In the world there are various bridges which are so old sometimes more than 50 years and more but still catering traffic in full swing. As we know when a human get older his or her body needs some supporting system to strengthen them like knee cap, walker or cane, medication etc. in same way bridges also need some external supports or precaution that make it life longer and this topic is very important in this respect. Old joints are generally become Worn out and depressed by wearing action of continues repetitive wheels. This joint with open joint makes concrete surface attacked by horizontal thrust of wheel which may cause concrete edge to damage continuously. Also dust directly get entered in to gap of joint and clogged it and make bridge movement resist.
Following are the images show real consequences
|Concrete edge damaged|
|Dust entrapped in joint|
Now there is only rectification may be done to replace it with new joint. There are various type of expansion joints are available but mainly two types joint namely Strip seal expansion joint and Elastomeric slab seal expansion joint are widely used in bridge construction. In this blog my prime concern is to share my experience of “Replacement of worn out expansion joint of old bridges with Elastomeric slab seal expansion joint”. After making a large research we found out about a special type of concrete which is nothing but concrete prepared with grouting cement. In market there are various type of grouting cement are available but we go for Conbextra GP2 (T) which is free flow, high strength , non-shrink , cementitious precision Grout. It is mainly used as grouting element but can also be used for anchoring work like in our case to anchor steel inserts to old concrete bridge deck. Now in your mind this question may arise that What is Steel insert specially in case of new learner so that it is my responsibility to first elaborate all parts of an elastomeric expansion joint. Here it is-
Description of Elastomeric Slab Seal Expansion Joint
Elastomeric Slab Seal Expansion joint is type of Reinforced elastomeric joint.
It has primarily these parts-
i) Steel Inserts : The elastomeric slab units will be fixed to the steel inserts correctly anchored in the deck concrete. Setting of elastomeric slab units with anchoring bolts directly embedded in deck concrete shall not be allowed.
ii) Anchorage : The anchorage shall either be loop anchors linked to the inserts by anchor plate or sinusoidal anchor bars welded with the horizontal leg of the steel inserts. For loop anchors with anchor plate, the thickness of the anchor plate will not be less than 12 mm. Diameter of anchor loops will not be less than 16 mm and the spacing of anchors will not be more than 250 mm. For sinusoidal anchors, diameter of bar shall not be less than 12 mm.
iii) Fixing Bolts : Fixing bolts and nuts will be made of stainless steel. Tautened nuts shall be locked by using lock washers.
iv) Elastomeric Plugs : The plug holes provided in elastomeric slab units to house fixing bolts shall be plugged with elastomeric plugs pushed in position after smearing adhesive on the surfaces.
v) Adhesives and Sealants : Special sealant to be decanted into the plug holes before plugging and special adhesive to be used for installation, shall be as per the recommendation of manufacturer.
Impact of Thickness
Thickness of the slab is most important factor over choice of type of expansion joint to use. Strip seal expansion joint is more durable, easy to install and economical also as compared to elastomeric but in our case we can’t opt for previous joint because of that slab thickness. As strip seal joint needs minimum of 200-230 mm slab thickness and in our case it was only 180 mm so that we go for elastomeric expansion joint which is much more costlier that strip seal.
|Thickness of steel insert|
1. Remove the old expansion joint by cutting from gas cutter or any other suitable method . After that we have to dismantle the joint with help of jack hammer (Concrete breaker) of 16kg as bridge deck is old so that higher weight of hammer may be risky.
|Dismantled by Jack hammer|
2. It should be dismantled upto the depth when the main reinforcement of slab is exposed.
3. Existing reinforcement of deck will be free from any foreign material. If possible can be painted with anticorrosive agent.
4.Place steel insert (with loop stud or anchors in our case) with proper with proper alignment and level.
5.Now welding of anchor stud is to be made with link bar (U-bend or S-bend) to main reinforcement. Extra reinforcement of same grade and specification may be used if needed in case any.
6.Place thermocol between gap of steel insert. Generally the grade of concrete used for concreting will be always same as grade of concrete as mother structure or higher.
7.Clean the surface of old concrete with air blower and apply Epoxy bonding agent (in our case Choksey-Master bond EP) to it just before concreting. To make concrete with GP2 , mix binder cement 60% with 40% aggregates (10 mm down aggregates for placing thickness more than 100 mm ). This proportion not fixed it may be designed using trial and error method. Maximum water cement ratio is 0.18 that is maximum 4.5 ltr water per 25 kg bag. Concrete mixer will be used for 3 mins mixing and pour it within 20 mins otherwise it will set.(For Grout cement concrete I will write a different blog in elaborate nature next time)
8.This concrete is early high strength concrete. Getting 55-65 mpa results only in 3 days with proper curing.
9.After getting full strength it will ready to receive elastomeric slab over it.
After fixing slab on it with bolt , plug holes should be sealed with sealant like (like master bond) and plugs will be pressed on it to get set.
10. Now it is ready for catering traffic.
I hope you will be happy to see this post if you think that I was wrong in any topic then drop me a mail I will incorporate that as requirement.
- Why replacement of bridge joint is needed?
- What is Elastomeric Slab Seal Expansion joint?
- Why slab seal not strip seal?
- What is conbextra GP2?
- Why not normal concrete shall be used?
- How much time for GP2 is to be cured?