Serviceability Design of Concrete Structures in Marine Environments

Carlos E. Ospina, PhD, PE, FACIBergerABAM Inc., Houston, TX

2014 Herbert J. Roussel Lecture

Organization• Introduction• Problem Statement• Serviceability Design of Marine Concrete

Structures– Flexural Crack Control– Deflection Control

• Durability Considerations• Conclusions and Recommendations

Intro - Serviceability Design• “Strength is essential…and otherwise unimportant” (Hardy Cross)

• Objectives of serviceability design in concrete structures:– Avoid excessive deformations and vibrations under service loads– Preserve structural functionality and visual appearance– Prevent corrosion of steel reinforcement– Maintain durability

• Most common serviceability limit states (SLS) in design of concrete structures– Cracking– Deflections

Problem Statement

Challenges in Serviceability Design of Marine Concrete Structures

• Heavy loads• Equipment and operations sensitive to

structural deformations• Harsh environment• Staged construction common• Specialized design guidelines/standards scarce

Excessive Deformations

Excessive Deformations

Problem Statement

Problem Statement

Problem Statement

Problem Statement

Problem Statement

Problem Statement• Serviceability design provisions abound in

building/bridge design codes. • Specialized design standards for marine

concrete structures are scarce.• How to interpret these for the serviceability

design of marine concrete structures?• What are their limitations and shortcomings?

Problem Statement• How to control cracking and deflections in

marine concrete structures in a practical manner?

• This presentation focuses on serviceability design provisions in ACI 318 and AASHTO LRFD and how they can be used for serviceability design of marine concrete structures

Flexural Crack control in RC Members

• Cracking can be controlled directly (crack width, w) or indirectly (smax).

• Marine terminal engineers seem particularly keen at requesting direct crack width calculations.

• Two schools of thought:– Flexural cracks induced by bond between rebars and

concrete– Flexural cracks NOT induced by bond. Crack spacing

affected by clear cover.

Crack Control Provisions• Gergely-Lutz (1967) (Statistical in nature)

• z-factor (ACI 318 prior to 1999 & AASHTO LRFD prior to 2004)

< 25,000 N/mm (interior exposure) < 30,000 N/mm (exterior exposure)

• Counterintuitive. Increase in dc leads to increase in z• Test beams had very small concrete covers

3max 000011.0 Adfw cs

3 Adfz cs

(w in mm, fs in MPa)

Revised Approach (Frosch 1999)• Bond is not a major parameter controlling crack

widths. Crack spacing depends mainly on concrete cover (Broms 1965).

• Max crack spacing is twice the minimum.

Crack Control• Frosch (1999)

– Direct Control:

– Indirect Control:

22

max 22

s

dE

fw c

s

s

2

2

max 22 c

s

s df

Ews

Crack Control• ACI 318-08:

– Based on Frosch’s model. Abandoned exposure conditions.

• AASHTO LRFD 2007 (DeStefano et al, 2003):– Adopted Frosch’s but distanced from ACI 318.– Preserved exposure condition effect (ge)

sc

s fc

fs

2803005.2

280380max

cs

e df

s 2600,122

(fs in MPa), cc in mm)

Crack Control in ACI 318-08 & ASSHTO LRFD 2007

0

100

200

300

400

500

0 50 100 150 200

Ba

r S

pa

cin

g, s

(mm

)

Concrete Cover, dc (mm)

Frosch, w = 0.44 mm

Frosch, w = 0.58 mm

ACI 318-08

AASHTO LRFD 2007

Observations to ACI 318 crack control rules

• ACI 318-08 does not report which limiting crack width it complies with.

• Apply smax eq. with judgment if cracking limits or regime differ from those aimed by ACI 318.

• Smax evaluated at fs = 0.67 fy. This is slightly higher than 0.6 fy used til 1999. The 0.67 stems from higher load factors adopted in the 2002 code. Increased stress should be tied to a larger (0.67/0.6 x w) limiting crack width.

Observations to ACI 318 crack control rules

• For the sake of fairness, ACI 318 warns designers to be cautious when using the crack control provisions when dealing with aggressive environments.

• This seems to imply exposure conditions do matter.

Exposure Condition

Maximum Allowable

Crack Width (mm)

ACI 318-95 and earlier versions

Interior Exposure 0.41 Exterior Exposure 0.33

ACI 224R-01*

Dry air or protective membrane 0.41

Humidity, moist air, soil 0.30 Deicing chemicals 0.18

Seawater and seawater spray, wetting and drying

0.15

Water-retaining structures † 0.10

CEB/FIP MC90**

Reinforced Concrete Members Exposure Classes 2 to 4 0.30 ‡

Exposure Class 1 See note ¥ De-icing agents on top of tension

zones of RC members See note ¤

BS 8110-97

Appearance 0.30

Aggressive environments 0.30

Limiting Crack Width

• w = 0.25 mm typically used in marine beam construction

Proposed Improvement to ACI 318• For indirect crack control, the limiting “w”

should be shown explicitly in smax equation.• Direct control:

• Indirect control:

000,190

5.2000,240max

sc

s fscs

fw

s

cs f

wc

f

ws

)000,190(5.2

000,240max

(w in mm, fs in MPa)

Proposed Improvement to AASHTO LRFD

• Similar concept.• Direct control:

• Indirect control:

cs dsfw 20000036.0max

cs

df

ws 2

600,278max

44.0

we

(w in mm, fs in MPa)

Proposed Crack Control Equations

0

100

200

300

400

0 50 100 150

Ba

r S

pa

cin

g, s

(mm

)

Concrete Cover, dc (mm)

fs = 280 MPa (40 ksi)

w=0.4 mmw=0.3 mmw=0.2 mm

Eq. 20

fs = 280 MPa (40 ksi)

w=0.4 mmw=0.3 mmw=0.2 mm

Eq. 20

sc f

wd

2

000,200

Modified ACI 318

Modified AASHTO LRFD

Crack Control in FRP-reinforced Concrete Elements

• Frosch’s model applicable.• Need to account for variable elastic modulus

and bond characteristics of FRP bars.• Larger w values allowed because of superior

corrosion resistance of FRP reinforcement.• Refer to ACI 440 standards. ACI 318 not valid

for FRP-reinforced concrete.

Deflection Control• Deflections can be controlled directly (through

direct D calculation) or indirectly (by specifying max span/depth ratio or min member thickness)

• Philosophy in ACI 318 is to waive direct deflection calculation if max span/depth ratio or min member thickness are complied with.

Direct Deflection Control• Direct calculation in terms of Ie

• Branson (1965):

• Bischoff (2005) improved Branson’s equation• Integration of curvatures (Ghali, 1994)

ec

mm IE

LMK

2

1 48

5

m

o

M

MK 2.02.11

gcra

crg

a

cre II

M

MI

M

MI

33

1

Indirect Deflection Control• Limiting curvature concept

Indirect Deflection Control• Indirect control in terms of curvature:

• Leads to indirect control through max span/depth ratio:

21 48

5LK mm

L

k

Kh

L m

ms

m

,1

1

5

48

dkdsmcm

m

smm

1

Max Deflections in ACI 318

Pile-supported Container Yard

• Appropriate Δ/L here?1/750 ~ 1/1000

“Span Length” in Pile-supported Beams

Pile

RTG Crane

Lo

Lef f

RTG Crane Runway

Note: Def lected shape exaggerated for clarity

Indirect Deflection Control• Minimum Thickness

ACI 318-08 Table 9.5(a)

Minimum Thickness, h Simply

supported One end

continuous Both ends continuous

Cantilever

Member Members not supporting or attached to partitions or other construction likely to be damaged by large deflections

Solid one-way slabs

L/20 L/24 L/28 L/10

Beams or ribbed one-way slabs

L/16 L/18.5 L/21 L/8

es and D/L Effect on Span/Depth Ratio

0

10

20

30

40

50

60

70

80

1.0 1.5 2.0 2.5 3.0

Static-to-Midspan Moment Ratio ,

m

o

M

M

sm = 0.0006

km = 0.254

rr Er = 1200 MPa = 0.9

Interior Span

Edge Span(Pinned/Continuous)

Simple Span

sm = 0.0013

h

L

sm = 0.0006

sm = 0.0013

m/L = 1/240

m/L = 1/1000

Staged Construction

Staged Construction

Staged Construction

• Book-keeping of bending moment growth is key to calculate crack widths, rebar stresses and concrete stresses as staged construction progresses

• Shored vs Non-shored construction also important

≠

Durability Considerations

Precast Concrete Elements• Construction per PCI MNL 116• 42 MPa Concrete Strength at 28 days• ASTM C150 Type II cement with C3A between 6 and 10% • Water-cement ratio ≤ 0.40• Minimum cement content of 400 kg/m3

• Max chloride ion content = 0.06% by weight of cement• Add Calcium Nitrate as needed (piles)• 75 mm clear cover at soffits• Aggregate needs to be innocuous (alkali-silica reaction)• Silica fume: max 8% cement replacement

Conclusions and Recommendations (I)• Crack and deflection control in concrete

structures can be done directly or indirectly. • Indirect control techniques require Designers to

know what limiting crack width/deflection is being controlled.

• Indirect (smax) crack control equations in ACI 318 and AASHTO could be more transparent. Need more explicit dependence on wmax.

Conclusions and Recommendations (II)

• The proposed modification to the ACI 318 crack control equation provides a means for controlling very narrow crack widths (narrower than the limits adopted by ACI 318 for buildings).

• ACI 318 crack control equation only works for steel-reinforced concrete. For FRP-reinforced concrete, refer to ACI 440.

• Frosch’s generalized equation for smax is a good tool for crack control for given limiting w.

Conclusions and Recommendations (III)

• Indirect deflection control typically expressed through max span/depth ratios.

• D/L limits given in ACI 318 are adequate for RC buildings but may be overly liberal for port structures with equipment sensitive to deflections.

Conclusions and Recommendations (IV)

• Indirect deflection control checks are appropriate for preliminary member sizing followed by detailed direct deflection calcs.

• Crack control in concrete elements cast in stages requires evaluation of crack widths and rebar stresses at every step.

• Designers need to adopt design codes carefully, understanding scope and limitations.

carlos.ospina@abam.com