ACI Crack Width Methods

ACI CRACK WIDTH METHODS

FOR WATER TIGHT STRUCTURES

1

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

1-ACI 224R-01

Some notes:-

  • 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

1-Appearance

2-Corrosion

3-Water tightness

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

image001

image002

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

image003

No distinction for interior/exterior exposure

For beams & one-way slabs:

image004

fs=0.6fy

Not for aggressive/water tight structures

C) EUROPEAN CODES


2-ACI 350-01

Same concept like ACI 224R-01


3-ACI 350-06

2 types of exposure:-

i)Normal

image005

ii)Severe

image006

image007

can be taken = 25

image009

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

image010

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)


image011

One comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s