Dear Dr...In the method of stitching, it is a normal practice to construct the widening part of the

bridge at first and let it stay undisturbed for several months. After that, concreting will then

be carried out for the stitch between the existing deck and the new deck. In this way, the

dead load of the widened part of bridge is supported by itself and loads arising from the

newly constructed deck will not be transferred to the existing deck which is not designed to

take up these extra loads.

One of the main concerns is the effect of stress induced by shrinkage of newly widened

part of the bridge on the existing bridge. To address this problem, the widened part of the

bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing

bridge so that shrinkage of the new bridge will take place within this period and the effect

of shrinkage stress exerted on the new bridge is minimized.

Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches.

To solve this problem, rapid hardening cement is used for the stitching concrete so as to

shorten the time of setting of concrete. Moreover, the stitching work is designed to be

carried out at nights of least traffic (Saturday night) and the existing bridge may even be

closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed.

Sometimes, longitudinal joints are used in connecting new bridge segments to existing

bridges. The main problem associated with this design is the safety concern of vehicles.

The change of frictional coefficients of bridge deck and longitudinal joints when vehicles

change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of

longitudinal joints in bridges is quite difficult.

Ok Dr Amjad

This is somewhat dependent on the relative ages of the sections to be joined. If you consider most of the creep and shrinkage has already occurred, then use a low shrinkage concrete mix for the stitch pour.

Thanks Eric Ancich 

The use of high early strength concrete is likely to make the shrinkage problems worse.  The stitch pour has to be designed as highly restrained and, therefore, be much more heavily reinforced.  The problem is analogous to pouring a deck slab over wide, closely spaced concrete slab decks.  The shrinkage of the new concrete is concentrated at the joints, and extra reinforcement is required  If you are subjecting the structure to de-icing salts, you should consider either Stainless Steel reinforcement of FRP reinforcement.  If possible, the deck should be sealed - see UK Highways Specifications.

I concur with Gordon Chirgwin.