Compressive Stress Values for Bricks and Blocks

[TheScullster]

Hi Guys

Does anyone have allowable compressive stress values for bricks and blocks used in 1970s construction please? The blocks I have heard described as slag blocks - a bit like aero in construction, but with black bits and solid chunks here and there. Bit nasty to drill, but more solid/heavy that the modern thermalite jobbies. The bricks I have heard described as Fletton (sp). Not sure if this is the correct name, but these are bog standard 1970 house bricks.

Reason:

I am looking at an RSJ for ceiling joist support with one end landing on an internal block leaf and the other landing on a "previously-external-now-internal" brick leaf. Need to assess end load support and any load spreading required.

TIA

Phil

[Tony Bryer]

Sounds like breeze blocks. I would treat them as 2.8N/mm2 blocks (this is the crushing strength: the actual stress you can put on them is a lot less than this)

Fletton near Peterborough was the place where the clay used to make them was first found. It contains shale that burns during the firing process thus reducing the amount of fuel required significantly (and this making the bricks cheaper). On spec housing they are generally laid frog down (to save mortar) so should be treated as 7N. Laid frog up the full 21N crushing strength can be assumed.

Lots about Fletton bricks in a (PDF) Competition Commission report at

[TheScullster]

Tony

Thanks for comprehensive reply!

One or two points arising:

Yes, now you mention them, the term breeze blocks is familiar - pretty sure that's what I'm dealing with. How can I use 2.8 N/mm2 for these, if the actual capacity is a lot less? Do you have reference links/texts for figures quoted? What is spec housing?

Phil

"Tony Bryer" wrote

[Tony Bryer]

Basically by reference to BS5628: the net allowable stress will be a function of the block material (as above), thickness, mortar, bearing type and assumed standard of workmanship. 0.7N/mm2 should be a safe conservative figure for 100mm blocks.

[TheScullster]

Thanks Tony Much appreciated

Phil