Mortar mix for bed under padstone

[Cory H]

Just wondering what mortar mix to use for under a padstone that's bearing a large steel beam? The padstone will be sitting on 4" blocks.

Would a standard 4:1 mix (no plasticiser) do the trick or would I be better off going with a 3:1 or sharp sand mix?

Thanks

 

[SpaceWorm]

You don't need to adjust the mortar strength under the padstone from what is specified for the rest of the wall. The engineer who designed it takes into account the mortar strength when designing the masonry bearing.

 

[Cory H]

Ok that's great. Thanks @SpaceWorm.

I presume the same is true for a retrofit operation where I've chiselled out a pocket for a padstone. The house was built in the 60s. Presumably a 4 to 1 mix with no plasticiser would be ok?

 

[SpaceWorm]

The Engineer who designed the beam should have checked the masonry bearing and made assumptions about the existing mortar (the new mortar will be attributed the same strength within the design). A reasonable assumption for the existing construction would be an M4 (iii) mortar, with the masonry strength calculated thus. If you use 1:3 to 4 for an M6(ii) mortar I don't think you will have any issues at all, but this really should be part of the Engineer's specification.

Do you know the beam size, wall thickness, material and end reaction in kN?

 

[^woody^]

In the context of padstones the bedding mortar is irrelevant.

The only purpose of a padstone is to deal with the bearing shear. After that the load just spreads out down the wall.

 

[SpaceWorm]

In the context of padstones the bedding mortar is irrelevant.

The only purpose of a padstone is to deal with the bearing shear. After that the load just spreads out down the wall.

The Designer needs to check the compressive strength of the masonry beneath the padstone, and the compressive strength of the mortar is a factor in determining the allowable compressive stress.

The term bearing shear always confuses me and I see it all the time on here with regard to masonry bearing and padstones. When you say bearing shear, are you talking about vertical shear in the beam being transferred to the padstone? The padstone is in compression, not shear.

 

[^woody^]

The compressive strength of the wall has little to do with whether a padstone is needed or not. Yes it needs to be taken account of, but its a separate thing

Bearing shear is basically when the face of the wall at the bearing cracks and slides from the pressure under the beam at that point (imagine snow sliding down a mountain). So to counter this, whatever is at the bearing needs to be strong enough - so it may be a padstone or it could be a brick - whatever is in the wall already, or more commonly a solid blue brick

 

[Cory H]

@SpaceWorm The beam is a 254x146x31 bearing 12.4kN on 100mm blockwork (cavity wall). The mortar wasn't spec'd by the Engineer but perhaps this is related to what @^woody^ and yourself said. I.e. - that the engineer assumes a mortar similar to what was used in the original construction (such as M6) will be used.
Thanks for your replies