6-inch slab would produce a dead load of 6 by 12.5 or 75 pounds per square foot, excluding the weight of forms. The recommended minimum construction live load to provide for the weight of crew members and equipment is 50 pounds per square foot of horizontal projection. If powered concrete buggies are used in concreting operations, it is recommended that 75 pounds per square foot be used as a minimum construction live load.

When concrete is placed in the form, it is in a plastic state and behaves temporarily like a fluid, producing a hydrostatic pressure that acts laterally on the vertical forms. If concrete acted as a true liquid, the pressure developed would be equal to the density of the fluid (150 pounds per cubic foot is commonly assumed for concrete) times the depth in feet to the point at which the pressure was being considered. However, plastic concrete is a mixture of solids and water whose behavior only approximates that of a liquid, and then for a limited time only. This lateral pressure is comparable to a full-liquid head when concrete is placed full height within the period required for its initial set. With slower rates of placing, concrete at the bottom of the forms begins to harden, and the lateral pressure is reduced to less than full-fluid pressure by the time concreting is completed in the upper parts of the form.

The effective lateral pressure, a modified hydrostatic pressure, has been found to be influenced primarily by the rate of placing and the temperature of the concrete mix. Other variables that have been found to have an effect on lateral pressure include consistency of concrete, amount and location of reinforcement, vibration, maximum aggregate size, and placing procedures. However, with usual concreting practices, the range of the effects of these variables is generally small and is either neglected or compensated for in design tables.

Adequate lateral bracing is extremely important to stability and safety in formwork construction; but all too often, it is treated carelessly or even omitted entirely. Formwork must be braced to resist all foreseeable lateral loads, such as those imposed by wind, dumping of concrete, or any other impact, such as starting and stopping of equipment. There are many types of braces that can be used to give forms stability. The most common type is a diagonal member and horizontal member nailed to a stud or wale. The diagonal member should make a 45-degree angle with the horizontal member. Additional bracing may be added to the form by placing vertical members (strongbacks) behind the wales or by placing vertical members in the corner formed by the intersecting wales.

Concrete forms must be constructed to resist the pressure exerted on them by the freshly placed concrete without deflection (side displacement) beyond a specified maximum. This maximum is very small; for a wall form, for example, the maximum deflection of sheathing, studs, and wales is not over 1/270th of the span.

Placing concrete exerts a very considerable lateral (side) pressure on the form sheathing. The pressure at the bottom of the freshly placed concrete is greater than that at the top and the pressure increases with the height of the form.

When designing formwork, you must ensure that the sheathing, the stud, and the waler spacing are designed to a given pressure (vertical rate of placement).

To determine the vertical rate of placement for concrete wall forms, you divide the quantity of concrete (mixer output) which is placed into the form in an hour (in cubic feet) by the horizontal area of the form space being filled. Suppose you are filling a wall section for a wall 30 feet long by 12 inches thick. The horizontal area would then be 30 square feet. See the formula below.

Let's take the hourly rate of the 11 S mixer (11 cubic feet per load) which has an hourly output of 4 to 8 cubic yards or from 108 to 216 cubic feet (depending on personnel) in a continuous operation. However, the quantity of concrete placed in the form per hour will depend on how continuous the mixer operation is and how rapidly the mix is transferred from the mixer to the form. This quantity you will have to determine according to your knowledge and circumstances at the

Continue ReadingIntegrated Publishing, Inc. |