www.sefindia.org

[swati_mayekar]

Hello All,


I need your help in calculating crack width as per IS 3370 (Part 2): 2009.

In annex A of said IS code, I am not getting how to compute estimated shrinkage strain, estimated total thermal contraction after peak temperature due to heat of hydration.

Also I am confused about the term 'nb' in formula of spacing. Which steel bars are we supposed to consider for this formula? Bars from both the faces or bars on single face?

The same clause A 1.4 gives formula of crack width for walls in normal climatic condition.
It is implied that the formula cannot be used for walls in abnormal climatic conditions.
In the entire code nowhere it is mentioned about walls in abnormal climatic conditions.
So can we say, the formula, { wmax = Smax X (alpha/2) X (T1 + T2) } holds good for walls in abnormal climatic conditions?
Can you all please guide me about when to consider T2 and when not to consider the same?

The formula,  { wmax = Smax X (alpha/2) X (T1 + T2) }, uses  coefficient of thermal expansion, then why coefficient of contraction is being discussed in the Annex A of above code?

Please solve my doubt.
Thank you.

[vikram.jeet]

IS 3370 part 2 (2009) - Annexure A

Clause on CRACK WIDTH DUE TO TEMPERATURE  &  MOISTURE
(MINIMUM REINFORCEMENT CRACK SPACING AND CRACK WIDTH IN RELATION TO TEMP  AND MOISTURE EFFECTS IN THIN CONC SECTIONS )

FOR LONG CONTINUOUS WALLS
FOR FLOOR SLABS


Regarding queries:

Estimated  shrinkage plus contraction strains due to fall of Hydration  temp ,  E = (width of crack )/ (SPACING of cracks )

SPACING of cracks Smax can be calculated = (Fct/Fb)* (BAR dia/2p)

Fct - Tensile strength of conc
Fb  - Average bond strength of conc
p  - steel area / Gross section conc area

WIDTH of fully develop crack due to Drying  shrinkage and Heat of hydration contraction in lightly reinforced walls and slabs .
E = [ Ecs + Etc - (100*10^-6)]

Esc = Estimated shinkage Strain
Etc  = Estimated total thermal contraction after peak hydration
Value 100*10^-6  = ultimate Strain in concrete in tension  

As per Code :
For Im-mature effective  coefficient of  thermal contraction  is taken as Half that of Mature concrete due to high creep Strain

Also for walls exposed to normal climatic conditions,  the shrinkage Strain less associated creep Strain is less than ultimate Tensile straining concrete (= 100x10^-6.


Even Taking Ecs = 100x10^-6
E = [  Ecs + Etc - ( 100*10^-6) ]

Hence E = Etc = crack width / crack spacing =Alpha * T1 /2
Crack width Wmax = Smax * Alpha * T1/2

Alpha - coefficient of thermal contraction  of Concrete
T1 - Fall in temp below Hydration peak temp
T1 = 30 degrees Celsius in Summer and 20 degree Celsius in Winter months

In case separation joints SPACING is more than 15 m  , Seasonal variation Of temp needs to be also included

Hence  crack width Wmax = Smax* Alpha *( T1+T2)/2


(Response  to yr queries  to be continued)

[vikram.jeet]

Response to queries:

(i) Computation of Total strain due to temp & moisture

Total strain for thin lightly reinforced walls and floor slabs
E = [ Ecs + Etc  - (ultimate conc strain in tension I.e. 100*10^-6 )]

For Imature  conc the above Formula to be used taking coefficient of thermal contraction as Half due to high creep Strain in immature conc .

For thin light reinforced walls & slabs :
Ecs will be less than ultimate conc strain in tension  , but even taking Ecs= UScs
E= Etc
Wmax = Smax*Alpha*(T1 + T2)/2
T2 can be ignored if joint spacing <15m.

(ii) Nb - nos of bars in width B
Since formulas pertain to thin members , I think  longitudinal reinforcement of both faces can be taken in wall members , but in ground slabs top face reinf . However more clarification needed from Code .

(iii) FOR abnormal climates , the temperature  T1 & T2 values can  be taken accordingly  .

(iv) T2 consideration - In case Joint spacing exceeds 15m , it needs to be consideredas clearly said in Code .

(v) FOR Immature concrete , both strains due to shrinkage as well as Hydration temp fall need to be considered in formula. However for mature concrete , shrinkage Strain is less than  ultimate Tensile conc strain and is approx taken same at the most . Hence it is only the Hydration  temp fall effect to cause strain and cracking thereby.

Just personal opinion looking to the Codal provisions . Any correction to views expressed will be appreciated.

[vikram.jeet]

ALSO PLEASE REFER EARLIER DISCUSSION ON SEFI OF 3RD SEPTEMBER 2013 .

Some members clarified certain points .

[swati_mayekar]

Thank you sir.