Re: why are there gaps in road surfaces?

Gaps, or joints, are installed in concrete roads for two reasons:  
Contraction and Expansion.    

First:  Concrete is a mixture of four substances:  Crushed stone, sand, 
portland cement and water.  When these are mixed, the materials in the 
cement "hydrate" -- a chemical reaction begins, resulting in the growth of 
crystals between the aggregates.  This makes a material that can be easily 
transported and poured into virtually any shape, which then becomes 
extremely strong.  

Concrete does not dry, as such -- the water gets used up in the reaction. 
One of the results of this reaction is that the concrete shrinks a small 
amount as it sets.  If the concrete structure is big enough and the right 
shape -- long but comparatively thin, like a wall or a road -- the 
concrete will crack.  There is nothing the engineer or contractor can do 
to stop it.  What they can do is place grooves in the surface every twenty 
to forty feet, to encourage the cracks to occur neatly and evenly.  This 
is done either by using a special trowel over the surface of the road slab 
to make a groove in the still-liquid concrete, or by saw-cutting a notch 
in the slab after it has set but before it has a chance to shrink, or by 
placing strips of wood on the inside of the forms for a wall pour to 
create a notch.  It is intended that the shrinking concrete will crack at 
the groove.  The builder also places smooth metal bars called dowels 
horizontally inside the concrete under the groove.  These work as guides, 
keeping the cracked concrete surfaces at the same level.  Here a couple of 
links:
 http://www.ctre.iastate.edu/pcc/index.cfm http://www.fhwa.dot.gov/legsregs/directives/techadvs/504030b.htm

One of the things the engineer in charge of construction has to do, along 
with checking the kind of mix being placed, making sure the consistency is 
correct, that the concrete is being placed properly, consolidated right, 
and finished correctly, is to check that these bars have been positioned 
properly and that the shrinkage grooves are being placed above them. Been 
there, done that.

The other reason roads require gaps is expansion due to temperature 
changes.  If you've examined a liquid-filled thermometer, you have seen 
how the fluid expands when it gets warmer, and pushes up the slender part 
of the tube.  All materials do the same thing to some extent, including 
concrete roads.  As the weather gets hotter, the road slab actually 
moves.  

To take care of this, road builders construct "expansion joints" in the 
roads.  These are spaced farther apart than the shrinkage joints -- about 
1400 feet.  They consist of a gap in the concrete, which is filled with a 
somewhat soft material, such as foam or asphalt.  As the road gets hotter 
and expands, it moves into the gap.  The asphalt gives, allowing the road 
to move.  As the road cools, it shrinks, opening up the gap.  But the 
asphalt gets beaten down by traffic keeping the joint full.  Every so 
often, maintenance crews have to come out and either grind down the 
asphalt or put some more in, to keep the surface level.

Asphalt roads develop gaps also.  Usually, this is because the asphalt is 
laid directly on top of a concrete surface that has existing gaps in it.  
These are called "reflection cracks" because they reflect what's 
underneath.  Asphalt shrinks and expands too, but because it is less rigid 
than concrete, joints are usually not installed in it.

Road builders would just as soon do away with gaps in the roads.  These 
allow water to get under the road surface, which causes deterioration of 
the subgrade.  This results in potholes (caused when water freezes, 
expands, and cracks up the road from below) excessive movement, sinking of 
the surface, crumbling of the edges of the gap, and other nasty problems.  
Different types of steel reinforcement, different types of concrete mixes, 
including short fibers of glass or steel in the concrete, have been 
tried.  Complete success has still eluded us.