Replies

1. 
   Guest_ | Jul 31, 1999 06:45am | #1

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   possibly they used high early strength concrete...or percent of design load met in 3 days...concrete takes many years tofully cure

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   Guest_ | Jul 31, 1999 06:50am | #2

   *Frank,Your gonna find that not too many things are built "text book". The concrete contractor in this case will be left hanging if someting goes wrong. I hope he has the experience to know what he is doing. You might be surprised how many things are done that the "experts" say won't work that work perfectly.Out in the field,Ed. Williams

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   Guest_ | Jul 31, 1999 06:56am | #3

   * Frank,

   If I could ask, do you know what they were building? This practice is used in soil that is fairly firm where the trench will retain it's shape. The most important aspect of this procedure is to bell/flair the bottom. This will prevent the mass of concrete from acting like a giant spike and driving itself in to the ground.

   Joseph Fusco View Image

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   Guest_ | Jul 31, 1999 07:14am | #4

   *Frank,What you described is a common practice here (Arkansas) and many other locations as well. Forms are required when soil conditions do not produce a clean trench with square walls. Trenching requires more concrete as a rule but saves on forming materials and labor. Concrete cures over time. However, most of the strength potential is achieved in a couple of days. A footing can take foundation blocks within 24 hours without problem. Concrete placed in typical footings will cure in most weather conditions without covering or treating with curing agent (At least I hope so, we recently put 30 yds of concrete in trench footings in 90 degree weather). Slabs are another thing. Because they have a wider surface to depth ratio (exposing more of the concrete to the sun and air) they should be covered or treated with a curing agent in hot weather. This holds the water in the concrete until it reacts chemically with the cement. Even so, concrete slabs placed in hot weather can be more prone to cracking or spalling. Concrete cures, it does not "dry." During the initial set up some water comes to the surface and is evaporated. This is normal. If, however, excesive water is cooked out the result is weak concrete and a tendancy to spall or crack. In warm weather additives are used at times to slow down the initial set up so that the finishers have time to do their work.

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   Guest_ | Aug 01, 1999 07:33am | #6

   *Isnt it amazing that no matter where you go.....concrete is different....so is foundation construction........but one thing will be the same no matter where you go....an 8/12 hip will always be cut on 8/17 on the framing square and the length will be the same for every span.......somethings must be and somethings can be....

2. 
   Frank_LaPiana | Aug 01, 1999 07:33am | #5

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   I saw something that surprised me the other day, and has made me very curious. It happened on a industrial site.

   A crew poured concrete footings in 95+ degree heat, directly into a trench. No forms, no plastic barrier, nothing. Just raw concrete (with rebar, of course) directly on dirt. It wasn't covered or watered and 3 days later they started building concrete block walls on it.

   Maybe my knowledge is out of date, but I thought that concrete had to "cure" slowly over a period of a week or more, and that if it dried out too quickly it would seriously weaken.

   So is this current practise? Does modern concrete + additives solve the curing problems?

   Thanks