TRANSPORTATION OF CONCRETE
The concrete in the mixer should be transported the concrete to the point where it should be placed as quickly as possible using a method that avoids segregation or loss of ingredients. The concrete must be placed before the set begins.
Attempts have been made to limit the time-lapse between mixing and compaction within the forms. The specifications, however, allow a maximum of two hours between the introduction of the mixing water to the cement and the aggregates, and the discharge, if the concrete is transported by truck mixer or mixer.
In the absence of a stirrer, this figure is reduced to a single hour. All of these, however, assume that the temperature of the concrete, when deposited, is not less than 5 ° C and not more than 32 ° C.
It has now been established that delays in concrete placement after the so-called initial set has taken place, are not detrimental and can increase compressive strengths, provided the concrete retains adequate workability to allow full compaction.
The requirements to be met during transportation of concrete are:
1. No segregation or separation of materials in concrete.
2. Concrete delivered to the point of placement must be uniform and of adequate consistency.
Segregation prevention is the most important consideration in the handling and transportation of concrete. Segregation must be prevented and not corrected after its occurrence.
Concrete is an inhomogeneous compound of materials of very different particle sizes and specific gravities, it is subject to internal and external forces during transportation and placement that tend to separate the constituents of the different material.
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Segregation can be avoided by ensuring that the direction of fall during pouring or falling of concrete is vertical.
When the discharge is at an angle, the larger aggregate is thrown to the opposite side of the container being loaded and the mortar is collected on the near side, resulting in segregation.
The plant required to transport the concrete varies according to the size of the work and the level at which the concrete is to be placed.
The main methods of transporting concrete from the mixer are as follows:
1. Barrows
(a) Wheelbarrows and handcarts
(b) Motorized trucks or motorized buggies or dump trucks
2. Dump trucks and trucks
3. Concrete mixer trucks and agitator trucks
4. Trash cans
5. The system of monorail or trolley or rails
The most commonly used method of conveying concrete using hand-to-hand trays is time-consuming, wasteful, and expensive. If the concrete is to be placed at or below the level of the mixer, steel carts are a better means of transportation.
The concrete can be unloaded from the truck to the required point. When concrete is to be placed well below general grade, such as basement slabs, foundations, etc., a wood or steel gutter can be used to drive the concrete into place. The trucks shown in Figs. 11.4
(a) and
(b) They are suitable for small jobs and where the duration of transport is short, and on muddy terrain. The average quantity that can be transported on a forklift is about 35 liters (80 kg).
Sometimes For relatively larger jobs, electric forklifts are used which are motorized versions of forklifts. Ordinary open steel body dump trucks and dump trucks are shown in Figs. 11.4
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(c) and 11.5
(a) can be used economically for tours of up to about 5 km. These trucks are suitable for dry mix only to avoid segregation and consolidation difficulties.
Travel time should be as short as possible. It is essential that the truck body is watertight to avoid the loss of fines. The concrete should be covered with tarps to prevent the concrete from being exposed to the sun, wind, and rain.
If the path is long, agitators should be used to avoid segregation. Steel buckets transported by rail or road can be used to transport
concrete for long distances and for large works such as dams, bridges, etc.
When using this method, it is necessary to see that
(I) the whole batch of the mixer is placed in the bucket, and
(ii) segregation is avoided while the bucket is being filled.
For the transportation of ready-mix concrete, mixer trucks and agitator trucks are used as shown in Fig. 11.5
(b). The monorail system is useful when ground conditions are not suitable for normal road traffic. In the monorail system, the rail can be easily provided at such a level that the concrete is poured directly into the formwork.
Basically, the system consists of a motorized trolley mounted on a single rail capable of reaching a travel speed of 90 m / min. The engine can be diesel or gasoline, without a driver.
Conveyors have also been used to transport fairly rigid concrete, but there is a tendency for segregation on steep slopes and at transfer points. When using conveyors, the concrete flow needs to be continuous to minimize the effects of segregation.
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On sites where concrete must be lifted up to 5 m, sloped walkways with one or two landings can be built to bring the concrete to the required level.
Another method of lifting concrete to greater heights is by using some type of forklift. The various types of hoists are chain hoists, platform hoists, or container hoists.
In chain hoisting, a chain sling is suspended from a pulley and is operated by an electric winch at ground level. The sling is attached to the container, which is then raised to a working level.
Trolleys and trolleys can be lifted by platform hoists operating on vertical steel guides. With some hoists, two platforms are provided, one lowers while the other raises.
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The main types of hoists used in tall buildings are tipping hoists, automatic dump hoists, double automatic hoists, and passenger/concrete hoists.
Tipper elevators, normally powered by direct discharge from the mixer, are raised and unloaded into a receiving hopper at the working level; from that point, forklifts or other means of transport can deliver the concrete to the forms.
On a modified dump hopper model, lower doors are provided, which open automatically to the required level, allowing concrete to be discharged through a drop chute into the floor hopper.
In tall buildings where the time required for moving containers and unloading concrete is long, containers with individual winch units are provided. When one container travels, the other fills, allowing for greater efficiency of the entire operation.
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In certain tall buildings where a passenger or passenger/freight elevator is essential due to the mechanical and other permanent services involved, a passenger elevator can be used in combination with concrete loading and unloading capabilities.
When the hoist is not used for the transport of concrete, the floor of the cage is perfectly free for the normal tasks of the passengers.
In an arrangement consisting of a raised tower and hoppers, the concrete is raised in buckets to the central tower and distributed through inclined hoppers from the top of the central tower.
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This system is suitable for large dam work. There is a tendency to segregate the dryer mixture, it may be necessary to install vibrators in the ducts.
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