The first subcontractor on a construction site is the excavator. His job is an important one. Proper footings depend on his ability to remove the soil to an accurate depth and to disturb the subgrade as little as possible. All footing subgrades should be thoroughly compacted whether fill is used or not. Too deep an excavation will demand fill, and no matter how well this is compacted, settling of the planned structure can occur and create unwanted cracks. Too high a grade, or an unlevel one will necessitate leveling the grade by hand, which is time-consuming and a costly.one.
Footing forms must be set to proper line and grade, and they must be well staked and braced. The footing concrete should be placed with a reasonable slump, to avoid a top scum-called laitance-which can result in a poor bond for the foundation walls to come later. It is recommended that the footing be keyed to reduce water seepage. Where the soil offers poor drainage, or on special projects, a water stop is recommended.
Before wall forms are erected, the footing of the surface should be clean and free of debris. The wall forms should be treated with form oil before erection so that the form oil will not run down the forms to the footing concrete.
Enough cannot be said regarding the necessity for well constructed wall forms, especially since solid form construction is a detail often overlooked by some concrete contractors. Forms must be snug and tight, firm and well braced, to avoid bulging or shifting. The form does not have to give way completely to cause cracks. Any movement of forms due to timber expansion, or to a loosening of nails or clamps, can create unnecessary problems.
Depending on the height of the wall, the concrete may be discharged directly from the concrete truck, be placed by means of buggies or by crane, or be pumped. Regardless of the method used, it is crucial, in order to avoid segregation, that the concrete be placed straight down- ward and not be allowed to bounce off the wall forms. Serious segregation can be avoided when using buggies or cranes by depositing the concrete into a hopper, or funnel-shaped receptacle, on top of the form. Below, and attached to the hopper, is a series of tapering metal sections known as an “elephant trunk,” which can be raised to keep the metal sections above the level of the deposited concrete as it rises in the form.
A concrete pump may be used on high-wall-form construction to avoid concrete segregation. The 4-in. rubber sections at the discharge end of the line can be lowered down into the form, reducing aggregate bounce off the reinforcing steel. Some concrete contractors use windows like openings located at different levels in the form. When the concrete reaches these levels, the openings are then closed with prefabricated sections. If contractors plan well, there is little delay in placing the concrete and the method does drastically reduce segregation. It is also necessary on high-wall concrete placement to first introduce several inches of grout on top of the footing at the base of the wall form. The concrete placement should follow promptly, while the grout is still in the plastic state, to eliminate the possibility of honeycombing at the base of the wall. Do not use high slump concrete when placing wall concrete, as it will usually result in unsightly sand streaking because of the bleed water’s rising to the surface between the concrete and the form, thus diluting the surface paste.
Do be certain the footing is clean and free of any debris. Do treat the wall forms with good form oil before erecting the wall panels, and let them drain before erection. Do use internal vibrators rather than external ones. External vibrators, popular in the manufacture of precast products, will place excessive strain on the wall forms. Compacting the concrete on low walls by rodding with a 2 X 4 may do a fair to reasonably good job but will not work well on higher pours.
Curing is one of the most important steps in placing concrete, especially for finished floors, but many people do not fully appreciate the need for good curing practices. The purpose of curing is to prevent the loss of the water present in the concrete mass. Such moisture must be maintained to effect the complete hydration of the cement paste. The greater the amount of water retained within the concrete mass, the higher the ultimate strength of that concrete.
Steps must also be taken to prevent the loss of concrete water to a dry subgrade. Wetting the subgrade thoroughly, but not so much as to create a muddy condition, is a help, but not always sufficient. The use of 4-mil polyethylene gives superior protection from subgrade absorption and also helps in preventing water penetration into the hardened concrete from the subgrade that can occur from hydrostatic pressure.
The ideal way of curing concrete floors is by means of water, either by ponding or by covering the finished concrete with water-saturated burlap, kept moist for at least 4 days. Keep the burlap constantly moistened, particularly when the slab cured under this method is exposed to the elements. Although this method has lost appeal because of the cost and time delay it involves, water-curing will greatly reduce volume change and shrinkage cracks. No curing method should ever be attempted until the concrete surface has set sufficiently to resist foot traffic. The widely used kraft paper should be placed flush with the concrete surface, be well overlapped, and be held down securely. Polyethylene sheeting resists flush placement against a concrete surface, resulting in a mottled appearance when removed.
When a liquid curing compound is to be applied to a concrete floor, it is important to know what is to be applied to this surface at a later date. Many liquid curing compounds become an integral part of the concrete and prevent adhesion of tile, cementitious materials, or epoxies. Where such applications are to be made later, an inexpensive, short lived curing compound is a better choice. Further, the choice of the color of the compound should be given consideration. If the concrete will be exposed to the sun the lighter colored compounds should be used to reduce heat absorption.