You have seven choices when determining how you will transport your concrete. Whether you choose to use a wheelbarrow, truck mixer, bucket, chute, conveyor, concrete pump, or a pneumatic placer will depend on a number of criteria which are described below. Figure 7 demonstrates some key tips that must be followed when handling concrete.
Wheelbarrow or motorized buggy
They are typically utilized on small jobs where the travelling distance is short and the terrain is good and level. You should discharge the whole batch being mixed and minimize any jolting or vibration during transport in order to avoid segregation of the mix. If the weather is hot and the distance is lengthy, it is very important that you provide some moist curing. When a wheelbarrow or motorized buggy is used for the first time, ten percent of cement and sand should be added to the first batch since the first batch of concrete mixed is left within the mixer and the mode of transportation.
Truck mixer
Truck mixers, as shown in Figure 5, are essentially free fall mixers mounted on a truck chassis. They typically handle either thoroughly mixed concrete or a batch of dry materials where water is added when the truck arrives on site. The size of each mixer ranges from 2.5 CY to 12 CY.
Truck mixers either contain centrally mixed concrete (mixed completely in a stationary mixer and then transferred to another piece of equipment for delivery) or will mix the contents during transport otherwise known as truck mixing. Central mixing is less restrictive than truck mixing and the goal is to obtain a "preblending or ribboning effect as the stream of materials flow into the mixer" (Beavers, 304R-11). Central mixing is often preferred since a greater load can typically be carried. The general rule of thumb in mixing is one minute for the first cubic yard and one-fourth of a minute for each additional cubic yard. In order to increase the production of a centrally mixed plant, most plants will do "shrink mixing." This involves mixing concrete to a point "where the plant slump meter indicates that the desired slump is predictable and then finish the mix on the way to the job site" (Dobrowolski, 18.8).
Truck mixing has less folding action when compared to a plant mixer. The total volume that a truck can handle is limited to 63 percent of the drum volume. It is recommended that "ten percent of the coarse aggregate and water [be placed] in the mixer drum before the sand and cement" is added in order to avoid packing of the materials at the head of the drum (Beavers, 304R-12). The general cycle time is 70 to 100 revolutions, however, if the materials have been adequately charged into the mixer, uniform concrete should be obtained within 30 to 40 revolutions.
The travel distance of a truck mixer can range from six to fifteen miles. This range can be extended by "dry batching." In this situation, the dry materials are added to the mixer and are delivered to the construction site. Water is added at the construction site and "must be added under pressure, preferably at the front and rear of the drum with it revolving at mixing speed, and then the mixing is completed within the usual 70 to 100 revolutions" (Beavers, 304R-12).
When these two type of operations are compared to one another, centrally mixed concrete is the most preferred. Some advantages are that the haul cycle time is shortened, wet mixes charge twice as fast as a dry charge, and there is no time loss in adjusting slump on the job or in the yard (Dobrowolski, 18.9).
If proper site access is provided for the truck mixer, the concrete can be discharged alongside the designated pour. Chutes are typically used during discharge. The time that is estimated for a full truck mixer to discharge all of its contents takes approximately five minutes.
Truck mixers can also discharge their contents into a concrete pump as shown in Figure 8. Keep in mind that the concrete mix must be adjusted for this type of transportation.
Bucket or steel skip
A bucket, as shown in Figure 9, is square or circular and needs the aid of a crane, cableway, or hoist so that it can be moved horizontally or vertically. This type of transportation has the capability to move large quantities of concrete (up to eight cubic yards at a time).
A steel skip is used with smaller mixers and holds up to 1 CY of concrete. There are two types, a wedge-shaped skip with a bottom opening and a lay back skip. They also need the aid of a crane, cableway, or hoist in order to be transported.
Chute
A chute should only be used in short lengths since concrete becomes more susceptible to segregation and drying out. When they are used, therefore, it is very important that the concrete mix is workable and cohesive and should be re-mixed at the lower end by passing down through a funnel-shaped pipe or drop chute.
Belt conveyor
A belt conveyor is used for continuous operations and it is important that the concrete be designed for this condition. It is recommended that low slump concrete be used for slower moving belts and higher slump concrete by faster moving belts. Although this type of transport can move approximately 100 - 350 cubic yards per hour, extra care must be given since the concrete mix is spread very thinly on the conveyor and is susceptible to loss of moisture during hot weather conditions.
Another concern is segregation. One way to minimize segregation (shown in Figure 7) is by discharging the concrete into a down pipe which feeds it on to the previously placed wet concrete. There are two types of discharges that essentially serve the same purpose. The first type provides end discharge from a special conveyor unit at the end of a line of conveyors; the second type involves a side discharge from a section of belt conveyor.
Belt conveyors are most commonly used in horizontal directions, but can be moved in the vertical direction (i.e. up to 35o) with some adjustments to the conveyors.
Concrete pump
The concrete pump, as shown in Figure 8, is a popular choice when transporting concrete because it is fast and often the most economical choice when challenged with a congested construction site. In fact, approximately "one-fourth of the concrete on building sites is moved through pipelines" (Young, 273). They can provide concrete in one continuous rate and are often utilized on large foundations. Approximately 100 cubic yards of concrete can be moved at one time in both the horizontal (approximately 1500 feet) and vertical (approximately 500 feet) direction.
Older concrete pumps use mechanical components to displace concrete, but today, most concrete pumps utilize a hydraulic system to displace the concrete. Hydraulic systems usually decrease the chance of blockages. The piping system can be of steel pipes or neoprene rubber pipes. Neoprene pipes will deteriorate quickly if not properly cleaned and maintained.
Two choices of pumps exist. The first are static pumps that are generally preferred when a large volume of concrete needs to be placed at a very long distance and there is limited flexibility at the discharge end. The second type, as shown in Figure 10, are mobile pumps. Since placement is assisted with articulated booms, concrete can be placed from one location to another between sites or on a given site.
Concrete that is chosen to be pumped must be workable. The ideal concrete should have a slump value ranging from three to four inches. There should also be a 3 to 5 percent increase in sand content and the aggregates should be round. Crushed aggregates can be used, but a higher sand content will need to be added in order to overcome the roughness of the aggregate. "A good check on the pumpability of a concrete mix is the determination of the total voids in the combined aggregates, and the aim should be to ensure that these voids constitute less than 25 percent of the total volume" (Murdock, 168).
In the event that blockages occur, the first thing that can be done is to reverse the pumping mechanism for a short time. If this does not clear the blockage, it must be physically identified. The quickest way to find the blockage is to ease the pipe joint couplings in order to see if there is any seapage. If none exists, move down to the next coupling. If there is no seapage then you have identified your blockage and can replace it with a clean section.
Pneumatic placer
Concrete can be transported through steel pipelines and compressed air by a pneumatic placer, as shown in Figure 11. This placer basically provides a separate batch of concrete that is shot at a considerable velocity. This is the key difference between pneumatic placement and concrete pump placement. This type of transportation is most common in tunnel work. The greatest capacity that they can provide is 1.5 CY of concrete.
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