QUESTION 4 20 marks i For large scale offshore concrete stru

QUESTION 4 (20 marks) (i) For large scale off-shore concrete structural components, cast on-shore, concrete needs to be pumped over a horizontal distance of 100m. The characteristic strength at 7 days should be 50MPa. Describe the special requirements of the concrete mix you willuse for such a situatio (10 marks) (i) The columns in a fire damaged reinforced concrete building are to be repaired using sprayed concrete. Describe a suitable method, giving details of the mix. (10 marks)

Solution

The following are the special requirements of concrete mix-

1)Since the concrete needs to be pumped for as long as 100m,light weight aggregate will be made used which are chemically stable.

2)Air entraining agent for easy workability(Transportation) can also be used so that the concrete can be pumped effeciently.

3)Since the structure is off-shore ,w/c ratio is kept minimum as possible.(Probably 0.38)

4)Admixtures for decreasing the initial setting time of concrete,Sulphate resistance,Rapid hardening to be used in such mixes.

5) For pump able concrete or congested reinforcement the coarse aggregate proportion may be reduced up to 10%.

Method of spraying concrete-:

There are 2 main methods of spraying concrete -

1)Wet method of spraying

2)Dry method spraying

Wet method of spraying concrete will be discussed in detail as follows-

The wet process makes use of the mixture containing cement, aggregates, and weighed volume of water for the whole mix. The whole mixture is made before the pumping process is carried out.

To bring the whole mix into position, high-velocity air is employed. This high-pressure air that is introduced into the nozzle plus the resultant velocity will spray the concrete to the area where it must be applied. The mix sprayed will be placed in a compacted state due to the momentum it gains through spraying.

The supply of wet mixed sprayed concrete can be either ready mix or carried out through site batching. These can be also supplied in dry condition (pre-blended materials) in the bags. For low volumes, these bags work best. Also in areas, large wet concrete volume delivery is restricted, this method is a good choice.

The water/cement ratio can be easily controlled with the help of wet process. A water/cement ratio of 0.45 can be easily attained using reducing plasticizers. The wet process gains higher strength characteristics due to the higher cement content.

Mainly the cement content used range from 350 Kg to 450 Kg per cubic meters. The cube strength that is finally obtained is 30N/mm2 and 60N/mm2 at 28 days.

The working hours of the mix can be extended up to 72 hours, with the help of hydration control admixtures. This provides greater flexibility. The cleaning of the whole system after each application can be prevented by this method.

The mix should attain good quality to facilitate better strength and durability. A proper balance between the fine and the coarse aggregate must be maintained to have optimum performance. This check will also help in the reduction of the rebound effect. All the above-mentioned requirements must be attained by the specialist contractor in charge.