Ready-Mixed Concrete
Ready-mixed concrete from a commercial plant is usually the easiest method of
obtaining a high quality concrete. However, it is necessary to specify the quality of
the concrete required for a specific application. Table D1.4 may be used as a guide
for ordering concrete from a ready-mix plant. Another common method of ordering
ready-mixed concrete is to specify the cement content per cubic yard of concrete
and the water content per bag of cement. The following guide is useful for being
sure you obtain the quality of concrete you need for specific types of use. Since
the specified water includes the water in the aggregate and since this water is
difficult to measure accurately the concrete plant can use a "slump test" to
determine the proper amount of water. Request a two to three inch slump. In the
absence of a "slump test" request the ready-mix plant to use a quart or two less
water per bag of cement; then at the site, add the minimum amount of water
necessary for placing and finishing; this should be a thick mud consistency.
Be sure you are completely ready to place the concrete when it is delivered since
delays will permit partial setting of the cement and reduced strength before it is
placed.
The quantity of concrete to order may be estimated from Table D1.4 and/or
D1.5. Ready-mix plants will assist you in estimating the quantity required. Order
five percent extra for waste and irregularities in surfaces which have been carefully
graded. As much as ten percent extra may be required for surfaces, which have
not been carefully graded.
Table D1.4:
Guide For Ordering Ready-Mixed Concrete
[Order medium-consistency concrete (3-inch slump)]
Job
F L A T W O R K:
Severe exposure (garages,
feeding floors, floors in
dairy plants)
Normal exposure (paved barnyards, floors for farm
buildings, sidewalks)
Mild exposure (building footings, improvements in mild
climates)
F O R M E D W O R K:
Severe exposure (mangers for
silage feeding, manure pits)
Normal exposure (reinforced
walls, beams, tanks,
foundations)
Mild exposure (improvements in
mild climates)
Specification
Minimum number of
Maximum number
bags of cement
of gallons of
per cubic yard
water per bag
concrete
cement
7
5
6
6
5
7
7-3/4
5
6-1/2
6
5-1/2
7
Determining the Amount of Concrete Needed
The cubic yards of concrete required for a job can be determined by calculating
the volume in cubic yards of the space inside the forms. To allow for spillage, etc.,
an extra 5- to 10-percent should be ordered.
Table D1.5:
Estimated Area That Can Be Covered With A Cubic Yard of Concrete
for Various Thicknesses
Thickness (inches)
Square Feet Per Yard of Concrete
3
4
5
6
8
10
12
107
80
64
54
40
32
27
NOTE: No Allowance has been made for waste.
For other thicknesses the following relationship can be used:
Square feet per cubic yard of concrete =
324/Thickness in inches