Concrete Sidewalk Primer

Posted by John B. Kelly

Everyone knows about the "cracks" on concrete sidewalks ("step on a crack/break your mother's back"), but very few people seem to know that most of these "cracks" are unnecessary.

Concrete is usually laid in large slabs that are 15-20 yards long. Wooden forms are used to contain the liquid concrete as it is poured, both at each end and along the sides. These forms are then removed after the concrete dries.

The gaps between the concrete slabs are filled with some sort of compressible material to allow slabs to expand and contract with weather conditions.

I don't seem to have a good photograph handy of one of these joints, but if you look carefully, they are pretty obvious, because there is a strip of rubberized material or some kind of resin. The other cracks, meanwhile, are quite shallow.

All those other cracks? These are usually called "control joints" or "weak plane joints," and seem to exist primarily for aesthetic reasons. A best practices guide recommends:

control joints (cut lines) should be provided at intervals of about 1.2 to 2 m [every 4 to 6.5 feet] transversely along the length of the sidewalk. The joint is a saw cut or trowel cut about one quarter the depth of the slab that provides a weak plane in the slab where cracking can occur without marring the appearance of the sidewalk on the upper surface.

A center line is also recommended:

As most cracking occurs longitudinally along the centreline, providing a control joint along the centreline will provide a plane for controlled cracking to occur without marring the appearance of the surface.

I have looked at a lot of concrete sidewalk, and indeed some cracks do run along these control joints, but most do not. In the last few years, I have noticed that the cracks have been getting wider and wider. A construction worker told me that this was to facilitate digging up a square of concrete without damaging adjacent panels.

In other words, almost all the cracks on a sidewalk are there for looks and construction convenience. The problem is that these control joints can be very hard on wheelchair users. Because the joints are perpendicular to the path of travel ( i.e., crosswise), our front tires tend to hit the little mini ditches at the same time.

The feeling is a lot like being in a car going over an old concrete bridge (tha-thunk tha-thunk tha-thunk, etc.), but wheelchair suspensions are closer to Conestoga wagons them a Lexis. And for people who are subject to back, neck, head or full body pain, these aesthetic little cracks are a nightmare.

Here is an example of how bad things have gotten. Last year, I led a little Saturday tour through the Fenway, and our group went up Gainsborough Street. Now Gainsborough is pretty much hopeless for a lot of reasons, primarily because of a terribly steep crossslope, but the new concrete laid in the last few years also has incredibly wide control joints.


photo: Gary Devino

Zachary Kern of the West Fenway came with us, and above is a picture of his front castor wheel entirely contained within a center line control joint.

And below is a piece of new sidewalk on Westland Avenue, with a ruler showing how wide these cracks have become.


In my next installment, I will show a very simple solution to this problem.