Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

36 Chapter Three
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Type I or Type III cement, weighting 150 lb per cu ft, containing no
prozzolans or admixtures, having a slump of 7 in. or less, and internal
vibration to a depth of 4 ft or less.
Effect of Weight of Concrete on Pressure
The unit weight of concrete for most structures is 145 to 150 lb per cu
ft. It is common to refer to normal-weight concrete as having a unit
weight of 150 lb per cu ft. However, the density of concrete may vary
from 100 lb per cu ft for lightweight concrete to 200 lb per cu ft for
high-density concrete. For concrete with densities other than 150 lb
per cu ft, the pressure can be found by multiplying the normal-weight
concrete (150) by the ratios of the densities. For example, if concrete
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weighing 150 lb per cu ft will produce a pressure of 1,200 lb per sq ft
under certain conditions, a low-density concrete weighing 100 lb per
cu ft should produce a pressure equal to (100/150) × 1,200 = 800 lb per
sq ft under the same conditions. Similarly, if a high-density concrete
weighing 200 lb per cu ft will be used, the pressure produced
will be equal to (200/150) × 1,200 = 1,600 lb per sq ft under the same
conditions.
Equation (3-5) may be used to determine the maximum pressure
produced by concrete having a density other than 150 lb
per cu ft.
P′ m
= [(D′)/(150)] × P m
(3-5)
where P′ m
= modified pressure, lb per sq ft
D′= density of concrete, lb per cu ft
P m
= maximum pressure for concrete whose density is 150 lb
per cu ft when placed under the same conditions
Vertical Loads on Forms
In addition to lateral pressure, vertical loads are also imposed on
formwork. These vertical loads are due to the weight of the newly
placed concrete and reinforcing steel, the weight of the form materials
and hardware that fasten the forms, the weight of tools and equipment,
and the weight of workers. For multistory structures, the design
of forms must consider the loads that are transmitted from all of the
higher floors.
In addition to the weight of concrete, the weight of the forming
material, hardware, and reinforcing steel must also be included in the
vertical loads that are applied to forms. The weight of the form material
and hardware will not be known until the formwork is designed.
The designer may estimate a weight of 10 lb per sq ft and then check
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