You said it all when you wrote that your company has been taking this "risk" for "plenty of years". The masons on the other side of town may have added accelerators or mixed hydrolic cement into the mix to speed up curing times. If you suspected weather to be an issue you should have specified the use of such additives in your contract or done up a change order for them to be used. It costs more to do this so if you are one of those lowest bid wins the job kind of guys you just learned a valuable lesson.

Coincidentally it takes 28 days for concrete to cure. You should have at the very least either let the foundation and block work set longer or framed the floor before back filling. This falls entirely on you for using very, very, poor building practices.

 

I cannot tell from your post whether the wall in question was entirely concrete block, or block over concrete. Also I cannot tell whether the part of the wall that collapsed was the concrete, the block, or both. Perhaps you have a photo or two to post? And I also can't tell from your post exactly what your DIY question is.

 

One small (I really mean, LARGE) detail in block laying often overlooked in today's 'hurry-up" world is that curing is more than simply some time passing. It is a process of chemical reaction that creates a bond between the blocks. Merely having the mortar dry out does very little for bonding strength.

Mortar is made up of a high percentage of lime. It is primarily this sticky lime that promotes the adhesion between the blocks. The Portland cement part of the mortar makes it 'hard', but really does little to actually 'bond' the blocks together.

What happens is that, much like wood glue, the wet lime laden mortar is 'drawn' (or sucked) into the porous block. The block acts a lot like a sponge. But there are two things that can hinder that drawing effect. One is blocks that are TOO dry. And the other is blocks that are TOO cold.

Too dry is usually not a concern at this time of the year, but I'll cover it here anyway.

Blocks need to have a high enough moisture content to keep the 'drawing' action at a reasonable rate. Very dry blocks will instantly suck the surface moisture out of fresh mortar causing it to become crusty and to begin to cure before it has a chance to absorb into the block. This is why good masons always spray a mist of water over their blocks before laying them in the summertime.

What do you suppose happens when the blocks are cold, below freezing? That's right, the water and lime content that might have gotten drawn into the blocks becomes ice. Ice goes nowhere. It hardens, along with the mortar, right there in a layer between the two block surfaces. Almost useless.

Include some of the correct additives, and the freezing problems can be overcome. The mortar will work as it is supposed to. Don't, and you can kick over a supposedly well set and cured wall with one kick of a big boot.

I think you can guess what may have happened in the case you're describing.

 

Is this some kind of machine foundation or a building addition?

I've backfilled some work the same day in summer, only long term equal load walls like a stoop or garge, etc..., but wouldn't try it in winter.

Odds are the other masons WERE using a different mortar, like maybe a Type M?

 

jomas has a good point. The ASTM C270 spec for mortar states in the appendix to use the lowest strength possible for the structural loads. Bad backfilling practices or bad material is a very common cause of early failure. High strength, like Type M is not always good. In my opinion, portland and lime is superior.

If you were sure everything was the same when you took the "risk", that just goes to show that it is a risk or things were not controlled to be the same.

Weather should not be a factor except for protection for the first day. Failure to brace before early backfilling is the GCs fault. Every good mason will not work without warm water and warm sand because it is just cheaper and gives better production and quality.

Dick

 

Concretemasonry,
The only choices you have for below grade would be Type M, type S, masonry cement (mixed with lime and sand), or portland (mixed with lime and sand). Type N is not rated for below grade and type O is for interior walls. I agree that portland is superior.

The OP chose to back-fill earlier than the he should have. The framing crew would not have had enough time to frame the floor in order to brace the structure. I understand that back-filling early is common place in the industry today, but that does not mean it is good building practice. I have not seen to many sites where warm water and sand where available. This seems like more of a lack of additives and bad practices than anything else.

WillieT,
Great explanation and way more detailed than I was willing to get.

 

Masonry Cement is not portland and lime pre-mixed. It is a special grind of cement with certain additives IN THE PLACE OF lime. I normally call pre-blended portland/lime "masons mix" or PCL mix.

 

Let's try not to get too hasty.
Masonry Cement Data Sheet

 

I stand corrected.