Crack Control

11

CRACK CONTROL REQUIREMENTS CRACK CONTROL REQUIREMENTS OF ACI 318OF ACI 318

S. K. GhoshS. K. GhoshS. K. Ghosh Associates Inc.S. K. Ghosh Associates Inc.

Palatine, ILPalatine, IL

www.skghoshassociates.comwww.skghoshassociates.com

22

Crack ControlCrack Control

Three reasons for limiting crack widths Three reasons for limiting crack widths in concretein concrete

Appearance (exposed to view)Appearance (exposed to view) Corrosion protection (exposed to Corrosion protection (exposed to

aggressive environments)aggressive environments) Water tightness (marine / sanitary Water tightness (marine / sanitary

structures)structures)

33

Crack ControlCrack Control Appearance Appearance limit crack widths on surface limit crack widths on surface

locate reinforcement as close as possible to locate reinforcement as close as possible to surface.surface.

Corrosion protection Corrosion protection use better quality use better quality concrete, increase thickness of cover.concrete, increase thickness of cover.

Water tightness Water tightness -- severe limits on crack width, severe limits on crack width, applicable only to specialty structures.applicable only to specialty structures.

A single provision not sufficient to address control A single provision not sufficient to address control of cracking for all three reasons.of cracking for all three reasons.

44

Correlation Between Surface Crack Correlation Between Surface Crack Width and CoverWidth and Cover

55

Crack Control Crack Control ACI 318ACI 318

From 1971 through 1995, ACI 318 From 1971 through 1995, ACI 318 restrictions concerning restrictions concerning distribution of tension distribution of tension reinforcement were based on reinforcement were based on limiting width of surface cracks.limiting width of surface cracks.

66

Crack Control Crack Control ACI 318ACI 318--9595For fFor fyy >> 40 ksi40 ksi

77

Crack Control Crack Control ACI 318ACI 318--9595

88


Equivalent no. of #9 bars = (3 x 1.00 + 2x 0.79) / 1.00 = Equivalent no. of #9 bars = (3 x 1.00 + 2x 0.79) / 1.00 = 4.584.58

A = 2dA = 2dcgcg x b / 4.58x b / 4.58ddcgcg = distance from extreme tension fiber to center of = distance from extreme tension fiber to center of

gravity of steelgravity of steel

Examples for Examples for determining Adetermining A

Bars 1, 2 and 3 are #9 Bars 1, 2 and 3 are #9 (area = 1.00 in2)(area = 1.00 in2)

Bar 4 and 5 are #8 Bar 4 and 5 are #8 (area = 0.79 in2)(area = 0.79 in2)

99


The numerical limitations of z = 175 The numerical limitations of z = 175 and 145 kips/inch for interior and and 145 kips/inch for interior and exterior exposure, respectively, exterior exposure, respectively, correspond to limiting crack correspond to limiting crack widths of 0.016 inch and 0.013 widths of 0.016 inch and 0.013 inch.inch.

1010


Method essentially encouraged Method essentially encouraged reduction of reinforcement cover, which reduction of reinforcement cover, which could be detrimental to corrosion could be detrimental to corrosion protection.protection.

Method severely penalized covers Method severely penalized covers exceeding 2 inches by reducing spacing exceeding 2 inches by reducing spacing or service load stress of reinforcementor service load stress of reinforcement

1111

Crack Control Crack Control ACI 318ACI 318--9595 Corrosion of reinforcement weakly related to surface Corrosion of reinforcement weakly related to surface

crack width limits of 0.013 to 0.016 inch.crack width limits of 0.013 to 0.016 inch.

Actual crack widths in structure highly variable. A Actual crack widths in structure highly variable. A scatter on the order of scatter on the order of ++ 50 percent is observed.50 percent is observed.

New approach intended to control surface cracks to a New approach intended to control surface cracks to a width that is generally acceptable in practice, but may width that is generally acceptable in practice, but may vary widely in a given structure.vary widely in a given structure.

Crack widths in the field cannot be predicted using Crack widths in the field cannot be predicted using ACI 318ACI 318--99 Eq. (1099 Eq. (10--5).5).

1212

ACI 318ACI 318--99, 0299, 0210.6.4 10.6.4 Crack ControlCrack Control

( )sc

s

f/3612s

c5.2f

540s

(10(10--5)5)

1313

ACI 318ACI 318--99, 02 99, 02 10.0 10.0 Crack Control Crack Control

s = centers = center--toto--center spacing of flexural center spacing of flexural tension reinforcement nearest to the tension reinforcement nearest to the extreme tension face, in. (where there extreme tension face, in. (where there is only one bar or wire nearest to the is only one bar or wire nearest to the extreme tension face, s is the width of extreme tension face, s is the width of the extreme tension face.)the extreme tension face.)

1414

ACI 318ACI 318--99, 02 99, 02 10.0 10.0 Crack Control Crack Control

ffss = calculated stress in reinforcement at = calculated stress in reinforcement at service loads, ksiservice loads, ksi= M / (= M / (jd jd x Ax Ass) )

-- ffss may be taken as 0.6fymay be taken as 0.6fycccc = clear cover from the nearest surface = clear cover from the nearest surface

in tension to the surface of the flexural in tension to the surface of the flexural tension reinforcement, in.tension reinforcement, in.

1515

Comparison of Max. Reinforcement Comparison of Max. Reinforcement Spacing for Crack ControlSpacing for Crack Control

318318--05 05 Crack Control Crack Control

Due to changes in both the load Due to changes in both the load

combinations and combinations and --factors in ACI 318factors in ACI 318--02, 02,

the servicethe service--level stress in reinforcement level stress in reinforcement

has increased. For consistency, the has increased. For consistency, the

default service stress level is increased.default service stress level is increased.

1717

318318--05 05 Crack Control Crack Control

s = 15 s = 15 (( 40,00040,000ffss

) ) -- 2.5 c2.5 cc c (10 (10 -- 4)4)

s s 12 12 ((ffss

))40,00040,000

Default value for Default value for ffss = (2/3) f= (2/3) fyy

1818

ACI 318ACI 318--02 02 7.7.5 7.7.5 Corrosive EnvironmentsCorrosive Environments

In corrosive environments or other In corrosive environments or other severe exposure conditions, amount severe exposure conditions, amount of concrete protection shall be of concrete protection shall be suitably increased, and denseness andsuitably increased, and denseness andnonporositynonporosity of protecting concrete of protecting concrete shall be considered, or other shall be considered, or other protection shall be providedprotection shall be provided

1919

ACI 318ACI 318--02 02 7.7.5.1 7.7.5.1 Corrosive EnvironmentsCorrosive Environments

For prestressed concrete members exposed to For prestressed concrete members exposed to corrosive environments or other severe exposure corrosive environments or other severe exposure conditions, and which are classified as Class T or conditions, and which are classified as Class T or C in 18.3.3, minimum cover to the prestressed C in 18.3.3, minimum cover to the prestressed reinforcement shall be increased 50 percent. This reinforcement shall be increased 50 percent. This requirement shall be permitted to be waived if the requirement shall be permitted to be waived if the prepre--compressed tensile zone is not in tension compressed tensile zone is not in tension under sustained loadsunder sustained loads

2020

ACI 318ACI 318--02 02 7.12 7.12 Shrinkage and Temperature Shrinkage and Temperature

ReinforcementReinforcement

... required at right angle to the ... required at right angle to the principal reinforcement to principal reinforcement to minimize cracking ...minimize cracking ...

2121

ACI 318ACI 318--020210.5 10.5 Min. Reinforcement Min. Reinforcement

for Flexural Membersfor Flexural Members

At every section of flexural member where At every section of flexural member where

tensile reinforcement is required by tensile reinforcement is required by

analysis,analysis,

dbf

200

dbf

f3A

wy

wy

'c

min,s

=

2222

ACI 318ACI 318--020210.5 10.5 Min. Reinforcement Min. Reinforcement

for Flexural Membersfor Flexural Members

The minimum reinforcement requirement The minimum reinforcement requirement need not apply if every section the need not apply if every section the area of tensile reinforcement provided area of tensile reinforcement provided is at least oneis at least one--third that required by third that required by analysis.analysis.

2323

ACI 318ACI 318--020218.8 18.8 Limits in Reinforcement of Limits in Reinforcement of

Flexural MembersFlexural Members

Total reinforcement must satisfyTotal reinforcement must satisfy

crn M2.1M

2424

ACI 318ACI 318--020210.9 10.9 Limits for Reinforcement of Limits for Reinforcement of

Compression MembersCompression Members

Area of longitudinal reinforcement Area of longitudinal reinforcement for nonfor non--composite compression composite compression members shall be not less than members shall be not less than 0.01... times gross area A0.01... times gross area Agg of of sectionsection

2525

10.6.7 10.6.7 Skin ReinforcementSkin Reinforcement

Width of cracks in the web may exceed crack widths at Width of cracks in the web may exceed crack widths at level of flexural tension reinforcement.level of flexural tension reinforcement.

2626

10.6.7 10.6.7 Skin Reinforcement Skin Reinforcement 89 89

If depth of web exceeds 36 in.If depth of web exceeds 36 in.

-- AAsk sk / ft /side face / ft /side face 0.012 (d0.012 (d--30) in.30) in.2 2 / ft/ ft

-- s s d/6d/6

-- s s 12 in.12 in.

2727


2828


Changed Changed

From:From: AAsk sk / ft /side face / ft /side face 0.012 (d0.012 (d--30) 30)

To:To: s s 1000 1000 AAbb / (d/ (d--30)30)

AAbb = Area of individual bar or wire, in.= Area of individual bar or wire, in.22

Above expressions are mathematically identicalAbove expressions are mathematically identical

2929


Changes made to simplify the crack Changes made to simplify the crack

control provisions for skin reinforcement control provisions for skin reinforcement

and make these provisions consistent and make these provisions consistent

with those required for flexural tension with those required for flexural tension

reinforcement.reinforcement.

3030

10.6.7 10.6.7 Skin ReinforcementSkin Reinforcement

In beams and joists with h > 36 in.In beams and joists with h > 36 in.

Skin reinforcement over h/2 from tension faceSkin reinforcement over h/2 from tension face

Spacing per 10.6.4 (cSpacing per 10.6.4 (ccc to side face)to side face)

May include skin reinforcement contribution to May include skin reinforcement contribution to MMnnbased on compatibility analysis based on compatibility analysis

Bar size not specifiedBar size not specified

3131

Fig. R10.6.7 Fig. R10.6.7 Skin ReinforcementSkin Reinforcement

3232

R10.6.7 R10.6.7 Skin ReinforcementSkin Reinforcement

The size of the skin reinforcement is not specified; The size of the skin reinforcement is not specified;

research has indicated that the spacing rather than research has indicated that the spacing rather than

bar size is of primary importance. Bar sizes No. 3 to bar size is of primary importance. Bar sizes No. 3 to

No. 5 (or welded wire reinforcement with a minimum No. 5 (or welded wire reinforcement with a minimum

area of 0.1 in.area of 0.1 in.22 per foot of depth) are typically per foot of depth) are typically

provided.provided.

Crack Control

Documents

Transcript of Crack Control