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Prefabricated Bridge Elements and Systems

Lightweight Concrete Benefits for PBES Deployment

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Slide 1. Lightweight Concrete Benefits for PBES Deployment

Reid W. Castrodale, PhD, PE
Director of Engineering
Carolina Stalite Company, Salisbury, NC

Speaker Notes:

My name is Reid Castrodale.

I am the Director of Engineering for Carolina Stalite, one of the companies that manufactures lightweight aggregate in the US. I am representing the lightweight concrete industry today, as we discuss the use of lightweight concrete in the deployment of PBES.

But before we get into the details of how lightweight concrete can be a benefit for accelerated bridge construction using PBES, I’d like to give you some information on lightweight aggregate and lightweight concrete.

 

Slide 2. Learning Outcomes

After completing this Module, you will be able to:

  • describe how lightweight aggregate is manufactured
  • identify the classifications of lightweight concrete
  • identify several advantages of using lightweight concrete for PBES bridges
  • recall several PBES projects where lightweight concrete was or could have been used
 

Slide 3. Development of LWC

Photo showing the first use of lightweight concrete on ships during World War I. The ship is visible in Galveston Bay off the east side of Pelican Island.
  • In early 1900s, Stephen Hayde discovered method to manufacture lightweight aggregates (LWA) from shale, clay and slate
    • Some bricks bloated during burning
    • Development of rotary kiln process began in 1908
    • Patent for expanding LWA using a rotary kiln process was granted in 1918
  • The first use of lightweight concrete (LWC) was for ships in World War I

Speaker Notes:

In the early 1900s, Stephen Hayde, a brick maker in Kansas City, noticed that some of his bricks bloated during firing. He realized that the expanded material could be used as a lighter aggregate for concrete and other uses, so he set out to develop a way to manufacture expanded aggregate.

When he was granted a patent in 1918, WWI was still underway and had created a shortage of plate steel for shipbuilding. He provided the use of the patent to the government so they could use lightweight concrete to build ships. The photo shows the launching of one of the first lightweight concrete ships, the USS Selma, in 1919. The ship is still visible in Galveston Bay off the east side of Pelican Island where it has been beached since it was taken out of service in the 1920s after providing satisfactory service. In areas of the ship where the concrete is undamaged, the 0.5" to 1.2" cover has done a surprisingly good job of protecting the reinforcing steel from corrosion.

 

Slide 4. Development of LWC, (Cont’d.)

  • Early use of LWC in a bridge project
    • San Francisco-Oakland Bay Bridge
    • Upper deck of suspension spans was constructed using LWC in 1936
    • Lower deck was rebuilt with LWC for highway traffic in 1958
    • Both decks are still in service
Photo of the San Francisco-Oakland Bay Bridge.
San Francisco-Oakland Bay Bridge construction. Photo shows the lower deck being reconfigured for highway traffic in 1958 using LWC.

Illustrates the early use of lighweight concrete for bridges.

Speaker Notes:

An early use of lightweight concrete for bridges is the upper deck of the suspension spans of the San Francisco-Oakland Bay Bridge. The deck was completed in 1936 using "all lightweight concrete" with a dry density of about 95 lbs/cf. Lightweight concrete was used again in 1958 when the lower deck was reconfigured for highway traffic. Both decks are still in service today.

An even earlier example of lightweight concrete for bridge decks is the original deck on the Lewis and Clark Bridge over the Columbia River between OR and WA. That bridge was completed in 1930 with a lightweight concrete deck, which served until it was replaced using SPMT technology with large precast deck panels in 2003, as mentioned in some of the other presentations on PBES. I’ll talk about this bridge again later in this presentation.

 

Slide 5. Structural LWA

Photo of lighweight aggregate composites and raw materials. The raw materials are being extracted from the ground using standard excavation or mining techniques. The raw material is then fed into a rotary kiln, very similar to those used to manufacture cement.  As the material moves through the kiln, it is heated until it reaches 1900 to 2200 deg. F at the lower end of the kiln.  At these temperatures, the material softens and gas bubbles form within it.  They transform into shale, clay or slate. Gas bubbles are formed in the softened material and are trapped when cooled, as shown.

LWA is manufactured

  • Raw material is shale, clay or slate
  • Expands in kiln at 1900 – 2200 deg. F
  • Gas bubbles formed in softened material are trapped when cooled

Speaker Notes:

Almost all of the structural lightweight aggregate used in the US is manufactured by expanding raw materials in a rotary kiln, using the same process that was developed by Stephen Hayde nearly a century ago. The raw materials used in the US are shale, clay and slate which are extracted from the ground using standard excavation or mining techniques. The raw material is then fed into a rotary kiln, very similar to those used to manufacture cement. As the material moves through the kiln, it is heated until it reaches 1900 to 2200 deg. F at the lower end of the kiln. At these temperatures, the material softens and gas bubbles form within it. The soft material is thick enough that the gas bubbles do not escape. When the heated material exits the kiln and is cooled, the bubbles remain as the material hardens, producing a porous lightweight ceramic aggregate.

 

Slide 6. Relative Density

Photo of each material soil, gravel, ESCS Agg., limestone, sand measured in beakers. This is a figurative representation showing the volume or relative density of different materials.  1 pound of each in aggregate density. It is clear that the expanded aggregate has twice the volume as the same mass of normal weight materials.

1 lb. of each
aggregate

  • Rotary kiln expanded LWA
    • Range from 1.3 to 1.6
  • Normal weight aggregate
    • Range from 2.6 to 3.0
  • Twice the volume for same mass
  • Half the mass for the same volume

Speaker Notes:

The expanded material has a relative density, or specific gravity, in the range shown, which is about half the relative density of normal weight aggregate. This is illustrated by the photo which shows the volume occupied by a pound of different materials. It is clear that the expanded aggregate has twice the volume as the same mass of normal weight materials.

Another way of looking at this is that the same volume of lightweight aggregate will weigh about half as much as normal weight aggregate. This is what happens with lightweight concrete, where normal weight aggregate is replaced with an approximately equal volume of lightweight aggregate that weighs about half as much.

It should be noted that the relative density and other properties of the expanded aggregate vary somewhat between sources of aggregate and types of raw material.

 

Slide 7. LWA is just a lighter rock!

  • LWA is lighter than NWA
  • But LWA still satisfies typical specifications required of NWA for use in most construction applications
    • Different gradations – AASHTO M 195
  • A non-concrete application for LWA
    • Geotechnical fill
    • Can be used on ABC projects

Speaker Notes:

It is important to realize that after expansion, structural lightweight aggregate is just a lighter rock! The high temperature firing of the raw materials converts them into a vitrified ceramic material that has a hardness equivalent to quartz.

Lightweight aggregate should satisfy the requirements for normal weight aggregates. The only exception is that the gradations are defined differently. The AASHTO materials specification for structural lightweight aggregate is M 195, which covers both coarse and fine gradations.

Lightweight aggregate has a higher absorption than normal weight concrete. Because of this, prewetting of the aggregate is important, especially when lightweight concrete will be pumped.

When lightweight aggregate is used in concrete, lightweight concrete results. The same batch plants and mixing procedures are used for lightweight concrete. The same admixtures can also be used for lightweight concrete. Mix design procedures can also be the same as for normal weight concrete. The absorbed moisture does not participate as mix water since it will not be released into the concrete until after hydration is underway.

A final notable difference between lightweight and normal weight concrete is that the air content of lightweight concrete is tested using the "roll-o-meter", or the volumetric method.

 

Slide 8. Geotechnical Use of LWA

  • LWA can be used as structured fill
  • LWA is free draining
Property LWA NWA or Soils
Compacted in-place bulk density 40-65 pcf 100-130 pcf
Bulk Loose density (dry) 30-50 pcf 89-105 pcf
Angle of internal friction 35°-45°+ 30°-38°
Abrasion resistance (Loss) 20-40% 10-45%
 

Slide 9. Pentagon Secured Entrance

LWA fill was used between MSE walls

  • Reduced anticipated settlement from 15" to 6"
  • Reduced settlement time from 180 days to 60 days
  • Enabled contractor to meet tight project schedule

Photo of the Pentagon Secured Entrance. LWA fill is being used between MSE walls.


 

Slide 10. LWA is just a lighter rock!

  • When LWA is used to make LWC
    • Same batch plants and mixing procedures
    • Same admixtures
    • Can use same mix design procedures
    • "Roll-o-meter" for measuring air content
  • LWA has higher absorption than NWA
    • Needs to be prewetted, especially for pumping
  • Visit ESCSI website or contact LWA supplier for more info on properties of LWA and LWC

Speaker Notes:

It is important to realize that after expansion, structural lightweight aggregate is just a lighter rock! The high temperature firing of the raw materials converts them into a vitrified ceramic material that has a hardness equivalent to quartz.

Lightweight aggregate should satisfy the requirements for normal weight aggregates. The only exception is that the gradations are defined differently. The AASHTO materials specification for structural lightweight aggregate is M 195, which covers both coarse and fine gradations.

Lightweight aggregate has a higher absorption than normal weight concrete. Because of this, prewetting of the aggregate is important, especially when lightweight concrete will be pumped.

When lightweight aggregate is used in concrete, lightweight concrete results. The same batch plants and mixing procedures are used for lightweight concrete. The same admixtures can also be used for lightweight concrete. Mix design procedures can also be the same as for normal weight concrete. The absorbed moisture does not participate as mix water since it will not be released into the concrete until after hydration is underway.

A final notable difference between lightweight and normal weight concrete is that the air content of lightweight concrete is tested using the "roll-o-meter", or the volumetric method.

 

Slide 11. Lightweight Concrete

  • LWA is used to reduce density of concrete
  • "All lightweight" – all aggregates, both fine and coarse, are lightweight
  • "Sand lightweight" – lightweight coarse aggregate and normal weight sand (most common)
  • "Specified density" – blend of NW and LW aggregate to achieve target density (SDC)
  • Density of LWC is checked during placement for QC

Speaker Notes:

Now we’ll turn from looking at lightweight aggregate to lightweight concrete, which, as I just mentioned, is made by using lightweight aggregate in concrete. While we have talked about the "unit weight" of concrete in the past, I’ll be using the term "density" for this presentation.

Two types of lightweight concrete are recognized in the design specifications:

The first type is "all lightweight concrete" in which all of the aggregate in the mixture, both coarse and fine gradations, are lightweight.

The second type, which is most common, is "sand lightweight concrete" in which the coarse aggregate is lightweight but the fine aggregate is normal weight sand. In almost all cases today, when someone mentions "lightweight concrete" or "structural lightweight concrete", they mean sand lightweight concrete.

A third type of concrete containing lightweight aggregate is usually called "specified density concrete". It uses a blend of lightweight and normal weight aggregates to achieve the desired density, which is usually in the range between "sand lightweight concrete" and normal weight concrete, i.e., from about 125 to 145 pcf. This type of concrete is most often used by the precast/prestressed concrete industry to address handling and shipping issues.

An important issue related to the use of lightweight concrete is that its density is specified. This is important because the structure has usually been designed using a reduced density. Therefore, the density of the fresh lightweight concrete is checked for acceptance as an important aspect of quality control during construction. This means that lightweight concrete gets more attention during batching and placement than most normal weight concrete mixtures, because the density of normal weight concrete is not specified and is therefore not an issue for acceptance.

 

Slide 12. Definitions

  • AASHTO LRFD Specs (Section 5.2)
    • Lightweight concrete: "Concrete containing lightweight aggregate and having an air-dry unit weight not exceeding 0.120 kcf..."
    • Normal weight concrete: "Concrete having a weight between  0.135    and 0.155 kcf"
  • Concrete that falls between these definitions is often called specified density concrete (SDC)

Speaker Notes:

Two definitions for concrete are repeated here from the AASHTO LRFD Bridge Design Specifications: lightweight concrete and normal weight concrete

An upper limit is given for the density of lightweight concrete, although for higher strengths, such as for prestressed concrete girders, sand lightweight concrete mixtures may exceed this limit by a small amount.

For normal weight concrete, the definition includes a range of densities. Since the density of normal weight concrete is most often between 140 and 150 pcf, a density of 135 may be achieved using specified density concrete with a blend of lightweight and normal weight aggregates.

The fact that the LRFD Specifications have a definition for lightweight concrete reveals that design using lightweight concrete is addressed in the specifications.

While not defined in the specifications, specified density concrete with densities between lightweight and normal weight concrete, can be used where a partial reduction in density is needed to achieve certain construction or design objectives.

 

Slide 13. Spectrum of Concrete Density

All LWC Sand LWC NWC
Specified Density Concrete (SDC)
90 – 105 pcf 110 – 125 pcf 135 – 155 pcf
LW Fine NW Fine NW Fine
LW Coarse LW Coarse NW Coarse

Density ranges shown are approximate

Must add allowance for reinforcement (typ. 5 pcf)

Speaker Notes:

This slide illustrates the concept that there is a range of concrete densities that can be achieved using lightweight aggregate.

The lightest concrete is "all lightweight concrete" which can have densities as low as 90 pcf for structural concrete mixtures.

The intermediate density concrete is "sand lightweight concrete" which typically has densities in the range of 110 to 125 pcf.

Strictly speaking, "specified density concrete" is any type of concrete which contains lightweight aggregate and is not "all lightweight" or "sand lightweight" concrete. But the term is most often used to achieve densities between sand lightweight and normal weight concrete.

The types of aggregate used to make the different types of concrete are also shown in this table.

The density ranges are approximate and depend on aggregate type, specified compressive strength and other mix parameters. Consult lightweight aggregate suppliers for densities that can be achieved for a specific project.

Please note carefully that the densities shown here are for plain concrete. The designer must add an allowance for reinforcement when determining the density to be used for reinforced concrete in dead load computations. This allowance is often taken as 5 pcf, but this may not be enough for heavily reinforced elements.

 

Slide 14. Specifying Density of LWC

  • "Equilibrium density" is defined in ASTM C 567
    • Density after moisture loss has occurred over time
    • Often used for dead load calculations
  • "Fresh density" used for QC tests during casting
    • Designer or supplier must specify
    • Must use for precast member weight at early age
    • May use for final design loads for large elements
  • Add reinforcement allowance to concrete density when computing dead loads (typ. 5 pcf)

Speaker Notes:

When specifying the density of lightweight concrete mixtures, two conditions should be considered.

The first is the "equilibrium density", which is the density that results after the concrete has been allowed to dry. The reason this is considered for lightweight concrete is that it contains more moisture because of the water absorbed when the lightweight aggregate is prewetted. In many cases, this density is used for dead load computations for service load conditions.

The second density is the "fresh density" or plastic density. This density is used for QC testing during construction. It must also be used for handling loads for PBES structures where precast elements are used, since it is greater than the equilibrium density. Shortly after casting, when precast elements are removed from the forms, very little drying has occurred, so the weight of the element is defined by the fresh density. Some also suggest that the fresh density be used for dead load computations at service load conditions, especially where element dimensions are relatively large and supplementary cementitious materials are used. Both of these factors result in a decreased potential for the migration of moisture out of the element, making the fresh density the reasonable, and conservative, choice for defining density of lightweight concrete.

The contract documents must clearly state the type of density that is intended for acceptance.

Furthermore, the contract documents should state the density used for dead load computations, which includes the reinforcement allowance.

 

Slide 15. DOT Specifications for LWC

  • Sand LWC for Bridge Decks
    • TennDOT includes in Standard Specifications
    • NCDOT, UDOT, etc. have std special provisions
    • Other states have project special provisions
  • All LWC
    • Has not been used in recent years
    • Special provisions have been developed for NCDOT

Speaker Notes:

Since lightweight concrete has been available to DOTs for use for about 80 years, some states have addressed the requirements for lightweight aggregate and lightweight concrete in their standard specifications or by special provisions. Some examples are listed here.

Sand lightweight concrete for bridge decks:

  • The Tennessee DOT includes sand lightweight concrete for bridge decks in their Standard Specifications.
  • Several states, including NCDOT and UDOT, have developed standard special provisions for sand lightweight concrete for bridge decks.
  • Other states have used project specific special provisions when lightweight concrete has been specified in a project.

All lightweight concrete:

  • This type of concrete has not been specified for quite a few years. However, its value is again being recognized, especially for situations such as truss rehabilitations where the reduction in dead load of a bridge deck can result in significant savings if modifications to a truss can be minimized or avoided.
  • NCDOT has a demonstration project with an all lightweight concrete deck for which a project specific special provision is being developed.
 

Slide 16. FRP Products for Infrastructure

Semi-LWC for Girders

  • INDOT allows in design manual (120-130 pcf)
    • Recurring special provisions being developed

Sand LWC for Girders

  • GDOT has special provisions (10 ksi at 120 pcf)
  • VDOT has special provisions (8 ksi at 125 pcf)

Approved aggregate lists

  • A number of states have approved LWA sources

Speaker Notes:

Semi-lightweight concrete for girders:

  • This is a term used by the IN DOT for specified density concrete. It has been widely used in the state for large pretensioned bridge girders. A standard special provision, also known as a recurring special provision in IN, is being developed.

Sand lightweight concrete for girders:

  • GDOT has built a demonstration project with 10 ksi sand lightweight concrete girders with a density of 120 pcf. Project specific special provisions were developed for the project.
  • VDOT has built 3 bridges for which sand lightweight concrete was used in the girders. Project specific special provisions were developed.

A key issue for most states is that an aggregate must be approved before it can be used in the state. Therefore, it is important for aggregates to appear on the approved aggregate list in a state. In a number of states, lightweight aggregate sources have been approved and appear on the approved aggregate list.

 

Slide 17. GDOT Special Provisions

  • Special provisions for 10 ksi sand LW HPC girders
    — Maximum air-dry density is 120 pcf
    — Size of LW coarse aggregate = ½ in.
      — Minimum cement factor = 650 lbs/cy
      — Maximum water-cement ratio = 0.330
      — Slump acceptance limits = 4½ ± 2½ in.
      — Entrained air acceptance limit = 5 ± 1½ %
      — Max. chloride permeability = 3,000 coulombs
  • Same as for NW HPC, except density & aggr. size

Speaker Notes:

As an example for special provisions for lightweight concrete, the GDOT special provision requirements for lightweight concrete girders are shown. The requirements are the same as for normal weight high performance concrete, except the maximum density is specified (120 pcf) and the maximum aggregate size is ½" rather than the 67 stone used for normal weight HPC.

 

Slide 18. Benefits of Using LWC

  • Reduced weight of precast elements
      Focus of webinar
    • Improves handling, shipping and erection
    • Can also improve structural efficiency
  • Enhanced durability
      Opposite of what many expect!
    • Reduced cracking tendency
    • Reduced permeability
    • Tighter quality control with a specified density
  • LWC can be used to achieve both accelerated construction and longer-life structures

Speaker Notes:

Now that you have been introduced to lightweight aggregate and lightweight concrete, the question must be asked, why should lightweight concrete be used for bridge structures?

The main reason is obvious: the weight of the structure will be reduced, and in bridges, the dead load of the structure is often a significant portion of the design loads. However, precast concrete elements for bridges are often very large. As a result, a reduction in weight affects the shipping, handling and erection of these elements. The structural efficiency of the bridge is usually improved by reducing the weight of the structure.

Another benefit of lightweight concrete for bridges is that lightweight concrete provides enhanced durability compared to normal weight concrete. This is because lightweight concrete has been shown to have a reduced cracking tendency and reduced permeability. These features are not expected by some engineers, since they think that a "porous" aggregate could not provide equal durability to normal weight concrete. However, research and field performance of structures, such as the upper deck of the San Francisco-Oakland Bay Bridge, demonstrate the improved performance.

In addition to the improved material properties of lightweight concrete that lead to increased durability, the quality of the concrete is also improved because of the increased attention to quality control required to achieve the specified density of the concrete.

Since the focus of this workshop is on prefabricated bridge elements and systems to accelerate bridge construction, the remainder of this presentation will focus on the effect that a reduction in the weight of concrete can have on PBES projects. Information on the other benefits is available elsewhere.

 

Slide 19. Enhanced Durability

  • Improved bond between aggregate & paste
  • Elastic compatibility
  • Internal curing
  • Reduced cracking tendency
  • Improved resistance to chloride intrusion
  • Increased fire resistance
  • Enhanced resistance to freezing & thawing
  • Good wear and skid resistance
  • Alkali-silica reactivity (ASR) resistance
 

Slide 20. Cost of LWC

  • Increased cost of LWA
    • Additional processing

Left photo showing the production of LWA. Right photo of materials in its raw state. The cost of greater than normal weight concrete bacause of the increased cost of the lightweight aggregate. The cost of production of lightweight aggregate is significant, because of the high temperature processing of the raw materials.


  • Shipping from the manufacturing plant

Left photo of the US Map and the shipping that occurs from manufacturing plants. Right photo of various structures. The cost of shipping of the finished aggregate may be significant.  However, it should be noted that lightweight aggregate can be shipped long distances relatively economically.  In some cases, lightweight aggregate has been shipped across the country or even overseas to projects where it becomes an important part of an economical structural solution.


Speaker Notes

But before moving on to look at potential benefits for PBES projects, we must first address the cost associated with the reduction in density of concrete.

For lightweight concrete, the cost is greater than normal weight concrete because of the increased cost of the lightweight aggregate.

The cost of lightweight aggregate is greater than normal weight aggregate because:

  • The cost of production of lightweight aggregate is significant, because of the high temperature processing of the raw materials.
  • The cost of shipping of the finished aggregate may be significant. However, it should be noted that lightweight aggregate can be shipped long distances relatively economically. In some cases, lightweight aggregate has been shipped across the country or even overseas to projects where it becomes an important part of an economical structural solution.
 

Slide 21. Cost Premium for LWC

  • Effect of sand LWC deck on cost of bridge
Sand LWC Premium / CY Cost Prem. / SF
$20 / CY $0.56 / SF
$35 / CY $0.97 / SF
$50 / CY $1.39 / SF
  • Cost / SF assumes 9 in. thick deck (average)
  • Premium depends on cost of LWA, cost of NWA being replaced, and shipping cost

Speaker Notes:

This slide shows a range of cost premiums for lightweight concrete for a bridge deck concrete.

The cost premium for lightweight concrete depends on the cost of the lightweight aggregate, including the shipping cost, and the cost of the normal weight aggregate that is being replaced.

Here it is shown that even though the cost premium for lightweight concrete may appear significant, when this cost is spread out to an equivalent cost per square ft of deck area, the cost is relatively minor. Using data reported on the FHWA’s NBI website, the average of the unit cost for bridges on the Federal Aid system for each state was $162/sf in 2009. For the same year, the average unit cost for bridges not on the Federal Aid system was $153/sf. This means that the additional cost for lightweight concrete for a bridge deck is less than 1% of the cost of a typical bridge. This cost does not consider any savings that may result from the use of lightweight concrete.

 

Slide 22. Cost Premium for LWC

  • Effect of sand LWC girders on cost of bridge
    • Assume $30 / CY cost premium for sand LWC
    • Girder spacing assumed to be 10 ft
Girder Type Cost Prem. / LF Cost Prem. / SF
PCBT-29 $4.97 / LF $0.50 / SF
PCBT-61 $6.63 / LF $0.66 / SF
PCBT-93 $8.35 / LF $0.84 / SF

Speaker Notes:

Areas: PCBT-29: 643.7; PCBT-61: 858.7; PCBT-93: 1082.7 in2

Ratio for example is based on 125/150 reduction from 899 plf.

 

Slide 23. Sample Girder Cost Analysis

  • Cost premium for sand LWC in Mod BT-74 girder
  • Assume $30 / CY = $6.83 / LF
  • Cost premium for LWC for 150 ft girder = $1,024
  • Cost reduction by using sand LWC girder
  • Shipping from plant to site = $811
  • NWC girder = 69 t; LWC girder = 58 t, or 11 t less
  • Drop 4 strands / girder @ $0.65 / LF ea. = $390
  • Total cost reduction = $1,201
  • Net savings by using sand LWC girder = $177
  • Speaker Notes:

    This slide shows a cost analysis for using lightweight concrete for a bridge girder that was proposed for an actual project.

    The girder was a 74" deep modified bulb-tee girder that was 150 ft long. Assuming a reasonable cost premium of $30/CY for sand lightweight concrete, the additional cost of the lightweight concrete for this girder would be $1,024 per girder.

    The prestressed girder manufacturer asked his shipper to provide an estimate for shipping the 150 ft girder from his plant to the project location about 300 miles away. The cost to ship a sand lightweight concrete girder (125 pcf) that weighed 115.3 kips was $811 less than the cost to ship a normal weight concrete girder (150 pcf) that weighed 138.3 kips. Not only did the lightweight concrete girder cost less to ship, but it also weighed 23 kips less, which could be a significant benefit for the fabricator for handling, or the contractor for erection.

    A further cost reduction was realized because the sand lightweight concrete girder design required 4 fewer strands. For the proposed design, an all lightweight concrete deck was used on the sand lightweight concrete girder, while a normal weight concrete deck was used on the normal weight concrete girder. The cost of an installed prestressing strand was taken to be $0.65 / ft. Therefore, the elimination of 4 strands resulted in an additional cost reduction of $390 for the sand lightweight concrete girder.

    The total savings for the lightweight concrete design compared to the normal weight concrete design was $1,201, or a net savings of $177 per girder. Therefore, the cost of the lightweight concrete has been paid for by savings without considering any other potential cost savings in the bearings, substructure or foundations, or for the contractor in erection.

     

    Slide 24. PBES Applications for LWC

    • Sand LWC & Specified Density Concrete
      • Use for any precast or prestressed conc. elements
    • All LWC
      • Can be used for any precast concrete element
      • Data not yet available for prestressed elements
    All LWC Sand LWC SDC NWC
    105 pcf 120 pcf 135 pcf 145 pcf

    These are fresh densities for concrete up to about 6 ksi
    Add 5 pcf allowance for reinforcement

    Speaker Notes:

    So what types of lightweight concrete should be considered for PBES applications?

    And what concrete densities should be used for preliminary evaluation of the benefits?

    Sand lightweight concrete can be used for any type of precast concrete element, including prestressed concrete girders. A typical fresh density for sand lightweight concrete with design compressive strengths up to about 6 ksi is shown in the table. A density of 125 pcf could be used for up to 10 ksi compressive strengths, which is the maximum reasonable design compressive strength for sand lightweight concrete.

    All lightweight concrete can also be used for any precast concrete element. While use of all lightweight concrete is probably feasible for prestressed concrete elements, data supporting this use is not yet available. A typical fresh density for design compressive strengths up to 6 ksi is shown in the table.

    Again, an allowance for the increased density due to reinforcement must be added to these densities for the computation of element weights and dead loads.

     

    Slide 25. Impact of LWC on PBES

    • Consider sample projects
      • Precast foundation elements
      • Precast pile & pier caps
      • Precast columns
      • Precast full-depth deck slabs
      • Cored slabs & Box beams
      • NEXT beams & Deck girders
      • Full-span bridge replacement units with precast deck
      • Bridges installed with SPMTs

    Speaker Notes:

    With this information in hand, we will look at a number of actual PBES projects where lightweight concrete was or could have been used. The difference in element weights for lightweight concrete and normal weight concrete will be computed.

    The full range of possible precast concrete elements will be considered as shown:

    • foundation and substructure elements;
    • deck and girder elements; and
    • full span elements with precast decks.

    The discussion is organized by element type.

     

    Slide 26. Mill Street Bridge, NH

    Photo of the Mill Street Bridge, NH.
    • Precast foundation elements
      • Project did not use LWC
    • Comparison for abutment footings
      • Abutment walls have similar weights
      % Chng. Weight as Des. Chng. Weight as Built Chng.
    150 pcf 0 39t 0 25t 0
    125 pcf 17% 32t 7t 20t 5t
    110 pcf 27% 28t 11t 18t 7t

    Speaker Notes:

    The replacement of the Mill Street Bridge in Epping, NH, has been frequently used as an example of a prefabricated bridge that was put in place very quickly and successfully. The project did not use lightweight concrete.

    Using the plans and shop drawings, the weight of the largest abutment footing piece was computed. The contractor proposed using more pieces to reduce the weight of the largest piece, so both the "as designed" and "as built" weights are shown, based on a concrete density, with reinforcement, of 150 pcf.

    For foundation elements, both sand lightweight and all lightweight concrete could be an option. Using sand lightweight concrete would have reduced the weight of the piece by 5 tons, or 17%, for the "as built" footing. Using all lightweight concrete would have reduced the weight of the piece by 7 tons, or 27% for the "as built" footing.

    While the reductions may not appear large, it should be noted that the contractor redesigned the footing segments to reduce the weight of the largest piece by 14 tons. After this redesign, the footing segments weighed about as much as all of the other pieces.

    The reduction in piece weights that is achieved using lightweight concrete could possibly reduce shipping costs, allow for a smaller crane, or allow a greater radius for the crane, which can be very important where crane placements may be restricted.

    • reduce shipping costs,
    • allow for a smaller crane, or
    • allow a greater radius for the crane, which can be very important where crane placements may be restricted.
     

    Slide 27. Okracoke Island, NC

    Photo of a bridge on a remote island of the Outer Banks of North Carolina utilizing precast elements to accelerate construction. As shown, materials and equipment are being delivered by a barge.
    • Precast pile caps
      • Project did not use LWC
    • End bent pile cap – 2 pieces
      • Size: 21 ft long x 3.67 ft x 3 ft
      • 3 pile pockets per piece
    Pile Cap Weight Change % Chng.
    150 pcf 16 t 0 0
    125 pcf 13 t 3 t 17%
    110pcf 12 t 4 t 27%

    Speaker Notes:

    The replacement of seven bridges on a remote island of the Outer Banks of North Carolina utilized precast elements to accelerate construction. All materials and equipment for the project had to be delivered to the island by barge. The project was not designed to use LWC.

    The weight of a typical precast pile cap for an end bent was computed using the project plans assuming a concrete density, with reinforcement, of 150 pcf. Each end bent was cast in two pieces that were 21 ft long and 3 ft deep.

    For substructure elements, both sand lightweight and all lightweight concrete could be an option. Using sand lightweight concrete would have reduced the weight of the piece by 3 tons or 17%. Using all lightweight concrete would have reduced the weight of the piece by 4 tons or 27%.

     

    Slide 28. Lake Ray Hubbard, TX

    Construction of the bridge over Lake Ray Hubbard in Texas. The photo shows a typical pier cap with 3 columns. This project did not use LWC.
    • Precast pier caps
      • Project did not use LWC
    • Typical pier cap on 3 columns
      • Size: 37.5 ft long x 3.25 ft x 3.25 ft
    Pier Cap Weight Change % Chng.
    150 pcf 29 t 0 0
    125 pcf 24 t 5 t 17%
    110 pcf 21 t 8 t 27%

    Speaker Notes:

    The construction of the bridge over Lake Ray Hubbard in TX is another project that is frequently used as an example of a project that successfully employed prefabricated bridge elements. The project did not use lightweight concrete.

    The weight of one of the 43 identical precast pier caps was computed using the project plans assuming a concrete density, with reinforcement, of 150 pcf. Each pier cap was constructed as a single piece that was 37.5 ft long with a 3.25 ft square cross section.

    For substructure elements, both sand lightweight and all lightweight concrete could be an option. Using sand lightweight concrete would have reduced the weight of the piece by 5 tons or 17%. Using all lightweight concrete would have reduced the weight of the piece by 8 tons or 27%.

     

    Slide 29. Edison Bridges, FL

    The Edison Bridge in Fort Meyer, FL constructed a number of years ago using precast columns. The maximum precast column weight was 45 tons.  Each H-shaped column was cast as a single piece.  Using the ratio of concrete densities, the weight of a sand lightweight concrete column would have been 37 tons, for a reduction of 8 tons or 17%.
    • Project did not use LWC
    • Precast columns
      • Max wt = 45 tons @ 150 pcf
      • Max wt = 37 tons @ 125 pcf
      • Using 128 pcf SDC could have eliminated pedestal for tall columns
    The Edison Bridge in Fort Meyer, FL constructed a number of years ago using precast caps. The maximum precast pier cap weight was 78 tons.  Each pier cap was cast as a single piece, using an inverted U-shape to reduce the weight.  Using the ratio of concrete densities, the weight of a sand lightweight concrete cap would have been 65 tons, for a reduction of 13 tons or 17%.
    • Precast caps
      • Max wt = 78 tons @ 150 pcf
      • Max wt = 65 tons @ 125 pcf

    Speaker Notes:

    The Edison Bridge in Fort Meyers, FL, was constructed a number of years ago using precast columns and pier caps. The project did not use lightweight concrete.

    The maximum precast column weight was 45 tons. Each H-shaped column was cast as a single piece. Using the ratio of concrete densities, the weight of a sand lightweight concrete column would have been 37 tons, for a reduction of 8 tons or 17%.

    I used the photo of the completed pier in a presentation several years ago. After the presentation, an engineer came up and asked if I knew why the tall column had been placed on a pedestal. He had been involved in the project and told me that the contractor’s crane could not lift the full-length column, so their standard methods of pier construction had to be changed and a pedestal had to be used. Scaling lengths off of the photo, I estimate that concrete with a density of 128 pcf could have eliminated the need for the pedestal.

    The maximum precast pier cap weight was 78 tons. Each pier cap was cast as a single piece, using an inverted U-shape to reduce the weight. Using the ratio of concrete densities, the weight of a sand lightweight concrete cap would have been 65 tons, for a reduction of 13 tons or 17%.

    Shipping a pier cap of this size on highways is often difficult because of the large weight and relatively short length. This combination can make it difficult to have enough room to place the required number of axles under the load to satisfy permitting requirements. With a short, heavy load, it may also be difficult to find a route for which bridges will not be overloaded since the short load length will place most of the load on a medium span bridge. Therefore, sand lightweight, or even all lightweight concrete, is a good solution for such large precast elements.

     

    Slide 30. Woodrow Wilson Br, VA/DC/MD

    Photo of the Woodrow Wilson Bridge in Washington DC.
    • Deck replacement with full-depth precast deck panels in 1983
    • Sand LWC was used for panels
      • Allowed thicker deck
      • Allowed widened roadway with no super- or substructure strengthening
      • Reduced shipping costs and erection loads
    • Sand LWC deck performed well until bridge was recently replaced to improve traffic capacity

    Speaker Notes:

    An early example of using precast concrete for accelerating bridge construction was the redecking of the Woodrow Wilson Bridge just south of Washington, DC, that was completed in 1983.

    For this bridge, precast deck panels were designed using sand lightweight concrete for the following reasons:

    • Since the concrete deck panels were lighter, the existing structure could support a thicker deck. The original normal weight concrete deck had been too thin, which had led to its early deterioration.
    • The reduced deck weight also allowed the roadway width to be increased several feet without requiring any modifications to strengthen the existing superstructure or substructure elements.
    • Finally, since the precast concrete panels weighed less, the cost for shipping the panels from the precast plant to the site, which was about 75 miles away, was reduced. The erection loads were also reduced.

    The specified "air-dry" density of the sand lightweight concrete was 115 pcf without reinforcement. The use of lightweight concrete reduced the weight of the panels by about 20%. The "air-dry" density is roughly equivalent to the equilibrium density discussed earlier.

    The new lightweight concrete deck, which was post-tensioned in both the longitudinal and transverse directions, performed well until it was replaced by a new structure to improve the traffic flow on this very busy interstate.

     

    Slide 31. Okracoke Island, NC

    Photo of a precast prestressed concrete cored slab in Okracoke Island, NC. The weight of a typical exterior cored slab unit for a 50 ft span was computed using the project plans assuming a concrete density, with reinforcement, of 150 pcf.  The cored slabs were 21" deep and 3 ft wide.  The exterior units were the heaviest because they use smaller voids to accommodate the reinforcement for the barrier.
    • Precast cored slabs
      • Project did not use LWC
    • 21" deep by 3 ft wide
      • 30 and 50 ft spans
    Ext. 50 ft spanWeightChange% Chng.
    150 pcf16.0 t00
    125 pcf13t3t17%
    125 pcf – Solid16.4 t0.4 t+3%

    Speaker Notes:

    We will now look at the main bridge elements for this project on the Outer Banks of North Carolina – the precast prestressed concrete cored slabs. These slabs were not designed to use lightweight concrete.

    The weight of a typical exterior cored slab unit for a 50 ft span was computed using the project plans assuming a concrete density, with reinforcement, of 150 pcf. The cored slabs were 217" deep and 3 ft wide. The exterior units were the heaviest because they use smaller voids to accommodate the reinforcement for the barrier.

    Since these elements were pretensioned, only sand lightweight concrete was considered as an option. The use of sand lightweight concrete would have reduced the weight of the slab by 3 tons or 17%. The option of using sand lightweight concrete and eliminating the voids, i.e., using a solid slab, was also evaluated in this comparison. For this case, the solid lightweight concrete slab weighed only slightly more than the normal weight cored slab. A solid slab would be more economical to fabricate, so the cost of the lightweight concrete may be offset by the savings in cored slab production costs.

     

    Slide 32. Okracoke Island, NC, (Cont’d.)

    Barriers on the bridges in Okracoke Island, NC. The photo shows a sand lightweight concrete barrier.
    • Precast barriers
      • Project was not designed with LWC
    • Contractor proposed casting barriers on cored slabs in precast plant
      • Sand LWC was used for the barrier
    Barrier Weight Change % Chng.
    150 pcf 13.7 t 0 0
    125 pcf 11.4 t 2.3 t 17%
    110 pcf 10.1 t 3.6 t 27%

    Speaker Notes:

    Now we will now look at one more element of the Okracoke Island project – the barriers. The original plans called for precast barriers to be used, made with normal weight concrete. However, the contractor decided that it would be better to have the barriers cast on the cored slabs in the precast plant, eliminating the operations of installing the barriers after the cored slabs were set. Sand lightweight concrete was used for the barriers to reduce the weight of the exterior slabs, which were now becoming even heavier with the addition of the barrier.

    The sand lightweight concrete barrier saved 2.3 tons per slab compared to the weight of a barrier made with normal weight concrete.

    If all lightweight concrete had been used for the barrier, it would have saved an additional 1.3 tons, or a total of 3.6 tons from the normal weight concrete barrier.

     

    Slide 33. Mill Street Bridge, NH

    Photo of the box beams used to replace the Mill Street Bridge in Epping, NH.
    • Precast box beams
      1. Project did not use LWC
    • NWC box beam weight governed crane size with 2 crane pick
    Ext. Box Beam Weight Change % Chng.
    150 pcf 69 t 0 0
    125 pcf 57 t 12 t 17%

    Using sand LWC for box beam would make beam pick nearly equal to NWC substructure elements.

    Speaker Notes:

    We will now take a look at the box beams used to replace the Mill Street Bridge in Epping, NH. The project did not use lightweight concrete.

    Using the plans, the weight of a the 36" deep box beam was computed, based on a concrete density, with reinforcement, of 150 pcf. From photos, it was clear that the contractor was using 2 cranes to erect each box. Therefore, each crane was required to lift about 35 tons, which exceeded the weights of other single elements on the project.

    Since these elements were pretensioned, only sand lightweight concrete was considered as an option. Using sand lightweight concrete would have reduced the weight of a box beam by 12 tons, or 17%. This would reduce the crane pick to about 29 tons, which is in the range of the maximum pick for the other normal weight concrete elements. The reduction in weight could also allow the cranes to boom out further in handling and placing the box beams.

     

    Slide 34. NEXT F Beams

    A graph comparison of section weights for NEXT 36 Beam series. These beams have 8 to 12 ft wide top flanges, and will greatly assist in making possible the rapid construction of short and moderate span bridges. There are two families of the NEXT beams.  The first, the NEXT F or Flanged section.  It has a 4" thick top flange which is designed as a form for an 8" thick cast-in-place structural slab.  The beams have been detailed with 4 different depths:  24", 28", 32", and 36".  This slide considers only the deepest section (36"). Considering densities of normal weight concrete and sand lightweight concrete that are shown (note: the design concrete compressive strength is 10 ksi, which increases the density), the weight per foot of the 36" deep sections are compared for 8, 10 and 12 ft flange widths. For all three widths, the use of sand lightweight concrete reduces the weight of the beam by 16%.  The PCINE design aids only provide maximum span charts for normal weight concrete beams, so the maximum spans for sand lightweight concrete beams are not known.  However, they are expected to be slightly longer than those for normal weight concrete. It is also noted that the widest (12 ft wide) sand lightweight concrete beam section weighs less than the narrowest (8 ft wide) normal weight beam section.  This means that the use of lightweight concrete could allow the use of wider beams, which may reduce the number of beams that must be transported to the job site.
    • Compare section weights for NEXT 36F
      • NWC @ 155 pcf; Sand LWC @ 130 pcf
      • No max. span charts for sand LWC

      • 16% reduction in weight for same width sections
      • 12 ft wide LWC is lighter than 8 ft wide NWC

    Speaker Notes:

    After considering cored slabs and box beams, we will now consider the new decked beam sections that have been developed by the PCI Northeast (PCINE) Technical Committee: the NEXT Beam series. These were mentioned very briefly in an earlier module.

    These beams have 8 to 12 ft wide top flanges, and will greatly assist in making possible the rapid construction of short and moderate span bridges.

    There are two families of the NEXT beams. The first, the NEXT F or Flanged section, is considered on this slide. It has a 4" thick top flange which is designed as a form for an 8" thick cast-in-place structural slab. The beams have been detailed with 4 different depths: 24", 287", 32", and 36". This slide considers only the deepest section (36").

    Considering densities of normal weight concrete and sand lightweight concrete that are shown (note: the design concrete compressive strength is 10 ksi, which increases the density), the weight per foot of the 36" deep sections are compared for 8, 10 and 12 ft flange widths.

    For all three widths, the use of sand lightweight concrete reduces the weight of the beam by 16%. The PCINE design aids only provide maximum span charts for normal weight concrete beams, so the maximum spans for sand lightweight concrete beams are not known. However, they are expected to be slightly longer than those for normal weight concrete.

    It is also noted that the widest (12 ft wide) sand lightweight concrete beam section weighs less than the narrowest (8 ft wide) normal weight beam section. This means that the use of lightweight concrete could allow the use of wider beams, which may reduce the number of beams that must be transported to the job site.

    Specified density concrete could also be a good solution for beams such as these, if specific limitations on shipping or lifting need to be overcome for a given project.

     

    Slide 35. NEXT D Beams

    A graph comparison of section weights for NEXT 36 D. The Deck section has an 8" thick top flange that is designed to be the structural deck in the completed structure.  For these girders, connections between flange tips must be made.  A waterproofing membrane and asphalt wearing surface would typically be applied to the bridge.  The beams have been detailed with 4 different depths, each of which is 4" deeper than the NEXT F sections:  28", 32", 36", and 40".  As for the NEXT F beam, this slide considers only the deepest section (40"). Because this beam is significantly heavier than the NEXT F section, a 12 ft width was not considered in the beam standards.  Therefore, only 8 and 10 ft widths appear in the PCINE design standards. Considering densities of normal weight concrete and sand lightweight concrete that are shown on the previous slide, the weight per foot of the 40" deep sections are compared for 8 and 10 ft flange widths.  The weight of a sand lightweight concrete section with a 12 ft flange is shown for comparison purposes. For sections with the same width, the use of sand lightweight concrete reduces the weight of the beam by 16%.  For the NEXT D beams, maximum span charts are given for both normal weight and sand lightweight concrete, but only for the 8 and 10 ft widths.  In all cases, maximum spans for the sand lightweight girders is slightly longer than the spans for the normal weight girders.
    • Compare section weights for NEXT 36D
      • 12 ft width not used to limit weight of NWC section
      • Max. span charts are provided for sand LWC
      • 16% reduction in weight for same width sections
      • 12 ft LWC is lighter than 10 ft NWC

    Speaker Notes:

    The second type of NEXT beam is the NEXT D or Deck section, which has an 8" thick top flange that is designed to be the structural deck in the completed structure. For these girders, connections between flange tips must be made. A waterproofing membrane and asphalt wearing surface would typically be applied to the bridge. The beams have been detailed with 4 different depths, each of which is 4" deeper than the NEXT F sections: 28", 32", 36", and 40". As for the NEXT F beam, this slide considers only the deepest section (40").

    Because this beam is significantly heavier than the NEXT F section, a 12 ft width was not considered in the beam standards. Therefore, only 8 and 10 ft widths appear in the PCINE design standards.

    Considering densities of normal weight concrete and sand lightweight concrete that are shown on the previous slide, the weight per foot of the 40" deep sections are compared for 8 and 10 ft flange widths. The weight of a sand lightweight concrete section with a 12 ft flange is shown for comparison purposes.

    For sections with the same width, the use of sand lightweight concrete reduces the weight of the beam by 16%. For the NEXT D beams, maximum span charts are given for both normal weight and sand lightweight concrete, but only for the 8 and 10 ft widths. In all cases, maximum spans for the sand lightweight girders is slightly longer than the spans for the normal weight girders.

    Again, it is noted that the widest sand lightweight concrete beam (12 ft wide) weighs less than the 10 ft wide normal weight beam and just a small amount more than the 8 ft wide section. This means that the use of lightweight concrete could again allow the use of wider beams, which may reduce the number of beams that must be transported to the job site.

     

    Slide 36. Deck Girders, NY

    Photo of deck girders that were recently used for a bridge in NY State. These deck girders are 41" deep and had a 5 ft wide top flange, which provided the deck for the structure when the girders were placed side-by-side.
    • Precast deck girder
      • Project did not use LWC
    • 41" deep deck girders with 5 ft top flange
      • 87.4 ft long girders
    Girder & Deck Weight Change % Chng.
    158 pcf 45 t 0 0
    130 pcf 37 t 8 t 18%

    NWC density was obtained from girder fabricator

    Specified concrete compressive strength = 10,000 psi

    Speaker Notes:

    We will now take a look at the deck girders that were recently used for a bridge in NY State. The project did not use lightweight concrete.

    These deck girders were 41" deep and had a 5 ft wide top flange, which provided the deck for the structure when the girders were placed side-by-side. This type of structure is very efficient, allowing unusually shallow structure depths. However, having the deck on the girder when it is fabricated, shipped and erected, means that the girders are very heavy.

    Using normal weight concrete with a density of 158 pcf, including reinforcement, the weight of the 87.4 ft long girders is computed to be 45 tons. The normal weight concrete density and reinforcement allowance used here were provided by the girder manufacturer. The higher density reflects the NYS DOT requirement for pretensioned girders to use 10 ksi concrete.

    Since deck girders are pretensioned, only sand lightweight concrete was considered as an option. Using sand lightweight concrete would have reduced the weight of a deck girder by 8 tons, or 18%.

     

    Slide 37. I-95 in Richmond, VA

    For the heavily traveled bridges carrying I-95 over James River through Richmond, VA., a is deck being precasted on the steel girders off-site. As shown in the photo, the units were transported to the bridge and placed.
    • Prefabricated full-span units
      • Steel girders and sand LWC deck
    • Maximum precast unit weight for current project
    Deck Weight Change % Chng.
    145 pcf 132 t 0 0
    120 pcf 116 t 16 t 12%
    105 pcf 106 t 26 t 20%

    Deck densities do not include reinforcement allowance

    Speaker Notes:

    The heavily traveled bridges carrying I-95 over the James River through Richmond, VA, were replaced a number of years ago using prefabricated full span units. This bridge has also been mentioned several times already in this webinar.

    The deck was precast on the steel girders off-site, then the units were transported to the bridge and placed. The bridge was designed with a sand lightweight concrete deck to reduce the weight of the precast deck units. The lightweight concrete decks are exposed to traffic, i.e. there is no wearing surface, and have performed very well.

    A second contract to replace nine smaller overpass structures just north of the James River Bridge has just been awarded. The bridges have again been designed using full-span units with a sand lightweight concrete deck on steel girders. Using the plans for one of the new bridges, the weight of a precast unit has been computed using the different concrete densities. Since the plans gave quantities for reinforcement, it was accounted for separately in computing the unit weight rather than including it in the density of the reinforced concrete. While the design uses sand lightweight concrete, comparisons are shown with respect to normal weight concrete to show the weight savings achieved.

    Although the precast decks are designed to be post-tensioned, all lightweight concrete is also shown as an option here.

    The use of sand lightweight concrete reduced the weight of the precast slab unit by 16 tons or 12%. The percent reduction is less than for other situations because the weight of the unit also includes structural steel.

    If all lightweight concrete had been used, the weight of the precast slab unit would have been reduced by 26 tons or 20%.

    The plans specify normal weight concrete for the barriers, so the precast unit weights shown were computed using normal weight barriers for all concrete types. However, if sand lightweight concrete or all lightweight concrete were used for the barriers, the precast unit weights would be reduced another 2 and 3 tons, respectively.

     

    Slide 38. Lewis & Clark Bridge, OR/WA

    Deck replacement on the Lewis and Clark Bridge. In the photo the deck is being supported by a truss. A system of SPMTs is being used to remove the existing deck and replace it with the new panels.
    • Deck replacement on existing truss
      • Sand LWC precast deck units with steel floor beams
      • Sand LWC density = 119 pcf
      • Max. deck unit weight = 92 t
      • Sand LWC saved about 14 t
    Deck replacement on the Lewis and Clark Bridge. It uses full span units comprised of steel beams and a precast concrete deck. As shown in the photo, the deck is supported by a truss. A system of SPMTs is being used to remove the existing deck and replacing it with the new panels.
    • Existing deck was LWC
      • Was in service 73 years
    Lewis and Clark Bridge. Photo of the original duck on the truss bridge. Opened in 1930, the deck was lightweight concrete.  It provided service for over 70 years.

    Speaker Notes:

    The Lewis and Clark Bridge is another commonly used example of how to replace a bridge deck using prefabricated elements. It is similar to the I-95 bridges in VA since it uses full span units comprised of steel beams and a precast concrete deck. The difference here is that the deck is supported by a truss. A system of SPMTs was used to remove the existing deck and replace it with the new panels.

    For this bridge, the deck was designed to use sand lightweight concrete. Using lightweight concrete reduced the weight of the largest precast unit from about 92 tons to 78 tons, for a weight savings of about 14 tons.

    It is also interesting to note that the original deck on the truss bridge, which was opened to traffic in 1930, was lightweight concrete. It provided service for over 70 years, a pretty remarkable record considering the northern location.

     

    Slide 39. Bridges set with SPMTs, UT

    Photo of a bridge being set with SPMTs.
    • 3300 South over I-215 – Built in 2008
      1. Sand LWC used for deck
      2. Less deck cracking than bridges with NWC decks
    • 3 bridges to be moved in 2011
      1. Steel girder bridges with sand LWC decks
      2. 200 South over I-15 – 2 spans @ 3.1 million lbs
      3. Sam White Lane over I-15 – 2 spans @ 3.8 million lbs
      4. I-15 Southbound over Provo Center Street
        1. 2 moves of 1.5 and 1.4 million lbs

    Speaker Notes:

    You have already heard about the experience of Utah in placing bridges using SPMTs. The initial bridges placed by SPMTs had normal weight concrete decks. However, the 3300 South bridge over I-215, constructed in 2008, was the first bridge installed using SPMTs that was designed with a sand lightweight concrete deck. Inspections of the deck after a few years in service revealed that this bridge deck had less cracking than the normal weight concrete decks constructed and moved in the same or earlier years.

    Three bridge moves in Utah have been announced for 2011. Two of them are going to be moved as 2 span structures. All three of the bridges have been designed with sand lightweight concrete decks.

    The use of lightweight concrete decks for these structures reduces the weight of the structure to be moved, but also reduces the weight of the portion of the bridge overhanging the SPMTs. This is expected to reduce the cracking in the decks during movement of the bridges.

     

    Slide 40. Graves Ave. over I-4, FL

    Photo of an SPMT completing a span repalcement using normal weight concrete.
    • Complete span replaced using SPMTs
      • Project did not use LWC
    • Comparison of weight for NWC and sand LWC
      • Appendix C in FHWA "Manual on Use of SPMTs..."

    Comparison with all LWC deck is not in Manual

    Girder Deck Weight Change % Chng.
    152 pcf 150 pcf 1,282 t 0 0
    127 pcf 120 pcf 1,049 t 233 t 18%
    127 pcf 105 pcf 996 t 286 t 22%

    Speaker Notes:

    The Graves Ave. Bridge over I-4 in FL is the final example we will consider. The bridge is 143 ft long and 59 ft wide and was one of the first spans installed in the US using SPMTs. The project did not use lightweight concrete.

    A comparison of using normal weight concrete and sand lightweight concrete for the prestressed concrete girders and deck on this bridge can be found in Appendix C of the FHWA’s "Manual on Use of Self-Propelled Modular Transporters to Remove and Replace Bridges," which is listed as a reference for this webinar. The results of the comparison are repeated in the first two lines of this table. It can be seen that the weight of the structure to be moved would have been reduced by 233 tons, or 18%, if sand lightweight concrete had been used for both the girders and deck. This reduced weight could affect the number of axle lines that would be required to move the structure.

    This slide also shows a third comparison which does not appear in the Manual. If all lightweight concrete had been used for the deck and sand lightweight concrete had been used for the girders, the weight of the structure would have been reduced another 53 tons, for a total reduction from the normal weight concrete design of 22%.

     

    Slide 41. Conclusions

    You should now be able to:

    • describe how lightweight aggregate is manufactured
    • identify the classifications of lightweight concrete
    • identify several advantages of using lightweight concrete for PBES bridges
    • recall several PBES projects where lightweight concrete was or could have been used
     

    Slide 42. Questions?

    • For more information on LWA and LWC

    Photos left to right: precast barrier,


    Speaker Notes:

    In conclusion, I hope that the information I have presented has demonstrated the potential for using lightweight concrete to reduce the weight of precast elements used to accelerate bridge construction.

    It appears that the advantages of lightweight concrete may be easiest to consider in design / build projects where the contractor, designers and precast fabricators can work together to optimize the design by considering handling, shipping and erection requirements.

    The lightweight aggregate industry would be glad to assist you as you make plans to implement PBES in your structures. Please use the information on the slide if you would like to contact us to obtain more information on lightweight aggregate or lightweight concrete

     

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