ACI CRACK WIDTH METHODS
FOR WATER TIGHT STRUCTURES
Crack width is a complex and tough topic. Most people still use 20 years old method defined in ACI 318-95. The situation becomes more complex if axial tension force and moment is combined. One of the examples is large water tanks above ground. This tutorial aims at explaining details and methods in different ACI documents. Latest method defined in ACI 350-06 should be used. Given the variability and non-linear behaviour in long-term deflection and crack widths, it is NOT NEEDED to go for detailed sophisticated calculations for these effects. You can imagine this as calculating something non-linear (crack widths or long-term deflection) from linear-elastic analysis. You have to have some approximations for that. No matter how detailed are your calculations, you still can’t predict for certain the long-term deflection and crack widths.
Three ACI documents for crack width; ACI 224R-01, ACI 350-01 & ACI 350-06
- Table 4.1 is based on Nawy findings.
- The table is just a general guide line.
- The table gives w=0.004″ or 0.10mm for water retaining structures.
- It is expected that portion of cracks will exceed these values by a significant amount.
- No relationship between level of cracking & corrosion in long-term.
- More cover can be used even if it yields larger crack width, against corrosion.
- ACI methods deal only with conventional concrete for crack width.
- Crack width is directly proportional to dia of bar & fs and inversely to area of steel.
- Three reasons for limiting crack widths
There are three methods mentioned in this document
A) ACI 318-95
Statistical method of Gergely & Lutz 1968
Covers up to 2.5″ only
z in any units
For two-way slabs see section 4.3 of ACI 224R-01
For shallow beams/thick one-way slabs: (w in inches)
Thick means L/D = 15-20
d used here will be distance to the center of bottom bar nearest to tension face.
ß=1.25 to 1.35 if cc≥1″
ACI 318-95 section 10.6 says use ß=1.20 & fs=0.6fy
ACI 340R has design aids for z
ACI 318-98 & earlier max z=175 kip/in for interior exposure based on 0.41mm probable crack width(0.016″)
ACI 318 max z=145 kip/in for exterior exposure based on 0.33mm probable crack width(0.013″)
B) ACI 318-99
No distinction for interior/exterior exposure
For beams & one-way slabs:
Not for aggressive/water tight structures
C) EUROPEAN CODES
Same concept like ACI 224R-01
2 types of exposure:-
can be taken = 25
where c is at service load
ß can be taken = 1.20 for h≥16″
& 1.35 for h<16″
where appearance is of concern & cover exceeds 3″, also check equation 10-7
Exposure defined as
Previous codes (ACI 350-01) puts following limits on z:
Normal exposure: z=115 kip/in (w=0.010″ or 0.254mm)
Severe exposure: z= 95 kip/in (w=0.009″ or 0.229mm)