Formwork Striking Times.pdf

7/27/2019 Formwork Striking Times.pdf

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Formwork striking timesor ground Proc .ngrs nstniructs Civ.granulated blastfurnace slag concrete: Nov..441-448Bldgs. 1994, 104,test and site resultsc.A . Clear, BSc, PhD, CEng, MICE, MIC7This Papersets out examples of how theformwork striking time for concretes con-taining up to 70% ground granulatedblastfurnace slag (ggbs) as part of thecement has been determined overUKggbs are used to enhance the durability ofwinter months. Such high proportionsofthe concrete exposed to a range of other-wise deleterious environments. The higherthe proportion of ggbs the slower the earlyage strength development becomes, andexamples include concrete with 0%, 50%,and 70%ggbs, where the majority of thedata is on 70% ggbs, as this s an area ofparticular interest. Results from bothfull-size tests using temperature matchedcuring equipment and n-situ concreteelements are described. These examplesinclude a practical rangef strengthgrade, aggregate type, formwork materialand section size useful to thoseresponsible forassessing the formworkstriking time of concrete.

The durabili ty benefits f using concrete con-Introductiontaining up to70% ground granulated blastfur-nace slag (ggbs) to BS 6699, n combinationwith Portland cement PC) to BS 12, mean thatit is specified for ran ge of structural concreteconcrete with enhanced esistance to severeelements. Combinations of ggb s andPC provideforms of chemical atta ck as well as reducingthe r isk of early-age thermal cracking. Thissachieved by the nature f its hyd ration w hichalso givesa s lower ear ly age s t rength development than the equivalent gradef concretemade with PC only. W here the proportionofggbs is restr icted to levels up to0% t h e s t r i k -ing times are not increased sufficiently to affectthe construction programme, under normal con-dit ions. For this reason, the str iking t imefconcrete containing up to0% g g b s i s o n lyon -sidered briefly in line w ith the limited investi-gation s that have een required. Results showthat for massive or medium-sized concrete con-struction, the use f 70% ggbs does not presenta practical limitation to the strikingime.Where 70% gg bs concrete is used in thinner,more exposed elements, some additional con-sideration may be required and this is dealtwith in detail . Before an asses sme nt is mad ef

what the actual in-situ strength developmentsf i ts of using high proportions f ggb s in con-likely to be, it is worthwhile to review h e bene-Crete.Durable concrete2. To ensure the long-term durabili ty ofreinforced and plain concrete structures,t i soften desirable to specify concrete containing70% ggbs. The most frequent applications are( 0 ) to minimize the risk of early age thermalcracking, or to reduce the amountf crackc on tro l r e i n f ~ r c e m e n t ~ . ~(Q) o provide resistance to sulphate.bearinggroundwaters above Class as defined inBRE Digest 363,s r to res ist attac kbyacids.(c) to increase greatly the resistance to chlo-r ide diffusion, and subsequent reinforce-ment corrosion where structures areexposed to de-icing sal ts or marineenvironment.63. Frequen tly, the types of element thatrequire either an enhanced resistance to chemi-cal attack or require aminimum risk of earlyage thermal cracking are large sections. Wherethis is the case, str iking t imes are not generallyproblematical . However, as th e wareness ofthe durabili ty advantages f using 70% g g b shas grown, so has the requirement to uset inthinner structural elements where str iking t imemay significantly affect the construction pro-gramme. When this occurs, i ts important toknow what strength is required to str ike form-work.

Concrete strength at striking4. Under normal conditions, an in-situ con-crete compressive strengthof 2 N/mm2 is con-sid ere d sufficient to prevent mechanicaldamage on str iking vertical formwork, and toensurerotectionro mccording to BS8110,9 a t least 10N/mm2, or twice the s t ress towhich the element is subjected, whicheveristhe greater, is required to str ik e orizontalformwork. In BS 8110. it is sta ted tha t thisstrength may be assessedby test on cubescured, a s near as possible, under the same con.dit ions a s the concrete in the element. Thi sstatement may be interpreted to include cubescured alongside the structure as ell as morerealistic simulations and therefore the various

BuildingBoardSiructural andStructural PanelPaper l OS17

closes 16January 1995Written discussion

Assistant TechnicalC.A . Clear.Marketing M anager,Civ i l and MarineSlag Cement Limited -441


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CLEARstrength requirements for striking formworkare only a general guide. Sometimes, i t isoreappropriate to select an alternative strengthrequirement for particular applications.Forthis reason, a summ ary f factors affecting thestriking time is useful.Factors affecting striking timethe following factors( a ) formworkdesign( b ) workmanship(c) in-situ concrete strength development.

5. The s triking ti me of concrete depends on

6 . Formwork design is relevant, a s even thestrongest concrete canbe damaged if a formhas not been designed to allow removal. Simi-lar ly, workmanship is important, as concretefany strength canbe damaged where an appro-priate release agent has not been used or whereoperatives do not adopt a suff iciently highevelof care in removing forms.str iking t imes8 which effectively summarize theearly age strength developm ent f PC andRHPC concrete as a function of: concrete grade;mean air temperature; concrete placing tem-perature; formwork type; and theminimumdimension of the se ction ca st. Ha rrison's tablesare based on the concrete strength developmentwith a safety margin.As a consequence, the use

7. Harrison provides tables of minimum

Fig. l . Early age of Harrison's tables may not give a realistics trength development idea of strength developm ent for a pa rticularof concrete mixes source of ceme nt, and do no t includeom -M1 M 5 , a t 20C binations of PC with ggbs. In addition, the

5c

4c

NE 3cEtsmz 2c

l a

a

//'

/f

II Mix, cementndggregateype

-A- M1 , 70% ggbs. Marinee 2, 70% ggbs. Granite+ 3, 70% ggbs, Limestone-0- M4. 50% aabs. Marine

1 2 3 4 5 6 7Age: days

properties of PCs have changed" over theyears.8. The combin ation of mean a ir tem-perature, concrete placing temperature, form-work material and the m inimum dim ension fthe section cast are important factors s theyeffectively control the early age temperaturehistory of the element. In a report coveringmethods of assessm ent of strikin g time,Harrison" states that cubesmade from con-crete sampled from that placed n an eleme nt,and cured at a tem pera ture rofile whichmatch es that of the in-situ concrete,willprovide the shortest striking ime. This tem-perature m atched cu ring TMC)" technique isuseful, as i t is the in-situ temperature historyfthe concrete element which controls its earlyage strength development. n addition, the TMCfulfils the BS 8110' recommendation to curecubes under conditions as lose as possible tothe concrete in the element. Before w orkwiththe TMC apparatus is de scribed, i t is w orth-while to show the elative effect of using up to70% ggbs in a nominal C40 grade concrete at aconstant curing temperature.Early age strength development at20C9. Figure 1 shows the strength developmentof concrete cubes stored in wate r at 20C up toseven days, for f ive concrete mixes as set outnTable 1.Mixes M1 -M3 contain 70% ggbs incombination with a marine gravel, granite andlimestone aggregate respectively.Mix M4 con-tains 50% ggb s and Mix M5 is a PC only con-crete, both in combination with a marinegravel. All concrete mixes w ere mixed and up -plied by ready-mixed concrete vehicles; theconcrete used for strength development wassampled from tha t supplied to the element cast.10. In the unlikely event that a real struc-tural concrete element was ured at a constanttemperature of 20C, then Fig. 1 shows that a2 N/mm z requirement for the striking f verti-cal formwork is met by all the mixes n lessthan one day. All mixes also achieve 10 N/mm2by two days, with the 0% ggbs mix making i tin one day. Wheremore than 10 N/mm2, say20 N/m m*, is required or the striking of hori-zontal formwork then Fig. 1 ndicates a strikingtime of two d ay s for the 50% ggbs concrete,and up to four days or concrete containing70% ggbs. How ever, Fig. 1 shows only thestrength developm ent a t a constant 0C andthus ignores the temperature history hich cansignificantly change the early age in-situstrength development.Early age temperature of concrete11. The early age tem perature history ofin-situ concrete is a function of( a ) ccncrete placing temperature( b ) minimum dim ension of section cast


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FORMWORK STRIKINGTIMES FOR GGBSCONCRETETable 1 . Detai ls of concrete mixes f o r TMC tes tsMix Cube st rength :ggregate type Admixture*lump:ax .ominalotalg b s : %

~ ~~

cementw /content: N /mm2

mre eradekg/m 90 day6 day8 dayoarsein e

M1

59.67.71.0imestonearine +P35.4540 3900363.3 59.92.8raniterushed rockP15.4540900261.96. 09.8arinearineP00.4540900

crushed rockM4

63.68.39.2arinearineP00.4 540 390557.07arinearineRA 75.4240 415 50

* SP = is a superplas ticizer (FEB SP3) andWRA is a normal water reducing or plasticizing admixtureCormix P4).t Representa t ive s i te resul ts .( c ) amou nt of ceme ntitious material(d ) type of cem entitious material( e ) ambient emperature(f) nsul ating prop erties of formwork.

12. During hydration all ceme nts generateheat, and this exothermic behaviour is normallythe most important factor affecting the earlyage tempera ture of concrete sections placed informwork, with a minimum dimension greaterthan 300 mm. Where more than 30%ggbs isused in concrete to replace thePC, then thein-situ temperature rises canbe significantlyreduced, and at 0% ggbs the ear ly age tem-perature rise can be as low as one third of th atcontaining PC only. However, the fac tors affe ct-ing the in-situ temperature rise of concrete areinterdependent. The higher the init ial concretetemperature, the thicker theminimum sectionsize and the greater the total cementit iouscontent, then the higher the temperature devel-oped. In BS 8110,9 i t is suggested that a suit-able method for assessing str iking t ime s tocure cubes alongside the structure, assumingthat the curing conditions f the cubes will berepresentative of the concrete in the struc ture.This method of str iking t ime assessment cangive extended str iking t imes,as the concrete inthe cubes is effectively cured at a tem peratu re

close to ambie nt, frequently significantly owerthan that in the structure which it is deemed torepresent. Harrison" states that for thick slabs,or for large or well-insulated sections, the'cubes cured alongside' method will indicateconservative str iking t imes.Temperature and strengkh13. A diagram of the TMC apparatus, wherethe temperature of the w ater in a cu ring ta nk ismatched to the temperature of the struc turalconcrete element, is show n n Fig. 2. The posi-tion and depth of the single probe within theconcrete is important as i t sho uld be at a posi-tion where the in-situ strength is required, nor-mally the cover zone at the pointof maximumbending moment or a similar point f maximumstress. Lit t le or no moisture shouldove in orout of a prop erly c ured s tructural e lementwithin formwork. To match this condition, theconcrete samples forTMC water bath areplaced in cube moulds which are then sealed.14. In Figs 3(a)-3(e), the early age tem-perature and temperature matched curedstrength developm ent of the five co ncrete mixesM1 -M5, placed in various elements, are shown.Mixes M 1 M3 and M5 were placed in 1m 3blocks as par tof a test regime to determine

LAoncrete probe StirrerControl unit Heatingelement

Structure Water bathFig. 2. Temperaturematched curing( TMC) quipment

443


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CLEAR--- Ambient

40 - -. - Concrete+ n-situ strength

40 r

0' I 1 I I I I 1

/' 0 1 2 3 4 5 6 7

Fig. 3. Temperature m atched cured in-s i tu s trength development in tes te l e me n t s : ( a )1m 3 block with 70% ggbs an d mar i n e aggr e ga te ; ( b ) m 3block with 70% ggbs an dgr an i t e aggr e ga t e ; ( c ) m 3 lock with 70%g g b sand l imestone aggregate; (d) 50 mm s l ab w i t h50% ggbs and mar ine

Age: days

ggr e ga t e ; ( e ) 1m 3 block with PC and mar ine aggregate44

tempera ture rises and differe ntials or theavoidanc e of early age thermal cracking nwalls exposed to a marine environment. Theseblocks were placed durin g October to December1992, and the additional TMC measurem entswere taken to verify th e likely strik ing timebased on a nominal requireme nt for 20 N/mm2.Mix M4 was placed in a 250 mm bridge decksection where an in-situ strength f 10 N/mm2was required a s quickly as possible duringdays in late winter to early spring, to takeon -struction loads.

15. The temperatures show n were recordeda t t h ecover zone, 75 mm in from the face forth e 1m3 blocks, where the requirem ent tomini-mize the risk of thermal cracking mean t theformwork wa s well insulated to prev ent th edevelopm ent of differential temperatu res. In thebridge deck, the temperatures ere measured50 mm down from the top surfaceof the250 mm thick slab . Also shown in Figs 3(a)-3(e) are the ambient temperature measurem entswhich were fairly typical for the times of theyear during which the testswere carried out.

16. Tab le 2 sho ws a s um ma ry of sectionsize, date cast , average ambient temperaturesand str iking t imesor requirem ents of 2 N/mm2,10N/mm2 and20 N/mm2.

17. Th e 1m cubes were cast to simulate awater retainingwall where the reus e of form -work programme of one to two weeks renderedof aca dem ic intere st the strik ing ime of up tothree days. The construction rogramme for thedeck slab wasmore restrictive, but the required1 0 N/mm2 waschieved within two days. Thistest wa s carried ut in early March which sim u-lated the most severe condition as the ambien ttemperatures were higher during the ollowingmon ths when the rest of the deck slab was com-pleted. The high cementitious contentof mixM4 was required tomeet the w/c requirementsand the 28 day cube strengths xceeded thespecified characteristic valueby a high margin.18. Th e PC resul ts for mix M5 gave suc hhigh early strengths over the f irst nightofplacing that the exactimes when the variousstre ngth levels were achieved was not deter -mined. However, at less than one day, suchhigh early strengths arenot generally requiredfor in-situ concrete in civil engin eering struc-tures, and theywould allow formwork removalearlier than would be advisable w ithout theimmediate applicationof a protective barrierfor proper curing.19. These results demons trate how usefulthe TMC appara tus is n ascertaining thein-situ strengthof concrete but it may notalwa ys be readily available to contractors orother e ngineers. It is po ssible to es timate thein-situ stren gth of concrete from stand ard cuberesults in combination with just the em-peratu re history of the in-situconcrete. Thusthe need for a full TMC equipment is replaced


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FORMWORK STRIKINGTIMES FOR GGBSCONCRETETable 2. Section size and striking time for TMC tes ts

Mix Minimum striking for a strength requirement ofverage ambient Date castinimumdimension temperature; "C 2 N/mm2

< l dayl dayl day04 Nov. 1992m5, Fig. 3(e)63 hours3 hours2 hours03 Mar. 1990 250m4, Fig. 3(d)38 hours 27ours8 hours2Nov. 1992m3, Fig. 3(c)72 hours2 hours1 hours0 Oct. 1992m2, Fig. 3(b)69 hours9 hours1 hoursDec. 1992m1, Fig. 3(a)

20 N/mm20 N/mm2

by a tem perature m easuring evice with dis-posable or recoverable thermocouplesor probesfor embeddin g in the concrete.

Equivalent age20. The princ iple of Equiva lent Age wa s seto ut by W ea ver an d S a d g r ~ v e ' ~n 1971 for Port-land cement concrete. Harrison1 4 verified therelationship in 1975 for temperatures in therange 7"-27"C during the assessm entof therisk of mechanical damag e to conc rete y earlyremoval of formwork. Wimpenny andEllis15verif ied that the EquivalentAge principleworks for a range of combina tions of ggb s an dPC. Simply stated, a concrete cured for aeriodT , at a temperatureof 0C has an E qu iva len tAge T c qwhen cur ed at 20C. The relationshipcan be expressed thus

Equivalent Age T e q=1 e-i6)'A twhere B is the average temperature, and t isthe increment of time at the avera ge tem -perature.5C fo r 70 hours, which attains a strengthof ,21. A simple example is a concrete cured a t

Table 3. Example of equivalent age calculationAge :days-1

2345

67-

Temperature:"C192834343027

2420

Averagetemperature"C23.531343228.525.522

AT,,:daysaysT c q :

0

2.9 1.71.2.20

1.9 4.81.8 6.61.5 8.11.3 9.41.1 10.5

say , 7 N/mm2; the sameoncrete will beexpected to achieve 7 N/mm ' if it is cured foreither: 46 hou rs at10C; or 32 hou rs at15C;or24 hours a t20C; or 19 hou rs at 25C; or 1 5hours a t30C.22. A realistic exam ple is set out in Tab le 3,using the temperature history f th e 1m sectionshow n in Fig. 3(c), the resu lts forM3, and 70%ggbs concrete with a l imestone aggregate.TMC in-situ strength23. In effect, when the in-situ concrete isthree days old, i ts Equivalent ge to concretecured at 20C is 4.8 days becau se i t was at ahigher temperaturemost of the time. The TMCin - s i tu s t r en g th a t t h r ee d ay s , a s sh o wnnFig . 3(c), is 33.5 N/mm 2. Inspect ion of F ig. 1,conc rete streng th at 0"C, gives an estimatedstreng th of 31 N/m mz for an Equivalent Age at20C of 4.8 days . T his exam ple show s th at asafe estimateof the TMC in-situ strength canbe achieved as the estimate is ower than theTMC in-situ strength.24. Figure 4 shows the relationship betweenthe estimated strength-the strength estimatedusing the in-situ temperature history and the20C cube strength curves shownn Fig. 1-and the TMC in-s i tu s t rength resu l ts fo r thefour mixesM 1 -M4. In the case of mix M 5, onlythe earlier agePC results up to wo days old areincluded, becauseat la ter ages theoncrete isa t i t smaximum streng th, when further curingwould not give an increase n strength, andthus the equivalent age principle ould notapply.25. From an inspection of Fig. 4, it isevident that the estimated strengths are eitherequal toor a little less than theTMC in-situstrength results, indicating that the estimatingmethod is safefor the concretemixes andminimum dimensions of sectio ns tested .Estimated strengths without TMCTMC tests to ascer ta in an accura te s t r ik ingtime where it is the critical factor controllingthe construction programme.However, in many

26. In general , i t is advisable to use the full


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CLEAR

Mix, cement and aggregate typeA M1, 70% ggbs, Marine0 M2, 70% ggbs, GraniteV M3, 70% ggbs, LimestoneQ M4, 50% ggbs, Marine+ M5, PC, Marine

+ A

I I I I10 20 300 50

In-situ strength from TMC results: Nlmm'

Fig. 4 . Relat ionship cases, the concrete strength estimatedrom abetween es t imated minimum concrete temperature and the earlys t rength and in -s i tu age s t rength development a t0C may be suffi-( t emperature matched cien t to show tha t the s t r ik ing t ime isnlikelycured) s t r ength to be a controlling factor n the con struction

programme.Estimating concrete temperatures instructures27. In Figs 5(a)-5(e), the resu lts are show nfrom five examples,E l to E5 respectively, fstructural elements where the early agestrength developmentof cubes at 20C and thein-situ temperatureof the element have enabledthe in-situ strength to e estimated up to twodays af ter casting. Table ummarizes the

Table 4 . Detai ls of s i te s tructural elements and concrete mixes

section details and the propertiesf the con-crete mixes.28. In Fig. 5(a) it is shown tha t an es t imatedstrengt h of 10 N/mmz wasachieved within twodays wi th a50% g gbs concrete, even thoughthe in-situconcrete te mperatu re did not exceed30C. This was acceptable to the constructionprogramme, but i t was evident that a 0% ggbsconcrete would not achieve the requiredstreng th in such a thin and xposed deck slab.29. Results from a 400 mm diam eter pileusing 70% ggb s concrete are show n n Fig. 5(b),where it is evident that the tem peraturef theground kept thepile temperature consistentlylow. In this case, thepile tempe rature was mea-sured l m below the top. An estimated strengthof 5 N/mm z was chieved in under two days,which was deemed sufficient to resist damagecaused by the plac ing of adja cent piles.30. Res ults from the cent re of a 500 mm

section cas t in 70% ggb s concrete using timberformwork are shown n Fig. 5(c). A streng thof7.5 N/mmz was required to support the false-work system for the erection of the formworkon top of the cu rrent section. The insul atingproperties of the timber form allowed thesurface concrete to chieve a tempe rature of20C within 12 hours. This meant an estimatedstre ngth of 7.5 N/m m2 wasachieved within24 hours.31. When concrete is slipformed , it isexposed to the elements ery quickly aftercasting. Fig. 5(d) shows the results for a 1mthick section where the concrete emerged fromthe form afte r 7 hours. Despite this, the con-crete surface temperature maintained0C andthe estimated strengthdevelopment was veryclose to the strength evelopment of standardcubes. In this example, there was no early agestrength requirement, as he experience of theslipform personne l, and their proven ru le ofthum b method-more like a ' thumb penetra-tion test '-was all that w as re quired o gaug ethe safe rate f movement for the slipform.

Struc tura l

k g / m 3Cimension w lcontent :empera ture :inimum

Admixture*lump:axominalo ta lgbs :verageormworkelement and mmre eradeementmbient

El

WRA, P4.5130300.2imber Wall,00 mm3WRA, 2117540250.6round Pile,00 mm2WRA, P45.4240150.0n precast s labslab, 250 mm

E4WRA, P4.5130 3300.0teel 3.55WRA, P4 7540800.5hours in slipformm

WRA P 4 is Cormix P4 and WRA 211 is Conplast 211, both are norm al water-reducing or plasticizing admixtures.46


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Nevertheless the example is useful, a s undersevere conditions of almost immediate exposureof the concrete an estimated surface strength f5 N/mm 2 was achieved t one day.3.5 m thick section is show n n Fig. 5(e) where aspecial steel form wa s used without insulation.Like the slipform , there w asno early agestrength requirement for str iking formwork butthe example is u seful as i t show sow the steelreduced the initial s urface tem perature of theconcrete and so the estimated strength wasonly about 2 .5 N/mmz a tone day. However, thesubsequent temperature r ise enabled the con-crete to achieve an estimated strengthf10N/mmz a t two days.Practical application33. In the above tests and examples, theconcrete used for ascertaining the early agestre ngt h, eith er at 0C or by TMC, wa ssampled from, or indicative of, hat supp lied tothe element cast from the ready-mixed concretetruck. In cases where the concrete strength dataare no t as represen ta t ive as th is , an addi t ionalfactor of safety for the strengthof concrete atstriking may be required.34. Similarly, the tempera ture history ofeach element has been m easured at a point rep-resentative of the position where the in-situstrength is required, normally at the reinforce-ment level near the co ncrete surfac e. As s hownby the examples, an exact early age strengthrequirement is not always specif ied but theywere included as useful information for thosewho may be interested in estimating the earlyage in-situ strength f similar sized elem entsusing up to70% ggb s concrete.General conclusions35. Th e TMC test results confirm that theprinciple of Equivalent Age canbe applied toestimate the ea rly age in-situ streng th f con-crete containing up to70% ggbs. Early age isup to an equivalent age f 7 days which isequivalent to a real age etween 4 and 6.5 d ay sfor 70% ggb s concrete cast in 1m thick sec-tions under cold winter conditions. Undersimilar am bient conditions, the C only con-crete produced higher concrete tem peratures atearly ages, and an equivalent agef 7 d ay s wasachieved in less than 3 days.36 . The results were obtained during thesp rin g of 1990 and the winter of 1992, to sim u-late a worst case for the concrete elementsunder construction in the United Kingdom.Owing to the somewhat variable natureof theBritish climate, care must e taken when theseresults are extrapolated to colder winters orcooler climates .

37. From the exam ples of winter concretingdescribed, it is evident that the in-situ early agestrength developm ent of C30+ concrete, con-

32. On a larger scale, the results from a

FORMWORK STRIKINGTIMES FOR GGBSCONCRETE6 mbient emperature-P- Concrete surface emperature+ 0C cube strength-0- Estimated concrete strength--- Required concrete strength

-I 30

5 -L

- 10

0 I l I 0

I I I0 0.5.5 2

Age: daysFig. 5. Examples of es t imated in-s i tu s trength withoutTMC: ( a )250 m mslab, cast on precast concrete planks,0% ggbs concre te ; (b) 00 mmdiameter p i l e , cas t in ground,0% ggbs concrete; ( c )500 mm wall, int imber forms, 0% ggbs c on c r e t e ; ( d ) m hick s l ipform wall , 70% g g b sconcre te ; (e ) 3.5 m thickcrosshead in s tee l formwork, 70% ggbs concrete -447


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taining up to70% ggbs, in sections with aminimum dimension greater than250mm , issufficient to achieve a striking time hich doesnot extend the construction programme. Whereonly up to 50% ggbs is used , the reduction nearly age development has not presented prob-lems sig nificant enoug h to prom pt uch fieldinvestigation, a s reflected in the examples.Structural elements which mayneed detailedconsideration are where the minimum dimen-sion of section cast is less than r around250 mm, particularly in combination w ith con-crete cast in non-timber formwork, suchssteel, slipform or cast against the ground. Inthese cases, i t is worthwhile to use a worst casein-situ temperature history, suchas a represen-tative minimum concrete temperature, to esti-mate the minimum striking time to see w hetheror not a full temperature matched curing testsrequired in order to obtain the most realisticvalue.Acknowledgementshelp and assistance f Trafalgar House Con-struction Special Projects Limited for the sitedata and other information provided for thisPaper.

38. The A uthor acknowledges gratefully the

References1. BRlnsH STANDARDSNsnTunoN.Specification fo rGround granulated blastfurnace slag or use withPortland cement. BSI, London, 1992, BS 6699.2. BRITISH STANDA RDSN s n m n o N . Specification fo r

Portland cement. BSI, London, 1991, BS 12.3. THE ONCRETEOCIETY. ass Concrete. ConcreteSociety, London, A ug. 1986, Digest No. 2.4. HARRISON. A. Early-age thermal crack control inconcrete. Construction Industry Research an dInformation Association, London, 1981, Report91 .

5. BUILDING ESEARCH ESTABLISHMENT.ulphate andacid resistance of concrete in the grou nd. BRE,Garsto n, July 1991, BRE Digest 363.6. BRITISHTANDARDSNSTITUTION.Code ofpracticefo r maritime structures. BSI, London, BS 6349:Par t 1: 1984, Last amen dmen t No. 4, July 1989.7. THE ONCRETEOCIETY.ormwork, a guide to goodpractice. Joint committee of the Concrete Societyand The Inst i tu t ionof Civil Engin eers, London,Aug. 1986.8. HARRISON. . Tables of minimum striking timesfo r soffit and vertical formwork. ConstructionIndustry Research and Information Association,London, 1977, reprinted March 1979, Report 67.

concrete, Part 1 : ode ofpractice fo r design andconstruction. BSI, London, 1985, BS 8110: Part 1.10 . THE ONCRETEOCIETY.hanges in the propertiesof ordinary Portland cement and their effects onconcrete. London, O ct. 1987, Techn ical Report 29(Repo rt of a Concrete Society work ing party).methods of assessment. Construction IndustryResearch and Inform ation Association, London,1987, Report 73,2nd edn.12. BRITISH STANDARDSNSTITUTION.ethod of tem-perature matched curing of concrete specimens.BSI, London, 1984, Dra ft for developmentDD 92.formwork -tables of curingperiods to achieve givenstrengths. Construction Industry Research andInformation Association, London, Oct. 1971,Report 36.14. HARRISON. . Mechanical damage to concrete byearly removalof formwork. London, Cement andConcrete Association, London, Feb. 1975, Techn i-cal Re port 42.505.15. WIMPENNY. and ELLIS . Th e effect of g gb s onthe temperature and strengthdevelopment in con-crete elem ents under ow ambient temperatures.P Y O C . Int. Conf. on Blended Cements in Construc-tion, Sheffield, 1991, Elsevie r Applied Science,London, 1991,222-235.

9. BRITISHSTANDARDS INSnTvnON. StYUCtUYal use Of

11. HARRISON. A. Formwork striking times-

13. WEAVER. and SADGROVE. M. Striking times of

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