The comments about good sub-base are dead on. Even if you put down an aggregate base, it should be rolled or hand tamped. Aggregate bases can be compacted up to 25% of their initial thickness. Concrete is usually specified as 6 bags portland per cu. yd. with no more than 6 gallons of water per bag (36 gallons per yard). The water is the key to cracks. The slab as it cures will develop tremendous shrinkage forces internally. The lower the water/cement ratio the less shrinkage and the fewer the cracks. Normally slabs are poured in 20 ft squares to control cracking but cracks do occur anyway, usually a foot away from the control joint. The fiber mesh folks made huge claims of strength when the product was introduced in the late 6o's. Most of the claims have proven to be false. The only accepted benefit of fiber mesh is that it absorbs or disapates the internal shrinkage forces thus helping to prevent cracks. Flexural strength, the ability to carry loads is usually attained by the addition of steel reinforcing bars. The usual spacing for a 6" thick slab on a good sub base would be #3 bars at about 12" centers each way. If the slab is to carry unusually heavy loads, the bars need to be #4 or thicken the slab another 2 inches. The bars to be effective must be held at a point at least 2" clear of the base not to exceed 3". This usually means driving bars to support the mat or buying bar chairs to hold the mat at the proper height in the slab. The bars must be tied together at at least every other intersection to prevent them from being displaced by pouring the concrete. In summary the best way to prevent cracks is to isolate immoveable surfaces, i.e. columns and walls and limit the water/cement ratio. Pour it as dry as can be worked. Wet self leveling concrete will in a month look like a spider web of cracks. Good luck with your project.......
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Today's Featured Article - New Life for an Old Allis - by Tyler Woods. My friend Jon, has an old '39 Allis Chalmers B. He thought it a marginal tractor that had long since served its time. She smoked terribly and never had much power but he couldn't afford another so he was limping along with what he had. Jon's Allis has a small front loader and though it doesn't carry much, it serves his needs. It was the hard starting and low power that made him think it was time to replace the old girl. Jon called me to help him discover why his tractor wouldn't start
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