The rammer usually employed consists of a block of iron or a stick of wood 6 to 8 inches square, weighing from 20 to 40 pounds. The concrete should be tamped until mortar flushes to the top, which secures a smooth surface and guarantees that the mass has been thoroughly consolidated. There are two tricks sometimes employed to make it appear that this condition has been fulfilled. One is to scrape off the mortar left adhering to the mixing board and throw it on top of the concrete already in place. The other is to shovel the concrete up higher than the finished surface and then bring it down with a rake, the stones being pulled out and the mortar left on top. Of course neither of these tricks should be accepted as a substitute for the proper amount of tamping.
Not infrequently loose fragments of stone are left upon the top of the foundation. If the concrete is well mixed and well tamped, there will be no pieces left on its surface; and if they are left there, they should be carefully picked off, since otherwise the paving blocks will not have a firm even bearing on the concrete. In Eng land the top of the concrete is floated with a thin film of either neat cement or rich mortar to secure a perfectly smooth surface upon which to place the wood paving-blocks.
When finally completed the concrete should be covered with sand, say an inch deep, to keep it from drying out. If the water is evaporated instead of uniting with the cement, the concrete will be materially weakened; concrete stored in a steam-heated room will attain to only about half the strength of cubes stored in water. Therefore if the weather is warm and dry, the sand covering the concrete should be sprinkled at intervals to keep it damp. No teaming or walking on the concrete should be permitted until it has firmly set—which is usually assumed to require from 4 to 10 days, depending on the activity of the hydraulic cement used. With reasonable care and ordinary cement, the pavement may be laid four or five days after the concrete is placed, although many en gineers to be on the safe side wait 8 or 10 days. The fact that the street is closed for a considerable time to allow the concrete to set, is the only objection, except possibly the cost, to this form of foun dation.