20 N/mm sq Blocks

[heating12numpty12]

I have a structural drawing that states that a pier is to be constructed of 20N/mm sq brickwork. The existing block work below ground I can easily dent with a screwdriver. Is it likely that these blocks are not 20N/mm sq?

The house is about 17 years old if that helps. I am also thinking I might have to call in the structural engineer to pass his expert opinion. But would appreciate any comments from those more in the know than me.

 

[^woody^]

No blocks have the compressive strength of bricks.

 

[denso13]

The dense concrete blocks are normally around 7N

 

[RonnyRaygun]

I have a structural drawing that states that a pier is to be constructed of 20N/mm sq brickwork. The existing block work below ground I can easily dent with a screwdriver. Is it likely that these blocks are not 20N/mm sq?

Most bricks are at least 20N/mm2, few concrete blocks are. However, the strength of the block or brick is not the whole story. The masonry panel or pier's strength is a function of the masonry unit strength, the mortar strength, and the size of the unit.

The blocks are probably 3.6 or 7.3N but being able to dent them with a screw driver doesn't make them weak in compression. You are confusing compressive strength with toughness.

20N bricks in M4 mortar will have a strength (fk) of 5N/mm2
7.3N blocks in M4 mortar will have a strength (fk) of 6.4N/mm2

 

[heating12numpty12]

Thank you for the repy. So I think I am hearing to be certain of 20N/mm2 I need the piers built up from the foundations in engineering bricks. Would that be correct?

 

[tony1851]

@op; 20N/sq mm bricks is not the same as 20N/sq mm brickwork.
The SE should be specific, and also spec. the mortar - as above, that has a bearing (no pun intended) on the overall 'strength'.

 

[tony1851]

7.3N blocks in M4 mortar will have a strength (fk) of 6.4N/mm2

As long as they are laid upright and not flat! - Go to the back of the class!!!

 

[RonnyRaygun]

Thank you for the repy. So I think I am hearing to be certain of 20N/mm2 I need the piers built up from the foundations in engineering bricks. Would that be correct?

No, not necessarily. A brick pier has probably been specified due to dimensional constraints. As long as the masonry strength of the blockwork is sufficient then no need to rebuild. Your SE should have done a check on the substructure blockwork too.

You don't need a strength of 20N/mm2 - your SE has specified 20N/mm2 bricks which should give the required strength with the correct mortar (which should also have been specified).

 

[heating12numpty12]

7.3N blocks in M4 mortar will have a strength (fk) of 6.4N/mm2

As long as they are laid upright and not flat! - Go to the back of the class!!!

I have found out that they are laid flat. I would have thought there would have been more compression strength with them laid flat (more surface area to displace the load). Is there an explaination for this reason?

 

[tony1851]

deleted

 

[tony1851]

I have found out that they are laid flat. I would have thought there would have been more compression strength with them laid flat (more surface area to displace the load). Is there an explaination for this reason?

For the usual size of block eg 440 x 215 x 100, in a given strength of mortar, the compressive strength of the masonry will be about 50% if the blocks are laid flat, than if the blocks are laid upright.
The mortar fails first under high load, and the deformation in the mortar causes tensile stress in the blocks, which eventually pulls them apart. Consider the mortar as the 'weak link'; the more mortar joints, the weaker the masonry in compression.

This is a badly-built pier in 7N block laid flat - note the third block down, which has failed.

 

[RonnyRaygun]

I have found out that they are laid flat. I would have thought there would have been more compression strength with them laid flat (more surface area to displace the load). Is there an explaination for this reason?

As Tony says, the blocks laid flat have about half of the strength of blocks laid upright. Masonry strength is derived empirically from tests under controlled conditions. There are large factors of safety to allow for site conditions etc.
The blocks laid flat may well still be OK as the pier will have a reduced capacity due to its slenderness. This will not be an issue for the substructure blockwork. Ask your SE to confirm. If it involves doing a calc he will probably just tell you to build up from top of concrete!