mortar too weak?

[swimmydeepo]

I've just started to rebuild the inner leave of an external wall (terrace house). I've decided to use engineering bricks for the first few rows starting just below the concrete floor but above ground.

The existing bricks (located between the foundations and the newly laid engineering bricks) are slightly damp; so, I've opted for a 5:1 mix plus a waterproofer. Now, 2 days after laying the first 3 rows, the mortar used for the engineering bricks still appears to be slightly damp and rather weak.

I am thinking of restarting from scratch, and use a stronger mix (e.g.

3:1) plus a waterproofer and a plasticiser as well.

Any comment greatly appreciated,

Thanks

Alex

[Weatherlawyer]

With it being slightly damp I'd say you have to find the reason for the damp. I would have thought the problem with the masonry was that the mix had dried too quickly in the ho weather.

Bricks -even engineering bricks, can absorb a lot of water and in hot weather will suck a mortar dry too quickly.

5:1 using portland cement, is a strong mix. Most use 6:1 but you can use 8 to one. The idea is to keep the bricks apart as muchas to bind them together.

That's so the stresses and loads go down at 45 degrees. The middle of the brick sitting equally on the two below it.

[swimmydeepo]

These are bricks located underneath the concrete slab and in direct contact with the earth on the sides and the foundations underneath (I can't see it, but I assume and hope there must be some kind of foundations somewhere under those slightly damp bricks). The house is rather old and I would rather prefer not to go down to the foundations to resolve the "damp problem"...and anyhow, the damp wouldn't affect the living area as I am using 3 courses of engineering bricks to stop any rising damp from underneath.

Thanks for your reply,

Alex

[Peter Lynch]

On the topic of weak mortar mixes. I recently decided to repair one of the walls of my standalone, block-built garage. Towards the base of the wall, the mortar was quite robust and in good condition. As I looked higher up the wall, I noticed that the mortar gradually changed colour and became a lot more friable. The mortar between the top couple of rows of block was so weak that I could remove it by brushing it with a normal stiffness paint brush. (I stopped before I removed too much :-)

My theory was that whoever built the garage 20+ years ago ran short of cement and mixed each bucketful weaker and weaker as he went up the wall.

At least now the wall has an outer coating of strong mortar. I can't say how long it will last - hopefully it won't fail before I sell the house!

Pete

[meow2222]

mortar takes a month to get its full strength up. 5:1 is more than strong enough. I take it youre using all new bricks, not salvo old ones. If youre using old you need a different mortar altogether.

I certainly wouldnt assume that. A lot of 19th century houses wrere built with sod all in the way of foundations.

So is there a damp problem or not? I'm not clear. The bricks in the soil will of course be damp, but this wont normally transfer itself to the living area. Think more explaantion is needed.

BTW one row of engineering bricks is more usual as a dpc.

NT

[swimmydeepo]

You are right I am using new engineering bricks but planning to use older common bricks above them...thinking of using a 6:1 mix for them.

You mean there are no foundations?

No there isn't a damp problem; I've simply assumed that laying new bricks on top of slightly damp ones would required some waterproofer or a new DPC. I've opted for waterproof mortar and a few rows of engineering bricks.

Thanks for your help,

Alex

[The Natural Philosopher]

Not at all. If anything it helps slow the cure and increases the strength.

[Chris]

Not always no.

[meow2222]

If youre using old bricks, there are several reasons not to use cement mortar. You can of course, but lime has several advantages, especially when used on houses with little in the way of foundations. I'd definitely suggest using lime instead for the commons.

The only way to know is to dig down. In 1800s and earlier houses varied greatly, some have good foundations, most have very little, some had none at all. So who knows. 3 bricks deep is a very common Victorian era foundation, and that may be all youve got.

Most 1800s houses had no dpc of any sort, and normally were fine. Damp handling on old properties is somewhat more complex than with new builds. There are a lot of old properties that have become damp as a result of unsuitable maintenance, but few that are damp due to original design.

NT

[Tim Lamb]

In message , [e-mail] writes

Oh! I'm about to re-use some well cracked Victorian soft reds which still have a thin coat of the original lime mortar. I had planned on a

5:1 mix using a proprietary (Rugby) masonry cement. Below damp is facing brick on a conventional foundation.

Am I heading for disaster?

regards

[meow2222]

I think I'm going to make a new thread for this one, as it keeps coming up. Disaster no, but there are several downsides to cement with old bricks. When cement mix is used 1:1:6 is more usual, 5:1 is too strong.

Have to run, will reply properly tomorrow in new trhread.

NT