Shrinkage Reinforcement for LARGE slabs.
Shrinkage Reinforcement for LARGE slabs.
(OP)
I've been reading a lot about using high levels of reinforcement to eliminate the need for saw cuts in large slabs. I learned that AASHTO uses this method so they only have cold joints at a hiatus from construction, no saw cuts. I've also seen that it is used for environmental structures (containment slabs) per ACI 350 (specifically section 7.12.2).
I was having difficulty selling this idea to some other engineers. So, I was wondering if I was wrong or if they were afraid of going outside their little box. They cited the research I'd done was mostly on roadways and may not be applicable to structural slabs. They also noted that ACI 350 Table 7.12.2.1 tops out at 40 ft. Where we had a 150' slab, it is far beyond the limits of this chart.
My rebuttal was that ACI also has slab design criteria that use similar language as AASHTO for slab design. And the 40' limit was because the need to increase rebar ratios reaches an asymptote at around that length. Thus no further increase is required except for certain soil conditions.
So, what more can I learn about this? Was I jumping too readily at something new? And what soil conditions do I need to worry about?
I was having difficulty selling this idea to some other engineers. So, I was wondering if I was wrong or if they were afraid of going outside their little box. They cited the research I'd done was mostly on roadways and may not be applicable to structural slabs. They also noted that ACI 350 Table 7.12.2.1 tops out at 40 ft. Where we had a 150' slab, it is far beyond the limits of this chart.
My rebuttal was that ACI also has slab design criteria that use similar language as AASHTO for slab design. And the 40' limit was because the need to increase rebar ratios reaches an asymptote at around that length. Thus no further increase is required except for certain soil conditions.
So, what more can I learn about this? Was I jumping too readily at something new? And what soil conditions do I need to worry about?






RE: Shrinkage Reinforcement for LARGE slabs.
Dik
RE: Shrinkage Reinforcement for LARGE slabs.
RE: Shrinkage Reinforcement for LARGE slabs.
The 0.6% is only for grade 40 steel. According to ACI 350, 0.5% is allowed for grade 60.
JAE,
I'm talking about both. Containment slabs are mostly slabs on grade. But when vertical vessels (for example) are placed inside the containment area, they have a wind load that must be resisted by the slab as a footing. This structural aspect of the slab only extends a few feet beyond the equipment base.
Because of this and other factors, placing top and bottom mats on a 12" to 18" thick slab is common. Because of equipment spacing, it is often more cost effective to extend the reinforcement across the entire slab. With #6 @ 12"o.c. this gives us about 0.4% for an 18" slab. That is so dang close to the 0.5% that I can almost taste it. (And for a 12" slab, we're already there.) So, either tighten the spacing a touch or upsize to #7 and we're at the 0.5%. I figured this was cheaper than the saw cuts.
So, does anyone know more about this?
RE: Shrinkage Reinforcement for LARGE slabs.
The problem with the higher strength is the concrete strain increases due to the smaller As... We have great difficulty getting Grade 40 in these environs... nearly 400 MPa (Grade 60), even for stirrups...
Dik
RE: Shrinkage Reinforcement for LARGE slabs.
Continuously reinforced pavements have been used for some years in highways, particularly Interstate construction. I think there have been problems with some of them. There are lots of patched areas on some of the pavements built that way in the South. Probably spalling due to corrosion. You don't see the cracks at 70 MPH, but they are there. Some of these now have asphalt overlays.
RE: Shrinkage Reinforcement for LARGE slabs.
ACI 360 has a chapter devoted to slabs on ground supporting building code loads.
One of the sections in ACI 360 states....... Reinforcing is a function of joint spacing and slab depth; and should be
located as close to the top surface as possible.
We had done 9" thick x about 200 feet long slab on ground without any joints for a pharmaceutical warehouse
supporting 60 feet high storage racks. It was reinforced top and bottom and the sequence of placement of
concrete was specified on the drawing. I was not involved with the design and do not have any more info.
RE: Shrinkage Reinforcement for LARGE slabs.
I have done large slabs on Grade (75m x 50m etc), my solution, stress them. If you prepare the correct sub grade and ensure you minimize restraints where they will be a problem it works great.
I know in the US you do your Post tension different however maybe consider this as an option. In Australia once you look at the cost of joints/ reduction in concrete thickness a Post tension slab on grade for large ware house is only marginally more expensive. If water tight design is required then this marginal expense is negligible
RE: Shrinkage Reinforcement for LARGE slabs.
The requirements for continuous reinforcement are similar in both those documents (keeping in mind dates of publication). But this was specifically a 350 application. We have an environmental containment slab for secondary containment in a chemical plant.
@Aaron,
Post-tensioning has a bad reputation in these parts.
DIK & Hokie,
I appreciate the use of 0.60%, but is there a standard/specification/code you can provide that indicates that?
You have about 40ksi vs 60ksi. I figured ACI had to have a reason for that distinction. So, I've been thinking about that. The only thing I can figure is that it has something to do with splice/lap and development length??? Shootin' in the dark here.
RE: Shrinkage Reinforcement for LARGE slabs.
I don't know why post-tensioning would have a bad reputation. That is a shame. Where are you located? If you mean unbonded post-tensioning, then I understand.
A bonded post-tensioned slab on ground would be the ideal solution for a containment in a chemical plant. Depending on the chemicals, you might also need an appropriate coating.
As to the 0.60% reinforcement ratio, that comes originally from requirements for watertight structures, and most of the early provisions were in British standards. But slab on ground design is still as much art as science. Much is based on experience.
RE: Shrinkage Reinforcement for LARGE slabs.
I was told it is a requirement by almost all the building departments around here on all new construction because of expansive soils and so forth. But they end up causing problems even when the soils aren't causing a problem.
RE: Shrinkage Reinforcement for LARGE slabs.
For an equipment slab, which is generally a one-off design (not miles of pavement), and is not subject to the frequent, rapid cyclic loading of pavement, there should be no need to excessively reinforce a slab on ground.
ACI 350 is meant to keep watertight structures watertight, by limiting crack widths. For a given stress level across the crack, the width of each crack is directly proportional to the area of steel crossing the crack, regardless of grade. Until the steel yields. If there is insufficient steel crossing the crack, the reinforcement could yield, rather than remaining elastic.
Slab performance will be dependent on other factors. Use the largest practical aggregate size. Use the least portland cement required. Keep water/cementitious materials ratios low. Cure the slab properly (wet curing or proper membrane curing). These measures will control shrinkage, the main reason for slab cracking, absent excessive loading.
When it comes to saw cutting a slab, it's only purpose is to control WHERE a crack occurs, not stop all cracking. The slab ultimately cracks beneath the saw cut. Also, if the design is for a true structural slab, don't saw cut it.
RE: Shrinkage Reinforcement for LARGE slabs.
I was considering the question about why the ratio would be different for 60ksi vs 40ksi rebar when it is the E that would be the controlling factor. Apparently not so. It just occurred to me to look at ACI 318 Section 7.12.2.1 which outlines minimum temperature steel.
It shows less reinforcement required for higher strength steel. The commentary reveals that the values are determined empirically. So, sorry, no theoretical understanding to be gained as to why. I suppose it is the same for the continuous reinforcement question.
RE: Shrinkage Reinforcement for LARGE slabs.
RE: Shrinkage Reinforcement for LARGE slabs.
RE: Shrinkage Reinforcement for LARGE slabs.