A level shovel of cement, two full shovels of sand, three full shovels of stone, enough water to make it workable, and voilà—the magic stuff around which the concrete profession revolves. My granddaddy's 1-2-3 concrete formula—passed on to me around age 12—was my introduction to the high-tech world of concrete. Fifty years later, most slab-on-grade projects feature some variation of this mix.
At 30 minutes old, a yard of 4-inch slump 1-2-3 concrete will typically contain about 6½ sacks of cement, 1850 pounds of stone, 1220 pounds of sand, and 300 pounds (36 gallons) of water. At 28 days, its 0.49 w/c will deliver a compressive strength of 4500 psi. Although a little fine and somewhat gap-graded, it'll pump if you need it to and perform about as well as most engineer-approved slab mixes.
If the traditional 1-2-3 formula just seems too unsophisticated to be handed down, then how should my grandson be introduced to the mysteries of concrete mix design and production? A simple image might still be the best starting place. Imagine a 6-inch non-air-entrained slab with its ingredients compacted into separate layers piled up according to their densities. How deep would each layer be and where would it occur in the stack?
The cement, in fact, would form an 11/16-inch layer at the bottom. Next would come a 2½-inch layer of stone, a 1 5/8-inch layer of sand, a 1 1/16-inch layer of water, and a 1/8-inch top layer of air. Because the imaginary layers associated with most modern floor mixes will all have about these same depths:
Rule No. 7a: In a typical non-air-entrained 6-inch slab, more than 11/4inch of the slab's depth is water and air.
Moreover, because the theoretical water-cement ratio required to hydrate the cement is only about 0.30, or only about three-fifths of the total water content, the following is deduced:
Rule No. 7b: In a typical non-air-entrained 6-inch slab, more than3/4inch of the slab's depth is water and air that serves no reliable purpose other than to make the mix workable.
Because the combined fluids (the cement plus water paste and air) are only about two-thirds as dense as the solids—as long as the concrete remains plastic—the stones and sand tend to sink, forcing the surplus fluids to bleed toward the surface. This natural segregation is seen in fast motion when concrete settles and bubbles in response to vibration. To avoid diluting the cement glue at the surface, all such bleed water must be removed (usually by evaporation) before finishing can proceed. However, because the slab's volume must decrease upon losing these fluids and such reduction is only further promoted by the initial float and trowel passes, it is apparent that:
Rule No. 7c: All slabs compress.
It was this fact that made the old +3/8-inch, -¼-inch ACI slab thickness tolerance so unrealistic, because every well-struck-off slab must inevitably end up thinner than its specified nominal thickness.
The ramping that regularly occurs at construction joints, often mistakenly attributed to curling, primarily results from this phenomenon. To avoid the concrete sprinkling normally required to build the sinking edges back up to form height:
Rule No. 7d: Shim the end of the straightedge riding the edge form up1/32inch for every inch of slab depth and intentionally strike the concrete along the edges higher than the form.
Pan or float the edges to pull the paste back into the floor, and the ramps will disappear.
Allen Face is the inventor of the F-number system, F-min system, Dipstick, F-Meter, D-Meter, and Screed Rail. He is also an ACI Fellow and a long-time member of ACI Committees 302, 360, and 117.