[C M Dordi]

Concrete Transportation and Placement

INTRODUCTION

After the concrete or mortar materials have been batched and mixed, the concrete I mortar mix has to be transported and placed in a formwork.
Concrete transportation has to be done from the mixing point to the placing point in the shortest possible time so that it does not lose its workability. Care should also be taken that the mix does not segregate or bleed but remains a cohesive mass.

Stiffening, bleeding and segregation of concrete while transporting can result in loss of strength, non-uniformity, porosity and poor durability. It is very important that uniformity of concrete is maintained after mixing. In most of the projects, in India, concrete and mortar are transported by a human
chain by head loads. This method of transportation is slow and prone to errors.


Delay in transportation can cause loss of workability and cohesiveness and thereby difficulty in placing and compacting the concrete. Cohesiveness and workability of the concrete mix should be adequate enough to cater to the mode or method and duration of transportation and placement.
Transportation and placement specification should be reviewed and necessary precautions for concreting at night or during periods of extreme heat, extreme cold, wind or rain must be taken. Concrete is required to be transported in both horizontal and vertical directions. Independent of method deployed whether it is using head loads or highly mechanised equipment it is very important that advance planning be done thereby preventing
any problems which could affect the quality of work.


MODE OF TRANSPORTATION
a) Fully Manual
b) Semi-manual and semi-mechanised
c) Fully mechanised


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[Manish Jain]

CHECKLIST FOR CONCRETE TRANSPORTATION

The Site Engineer has to keep a watch on several points during transportation so that the concrete workability and its uniformity remains unaffected. The checks are
follows:

If the concrete is transported by transit mixers it is necessary to maintain revolutions within the prescribed limits (70 to 100 revolutions) at mixing speed. Any additional revolutions should be at agitating speed only. This is necessary to avoid segregation due to overmixing.

The water tank on the transit mixer is at times misused by the mixer operator who often add water in the mixer without consulting the Engineer at site. It is therefore advisable to keep these water tanks empty and addition of water and other ingredients preferably be done at the batching mixing plant only. 

Some specifications limit the time within which concrete must be placed. It is felt that time limit is unnecessary as long as concrete can be properly placed , fully compacted and adequately finished without any need for adding more water. Addition of water to cater to drop in workability due to delay in  transportation is prohibited. Any addition in the concrete should be done after consulting the Engineer in charge and keeping the water to cement ratio of the mix unchanged. 

During hot weather and / or strong winds concrete must be covered if transported in open containers such as trucks, dumpers, wheel barrows or gamelas. This is necessary if the duration of the transportation is considerably long.

Similarly the concrete during the monsoon period should be protected from water contamination if transported in the modes described above.

The concrete containers get coated with mortar during loading, transporting and unloading process. The coating must be removed and hosed off with water jet at the earliest possible break, to prevent further mortar caking up above the dried mortar on the container surfaces. This is special ly observed at the sharp corners.

Contamination of concrete by oil , dirt or wind blown dust must be prevented. 

Container should be leakproof to prevent loss of mortar or cement slurry during transportation . Baskets to carry concrete by head loads should not be permitted.

Concrete transported through long distances may segregate due to vibrations, incorrect use of chutes, placers, pipelines or bumps due to rough road surfaces etc. The mix if found segregated should be remixed before placing.

If the concrete mix is not transported quick enough or due to any other problem the mix placement is delayed, there is a drop in workability and mix stiffens. If such delays are anticipated then the mix should be designed with a higher workability or using workability retention admixtures. In a massive pour if the gap between the subsequent layers is planned at 30 to 45 minutes, the concrete mix should be designed having higher workability and using proper concrete additives, in such a manner that even upto 1 hour 30 mts. the concrete in the earlier (lower) layer can be easily compacted.

for more details visit or download

 concrete transportation and placement fd.pdf