SECTION VI. HANDLING AND TRANSPORTING
5-33. Concrete consistency depends on the conditions at placement. Handling and transporting methods can affect its consistency. Therefore, if placing conditions allow a stiff mix, choose equipment that can handle and transport such a mix without affecting its consistency. Carefully control each handling and transporting step to maintain concrete uniformity within a batch and from batch to batch so that the completed work is consistent throughout.
5-34. Figure 5-6 shows several right and wrong ways to handle concrete to prevent segregation of the aggregates and paste. Segregation causes honeycomb concrete or rock pockets. Segregation occurs because concrete contains aggregates of different particle sizes and specific gravities. When placed in a bucket, the coarser particles tend to settle to the bottom and the water rises to the top.
Figure 5-6. Concrete handling techniques to prevent segregation
5-35. The three main requirements for transporting concrete from the mixing plant to the job site are:
5-36. Concrete delivery equipment must be capable of handling 100 percent of the mixer capacity to meet peak demands.
5-37. Wheelbarrows or buggies (see Figure 5-7) are the most practical and economical to deliver concrete for foundations, foundation walls, or slabs on or below grade. If available, power buggies (see Figure 5-8) are best for longer runs. Both wheelbarrows and buggies require suitable runways. If possible, arrange them so that the buggies or wheelbarrows do not need to pass each other on a runway at any time. When placing concrete below or at approximate grade, set 2-inch plank runways directly on the ground to permit pouring the concrete directly into the form. View 1 in Figure 5-9 below shows a runway along a wall form that is almost level with the top of the form sheathing. To provide room for the ledger, the top wale is at 1 foot below the top of the concrete. A runway can be made economically from rough lumber consisting of 2 by 10 planks, supported by 4 by 4s, and spaced on 6- foot centers. Nail the 1 by 6 ledger on the form side to the studs on the wall form. Always brace runways securely to prevent failure. A runway for placing concrete on a floor slab also consists of 2 by 10s supported by 4 by 4s placed on 6-foot centers. Such runways are built from either the formwork or the ground. When placing concrete about 5 to 6 feet above grade, economically construct inclined runways (see view 2 of Figure 5-9) for buggy or wheelbarrow use. This will allows wheeling the concrete up inclined runways having a slope of 10:1. If a large quantity of concrete more than 5 to 6 feet is elevated, it is more economical to use elevating equipment, such as a bucket and a crane. If the difference in elevation from the runway to the bottom of the structure is large, use a hopper and chute like the ones shown in Figure 5-10 to prevent segregation. The slope of the chute should normally be 2:1 or steeper for stiff mixes.
Figure 5-7. Handling concrete by buggy
Figure 5-8. Handling concrete by power buggy
Figure 5-9. Runways for wheelbarrow or buggy use
Figure 5-10. Hopper and chute for handling concrete
5-38. Timely delivery of enough concrete on large projects requires careful planning and the selection of the right type of equipment for the purpose.
5-39. Ordinary dump trucks are not designed as concrete carriers although they are commonly used to deliver concrete on large projects in the theater of operations. Exercise care in using them because no means of preventing segregation is provided as it is on mixers or agitator trucks. Even if you use a stiff mix and an air-entraining agent to reduce segregation, keep hauling distances as short as possible, maintain slow speed, and utilize smooth roads to reduce vibration.
5-40. You can use either transit-mix or ready-mix trucks as agitator trucks to deliver premixed concrete. A transit-mix truck is a concrete truck (M919) where materials are mixed after they arrive on the site. A ready-mix truck comes from the batch plant to the site already mixed, ready for placement. The load capacity of a ready-mix trucks is 30 to 35 percent greater than transit-mix trucks, but the operating radius of the ready-mix trucks is somewhat more limited. The trucks discharge concrete either continuously or intermittently from a spout or chute that moves from side to side.
Buckets. Figure 5-11 shows a crew using a bucket and crane to place concrete. Buckets in 2 yard capacities are standard TOE and Class IV items in combat heavy engineer battalions. They are either square or cylindrical with a clamshell door or gate at the bottom. The doors or gates are hand operated for flexibility in discharging the bucket.
Pumps. When limited space prevent other more conventional means of delivery, use a heavy-duty piston pump to force concrete through 6-, 7-, or 8-inch pipeline as shown in Figure 5-12. Pumps operated by a 25 horsepower (HP) gasoline engine have a rated capacity of 15 to 20 cubic yards per hour. Larger equipment with a double-acting pump has a rated capacity of 50 to 60 cubic yards per hour. Both machines can pump a mix having 2 or more inches of slump and force their rated capacities up to 800 feet horizontally, 100 feet vertically, or any equivalent combination of these distances. However, a 90-degree bend in the pipeline decreases horizontal delivery distance by 40 feet, and each foot of vertical lift decreases horizontal delivery distance by 8 feet. When starting the pump, lubricate the pipeline with a light cement grout first. Then make sure the pump receives an uninterrupted flow of fresh, plastic, unsegregated concrete having medium consistency. Maximum aggregate size is 3 inches for the 8-inch pipeline, 2 1/2 inches for the 7-inch pipeline, and 2 inches for the 6-inch pipeline. The discharge line should be as straight as possible, with a 5-foot-radius bend. Take appropriate steps to cool the pipe in hot weather for smooth concrete flow. An interrupted flow in the pipeline can seriously delay the concrete pour, causing undesirable joints in the structure. A deflector or choke (restricted section) can be used at the discharge end of the pipeline to direct and control the discharge flow. Be sure to thoroughly flush both the pump and line with water after each use. One disadvantage of using pumps is that the concrete mix is usually more expensive due to the smaller aggregate required to pass through the pump, resulting in a correspondingly higher requirement for expensive cement.
Figure 5-11. Placing concrete using a bucket and crane
Figure 5-12. Piston pump and discharge pipeline
|David L. Heiserman, Editor||
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Revised: June 06, 2015