why the Expansion joint top level be bit below the finished road level over the bridges?

Expansion joints in bridges are usually constructed using either armor & strip seals or preformed joint seals (inset picture, below). The sealant is typically set below the pavement surface by 1/4" or so to keep snow plows & vehicle wheels from coming into contact with them and tearing them out. You don't want to set the joint filler too low since it will collect debris that will eventually get too thick to self-cleans itself during rainfall. Hope this helps answer the question: Cheers!!!

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Expansion joints in bridges are usually constructed using either armor & strip seals or preformed joint seals (inset picture, below). The sealant is typically set below the pavement surface by 1/4" or so to keep snow plows & vehicle wheels from coming into contact with them and tearing them out. You don't want to set the joint filler too low since it will collect debris that will eventually get too thick to self-cleans itself during rainfall. Hope this helps answer the question: Cheers!!!

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thank you very much. but one more thing i need to clarify.
if the joint width is very narrow then we can protect the joint top from the impact of vehicle wheels by recess the joint top below the pavement surface level. in other hand. in case, if the joint width is very high i hope the impact will be very high from vehicle wheel load
 
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thank you very much. but one more thing i need to clarify.
if the joint width is very narrow then we can protect the joint top from the impact of vehicle wheels by recess the joint top below the pavement surface level. in other hand. in case, if the joint width is very high i hope the impact will be very high from vehicle wheel load

PANDIYARAJAN jAYARAMAN I certainly agree with you. Unfortunately, the gap size is typically determined by the pavement and/or structural elements on either side of it and what type of expansion/contraction properties they have, so we don't always have the ability to make the joint smaller. I have seen pavement expansion joints sized anywhere from 2" to 4", and even at 4", vehicles can travel over the joint without causing damage to the joint or seal below it. Once you get a gap larger than that, say on a large-span bridge, we start seeing the need to use structural expansion-type solutions, like finger joints or modular joints because of the higher amount of movement. Cheers!!
 
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