Thermal cracking of reinforced concrete 1

[sybie99]

Hi Guys

I have a simple question. I know that when looking at flexural cracking of concrete in a cantilever wall, the cracks will be horizontal cracks in the tension face. When checking the crack widths for cracking as a result of thermal shrinkage, which is induced by internal or external restraint, the cracks can be horizontal or vertical. What I want to know is which rebar is used to limit thermal cracking, rebar parallel or perpendicular to cracks? My understanding is that thermal shrinkage cracks will run parallel to the rebar (more or less midway between bars) which act as restraints, thus controlling crack spacing and therefore crack widths. Is my understanding correct? I am using BS8007 to calculate crack widths.

Thanks

Seb

 

[dcarr82775]

The steel running perpendicular to the crack will control the growth of the crack, the bars parallel to a crack do nothing to restrain that crack.

 

[VoyageofDiscovery]

Look into this as there are apparently are flaws in BS8007.

http://www.innovationandresearchfoc...oach_for_controlling_cracking_in_concrete.asp

HTH

VoD

 

[hokie66]

I think we need to get the terminology correct. In a cantilever retaining wall as described by Seb, the cracking which develops vertically is predominately drying shrinkage cracking, not thermal cracking. For a description of thermal cracking, read the following:

http://www.nrmca.org/aboutconcrete/cips/42p.pdf

Drying shrinkage cracks in the stem of retaining walls typically occur at about 3 metre spacing, and are due to restraint by the footing. The propagation of these cracks can be controlled to an extent by using a greater amount of horizontal reinforcement just above the footing.

 

[Ron]

Agree with hokie66...thermal cracking is rare. Drying shrinkage is common. As he noted, look at the pattern. Drying shrinkage will be characterized by regular cracking patterns, usually spaced at consistent intervals, whether in reinforced or plain concrete.

 

[slickdeals]

Agree with Ron and Hokie66. Drying shrinkage cracks are regular and repeat at constant intervals. I recently saw them even on non-shrink grouts.

 

[patswfc]

In the UK we refer to drying shrinkage cracks as early age thermal cracking.

sybie99, dcarr82775 reply is correct.
Have you got a copy of the CIRIA document 'early age thermal cracking in concrete'? If not get hold of it its very useful.

 

[VoyageofDiscovery]

Drying shrinkage is not early age thermal cracking as drying shrinkage is typically after 5-7 days.

Early age thermal cracking is not addressed quantitatively to date within the codes in North America. ACI mentions it but doesn't give too much guidance on it. In North America concrete mix is proprietary and therefore the controls for mix design are placed on the concrete suppliers and thus we provide performance specs to restrict cracking.

I believe the issue of early age thermal cracking you are referring to is dealing with some form of water retension.
If not then just dealing with drying shrinkage should be adequate.

HTH

 

[VoyageofDiscovery]

oops! retension should be retention

 

[hokie66]

patswfc,
Your explanation sounds strange, but I'll accept it. I wish we all spoke the same language. The different terminology is probably because the British treatment of concrete evolved quite separately to that in the US, and while most countries used ACI as the basis for their codes, the UK did not.

Back to the original question...my practice is to beef up the horizontal bars at the base of a concrete wall to try to deal with the restraint cracking which inevitably occurs. Seems to work.

 

[patswfc]

Hokie and VofD,

Yeah the different in terminology is a pain in the backside (ass)!

From what you have written my understanding is that what you call drying shrinkage cracking is cracking caused by restraint at the edges of the concrete pour due to previously cast elements (slabs and walls) preventing free contraction of the concrete.

I assume that you refer to thermal cracking as cracking caused by temperature gradients within the concrete section, that causes different rates of expansion and contraction within the section (internal restraint) and thus cracking. This is more of a problem in thicker sections.

In the UK we call both of these phenomena 'early age thermal cracking'. The first being caused by external restraint and the second by internal restraint.


Hokie as to your method of dealing with drying shrinkage I agree, although depending on pour sequences additional rebar may be required in other locations. E.g. See figure b on the attached from BS8007 (British code for concrete water retaining structures) which is a situation were additional vertical rebar would be required at the panel edges.

 

[hokie66]

patswfc,
Absolutely correct. I just think the "thermal" in relation to external restraint is misleading, because the shrinkage occurs in all temperature conditions. Just calling it restraint cracking would suit me.

Your illustration from BS8007 is illustrative of something that should be included in all texts (and not just for water retaining structures), as many engineers only learn about these type cracks when they see them for the first time when it is too late to do anything about it.

 

[londonkid]

there is a new CIRIA guide to this - Ciria 660. Superceeds the previous ciria report and the guidance given in BS8007. Is applicable to eurocodes also. Deals with early age cracking (internal and external restraint) and also long term shrinkage.

 

[rowingengineer]

I have attached a file that will hopefully bring everyone onto the same page.

An expert is a man who has made all the mistakes which can be made in a very narrow field