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Robin T and better wall construction

from Robin T

Towards a more effective wall construction

image rt01image001.jpgAny opinions aired here which concern the structural load bearing integrity of a building must be designed, checked and approved by a licensed Thai civil engineer, That’s the law. It is very foolhardy for a falang to by-pass any such procedure.

I have puzzled for some time over the building methods used for concrete blocks. Mostly one sees the stretcher bond arrangement of traditional brickwork. This is a hangover from the times when foundations were unreliable and apt to settle at different rates. Victorian brickwork in London used lime and sand mortar on workers’ dwellings with some Portland cement added on higher cost housing. One did not align bricks on top of each other because this would make a very weak wall and it would fail down the vertical weak mortar lines.

London is a huge bowl of clay which is notorious for heaving and sinking during the wet and dry seasons. High quality housing had basements and used massive strip concrete foundations (around 1 metre square in section) and buried at least 2 metres below the surface. It was a costly form of construction certainly not used on cheap dwellings.

However, one can see numerous examples of cheap workers’ tenements built around 1880 to 1910 where foundations were made from crushed rock piled into shallow trenches. Over the years the brickwork courses above moved up and down and formed crazy wave-like patterns. Yet, the walls remain sound even today, although they often bulged and were reinforced with many tie- plates passed right through the building to hold the sides together. The lime mortar is weak and somewhat plastic and allows the flexibility needed so that the bricks don’t crack. It’s worked for 200 years!

In Thailand we have the reverse case, where the cement mortar is almost certainly stronger than the blockwork. Indeed the blockwork only gets any real strength from the cement rendering of 10mm either side. !@— image rt01image002.jpgTypical blockwork, shown without columns !@— image rt01image005.jpgConstruction of a typical ground beam, at least it is wrapped around the column in this case. Notice the rebar is too near the formwork. Rebar should be at least 20mm from the edge.

This is the kind of thing that happens when you don’t include construction details in your design plans. !@—

Pre-cast Columns

This is one of my pet hates. IMHO never use these for a dwelling, only for a shed. image rt01image007.jpg !@—

My wife’s village house was constructed 10 years ago and is now suffering all kinds of structural defects (see my article on termite for horror photos.

It’s still a very prevalent for of construction up country, to be avoided at all costs.

Laterite

The subsoil, you may encounter is usually a type of laterite (mixture of iron oxide, clay and sand). This material turns into a slippery sludge when inundated by monsoon rains. It rapidly dries out however and within a week or so into the dry season it becomes rock hard – literally. Strangely, however, the amount of sludge formation (called shiggy in the Middle East) doesn’t continue down indefinitely. Try and observe this yourself during the rainy season. Pick a suitable patch where rainwater has collected in a shallow pool. Try pushing a 15mm wooden rod (a dowel piece which has a flat end) into the centre of this puddle. If you want to get a bit scientific about it, then note down the date and use a suitable weight of say 10 kgs and measure how far the rod penetrates into the ground over the next 3 months.

Publish your results here and get a commendation for scientific research!

You should find that the ground will only soften to a limited extent (say 200mm). Why is this?

Well you can carry your investigation a bit further by collecting a bucket of the soil and mixing it up with water in a clear glass bowl. Observe what happens over 24 hours as the mixture settles out. Publish your theories here?

So what does this mean for you?

image rt01image009.jpg !@— In the foreground is my future swimming pool!

This “Red house”, was originally built in the middle of a pond on another plot. I decided to relocate it to our farm. So it was dismantled and rebuilt on six pre-cast columns. In this state it was very shaky and I feared that a strong wind would blow it over. The downstairs blockwork was quickly constructed between the columns which made the whole place very solid. We did not use my preferred method of column construction but at least the ground beam was properly wrapped around the columns (see above).

How to build a better wall

Lets consider some better alternatives which I have seen used in Thailand

image rt01image013.jpg !@—

The aligned block method

Here the blocks are laid on top of each other in line supported by a ground beam. A 3 hun (8mm) length of rebar is laid along every second course. In theory you wouldn’t need any columns for 4m x 4m simple room. In practice, your building must have a minimum of 6 columns (hok ton) to be considered as worthy of registration for house papers by the local authorities. My local worthies have been most helpful and agreeable in all matters of building regulations and we got our house papers remarkably quickly ( a few weeks or so) and negligible tea money, but we did hold a little house warming BBQ which was gladly offered (a total contrast to the horror stories reported about Pattaya in this site).

image rt01image014.jpg !@—

Note that the gaps in the rendering are shown only as and example. Your own wall would be fully rendered flat (though you could scribe some coursework lines in the surface – this can produce an attractive effect)

What about Corner Blocks

Well, here you have to use a disc cutter (not a hammer and chisel – insist that your tradesmen have their own proper tools else get someone else).

You can cut alternate block widths from each side so that they form teeth that fit together BUT REMEMBER always turn the cut end in towards the existing wall because it will be weak and hollow at that edge.

This method can work because * The mortar is stronger than the blocks.
* Furthermore I have seen a skim of cement rendering applied simply to smooth the surface and make it easier and more economical to paint (bare blocks are very difficult). Of course you can apply a full 15mm plaster to the wall both side as usual * This also works because there is a structural ground beam cast and tied into the columns. If using this method for a perimeter wall, then a top beam of similar construction should be used also. * You can design the spacing of the columns so that no cutting of blocks is required, saving time and spoiled blocks * It’s much quicker to lay blocks like this.

I have seen this used for building perimeter walls 2 meters high around my local Bank (they wouldn’t use a Cheap Charlie system would they?)

What about Columns

image rt01image016.jpgIf the use of pre-cast columns is a bad idea then what to do? Using proper formwork and pouring your columns in situ, however, is more expensive and time consuming, so here is an alternative I have seen that is a fair compromise. Here you see the columns placed inside and they are strapped to the wall with 6mm rebar wrapped around the column and tied to the rebar along the courses of the blockwork. I would expect also that these courses would contain a length of rebar tied along to the next column. If this is done every second course it should be quite strong. I would recommend chipping the sharp edge off the corners of the columns where the rebar is to be wrapped (just enough to make a suitable radius for bending. !@— image rt01image019.jpg !@—

This might look ugly but you could easily plaster over this if desired. If you are going to do this then I recommend that you wrap some chicken wire around the columns to make plastering easy and strong.

This shows the columns fully tied to the ground beam

The problem occurs when trying to tie the pre-cast column to the outer ground beam. What I recommend is that you order your pre-cast columns with a 10 mm hole cast into the column right through, at ground level (very easy, no extra cost).

You can then grout in a stub of 8mm rebars into the hole and this can be cast into the ground beam.

!@— image rt01image021.jpgexterior view !@—

BTW I always prefer to have my columns laid 1 meter below grade rather than the traditional 0.5 meter – insist on it and write it into your build spec (they don’t like the extra effort, but its your money).

I don’t think this method would be suitable for a two storey dwelling as the columns may not have enough lateral stability but you might get away with a wooden upper floor? As per traditional design.

So numerous cost and time advantages and no disadvantages that I can see – any comments please?

To follow

  • The suspended floor design (virtually tank proof)
  • All the benefits of commissioning a full design pack
  • Build your own adobe rondavel
  • Some novel approaches for your dream house
  • Building a swimming pool
  • Security features you should consider
  • Roofers guide
  • Plumbing
  • Basic electrical systems

Until the next time

Design, design, design (not on the back of an envelope or on the fly). Proper detail design can save you time and money. You also have a set of house blueprints which go with the sale of your house and add to its value. A good design is almost certain to recoup the initial extra cost (around 10% of total construction budget).

I was sitting in a coffee shop the other day shooting the breeze with the guys – as you do. One of our numbers, a Republican redneck from North Carolina, was spouting about his dream house to be built down by the sea on some island. He had a budget of 2-3 Million Baht.

“How about starting your preliminary design work now? I said.

“I know what I want and my wife’s uncle is a builder”, he replied.

He didn’t even want to know about this website and he would not accept that 10% spent up-front on design specs could save more than that in construction costs later on.

“What’s it like in your home town for building something” I asked.

“There are dozens of inspection stages”, he said, “if you take a dump, City hall has to come out and inspect it before your can pull up your cacks”.

By this time the sun was over the yard arm and I had to retire to my favourite tin table in soi 18 and calm my nerves with a large Archa (25 baht from Yai Pun, dear old noodle granny who is guarded by a large battle-scarred street dog) – so carry on cowboys.

1 Comment

  1. RobinT, I am interested in building a small bungalow similar to your “Red House” to live in while the main house gets built and as a future guest house. Thw wife and I like the traditonal Thai style of putting the house up on “stilts” and parking the car beneath. You mentioned having some issue regarding the sturdiness of the house before you filled in with some walls on the ground level. Is that only because the columns were pre-fab? Is it possible to put a 35-40 sq. meter house up 2 meters off the ground without any ground level walls or should at least some portion of the ground level be walled in for extra support? Also, do you have any general figures on pricing for such a project? I’d also be interested in seeing more pics of the “Red House” if you have any. Thanks very much.

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