Slab-on-Grade 1

[dik]

I'm looking at a current problem.

Has anyone encountered a reinforced slab-on-grade/ground where the reinforcing steel continues through the sawcut and the reinforcing area is sufficient to interfere with normal shrinkage cracking?

 

[erkansamhal]

dik,

I am familiar that kinds of slabs-on grade but, we are cutting the upper reinforcing bars just under the steel angles 2 days later after concreting...The main problem I think not using any formwork two sides of the slab-on grade. The second one is whether one of the dummy joints or working joints can serve as an expansion joint where the weakest cross section.

See You,

ERKAN

 

[dik]

Erkan:

Two days is too late for sawcutting. Sawcutting should occur about 8 or 10 hours after the slab is finished. After this period, the microcracking has likely determined the location of the cracks and sawcutting just puts more lines on the slab.

Can you empty your mailbox?

 

[JAE]

dik
We have placed slabs with continuous reinforcing placed without any regard for where the sawcuts occur. Concrete will usually crack prior to fully engaging the reinforcing. We've had pretty good results doing this.

You often hear the concept of reinforcing not eliminating cracks, just keeping them tight.

I would think the best detail would be to terminate some or all the reinforcing at the joint, but this involves extra cost and layout and I'm not convinced that it really makes a huge difference.

 

[dik]

There's little doubt that the addition of reinforcing 'holds' the concrete together. Cracking is generally finer and more distributed. For placing reinforcing, I've staggered bars that are about 95% of the length of 2 panels and the end panels are the only ones that have 'short' bars. With reinforcing steel, the time that sawcutting occurs is still critical; most of the badly cracked slabs I've encountered are a result of this. The other main cause is being restrained.

I was wondering if it is possible to have 'too much' reinforcing that it affects the cracking pattern (minimizes or negates the effects of sawcutting).

Most slab on grade construction I use has a single layer of rebar located a sawcut depth below the top of the slab. This keeps it near the top (you can't see the cracks on the bottom <G>) where it is most effective and keeps it from being cut.

 

[jheidt2543]

About ten years ago the State of Wisconsin replaced major sections of the I-90 Interstate using &quot;continuously reinforced pavement&quot;. No joints whatever (except at the end of very long pours), but the pavement was heavily reinforced. I drive the road regularly and don't notice any cracking (but at 70 mph it might be kinda hard to notice <g>). You might try searching the web for research information on this method, if you have further interest.

Most of the slab-on-grade floors I see use mesh reinforcing not rebar. If the subgrade is good soil and prepared well there is little reason to heavily reinforce a slab-on-grade. As dik and JAE have noted above, the key is to saw the joints as soon as possible after finishing the slab. The joint spacing is also critical, roughly square sections in 12'-20' lengths depending on thickness of the slab. The &quot;Soft Cut&quot; saw method is really the way to go.

 

[dik]

jheidt2543:

How do they accommodate thermal movement? Even if the road is extruded using 0 slump to reduce shrinkage. Maybe they use the rebar to minimize crack widths and the weight of the road to hold it down in the summer <G>. Wisconsin could have temperatures ranging from -40? to +100 (F Scale). I know they do that with welded rail, but I still don't know how it works. I have a picture of a bridge in Winnipeg where the concrete curb and median went over an expansion joint. The thermal movement lifted the median by approx 6&quot;.

6m or about 20' is OK for a 10&quot; slab.

Is the Soft Cut saw the one with the preloading plate behind the blade?

 

[Ron]

dik...The area of reinforcing will almost always interfere with shrinkage. The key, as JAE noted, is causing the cracks to occur where you want them. The key to this is timing. For reinforced slabs on grade, the stress distribution gets a bit wacky during initial curing and shrinkage. As the rebar engages (again, noted by JAE), the shrinkage is restrained. If you examine closely, you'll see more cracking in reinforced slabs than non-reinforced, but the cracks will be closer together and much tighter. Sometimes even difficult to see.

Make your sawcuts even sooner than you would with unreinforced as the rebar engagement issue is already occurring. This is where the Sof-Cut saw is useful. You should also make the joints closer (yes, closer, not farther apart!), again accommodating the shrinkage stress distribution difference.

 

[dik]

Thanks, Ron

 

[dik]

and JAE...<BSG>

 

[jheidt2543]

dik,

I don't understand the theory behind the continuously reinforced pavement either. I'm sure that there have been studies of it and there may be information available on the Wisconsin DOT website.

The Soft Cut saw is a small saw about the size of a hand circlular saw with a 5&quot; (I think) diamond blade. As soon as the slab is finished, the saw cuts are made. Thus, the control joints are in place before most of the shrinkage stresses have built up. In past &quot;normal&quot; practice, we would use a large walk behind paving saw with a 16&quot; - 36&quot; steel or fiber blade and make the cuts 8 - 24 hours after finishing.

 

[dik]

jheidt2543:

24 hours is also too late. I often caution contractors when they are working on a slab that it may be that the sawcutting occurs at 2AM... so that they can consider the time they are casting the slab...


This is from one of our Canadian concrete codes...

A23.1-94
Concrete Materials and Methods of Concrete Construction
Methods of Test for Concrete

20.1.5
Where construction joints are specified in watertight construction, all specified layers of reinforcement shall be continuous across the joint. The type, size, location, and material of waterstops shall be specified by the Owner. Joints in waterstops shall be made in accordance with the manufacturer’s directions. Joints in waterstop material shall be as watertight as the continuous material and shall have a permanent strength and flexibility not less than 50% of that of the continuous material.

20.2 Control Joints

20.2.1
Crack-control joints in slabs-on-grade shall be cut, when specified, using power-driven abrasive or diamond blades. Cutting of joints shall begin as soon as the concrete surface has hardened sufficiently to resist ravelling as the cut is made, and before shrinkage cracks form in the concrete. Alternatively, control joints may be formed by placing strips in the forms or by hand tooling of plastic concrete. Crack-control joints in concrete walls shall be formed or cut. Control joints shall be filled with a specified material if required by the Owner. The depth of control joints shall be between a quarter and a third of the thickness of the slab or wall.
Notes:
(1) The proper time for cutting will vary from 6 to 18 h after placement of the concrete, depending upon several
factors such as mix proportions, ambient conditions, and aggregate hardness. For further details, refer to AC/
Standard 302.1 R.
(2) To minimize uncontrolled cracking for slabs-on-grade, control joint spacing of 24-36 times the slab thickness, to
a maximum of 5 m, is recommended for panel shapes approximate/y square.
(3) Where slabs-on-grade are subject to small hard wheeled traffic, saw-cut joints should be filled with an epoxy
resin material of 100% solids, with a minimum Shore-D hardness of 50 (ASTM 02240) and elongation of 6%.
These materials should be used where only minimum further movement may be expected, particularly in interior
slabs, typical/y 3-6 months after concrete placement.

20.2.2
Where reinforcement continues through a control joint, the cross-sectional area of the steel shall be reduced at the joint location as determined by the Owner.

20.3 Expansion Joints and Isolation Joints
Expansion joints and isolation joints shall be located and detailed by the Owner.
Note: Slabs-on-grade should be separated structurally from other building elements to accommodate differential
horizontal and vertical movement. The joint should extend the full depth of the slab.