value for 3/7/28 d below.

Some general recommendations of the steps needed if you have a design sensitive to creep and shrinkage are below. This is a from a 1973 study, so does not have current admixtures. I did not find any other free documents that might include current practices, but I did find a number you could pay for.

At this point you probably think, but everyone uses high early admixtures and strips forms as we did. The key difference is most do not use designs with this span/depth ratio. To show the difference I reran the design if the slab were 15" thick. I am not suggesting that is an appropriate design nor cost effective. It is merely to illustrate the benefit of using the span/depth ratios suggested. This analysis was subjected to the same load case, and the t0 = 5d.