Under water concreting

Under water concretingSubmitted by:Objectives1.Introduction.2.Types of material used and its mix proportions.3.Concreting and Concreting Methods.4.Precautions required during under water concreting.5.Under water repair works.6.Conclusion.7.Reference.

INTRODUCTION There are often situations, such as basements, sewerage and marine works, in which concrete is to be placed underwater. In diaphragm wall construction, concrete is placed in a trench filled with bentonite slurry.The concrete should be plastic and cohesive but should have good flow ability. The proportions of fine and coarse aggregates should be adjusted to produce the desired workability with somewhat higher proportion of fine aggregates than used for normal conditions.

Performance requirements for UWC:

Workability & self compactionCohesion against washout, segregation & laitance formation Low bleedingLow heat of hydrationControlled set timeCompressive strength Adequate bond

TYPES OF MATERIAL USED AND ITS MIX PROPORTIONS.

CementCoarse aggregatesFine aggregatesAdmixture

CEMENTOrdinary Portland cement: OPC having not more than 10% tri calcium aluminates is suitable for underwater concrete construction where the sulphate content of ground water does not exceed 1200ppm and for marine structures which are permanently submerged.Sulphate resisting Portland cement: SRPC with its reduced tricalcium aluminates content should be used where the sulphate content of ground water exceed 1200ppm. Its use in marine structures in the splash zone and above is less straight forward.

Low heat Portland cement: Large pores of concrete cast underwater are particularly susceptible to thermal cracking as relatively high cement content concretes are used. LHPC not only reduces the rate of heat evolutions but also provides protection against sulphate attack owing to the low level of tricalcium aluminate in this cement. The use of cement replacement materials is an alternative method of reducing the thermal effects and provides additional benefits.

. Aggregates: As it is impossible to achieve detailed visual inspection during the placing of the underwater concrete , and it is usually necessary for the concrete to flow and self compact , it is important to select aggregate and grading which are particularly resistant to segregation and bleeding and which and have high cohesion. Coarse aggregates: It is well known that rounded aggregates achieve more dense packing and have reduced water demand for a given degree of workability than do crushed rock aggregate.Maximum Size - 1.5-2 inc (100% shall pass a 75 mm sieve )Fine aggregates :

The only special requirement for sand fraction over and above those needed for normal concreting mixes is that there should be significant proportions with a particle size less than 300 At least 15 20% of sand fractions should pass a 300 m sieve as this necessary to enhance the cohesive property of concrete to be placed underwater. When suitable sand are unavailable it is necessary to increase significantly the cement content of mixes , or add pulverized fuel ash or ground granulated blast furnace sag .

The sand for the intruded grout shall be well graded, preferably of round grains and shall conform to the following gradation: Passing 1.18 mm sieve 95 - 100% Passing 600 um sieve 60 - 85% Passing 300 um sieve 20 - 45% Passing 150 um sieve 15 - 30% Passing 75 um sieve 0 - 10%

ADMIXTURES: Admixtures are the chemical compounds in concrete other than hydraulic cement, water and aggregates, and mineral additives that are added to the concrete mix immediately before or during mixing to modify one or more of the specific properties of concrete in the fresh or hardened state.

The Anti- washout admixturesHydrocemMelloseRheomac uw-450

:Concreting methods :The following are the principal techniques which have been used for placing concrete underwater. Tremie method Bucket placing Placing in bags Pre packed concrete

Tremie Method : A tremie is watertight pipe, generally 250mm in diameter, having a funnel shaped hopper at its upper end and a loose plug at the bottom or discharge end. The valve at the discharge end is used to de-water the tremie and control the distribution of the concrete .placing it is built up in 1to 3.5m sections. Thetremie is supported on working platform above water level, and to facilitate the placing it is built up in 1 to 3.5 section.

During the concreting, air and water must be excluded from the tremie by keeping the pipe full of concrete all the time; and for this reason the capacity of the hopper should be at least equal to that of the tremie pipe. In charging the tremie a plug formed of paper is first inserted into the top of the pipe. As the hopper is filled the pressure of fresh concrete forces the plug down the pipe and the water in the tremie is displaced by concrete.

Bucket Placing :

This method has the advantage that concreting can be carried out at considerable depths. The buckets are usually fitted with drop-bottom or bottom-roller gates which open freely outward when tripped as shown in fig. The method permits the use of slightly stiffer concrete than does tremie method.

(a) FILLED(b) DISCHARGING

BOTTOM OPENING BUCKETPrecautions required during underwater concreting :

Dry ingredients should not be dumped into water, nor should the concrete be allowed to fall water from any height.No tamping, ramming or compaction of concrete should be done until concrete surfaces raises above water level.The concrete mix should be rich and have 10% to 30% extra cement content of the concrete should not be less than 380 -390 kg/m3 .The aggregates should properly graded to reduce voids to a minimum and produce a concrete mass of excellent plasticity and strength.Concrete should not be placed in very cold water, as this causes hardening problems.

Under water repair works:

Defining the AreaCleaning the Affected AreaEquipment HaulageUse AWAPersonnel requirementsInspection methodsEnvironment consideration

Conclusion: Concrete is often placed beneath water surface for the construction of offshore platforms , docks , harbors , bridge structures etc . . We should take serious precautionary measures and care while concreting beneath water level . Since w/c plays an important role in attainment of strength of concrete.There may be possibilities of segregations , bleeding etc .Thus concreting underwater should be carefully controlled and carried out.

References:

Bryant Mather , Effect of Marine Water on Concrete ,U. S. Army Engineer Waterways Experiment Station CORPS OF ENGINEERS Vicksburg, Misissippi, December 1964.Rui Miguel Ferreira ,Probability Based Durability Analysis Of Concrete Structure in Marine Environment, November 2004."Chemistry and the oceans." Special report, Chemical and Engineering News vol 42, No. 22 (1 June 1964), pp 1A-48A. American Concrete Institute, ACI Standards, 1063, Detroit, Mich..THANK YOU