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I
I I ·1
I I I I I I I 'I I I I I I
FIELD OBSERVATION of FIVE
LIGHTWEIGHT AGGREGATE
PRETENSIONED PRESTRESSED
CONCRETE BRIDGE BEAMS
Iowa Highway Research Board
· Project HR 104
Iowa State Highway Commission
Ames, Iowa
POR llTTIR HIGHWAYS
BEAM NO.
152
153
154
155
156
CORRECTION
HR-104, "Field Observations of Five Lightweight Aggregate Pretensioned Prestressed Concrete Bridge Beams."
Please replace Table #5, page 36 with the following.
(All Cambers are in inches)
CAMBER. PRIOR TO CAMBER AFTER INITIAL CAMBER SLAB PLACEMENT SLAB PLACEMEN'T' FINAL
PRED. MEAS. PRED. MEAS. PRED. MEAS. PRED. CAMBER
MEAS.
2.50 2.50 3.20 3.10 1. 20 1.05 0.70 + 0.25
2.50 2.50 3.20 3.15 1. 20 1. 05 0.70 + 0.40
2.50 2.50 3.20 3.00 1. 20 0. 70 0.70 + 0.20
2.50 2.50 3.20 3.00 1. 20 0.60 0.70 - 0.20
2.50 2.70 3.20 2.85 1. 20 0.65 0.70 + 0.10
a) as of January 14, 1969
b) Figure 16, page 26 shows how the beam has developed a negative camber.
(a)
(b)
I I I I I "Field
I I I I I I I I I I I I I I
Observation of Five Lightweight Aggregate Pretensioned Prestressed Concrete Bridge Beams
Final Report
By James A. Young
Research Department Iowa State Highway Commission
Ames, Iowa
Iowa Highway Research Board Project No. HR-104
•I!'
I I I I· I I I I I I I I I I I I I I I
Chapter
1 2
3
4
TABLE OF CONTENTS
List of Tables List of Figures
INTRODUCTION AND SCOPE EXPERIMENTAL PROCEDURES
2.1 Concrete Mix 2.2 Instrumentation 2 .3 Beams 2.4 Field Deflection Measurements
DISCUSSION OF RESULTS
3~1 Laboratory Tests 3.2 Camber Development
OBSERVATIONS ACKNOWLEDGEMENTS LIST OF REFERENCES APPENDIX
Page
ii iii
1 4
4 4 6
15
21
21 22
36 37 38 39
I I I I I I I I I I I I I I I I I I I
Table
1
2
3
4
5
LIST OF TABLES
CONCRETE MIX QUANTITIES FOR LIGHTWEIGHT CONCRETE BRIDGE BEAMS
STRENGTH AND AGE OF CYLINDERS FOR GROUP I BEAMS
STRENGTH AND AGE OF CYLINDERS FOR GROUP II BEAMS
COMPRESSIVE STRENGTHS FOR LIGHTWEIGHT CONCRETE CYLINDERS
COMPARISON OF PREDICTED AND MEASURED CAMBER AND DEFLECTION VALUES
ii
Page
5
8
8
21
36
I I I I I I I I I I I I I I I I I I I
Figure
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
LIST OF FIGURES
Comparison of Old and Proposed New Standard Crossing
Detail and Location of Brass Plates
Development of camber immediately after release in Beam number 152
Development of camber immediately after release in Beam number 153 ·
Development of camber immediately after release in Beam number 154
Development of camber immediately after release in Beam number 155
Development of camber immediately after release in Beam number 156
Development of camber in Beam number 155 for short intervals of time at release.
Beam layout on test bridge
Calculation of correction to be applied to compensate for the rotation of the beam ends during camber development.
Set-up during deck placement
Procedure in reading rods
Camber development with respect to time for Beam number 152
Camber development with respect to time for Beam number 153
Camber development with respect to time for Beam number 154
iii
Page
2
7
9
10
11
12
13
14
16
18
20
20
23
24
25
I I I I I I I I I I I I I I I I I I I
Figure
16 Camber development with respect to time for Beam number 155
17 Camber development with respect to time for Beam number 156
18 camber development for a typical beam
19 variation of camber in beam number 152 during the deck placement
20 Variation of camber in beam number 153 during the deck placement
21 variation of camber in beam number 154 during the deck placement
22 Variation of camber in beam number 155 during the deck placement
23 variation of camber in beam number 156 during the deck placement
iv
Page
26
27
28
29
30
31
32
33
I I I I I I I I I I I I I I I I I I I
CHAPTER I
INTRODUCTION AND SCOPE
1
The use of lightweight aggregates in pretensioned ·prestressed
concrete beams is becoming more advantageous as our design criteria
dictate longer span concrete bridges. Bridge ~earns of greater lengths
have been restricted from travel on many of our highways because the
weight of the combined beams and transporting vehicle was excessive,
making hauls of any distance prohibitive. This, along with the fact
that new safety requirements necessitate the use of longer spans in
grade separation structures over major highways, prompted the State
of Iowa to investigate the use of lightweight aggregate bridge beams.
Until recently, it was possible to use 67' bridge beams in the
two interior spans of a four span overhead crossing over interstate
highways in Iowa. The new safety standards require that any obstruc
tion such as columns or abutments be at least 30' beyond the out
side edge of the pavement. This requirement means that beams for
the two interior spans must be increased to at least 87' in length
on a right angle crossing. If it should develop that a skewed cross
ing would be necessary, the length of the beams could conceivably
be 90-95 feet in length. Figure I shows the relationship between
typical new and old overhead crossing standards.
A series of three projects was started to investigate the pos
sibility of using lightweight aggregate with natural sand fines in
pretensioned prestressed concrete bridge beams. These projects were
--------- ----------
~a~ I . I v; ! :------------~--~------
i I
I. ~ ;
-l~ I
i 24'5/ob I r G' i-___.------------:-___ '
'/v/1~'/- Cf. l·
---,,,===t===r---.:......- ~
' I 74 -o <t: to cf
I f---------------------------- 2 I G '- i
Standard at Present
3 9'-1
I I ....--~="==r===-----..___JJL..~-----:"=="'-'===~~----....jj !I
L __ B_Y_~:_G ___
88 '~-~::-1
-_4. ___ 8~7'G .j. •8'o ~ ~-
Proposed New Standard
Figure 1. Comparison of Old and Proposed New Standard Crossing
I I I I I I I I I I I I I I I I I I I
3
basically designed to investigate the feasibility of using light
weight aggregate bridge beams in the State of Iowa and to determine
the properties of the material which are ,essential for design pur
poses. The three projects, which were started at approximately the
same time are: "Creep and Shrinkage of Lightweight Aggregate Con
crietes;' "Time Dependent Camber and Deflection of Non-Composite and
Composite Lightweight Prestressed Beams," and "Field Observation of
Five Lightweight Aggregate Pretensioned Prestressed Concrete Bridge
Beams".
The first two are under the supervision of the. Civi~ Engineer
ing Department, at the University of Iowa, the third project is
the subject of this report.
The objective of this project .. is the collection of field
deflection measurements for five pretensioned prestressed light
weight aggregate concrete bridge beams fabricated by conventional
plant processes; also the comparison of the actual cambers and deflec
tions of the beams with that predicted from the design assumptions.
The test bridge is located on County Road "W" over Tipton
Creek in Hardin County, Iowa. The bridge was designed by Mr. P. F.
Barnard, Consulting Structural Engineer, Ames, Iowa and the beams
were fabricated by Prestressed Concrete of Iowa, Inc., Iowa Falls,
Iowa.
The Situation Plan and Superstructure Details of the test bridge
are shown on pages A-1 and A-2 respectively in the Appendix.
I I I I I I I I I I I I I I I I I I I
2.1 Concrete Mix
CHAPTER II
EXPERIMENTAL PROCEDURES
4
One of the essential parts of any project involving
concrete is the proper proportioning and mixing of the necessary
constituents.
Three possible sources of lightweight aggregate were suggested
for use on this project and they were tested by the Material Test
ing Laboratory at the Iowa Highway Commission. Aggregate A was
eliminated on the basis of a very low durability of its beam samples.
Aggregates B and C, using air dry aggregate, had durability factors
of 100 and 97 respectively. It was noted, that even though aggre
gates B and C had durabilities which were acceptable, the beam made
with aggregate B crumbled around the edges at one end. Based on
these results Aggregate C, known by the brand name Idealite, was
selected.
Table 1 shows the Mix Design Objectives, Ingredients and Pro
cedures for this project.
2.2 Instrumentation
A permanent set of reference points was established on
each beam at a distance of 22" from each end and at the midspan.
The distance of 22" was used so that the reference points on the
end would not be covered by the abutment diaphragms when the bridge
is complete. These reference points consisted of 3~" x2" brass
p;Lates cast to the bottom flange of the beam. A ~" diameter hole
I I I I I I I I I I I I I I I I I I I
TABLE I CONCRETE MIX QUANTITIES FOR LIGHTWEIGHT CONCRETE
' BRIDGE BEAMS
MIX DESIGN OBJECTIVES
Concrete Quantity l~ cu. yds.
Concrete Strength @ 28 days 5000 psi
Unit Weight, Maximum' Air-Dry (117) pcf
Air Entrainment (5+ 1) %
MIX INGREDIENTS
Cement (Type 1) 1058 lbs.
Natural Sand 2093 lbs.
Idealite Aggregates (60% of 3/4" to 5/16" and 1230 lbs. 40% of 5/16" to #8)
water 52.5 gal.
Darex'@ 7/8 oz. per sack of cement 9.75 oz.
Pozzolith 31.5 oz.
MIXING PROCEDURES
1. Proportion sand and Idealite.
2. Add 26 gallons of water.
3. Mix for approximately two minutes.
4. Proportion the cement.
5. Add six gallons of water.
' 6. Add Darex AEA in 3 gallons of water.
7. Add Pozzolith with remaining water while adjusting to 2~" slump.
5
I I
was tapped in the plate: the hole was used to receive a ~" SAE -
I fine thread bolt on which the level rod is seated when the camber
I readings are taken. Figure 2 shows the position of the plates
I I I I I I I I I I I I I I I
on the beam. A level rod (reading to 0.005') and a precise level
was used on all measurements. The camber and deflection are deter-
mined on the basis of relative displacement between the reference
points.
2.3 Beams
Five pretensioned prestressed concrete beams were cast I
'
in 2 groups; the first group of 3 was cast on April 15, 1968 and
steam cured for 40 hours; the second group of 2 was cast on April
19th, 1968 and cured 67 hours in steam. The beam detail and data
sheet is shown on page A-3 in the Appendi~.
Group I, consisting of beams numbered 152, 153, & 154 was to
have been released* on April 16th, however the cylinder strength
did not reach 4500 fc' and the beams were not.' released until the
17th of April. Table 2 shows the strength and age of the cylinders
at the time of testing.
*released - is defined as the time at which the pretensioned cables are cut and the stress is transferred from the prestressing steel to the concrete.
- - - - - - - - -:~
t ! t I I I
~--------,-,
l I : I
'
±'-<"om - ~"-' ,.
I !
;
: I
i ' l
y ----tb-----------~ i .__~i _________ __,il ~ ~
I 1.i
ffi
, - r-.. , JU 4: ,_ 8
87'-o
-..I!_ e--3 l 8 I
1-ft , ""'1' I ?J..
·r<-------1--
/ _1/<P f)Q/e fapped :!"or 1 ''""' rA f:, r-,n· ·"' i z ~.,,.I Ir.- I '-
fhread b o If.
Detail of Plate
- - - - - - - - -I I
I i
11 ti I ,
! I i '
I i \
i l I
4 I,_ 8
~I I
I
t i
_,,.If 3eanno \ J
' i I 3r :
"' ' r'' ass---+-· ·--1
<:
--::} !
r:b------- _____ L
" A
, '. _ ,__eve,1n9 __ _ rod
-__J ''<P 60 If ~--.J
Rod position on Reference Bolt
Figure 2, Detail and Location of Brass Plates
-
I I I I I I I I I I I I I I I I I I I
8
Table 2, STRENGTH AND AGE OF CYLINDERS FOR GROUP I BEAMS
Cylinder No. (a) Beam No. Age (Hours) Strength (psi)
152A 152 24 4310 152B 152 48.5 5160 153A 153 40.5 4460 153B 153 48.5 4480 154A 154 40.5 4420 154B 154 '48. 5 4950
(a) Note: All cylinders except 152A were tested at time of
release.
Group II, consisting of beams numbered 155 and 156, was cast
on the 19th, and released on the 22nd of April. At the time of
release, the test cylinders exhibited the strengths and ages as
shown in Table 3.
Table 3, STRENGTH AND AGE OF CYLINDERS FOR GROUP II BEAMS
Cylinder No.
155A 155B 156A 156B
Beam
155 155 156 156
Age (Hours)
67
67 67
Strength (psi)
5130
4350 4360
When the beams were released, readings were taken at short
intervals of time to show the development of camber with respect
to time. The graphs in figures 3-7 show the camber development
immediately after release. Figure 8 shows how the camber developed
in beam number 155 over very small increments of time immediately
after release. During the first minute and 45 seconds the beam
camber held at a constant value of about 0.10 inch. It would appear
that the bond and friction between the beam and steel pallet restrained '
-- -- 3. 0
1.0
w
- - - - - - - - - - - ---- - -
0
20 40 60 80
TIME, MINUTES
CAMBER vs
TIME BEAM NO. 152
100 120 140 160
-
- 1 - - - - - - - -· 3.0
0 0
1.00
20 40 60 80
TIME, MINUTES
- - -CAMBER
vs TIME
BEAM NO. 153
100 120
- -
140
- -
160
-
I-' 0
-
\fl
~ CJ z !·-!
l - - - - - - - -3. 00 ..
0
- - -CA.MEER
vs TIME
-BEAM NO. 154
- - - ...
G
2.00 1--~--::---------=-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~----i 0
0 G
1.00 ;..
I • . • I . ' 20 40 60 80 100 120 140 160
rrIME, MINUTES
-
- - - - - - - - -3. 00 ,.,.
2.00
0
(
l. 00 r-
I I
20 40 60 80
TIME, MINU'l'ES
-
I
100
- -CAMBER
vs TIME
-BEAM NO. 155
- I
120
- - - - --
..
I I
140 160
- , 3.00
1. 00
! I
- - - -
20 40
- - - -
60 80
TIME, MINUTES
- -CA!\IBER
vs TIME
-3EAt<i ~::c'. 1S".:
.100 120
- - - - -
140 160
.•
-
.. ~ li:i
~
l 3.00
~ u 1.00
- - - - - -
1.0
- -· - - - - -
2.0
TIME, MINUTES
CAMBER VS TIME IMMEDIATELY AFTER RELEASE
BEAM NO. 155
3.0
- - -
4.0
I I I I I I I I I I I I I I I I I I I
15
the beam from deflecting upwards during this interval of time.
Once the beam overcame the bond and friction deflection upwards
was rapid. The deflection proceeded to increase until it leveled
off at about 1.80 11 only 2 minutes after the bottom flange was
separated from the pallet.
During the 2 months immediately after casting, the beams
were stored outside. The temperature varied during this period
from a low of 30°F on the 24th of April to a high of 89°F on the
7th of June. Camber measurements were taken during the period of
22 April to 7 June at an average of once every 8 days.
On June 10th the girders were moved from their plant storage
location to the bridge site. A final set of readings was taken
after the beams were seated on the abutments and before any super
imposed load was applied. Figure 9 shows the beam layout on the
test bridge.
On June 21st the deck was placed on the bridge. As the deck
concrete placement progressed, deflection measurements were taken
at 30 minute intervals. The placement of the deck took approx
imately five hours without any major delays. Readings were taken
at 30 minute intervals over this entire period.
2.4 Field Deflection Measurements
The observation of the deflections of the center of the
beams is one of the prime objectives of this report.
-------------------cf: Pa,-, ~,:nq L.. ~·--'/
Vv' :. ·;...,U~ --, .,-1..,,1._~
3 1 ~eam 155 I
F ~I I I '
-" -· ., ! : ; r 8.eom "1s0 i
! -·
4-- -1- ..
' ()d --~ '"'- we; : I i
" I I r-8eam 4
15Z \
: i - " 1 ==r. ..
i
' I JI.
r 8eam /53 I i
.L... .. - . ~I-
L
3.eom tt
r 15 '+ I
\ !
.. ..:.+I· ~ Ii
~] i
Figure 9, Beam Layout on Test Bridge
I I I I I I I I I I I I I I I I I I I
17
While the beams were in the storage yard all camber measure
ments were made by placing a level rod on the reference bolts and
taking readings with a precise level. When the readings at each
point had been taken it then becomes necessary to convert the
displacement of the centerline to inches and add a correction
to compensate for rotation of the end bolts about a point on the
end of the beam. Figure 10 shows how the compensation is figured.
The calculation of the correction involved one assumption which ·
can be justified. rt was necessary to assume that when the cen
ter of the beam deflected vertically upward, the reference bolts
displaced vertically also, rather than on an arc about the rota-
tion point which is the neutral axis. The rotation about the
neutral axis, for a vertical deflection of 3.2" at the centerline,
0 amounts to an angle of 0 21'.
When the assumption of vertical displacement of the end bolts
is employed it is found the bolts will move vertically 0.13" when
a deflection of 3.2" is observed at the center. The horizontal
movement, X, would then be 0.0008 in, therefore, the assumption
of vertical displacement is justified.
After the beams had been set on ~he abutments readings were
taken on all reference points before any load was superimposed.
When formwork for the deck was completed it became very dif
ficult to read the rods by setting them on the top of the bolts.
At this time, a single hole was drilled in the rods at the top so
I I I I I I I I I I I I I I I I I I I
18
Figure 10
d ..
122" I : > .5Z2 ·I In calculating the additional vertical displacement due to
rotation, it is assumed that the bolt at the end moves vertically in direct proportion to the displacement at the centerline. The preceeding page served as a justification of this assumption.
The correction, y is given by:
y = 22 /),, 522
t1 = Camber
Li = y+d, where d is the rod reading
y = 0.0421 (y+d)
y = 0.0421 d 0.9579
y = 0.0440 d
l .6
I I I I I I I I I I I I I I I I I I I
19'
they could be bolted to the centerline of the five beams and
allowed to hang free for ease of reading. This method was
employed to check camber measurements during the entire period
of time the deck was being placed. Figures 11 and 12 show the
set up during the deck pour. Readings were taken at short inter
vals of time and periodic checks were made on the bolts ,at the
end of each beam to note the vertical displacement.at these points.
I I I I I I I I I I I I I I I I I I I
Figure 11 Set-up during deck placement
Figure12, Procedure in reading rods
20
I I I I I I I I I I I I I I I I I I I
21
CHAPTER III
DISCUSSION OF RESULTS
3;1 Laboratory Tests
In addition to the cylinders which were cast at the
time the beams were made, another series of cylinders were made
to study the strength development pattern of the lightweight con-
crete. The cylinders were cured exactly as the beams~ the first
set was cured for 40 hours in steam, the second set 67 hours in
steam. Table 4 shows the properties as they were determined.
It should be noted that the f 'c of the test cylinders cured for
67 hours averaged slightly less than that of the test cylinders
cured for 40 hours.
Table 4, Compressive Strengths of Lightweight Concrete Cylinders
a Date Cast Age f 'c E
(days) (psi) (psi xc106)
4/15/68(b) 7 5125 3.10 4/15/68 14 5560 3.23 4/15/68 28 5980 3.35 4/15/68 58 6360 3.46
4/19/68(c) 7 4915 3.04
4/19/68 14 5570 3.23 4/19/68 54 5925 3.34
a. Computed by E = 33 ~w3. f~1
where w has an average value of 120 lb/ft~ c
b. 40 hours steam cure.
c. 67 hours steam cure.
I I I I I I I I I I I I I I I I I I I
22
3.2 Camber Development
Figures 13 to 17 indicate the pattern of the camber
development from the time when the prestress was transferred to
the beams until about 200 days after the deck was placed. Figure 18
shows the development of camber in a typical beam. The deck was /·
1cast with normal weight sand and gravel concrete.
There was a slight difference in the camber development between
the two sets of beams. Group I, consisting of beams 152, 153, and
154, had developed a larger value of camber, prior to deck placement,
than the Group II beams. The beams in Group I were about 0.1" below
the value of 3.20" which had been predicted for them. The beams
in Group II had a value which was approximately 0.3" below the pre-
dieted value of 3.20". The design calculation for the beams and
the camber predictions are shown on pages A-4 and A-5 in the
Appendix.
During the time the deck was being placed continuous readings
were taken on the reference bolts as described in Chapter II, sec
tion 4. The graphs in figures 19 to 23 show how the camber of
the beam varied during the deck pour.
The pour started at the west end of the bridge and proceeded
east across the bridge. The initial set of readings was taken
with the finish machine "in place" on the west end of the bridge.
The apparent rebound near the end could be attributed to the
- - - - - - - - - - - - - - - -- - l -CAMBER DEVELOPMENT vs
3.00 TIME BEAM NO. 152
2.00
1.00
0
0
40 80 120 160 200 240 280 320
TIME, DAYS SINCE BEAM RELEASED
- 1 - - - -3.00
2.00
t.f}
rx:i ::r: 0 z H
.. ~ rx:i i:x:i ~ ~ 0
1. 00
40 80
- - - - -
120 160 200
- - -CAMBER DEVELOPMENT
vs TIME BEAM NO. 153
-
240 280
TIME, DAYS SINCE BEAM RELEASED
- - -
320
-----------------3.00
2.00
CJ)
~ () z H
.. 0:: rx:i IJ'.l :E: .:r: ()
1.00
40 80 120
0
160 200
CAMBER DEVELOPMENT vs
TIME BEAM NO. 154
0
240 280
TIME, DAYS SINCE BEAM RELEASED
320
-
. "' Ul
- 1 - - - - -3.00
2.00
CJ) µ::i :r: C) :z; H
.. p:: µ::i
~ .::i:; C)
1.00
40 80
-
120 0
- -
160
- - - - -CAMBER DEVELOPMENT
vs TIME BEAM NO. 155
240 200-e-~----
280
0 TIME, DAYS s"'INCE BEAM RELEASED
- - -
320
- -, I
3. 00:
2.00
-- _l. 00
I-' -....]
- --------------
0
CAMBER DEVELOPMENT vs
TIME BEAM NO. 156
-
--~----__,..__ _____ 0 __ ...._-:--------"""--'.::-------LO~·--------J.__ ______ ---1 ________ -L ________ ....L __ ~--~....I' ~ 120° 160 200 240 280 320 -...J 40 80
TIME, DAYS SINCE BEAM RELEASED
- -, 3.00
2.50
2.00
VJ r.::i p:; CJ z H
.. r:r: µ:i (:Q :E: ,:l; CJ
1. 00
. o. 7·J
f':j I-'·
l.Q s:: Ii CD
I-' ())
- - -DECK
PLACEMENT
3.20 -
0 0
N
- - - -
TI.ME
- - - -PREDICTED CAMBER
vs
-'rIME FOR A TYPICAL BEAM
- - -
0 l[) . 0
"' 00
-· -
(()
rz.1 ::r:: 0 z .H
3.00
2.00
1.00
---------------
~o t/JO 120 160
TIME,
CAMBER VS T!ME DURING DECK POUR
BEAM NO. 152
200 240
MINUTES
290 3i0
Figure :19.'.o
160
l'V l.D
- -3.00
2.00
U)
Iii :r: tJ z H
~
p::; Iii Ill ~ ~ tJ
1.00
---------------
40 2-0B'
TIME, MINUTES
CAMBER VS TIME DURING DECK POUR
BEAM NO. 153
Figure -20
w 0
- - , - - - -3.0
2.00
U)
i:z.l ::r:: () z H
.. o:; li:i
~ .:x; ()
1.0
- - - -
I-20
TIME, MINUTES
- - - - - -CAMBER VS TIME DURING DECK POUR
BEAM NO. 154
-280
Figure .21_
-
------------------'
3.00
2.00
120 160
TIME, MINUTES
CAMBER VS TIME DURING DECK POUR
BEAM NO. 155
200 320
Figure 22
360.
- - - - - - - - - -3.0
2.00
F6-0
TIME, MINUTES
- - - -
CAMBER vs TIME DURING DECK POUR
BEAM NO. 156
- - -
Figure 23 ·
I I I I I I I I I I I I I I I I I I I
34
finish machine being removed at about the 280 minute mark and the
last 2 readings on each beam taken without the finish machine on
the deck.
No cylinders for the Group II beams were broken at the 28
day mark, however we can get a comparison of the strengths and
calculated modulus of elasticity for Group I and Group II, at
58 and 54 days respectively. The 435 psi difference in f 'c and
6 0.12Xl0 psi difference in modulus of elasticity are not signif-
icant to where this data could be used to explain the difference
in camber between Group I and Group II.
A factor which would have caused a difference in the camber
was the different conditions under which the beams were cured.
The time spent in raising the steam to curing levels was different
for the two groups. It took nearly 4 hours to raise the steam on
Group I and only 2~ hours to raise the steam on Group II. Group I
was steam cured 40 hours while Group II was cured 67 hours. These
conditions could have had an effect on the creep characteristics
and could change the pattern of camber development completely.
There are many inherent factors in concrete, especially in
lightweight concrete, by which the camber is affected. Aggregate
type and shrinkage characteristics are just two more factors
which could affect the camber development.
I I I I I I I I I I I I I I I I I I I
35
The effect of the deck placement is to rapidly decrease the
camber under the action of the dead load (deck). Initially it
was estimated that 3.20" of camber would be in the beam at the
time the beams were to be set. It was predicted that the dead
load would cause the beams to deflect about 2.00" thus leaving
approximately 1. 20" of camber in the beam after the dead load
was applied. Creep and shrinkage will cause the beam to deflect
approximately 0.50" over a period of time.
I I I I I I I I I I I I I I I I I I I
36
Chapter IV
Observations
In looking at the results, it is evident that the work done
under Iowa Highway Research Board, Project HR-104 is consistent
1 with work done by others. Camber (inches) vs time (days after
release) has indicated that the results of this work are fairly
consistent with the predicted values.
The method of measuring camber that was used was rather sim-
ple yet it afforded very good results. The following table com-
pares the predicted results with the measured results of various
stages in the project development.
Table 5, COMPARISON OF PREDICTED AND MEASURED CAMBER AND DEFLECTION VALUES
Initial Final Slab D.L. Deflection after Camber Camber(a) Deflection Slab in Place
Beam No. Pred. Meas. Pred. Meas. Pred. Meas. Pred. Mea:s.
152 3.20 3.10 0.70 0.25 2.00 2.10 1. 20 1. 05 153 3.20 3.15 0.70 0.40 2.00 2.10 1.20 1. 05 154 3.20 3.00 0.70 0.20 2.00 2. 30 1. 20 0.70 155 3.20 3.00 0.70 0.20 2.00 2 .35 1. 20 0.60 156 3.20 2.85 0.70 0.10 2.00 2.25 1.20 0.65
.I a) as of January 14, 1969
The initial values of camber appeared to be slightly lower
than what was predicted. The dead load deflections were some-
what larger than the 2.00 inches which was initially calculated.
It appears the creep and shrinkage varied somewhat from their
predicted value of 0.5",
I I I I I I I I I I I I I I I I I I I
37
ACKNOWLEDGEMENTS
The author wishes to thank Mr. Charles Pestotnik, Bridge
Engineer, Iowa State Highway Commission, Mr. Henry Gee, Assistant
Bridge Engineer, Iowa State Highway Commission and Mr. James
Boehmler, Jr., President of Prestressed Concrete of Iowa, Inc.,
Iowa Falls, Iowa; without the aid of these men the preparation
of this report would have been very difficult.
I I I I I I I I I I I I I I I I I I I
38
REFERENCES
1. Furr and Sinno, "Creep in Prestressed Lightweight Concrete",
Texas Transportation Institute, October, 1967.
I I I I I I I I I I I I I I I I I I I
APPENDIX
39
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FILE NO i?3087
-
~ I w
- - - - - - -
:------------------·------
3EAM DATA -,- .'
:o-:-o. /n/.~ 10/ ~-,,..esf,,...e s s BG 7 I
I II
2 ¢
, No. of" ;fro /qn f sfrand..r 22 ! ' No or- def7ecfed sfrands 8
/n. 1 o' d:(/ec . .:.1on ~ir--_-... -o-~-·c_r--e-.i.-. -e-----------c-. -lj-.. -'---I-!-. -0-... -? -
;
I /3eam · 1.. • 1 vve10,11r :ans /{j. 35
:
- - - - - - - - - - -i '- 3
' ' r-----1
--r----+-- -0 0 -&' ~
-----, ----r lJ) .
----· _} __ _ ~i-~-
• :; J ~z
.__. -h
~
-.._ (). - I
8 t; I c J ... ,. ; .... z. cy-J
, __ l '3.
~--- ~--·-:-:·!-: -'-, ---1--r.> ..... ~ •.• ~ i _._._.a-•••-• c.r:_i l ~- ., .••. ·• ......... ---1---~l-
?' ' ' 1 r@ ? : ? :., -z _J - --- . . -'. ... ~ '-
~---L~_Z __ -~
_________ _z:-._~
Gross oeam c " I: ~·? s areQ __... I . i
---------·--------'
Tran-:;rormed .20,!09
4
6eam I ,+. }r)
(CV
8 " n = Lit ~ z;.47 :...;, = ;9"53 , /)
Transformed i20/0391i/ beam r @end .;.
0 Upper -!lgure /s t/1= beam camber af r::/eose. ~ower -... .. I-' I • • I ·' :-1qLJre is rne on:1c1.-:>area
I
beom camber :"usf 6erore ., slao LS placed.
" n = a r_; t. = 2 .:. 0 7 U. : I
IQ "9 .3 I c.
.. ... .. v...;t. of s/ab :... ewer Tlaur--e. I
cJef!echon due ro
L
I I I I I I I I I I I I I I I I I I I
DESIGN CAMBER AND DEFLECTION AT MIDSPAN
(1)
<; M1aspan
_f_________ . --·----·· r· ,. · N ·.~: ()_f 6m. -----------·-· _______ _
QJ ! I
,,.
---..-~~-i-------
I
GJ ~~~~~t-··· ~~:t~il~~;~-·- 3~:~:i. Midspan camber at release of prestress:
~. = (0.97 Pi) (L2 ) (Eel) (IB)
5 (MB) (L 2
)
48 (Eel) (IB)
~-,;.··
Pi = total initial prestressing force in lb.
0.97 Pi = assuming 3% loss of initial prestressing force due to stress relaxation in steel before release.
L = beam span length in inches.
Eel = 2.9lxl06 psi at f 'c = 4,500 psi
IB = average transformed I of beam in inch.4
~=moment at midspan (in-lb.) due to wt. of beam.
= ( 0 • 9 7 ) ( 8 6 7 I 0 0 0) ( 86X12 ) 2
.d. (2.9lxl06) (120,400) (0.0983xl4.33
+ 0.0267x6.2) 5 ---48
A-4
:,
(4,803,702) (86xl2) 2
(2.9lxl06 ) (120,400) = 2 ~ 5 i
I I I I I I I I I I I I I I I I I I I
(2) Midspan camber at time just before placing concrete slab:
5 48
0.82 Pi
(0.0983ec + 0.0267ee )
= assuming 18% loss of initial prestressing force due to stress relaxation in steel, and creep and shrinkage in concrete.
Ec2
= 3.06xl06 psi at f'c = 5,000 psi.
C = coefficient for creep effect = 1.8
[ (0.82) (867,000) (86xl2)2
= [(3.06xl06) (120,400) . (0.0983xl4.33
+ 0.0267x6.2) -~5-48
(4,803,702) (86xl2)2l =
( 3 • O 6x 1O6 ) ( 12 0, 4 0 O) J 1 • 8 II i 3.2
. (3) Midspan deflection due to weight of slab:
= 5 48
. (Ms) (L2)
(Ec2) (IB)
Ms = moment at midspan (in-lb) due to wt. of slab.
= 5 48
(6,789,528) (86xl2)2 (3 • 06xlo6 ) ( 120 I 400)
= II
2. 04i
(4) Midspan deflection due to creep and shrinkage in slab:
= 25% x 2~04 = 0~5~
A-5
top related