SECTION V. MIXING
5-18. Established and well-defined concrete-mixing procedures must be followed to produce good quality finished concrete. Never become overconfident in this phase of concrete construction, whether caused by lack of competent and conscientious supervision or inattention to detail. Whoever is in charge of construction must know the concrete-mixing procedures and ensure that they are followed. The extra effort and care this requires are small in relation to the benefits in terms of strength consistency and finished appearance.
MEASURING MIX MATERIALS
5-19. Concrete of uniform quality requires measuring the ingredients accurately for each batch within these percentages: cement, 1 percent; aggregate, 2 percent; water, 1 percent; and admixtures, 3 percent. Equipment should be capable of measuring quantities within these tolerances for the smallest batch used, as well as for larger batches. Periodically check equipment for accuracy and adjust when necessary. Check admixture dispensers daily for errors in admixture measurements--particularly over-dosages; this can cause serious problems in both fresh and hardened concrete. Always measure the following:
Cement. Concrete mixes normally call for sacked cement as the unit of measure, although bulk cement is common practice in commercial construction requiring large quantities. Bulk cement is stored in bins directly above a weighing cement hopper and discharged from the hopper. However, because bulk cement requires special equipment for transport, sacked cement is used almost exclusively in troop construction, particularly in the theater of operations.
Aggregate. Measure aggregate for each batch accurately either by weight or by volume. Measurement by weight is the most reliable because the accuracy of volume measurement depends on an exact knowledge of the amount of moisture in the sand. Nevertheless, sometimes measurement by volume is more practical.
- Measurement by weight. On comparatively small jobs, you can use platform scales placed on the ground to weigh aggregate. Construct runways as shown in Figure 5-1, so that a wheelbarrow can run onto one side of the scale and off the other easily. With practice, you can fill a wheelbarrow so accurately that adding or removing material to obtain the correct weight is seldom necessary. Always place the same weight of aggregate on each wheelbarrow so that the quantity per batch equals the same number of wheelbarrow loads. Do not load a wheelbarrow to capacity.
- Measurement by volume. Measure aggregate by volume using a 1-cubic foot measurement box built on-site or a wheelbarrow. Wheelbarrows having from 2 to 3-cubic feet capacity are also available in the engineering units. A simple way to mix batches is the 1:2:3 method. This means that each batch contains 1 part cement, 2 parts sand, and 3 parts aggregate, regardless of the units of measure used (shovels, cubic feet, wheelbarrows, and so forth).
Water. Measure mixing water accurately for every batch. If the aggregate contains too much moisture, be sure to take this into account when adding mixing water. The water tanks on machine mixers often have automatic measuring devices that operate like a siphon.
Figure 5-1. Measuring aggregate by weight
5-20. Although a machine generally does the mixing, hand mixing sometimes may be necessary. A clean surface is needed for this purpose, such as a clean, even, paved surface or a wood platform having tight joints to prevent paste loss like the one shown at the top of Figure 5-2 below. Moisten the surface and level the platform, spread cement over the sand, and then spread the CA over the cement as shown at the top of Figure 5-2. Use either a hoe (see the middle of Figure 5-2) or a square-pointed D-handled shovel to mix the materials. Turn the dry materials at least three times until the color of the mixture is uniform. Add water slowly while you turn the mixture again at least three times, or until you obtain the proper consistency. Although one worker can mix 1 cubic yard of concrete by hand in about 1 hour, this is not economical for batches of more than 1 cubic yard. Instead, two workers facing each other should work their way through the pile, and keep their shovels close to the platform surface while turning the materials. You can also mix in a hoe box shown at the bottom of Figure 5-2.
Figure 5-2. Hand mixing of concrete
MACHINE MIXING AND DELIVERY
5-21. The methods of mixing and delivering concrete ingredients and the types and sizes of equipment available vary greatly. Power-concrete mixers normally produce one batch about every 3 minutes including charging and discharging. Actual hourly output varies from 10 to 20 batches per hour. A mixer's cubic foot rating usually reflects the number of cubic feet of usable concrete that the machine mixes in one batch. Most mixers can handle a 10 percent overload. The stationary 16-cubic-feet mixer and the M919 Concrete-Mobile-Mixer unit (see Figure 5-3) are table of organization and equipment (TOE) equipment in engineer construction battalions, and are well-suited for troop construction projects.
Figure 5-3. M919 Concrete Mobile Mixer
5-22. Mixing methods include--
Site mixed. Method used for delivering plastic concrete by chute, pump, truck, conveyor, or rail dump cars.
Central-plant mixed. Method used for delivering plastic ready-mix in either open dump trucks or mixer trucks.
Central-plant batched (weighed and measured). Method used for mixing and delivering "dry-batched" ready-mix by truck.
Portable-mixing plant mixed. Method used for large building or paving projects distant from sources of supply.
5-23. Mixer types include:
CENTRAL MIX PLANTS
5-24. Either stationary mixers or a battery of trailer-mounted mixers usually makes up a central-mix plant which is normally a gravity-feed operation. A clamshell bucket crane, conveyor belts, or elevators carry materials to a batching plant set high enough to discharge directly into dump trucks or other distribution equipment. Mixing time and mixing requirements do not differ much from those already discussed, but you must take special precautions to make sure that the concrete has the proper characteristics and workability upon arrival at the work site. Be especially careful to avoid segregation when using dump trucks.
SITE MIX PLANTS
5-25. The proper location of mixing equipment and materials at the site can yield large savings in time and labor. Figure 5-4 below shows a typical on-site arrangement of mixer and materials. Always locate the mixer as close to the main section of the pour as possible. On a concrete wall project move the mixer to each wall in sequence to reduce transportation distance and time.
Figure 5-4. Hand mixing of concrete
Store aggregate and sand as close to the mixer as possible, without interfering with concrete transportation. The location of the mixing water depends on the type of water supply available. If water is piped in, use a hose to carry it to a barrel near the mixer. If you are using a water truck or trailer, park it next to the mixer.
OPERATING THE 16-CUBIC FOOT MIXER
5-26. Table 5-3 below gives the physical characteristics of a typical 16-cubic-feet mixer. Normal operation of the mixer requires ten soldiers and one noncommissioned officer. The crew operates the mixer and handles the aggregate, sand, cement, and water. The noncommissioned officer, who must be competent, supervises the overall operation. The crew should produce about 10 cubic yards of concrete per hour, depending on their experience, the location of materials, and the mixer's discharge rate. You would need at least one platoon to operate an overall project like the one in Figure 5-4 above.
Table 5-3. Physical characteristics of a typical 16-cubic feet mixer
Charging the Mixer
5-27. Mixers can be charged in two ways: by hand or with a mechanical skip (see Figure 5-5 below). When using the skip, deposit the aggregate, cement, and sand (in that order) into the skip and then dump it into the mixer while water runs into the mixing drum. Place the sand on top of the pile in the skip so that you do not lose too much cement as the batch dumps into the mixer. A storage tank on top of the mixer measures the water in the drum a few seconds before the skip dumps. This discharge also washes down the mixer between batches.
Figure 5-5. Charging a 16 cubic-foot mixer
Discharging the Mixer
5-28. When the mix is ready for discharge from the mixer, move the discharge chute into place to receive the concrete from the drum. Concrete that is somewhat dry tends to cling to the top of the drum and not drop onto the chute in time. Very wet concrete may not carry up high enough on the drum to drop onto the chute. Correct these problems by adjusting the mixer speed. Increase the speed for very wet concrete and decrease the speed for dry concrete.
5-29. The mixing time starts when water runs into the dry mixture. This is normally during the first quarter of the mixing period. The minimum mixing time per batch of concrete is 1 minute unless the batch exceeds 1 cubic yard. Each additional cubic yard of concrete, or fraction thereof, requires an additional 15 seconds of mixing time.
Cleaning and Maintaining the Mixer
5-30. Clean the mixer daily, if it operates continuously, or following each period of use if it operates less than 1 day. The exterior cleaning process goes faster if you coat the outside of the mixer with form oil before you use it. Knock off all accumulated concrete on the mixer exterior and wash it down with a hose. Mixer blades that are worn or coated with hardened concrete provide less efficient mixing action. Replace badly worn blades, and do not allow hardened concrete to accumulate in the mixer drum. Clean it out whenever you shut down for more than 1 1/2 hours. To do this, place a volume of CA equal to one-half the mixer capacity in the drum and allow it to revolve for about 5 minutes. Then discharge the aggregate and flush out the drum with water. Never strike the discharge chute, drum shell, or skip to remove aggregate or hardened concrete, because concrete adheres more readily to dents and bumps. Diligence in cleaning the drum is important because hardened concrete in a mixer absorbs mixing water during subsequent batches, diminishing the effectiveness and composition of the concrete.
OPERATING THE CONCRETE-MOBILE MIXER UNIT
5-31. The Concrete-Mobile-Mixer unit is a combination materials transporter and an on-site mixing plant. Table 5-4 below gives its physical characteristics and overall dimensions. The special body is mounted on a model M919 truck chassis. The unit carries enough unmixed material to produce up to 8 cubic yards of fresh concrete. Because the unit is precisely calibrated, you can produce mixes that meet or exceed both the ACI and the American Association of State Highway and Transportation Officials (AASHTO) standards for design strength. The unit operates on either an intermittent or continuous basis, although continuous operation depends on raw material availability at the site. Certain control settings for the mix operations vary from truck to truck and from site to site.
Table 5-4. M919 Concrete-Mobile-Mixer unit
5-32. When fresh concrete is left standing and not poured, it tends to stiffen before the cement can hydrate to its initial set. Such concrete is still usable if remixing makes it sufficiently plastic to be compacted in the forms. To remix a batch, carefully add a small amount of water, and remix the concrete for at least one-half of the minimum required mixing time or number of revolutions. Then test the concrete to make sure that it does not exceed the maximum allowable W/C ratio, maximum allowable slump, or maximum allowable mixing and agitating time. Do not add water randomly because this lowers concrete quality. Remixed concrete tends to harden rapidly. Any fresh concrete placed adjacent to or above remixed concrete may cause a cold joint.
|David L. Heiserman, Editor||
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Revised: June 06, 2015