Bridge Design to AS 5100Sydney May 25th 2005

Using High Strength Concrete

with AS 5100 opportunities and restrictions

Introduction• Increasing international use of HSC in bridges

• Mainly in response to durability problems; de-icing salts; freeze-thaw conditions

• Focus of this paper - direct economic benefit

• Saving in materials

• Reduced construction depth

• Reduced transport and erection cost

Overview• What is High Performance Concrete?

• Use of HPC in Australia

• Economics of High Strength Concrete in N America

• HSC in AS 5100 and DR 05252

• Case Studies

• Future developments

• Recommendations

What is High Performance Concrete?

"A high performance concrete is a concrete in which certain characteristics are developed for a particular application and environments:

• Ease of placement• Compaction without segregation• Early-age strength• Long term mechanical properties• Permeability• Durability• Heat of hydration• Toughness• Volume stability• Long life in severe environments

Information on H.P.C.

“Bridge Views” – http://www.cement.org/bridges/br_newsletter.asp

“High-Performance Concretes, a State-of-Art Report (1989-1994)”

- http://www.tfhrc.gov/structur/hpc/hpc2/contnt.htm

“A State-of-the-Art Review of High Performance Concrete Structures Built in Canada: 1990-2000” - http://www.cement.org/bridges/SOA_HPC.pdf

“Building a New Generation of Bridges: A Strategic Perspective for the Nation” -

http://www.cement.org/hp/

Use of H.P.C. in Australia

• Maximum concrete strength limited to 50 MPa until the introduction of AS 5100.

• Use of HPC in bridges mainly limited to structures in particularly aggressive environments.

• AS 5100 raised maximum strength to 65 MPa

• Recently released draft revision to AS 3600 covers concrete up to 100 MPa

Economics of High Strength Concrete

0.6 in diameter strands 0.5 in diameter strandsSpacing, m 3.4 2.7 2.1 1.5 3.4 2.7 2.1 1.5

SectionAAASHTO 90 83 83 83 76 76 69 69

Type IAAASHTO 90 90 90 83 76 76 76 76

Type IIAAASHTO 83 83 83 76 76 69 69 62

Type IIIAAASHTO 83 83 83 83 62 62 62 62

Type IVNU1100 83 76 76 76 62 62 62 62

NU1350 83 76 69 69 62 62 62 55

Table 1 Maximum effective girder compressive strength, after Kahn and Saber (34)

Economics of High Strength Concrete

• Compressive strength at transfer the most significant property, allowable tension at service minor impact.

• Maximum spans increased up to 45 percent• Use of 15.2 mm strand for higher strengths.• Strength of the composite deck had little impact.• HSC allowed longer spans, fewer girder lines, or

shallower sections.• Maximum useful strengths:

• I girders with 12.7 mm strand - 69 MPa• I girders with 15.2 mm strand - 83 MPa• U girders with 15.2 mm strand - 97 MPa

Economics of High Strength Concrete

AS 5100 Provisions for HSC

• Maximum compressive strength; 65 MPa

• Cl. 1.5.1 - Alternative materials permitted

• Cl 2.5.2 - 18 MPa fatigue limit on compressive stress - conservative for HSC

• Cl 6.11 - Part 2 - Deflection limits may become critical

• Cl 6.1.1 - Tensile strength - may be derived from tests

• Cl 6.1.7, 6.1.8 - Creep and shrinkage provisions conservative for HSC, but may be derived from test.

AS 5100 and DR 05252Subject

AS 5100 DR 05252 AS 5100 DR 052521.1.2 1.1.2 Concrete srength and

density range25-65 MPa, 2100-2800 kg/m3 20-100 MPa, 1800-2800 kg/m3

1.5.1 - Use of alternative materials Alternatives allowed Clause removed

2.2 2.2 Strength reduction factors Phi reduced for ku > 0.4 Phi reduced for ku > 0.375

2.5.2 - Fatigue provisions Maximum stress under fatigue loading = 18 MPa

Not included

6.1.1(b,c)

3.1.1.2(b) Tensile strength From compressive strength or tests From flexural or tensile tests, upper and lower bound factors applied if compressive strength used

6.1.2 3.1.2 Modulus of elasticity Proportional to square root fc Revised for higher strength grades6.1.7 3.1.7 Shrinkage Default basic shrinkage strain

independent of concrete strengthAutogeneous and drying shrinkage calculated separately, both related to concrete strength

6.1.8 3.1.8 Creep Basic creep factor constant for f'c >= 50 MPa

Basic creep factor increased for f'c = 40, 50 MPa; reduced for f'c >= 80 MPa

6.4.3.3 3.4.3.3 Loss of prestress due to creep

Default creep factor uses prestress force before time-dependent losses.

Default creep factor reduced to 80% of AS 5100 value

8.1.2.2 8.1.3 Rectangular stress block Stress = 0.85f'c Stress = (1.0-0.003f'c)f'c with limits of 0.67 and 0.85

8.2.7.1 8.2.7.1 Shear strength of beams excluding shear reinforcement

Shear strength proportional to f'c1/3 f'c

1/3 limited to 4 Mpa, ie no increase in shear strength for f'c > 64 MPa

8.2.8 8.2.8 Minimum shear reinforcement

Independent of concrete strength Increased area for f'c > 36 MPa

8.6.1(a) 8.6.1(a) Minimum steel area in tensile zone

3ks(Act/fs) Cl 8.1.4.1 (minimum strength requirements) applied

9.1.1 9.1.1 Minimum tensile steel in slabs

Independent of concrete strength Increased area for f'c > 30 MPa approx

Clause Provisions

AS 5100 and DR 05252

Main Changes:• Changes to the concrete stress block parameters for

ultimate moment capacity to allow for higher strength grades.

More detailed calculation of shrinkage and creep deformations, allowing advantage to be taken of the better performance of higher strength concrete

Shear strength of concrete capped at Grade 65.

Minimum reinforcement requirements revised for higher strength grades.

Over-conservative requirement for minimum steel area in tensile zones removed.

Case Studies

• Concrete strength: 50 MPa to 100 MPa

• Maximum spans for typical 3 lane Super-T girder bridge with M1600 loading

• Standard Type 1 to Type 5 girders

• Type 4 girder modified to allow higher pre-stress force:

Increase bottom flange width by 200 mm (Type 4A)

Increase bottom flange depth by 50 mm (Type 4B)

Increase bottom flange depth by 100 mm (Type 4C)

Case Studies

Compressive strength at transfer = 0.7f’c.

Steam curing applied (hence strand relaxation applied at time of transfer)

Strand stressed to 80% specified tensile strength.

Creep, shrinkage, and temperature stresses in accordance with AS 5100.

In-situ concrete 40 MPa, 160 mm thick in all cases.

Assumed girder spacing = 2.7 m.

Case Studies

Type Depth Max NoStrands

A I Yc A I Ycmm mm2 mm4 mm mm2 mm4 mm

1 750 454,084 3.280E+10 389 887,584 7.685E+10 604 502 1000 491,409 6.675E+10 515 924,909 1.412E+11 779 503 1200 531,021 1.043E+11 613 964,521 2.112E+11 913 504 1500 573,592 1.756E+11 776 1,007,092 3.357E+11 1,122 505 1800 616,426 2.658E+11 946 1,049,926 4.886E+11 1,331 50

4A 1500 680,172 1.971E+11 627 1,113,672 4.383E+11 998 824B 1500 592,772 1.798E+11 760 1,026,272 3.488E+11 1,106 624C 1500 618,612 1.840E+11 743 1,052,112 3.632E+11 1,088 74

Section PropertiesPrecast Composite

Super-T Maximum Span

25

30

35

40

45

50

55

18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 36.00 38.00

Maximum Span, m

Nu

mb

er

of

Str

an

ds

Type 1

Type 2

Type 3

Type 4

Type 5

50 MPa

65 MPa

80 MPa

Super-T Maximum Span

45

50

55

60

65

70

75

80

85

33 34 35 36 37 38 39 40

Maximum Span, m

Nu

mb

er

of

Str

an

ds

Type 4

Type 4A

Type 4B

Type 4C

50 MPa

65 MPa

80 MPa

Case Studies - Summary

Significant savings in concrete quantities and/or construction depth.

• Grade 65 concrete with standard girders.

• Grade 80 concrete with modified girders and Type 1 and 2 standard girders.

• More substantial changes to beam cross section and method of construction required for effective use of Grade 100 concrete.

Future Developments

• Strength-weight ratio becomes comparable to steel:

0

5

10

15

20

25

30

35

40

45

Structural steel Concrete High strengthconcrete

Lightweight HSC

Strength-Weight Ratio

Future Developments

Recommendations65 MPa to be considered the standard concrete grade

for use in precast pre-tensioned bridge girders and post tensioned bridge decks.

The use of 80-100 MPa concrete to be considered where significant benefit can be shown.

AS 5100 to be revised to allow strength grades up to 100 MPa as soon as possible.

Optimisation of standard Super-T bridge girders for higher strength grades to be investigated.

Investigation of higher strength grades for bridge deck slabs, using membrane action to achieve greater spans and/or reduced slab depth.