A good article on crack width & waterproof concrete...

[Alan Ambrose]

https://www.si-eng.org/post/a-journey-of-designing-self-healing-concrete#:~:text=The allowable crack width for,are harder to self heal.

 

Suggests, for the typical self-build single storey basement (say, roughly 4m deep & 0.25m walls - i.e. 'pressure gradient~=16'), the optimistic value is 0.2mm based on the old BS8007 and the new C766 and the pessimistic value is 0.15mm from EC2-3.

 

 

[saveasteading]

I have skim read  this but may try again.

In case it is of any interest here is my experience.

I have designed large, above ground,  water tanks for industry and for a hospital. I recall they were about 3m deep.

Both were built and I heard no more, so I think they worked.

The design codes at the time covered how to control the cracking. Maybe the codes have changed but it worked back then, and concrete will still behave the same way.

 Basically we used a lot of small reinforcing bars, so that the inevitable shrinkage resulted in micro-cracking.... millions of cracks that are too small to see, and water can't get through.

Did it heal? Probably.

 

These tanks could have been sealed on the inside but were not. The evaporation of water was not considered an issue, and perhaps that causes 'healing' as water soaks through. 

 

For a garage or other basement, the water is on the outside and it would be easy to tank the inside, but it might come off.

A big basement I recall doing, we tanked the outside of the wall and slab using a bentonite filled sheeting.

[Alan Ambrose]

@saveasteading Thanks, that is interesting. The info I've been given is that it's largely down to the quantity of steel. Nobody said anything about the density of the mesh - although small mesh sounds logical.

 

The bentonite filled sheeting sounds like Sika BentoShield or similar?

 

Did that all work well?

[saveasteading]

20 minutes ago, Alan Ambrose said:

it's largely down to the quantity of steel.

Yes. In a structural slab  the cross section of steel is what resists the shrinkage, and in a structural wall or slab the cross section is chosen to provide the tensile strength required.

But by using more bars of smaller diameter, at close centres, adding up to the same sort of c/s ,  the cracks can be made smaller ( in both directions)

This was a very long time ago but I think I selected small mesh and tied lots of small bars to it, for ease of construction.

If it works, then the tiny cracks are not visible. I expect they do 'heal' as a bonus.

 

I probably specified  concrete with smallish aggregate to ensure it flowed around all those bars. 

I hope the concrete was poured without extra water added on site, or it would defeat the object.

[SteamyTea]

Not read the article (yet), but in the composite plastics world, fibres (usually glass, but can be organic or metallic) are added to reduce cracking.

These fibres can be of varying length but generally no more than 12mm, but they are very thin, around 10 micron (10, 1 thousands of a millimetre, or 100^thmm), but are generally bundle into 200 fibre 'lots'. This a more to do with the manufacturing of the fibres than any mechanical calculations.

These are the same fibres that are used in CSM (chopped strand mat) but shorter, and usually without the chemical binder to hold them together.

There has been attempts to injection mould polymers with these fibres in, but because of the flow characteristics, it has never been totally successful, so tends to be used in DMC (dough moulding compounds) and hand layup.

They work by spreading out on the mould surface (or on the gelcoat) and fill small gaps and can allow for a 'resin rich' mixture which also helps fill voids.

Once curing has taken place, they are locked in place, but because of the random positioning, they act as a good bridge between the exposed surface and the main reinforcement (usually CSM, weave or a core material).

 

The same fibres are often used in concrete mixes to reduce micro cracking, but not excessive cracking caused by shrinkage (usually an incorrect concrete mix).  I have never checked, but I suspect they work in the same mechanical manner i.e. helping to fill voids and micro-reinforcement.

 

Short fibres/filaments/wires/rods are not there to make a non structural material a structural material, if the was the case, ropes and multi-strand cables would be made from very short lengths.  If you look at an old suspension bridge, or boat's mast stay, you may notice that some of the wires in the bundle have come loose, but the bridge, or mast, is still in place.  This is because there is a bit of a safety factor built in on the overall CSA (cross sectional area) tensile strength, but mainly because the side of the strands are adjacent to other strands, this causes resistance to movement (friction) which adds to the overall strength (and changes the failure characteristics).  It is why they are twisted together.

 

Because engineers (not the type of engineer that fixes your washing machine) are clever people, very small fibres can be made into sealed tubes, filled with a polymer.  If there is enough movement, the fibre tube brakes, releasing the polymer, with then leaks out and fills the micro-crack, then the polymer hardens, restoring the mechanical properties of the material.

(expletive deleted)ing clever that is. 

Edited July 26 by SteamyTea

[SteamyTea]

Just noticed that @Gus Potter has awarded me the equivalent of a gold star that McDonald's employees get.

Gus, tell us a bit more about secondary reinforcing, reinforced concrete.

My interest in concrete is just a hobby for me to help improve my chemistry, not my mechanical engineering skills (very long time ago I studied it).

[Alan Ambrose]

>>> a gold star 

 

The buildhub olympics, yeah...

[Gus Potter]

6 hours ago, Alan Ambrose said:

Suggests,

The following I hope gives you a flavour of the design but also the liability that lies with the designer and that, I hope, will help folk on BH get their head around some of the SE type fee costs.

 

Well done @Alan Ambrose for digging out this article. The author takes one of the CPD courses on Eurocode concrete design I have been on and interrogates it in a bit more detail.. and makes a good job of it too!

 

For me this is part of my day job so am familiar with the terms / design calcs etc and how the Euro codes go into more detail and the theory behind it.. such as restraint conditions, ageing, restraint at slab ends.. a long list.

 

But for all on build hub for the critical thing to take away from this is the bit at the end of the article copied below.

 

"Firstly, the design need to be realistic. Blindly throwing reinforcement at it can do more harm than good. Good detailing is particularly important for complex geometries. What’s more important is to try and have simple geometries in the first place".

 

In other words you can have a fab house but keep the underlying structure as simple / stupid and buildable as you can. That drives down cost, reduces risk and lets you spend your cash on the thing you get to see and enjoy.

 

15 minutes ago, SteamyTea said:

Just noticed that @Gus Potter has awarded me the equivalent of a gold star that McDonald's employees get.

Hiya @SteamyTea If I could I would give you a double cup award.. I've learnt loads from you..

 

For all there are some folk like Steamy and to do a name check @MikeSharp01 who are fantastic educators.

24 minutes ago, SteamyTea said:

Gus, tell us a bit more about secondary reinforcing, reinforced concrete.

I'll give it a go and try and cover some of the concepts for folk on BH. but excuse the spelling and grammer please!

 

For all the below applies to lots of other things you might do not just basements.

 

Ok take a simple concrete reinforced beam spanning between two walls loaded from above. The bottom of the beam is in tension and the top in compression. Concrete is quite "stiff" which means that under compression is does not compress much. The bottom of the beam is in tension. Concrete is not so good at resisting tension so we introduce steel rebar in the bottom of the beam which has lots of tensile strength and now we have a reinforced concrete beam.

 

But steel is quite stretchy compared with concrete. For it to work the steel rebar needs to be bonded to the concrete at the bottom of the beam. For the beam not to fall down the tensile forces in the top of the beam need to balance the tensile force in the bottom steel rebar. But to achieve the balance the steel needs to stretch first thus we get cracks in the bottom of the beam. The same principles applies to basement walls. Call this behavoir primary cracking. To limit cracking we add more rebar area so the steel does not stretch as much.

 

In other words when we design reinforced concrete we make sure we have enough rebar so it does not fall down.. then check that we are limiting the amount the steel stretches by so we limit cracking.

 

Now it gets complicated.

 

In a reinforced concrete wall we have a large area of concrete that as @Alan Ambrose article points out undergoes a number of "experiences".

 

Concrete goes through a number of phases when you pour it in to say a basement wall.

 

Broadly speaking cement (part of the component in concrete) needs to start getting it's act together within 4 -8 hours ( temperature dependeant) and this is covered in the BS standard for example. This means that the chemical transition gets underway. This causes a lot of heat which makes the concrete expand. After say 24 -48 hours the concrete starts to "bind".. the aggregates, cement any additives and we get what is called plastic shrinckage.. which can be quite a lot.. in other words the concrete is still in quite an excited state for a better word.

 

After a bit of time the concrete starts to dry out and we get what we call drying shinkage. On a raft slab.. say on an EPS raft slab we also get funny behavoir at the slab corners call "curling".. which is basically all of the above having a laugh with us.

 

@SteamyTea The secondary reinforcement is primarily intended to deal with the behavoir of the concrete during the curing and drying out process.

 

It is a bit of a guess but if you look after your concrete for say a month after it's poured you can do a lot to help yourself in terms of getting what you have paid for. Quality of workmanship is key.

 

I've copied below a bit out of one of my specifications that I wrote for a self builder for a floor slab. The idea of this is to provide simple instructions for what you can do on site to address what is a very complex problem.

 

 

Designer liability:

 

From the above you can see that this is fraught with liability if you are say an SE / designer like me. My PI insurer goes nuts and wants to know exactly how much liability I'm taking on. @Alan Ambrose article deals with Eurocode design which if applied well results in lean and cost effective design.. but it is predicated on all the Contractors etc doing their bit very well.. the construction industry in the Uk is not often up to this standard. So when it goes wrong I'm for example the first easy target.

 

Now folks if want to pay me for taking on that liability then I'm fine with that.. but the only way I can do that is if I come to site often to make sure that the contractor is doing "EXACTLY WHAT I REQUIRE" no if's or buts!

 

However while that sounds great you will need to pay for an experienced contractor who is used to having someone like me holding thier feet to the fire.

 

Self building is about finding the right compromise.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

6 hours ago, Gus Potter said:

I've learnt loads from you

Thank you.