Concrete transport: a new method proposed10 August 1998
Belt-conveyor distributors are commonly used for concrete transport in dam construction. Shimizu proposes using a rising tower, which demonstrated large capacity and simple operation at Japan’s Mizukami dam
The rising tower method of transporting concrete was developed by Tokyo-based Shimizu Corporation. It was trialled at the Mizukami dam in Nagano, Japan, in 1996 when it showed satisfactory performance with no deterioration in concrete quality. It was later used in 1997 during the construction phase of the dam.
Rising tower method
In the new method concrete is carried to the foot of the rising tower in a transfer car loaded into a bucket. It is lifted to the top of the tower and the concrete is then transferred from the bucket to a belt feeder through a hopper.
The belt feeder transfers concrete horizontally to the body of the dam, and finally releases concrete to the dump truck waiting at the end of the feeder. The dump truck carries the concrete to the site of placement, running on the body of the dam.
The belt feeder carries the concrete horizontally, at the level of the top of the dam. It needs to be lifted upwards as the body of the dam rises. The feeder is therefore placed in a frame named the climbing stage, which rises along the tower. The stage climbs by 1.5m stages.
The height of the tip of the feeder can be set freely because the feeder can angle either upward or downward within the range -5° to +5°.
The concrete transport capacity is determined by the elevation cycle time of the concrete buckets.
The rising tower method could be used at the Three Gorges construction project in China. The present construction plan at this site is based on the use of six belt-conveyor type distributors, with a loading belt conveyor used to supply concrete to distributors from the ground.
In using the rising tower method, different approaches would be required at different elevations:
•At low to medium elevations, concrete mixed at a concrete plant is carried to the rising tower by 20t dump trucks, then lifted horizontally to the required height. The concrete is then transported to the distributor by a connecting conveyor.
•At higher elevations, concrete is transported by the rising tower to the elevation where it is to be placed. It is then transferred to a concrete bucket, and a portal crane is used during concrete placement.
Does the rising tower method offer sufficient transport capacity? The nominal capacity of belt-conveyor distributors is listed in Table 1. It can be seen that the transport capacity varies with the maximum aggregate size — capacity is greater when the maximum size is 80mm. Under the present plan, if the point where the loading belt receives concrete is set at a fixed point 500m from the distributor, the loading belt angle becomes greater as the elevation of the concrete placement increases, and transportation capacity is decreased (see Figure 1).
The concrete trans-port capacity of the rising tower method is shown in Figure 2.
The capacity to carry concrete to a point at elevation 152.50 was calculated based on the performance of portal cranes generally available in Japan. The difference in transportation capacity is attributable to the difference in cycle time of concrete bucket hoisting.
A comparison of the transportation capacities of the two methods is compared in Table 2 and is also illustrated in Figure 3:
•At lower to medium elevations, the volume of concrete to be transported for placement during the dam construction at the Three Gorges project is estimated at 200-300m3/h at normal time. The maximum volume will be 400m3/h. However, the present plan cannot carry sufficient volume, with the assumptions used above.
In the case of concrete with mix-proportion 4 the amount trans-ported is small. The new plan could transport the necessary concrete volume regardless of the mix proportion.
•At higher elevations, the new plan, which can retain the time required for hoisting by the portal crane at a low level, provides around twice the transport capacity of the present plan.
This case study is based on a number of assumptions, but does not dismiss the present plan. The authors believe the present plan could work more efficiently with modifications, and indeed hear that the problem of decreasing concrete capacity due to the inclination of the conveyor belt has been solved.
The authors offer the rising tower plan as an option for future dam construction in China.
|Specifications of the rising tower|
|•Tower: Four masts of square cross-section 1900x1900mm, length 6000mm •Elevating bucket: 2xcapacity 9m3 •Winch: Maximum hoisting speed 120m/min Maximum lowering speed 120m/min Motors 4x400kW controlled by thyristor Leonard method •Concrete hopper: Two of capacity 9m3 •Belt feeder: Length 13m Belt width 120mm Belt speed 50m/min Conveyor motors 2x55kW Capacity (concrete)800m3/h Rising or falling angle -5° to 5° Motor for angling 2x15kW •Climbing equipment 4xjack-up cylinder 8xlock cylinder|