When bins are employed, steam pipes are used to thaw out the materials.

Methods of covering concrete to protect it from light frosts include the use of sacking, shavings, straw, and manure. In cold climates, frame buildings that completely house in the construction are frequently erected. Such buildings are heated and the temperature kept well above the freezing-point.

Placing Concrete Under Water.-Mixed concrete if emptied loose and allowed to sink through water is destroyed; the cement paste is washed away and the sand and stone settle on to the bottom more or less segregated and practically without cementing value.

3'01

Concrete

FIG. 16. Tremie

To overcome these difficulties, the following methods are employed for depositing concrete under water:

1. Depositing in (a. Bottom dumping.

closed bucketsb. Revolving buckets.

2. Depositing in ( a. Bottom dumping bags. bags........b. Bags to be left in the work. 3. Depositing through a tremie.

4. Grouting submerged stone.

Buckets for depositing concrete under water are provided with covers, so that the water cannot flow in and wash out the cement as the material is being lowered. Bottom dumping buckets also possess an unlocking device to open the bottom doors and allow the concrete to pass out. Revolving buckets are turned upside down before emptying.

Two methods of depositing concrete in bags are available to the engineer. In the first method a bag of heavy tight-woven material is filled with concrete and emptied at the bottom, the bag serving like the buckets as a means of conveyance.

In the second method bags of paper or Concrete under Water. loose-woven gunny-sack are employed. The

Tube for Depositing

bags are filled with concrete and are left in the work, the idea. being that the paper will soften or the cement ooze out through the openings in the cloth sufficiently to bond the separate bagfuls into a solid mass.

A tremie consists of a tube of wood or sheet metal, which reaches from above the surface to the bottom of the water. It is operated by filling the tube with concrete and keeping it full by successive additions, while allowing the concrete to flow out gradually at the bottom by slightly raising the tube to provide the necessary opening.

Masses of gravel, broken or rubble stone deposited under water may be cemented into what is virtually a solid concrete by charging the interstices with grout forced through pipes from the surface. The grout employed is a 1 : 1 mixture of Portland cement and sand, with sufficient water to form a thick paste. This is readily forced through 2 in. pipes into depths of 50 ft. and over.

In heavy subaqueous operations concrete is also placed by constructing a coffer dam around the site, pumping out the water, and working in the dry or by placing large specially prepared blocks by means of derricks, the setting being done by divers. A new method has recently come into use for building subaqueous concrete walls, by means of pontoons constructed on shore, floated into place and sunk by means of ballast.

CHAPTER VIII

FORMS FOR CONCRETE CONSTRUCTION

Kinds of Forms.-Pressure of Concrete on Forms.-Dressing and Lubrication of Forms.-Design of Forms.-Removing Forms.-Cost of Forms.

THE design and construction of forms are among the most difficult of the problems imposed upon the worker in concrete.

Forms should be stiff, strong, and economical in labor and materials. They should be built with a view to economy in taking down rather than to cheapness in erecting or in first cost. Roughly built forms which cannot be removed without being ripped to pieces are always expensive.

Kinds of Forms.-The principal kinds of forms in general use are as follows:

1. Simple braced forms.

2. Wired and bolted forms for walls.

3. Forms made of studding and matched boards.

4. Panel forms.

5. Column forms and braces.

6. Forms for beams and slabs.

7. Arch centres.

8. Special, collapsible facing forms and templets.

Forms are most commonly constructed of wood, which must be planed and oiled to present a smooth surface, since the concrete takes the impress of any irregularity that presents itself. Stiff, close-grained woods are the best, such as white pine, yellow pine, spruce, Oregon pine, or redwood. Hemlock should not be employed, as it is rough, splintery, and weak. Oak is hard to nail, expensive and imprints grain marks on the concrete even when the form is well oiled.

Forms should be constructed in such a way as to avoid the use of nails whenever possible. Braces are seldom less than 1 in. thick and it takes hard driving to get spikes through them. When

ever possible, blocks or wedges held in place by thin nails, should be substituted for the large spikes so often employed.

Lagging and panel strips are made of 1 1/4 to 2 inch stuff, short struts and braces of 2 x 4 inch timber, while long struts range from 4x4 to 8 x 8 inch sectional area.

Simple braced forms are used for foundations, retaining walls, and ordinary construction. They consist of from 1 to 1 1/2 inch boards, which are supported by 2 x 4 inch studs, set about 2 feet apart. The studs are also braced with 2 x 4 inch diagonals. The diagonal braces are held in position by posts driven into the ground.

lin Boards

2x4 in. Cleats

FIG. 17.-Simple Forms for Cellar Walls. This type is faulty in that the braces are nailed to the sides.

"As a rule it is best to drive a line of posts and to lay against them a heavy timber or thick plank. This provides a stiff support against which braces may be placed at any point when needed. At any sign of giving way in the forms, intermediate braces may be quickly introduced without the delay consequent upon driving new posts." "Bracing is not good practice for the holding of wall forms in place." Failures of such forms are frequently caused by the giving way of the posts due to the yielding of earth. Earth is a poor material to depend upon for holding forms rigid, and bracing is only excusable when the form can be secured from but one side and that usually the outside. In all narrow forms, the studding on opposite sides should be tied together by bolts or wires.

In all braced forms, the posts against which the ends of the diagonals rest should be driven deep. "They should also be driven as nearly vertical as possible. The usual way is to drive them on a slant," but experience has shown that vertical posts are the stiffer, especially when the ground is poor. "The top soil is seldom able to carry much of a load," hence the brace should be driven deep in order that it may obtain sufficient anchorage.

Wired and Bolted Forms.-Forms, when used on both sides of a narrow wall, should be tied together by wires or bolts. The wire

is preferably passed twice through the forms, the ends twisted together and any surplus cut off with nippers, while the wire is tightened by twisting the two strands together inside of the forms, a stick being employed for the purpose. Before it is drawn up, a wooden spacer of length equal to the required width of the wall is placed beside the wire, where it is left until the concrete reaches that height, after which it is removed.

Wired forms are much more secure than those which are merely braced. They possess, however, the following objectionable features: