Over several years, one of my ready-mixed concrete customers has been building an animal preserve in the high desert area of Los Angeles County. I've supplied all of the concrete for flatwork. The customer requests a six-sack, 3/8-inch pea-gravel mix that's placed through a small-line pump because trucks can't reach the placement area.

The customer doesn't tool in any contraction joints and doesn't use fillers for isolation joints because he believes animal feed would collect in these joints, rot and possibly harbor bacteria that could infect the animals. However, he complains to me about flatwork cracking, insisting that it's a concrete problem. I have had no luck explaining to the customer that random cracks are unavoidable if there are no joints in the concrete.

Under these circumstances, do you have any recommendations for a better mix design that would shrink less but could still be pumped? Or do you have any suggestions for methods he could use to achieve crack-free and joint-free concrete?

To produce a given slump, pea-gravel concrete requires more water than concrete made with larger coarse aggregate, shrinks more and is more likely to crack. Concrete made with the larger aggregate and placed in an unjointed slab will also crack, but perhaps in fewer locations. You could recommend that he use a larger pump, thus allowing you to supply concrete that shrinks less, but that alone won't eliminate random cracking. If the owner is willing to use a larger-diameter pump line, you could also suggest adding steel fibers to the concrete. Depending on the size and shape of the flatwork sections being placed, the steel fibers might eliminate most cracks.

To allow jointing the flatwork but still allay the concern about bacteria in the joints, you could suggest adding synthetic fibers containing an additive that inhibits microbial growth.

Other mix-design options include using a shrinkage-reducing admixture or a shrinkage-compensating cement in the concrete. Both of these would significantly increase cost, the former because the admixture is relatively expensive and the latter because it requires enough reinforcing steel to restrain expansion of the concrete, producing a tensile stress in the steel and a compressive stress in the concrete.

Another option is post-tensioning the concrete flatwork, although this probably isn't economical either.

You could also suggest sawing contraction joints in the concrete, then filling them with a joint sealant to keep out the animal feed. Instead of joint-filler material, tell him to try using a thin layer of roofing felt to break the bond at the joint. If the contact surface isn't too rough, this should allow vertical movement of the slab, although it won't permit horizontal movement if the slab expands.