uye ohbe

8/10/2019 uye ohbe

http://slidepdf.com/reader/full/uye-ohbe 1/35

ABSTRACT

High performance concrete(HPC) is that concrete which meet special performance

and uniformity requirements that cannot always be achieved by conventional material

,normal mixing, placing and curing practices .

Architects, engineers and constructors all over the world are finding that using HPC

allows them to build more durable structures at comparable cost .HPC is being used

for building in aggressive environments, marine structures, highway bridges and

pavements, nuclear structures, tunnels, precast units

This reports aims to discuss the application of HPC particularly for bridge structures

.The use of HPC was found to have added advantages compared with normal concrete

in areas of strengths , service life, construction time, economy ,etc.

An experimental study on “Behavior of instrumented prestressed high performance

concrete bridge girders” by Hazim M. Dwairi, Mathew c. Wagner, Mervyn

J.Kowalsky, Paul Zia is also discussed as case study

8/10/2019 uye ohbe


1. INTRODUCTION

1.1 GENERAL

Concrete is considered as durable and strong material. Reinforced concrete is one of

the most popular material used for construction around the world. Reinforced concrete

is exposed to deterioration in some regions especially in costal regions. There for

researchers around the world are directing their efforts towards developing a new

material to overcome this problem. Invention of large construction plants and

equipments around the world added to the increased use of material .This scenario led

to the use of additive materials to improve the quality of concrete. As an out come of

the experiments and researches cement based concrete which meets special

performance with respect to workability, strength and durability known as” High

Performance Concrete” was developed

1.2HIGH PERFORMANCE CONCRETE

High performance concrete (HPC) is that which is designed to give optimised

performance characteristics for the given set of materials, usage and exposure

conditions, consistent with requirement of cost, service life and durability

The American Concrete Institute (ACI) defines HPC „„as concrete which meets

special performance and uniformity requirements that cannot always be achieved

routinely by using only conventional materials and normal mixing, placing, and

curing practices.”

High performance in a broad manner can be related to any property of concrete. It can

mean excellent workability in the fresh state like self-levelling concrete or low heat of

hydration in case of mass concrete, or very rigid setting and hardening of concrete in

case of sprayed concrete or quick repair of roads and airfields, or very low

imperviousness of storage vessels, or very low leakage rates of encapsulation

containments for contaminating material.

8/10/2019 uye ohbe


HPC is composed of the same material as normal concrete, but it has been engineered

to achieve enhanced durability or strength characteristics, or both, to meet the

specified demands of a construction project. The main ingredients of high

performance concrete are cement, fine aggregate, coarse aggregate, water, mineral

admixtures and chemical admixtures

If the structure of normal strength concrete (NSC) is compared with high performance

concrete (HPC) one notes several differences: The matrix stiffness of HPC is larger

than NSC and approaches the stiffness of the aggregate, the bond strength between

matrix and aggregate is higher for HPC, matrix tensile strength is higher, Reduced

internal cracking in terms of number of cracks and size of intrinsic cracks before

loading. These aspects show that HPC is more elastic and more brittle than NSC.

Figure 1 shows a schematic representation of the stress-strain curve from a uniaxial

test along with the simplified crack pattern

HPC has a greater Young‟s modulus than NSC and the post-peak softening branch is

steeper. High Strength Concrete (HPC) is more homogeneous than normal strength

concrete (NSC). Initial flaws like pores, cracks and interfacial delamination in HPC

are smaller and less numerous than in NSC. This makes HPC more stiff and elastic as

compared to NSC

HPC does not simply mean high strength concrete (HSC), but also includes other

enhanced material properties such as early-age strength, increased flow ability, highmodulus of elasticity (MOE),low permeability, and resistance to chemical and

8/10/2019 uye ohbe


8/10/2019 uye ohbe


Table 1. Materials Used in High-Performance Concrete

1.2 HIGH PERFORMANCE CONCRETE BRIDGES

For many years, high-strength, high-performance concrete has been used in the

columns of high-rise buildings. However, in recent years, there has been increased use

of high-performance concrete (HPC) in bridges where both strength and durability are

important considerations. The primary reasons for selecting HPC are to produce a

more economical product, provide a feasible technical solution, or a combination of

both

High performance concrete bridges include two key elements: total precast bridge

systems that can dramatically improve construction speed and high performance

concrete that can improve durability and structural efficiency. In HPC bridges, these

improvements are achieved at no cost premium and often at a reduced initial cost.

8/10/2019 uye ohbe


Designing with HPC components can drastically reduce construction time because

various precast components can be combined to allow a truck-to-structure systems

approach without waiting for site forming and curing. Full depth precast decks are

being used on both new and rehabilitated bridges. The cost for this approach can

result in overall savings due to more efficient designs that permit longer spans or

fewer girders and/or piers. HPC can be used effectively in virtually all bridge

components to aid in minimizing construction and future maintenance. HPC

components can include piles and pile caps, piers and column bents, abutments,

decks, and rails and barriers. HPC uses the same materials as typical concrete but is

engineered to provide higher strength and better durability. These attributes can be

varied to align with the design‟s needs. They will be affected by environmental and

geographic conditions and the specific bridge components (that is, substructure,

beams or deck).

Fig 2 Cross section of the pier elevation shows the main compmnents of a bridge

system

8/10/2019 uye ohbe


1.2.1ADVANTAGES OF HPC

Overall, the advantages accruing from higher durability and/or additional strength

include a variety of benefits:

• Longer service life thanks to higher durability and lower chloride penetration. When

needed, bridge life can extend to 100 years or even more.

• Lower maintenance and inspection requirements, especially since the bridge requires

no painting or rust protection. This savings grows with the bridge‟s longer service

life.

• Longer spans, which can reduce costs by eliminating piers or allowing the use of

concrete beams instead of steel beams.

• Wider beam spacing, reducing the number and cost of beams.

• Shallower beams due to higher concrete strength.

• Improved mechanical properties such as greater tensile strength.

• Rapid construction due to the ability to factory-cast components while site work is

underway and the ability to erect pieces upon delivery. These benefits cut the time

necessary for disruptions to local traffic.

• Predictable performance and close tolerances for precast members due to the high

quality achieved through PCI certification and casting under controlled conditions in

the plant.

In general, HPC components can produce lighter, longer precast pieces and smaller-

diameter columns that creep less. This means span lengths can be lengthened and

under clearances can be maximized.

8/10/2019 uye ohbe


2. CASE STUDY

Behavior of instrumented high performance concrete bridge girders byHazim M.

Dwairi, Matthew C. Wagner, Mervyn J. Kowalsky, Paul Zia

2.1GENERALIn this study a comprehensive monitoring of the behavior of four prestressed high

performance concrete (HPC) bridge girders, with higher compressive strength, during

construction and while in-service, is presented. The monitoring program covered

instrumentation and monitoring of a series of four girders during the casting

operation, after construction, under the effects of traffic and thermal loads, as well as

under controlled

load conditions. Information regarding transfer length, prestress loss, heat ofhydration, compressive strength, modulus of elasticity (MOE), modulus of rupture

(MOR), creep, shrinkage, coefficient of thermal expansion, and chloride permeability

of the concrete used is obtained and presented. Furthermore, the in-service monitoring

and controlled load tests and details regarding thermal expansion, bridge stiffness, and

load distribution factors are also presented. This paper provides details of testing of

the concrete properties and field instrumentation of the bridge girders as well as a

discussion of service level monitoring and controlled load testing. Comparisons are

made between experimental and theoretical results

8/10/2019 uye ohbe


Fig.3 shows the bridge for three southbound lanes under construction, which forms

the basis for the work described in this paper. Figs.4 and5 show the plan of the bridge

and a typical cross section, respectively.

Fig. 3. US 401 Southbound Bridge over the Neuse River, Raleigh, NC.

Fig. 4. US 401 Southbound Bridge over the Neuse River plan view.

8/10/2019 uye ohbe


8/10/2019 uye ohbe


elasticity (MOE), modulus of rupture (MOR), creep, shrinkage, thermal properties,

and chloride permeability. In all cases, the concrete samples were taken from batches

of material used in four instrumented bridge girders. Two were AASHTO Type IV

girders (designated as A4 and B4) and two were AASHTO Type III girders

(designated as C4 and D4) as shown in Fig 4

Table .2

2.3.1. COMPRESSIVE STRENGTH, MODULUS OF ELASTICITY AND

MODULUS OF RUPTURE

Numerous 102 * 203 mm cylinders, six 76 * 76 *286 mm prisms, and six 152 * 152

*508 mm (6 6 20 in.) prisms were cast for the material testing. The specimens were

cured along side with the girders to keep the curing temperatures for the specimens as

close as possible to those of the actual girders. One concern in the use of HPC is that

standard cylinder tests may not provide an accurate measure of the in-place strength

of the concrete. High temperatures generated during hydration can affect the in-place

strength of HPC. For this reason, it was desirable to match-cure the cylinders that

were to be used for compression tests and the determination of the modulus of

elasticity. The match curing was performed with the help of the FHWA Mobile

Concrete Laboratory. Four cylinders were matchcured for each girder, for a total of

sixteen specimens. The match-cured cylinders were prepared in the same manner as

standard non-match-cured cylinders.

The performance requirements of the HPC and the laboratory test results are listed in

Table 2. Table 3 shows the mix proportion of the concrete that was used for the

girders. The slump and the air content of the concrete are shown in Table 4. Both the

air content and the slump met the requirements of the specifications .Table4

8/10/2019 uye ohbe


summarizes the compressive strength of match cured and non-match-cured cylinders.

The average compressive strength of the concrete for both Type III and Type IV

girders met specifications as shown in Table 2. It is noted that by using silica fume the

concrete gained strength rapidly at early age and after 56 days there was virtually no

increase in strength.

Table. 3

Table.4

8/10/2019 uye ohbe


Table.5

The measured and predicted results for the modulus of elasticity (MOE)are shown intable.6

MOE of concrete calculated by the equations

√ (ACI Code) (1)

√ + (ACI 363) (2)

By using obtained compression strength and the measured unit weight of

concrete,24kN/m3 ,the predicted values of MOE are shown in table.6

Table.6

8/10/2019 uye ohbe


The modulus of rupture (MOR) for the HPC was determined according to ASTM C

78 – 84. It was found to be 1.25 MPa for the Type III girder and 1.11 MPa for the Type

IV girder. According to the ACI Code [5], the modulus of rupture may be calculated

by

√ Mpa (3)

Using an average value of 72.4 MPa for f‟c, Eq. (3) gives a value of 5.28 MPa ,which

compares very well with the test results. Note that MOR was not specified as a

performance criterion.

2.3.2. PERMEABILITY

The rapid chloride permeability testsaccording to AASHTO T 277 were carried out in

order to get permiability value. Standard 102 * 203 mm cylinders were sliced into

thirds and the top and middle thirds were used for testing. The results are given in

table .7

Table.7

8/10/2019 uye ohbe


8/10/2019 uye ohbe


2.4. Girder behavior during casting

2.4.1. OVERVIEW OF INSTRUMENTATION PLAN

A single line of girders was instrumented in order to monitor temperature and strains

within the girders, as shown in Fig. 4. There are five girder lines, each with fourspans: 28.0, 28.0, 17.5, and 17.5 m (91.9, 91.9, 57.4, and 57.4 ft). The longer spans

use AASHTO Type IV girders and the shorter spans use AASHTO Type III girders.

The instrumented girders are designated A4, B4, C4, And D4.

The use of the HPC mix eliminated one line of girders and increased transverse girder

spacing from the original design using the conventional concrete. The strength

requirement for the girders was 69.0 MPa (10,000 psi) to 76.0 MPa (11,000 psi) at 28

days, and the average strength of the tested cylinders met the requirements as shown

in Table 2.

In order to monitor the temperature gradients within the girders both during the curing

period and the long-term testing, a total of 22 thermocouples (Omega FF-K-24) were

placed at five cross sections of each of the four girders. Ten thermocouples were

placed at mid-span, three at 1/4 L, three at 3/4 L, and three at a distance of L/ 50 from

either end (where L is the girder span. To measure strains in the concrete, a series of

Vibrating Wire ages (VWGs) (Roctest EM-5) were placed at the center of gravity of

the prestressing strands at mid-span and 1.524 m (5 ft) for girders C and D, and 1.219

m (4 ft) for girders A and B, in either side from mid-span to measure long-term

strains. Prestressing force was measured with load cells and transfer length was

determined from strains measured by using an embedded steel bar with attached strain

gages.

2.4.2. PRESTRESS FORCE

Load cells (Strainsert model PC-50 with 220 kN (50,000 lb) capacity) were placed at

the dead end of the casting bed to measure the prestressing force after tensioning, as

well as after curing, and immediately prior to detensioning of the strands. The load

cells were placed on four strands on each of the casting beds (Type III and Type IV).

Tables 8 and 9 summarize the load cell readings at various times after tensioning.

8/10/2019 uye ohbe


Table.8

Table.9

2.4.3. TRANSFER LENGTH

The transfer length of prestressing strands at the ends of the girders was determined

by measuring the strains in a „„strain gage bar” (SGB) embedded in each girder. Bars

used in girders C4, D4, and B4 had eight strain gages, while the bar used in girder A4

had nine strain gages. The gages were read with a standard strain gage indicator after

the concrete had cured and just before detensioning, in order to obtain the initial

readings. The gages were then read immediately after detensioning. The change in

strain for each gage is plotted in relation to its distance from the end of the girder in

Figs. 10 and 11. The average strain is also plotted in these figures. To determine the

transfer length from the measured strains, a method similar to that proposed by Ohand Kim [10] was followed to establish the strain plateau, which was obtained by

drawing a horizontal line at 95% of the maximum value of the plotted average strains.

The transfer length is taken as the horizontal distance from the origin (i.e. the end of

the girder) to where the horizontal line intersects the plotted average strain profile (see

Figs. 10 and 11). For the Type III girder, a transfer length of 0.711 m (28 in.) was

estimated, while 0.660 m (26 in.) was estimated for the Type IV girder.

8/10/2019 uye ohbe


Fig 6. Transfer length for Type III girder.

Fig 7. Transfer length for Type IV girder

8/10/2019 uye ohbe


2.4.4. THERMAL GRADIENTS DURING CASTING

Figs. 12 and 13 represent a sample of the girder curing temperatures along with the

ambient temperature.

Fig. 8. Thermocouples 1 – 5 for girder C4 at mid-span.

Fig 9. Thermocouples 6 – 10 for girder C4 at mid-span.

8/10/2019 uye ohbe


2.4.5. CONCRETE STRAINS DURING CASTING

Concrete strains were recorded by using embedded VWGs. A typical result is shown

in Fig. 14 for a series of vibrating wire gages. It is noticed that the strain values

change as the heat of hydration develops.

Fig.10. VWG strains for girder D4.

2.4.6. PRESTRESS LOSSES

Table 10 shows that the prestress loss due to elastic shortening based on the strain

measurement is less than predicted, especially for the Type III Girder. It is suspected

that the gages failed to record the entire compressive strain during detensioning; this

could be due to some reasons such as inadequate consolidation of concrete around the

embedded gages or failure of the gage itself. By using the predicted loss due to elastic

shortening, it appears that the total prestress loss for the Type IV Girder given in

Table 11 is slightly overestimated.

8/10/2019 uye ohbe


Table.10

Table.11

8/10/2019 uye ohbe


8/10/2019 uye ohbe


2.5. IN-SERVICE BRIDGE BEHAVIOR

As previously noted, only girder-line 4 shown in Fig 10 was instrumented.

Fig10.Plan view

In this phase of reserch three types of instruments were utilized in this phase, twelve

previously installed thermocouples (Omega FF-K-24) were retained and an additional

two thermocouples were placed in the deck at a distance of L/4 from the supports in

spans A and D, as shown in Fig. 12. Twelve of the previously installed EM-5

Vibrating Wire Gauges (VWGs), were retained and additional VWG‟s were placed in

the deck at supports. Finally, one additional LVDT was used at each abutment and

two extra LVDT‟s were used at the expansion joint to measure the longitudinal

movement of the girder. All instruments were connected to CR23X Campbell

scientific data-loggers, placed at bent diaphragms under the bridge and powered by

solar panels. Two data-loggers were used one for spans A and B, and the other for

spans C and D. The data was recorded every 4 h over a period of four month under

normal traffic loading, at each period the data-logger recorded the instruments‟

readings for five minutes. The data-logger records up to 1500 readings per second.

The monitoring of the bridge started two months after it was opened for traffic. Fig.

11shows span C and D end displacements due to thermal effects in addition to

differential displacements measured by LVDTs during the four month period.

8/10/2019 uye ohbe


8/10/2019 uye ohbe


thermocouples number 3, 4, and 5 for Girder D4 and thermocouples number 6, 7, and

8 for Girder C4 as shown in Fig. 12

Fig. 12. Retained thermocouples of girder cross-sections along girder-line 4 and

locations in each girder cross-section.

The differential displacements represent the difference between the LVDT‟s reading

at anytime and its reading when the lowest temperature was recorded. The

displacements measured by the LVDTs placed at the end of the girders show

additional end displacements due to traffic loading; however, maximum girder end

than 0.0064 m (1/4 in.). End displacement of girders caused by end rotations due to

temperature gradient along the depth of the girder cross-section was found to beminimal and it has insignificant effect on total end displacements.displacement due to

thermal effects and traffic loading was less

2.6. CONTROLLED LOAD TESTING

Two static live load tests were conducted on this bridge, the first test took place

before the bridge was opened for traffic and the second test was eight months after the

bridge was put in-service. In both tests, a five-axle truck was used (Type 3S2

AASHTO designation), loaded roughly to full capacity in one run and to half capacityin the second; the truck and it is total weight are shown in Fig. 13

8/10/2019 uye ohbe


Fig. 13. Configuration and weights of the truck used in live load tests.

The truck was positioned on 10 different locations as shown in Fig. 14 to maximize

moment at mid-spans and supports, and to estimate load distribution factors by

positioning the driver‟s wheel on Girder -4 in one run and the passenger‟s wheel on

the adjacent girder in another.

8/10/2019 uye ohbe


Fig. 14. Truck loading positions.

In addition to the internal gages alreadyembedded in the girder (VWG‟s and

LVDT‟s), two temporary string potentiometers were placed under the bridge at mid-

span D and mid-span A to record maximum deflections. The live load tests were

performed by placing the loaded five-axle truck on the desired location, when the

truck and trailer came to rest at the designated loading position, instruments readings

were recorded for a period of 30 s. The truck was then moved to next position without

unloading the bridge, and then readings were recorded in the previous manner.

8/10/2019 uye ohbe


Fig.15. Strain distribution versus cross-section depth, for girder Type-IV at the

support between span A and B due to different loading positions.

Fig.15.represents the strain distribution at the support between span A and B due to

the different loading positions. It is clear that strains due to half-loaded truck are

approximately half of the strains due to the fully loaded truck, which indicates that the

girder behaved elastically as expected. Load versus strain for both tests is shown in

Fig. 16. Strain is measured at mid-span D due to loading position 1 and at the mid-

depth of deck-slab at support between spans C and D due to loading position 2. The

strains recorded in the second live load test are larger than those recorded in the first

live load test, possibly indicating some minor softening of the system due to micro

cracking in the tension zone.

8/10/2019 uye ohbe


Fig. 16 Load versus strain for both live load tests; (a) strain measured at mid-span D.

(b) strain measured at support between span C and D.

A comparison between experimental and calculated strains is shown in Figs.17 and 18

.A simplified model was used for strain calculations; every two spans were assumed

to be a continuous beam, although, joints constructed between spans do not guarantee

full rigidity. Load distribution factors were obtained according to AASHTO

provisions and axle loads were distributed accordingly.

8/10/2019 uye ohbe


Fig.17. Experimental and calculated strain values due to half-full and full trucks.

Fig. 17 a and b show measured and calculated strains at mid-spans D and A,respectively. Calculated strains were higher than measured strains for both spans and

8/10/2019 uye ohbe


for different loading values and positions. As a result, one can conclude that the load

distribution factors given by AASHTO are higher than the actual load distributions.

Fig.18. Experimental and calculated strain distribution along girder Type IV at

middle support between spans A and B due to loading position 5.

Fig. 18 shows the strain distribution along the cross-section depth at the middle

support between spans A and B. Note that the calculated neutral axis depth was found

to be smaller than the actual one. The slight drop in the neutral axis depth between

live load test one and two could be attributed to minor cracking in the bridge deck and

diaphragm. Another comparison can be made in terms of middle span deflection. Fig.

19 a and b shows measured and calculated deflections at middle spans D and A,

respectively. Again, the calculated deflections were found to be higher than the actual

recorded values.

8/10/2019 uye ohbe


Fig. 19. Experimental and calculated deflections due to half-full truck (250 kN).

8/10/2019 uye ohbe


3. DISCUSSION

This research examined the material properties and behavior of four prestressed HPC

girders during casting and initial curing as well as during service. Based on this

research, the following conclusions can be drawn:

1. In general, the HPC used in this research was of good quality.

. The concrete performed well and met the expected results in compressive

strength, modulus of rupture, and creep.

2. The modulus of elasticity of the concrete was lower than expected,

3. The results of the rapid chloride permeability test were higher than expected. The

higher values of permeability are most likely the results of higher cement content of

the HPC mix and the use of heat and air cure of the test specimens. Both of these twofactors could cause more and larger pore structures of the concrete paste, which would

in turn increase the permeability of the concrete.

4. During concrete curing, the temperature measured by the embedded thermocouples

showed that peak temperatures occurring 7 – 8 h after casting never reached more than

Therefore, there was no danger of thermal cracking.

5. Based upon the load cell readings (Tables8and 9), practically there were no

changes of the initial prestressing force up to the time of detensioning. Therefore the

measurement suggested that there was no loss of prestress due to strand relaxation

prior to detensioning.

6. Upon detensioning, the transfer lengths for the 0.015 m (0.6 in.) strand were found

to be 0.711 m and 0.660 m respectively, for Type III and Type IV girders. These

values are slightly less than the standard design value of 50 times the strand diameter

or 0.762 m

7. The calculated prestress loss due to elastic shortening was 82.7 MPa for the Type

III girders and 124.8 MPa for the Type IV girders. Total prestress loss was 179.3

MPa), i.e. 12.9%, for the Type III girders and 262.7 MPa, i.e. 19.1%, for the Type IV

girders.

8. The predicted camber compared closely with the measured camber. The close

prediction was possible because the use of load cells at the anchoring end of the

prestressing bed provided a more accurate value of the prestressing force at

transfer than the normally assumed prestressing force based on estimated loss of

prestress.

8/10/2019 uye ohbe


9. Girder end displacements were caused mainly by thermal effects with small effect

due to traffic loading, while displacements due to end rotations could be neglected,

however, maximum total girder end displacement was less than a quarter an inch.

10. The calculated strains and deflections based on AASHTO load distribution factors

were found to be higher than actual recorded data.

8/10/2019 uye ohbe


uye ohbe

Documents