Minimum Reinfocement Criteria For Crack Control

[abdulqadeer29]

The minimum amount and spacing of reinforcement to be used in structural floors, roof slabs, and walls for control of temperature and shrinkage cracking is given in ACI 318 or in ACI 350R. The minimum-reinforcement percentage, which is between 0.18 and 0.20%, does not normally control cracks to within generally acceptable design limits. To control cracks to a more acceptable level, the percentage requirement needs to exceed about 0.60% (REFRENCE ACI COMMITE REPORT 224R-01)

 

 

 

 

So according to above statement , should we follow 0.60%, to be on more safe side??

 

[Ayesha]

Good Question. Depends on what the situation is. You should follow the above quoted criteria where cracking would be a failure criteria in terms of serviceability or strength. For example, a leaking water tank  would not serve its purpose and would be a serviceability limit state. So, in such cases, yes use a 0.6%, but for flexural members (slabs, footings are beams), cracking is expected for balanced and under reinforced sections and hence such a limit is not required.

[Badar (BAZ)]

The statement taken out from ACI document only covers shrinkage cracks in reinforced concrete structures. Hence, it is regarding serviceability limit states. 

In order to fully understand it, one should know what they mean by 'acceptable level', or 'acceptable design limits'. There must be a paper regarding that, which needs to be studied.

You should check latest ACI code; I think.  they have not included this provision in the latest code. They are continuing older provision of 0.18/0.2 %. It look like as if they have not taken the recommendation of this committe seriously.

 

It is a confusion.

[Khawaja Talha]

 

The minimum amount and spacing of reinforcement to be used in structural floors, roof slabs, and walls for control of temperature and shrinkage cracking is given in ACI 318 or in ACI 350R. The minimum-reinforcement percentage, which is between 0.18 and 0.20%, does not normally control cracks to within generally acceptable design limits. To control cracks to a more acceptable level, the percentage requirement needs to exceed about 0.60% (REFRENCE ACI COMMITE REPORT 224R-01)

 

So according to above statement , should we follow 0.60%, to be on more safe side??

 

According to ACI 318-08 in the commentary of section 7.12.1.2 its said

"Where structural walls or columns provide significant

restraint to shrinkage and temperature movements, the

restrain of volume changes causes tension in slabs, as well as

displacements, shear forces, and flexural moments in columns

or walls. In these cases, it may be necessary to increase the

amount of slab reinforcement required by 7.12.2.1 due to the

shrinkage and thermal effects in both principal directions (see

References 7.7 and 7.16)."

So according to these two references we are allowed to increase the minimum steel ratio depending upon the restraint of the structure. Now the question is to which extent are we allowed to increase this ratio to which you quoted ACI COMMITE REPORT 224R-01.

What reference 7.16  ( Gilbert, R. I., “Shrinkage Cracking in Fully Restrained Concrete Members,” ACI Structural Journal, V. 89, No. 2, Mar.-Apr. 1992, pp. 141-149.)* quoted by ACI R7.12.1.2 suggests is that minimum steel ratio should be greater than 0.45%. For through understanding please download the paper given below and read it but for brevity let me quote the statement from the discussion of this paper

"For a maximum design crack width of 0.3 mm (as is commonly specified in codes of practice), it appears that for the restrained slabs tested in this study a reinforcement area of greater than about 270 mm2 (ρ = 0.0045) would be satisfactory"

 

* you can also download it from https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CC4QFjAA&url=http%3A%2F%2Fwww.scribd.com%2Fdoc%2F48305786%2Fshrinkage-cracking-in-restrained-reinforced-concrete-members&ei=MtooUtmAIpDLswb-zIHgAQ&usg=AFQjCNFdUzeJmxmkPw3K5S1_oXde_bak4g&sig2=DXzeQ2UG9dwzrB81bTZ03g

48305786-shrinkage-cracking-in-restrained-reinforced-concrete-members.pdf

[zenith.international]

In our opinion, the observation made below is correct and thus a designer is allowed to increase the minimum steel ratio depending upon the restraint of the structure. 

"Where structural walls or columns provide significant restraint to shrinkage and temperature movements, the restrain of volume changes causes tension in slabs, as well as displacements, shear forces, and flexural moments in columns or walls. In these cases, it may be necessary to increase the amount of slab reinforcement required by 7.12.2.1 due to the shrinkage and thermal effects in both principal directions (see References 7.7 and 7.16)."

 

Thus for crack control (as serviceability limit) one can use the following

 "For a maximum design crack width of 0.3 mm (as is commonly specified in codes of practice), it appears that for the restrained slabs tested in this study a reinforcement area of greater than about 270 mm2 (ρ = 0.0045) would be satisfactory"

 

Best wishes

Zenith International
 

[Ghulam Abbas Gazer]

I have lot of experience construction of poultry shed ,for slab commonly structural engineer design 4" thick & steel reinforcement 6mm dia bars @ 6" c/c(it will be 0.18 to 0.20 %) i check that minimum cracks occur in the slab but they cannot cause of any mishap a poultry shed normally having 22000 sft covered area ,problem is that if they follow the minimum limits 0.60 % it will be highly effected on cost & client move to TT slab or prefabricated structure.It will be a loss for contractor & construction labor.             what will be solution of this problem.

[zenith.international]

Ghulam Abbas Gazer:

It is recognized that you have lot of experience in construction of poultry shed slabs and that commonly structural engineer design 4" thick slabs with steel reinforcement 6 mm dia bars @ 6" c/c that comes to 0.18 to 0.20 % reinforcement.

 

It is not clear why the minimum limits 0.60 %  is followed ?

Your other part of the question is not clear.  Please clarify.

 

Best wishes

Zenith International

[Syed Umair Haider]

It seems that ACI is clear in its provision and usage of 0.6% rather than 0.2% reinforcing ratio for temperature & shrinkage crack/stress control could not be justified for general structures and is only applicable in cases where crack width is a design consideration.

 

As ACI -350 table 7.12.2.1 recommends only for "structures subjected to environment or required to be liquid tight" a reinforcing ratio range from .003 to .006 for Grade 40 & .003 to .005 for Grade 60 steel as a function of length between movement joints from 20' to 40'.(see ACI 350 7.12.2.1).

Thus the extreme ratios ".005" & ".006" represents the maximum temp & shrinkage reinf ratio for "structures subjected to environment or required to be liquid tight" where movement joint is not provided.This maximum ratio is mentioned in ACI 224 and is supposed to be followed when crack width is a design consideration for eg in structures as defined above.(note here structures subjected to environment could simply refer to outdoor structures in accordance with CIRIA technical note 107 section

 

2) On the other hand as long as general structures is concerned (say building structures) crack width is not a design consideration in accordance with same code (ACI 318) and here provision of minimum reinf (0.18-0.2%) intends to limit shrinkage & temperature stresses rather than crack widths within tolerable limits (see ACI 318-11 7.12.1) which is claimed to be observed for many years satisfactorily controlled with these ratios.

 

As long as design consideration of shrinkage & temperature effects is concerned in general structures, we include the effects of shrinkage & temp cracks indirectly in serviceability limit states as

 

1, In the form of additional vertical deflection due to creep & shrinkage of structural members (see ACI 435R-95 table 2.1).

 

2, Effective stiffness (see ACI 318-11 10.10.4) to be used in the analysis of lateral deflection accommodates the stiffness reduction due to shrinkage & temp cracking.

 

3, ACI 318 also recommends to analyze for the severity of stresses arising from temp & shrinkage effects (see ACI 318-11 8.2.4 & 9.2.3)

 

Hence, usage of 0.18-0.2% reinf and 0.3-0.6% reinf is justified for general and special structures respectively.

[Hasan Tariq]

If we have tank size of 16x9x4 ft and we have to provide the steel in horizontal direction which is temperature shrinkage steel .Do we go for 0.003 or 0.006 of the gross area .?

[Syed Umair Haider]

.003

[Muhammad Imran Zafar]

Well it depends on a lot of factors, however limit prescribed by ACI committee is fine. In order to get a good understanding of crack control I would like to refer Advanced reinforced concrete design by P.C. Verghese. Please go through the page 30 topic shrinkage and thermal cracking where Author has derived the minimum percentage of steel to control shrinkage cracks. He also recommends 0.003 percent. So on both faces it comes to 0.006 which is exactly equal to recommendation of ACI 224 R-01

 

http://books.google.com.pk/books?id=uXqXP6pEEsUC&pg=PR3&lpg=PR3&dq=advanced+reinforced+concrete+design+by+p.c.+varghese+pdf&source=bl&ots=hqxOR_udqx&sig=Fpn3F0WMD8UVONN1Sax43r5C5vU&hl=en&sa=X&ei=Zc9gVLOeI8yu7AaD0IGgCA&ved=0CB8Q6AEwATgK#v=onepage&q=advanced%20reinforced%20concrete%20design%20by%20p.c.%20varghese%20pdf&f=false

[UmarMakhzumi]

So looks like today, I came across the same problem that is being discussed here. I had to provide a design criteria for a buried concrete pit. I will summarize my findings below for the benefit of everyone. This applies to structural members that are subjected to environmental exposure conditions or that are required to be liquid tight. 

1) The first step is to calculate flexural demand in the walls of concrete pit/ water tank based on all possible conditions. For the case of buried concrete pit, it included, empty condition (no fluid in the pit) , operating condition (full of liquid), test condition (no backfill  around the pit and it is full of liquid) etc. Buoyancy checks should also be performed.

2) Compare required flexural reinforcement against minimum reinforcement ratio = 0.006 per ACI 350, Table 7.12.2.1 & ACI 224 , Section 3.5 and provide whichever is the maximum. The ratios provided are basically temperature and shrinkage reinforcement ratios based on gross section so provide half of the reinforcement at each face.

3) Satisfy maximum crack width of water-retaining structure = 0.10 mm, ACI 224R-01 Table 4.1 based on the reinforcement already provided. If the reinforcement is inadequate, increase the reinforcement till this requirement is met. To meet this requirement, smaller bars should be used with close spacing.

Now a few comments on the the excellent discussion above.

@Khawaja Talha post above is applicable for all normal cases where there is a restraint to shrinkage and temperature movements only. If you have a condition like that, you need to provide 0.45% reinforcement ratio in your slabs. Example of a situation where this would be applicable will be a structure where movement or expansion joints haven't been provided at industry standard spacing. But if you want to meet liquid tight start with 0.6% as a minimum and work your way as suggested above.

Other posts above explain the same things in a slightly different manner but all good.

Thank you

 

[shahidthanvi]

On 8/13/2019 at 3:27 PM, UmarMakhzumi said:

So looks like today, I came across the same problem that is being discussed here. I had to provide a design criteria for a buried concrete pit. I will summarize my findings below for the benefit of everyone. This applies to structural members that are subjected to environmental exposure conditions or that are required to be liquid tight. 

1) The first step is to calculate flexural demand in the walls of concrete pit/ water tank based on all possible conditions. For the case of buried concrete pit, it included, empty condition (no fluid in the pit) , operating condition (full of liquid), test condition (no backfill  around the pit and it is full of liquid) etc. Buoyancy checks should also be performed.

2) Compare required flexural reinforcement against minimum reinforcement ratio = 0.006 per ACI 350, Table 7.12.2.1 & ACI 224 , Section 3.5 and provide whichever is the maximum. The ratios provided are basically temperature and shrinkage reinforcement ratios based on gross section so provide half of the reinforcement at each face.

3) Satisfy maximum crack width of water-retaining structure = 0.10 mm, ACI 224R-01 Table 4.1 based on the reinforcement already provided. If the reinforcement is inadequate, increase the reinforcement till this requirement is met. To meet this requirement, smaller bars should be used with close spacing.

Now a few comments on the the excellent discussion above.

@Khawaja Talha post above is applicable for all normal cases where there is a restraint to shrinkage and temperature movements only. If you have a condition like that, you need to provide 0.45% reinforcement ratio in your slabs. Example of a situation where this would be applicable will be a structure where movement or expansion joints haven't been provided at industry standard spacing. But if you want to meet liquid tight start with 0.6% as a minimum and work your way as suggested above.

Other posts above explain the same things in a slightly different manner but all good.

Thank you

 

excellent discussion above and this is what I usually practice for buried RC water tank. Just to add to Mr syed umair excellent discussion ACI 350-20 is now released and chapter 7 is shifted to chapter 12 where the min reinf criteria is also modified to min, max restraint instead of spacing of movement joints etc. Regarding Er Makhzumi explanation, I would add one imp point that ACI 224, althu mentioned 0.1mm crack width for liquid tight strs, however ACI 224 is not applicable to environ.. engg strs as mentioned in its chapter 04, "instead recommendations fo ACI 350 are followed, fs CHECK in steel, reinf spacing and dia etc. Also I found load applied for shrinkage stresses in some advanced models from high level consultts althu i don't have full knowledge how to calculate that load, any feed back for it would be appreciated