to get 600. Finally, multiply the 600 by 0.25 to determine
the volume of concrete required for the pad which, in
this case, is 150 cubic feet.
Concrete is ordered and produced in quantities of
cubic yards. To calculate the number of cubic yards
required for the pad, divide the cubic feet of the pad by
27. This is required because there is 27 cubic feet in 1
cubic yard. Therefore, the concrete pad described in the
previous paragraph, which has a volume of 150 cubic
feet, requires 5.56 cubic yards of concrete: 150 cubic
feet divided by 27 = 5.56 cubic yards.
Concrete projects often present varying degrees of
difficulty; therefore, extra concrete is required to
compensate for these difficulties. Once the total number
of cubic yards of concrete is computed, add a little extra,
normally 10 percent, to compensate for waste. To
calculate the excess needed, multiply the cubic yards by
.10 (10 percent). In the above case, multiply 5.56 cubic
yards by .10 to get 0.556 cubic yards. Add the 0.556
cubic yards to the 5.56 cubic yards for a total of 6.116
or 6.12 cubic yards required for the concrete pad.
Batching is the process of weighing or
volumetricall y measuring and introducing into a mixer
the ingredients for a batch of concrete. To produce a
uniform quality concrete mix, measure the ingredients
accurately for each batch. Most concrete specifications
require that the batching be performed by weight, rathcr
than by volume, because of inaccuracies in measuring
aggregate, especially damp aggregate. Water and liquid
air-entraining admixtures can be measured accurate]y
by either weight or volume. Batching by using weight
provides greater accuracy and avoids problems created
by bulking of damp sand. Volumetric batching is used
for concrete mixed in a continuous mixer, and the
mobile concrete mixer (crete mobile) where weighing
facilities are not at hand.
Specifications generally require that materials be
measured in individual batches within the following
percentages of accuracy: cement 1%, aggregate 2%,
water 1%, and air-entraining admixtures 3%.
Equipment within the plant should be capable of
measuring quantities within these tolerances for the
smallest to the largest batch of concrete produced. The
accuracy of the batching equipment must be checked
and adjusted when neccessary.
Concrete should be mixed until it is uniform in
appearance and all the ingredients are evenly
distributed. Mixers should not be loaded above their
rated capacities and should be operated at
approximately the speeds for which they were designed.
If the blades of the mixer become worn or coated with
hardened concrete, the mixing action will be less
efficient. Worn blades should be replaced and the
hardened concrete removed periodically, preferably
after each production of concrete.
When a transit mixer (TM) (fig. 7-1) is used for
mixing concrete, 70 to 100 revolutions of the drum at
the rate of rotation designated by the manufacturer as
mixing speed are usually required to produce the
specified uniformity. No more than 100 revolutions at
mixing speed should be used. All revolutions after 100
should be at a rate of rotation designated by the
manufacturer as agitating speed. Agitating speed is
usually about 2 to 6 revolutions per minute, and mixing
speed is generally about 6 to 18 revolutions per minute.
Mixing for long periods of time at high speeds, about 1
or more hours, can result in concrete strength loss,
temperature rise, excessive loss of entrained air, and
accelerated slump loss.
Concrete mixed in a transit mixer should be
delivered and discharged within 1 1/2 hours or before
the drum has revolved 300 times after the introduction
of water to cement and aggregates or the cement to the
aggregates. Mixers and agitators should always be
operated within the limits of the volume and speed of
rotation designated by the equipment manufacturer.
Overmixing concrete damages the quality of the
concrete, tends to grind the aggregate into smaller
pieces, increases the temperature of the mix, lowers the
slump, decreases air entrainment, and decreases the
strength of the concrete. Also, overmixing puts needless
wear on the drum and blades of the transit mixer.
To select the best mixing speed for a load of
concrete, estimate the travel time to the project (in
minutes) and divide this into the minimum desired
number of revolutions at mixing speed-70. The results
will be the best drum speed; for instance, if the haul is
10 minutes, 70 divided by 10 equals 7. With this drum
speed, the load will arrive on the jobsite with exactly 70
turns at mixing speed, with no overmixing of the
concrete mix and no unnecessary wear on the