fibermesh reinforced concrete slabs on grade 2

[ET]

I am trying to find information to determine the maximum recommended spacing for control joints.

 

[MathMan]

I would suggest that you try to locate a handbook called &quot;Concrete Floors on Ground&quot; published by the Portland Cement Association. It goes into great detail on the design of slabs-on-grade.<br><br>In a reference table in this booklet, the maximum control joint spacing is given as follows (for 3/4&quot; aggregate):<br>For concrete slump 4-6&quot;: slab thickness times 24<br>For concrete slump &lt;4&quot;: slab thickness times 36<br><br>In practice, control joint spacing tends to be somewhat larger than these values. Employing good curing practices (moisture control, temperature control, not finishing the surface excessively) will reduce dependence on control joints for crack control. Reinforcing also reduces the need for having such tight spacing. Generally, for reinforced slabs, 16'-20' spacing would not be unreasonable.<br>

 

[Ron]

Jointing for fiber supplemented concrete is not significantly different than other concrete, though the fiber suppliers would have you think otherwise!&nbsp;&nbsp;Common sense must still prevail.<br><br>Consider that thin slabs on grade have large surface areas relative to their cross sections and shrinkage is exacerbated by this.&nbsp;&nbsp;Keep the jointed areas to somewhat less than 400 square feet.&nbsp;&nbsp;This would be a MAXIMUM 20x20 joint pattern.&nbsp;&nbsp;Even this is too large under a couple of conditions; i.e., if the slab is less than 6 inches thick or if placed under fast drying or setting conditions.&nbsp;&nbsp;Would prefer to see joint pattern of 15x15 or so, trying to maintain 1:1 length-to-width ratio.&nbsp;&nbsp;Do not exceed 1.2:1 ratio, as random cracks increase with larger ratios.<br><br>Next, consider the timing of sawcuts.&nbsp;&nbsp;Saw the joints quickly!!&nbsp;&nbsp;As soon as the concrete is finished and will support the sawing without aggregate raveling, get on it. This can be anywhere from 6 to 12 hours after placement.&nbsp;&nbsp;If you wait more than about 12 hours, you probably will already have some cracks, whether you see them or not.&nbsp;&nbsp;Cut the joint to a depth of 20 to 25 percent of the thickness.

 

[daski]

An engineering standard from an international chemical company states that rules of thumb for joint spacing are:
plain concrete (no wwf or rebar):
24xthickness for aggregate < 3/4&quot;, 15 ft. maximum
30x thickness for aggregate > 3/4&quot;, 15 ft. maximum
36 x thickness for low slump conrete(undefined), 15 ft. max

for reinforced slabs, 48 x thickness [sig]<p> <br><a href=mailto:markdaski@aol.com>markdaski@aol.com</a><br><a href= > </a><br> [/sig]

 

Has anyone experienced curling of fiber reinforced floors? Floor in question is a 6&quot; floor with saw cuts at approx 14' o.c/. 4500 psi low slump (3&quot concrete, 3/4&quot; agg.

 

[JAE]

I don't think the fiber reinforcing would add to curling. We've used fiber reinforcing on numerous slabs and had great results. Was the slab placed in sun, high wind, etc.?

 

[Ron]

I agree with JAE...curling more likely caused by differential moisture content between top and bottom of slab. Was it placed with plastic vapor barrier? If so, with such a low slump, then accumulation of moisture would only be a bottom, with significant drying at top. Result would be &quot;cupping&quot; curl.

 

[emmgjld]

In addition to the above comments, I will suggest that appropriate steel reinforcement (moderate to heavy) placed at the UPPER 1/3 of the Slab thickness may be required to control Curling.

Curling of Slabs on Grade is also dependent upon the amount of material in the mix, usually very fine, which can shrink. It is appropriate to minimize the amount of finer particles, to include cement. I have found higher strength concrete experiences higher shrinage.

If possible, reduce the Design Strength to allow less cement. The use of larger aggregate decreases the relative amount of past. so use a larger maximum aggregate. It may be possible to increase the coarse aggregate percentage, as workability in a 6&quot; slab may not be quite as critical.

The minimal use of Chemical Water Reducers may be appropriate. Careful inspection of the Chemical 'Tech Sheets' may reveal that the chemical addition may increase the amount of concrete shrinkage.

Curing is extremely important. Curing starts with the subgrade preparation and ends several days after the concrete is placed.

I recommend careful reading of ACI 302.1R-96 & 360R-92(97) with extra attention to Chapter 9, Reducung the effects of Slab Shrinkage and Curling.