7.3 N/mm2

[gh483]

There is something don't understand about concrete block strength vs brick. Most of the blocks on sale are advertised as maximum 7.3 N/mm2 (occasionally I see 10 N/mm2 but rarely more). This goes for the dense, hard to handle blocks as much as medium density or even aerocrete. I am no expert but think that the number 7.3 comes from one of the standards or building regulations that would apply to low-rise construction.

However, on the other hand the compressive strength of bricks varies but is usually stated as 20, 30 or more. Setting aside whether 7.3 is good enough for anything up to 3 storeys using typical designs, how can blocks ever be considered stronger than or equivalent to brickwork, as many believe? Also, what are dense concrete blocks used for if medium or light weight should be just as good?

 

[jeds]

Dense blocks are cheaper so are used where thermal properties are not a factor. Bricks are generally clay, which produces higher compressive strength. Blocks are not comparable to bricks for compressive strength.

 

[gh483]

There is quite a widespread opinion that concrete blocks are as strong as or even better than brickwork, or at least that was the impression I had. Is this a myth? Also it is certainly possible to produce high strength concrete with compressive strength up to 40 N/mm2, so why aren't blocks with these properties widely available?

 

[tony1851]

The 20N and 30N figures you quote for bricks seems at first glance much stronger than the 7.3N block.

There are far more mortar joints in a square m of brickwork than in a square m of concrete blockwork. The mortar, being often of variable quality, is a source of weakness, so the design codes allow for this in their safety factors.

Therefore even though an individual brick may appear stronger than a block, in terms of an equal-size wall, the overall strength in compression will be similar.

 

[tony1851]

There is quite a widespread opinion that concrete blocks are as strong as or even better than brickwork, or at least that was the impression I had. Is this a myth? Also it is certainly possible to produce high strength concrete with compressive strength up to 40 N/mm2, so why aren't blocks with these properties widely available?

Concrete blocks of that strength would be expensive, very heavy for the bricklayer to lift, and not necessary for the relatively small loads in domestic buildings.

 

[jeds]

compressive strength up to 40 N/mm2, so why aren't blocks with these properties widely available?

Apart from Tony's point that they would be extremely heavy and difficult to lay, the simple answer is they are just not needed. What would you use them for?

 

[RonnyRaygun]

It's very unusual to use blocks with a compressive strength greater than 10.4N/mm2 (except as padstones).

If they are required it's more cost effective to thicken the walls or build panels in engineering brick and stronger mortar.

If that's still not satisfactory, it's usual to install a steel column instead as loads of this magnitude are usually beneath a steel beam.

 

[phatboy]

My local quarry do a 22.5N/mm2 concrete block! Only £40 per pack more than a standard 7.2......

 

[gh483]

Good explanations, cheers.

 

[George_engineer]

To add to tony's spot on explanation on the role of mortar joints. The compressive strength is also just a simple factor of how bricks are made and what they are made of.

If you could make a brick look like a brick, had the right properties (for example, frost resistance and water absorption) and be, say 12N/mm^2 (and it cost less to make) then you'd do so. But no-one has figured out how!

Stock bricks that were used on the internal face of solid walls - so a similar role to concrete blocks today - tend to crumble and be misshaped. But even then you rarely find them going much less than 20N/mm^2.