LIGHTWEIGHT CONCRETE BENEFITS FOR
PREFABRICATED BRIDGE ELEMENTS &
SYSTEMS (PBES) DEPLOYMENT
Reid W. Castrodale, PhD, PE
Director of Engineering
Carolina Stalite Company, Salisbury, NC
Development of LWC
• 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
Development of LWC
• 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
Structural LWA
• 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
4
Relative Density
• 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
1 lb. of each aggregate
LWA is just a lighter rock!
• LWA should satisfy NWA specifications
– Except different gradations – AASHTO M 195
• LWA has higher absorption than NWA
– Needs to be prewet, especially for pumping
• For LWC
– Same batch plants and mixing procedures
– Same admixtures
– Can use same mix design procedures
– “Roll-o-meter” for measuring air content
Lightweight Concrete
LWA is used to reduce the density of concrete
• “All lightweight” – all aggregates, both Most
fine and coarse, are lightweight
common
• “Sand lightweight” – lightweight coarse
aggregate and normal weight sand
• “Specified density” – blend of NW and LW
aggregate to achieve target density (SDC)
• Density of LWC is specified, so it must be
measured during placement for QC
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)
Spectrum of Concrete Density
All LWC
Sand LWC
SDC
NWC
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)
Specifying Density of LWC
• “Equilibrium density” is defined in ASTM C 567
– Density after moisture loss has occurred
– Often used for dead load calculations
• “Fresh density” used for QC tests during casting
– Use for handling loads at early age
– Suggest using for final design loads in large elements
• Add reinforcement allowance to concrete density
when computing dead loads (typ. 5 pcf)
DOT Specifications for LWC
Sand LWC for Bridge Decks
• TennDOT includes in Standard Specifications
• NCDOT, UDOT, etc. have std special provisions
• VDOT & other states have proj. special provisions
All LWC
• Has not been used in recent years
• Special provisions are being developed for
NCDOT
DOT Specifications for LWC
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
GDOT Special Provisions
• Special provisions for 10 ksi 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
Benefits of LWC
• Reduced weight of precast elements
– Affects handling, shipping and erection
– Can also improve structural efficiency
• Enhanced durability
– Reduced cracking tendency
Opposite of what
many expect!
– Reduced permeability
– Tighter quality control with a specified density
Focus for this presentation
– Reduced weight for PBES deployment
Cost of LWC
• Increased cost of LWA
– Additional processing
– Shipping from the manufacturing plant
Cost Premium for LWC
• Effect of sand LWC on cost of bridge
LWC Premium / CY
Cost / SF
$20 / CY
$0.56 / SF
$30 / CY
$0.83 / SF
$40 / CY
$1.11 / SF
– Cost / SF assumes 9 in. thick deck (average)
– Premium depends on cost of LWA, cost of NWA
being replaced, and shipping cost
Sample Girder Cost Analysis
• Cost premium for LWC for Mod BT-74 girder
– Assume $30 / CY = $6.83 / LF
– Cost premium for LWC for 150 ft girder =
$1,024
• Cost reduction by using LWC
– 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. =
– Total cost reduction =
• Net savings by using LWC
$390
$1,201
$177
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
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
Mill Street Bridge, NH
• Precast foundation elements
– Project did not use LWC
• Comparison for abutment footings
– Abutment walls have similar weights
%
Weight
Weight
Chng.
Chng.
Chng. as Des.
as Built
150 pcf
0
39 t
0
25 t
0
125 pcf
17%
32 t
7t
20 t
5t
110 pcf
27%
28 t
11 t
18 t
7t
Okracoke Island, NC
• 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
Chng.
% Chng.
150 pcf
16 t
0
0
125 pcf
13 t
3t
17%
110pcf
12 t
4t
27%
Lake Ray Hubbard, TX
• 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
Chng.
% Chng.
150 pcf
29 t
0
0
125 pcf
24 t
5t
17%
110 pcf
21 t
8t
27%
Edison Bridges, FL
• Project did not use LWC
• Precast columns
– Max wt = 45 tons @ 150 pcf
– Max wt = 37 tons @ 125 pcf
• Precast caps
– Using 128 pcf SDC could have
eliminated pedestal for tall
columns
– Max wt = 78 tons @ 150 pcf
– Max wt = 65 tons @ 125 pcf
Woodrow Wilson Br, VA/DC/MD
• 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
• LWC deck performed well until bridge was
recently replaced to improve traffic capacity
Okracoke Island, NC
• Precast cored slabs
– Project did not use LWC
• 21” deep by 3 ft wide
– 30 and 50 ft spans
Ext. 50 ft span
Weight
Chng.
% Chng.
150 pcf
16.0 t
0
0
125 pcf
13 t
3t
17%
125 pcf - Solid
16.4 t
0.4 t
+3%
Okracoke Island, NC
• 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
Chng.
% 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%
Mill Street Bridge, NH
• Precast box beams
– Project did not use LWC
• NWC box beam weight governed
crane size with 2 crane pick
Ext. Box Beam
Weight
Chng.
% Chng.
150 pcf
69 t
0
0
125 pcf
57 t
12 t
17%
– Using LWC for box beam would make beam pick
nearly equal to NWC substructure elements
NEXT F Beams
• Compare section weights for NEXT 36 F
– NWC @ 155 pcf; Sand LWC @ 130 pcf
– No max. span
NEXT 36 F
charts for sand
LWC
1700
1592
– 16% reduction in
weight for same
width sections
– 12 ft wide LWC
is lighter than
8 ft wide NWC
Weight per Foot (lbs)
1600
1489
1500
1400
16%
1385
1335
1300
1249
1200
1162
1100
8 ft
10 ft
12 ft
8 ft
10 ft
1000
NWC
LWC
12 ft
NEXT D Beams
• Compare section weights for NEXT 36 D
– 12 ft width not used to limit weight of NWC section
– Max. span charts
NEXT 36 F
are provided
for sand LWC
2200
2100
2000
– 16% reduction in
weight for same
width sections
– 12 ft LWC is
lighter than
ft NWC
Weight per Foot (lbs)
2000
1900
1800
16%
1851
1793
1677
1700
1600
1504
1500
8 ft
10 ft
12 ft
8 ft
10 ft
1400
NWC
LWC
10
12 ft
Deck Girders, NY
• 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
Chng.
% Chng.
158 pcf
45 t
0
0
130 pcf
37 t
8t
18%
NWC density was obtained from girder fabricator
Specified concrete compressive strength = 10,000 psi
I-95 in Richmond, VA
• Prefabricated full-span units
– Steel girders and sand LWC deck
• Maximum precast unit weight for current project
Deck
Weight
Chng.
% 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
Lewis & Clark Bridge, OR/WA
• 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
¢ STRINGER
0.02'/FT.
17'-1"
¢ STRINGER
3'-0"
(TYP.)
7"
1¼" MMC
OVERLAY
– LWC saved about 14 t
5'-6"
(TYP.)
0.02'/FT.
• Existing deck was LWC
**Note – New panel weighs about 5% less than original**
– Was in service 73 years
2" 9"
Bridges set with SPMTs, UT
• 3300 South over I-215 – Built in 2008
– Sand LWC used for deck
– Less deck cracking than bridges with NWC decks
• 3 bridges to be moved in 2011
– Steel girder bridges with sand LWC decks
– 200 South over I-15 – 2 spans @ 3.1 million lbs
– Sam White Lane over I-15 – 2 spans @ 3.8 million lbs
– I-15 Southbound over Provo Center Street
–2 moves of 1.5 and 1.4 million lbs
Graves Ave. over I-4, FL
• 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 …”
Girder
Deck
Weight
Chng.
% 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%
Comparison with ALWC deck is not in Manual
Questions?
For more information on LWA and LWC
• Contact Reid Castrodale: rcastrodale@stalite.com
• Visit the Expanded Shale, Clay and Slate Institute
website: www.escsi.org
• Contact local LWA suppliers: listed on ESCSI website