GROUTING WORK OF PSC GIRDER BRIDGES

 Grouting work of PSC Girder Bridges 

1.0 General

The purpose of Grouting is to provide permanent protection to the post-tensioned tendon against corrosion and to develop a bond between the pre stressing Strand and the concrete. Grout also fills up void spaces, expelling the water collection therein.This is one of the major activity in Major Bridge Construction.

2.0 Material Required for Grouting

a)  Water:  Only clean water to be used.

b)  Cement: OPC (Ordinary Portland Cement), Grade 43 approved by the Engineer to be used.

c) Admixture: The admixture to be used shall be Alumina free and shall have the requirements as given in specification.

 

3.0 Equipment Required for Grouting

                           BBR Grout Mixer (MMJ-100) and colloidal mixer 

BBR Grout mixer, which consists of twin tanks, carries out the grouting operation. One tank is used for mixing while the other tank serves as an agitator. The top tank is connected to a colloidal mixer. The Grout flow is distributed to the bottom tank and on to the Swibo mixer through a three-way valve. Cement and water is mixed in the top tank in the water cement ration 0.38 to 0.42. This mixture is passed through the Swibo mixer and the top tank. This cycle is repeated for two minutes. After this, the grout is passed on to the agitator tank in the bottom. From this tank this grout mix is pumped to the tendon duct. A by-pass valve is fitted between the grout mixer and the tendon inlet. The following are the specifications of the grout mixer.

 

4.0 Grouting Material and Admixture

a) The cement to be used for grouting is OPC (Grade 43) obtained from manufacturers as approved by the Engineer.

b)  The admixture shall be free of Alumina. It shall be a plasticizer and a retarding agent having the following requirements.

i.                   Flowability at a given water / cement ratio.

ii.                 Reduction of bleeding water.

iii.              Prevention of segregation in high pressure grouting.

iv.              Retarding in the setting of grout.

v.                 Expansion of grout of compensate for shrinkage.

Conbex 100, a product to be used after approval by the Engineer.

          i.        Capacity of mixing                  =       100 L each time.

          ii.       Delivery of Pump                    =       approximately 1400 L.

          iii.      Maximum injection Pressure   =       5 Kg / Sq CM.

c)                Air Compressor

Water used for cleaning the tendons are flushed by Compressed Air. An air compressor is also kept ready to flush out grout from the tendon as an emergency measure.

The capacity of the compressor is 300 CFM.

5. Pre Grouting Operations

a)  Grouting is carried out for the tendon after stressing. This is done as soon as possible but not later than two weeks after completion of stressing of the tendon. Whenever this stipulation cannot be maintained, adequate protection measures against stress corrosion are to be taken for the protection of the tendon by injecting water-soluble oil.

b)  The protruding Strands are cut with a disc cutter not closer than 25 mm from the face of the Anchor Head.

c)  A Grout Cap with air vents is fixed to the anchor head by fasteners.

d) Check all grout vents, Inlet and outlet pipes.

e)  The tendon duct is washed well with water and then this water is flushed by compressed air.

f)  Before grouting the following tests are conducted on grout mix.

i) Flowability check: This is done using a Flow Cone Apparatus. The time taken for the flow of pre-measured quantity of grout is noted which should be in the range of 13 to 18 seconds.

ii) Shrinkage and Bleeding test: Grout mix is poured into a transparent beaker. Shrinkage and bleeding are measured and recorded.

iii) Compressive Strength: Grout cubes of size 100 mm x 100 mm x 100 mm are taken for different W/C ratios and the compressive strength is tested after 7 days.

The test results of the grout shall have the following specifications.

Compressive Strength  = not less than 17 Mpa at 7 days.

Shrinkage                       = Max,-3%

Bleeding at 3 hour                   = Max, 2%

6.0 Grouting Operations

a)  Grouting operation to be started after completion of Stressing and clearance by the Engineer. Once all tendons are prepared for grouting, the span is ready for grouting. Before grouting, water will be removed and duct will be flushed with compressed air.

b) Water, cement and admixture required for the span are kept ready within the accessible distance.

c) The grout mixer is started and checked. The cement and water is mixed in the top tank of mixer in the water / cement ratio of 0.38 to 0.42. This mix is passed on to the bottom tank 

d) Another fresh grout is mixed in the top tank and once this is ready, the grouting of tendons is started.

e) The temperatures of grout is measured, which shall not exceed 25 deg centigrade and not lower than 10 deg centigrade.

f) Grout is injected into the tendon in a continuous manner without any interruption.

g) Grouting is started with a low pressure. As the grout fills up and flows forward, the pressure increases. As the grout flows forward, it fills up and comes out through the grout vent opening. Once the grout of good consistency comes out, the vent openings are closed.

h)The grout comes out through the outlet of the tendon. The consistency of the grout is compared with that of grout at the injection end.

i) At this stage, pumping is stopped. The outlet is closed and the pumping is resumed. The pressure increases and is allowed to buildup up to 5 kg / Sq. Cm. This pressure is require to maintain about 1 min.

j) After this the ball valve is closed and grout pipe is shifted to another tendon.