Time for Setting and Hardening

TIME FOR SETTING AND HARDENING The time to be allowed for the necessary set ting and hardening of the concrete before the forms are taken away, plays a very important part in the success of the structure. Mr. Edward Godfrey, in his volume on "Concrete," says: "The time that should elapse between the placing of concrete and the removal of the forms depends upon a number of things, among which are the consistency of the concrete, the richness of the mixture, the load sus tained, and the temperature and atmospheric humidity. Wet concretes require longer to harden than dry con crete. Lean concretes require longer than rich ones. Concrete hardens more slowly under water or in a sat urated atmosphere than in dry air. Low temperatures delay the setting of concrete. If the temperature be below freezing, the setting may be suspended. Failures have resulted on account of forms being removed from concrete which was frozen and which appeared to have hardened through setting.

"Another error apt to be made is to mistake drying for setting. Drying is not a necessary accompaniment to the hardening of concrete. In fact, if the concrete is too warm and the air too dry, the early drying of the con crete that will result will be detrimental to its strength. Concrete should not be allowed to dry out until it has stood for several days.

"Concrete receives its set when it reaches the state where a change of shape cannot be produced without rup ture. This requires from a few minutes, in rich mortars of quick-setting cement, to several hours, in lean mix tures. A common way of determining when concrete has set is by pressure of the thumb nail. After the set has taken place, the concrete continues to harden and gain strength for months, and sometimes for years. In ordi nary weather, nearly the full strength is attained in six or eight weeks. Loading tests may be made at this stage. Strength necessary to support its own weight is reached at varying periods, depending upon several conditions.

"In counting the time that concrete should stand be fore removing the forms, those days when the tempera ture is at or below freezing should be counted out, or at least allowance should be made for almost total suspen sion of the hardening process during those days.

"It is safe to remove the forms from mass work receiv ing at the time no load except its own weight, in from one to three days; in warm weather with dry concrete, one day; in cold or wet weather or with wet concrete, more time. When the concrete will bear the pressure of

the thumb nail without indentation, it is ready to sup port itself in this class of work. Thin walls should stand two to five days. Slabs of reinforced concrete should stand about one to two weeks of good weather before being called upon to support their own weight. Slabs of long span may require more time than two weeks. At the same time that the slab centering is removed, or even before it is taken down, the forms on the sides of beams and girders can be removed, leaving the supports of the bottoms in place for a longer time. This will afford an opportunity to inspect the surface of the beams and girders, and to plaster up any cavities before the con crete is too hard. Where practicable, it is well to leave the shores under beams and girders for three or four weeks. Large and heavy beams should be allowed to stand longer than short ones, because the dead weight is a greater fraction of the load they are designed to carry.

"Column forms may be removed in a week or so, if the entire weight of the beams is supported by shores close to the columns; otherwise three weeks or more should be allowed.

"Arches of small span may have the centering re moved in one to two weeks. Large arches should harden a month or more. Where practicable, it would be well to leave the concreting of the spandrel wall of an arch span until the arch ring has hardened and the forms are removed. The settling of the arch often cracks the span drel wall, and gives an unsightly appearance to the bridge.

"Ornamental work should have the forms removed as soon as possible, so that defects can be plastered up, and so that swelling of the wood will have less time to act.

"Falsework should be removed carefully, without jar to the concrete either by hammering on the boards or by dropping heavy pieces on the floor below. The sup ports should not be removed when any unusual load is on the floor. Materials should not be stored on floors that are not thoroughly hardened and self-supporting.

"Concrete reinforced work should ring when struck with the hammer, before the supports are removed."