643. The principles governing the laying of walks apply also in a general way to the construction of floors that rest directly on the ground.

For residences, basement floors may be laid with three inch base of concrete and one-half inch wearing surface. The thickness of sub-base will depend upon the character of the soil. Where natural conditions do not assure good drainage of the foundation, this should always be provided for by either a blind stone or tile drain laid around the outer edge of the building and leading to the sewer or other outlet. The finished surface of the floor should always have a slight slope toward the center or one corner of the basement, and a trapped sewer connection set at this lowest point in such a way that it is accessible for repairs and cleaning.

1 Engineering News, March 4, 1897.

644. Wet Basements

Where much ground water is encountered, and especially where a basement is subjected to a head of water from without, special precautions must be taken in building the floor. The concrete must be made thick enough so that its weight and the arch action set up, shall be able to withstand the upward pressure of the water. In building such a floor it is necessary to keep a sump hole, preferably in the center, towards which the construction proceeds from the sides. A pipe placed in the sump hole permits pumping until the concrete is laid about the pipe, when the latter may be filled with rich cement mortar. In such cases the side walls of the basement should be plastered with Portland cement mortar on the outside and special care taken in joining the floor to the wall.

645. Size Of Blocks

As the changes in temperature in a building are usually much less than in open air, the blocks of concrete may be of much larger size, say ten feet square, and many basement floors are laid without any joints, though sooner or later they will probably crack if so laid. In factories for certain purposes, however, the floors may be subjected to greater changes in temperature than walks laid in the open air. In such cases the blocks should not be more than three or four feet on a side, and the joints may well be filled with asphalt, especially if water-tightness is desired.

646. Stable floors may be made of six inch cobble or broken stone sub-base, six inches of concrete made with mortar containing three parts sand to one cement, and one inch of top dressing containing three parts sand (mixed sizes) or crushed granite to two parts cement.

Factories having heavy machinery with much vibration require strong floors. Such a floor may be made of six inches of cobble stone sub-base covered by six inches of a lean concrete made with one-to-four mortar, and above this, three to five inches of rich concrete made with mortar containing two and one-half parts sand to one cement, and one inch of top dressing, equal parts cement and sand or cement and crushed granite.

647. Example And Cost

In the construction of the new printing building for the Government Printing Office at Washington, the basement floor is nine inches thick, made as follows :1

1. Concrete sub-base, six inches thick of one part natural cement, two parts sand and four and one-half parts broken brick.

2. Concrete base, two and one-half inches thick of Portland cement one part, sand two parts and fine broken gneiss four parts.

3. Top dressing, one-half inch in thickness, of two parts sand to one part Portland cement.

The Cost of this floor was about $1.50 per square yard, or about seventeen cents per square foot.