Production
Tip
Lightweight Block Production
Throughout the concrete masonry
industry there are many producers using large quantities of lightweight
material. The production of lightweight units requires extra attention
to the mixing procedures because of the unique properties of lightweight
aggregates.
In order to be considered lightweight,
the aggregate must be composed predominately of lightweight cellular and
granular inorganic material. In general terms, any air-dried concrete
with a density of 105 pounds or less per cubic foot
(1680 kg/cu m) is considered lightweight.
The various types of structural
lightweight materials are described in ASTM C331. The classifications
follow:
Manufactured lightweights:
These are aggregates prepared by expanding, pelletizing or sintering
products such as blast furnace slag, clay, fly ash, shales and slates.
Natural lightweights:
These are aggregates prepared by processing natural materials such as
pumice, scoria, tuff and diatomite.
By-product lightweights:
These are aggregates consisting of end products from coal or coke
combustion.
Care should be taken in transporting,
storing and retrieving lightweight aggregates because of their
segregation characteristics. Segregation can occur in bins, during
transportation to the yard, or while conveying the material from one
point to another in your facility. Many companies wet the lightweight
material to minimize segregation.
Lightweight particles have a thirst
that must be satisfied to get the best use of the cement, which acts as
the binder in the mix. Proper hydration of the lightweight aggregate
will produce concrete that is strong and less likely to chip or spall.
The unique qualities of lightweight aggregates require a different
mixing procedure than is used for regular sand and gravel mixes.
It is important to know the percent of
moisture content in the lightweight aggregates being utilized in
production of the concrete masonry units. This can be done using a
simple calculation. Grab a handful of the aggregate and weigh it. Oven
dry the aggregate and weigh it again. Subtract the dry weight from the
wet weight and divide by the dry weight, multiply by 100 to discover the
percent of moisture contained in the sample.
| A
(weight of wet aggregate) - B (weight of dry aggregate) |
X
100% = % moisture in the material |
| B |
Lightweight particle
The quantity of mixing water should be adjusted based on this
information.
The scales can now be adjusted to
compensate for the moisture that is already contained in the lightweight
aggregate. This helps to keep the cement to aggregate ratio consistent
and the yield of concrete products per mix more accurate.
After considerable research, it is now
accepted that the following mixing sequence for production of
lightweight concrete masonry units gives optimum results.
Safety First: Always follow lockout/tagout
and, if applicable, confined spaces procedures before starting the
mixer. Contact federal or state OSHA to determine if your mixer is
classified as a confined space.
Lightweight Mixing
Instructions
1. Charge (load) mixer with all
aggregates
2. Add ½ to ¾ of the total mixing
water based on the absorption characteristics of the aggregate used. Use
of a gauge or metering device is recommended for accurate measurement.
The water should be pure enough to drink. Water should be added from
more than one distribution point such as a spray bar to produce a
uniform mix.
3. Mix for a minimum of 30 seconds.
4. Add all cementitious materials.
5. Add the balance of the mixing water.
6. Continue mixing for a minimum of 2
to 4 minutes in a ribbon mixer.
7. When tempering water is required to
return the mix to proper consistency, mix for an additional minute.
8. Test the first batch for the
moisture requirements and consistency.
Follow this mixing procedure for
lightweight production to produce strong, aesthetically pleasing
concrete masonry units and the potential for increased yields.
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