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Concrete Cracking and CIRIA C660

Concrete Cracking and CIRIA C660

Concrete Cracking and CIRIA C660

(OP)
Hi All

I need to design a slab on ground to Ciria C660. The slab is only 150mm thick and not for liquid retaining. Sparing everyone the queries..Ciria C660 is necessary to limit crack widths and for other reasons.

In my case, I have determined from Ciria that I need diameter 12 bars at 150mm centres. From what I have read, Ciria C660 appears to cater only for reinforcement on each face since the formula for Asmin is based on h/2. Note: my modification factors kc and k are both 1.0 and I have external restraints. Thus, Act (the area of concrete in the tensile zone) is always h/2 for the top face and h/2 for the bottom face (regardless of section thickness).

PROBLEM: There appears no way to have a single top layer of reinforcing even for a "thin" slab on ground?? if so how does one determine this from Ciria? In other words for my 150mm slab, I have a Act based on 75mm for the top AND Act based on 75mm for the bottom. Is this implying that I NEED to provide dia 12 bar at 150 centres for both faces??

QUESTIONS in relation to the 150mm slab on ground
A) Since h/2 is referred to, is ciria c660 implying that the bottom face requirement for reinforcement can be ignored if it is not visually important or not liquid retaining.??
B) Is ciria c660 implying that the quantities of steel must be added together but can be placed in the top mat (if so desired). ??
c) Is ciria c660 stating that within the top and bottom Act zones...reinforcement must always be placed, ..no matter what the application or section thickness.??


Solution for Option A does not sound right to me! as does Option B solution. Surely Option C solution is not realistically expected for thin slabs!!
Would appreciate help from people already familiar with Ciria C660 requirements (in particular section 3.3.1)

RE: Concrete Cracking and CIRIA C660

Crack control reinforcement to control drying shrinkage cracks (or early age thermal cracking as it is called in the UK), is provided on the gross section of concrete. These cracks are direct tension cracks, not flexural cracks. But not being familiar with the referenced CIRIA standard, I can't comment further. Perhaps someone from the UK will chime in.

RE: Concrete Cracking and CIRIA C660

Its been a while since I've looked at this and i would have been using BS8007 and CIRIA Report 91 (older version of C660).
It used to be that for thin slabs (<300mm) there was no bottom zone requiring min steel.

Has this changed in C660 and EC2.


RE: Concrete Cracking and CIRIA C660

As hokie66 notes, the full cross section is to be considered in determining crack control reinforcement. Your example seems to divide upper and lower reinforcement sections. Keep in mind that slabs on grade are generally restrained at the bottom, not the top. The top is free to shrink. Such differential shrinkage can lead to uncontrolled cracking.

You do not give the purpose of your design. You mentioned the slab is not for liquid confinement, so why can't you have control joints in the slab to control cracking. Further, you do not give loading information, so unless the slab is inordinately point loaded, flexural reinforcement is not likely required.

Perhaps I am not understanding your post, but it seems that you are treating the top and bottom cross sections separately. Though I'm not that familiar with C660, it would appear from Bamforth's papers on the subject, "h" is the full depth of the slab. Do a search for his papers. He has written extensively on the subject.

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