The Design and Construction of
Precast Concrete Structures
Precast & Prestressed Floors
and Composite Slabs
Precast & Prestressed Floors
and Composite Slabs
Hollow core floor units & slab fields
Double tee units
Half-slab (precast + insitu topping)
Composite floors
Load v span data
Spreadsheets for design of units
and slab fields
Dutch trade association claim fixing
rates of 2000 sq.m per week
40 x 11 feet wide hollow core onto precast walls
at MGM hotel, Las Vegas, 1992
European production = 25 million sq. m per year
75% is 150 250 mm deep
Double tee units twice the price but up to 4 x
capacity than hollow core
Prestressed half-slab popular for housing
and awkward shapes
Propping required over 5 m
PRESTRESSED HOLLOW CORE FLOOR UNITS
400 3600 mm wide; typically 1200 mm
90 730 mm deep; typically 150, 200, 250, 300 mm
self weight 1.5 to 5 kN/sq.m
void ratio 40 60 % of solid section
spans 6 20 m (economical range)
Longitudinal 30 mm
pretensioning No shear or flanges and
strand or wire torsion links webs
Shear key
profile
1195 mm
Are the deeper units beams or slabs?
Should they abide by normal RC rules?
Extrusion or slipformed
100-150 m long bed; no-slump mix grade C50-60
Extrusion - rotating screws turn opposite hands
Extrusion circular mandrels make the holes
Extrusion circular mandrels make the holes
Slipforming shear compactor (hammers down
flanges and webs)
Sliding
motion t
o
and fro
Curing temperature contours (c Branco, Lisbon)
Sliding
motion t
o
and fro
Splitting cracks due to sawing
restraints as prestress in transferred
At 16-18 hours, circular saw cuts
to length +10 to -15 mm
The new carousel system for continuous
production of 2 x 1.2 m wide units
e.g. Spancrete, USA
Bison, UK (2006)
Italian machinery
Factory 500 m long, including labs and prep
The new carousel system for continuous
production of 2 x 1.2 m wide units
Sawing room
Steam curing
ID chip marker under covers
Tensioned wire
10-14 mm coarse Mixer & re- Cement and
and fine agg cycled slurry admixtures
Moving steel beds collect the tensioning wires
Steel plates move
across onto roller beds te
u
in
r m
pe
m
8
1.
5 mm indented wire
tensioned over 120 m
Continuous concrete delivery for
2 x 120 m long extrusions
Production at the rate of 4.8 sq. m per minute,
yielding 1200 sq. m per day
Steam curing under cover for 16 hours.
Transfer strength = 40 N/mm 2
Identification chip is automatically
glued onto top of unit
Units cut to length +
- 10 mm accuracy
Final lifting into stockyard
400 450 deep units have a new market
for 16 m long clear span car parks
Too much plasticiser !! in 450 deep units
Actually air-entrainment agent is used by
several producers as a plasticiser
Single storey supermarket podium and car park
Span/depth ratio = 40
Italian variation (c. ASSAP)
600 mm wide prestressed units in Budapest
7-wire helical strand (1750 MPa) gives good
bond in the important transmission zone
Locks in
Strand pull-in:
An important indicator
of success
Should be about 1 mm,
irrespective of length
Theoretical pull-in limit
for zero prestress
PL
AE
Use a linear scale
between the extremes
700 mm deep units in Italy, often used at 25 m
span for tunnel cut-and-cover
For units > 500 mm deep, mesh is rolled out to
reinforce the outer webs
Bearing onto neoprene or mortar for spans
more than about 15 m onto insitu or masonry
20 m long ASSAP unit
Section Analysis for prestressed
- Pretension and losses (about 18-25%)
- Service moment (bottom tension critical)
- Ultimate moment (usually > service x 1.5)
- Ultimate shear uncracked & flexurally cracked
- End bearing and transmission length
- Deflection and camber (long-term, creep)
- Live load deflection after installation
Msr = (f bc + f ct ) Z b
Mur = 0.95 fpu A ps (d-dn )
Vco = 0.67 bv h f t 2 + 0.8 f cpx f t
Imposed load
Allowable span
Bearing limit
Imposed load
Handling limit
span/depth = 50
Span
Bearing limit
Imposed load
Handling limit
span/depth = 50
Span
Bearing
Shear
Imposed load
Service
moment
Deflection
Handling
Span
Service moment
Possibly shear ? control
Deflection
control
Routine bending tests to 25% overload
with 95% recovery
Flexural cracks extend rapidly through the
section due to narrow webs
Approx
200 mm spacing
Effective stiffness changes
with increasing load
Load
Deflection
2 to 6
EIeff = EI c + (EIu - EI c ) (M/ Mcrack )
Watch out for flexural-shear (Vcr ) failure !
Decompression
Vco point
Vcr
Vcr = 0.55(1-loss) vc b v d + M o Vu
Mu
P
UDL
SF diagram
Vcr failure
here
Failure of solid prestressed unit during
pouring of insitu topping
Cover to the tendons
100 mm
50-55 mm
Shear failure in 400 deep units
(c. Engstrom, Sweden)
Shear searches
out the
weakest web
Shear capacity of single webs more reliable
Shear bond crack stops
Aswad tests on edge loads (PCI JOURNAL)
Very brittle in this mode !
Lateral load distribution
Shear keys in
longitudinal joints
LINE LOADS
PARL TO SPAN
POINT LOAD
PER UNITS
LATERAL
DISTRIBUTION
SIMPLY
SUPPORTED
TIE STEEL
distribution
Edge loads
FIP data (Van Acker 1984)
Edge loads
distribution
FIP data (Van Acker 1984)
30%
2
Reinforced Hollow Core: 600 mm wide
Housing and office spans up to about 5.5 m
Self weight approx 270 kg/m
Must use partial cracked stiffness and quasi-
permanent live load ( = 0.3) for deflections
Made upside down. 6 no. T8-T20 bars
Two pass of concrete, grade C40
Cores withdrawn immediately. Cycle takes
less than 3 minutes. Equipment maintenance
approx. 50% of extrusion m/c costs
24 hour drying and humid curing
DOUBLE TEE FLOOR UNITS
2400 3000 mm wide
300 2000 mm deep; typically 400 - 800 mm
self weight 2.6 to 10 kN/sq.m
void ratio 70 80 % of solid section
spans 8 30 m (economical range)
Double tee units mostly prestressed,
but RC if manufacturer prefers
Bearing pads required 150 x 150 x 10
Double tee long-line casting
Prevent settlement cracks at the top of the web
Double tee - concrete train in Germany
Self topped units with 120 mm thick flanges
Double tee self compacting concrete pour.
1 batching + 4 workers = 240 sq.m per day
equates to product cost / salary ratio = 20
Half joint box and
confinement U bars
Half joint lowers the centroid of the floor plate
Site weld to adjacent flange
Make a small saw cut
to prevent spalling
Composite floors required for double tee,
but optional for hollow core
Surface laitence due to cutting slurry
More than 0.5 mm thick
50 mm minimum at the highest point, increasing
(with slab and beam cambers) to about 80 mm
Composite design 2 stage approach
MINIMUM STRUCTURAL DEPTH
FOR 6 10 m
SAME LOAD AND SPAN
CONDITIONS
m
15
6-
FLOOR SPAN
= 1.0 TO 1.5
BEAM SPAN
Vibrations and Natural Frequency of
Precast Concrete Floor Elements
Offices and industrial buildings -
machines, vibrations through the ground, footfall
Sports halls -
human activities (aerobic, jumping, dancing)
Concert halls
Grandstands -
human activities (jumping, stamping)
Vibration & natural frequency The most
important
vibration
property of a
floor is its
natural
frequency fn,
or frequency
of its first
mode.
Vibration & natural frequency
Vibration & natural frequency
Egenfrekvenser DT-elementer
16
14 DT-200/50
12 DT-300/50
Frekvens f [Hz]
DT-400/50
10
DT-500/50
8
DT-600/50
6
DT-700/500
4
DT-800/500
2 Grense
0
2 4 6 8 10 12 14 16 18 20
Spennvidde [L [m]
Vibration & natural frequency spreadsheet
Summary
Strength v economic cost indicator
Depth Cost index Strength index* Popularity
Hollow 150
Core 200 1 1 1
300
400
Double 500
Tee 800
* Based on service moment of resistance
Summary
Strength v economic cost indicator
Depth Cost index Strength index* Popularity
Hollow 150
Core 200 1 1 1
300
400
Approx 120 kNm per unit
Double 500
Approx 25 supply, 30 fixed
Tee 800 per sq.m
* Based on service moment of resistance
Summary
Strength v economic cost indicator
Depth Cost index Strength index Popularity
Hollow 150 0.9 0.65 0.5
Core 200 1 1 1
300
400
Double 500
Tee* 800
* + 75 mm structural topping
Summary
Strength v economic cost indicator
Depth Cost index Strength index Popularity
Hollow 150 0.9 0.65 0.5
Core 200 1 1 1
300 1.3 1.9 0.1
400 1.6 3.0 0.3
Double 500 2.0 3.0 0.2
Tee 800 3.5 5.8 0.1
Summary
Strength v economic cost indicator
Depth Cost/Strength ratio
Hollow 150 1.40
Core 200 1
300 0.68
400 0.53
Double 500 0.67
Tee 800 0.60
