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  1. #1
    I am not a cat
    nidhogg's Avatar
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    Road construction questions

    My own personal theory is that there is nothing that members of this board cannot answer. Always been quite amazed to be honest.

    So, this question has been bugging me. The roads around my Moo Baan got tore up pretty badly in the big flood a few years ago. They finally got around last month to fixing them (!!). The old road was standard tarmac. So, instead of re-doing that, they worked in about 50 meter strips, half the road at a time. On top of the old road they put down a sand base, laid iron mesh on that, another layer of sand, then poured an inch or so of concrete on top of that. Means that the new surface is about 10cm or so higher than the original road, but as they filled back right to the property wall, it all worked out somehow. And now looks a lot better.

    After they had let the concrete set, a day or so later they cut a channel into the concrete across the width, about 1 cm wide and half a cm deep using some special grinding machine. The cuts were made every 10 meters or so. They then poured black gunk into the ditch, together with the bit where the two half of the road met, along the center line.

    Onto the question: Why. Why cut the grooves into the concrete? What purpose does it serve?

    A followup would be why the black gunk. The thing is, in many places there is no black gunk, probably mixed with dust or the like and has now gone. So its probably not vitally important, but what is the purpose of it?

    Answers will be appreciated, cause it irks the hell out of me not knowing.

    Cheers.

  2. #2
    Thailand Expat
    Mendip's Avatar
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    I would say the grooves cut into the concrete could be to limit the propogation of cracks... a developing crack should stop at a groove.

    The black gunk sounds like tar. Maybe just to give a smooth profile?

  3. #3
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    Heat expansion? Do the cracks allow the concrete to expand in the heat of the day without cracking? The black gunk is an aid to dampen tyre noise going over the cracks.

  4. #4
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    The cuts are usually made with a diamond saw as that will cut the concrete and rebar. This is to allow for thermal expansion of the concrete. Asphalt is usually poured into the cuts as a seal. The sealing material must be elastic so that it can expand and contract while maintaining the seal.

    The concrete would crack and buckle without the expansion joints.

  5. #5
    Thailand Expat Pragmatic's Avatar
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    When they use concrete they should only build in slabs of a certain size and then put in an expansion joint to prevent cracking. Here they cheat because the people at the Amphur know FA about road build so the contractors just cut the concrete, or imprint a supposed gap, and pour bitumen in it to look as if an expansion joint was done. The thing they do not use here when building roads is to put a weight bearing sub base in and that's why Thai roads fcuk up after a rain season. It's all down to cutting corners.

    Just as an extra on this subject. Why do Thais put un-compacted sand down before laying wet concrete? And why do they spray a road with emulsion and then put sand on it before tarmacking? The emulsion is to bond the old surface to the new. Monkey see. Monkey do?
    Last edited by Pragmatic; 23-02-2020 at 02:40 PM.

  6. #6
    Thailand Expat David48atTD's Avatar
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    What the guys have mentioned above. The Cuts in the road, refilled with a flexible compound is to cater for the expansion and contraction of concrete.

    Easily demonstrated in a common footpath.



    The grooves pressed into the concrete cater for some of the expansion and the rubber (thick black line) allows for more expansion.
    Someone is sitting in the shade today because someone planted a tree a long time ago ...


  7. #7
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    One can find the answer when looking on the bridge: there are always pretty gaps in-between the separate sections in the bridge length. And when we look underneath, we see that the sections are sitting on rollers enabling the expansions/contractions the concrete with its reinforcement is very prone to.

    In my moo baan I see transverse cracks in the concrete surface every 10m or so, not straight. I suppose that they did the mixture with too much water - it's easy to spread it up without much hand power. So after the drying the whole mass had loosen the huge volume of the water, hence shrunk.

  8. #8
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    The cracks are to account for expansion and contraction as mentioned. The idea is you cut down about 1/4 to 1/3 the depth of the concrete and then its will crack down the its you made rather than randomly break up at any old angle, You'll see i did the same on the BiL car...erm Sala.

    Have to say though that from the description that road won't see out 3-5 years, especially if you get some lorry traffic occasionally.

    here's a linky

    Contraction joints should be placed to produce panels that are as square as possible and never exceed a length to width ratio of 1 ½ to 1. Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness.

    https://www.thebalancesmb.com/types-of-concrete-joints-845022

  9. #9
    Thailand Expat Pragmatic's Avatar
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    Quote Originally Posted by Pragmatic View Post
    And why do they spray a road with emulsion and then put sand on it before tarmacking?
    I'm gonna answer my own question now I've had time to think. As I said the bitumen emulsion put down before resurfacing is to bond the old surface to the new. Being as the men have to walk on the emulsion ones boots become coated in the new tarmac and it's got to be cleaned off and it's a bastard. What we do in a sensible country is to cover ones boots in diesel to stop this. The Thais use sand to do the same but they put it on the whole road to stop the delivery tyres lorries sticking. In doing so the new surface does not bond. Best I could come up with.

  10. #10
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    ^ Prag they are always on to a loser on the little village roads, you see them when they crack up and there is barely 1 inch of tarmac, the other 3 inches surfaces in the headmans bank account.

  11. #11
    Thailand Expat Pragmatic's Avatar
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    Quote Originally Posted by Pragmatic View Post
    Why do Thais put un-compacted sand down before laying wet concrete?
    Any replies as to why?

  12. #12
    I am not a cat
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    Thank you all. As said, i think there is little the collective wisdom of the board cannot shed light on. Looks like it is a fucked up imitation of a real job. As said, the cut is only half a cm deep, so it does not go through the concrete completely, and certainly not throgh the rebar.

    I do appreciate everyones comments though, as it irked me no end.

  13. #13
    Thailand Expat Pragmatic's Avatar
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    Quote Originally Posted by nidhogg View Post
    As said, the cut is only half a cm deep, so it does not go through the concrete completely, and certainly not throgh the rebar.
    As I said I think it to be just show and there to make it look as if they've done the job correctly. They did similar to where I live. They poured concrete but poured more than the slab required. Thus they had to make it look correct. They did that by using a piece of rebar to make an indent in the concrete once it had start to set. Then in the indentation they poured bitumen to make it look like they'd done 2 separate slabs. Different gangs, different ways of doing it.

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