Re: Why are joints placed in sidewalks and not in most asphault surfaces?

Joints are placed in sidewalks and most other large slabs of concrete to 
provide for the expansion and contraction of the concrete.  Concrete is a 
brittle material; chemically it is a type of ceramic, and ceramics like 
glass or tile, do not stretch much before they break.  Thus, if you don't 
put expansion joints in concrete at regular intervals, the concrete will 
break and form cracks wherever it needs to.  We don't think about it, but 
each expansion joint is simply a crack in the concrete.

When crude oil is refined, it is heated to high temperature, and most of 
the hydrocarbons are vaporized and turned into gasoline, motor oil, etc.  A 
fraction of the crude oil consists of very large molecules, typically 
compounds with multiple ring structures.  These compounds are black, 
tar-like materials which appear solid at room temperature, but soften as 
they are heated.  This material is typically called asphalt.

Sidewalks and streets are often covered with a layer of asphalt mixed with 
small stones which we typically think of as "asphalt."  Since the 
hydrocarbon is much more expensive than the stone, the added stone reduces 
the cost of the mixture, plus it helps to give structure and strength to 
the mixture, with the asphalt binding everything together and making the 
surface water repellant.

When a new street or sidewalk is paved, you might notice on a hot summer 
day that your kick stand from your bike will sink or make an impression 
into the asphalt paving.  Although the asphalt appears pretty solid, it 
still retains some flexibility, particularly if it can move slowly.  So, 
new asphalt can more easily expand and contract as it heats and cools, 
making expansion joints less necessary.

I will let you in on one other little secret.  Asphalt reacts with oxygen 
in the air and becomes more brittle with time, making older asphalt more 
likely to crack.  The average asphalt road lasts about 7 years before it 
needs to be repaved, whereas the average concrete road lasts maybe three 
or four times longer.  So, maybe expansion joints are not put into asphalt 
paving in part because it won't last long enough to justify their use.