Editorial

1997 ASBI Leadership Awards 3

STRUCTURAL LIGHTWEIGHT CONCRETE BRIDGES

The California Department of apparent deterioration of the deck

Transportation (Caltrans) has used concrete. A 1986 Design Policy

expanded shale structural lightWeight Memo suggests the use of structural

concrete for bridge construction as a lightWeight concrete in deck

substitute for normal weight concrete replacement and rehabilitation at

for both older bridge decks and locations where local aggregates are

widenings, and new bridge unsuitable, as a cost effective material

construction on the California State for long span structures, and in

Highway System for the past forty five seismic regions where superstructure

years. Early use was primarily for deck dead load needs to be reduced.

elements to reduce the dead load The outstanding performance of

imposed on supporting these lightWeight concrete bridges

superstructures, bents, abutments and under heavy traffic, and the close

foundations. The additional weight competition in alternative bidding

imposes severe problems on suggests that lightWeight aggregate is a

foundation design in a highly active material which should be considered

seismic zone. Since it is an accepted in future bridge designs, especially in

bridge design philosophy that the earthquake regions where dead load is

most economical design balances such an important factor in seismic

superstructure and substructure costs, design. The known consistent creep,

it is just as logical to use structural shrinkage and modulus properties of

lightWeight concrete in non-seismic lightWeight aggregate remove any

zones, especially where foundation doubts about performance as our

materials are weak. LightWeight

concrete can be produced with a unit

weight of 120 pounds per cubic yard,

affording a 20 percent weight

reduction over normal weight

concrete. A total of 15 major

California bridges have been designed

with structural lightWeight concrete

decks, and several bridges use

structural lightWeight aggregate

concrete for the entire superstructure.

T wo of those have been in service for

several years and eight have been in

place in excess of 30 years with noEditorial byJames Eo Roberts, Caltram

latest technological developments.

Questions regarding the shear strength

and ductile performance of structural

lightweight concrete prompted research

at the University of California at San

Diego, financed by the Department.

The Carquinez Bridge site is on

Interstate 80 at Vallejo where two

bridges carry the east and west bound

lanes. The west bound bridge was

erected in 1927 and is severely

overloaded by the current truck loads.

A new westbound bridge has been

financed and design studies are

undetway. Several alternatives were

studied, including a structural

lightweight concrete segmental bridge.

In any bridge constructed at this site

the decks will be constructed of

structural lightweight concrete.

The Benicia-Martinez site is on

Interstate 680, upstream of the

Carquinez site, and parallel to the

bridge which was recenrly widened.

This 7200 foot bridge will carry five

lanes of northbound traffic and the

older, widened bridge will carry the

southbound lanes. Design alternative

studies were completed by four

separate consulting firms to determine

the two most competitive. Studies were

conducted for a structural steel truss,

similar to the existing bridge, a

structural steel box girder, a concrete

structures have shown. The industry

advances in controlling lightweight

aggregate moisture content have

considerably reduced the handling and

finishing problems of earlier years.

The Napa River Bridge (Fig. 1) on

State Highway 12 at the southern

entrance of the world famous Napa

Valley wine country is an aesthetic

award winning 2230 foot, thirteen

span continuous post tensioned,

prestressed structural lightweight

aggregate concrete box girder

superstructure spanning the Napa

River on lOO foot high normal weight

concrete piers. The superstructure

contains 11,000 cubic yards of

structural lightweight concrete utilizing

expanded shale aggregate and is

supported on spans up to 250 feet. The

lightweight concrete alternative design

was bid lower than a structural steel

girder alternative design in 1974. This

bridge has been in service on a busy

four lane arterial state highway for over

20 years and has not shown any signs

of maintenance problems.

Preliminary plans to bridge two large

bodies of water in the San Francisco

Bay Area with long span structures over

1.5 miles in length has prompted the

Department to review and update the

overall policy on use of structural

lightweight concrete, incorporating the

Figure 1

Overall View of

Napa River Bridge.

and steel cable stayed bridge, and a

structural lightweight concrete

segmental box girder bridge. The

structural steel truss and the structural

lightweight concrete segmental box

girder bridge were the two most

competitive designs. Confirming cost

estimates were conducted by a fifth,

cost estimating specialty consulting

firm to remove any doubt from the

comparisons. Each bridge is composed

of a series of 528 foot spans supported

on normal weight piers ranging up to

250 feet from bedrock to deck.

Structural lightweight concrete was

planned to be used for the decks and

superstructure on both alternatives,

with polyester concrete overlay wearing

surfaces. In 1996 the decision was

made to complete design of only the

structural lightweight concrete

alternative, after bids on some nearby

structural steel bridges showed that

material not to be competitive with

concrete in this region.

Concerns over the shear strength and

ductile performance of structural

lightweight concrete in a seismic event

prompted the Department to initiate a

research project at the University of

California at San Diego. This

lightweight concrete testing program is

being conducted in three phases; first

to determine the shear strength of

structural lightweight concrete, second

to investigate the flexural strength and

ductility, and third to investigate the

dynamic behavior of structural

lightweight concrete. The results of the

first two phases are available now and

are discussed in a separate article in this

newsletter.

The performance of existing bridges

and the research results to date indicate

that structural lightweight concrete

using expanded shale aggregate is a

viable alternative to normal weight

concrete, especially where dead load is

a design consideration. It can also be

used in columns with dependable,

predictable behavior in seismic zones.

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This is supported by test observations

which showed that cracks form

through the aggregate instead of

around them, which implies a

reduction in aggregate interlock.

weight concrete, 3. normal lightweight

concrete {using lightweight aggregate

and normal weight sand), and 4. high

strength lightweight concrete. Control

cylinders were cast from each mix to

measure shrinkage and adjust the

measured creep strains to obtain a

more precise estimate of creep strain

for the various mixes. It is clear from

these tests that High Strength

Lightweight Concrete strained the

least, while the lightweight batch

strained the most. Further tests will be

conducted during the bridge design

phase to confirm the creep, shrinkage

and modulus coefficients for the mix

design proposed. It is essential that

these tests be conducted for each mix

design during the design phase of a

project.The results of this research to date

indicate that structural lightweight

concrete using expanded shale

aggregate is a viable alternative,

especially where dead load is a design

consideration. It can be used in

columns with dependable, predictable

behavior in seismic zones. These tests

confirm that shear strength of

structural lightweight aggregate

concrete is not a major issue and can

be dealt with in the design.

Flexural Strength and Ductility Tests

Results of the flexural strength and

ductility tests suggest that the initial

cracked section stiffness of a

lightweight concrete member can be

conservatively reduced by 15% from

the stiffness of a normal weight

concrete column. This would result in

an increase in elastic displacements in

a moderate earthquake. For design for

the ultimate limit state the reduced

stiffness would not playa role.

However, the use of force based design

would likely result in an inaccurate

estimate of displacement. Therefore

the use of direct displacement based

design is recommended.

Based on these tests it can be

concluded that the hysteretic damping

of structural lightweight concrete is

essentially the same as for normal

weight concrete. For direct

displacement based design damping

relations for normal weight concrete

can be applied without modification

for lightweight concrete. Analysis of

these test results indicate that the

ultimate concrete compression strain is

not affected by the type of concrete,

and that estimates of displacement

capacity with the same degree of

conservatism as for normal weight

concrete can be obtained for light

weight concrete.

Long Term Creep Tests

In conjunction with the shear and

ductility tests the UC San Diego team

also conducted a series of long term

creep tests of the lightweight aggregate

concrete, using several different mixes,

and expanded shale lightweight

aggregate. Tests were conducted on 1.

lightweight concrete (using both light

weight aggregate and sand), 2. normal