Double wall by cinder blocks - load bearing construction

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Double wall by cinder blocks - load bearing construction

Postby Klondyke » Sat Apr 21, 2018 11:06 pm

There is a general misconception about simple concrete blocks - cinder blocks – clinker blocks – 40 x 20 x 7 cm that they are not strong enough for any building structure. These blocks are made locally in villages, thus supporting the local workmanship, avoiding long distance transportation.

It needs just to look around seeing that almost all boundary fences are built by such blocks, hardly witnessing that such wall would be ever broken. Even if the foundation had not been properly made and collapsing, the wall will tilt at the whole length, not breaking apart.

Who has ever tried to break and remove such wall will know how hard it is. If we use such wall erected in double cladding with a cavity in-between the wall can be used as load bearing thus avoiding the need to provide a column structure with even more difficult beams above. To make the structure more sturdy a steel wire is laid under the blocks (I used 3 mm wire under every 3rd layer). Then, in the corners and in the middle of a long wall steel bars are vertically inserted, the cavity cast by concrete, thus providing the same function as a column.

The cavity in-between the 2 blocks (3, 4 cm distance) can be left empty or filled with a suitable insulation material, I have used wooden sawdust. Such wall protects the heat flow from outside, perhaps much better than a single wall of AAC blocks. An argument can be that a heavy concrete structure accumulates more heat than a light porous block. Once the house is kept closed during daytime the inner blocks do not heat up, then the difference of few centigrades can be neglected.

Once the wall is erected to the height a concrete reinforcement ring is formed in-between blocks or bricks with embedded steel profile prepared for welded connection of roof structure.

Instead of difficult digging of holes for columns the foundation for the walls is created by a trench (40 – 50 cm wide) with blocks at sides (instead of wooden formwork), with embedded reinforcement bars and filled with concrete in a reasonable depth of the terrain. Such ring around the house – and subtle ones under the inside walls – have much larger total bearing surface than the number of the pads under the columns.

In this technique I have built extensions (7x7m) to my house I bought in ruins 13 years ago. Also 3 kiln drying chambers 12x12m with furnaces and blowers (survived few fire blasts). And also my swimming pool 12x3m, 1.5m deep, where the double block wall cavity was filled by concrete, the pool finished within 6 weeks - described in a thread viewtopic.php?f=19&t=4736

All these structures had survived two earthquakes when water flown with 10cm wave over
the swimming pool edge.

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Rough cost for walls and foundation of a house 12x12m, 3 m high, double walls:

4x12m (wall length) x3m (wall height) x 2 (double wall) - 10 windows/door @5m2
288m2 - 50 m2 = 238 m2
Calculating 15 blocks to 1 m2 (normally we need 12.5) 238 x 15 = ca. 3,500>4,000 blocks
For foundation + 1,000 blocks

5,000 blocks @ 6 Baht 30,000
foundation trench digging 7,000
steel bars 20,000
sand 4 trucks @3m3 2,000
cement 100 bags @ 110 11,000
total walls 70,000

workmanship: 4 people @ 500 daily for 50 days = 100,000
Within 2 months the house is prepared for roof structure.

These two months would not be enough when making a traditional column (sau) construction with formwork for beams.
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Re: Double wall by cinder blocks - load bearing construction

Postby canopy » Sun Apr 22, 2018 7:42 am

The cavity in-between the 2 blocks (3, 4 cm distance) can be left empty or filled with a suitable insulation material, I have used wooden sawdust.

In Thailand biochar from rice husks would be a superior material. It doesn't settle so much, does not rot, and bugs aren't interested in it.

perhaps much better than a single wall of AAC blocks

But then again a *thick* AAC block wall can be load bearing and have superior thermal performance. You can calculate the r-values. To do even better, a sufficiently thick straw wall or something of that nature could be used.

Once the house is kept closed during daytime

Most may not want to give up the natural ventilation. Yeah it may stay well cooler than 40C inside keeping it closed but could gradually heat to uncomfortable levels as the day progresses.

A few more comments:

It's good to keep block work above grade. A 20cm step up is good.

I don't think in the 21st century guesswork is necessary for designing a load bearing structure. Some areas may be over designed resulting in wasted materials sitting on the ground doing nothing. And other areas that needed more reinforcement can be missed resulting in weak points. We can do better than a trial and error approach of "I tried it and it worked so far". And think that for every success story how many failure stories we don't see.
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Re: Double wall by cinder blocks - load bearing construction

Postby Klondyke » Sun Apr 22, 2018 9:31 pm

canopy wrote:
Once the house is kept closed during daytime

Most may not want to give up the natural ventilation. Yeah it may stay well cooler than 40C inside keeping it closed but could gradually heat to uncomfortable levels as the day progresses.


Of course, the house is never hermetically closed. However, even now, the most hottest days of the year, the temperature inside does not come over 32 deg. Then, when a/c used (not every day, then mostly at night for sleeping) the rooms are cooled down to just 26 deg. For these few deg. gradient it's not worth to have a headache with that or that material, not speaking about double or triple windows as I sometimes read.
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Re: Double wall by cinder blocks - load bearing construction

Postby Ians » Mon Apr 23, 2018 8:51 pm

The hottest room in my house build with red brick, double cavity, rendered on both sides with all rooms vented into the cavity and exhausting into the roof space will reach about 30c after a few days of long periods and drop back to the overnight outside temp.
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Re: Double wall by cinder blocks - load bearing construction

Postby Klondyke » Tue Apr 24, 2018 8:51 am

^That's an interesting solution. Can you disclose more details about the cavity ventilation, location, sizing, dimensions of the openings, protection covers, etc?
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Re: Double wall by cinder blocks - load bearing construction

Postby Ians » Wed Apr 25, 2018 11:08 am

The wall vent has removable front with separate insect screen.

The exit from the cavity wall is above the ceiling and exhausts into the roof space.

20180424_093351.jpg
wall vent from Global House
P1100524[1].JPG
exit from cavity above ceiling into roof space
P1100524[1].JPG (33.88 KiB) Viewed 125 times


This idea was used in all cavity brick houses in Aust. in the early days and it really works, I checked for air flow from the rooms into the vent using a burning incense stick - it showed considerable flow in all cases.
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Re: Double wall by cinder blocks - load bearing construction

Postby Klondyke » Wed Apr 25, 2018 7:18 pm

^Thanks, a good and simple idea.
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