I’ve noticed in my walkabouts about town that it is customary (or was) for the contractor to impress their name and the date on sidewalks that run along city streets and rights-of-way.
There are numerous runs from the 1950’s that are in good shape. Even, in remarkably good shape, from 1929. What has happened to concrete in the ensuing years, that seemingly condemns driveways/parkways/sidewalks to a short, cracked lifestyle? Trees and other inevitable construction is one answer, but clearly the longetivity of a slab of concrete is at issue here. The reason I ask is of course primarily Fighting Ignorance, although there is also a pecuniary concern.
As a taxpaying townie, have to pay for sidewalk that does not meet homeowners ass./City/County/State/Federal/International/UN/golly-knows-what standards, I’m sure it’s all down at the courthouse.
They make them too thin these days. This means they can flex more due to loading and changes in climate.
I recall my materials proffesor in school complaining that American roads were always being reworked because of the use of asplaut instead of concrete. Of course, he had his own concrete business so that may have swayed his judgment.
Also, the set of sidewalks that Common Tater’s noticed from the 20’s to 50’s are members of the set of sidewalks that were made well enough not to degrade. All, well most, of the crappy ones they laid down back then are gone.
It could also be a failure to do an adequate job of compacting the underlying soil. The ground under the walkway is what really supports the load, the concrete being the protection to keep the ground dry and prevent wearing it into a rut.
Failure to compact the soil allows high and low spots to develop and this can put tension on the concrete which has little tensile strength.
That is one answer I have heard before, that much more preparation prior to the pouring of cement was performed. Another is that the riverbeds were more or less mined for the aggregate and that cheaper mixes are used today. Sand that expensive?? I dunno.
Most of the ones in my neighborhood were put down in 1927, but I don’t remember the name of the paving contractor. Near my old apartment, a few blocks away from my current one, a square was replace in 1949, and a couple put their names in the cement, and also at some point a cat walked across it. A long dead cat whose bones are probably dust now.
I also know a place visited by someone named Lorraine in 1938.
A significant difference between earlier sidewalks (and roads) of concrete and current ones involves the curing process. Back when you could pay labor a bit over a dollar for a ten-hour day or 80 cents an hour for an eight hour day, they used to have guys keep the concrete watered for several days after pouring to slow the curing process, making the concrete stronger. When labor costs went up, someone developed a chemical additive to concrete that was supposed to do the same task. (It does, but it is not as good as actually keeping the material moist.)
Boulder Dam is actually stronger, today, than when it was poured because the water backed up against it continues to promote the ongoing curing process. Sidewalks, streets, and driveways do not get the same attention.
Good point. Concrete takes a long time to reach its maximum strength and might even be asymptotic to some unreachable ultimate.
Concrete is a batch process and unless careful controls are used on the mixing process there can be considerable variation in properties from batch to batch. The ratio of water to cement for maximum strength is too dry for easy handling in mixing trucks so excess water is used. Maybe when it was mixed on site it could be dryer and therefore stronger.
Even so, the concrete in a sidewalk doesn’t do much to support the load. The ground does most of that and if the ground settles the concrete fails. Notice that the concrete high dams, like Hoover, are arches which put all of the concrete in compression. Concrete is considered to have no tensile strength in designing structures. Reinforcing steel provides the tensile strength in reinforced concrete.
Where are all our civil engineers? (We seem to be stuck with mostly the uncivil.)
Concrete is a mix of aggregate (gravel), sand and portland cement. I think that they used more cement in the old days. When I ordered concrete with four sacks of cement per cubic yard for a garage, my concrete guy complained that he used less than three sacks per yerd for house foundations (“I know, I’ve seen your house foundations.”) I’d guess that the
More than that though, I think that what you’re seeing is a combination of time and a change in how people approach construction.
Concrete tends to harden over time, but not by much. More noticeable is that the top layer of portland powders up and blows away over time, leaving something that looks a looks a lot more like rock since the sand is more exposed.
The folks in the old times took more pride in their work and would spend time shaping the edges. The stamp with the master’s name and the year was probably a cost savings thing for the city: instead of paying an inspector to drive around and look at sidewalks, they could just look at the finished product and not use the guy whose sidewalks had fallen apart. More importantly, people wouldn’t cut corners on something that was going to bear their name. Nowadays, concrete is a competitive business and the companies tends to be fly by night operations. Besides, who wants to spend extra money on a sidewalk?
I think there is too much anecdotal evidence and too much nostalgia to get a factual answer.
Concrete can be very high tech now, and the best concrete of today is better than yesterday’s. The mixes can be computer controlled and fiber-mesh types combined with better sealants and better rust resistant rebarb can walk all over any old concrete mix.
The problem now is that there are MANY more codes and requirements that a builder and buyer must meet, and something has to give to get smoke detectors, sprinklers and numerous other enviro and hazzard issues addressed during construction. The budget is spread so thin between HVAC systems, double pane windows, etc. Consider the types and am’t of appliances and everything else homes and business must have, and you can see that to have bigger modern homes, things like expensive woodworking, expert masonry and finish carpentry are often done with ‘base’ or ‘builder grade’ workmanship and materials.
Builders/developers don’t use the best. When having a home built, meet the cement contractor, demand fiber mesh, rebarb and LEGIT expansion joints every X number of feet and see what it costs. You can have cement that is state of the art, but it just ain’t included in the base price.