CRACK FORMATION AND ITS MITIGATION

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25 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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CRACK FORMATION

AND ITS MITIGATION

IN

POST
-
TENSIONED CONCRETE

STRUCTURES

Procedure


Crack Types and Causes



Crack Mitigation



Structural Evaluation of Cracks



Repair Cracks



Conclusions and
Recommendations

Crack Types and Causes


Major Causes of Crack
Formation


Deficient Design


Bad Detailing


Poor Workmanship



Shrinkage


Creep


Elastic Shortening


Temperature


Crack Types and Causes


These factors, when combined,
Lead to Crack Formation:


Slab Tends to Shorten


Walls/supports restrain free
movement of slab


Tension developed in slab due to
restraint exceeds slab’s tensile
capacity

Factors Causing Shortening


Shrinkage of Concrete


Creep of Concrete Due to
Precompression


Elastic Shortening Due to
Precompression

DESCRIPTION

PERCENTAGE

%

SHRINKAGE


(SH)

CREEP



(CR)

ELASTIC SHORTENING

(ES)

TEMPERATURE


(ES)

TOTAL

66

11

7

16

100

* For a parking structure in southern California

Contribution of Different Factors to
Typical Slab Shortening*

LONG
-
TERM SHORTENING OF A TYPICAL SLAB,

CONCRETE SHRINKAGE 450 micro
-
strain

CONCRETE STRENGTH 4000 psi

PRECOMPRESSION 150 psi

SLAB THICKNESS 8 inch

IF FREE TO MOVE

1.6 in.

0.8 in.

SHORTENING

200 ft.

SLAB

100 ft.

Long
-
Term Shortening

HYPOTHETICAL SHORTENING TENSION

APPROX.


+1000 PSI

PRECOMPRESSIONING DUE TO POST
-
TENSIONING


-

150 PSI

NET HYPOTHETICAL TENSION




+ 850 PSI

If Shortening is Fully Inhibited

CALCULATE TENSION 850psi > TENSILE


CAPACITY 475psi


THEREFORE, TO AVOID CRACKING,


SLAB MUST BE ALLOWED TO SHORTEN

Crack Characteristics


Prominent characteristics of cracks
in post
-
tensioned slabs compared to
regular reinforced concrete


Cracks are lesser in number


Cracks are wider


Cracks are normally longer and
continuous


Cracks commonly do not coincide with
locations of maximum moments


Cracks occur at axially weak sections

STRENGTH/SHRINKAGE

REINFORCED CONCRETE

SLAB

CREEP/SHRINKAGE

SLAB

CRACKS

SLAB

COL.

CRACKS

POST
-
TENSIONED

Reflected Ceiling View of Slabs

Axially Weak Sections


Construction Joints


Cold Joints


Path With Reduced Cross
-
Sectional Area


Paths With Fewer Rebar


Discontinuities in Slab

This Presentation Reviews
Cracks in:


Slabs


Columns


Walls

VIEW OF REFLECTED CIELING SHOWING
CRACKS IN PT SLAB (VILLAGE
SERRAMONTE, CA)

WALL

CRACK

COLUMN

SLAB

150'

88'

Crack in Slabs

Cracks in Slabs


Slab Cracks are Grouped into


Overall cracks


Due to poor layout of supports


Due to irregularities in slab geometry


Localized cracks

PT SLAB (VILLAGE SERRAMONTE, CA)

VIEW OF REFLECTED CEILING SHOWING CRACKS IN

WALL

SLAB

COLUMN

CRACK

188'

88'

Crack in Slabs

(a) SLAB FREE TO MOVE

CONCRETE COMPRESSION = TENDON TENSION

TENDON

F

Forces in Slab

(b) SLAB RESTRAINED AGAINST MOVEMENT

CONCRETE COMPRESSION < TENDON TENSION

TO WALL

COMPRESSION

DIVERSION OF

F

WALL

SLAB

Forces in Slab

CROSS BEAMS

DUE TO RESISTANCE BEHIND

CRACK FORMATION IN SLAB ON GRADE

CRACKS

OF SOIL

RESISTANCE

PASSIVE

PT

SOG

PT

Slab on Grade

IRREGULAR SLAB PLANS SHOWING

(c)

SLAB

(a)

SLAB

(b)

SLAB

CRACK FORMATION

Irregular Slab Plans

TIED TO CORNER WALLS

CRACKS AT SLAB CORNERS WHICH ARE

WALL

SLAB

CRACK

Corners of Slab

CRACK FORMATION AT CORNERS OF

INTERIOR

OPENINGS

OPENING

SLAB

CRACK

WALL

Corners of Slab

Cracks in Columns


Column Cracks are Categorized
into:


Short Columns


Regular Columns


Architecturally Oversized
Columns

LEVEL OF PARKING STRUCTURE

CRACKING IN SHORT COLUMN AT SPLIT

RIGIDLY CONNECTED COLUMN

RELEASED COLUMN

SLAB

COLUMN

COLUMN

SLAB

PT FORCE

TENDON

DOWEL

CENTRAL

RELEASE

SHORT

Cracks in Columns

NO JOINT

TIED TO WALL

(a) SIDE VIEW OF COLUMN

COLUMN

SLAB

DIRECTION

MOVEMENT

WALL

CRACKS

PARTIAL HEIGHT

(b) FRONT VIEW

FULL HEIGHT

OPENING

A

WALL

COLUMN

B

SLAB

Wall
-
Column Release

CRACKS IN END COLUMNS OF
LONG BUILDINGS

PT SLAB

MOVEMENT

COLUMN

COLUMN

Columns

LOCATIONS OF POTENTIAL DISTRESS DUE
TO SHORTENING OF POST
-
TENSIONED
SLABS IN MULTISTORY BUILDINGS


LOCATIONS

LEVEL

PLAZA

DISTRESS

SHEAR WALL

SLAB

PT

LOWER COLUMN

DISTRESS AT

UPPER LEVELS

SHORTENING OF

Shortening

RELEASE DETAIL FOR EDGE COLUMNS OF
SUNNYVALE SHOPPING CENTER,
SUNNYVALE, CA


PT SLAB

1/2" INCH NEOPRENE

#7 DOWEL

24" DIA. COLUMN

MATERIAL

HOOK

5"

3"
-

DIA COMPRESSIBLE

8"

Release Detail

Cracks in Walls


Due to overall behavior of wall


Due to local distress

CRACKS IN WALL DUE TO SLAB MOVEMENT

DIRECTION OF SLAB MOVEMENT

CRACKS

WALL

END OF

FOOTING

WALL

SLAB

Cracks in Walls

AT CORNER

ADD REINFORCEMENT

MOVEMENT

SLAB

SPALLING OF CONCRETE MASONRY UNITS
(CMU) AT SLIP JOINTS AND RECOMMENDED
ADDED REINFORCEMENT


CMU WALL

DEFLECTION

SLIP JOINT

PT SLAB

Concrete Masonry Units