Expansion Joints in Concrete

[sybie99]

I am detailing a retaining wall and placing contraction joints at 15m centres with horizontal rebar terminating either side of the joint. This should control any shrinkage cracking. Some people/codes recommend an expansion joint every 90 feet or so, why would this be? My question is why are expansion joints ever used in concrete under “normal” conditions? Contraction joints yes, but expansion joints?
After placing concrete dries out and becomes less dense. Shrinkage cracks form. Now if the concrete at some future date were to be heated, can it actually expand to increase in volume? I just cannot see how this is possible. As shrinkage cracking occurs the concrete element is in effect split into segments. Would each segment that is formed between cracks not just expand and fill up the cracks? Can concrete actually increase in volume from its volume at placement?

I can understand that if the concrete was placed at a very low temperature and at some point in future it is much hotter, say 30 degrees Celsius. But has anyone actually witnessed any serviceability issues as a result of expanding concrete? Am I missing something here?

For a material such as steel which does not crack or changes to a less dense material, expansion is of course understandable. I just do not see concrete to behave the same way even though they both have similar thermal expansion coefficients as a section of steel will lengthen at any temperature rise, whilst a section of concrete cannot lengthen at any temperature rise as the cracks act as small expansion joints

 

[Ron]

Your premise is correct, and in most cases you don't need expansion joints; however, in long, narrow, thin sections of concrete (some pavements, sidewalks, etc.), there is enough linear expansion during temperature extremes (from placement to high ambient), coupled with impacted control joints that have too much compression resistance (sealants harden with time) that buckling can occur.

In general though, the volume of concrete (using normal portland cement) is greatest just after placement.

 

[hokie66]

I never call a joint in a concrete structure an "expansion joint" for the reasons you outlined, sybie. But "movement joints" are often required for other reasons than expansion.

 

[JAE]

Yes - for a long cantilever retaining wall we use contraction joints every 25 feet and an expansion joint every 100 feet. The expansion joint certainly doesn't do anything for you to deal with drying shrinkage. But as hokie66 suggested, there are thermal expansions, foundatin settlements, etc. that would distress the concrete without the expansion joint to allow some level of free movement between lengths of wall.

With initial drying shrinkage you can get separation between contraction/control joints but sometimes debris can get into these joints and thereafter inhibit full closure of the joint. Thus the EJ helps.

 

[sybie99]

Thanks for the replies guys, it does give me a better understanding on the purpouse of the joint.

Cheers

 

[BAretired]

I saw an example of a concrete pavement in Edmonton, Alberta which buckled in the hot sun a few years ago. It is an infrequent sight, but it can happen. In this case, the slab was butting up to a building foundation on one side and the city pavement on the other with no expansion joints between.

I have never seen a retaining wall buckle, but if pavement can do it, why not a retaining wall?

See this link:

http://www.google.ca/search?hl=&q=b...rlz=1B3GGLL_en-GBCA404CA404&ie=UTF-8&aq=0&oq=

BA

 

[hokie66]

Point taken, but concrete pavements do get very hot. They buckle even if they have movement joints, because the joints fill with debris as JAE suggested for the walls. Usually the expansion moves the perimeter of the pavement, and frequently causes distress in kerbs, etc. which border the pavement rather than causing buckling of the slab itself. But uplift can happen.