Is concrete stronger when it is cured slower? In other words, we are in debate as to whether or not the current weather, cold wet and rainy, will effect the concrete. We have heard that it is better to do it in these conditions, but it sounds fishy. So, is it best to pour concrete when its cold or hot outside???
Yeah, how fast it cures definitely affects strength. In fact, when we do concrete design, we have to find out the average humidity of the area in which it will be poured to get an reasonably accurate assessment of the ultimate strength. Concrete is stronger in higher humidity environments because the concrete has more time to cure (for the chemical processes that make it concrete) than it would in an arid environment. The makers of prestressed high strength concrete beams often use techniques like “moist curing” and “steam curing” where they can control the level of humidity.
If where you’re pouring concrete is just slightly rainy, it’ll be fine. If it’s raining cats and dogs, it might mess up the surface but the conrete will still set. Concrete will even cure under water because it’s a chemical reaction and not a drying process.
Since this is a question with a factual answer, I’ll move it to General Questions for you.
Cajun Man
for the SDMB
Not only will it cure underwater, it will be a stronger final product that way.
There are all kinds of things you can add to concrete to change the way it cures: retarders for when it’s hot, accelerators for when it’s cold, air entrainers to make a lighter weight concrete, etc. There are very few weather conditions that can’t be addressed by the proper concrete mix.
In which case what will you do with the question about a cotton ball the size of Texas, which does not burn up when it hits the atmosphere? 
I’m no materials expert - I suspect these other fellows are - but here’s what I see as being a critical factor: what are you using the concrete for? True, the longer you keep the concrete moist, the stronger it will set up - but it’s good to keep a sense of proportion about the whole thing. If the concrete is “only” for a walkway, moderate strength concrete will suit. Foundations? Check your local building code (and/or building department) for required strength and testing.
As important as anything is the initial water/cement ratio - concerning which, check with your cement supplier or other expert. The purpose of a moist cure is to keep surface water from evaporating away until the concrete sets up mostly: for consistency, the entire block of concrete should maintain the same water/cement ratio. There’s different trouble you can go to if this is really important and the air is dry: cover the concrete with plastic sheeting (wet burlap?); keep watering down the concrete; or spray curing compound over the exposed concrete. YMMV - check with local experts regarding availability and costs.
[slight hijack] I’ve heard about underwater concrete (called tremie?), never seen it done - and I’m a little curious myself: the very top surface probably mixes with the top water, some, right? This would maybe leave a surface layer with sub-optimal w/c? But presumably this isn’t a big deal? [/slight hijack]
Oh, and I wouldn’t worry about cotton balls the size of Texas. It will probably never happen. :rolleyes:
Tremie is when you use a pipe to push concrete into the bottom of a slurry or liquid. You keep on pumping until the concrete comes out of the top. If the liquid surrounds the whole operation, you would probably have loss of strength in the top inch or two. Otherwise, the concrete displaces the slurry/liquid, and there are no worries.