Casting Box Girder Segments_tcm45-342871

7/31/2019 Casting Box Girder Segments_tcm45-342871

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Producing bridge seg-

ments in a cast ing yard

aw ay from th e br i d g e

c o n s t ru ction site is a m a-

jo r advan tage of se gm en tal co n -

c ret e b ri d g e s. The ca stin g yard

b rings factor y- c o n t ro lled pro d u c-

tion t echn iques, efficiency, qu ality

c o n t ro l, an d tim e s avings t o b ri d g e

c o n s t r uc tion . Fa b ricat in g b ri d g e

segments in a separate area also

re m oves casting op eration s fro mthe construction crit ical path and

red uc es th e ove rall con str u c t i o n

time.

Re g a rdles s of t he pr oject loca-

tio n or size, a con tra c t o rs ca stin g

y a rd for bridge segm en ts h as seve r-

al essen tial feat ure s. These inc lude

d e l i ve ry an d sto rage are a s, a co n-

c rete b atch p lant, a rebar cage as-

semb ly area, one or m ore casting

c e l l s, stea m cu rin g fac ilities, ge o-

m e t ric co nt rol sta tions, an d seg-

m ent storage an d ha nd ling facili-t i e s. The size of th e yard d epe n ds

on th e size of the job and the re-

q u i red rat e of se gm en t pro d u c t i o n .

A typical production rate is four or

f i ve segm en ts p er five-d ay w ork

we e k .

Forming Systems

Two m ethods of segment cast -

ing are available to the con tra c-

t o r. Lon g-line or shor t-line form s

c a n b e u s e d d e p e n d in g o n t h e

a rea avai lable for the cast in gy a rd and the geom et ry of th e

b ridge sp an s.

In long-line casting, all seg-

m en ts are cast on a so ffit the fu ll

length of the can tilever (or half-

length if the cant ilever is symm et-

rical). All geom etric con trol is ac-

com p lish ed wh ile con stru c t i n g

th e soffit, greatly sim p lifying con -

t rol during segmen t produ ction. A

full soffit constru cted for th e long-

line m eth od , h owe ve r, re q u i res a

l a rge area, a n d the soffit m ight on -

ly be u sed o nce bec ause it is diffi-

cu lt to acco m m od ate va ri a t i o n s

for differen t bridge spa n s.

With short-line casting bed s the

f o rm is station ary while th e ind ivid-

ual segmen ts move from the cast-

ing position to the m atch-castin g

p osition to sto ra g e. Ad van tages o fs h o rt-lin e castin g are mu ch sm aller

sp ace re q u i re m e n t s, ce ntr a l i ze d

p rod uctio n, a dap tab ility to va ri a-

tion s in bridge geometry, and the

abili ty to reuse the forms many

t i m e s. Casting br idge segm en ts us-

ing a sho rt-line bed re q u i res a ccu -

rate placem ent of the m atch-cast-

ing segment and post-casting

g e o m e t ry ob serva t i o n s. Precise su r-

veying skills and equipment are

need ed to me asure elevations and

alignm ent s within 0.001-foot toler-a n c e s.

Match ca sting is a type o f cast-

ing metho d where a n ew segmen ts

f resh co n crete is cast against t he

h a rde n ed con cre te of a pre v i o u s l y

p rodu ced segmen t . Both short -

l in e and lon g-lin e casting opera-

tions can use match casting. By

casting against the h ardened sur-

face of the p revious segmen t, the

jo in t is a lm ost in vis ib le wh en the

segmen ts are rea ssemb led du ri n g

e rect ion of th e br i d g e. A b on db reaker (usua lly chem ical com -

pou nd s or a m ixture of wax, soap,

and talcum powder) app lied to the

h a rden ed co ncrete s urface en sure s

that th e segm en ts will sepa ra t e.

Re g a rdless o f the form ing system

used in the casting yard, follow ba -

sic recom men ded practices to pro-

duce quality concrete segments

an d ach ieve de sired p ro d u c t i o n

ra t e s. These ba sic p ractices ap p ly

to both long-lin e an d short - l i n e

casting methods.

Rebar Cages and Post-tension-ing Ducts and Hardware

To inc reas e segm en t pr o d u c t i o n

ra t e s, som e con tra cto rs pre f a b ri-

cate rebar cages with p ost-tension-

ing ducts an d h ard w a re alread y in-

stalled. Custom jigs an d tem platesa re typically used to facilitat e initial

a s s e m b l y. Fina l adju stm en ts m ad e

in the casting cell account for

m oveme nt du ring tra n s p o rt or

slight va riations in sha pe, re i n-

f o rcemen t, or post-ten sioning re-

q u i re m e n t s.

Du ring fabrication , avoid con -

flicts between the rebar and the

post- tensioning du cts and h ard-

w a re. Proper alignm en t of th e post-

tensioning ducts is more impor-

tan t th an reb ar location . Po s i t i o np ost- ten sion ing du cts corre c t l y,

then make local adjustmen ts to the

rebar as needed. When using

e p ox y-coa ted re b a r, avo id da m ag-

in g th e coa ting . Shop dra w i n g s

should show details of the post-

ten sionin g ha rd w a re system,

d u c t s, re i n f o rcin g bars , an d an y

special construction details.

A com bination of chairs, spac-

er ba rs, a n d tie wire is re c o m-

m ended t o m a i n t a i n pos t - t en -

sionin g du cts at the corre c te l e vation an d secure them to the

re i n f o rcin g ste el. Usin g tie wire

alone ma y cause deform ation of

the rebar cage u nless braced by

loca l t ra n s verse re b a r. In de n sely

re i n f o rce d an ch or zo n e s, p lac e

an d consol idate the concre t e

p roperly to el im inate hon ey-

c o m b i n g .

Ca s t in g Box Gir d e r Segm e n t s

BY ALAN J. M ORETON AND H. HUBERT JANSSEN

Followin g ba sic castin g yard p rocedures will

a ssu re qu a lity concrete bridge segm ents


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Placing Concrete

Fo l l ow goo d con crete placem en t

p ractice to en sure a qu ality pro d u c t .

Be f o re p lacin g con cre t e, be su re

f o rm s are th orou ghly cleane d, a ll

join ts are tight an d sealed , and post -

tensioning ducts are aligned an d se-

c u re. The forms sho uld be lightly

oiled for easier stripping and the

face of the match-cast segmentcoated with a suitab le bond bre a k e r.

Use skips, chu tes, or pu mp s to

d e l i ver con crete with ou t lettin g it

fall a gr eat dist an ce. Th is p re ve n t s

c o n c rete segregation, da ma ge to

post-ten sioning du cts, an d re b a r

displacemen t. Thou gh concre t e

d e l i ve ry s ho uld be as con tinu ous

as p ossible, short waits are occa-

siona lly ne cessary, esp ecially after

placing th e bo ttom slab an d we b -

c o rn er con cre t e. A sh or t int er va l

a l l ows th e con crete to set up

eno ugh to suppo rt the weight of

the rest of the web concrete with-

ou t crea tin g cold join ts. Re t a rd i n g

a d m i x t u res are often u sed to h elp

simplify the o perat ion.

T h o roughly conso lidat e th e con -

c ret e u sing int ern al vibra t o r s. Vi-

b rators should b e pu shed into the

c o n c rete n o m ore th an abo ut 2 feet

an d sho uld be withd rawn slow l y

f rom the sam e location. Moving th e

v i b rator sideways in the co ncrete ort ryin g to m ove con crete with the vi-

b rator can lead t o po or con solida-

tion an d ho ne ycomb ing. Avo i d

contact between the vibrator and

rebar or post-tensioning ducts,

which can cause damage or dis-

placement .

Be sure the concrete is thoro u g h l y

com pacted, especially in a wkward

a reas such as corn e r s, s p ira l s, a nd

heavily re i n f o rced an chor zo n e s.

Placement SequenceA good p lacem en t pr o c e d u re

shou ld pre vent the concrete p laced

in th e b ottom of the web fro m

spilling in to the b ottom slab. Move-

men t of the web concrete can easi-

ly displace rebar an d po st-tension-

ing du cts a nd can pu ll concre t e

aw ay from th e hea vily re i n f o rc e d

botto m an chors or the web itself. A

p roper seque nce will m inimizef l ow of th e con crete after it h as

been placed.

Place the first concrete in the

midd le portion of the bottom slab,

leaving abou t 6 to 12 inche s clear of

the side forms at the bottom of the

we b s. De l i ver bo tto m slab con cre t e

t h rough a trap in the t op slab soffit

or throu gh th e b ulkhead en d. Ne x t ,

place concrete in the webs and

consol idate i t around the bot tom

c o rne rs to comp lete the bottom

s l a b. Con tinu e placing concre te inth e we b s, working u p to t he top

s l a b. Fi n a l l y, p lace th e con cre te in

the t op slab, workin g from the cen -

ter an d outside e dges tow a rd the

we b s.

Finishing and Curing

A high-q ua lity finish of th e top

s u rface is essen tial in sup erstru c-

t u re segm en ts wh ere it also fun c-

tion s as t he riding su rf a c e. The on ly

o p p o rtu nity to ach ieve a sm oot hs u rface is dur ing the castin g opera -

tion. Grinding rou gh surfaces after

c o n s t ruct ion redu ces co nc rete cov-

er and adds t ime and expense to

th e pro j e c t .

Mechanical f inishing equip-

m ent provides a sm ooth r i d i n g

s u rface if use d pro pe rly by

t ra ined an d exper ienced opera-

t o r s. Be su re to fill in de p re s s i o n s

an d re m ove high areas to achievea ve ry u niform, d en se, an d leve l

s u rf a c e. Both rollin g an d vibra t o-

ry screeds work well to pr odu ce a

smo oth finish.

Mechanical screeding should be

f o l l owed by a straigh t edge use d to

check and correct any low and h igh

a rea s (Fi g u re 2 ). The stra ight ed ge

also prod uces an accu ra t e, leve l

s u rface acro ss th e to p of th e se g-

ment f rom the bulkhead to the

m atch-cast segmen t. If han d fin-

ishin g th e su rf a c e, en sur e a leve ls u rface by using a stro ng, stra i g h t

s c reed th at extends from the top of

the bulkhead to the top of the

match-cast segmen t.

To ac hieve typ ica l pr o d u c t i o n

ra t e s, the curing process in the cast-

ing cell cann ot b e longer th an fro m

the com pletion of the casting in the

e venin g to the start o f survey an d

s t ripp ing the n ext m orn ing. Cu ri n g

p ro c e d u res depend on the concre t e

mix an d the environm ental condi-

tions after casting. Comm on pra c-

tice is to cover the segmen t with tar-

paulins an d a pply steam to maintain

a cont rolled t emp era t u re and hu-

m i d i t y. Other successful m eth ods in-

clude wet b urlap, curing blan kets,

an d fog spra y s.

Althou gh cur ing pro c e d u res ma y

Figure 1. Mat ch cast ing against t he hardened surface of the previous segmentleaves an almost invisible joint w hen the segments are assembled.


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d i f f e r, t he segm en t m u st re-

ma in in favo rable curing con-

d i t i o n s, su ch as u nde r we t

b u r l a p, after strip p ing th e

f o rm .

Striking Forms

Re m oving se gme nt form s

begins after the concrete ha s

reach ed spe cified stre n g t h ,typically 2500 ps i in com pre s-

sion. At th is strength it is usu-

ally possib le to ease off the

side form s, re m ove th e core

f o rm , an d pu ll ba ck the

match-cast segment (if the

to p slab is self-sup p ort i n g ) .

The segment can also be

m oved on its pallet , but not

lifted . If th e segm en t re i n-

f o rcem en t cann ot carry the we i g h t

of the u nsup ported top slab at 2500

psi, tra n s verse po st-ten sionin gm ust b e stressed in full or in pa rt .

This re q u i res a h igher con cre t e

s t ren gth, u su ally 4 000 p si. Tra d i-

t i o n a l l y, qu ality- c o n t ro l cylinder s

h a ve been bro ken to ve rify the con -

c rete strength for th ese interm e d i-

ate op era t i o n s.

Re m ove the form s ca refully since

the con crete is mo re susceptible to

spalling and oth er dam age at an

early age. Most casting cell form s

a re re m ovable in who le p ieces. De-

lay re m ovin g spe cial blockou tf o rm s for as long as po ssible to

a void d am aging blockout ed ges.

St riking an d pu lling ba ck th e

match-cast segment should be

done with particular care. If the

bon d b reaker has n ot been p ro p e r-

ly ap plied, por tion s of eithe r seg-

m en t can b reak off. Shea r keys are

esp ecially vuln era b l e. The str i p p i n g

c rew also n eeds to exam ine and

understand the movement m echa-

n ism. Do no t lift the n ewly cast

segment by loosening jacks and

tilting the pa llet. This motion can

easily dam age the sh ear keys. The

sam e rule applies when p ulling the

ne w segme nt away from th e bulk-

h e a d .

Understanding basic segment

casting techniques reduces costs

an d d elays associated with lack of

un ders tan ding of critical pro c e-d u res an d ove r-em ph asis of n on -

c ritical item s. Fo l l owing th ese ba sic

p ro c e d u res in th e pro du ction of

b ridge segm en ts will increase qu al-

ity assurance for the entire con-

s t ruct ion team .

Editors Note

The material for this article is excerpt-ed from the Guide to the Constructionof Segmental Bridgescommissionedby the Florida Department of Trans-portat ion to HDR Engineering Inc.

Alan J. Mo ret on, P.E., is vice pre s i-

dent and technical director for seg-ment a l and concrete cable-stayedbridge s with Parsons Brinck erh o ff ,

Tampa, Fla., and former state struc-t u res enginee r with t he Florida D e-

p a r tm en t of Tr a n s p o r t at io n, Of f i c eof Construction.

H. H uber t Janss en, P.E., is pre s i -

dent of Janssen Spaans Engineer-ing, a consulting engineering firm inIndianapol is special izing in br idge

design.

Fi g u re 2 . A high-qual it y finish of t he t op sur f a c eis essential in superst ru c t u re segments whereit is also the riding sur f a c e .

P U BL I C ATION # C950068

Copyright 1995, The Aberd e e n G ro u p ,

All rights re s e r v e d

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