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Creep in Concrete: Outline of the Problem and Proposed DBR Research Program
NATIONAL RESEARCH COUNCIL CANADA
D I V I S I O N O F B U I L D I N G RESEARCH
C R E E P I N CONCRETE: OUTLINE O F
THE
PROBLEM AND PROPOSED DBR RESEARCH PROGRAMW
,G. PlewesReport
No,
93
of the
Division of Building Research
O t t a w a Jul-y
1956
CREEP I N CONCRETE; . . . . - . . OUTLINE . - .. . .. . . ..OF . . . . - THE__PROBLEM . . .- AND ~
When c o n c r e t e i s s u b j e z t e d t o a c o n t i n u o u s l y a p p l i e d s t r e s s it undergoes a time-dependent deformation commonly c a l l e d c r e e p , T h i s defamation i s c o n s i d e r e d d i s t i n c t from
thos.s dimenvional cberge.8 accompanying shnr2nkage and v a r i a - t i o n s i n l o a d , t e m p e r a t u r e o r mof stul72 i n c o n c ~ e t e .
Aht,?-mugh ~ccana!~de~able s a . s e . a ~ c h h . a s been devoted t o t h e s u b J e s t , t h e y e i s s t i i l a lack of complete and e x p l i c i t d a t a f o r use by d e s i g n e r s who shou.ld take c r e e p i n t ~ account i n t h e i s d . e s i g n a , The number of v a r l . a b l e s
-
even c s n t r o l l a b l e v a r i a b l e s-
is eo l a r g e t h a t t h e r e has been d i f f i c u l t y i nobtaSnl.ng ccmplete s o ~ ~ a l a t i a w of' t h e r e s u l t s of v a r i o u s I n v e s t 1 g a t . o ~ ~ workLag independently
.
Committee 209 of t h e American C o m . s e t e I n s t S t u t e Volume Changes and P l a s t i c Flowi n Concrete2. h s a+;tj'e.mntfng tc r . a 8 s t l f y t.;!~,,i~ s i t u a t i ~ n by keeping I , ~ t e r e s t e d member :zgcru...-ha ka,fo@rn?d of ea.ch o t h e r s
work ?or t'.aa:r mutual bl;nzfit,
Casaa have be9n 'hsought t.o ths a t t e n t i o n . of t h e D i v i s i o n i n which cyaep m y kave been a fastor c o n t r i b u t i n g t o abncarm..al C.s?fls'ztl.on and c r a c k i n g of canzkete s t r u c t u r a l membe~s, l.rr most Ln2.6s,n@es fn t h e pLwsence 31% o t h e r c a u s e s ,
such a s d s y i n g a h r i n k a g e , tempesat,urz changes, o r improper r e i n % o s c e m e ~ . t , S i n c e t h e l a s t - m e n t i o n e d f a c t o r s a r e a l r e a d y being skud2ed by t h e D%visio.n, hZ has beeri thought f o r some time thst an 1 : n v e s t i g a t i o n of e r e e p should be undertaken as
w e l l , i n v o l v i n g a t Least a rev%..=w of data e x i s t i n g i n c o n c r e t e l i t e r a t u s a anc? p a s s i b l y aome 3,aP=a::jlnata~y r e s e a ~ z h , The o b j e c t of t h i s n o t e is a s u r v e y c n h e p r c b l m . p ~ e a e d i n g t h e commence- ment of s u z h a program of s t u d y .
The s t u d y of 8csseg :cf e o n c r s t e f a l l s i 3 t o two main aspectrs: the phyaxcal-s!~ie.r,l c . a Z e x p l . m a t 2 o n of c r e e p ( r e f e r r e d t o a s t h , ? o r l % a of creep:; ac..d t h e ol.~.t.wa~d, q u - a c t i t a t i v e
e f F e z t a r,? c r a s p r e l a t e d t o i t s c o n t ~ . l b i ~ t . % n g c a u s e s ( o f p a s t i c . ~ . I . a ~ i , n t , ~ ~ e s t t o t h e d e s i g n e r ] , Both a a p e s t a w i l l be d e a l t w i t h 5r.i.ittHy LE?. tBais n o t e , Following t ' h i s , a summary
of i n f o r m a t l o n o b t a i n e d by ~ o ~ ~ e u y o n d e n z e w i t h v a r i o u s agen- c i e s will be p r e s e n t e d , and f.l;.maLLy a proposed program f o r t h e s t u d y o f c r e e p c? coni,~aete at t h e D i v F s k o n of B u i l d i n g
A REVIEW OF SOME ASPECTS OF C m E P
From t h r e e b i b l i o g r a p h i e s a t hand, an index of 125 r e f e r e n c e s d e a l i n g w i t h c r e e p i n c o n c r e t e has been r e a d i l y assembled. It i s expected t h a t a more thorough s e a r c h w i l l r e v e a l many more. An examinat i o n of even a few of t h e p a p e r s shows t h a t a l a r g e number of i n f l u e n c i n g f a c t o r s a r e involved i n t h e time-dependent deformation which c o n c r e t e undergoes w i t h l o a d and t h e f o l l o w i n g n o t e s from two l i t e r a t u r e reviews
i n d i c a t e t h e scope of p r e v i o u s i n v e s t i g a t i o n s :
I. "Theories of Creep i n Concrete" by A . M . N e v i l l e Canterbury U n i v e r s i t y College, C h r i s t c h u r c h , N z . , J o u r n a l , American Concrete I n s t i t u t e , Vol. 2'(, No. 1, S e p t . 1955, p.47-60, T h i s r e c e n t review of t h e t h e o r i e s of c r e e p i n c o n c r e t e o u t l i n e s v a r i o u s hypotheses advanced by d i f f e r e n t a u t h o r s t o e x p l a i n c r e e p . The a u t h o r c l a s s i f i e s them as f o l l o w s : Mechanical deformation t h e o r y .
-
I n t h i s t h e o r y c r e e p i s a t t r i b u t e d t o a deformation of t h e c a p i l l a r i e s w i t h i n acement p a s t e under l o a d a g a i n s t t h e p r e s s u r e of w a t e r and
a i r phases w i t h i n i t . When t h e load i s removed t h e a i r
and w a t e r p r e s s u r e s t e n d t o r e s t o r e t h e c a p i l l a r i e s t o t h e i r o r i g i n a l shape. Upon a p p l i c a t i o n o r removal of l o a d t h e r e i s a "tendency of t h e whole system towards maximum s t a b i l i t y " . The t h e o r y i s a t t r i b u t e d t o F r e y s s i n e t
.
P l a s t i c t h e o r i e s . - Vogt, Binghm and R e i n e r , G l a n v i l l e ,T r o b s t and o t h e r s have e n t e r t a i n e d t h e i d e a t h a t c r e e p i s due t o s l i p p a g e a l o n g p l a n e s i n t h e c r y s t a l l a t t i c e i n a manner similar t o t h e deformation of m e t a l s . The t h e o r y i s n o t upheld by c o n s i d e r a t i o n s of c o n s t a n t volume s i n c e c r e e p i n c o n c r e t e i s accompanied by a volume r e d u c t i o n .
Viscous t h e o r i e s ,
-
Thomas, A r n s t e i n and Reiner and o t h e r s h o l d t h e t h e o r y t h a t t h e c e m e n t i t l o u s m a t e r i a l i n c o n c r e t e behaves s i m i l a r l y t o a v i s c o u s l i q u i d when it i s loaded and t h a t t h e a g g r e g a t e i s a n i n e r t suspension which does n o t flow under l o a d . There i s a g r a d u a l t r a n s f e r of t h e l o a d from t h e cement t o t h e a g g r e g a t e which a c c o u n t s f o r t h e observedg r a d u a l r e d u c t i o n i n c r e e p , N e v i l l e t h i n k s t h a t t h e a g g r e g a t e s a l s o c o n t r i b u t e t o c r e e p due t o i n c r e a s e d e l a s t i c deformation caused by t h e s h i f t i n g of l o a d and i n some c a s e s t h e r e may be flow i n t h e a g g r e g a t e . The v i s c o u s t h e o r y does n o t e x p l a i n t h e r e d u c t i o n of volume of c o n c r e t e under l o a d .
Seepage t h e o r y .
-
A t h e o r y c r e d i t e d p r i n c i p a l l y t o Lynamc o n s i d e r s t h a t w a t e r i s contained i n c o n c r e t e i n t h r e e forms: chemically combined water; c o l l o i d a l o r absorbed water; and f r e e w a t e r mechanically h e l d w i t h i n t h e c a p i l l a r i e s of t h e g e l . Both shrinkage and c r e e p a r e considered t o r e s u l t from
" t h e l o s s of c o l l o i d a l w a t e r , though i n one case w a t e r may be e x p e l l e d by p r e s s u r e and i n t h e o t h e r drawn out by evapora- t i o n . They a r e , t h e r e f o r e , i n t e r - r e l a t e d phenomena.
Matswnoto found no moisture movements i n a g g r e g a t e s , but Jones observed such movements i n some a g g r e g a t e s .
The f o l l o w i n g e x p l a n a t i o n of t h e c r e e p p r o c e s s a t t r i b u t e d by N e v i l l e t o Ross and Seed i s i n t e r e s t i n g , e s p e c i a l l y i n view of c o n s i d e r a t i o n s of t e s t c o n d i t i o n s .
"The vapour p r e s s u r e of t h e l i q u i d i n a g e l i n c r e a s e s immediately upon a p p l i c a t i o n of a l o a d . To r e s t o r e
e q u i l i b r i u m w i t h t h e e x t e r n a l c o n d i t i o n s , moisture i s e x p e l l e d from t h e g e l . Now i n a system composed of an e l a s t i c s o l i d and a v i s c o u s f l u i d and s u b j e c t e d t o p r e s s u r e , t h e f l u i d w i l l tend t o squeeze o u t , throwing a p r o g r e s s i v e l y i n c r e a s i n g s t r e s s on t h e s o l i d . The p r e s s u r e on t h e f l u i d w i l l correspondingly d e c r e a s e , and t h i s w i l l r e s u l t i n a r e d u c t i o n of t h e r a t e of e x p u l s i o n of t h e f l u i d . " " I n a d d i t i o n t o being reduced by t h e g r a d u a l t r a n s f e r of load t o t h e s o l i d framework, t h e r a t e of e x p u l s i o n of w a t e r i s a l s o a f f e c t e d by t h e r a t e a t which t h e e x p e l l e d w a t e r can seep away. T h i s depends on t h e moisture
g r a d i e n t between t h e g e l and t h e surrounding medium, and on t h e q u a n t i t y of moisture In t h e g e l . It i s apparent t h a t c r e e p and shrinkage w i l l be a f f e c t e d by any f a c t o r a f f e c t i n g t h e s e c o n d i t i o n s , i . e . t h e y w i l l be a f u n c t i o n of many v a r i a b l e s , such a s water-cement r a t i o , type and f i n e n e s s of cement, mix p r o p o r t i o n s , temperature, age of c o n c r e t e , compaction, c u r i n g c o n d i t i o n s , s i z e of specimen and presence of admixtures".
E l a s t i c a f t e r - e f f e c t t h e o r i e s .
-
Maney h e l d t h a t t r u e c r e e p i s p r a c t i c a l l y n o n - e x i s t e n t and t h a t t h e e f f e c t of l o a d i n g i s simply a n e l a s t i c change due t o non-uniform shrinkage of t h e c o n c r e t e .Picket considered t h e r a t e of d r y i n g important from t h e p o i n t of view of r e d i s t r i b u t i o n of s t r e s s e s but consid- e r e d t h a t t h e ef'rects were i n a d d i t i o n t o c r e e p ,
Lea and Lee p o i n t out t h e d e s i r a b i l i t y of c a r r y i n g out c r e e p t e s t s on specimens t h a t have reached moisture e q u i l i b r i u m w i t h t h e surrounding a i r .
N e v i l l e ' s c o n c l u s i o n s .
-
The a u t h o r s t a t e s t h a t t h e r e i s "no c o n c l u s i v e e x p e r i m e n t a l evidence a s t o t h e e x a c t mechanism of c r e e p " and s u g g e s t s t h a t it i s due t o more t h a n one c a u s e . He advances t h e f o l l o w i n g h y p o t h e s i s t o r e c o n c i l e t h e seepage and t h e v i s c o u s t h e o r i e s :"What i s normally measured as c r e e p c o n s i s t s of ! t r u e t c r e e p ( v i s c o u s i n n a t u r e , w i t h a g r a d u a l t r a n s f e r of l o a d t o t h e a g g r e g a t e ) and i n c r e a s e d s h r i n k a g e (seepage due t o e v a p o r a t i o n and e x t e r n a l f o r c e )
.
The magnitude of c r e e p depends on t h e f o r c e a p p l i e d and t h e p r o p e r - t i e s of c o n c r e t e , but i s independent of f a c t o r s such as c u r i n g c o n d i t i o n s , humidity, t e m p e r a t u r e , e t c . The v a r i a t i o n i n t h e s e a f f e c t s s h r i n k a g e o n l y " .11. "Review of L i t e r a t u r e on Creep i n Concrete". U . S . Army Corps of lingineers, Waterways Experiment S t a t i o n ,
Oicksburg, Miss. Misc
.
Y a p e m ~ - - 6 - 1 3 2 , June1955,
24p. T h i s l i t e r a t u r e review p r e p a r e d by t h e U . S . Army Corps of Engineers a p p e a r s t o be concerned more w i t h t h e o u t - ward m a n i f e s t a t i o n s of c r e e p r a t h e r t h a n t h e t h e o r e t i c a lc o n s i d e r a t i o n of why c o n c r e t e c r e e p s . The a t t e m p t s of v a r i o u s i n v e s t i g a t o r s t o f i t mathematical c u r v e s t o observable d a t a a r e p r e s e n t e d
.
The f o l l o w i n g c o n c l u s i o n s , quoted from Davis and Davis, a r e t y p i c a l of t h e t y p e of o b s e r v a t i o n c o n s i d e r e d s i g n i f i c a n t :
"From t h e s e t e s t s , it h a s been determined among o t h e r t h i n g s t h a t t h e flow of p l a i n c o n c r e t e under a c o n s t a n t l y s u s t a i n e d compressive s t r e s s (1) c o n t i n u e s t o i n c r e a s e , a t f i r s t r a p i d l y and t h e n more slowly, f o r a long t i m e , c e r t a i n l y f o r s e v e r a l y e a r 3 ( 2 ) i s g r e a t e r f o r c o n c r e t e i n a n atmosphere of low humidity t h a n f o r an i d e n t i c a l c o n c r e t e i n a n atmos h e r e of h i g h humidity o r when f u l l y s a t u r a t e d w i t h w a t e r
(37
i s m a t e r i a l l y a f f e c t e d by t h e c h a r a c t e r of t h e a g g r e g a t e being s u b s t a n t i a l l y l a r g e r f o r sandstone c o n c r e t e t h a n f o r l i m e s t o n e c o n c r e t e( 4 )
i s l e s s f o r r i c h mixes t h a n f o r l e a n ones( 5 )
i s l e s s t h a n f o r c o n c r e t e having a h i g h modulus of e l a s t i c i t y t h a n f o r one having a low modulus of e l a s t i c i t y ( 6 ) i s r e a t e r f o r a h i g h water-cement r a t i o t h a n f o r a low one and (77 i s g r e a t e r when l o a d i s a p p l i e d a t an e a r l y age t h a n when it i s a p p l i e d a t a l a t e r a g e " . A l t o g e t h e r s i x t y - f o u r c o n c l u s i o n s were p r e s e n t e d by t h e s e a u t h o r s .The remainder of t h e review i s concerned p r i n c i p a l l y w i t h a d i s c u s s i o n of s e v e r a l c o n t r o v e r s i a l a s p e c t s of c r e e p which a r e t o o involved t o t r e a t h e r e . Some of t h e s e a r e : t h e e f f e c t of P o i s s o n ' s r a t i o , t h e degree of a s s o c i a t i o n
between s h r i n k a g e and c r e e p , t h e e f f e c t of a v a r i a b l e modulus of e l a s t i c i t y , and t h e change i n c r e e p r a t e w i t h a change i n u n i t s t r e s s .
RESEARCH AT OTHER LABORATORIES
Before d e c i d i n g t o embark on any long-term program of t e s t i n g and r e s e a r c h , a survey w a s made by correspondence w i t h a number of o t h e r r e s e a r c h o r g a n i z a t i o n s w i t h a t h r e e f o l d
purpose :
1. To determine t h e s o u r c e s of r e l i a b l e and u p - t o - d a t e i n f o r m a t i o n on creep;
2 . To f i n d out what t y p e of r e s e a r c h i n t o c r e e p i s
being c a r r i e d out elsewhere and, i n t h e l i g h t of t h i s , i n what d i r e c t i o n t h e e f f o r t s of t h e D i v i s i o n might be most u s e f u l l y employed; and
3 .
To o b t a i n d e t a i l s of t h e t y p e s of t e s t a p p a r a t u s used by o t h e r s and t o p r o f i t from t h e i r e x p e r i e n c e ,A summary of t h e r e s u l t s of t h e s u r v e y a r e g i v e n i n Table I .
S e v e r a l o t h e r a g e n c i e s on t h i s c o n t i n e n t and i n o t h e r p a r t s of t h e world a r e a c t i v e l y engaged i n s t u d i e s of c r e e p but were not covered i n t h i s s u r v e y . The American Concrete I n s t i t u t e Committee 209, Volume Change and P l a s t i c Flow i n Concrete, i s engaged i n t h e c o r r e l a t i o n of a l l a v a i l a b l e d a t a on c o n c r e t e c r e e p and i t s r e d u c t i o n t o a form i n which it can be used by d e s i g n e r s . Contact w i t h t h i s committee h a s been e s t a b l i s h e d .
Types of Creep T e s t s
Creep t e s t s have been made e i t h e r by t e s t i n g small
beams i n bending o r by t e n s i o n o r compression t e s t s on specimens of a size irathe o r d e r of t h e o r d i n a r y
6-
by 12-inch c o n c r e t ec y l i n a e r . The former t y p e y i e l d s u s e f u l i n f o r m a t i o n f o r t h e p a r t i c u l a r c a s e of f l e x u r e but t h e l a t t e r i s of more g e n e r a l a p p l i c a t i o n . I n e i t h e r c a s e , f o r a n e x t e n s i v e program of r e s e a r c h , a l a r g e number i f d u p l i c a t e a p p a r a t u s a r e r e q u i r e d capable of m a i n t a i n i n g l o a d s as h i g h a s 60,000 pounds o r more on t h e specimens f o r s e v e r a l y e a r s . I n most c a s e s t h i s i s
accomplished by a p p l y i n g t h e l o a d through v e r y heavy c a l i b r a t e d s p r i n g s o r by some h y d r a u l i c method.
F a c t o r s Involved i n Creep
It i s n o t t h e i n t e n t i o n i n t h i s r e p o r t t o d e a l i n
d e t a i l w i t h t h e p h y s i c a l - c h e m i c a l f a c t o r s involved i n c o n c r e t e c r e e p but it i s i n t e r e s t i n g t o n o t e some of t h e v a r i a b l e s t h a t
are mentioned i n t h e correspondence as being s i g n i f i c a n t , These
are: t h e age of t h e c o n c r e t e a t t h e time of l o a d , t h e magnitude of t h e l o a d , t h e d u r a t i o n of t h e l o a d , r e p e t i t i o n of l o a d i n g , p h y s i c a l p r o p e r t i e s of t h e a g g r e g a t e , g r a d a t i o n of t h e a g g r e g a t e , f i n e n e s s modulus, cement f a c t o r , water-cement r a t i o , w o r k a b i l i t y , c u r i n g and m o i s t u r e c o n d i t i o n s d u r i n g t e s t ,
TABLE I
S m m r y of Infornation Obtained from a Correspondence Survey of Various
Research O r g a n i z a t i o ~ Engaged i n Creep Tests on Concrete Name of Organization Corps of Engineers U.S. A m y , l ~ ~ a t e r w a p ExperLment S t a t i o n , Vicksburg, Miss. Portland Cemant Association, Chicago, Ill. l bOf Concrete Tested Ordinary concrete with and without a i r entertairnnent
Lightweight concrete with sand and g r a v e l comparison specimens Dept. of S c i e n t i f i c M i n a r y
I
and I n d u s t r i a l Research Bldg. Research Stn. ,Garston, Size of Specimen Cylinders 6" d i a . by 16" long, h e m t i c a l l y sealed ln Neoprene jackets. A c o n t r o l s p e c h e n made f o r every cylinder t e s t e d Cylinders, 6" d i e . by 12" long. Cap@ with a carping com~ound Watford, Herts, England U s t e r i a l s Division Apparatus f o r Applying Load Special loading frames, Load applied t o c y l i n d e r s i n p a i r s by means of hydraulic r a m (Fig. 1 )Load frames incor- porating heavy springs. 3 c y l i n d e r s i n each frame. Plane s t e e l p l a t e s between c y l i n d e r s (Fig. 2 ) Cylinders C y l i n d e r s 3 , 4 a n d 6" diameters. Length not g r e a t e r Method of S t r a i n Measurement Carlson s t r a i n m t e r s embedded a x i a l l y ihitmore gauges on 3 Rauge l i n e s e q u a l l y spaced about t h e cylinder circunr ference 2 r i v e r pravels Loading frames f o r s i n g l e cyl. Loads maintained by springs i n tension (Fig. 3 ) Simi1artoP.C.A. a ~ p r a t u s Other Features Program j u s t commencing. Cylinders stored and t e s t e d a t 73.Li2O~. Cy1in;lers t e s t e d a t s t r e s s e s of 1C0, 320, rCQ and 640 p.8.i.
Two concrete s t r e n g t h ran-s t e s t e d : 3-450C p.s.i. range t e s t e d a t s t r e s s of bCO p.s.i. 7-9000 r.s.1. range t e s t e d a t s t r e s s of 2000 p.s.i. Lab. temp. and humidit controlled a t 7 3 f 2 . ~ . and &2% RH. Loads a r p l i e d a f t e r 7 d a p fog curing o r a t 78 days following a 21 day dryin& period Roller-mirror exten- someter. Recommend use of v i b r a t i n g wire gauges f o r f u t u r e t e s t s Whitmore gauges
U.S. N a v a l C i b i l Eng. 1 limestone aggregate
Research and Evalua- 1 lightweight expanded
Frames f o r loading 3 beams a t once a t 3 l e v e l s of s t r e s s (Fig. 4 ) Loading f r a w f o r two cylinders. Load a r p l i e d through a hydraulic system (Fig. 5 ) Maximum s p r i n g c a p a c i t y 1 0 tons. Reconanend c o n t r o l of laboratory temperature and RH. 5000, 7000 and 10,000 p.s.1. concrete t e s t e d a t s t r e s s e s of E l e c t r i c a l r e s i s t a n c e s t r a i n gauges ( ? ) Capacitance gauges 0.3, 0.5 and C.7 f ' c respectively. Storaee conditions, 20, 50 and lOOSRH. and 73.4 F.
Not a t present doing any creep t e s t s
Long term f l e x u r a l deformations studied only
hork c l 0 3 e 4 c o r r e l a t e d with t h e a c t i v i t i e s of two o t h e r l a b o r a t o r i e s i n Germany. A l l
f a c t o r s c l o s e l y controlled, even t o t h e use of t h e same
cement and aggregate a t t h e d i f f e r e n t l a b o r e t o r i e s than 4 x d i e . S m l l concrete beam Cylinders, 6" d i a . by 30" long (approx. )
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t i o n Laboratory Con- shale. About 2CO s t r u c t i o n Battalion s ~ e c i m e m Center, Port Nuenem,
C a l i f o r n i a U.S. Dept. of Comnerce, National Bureau of Standards, 'dashington, D.C. Conunommalth Scien- t i f i c and I n d u s t r i a l Research Organitation, Division of Building Research, Victoria, A u s t r a l i a Technical University, Munich. Gemrry Ordinary Various
PROPOSED PROGRAM OF CREEP STUDY AT THE DIVISION OF BUILDING RESEARCH The u l t i m a t e purpose of i n v e s t i g a t i o n s i n t o t h e c r e e p of con- c r e t e under l o a d i s t o produce d a t a of a type which d e s i g n e r s can use t o t a k e c r e e p i n t o account i n t h e i r d e s i g n s , T h i s type of i n f o r m a t i o n
would c o n s i s t of t a b l e s , c u r v e s , o r formulae t h a t could be a p p l i e d i n a p r a c t i c a l way without r e f e r e n c e t o t h e o r e t i c a l c o n s i d e r a t i o n s of why c o n c r e t e undergoes a time-dependent deformation under l o a d .
I n s e t t i n g up experimental work t o d e r i v e such i n f o ~ m a t i o n , however, t h e o r e t i c a l c o n s i d e r a t i o n s can s c a r c e l y be avoided i f t h e
e x p e r i m e n t a l c o n d i t i o n s a r e t o be p r o p e r l y chosen. For t h i s r e a s o n i t i s proposed t o begin t h e s t u d y of c r e e p a t t h e D i v i s i o n of B u i l d i n g Research w i t h a review of t h e most important l i t e r a t u r e on t h e s u b j e c t , This w i l l
provide an i n t e r p r e t a t i o n of p u b l i s h e d c r e e p d a t a w i t h i t s i m p l i c a t i o n s i n d e s i g n and a l s o form a background f o r e x p e r i m e n t a l work which may be undertaken.
Creep t e s t s a r e i n h e r e n t l y of l o n g d u r a t i o n and f o r t h i s s e a s o n a l o n e no one l a b o r a t o r y i s prepared t o make a n e x h a u s t i v e study of t h e s u b j e c t . T e n t a t i v e l y t h e D i v i s i o n i s contemplating a program of c r e e p t e s t s on l i g h t w e i g h t c o n c r e t e s i n c e t h e work would be complementary t o s t u d i e s of s t r e n g t h , d u r a b i l i t y , and dimensional changes t h a t a r e
a l r e a d y underway h e r e . A d e c i s i o n and f i n a l d e t a i l s a r e s e s e r v e d u n t i l completion of t h e l i t e r a t u r e review, but t h e t e s t s would be c a r r i e d out under s e l e c t e d c o n d i t i o n s , depending on a v a i l a b l e f a c i l i t i e s and b e a r i n g
in mind work being done e l s e w h e r e , For i n s t a n c e , i f some o t h e r agency
i s c a r r y i n g out an i n v e s t i g a t i o n of s i m i l a r m a t e r i a l s under r e l a t i v e humidity c o n d i t i o n s of 50 and 100 p e r c e n t t h e n t e s t s might be conducted a t t h e D i v i s i o n under 20 and
75
p e r c e n t r e l a t i v e humidity t o f i l l t h e gaps i n t h e r a n g e . I n a d d i t i o n , i t may be d e s i r a b l e t o do some t e s t i n g of l o c a l m a t e r i a l s on a more c a s u a l b a s i s ,It i s planned t o begln immediately t o d e s i g n , c o n s t r u c t , and t e s t one o r more p r o t o t y p e t e s t a p p a r a t u s , Most l i k e l y i t w i l l t a k e a
form similar t o one of t h o s e shown i n F i g s , 1, 2 ,
3 ,
o r 5 w i t h t h e l o a d s maintained by s p r i n g s o r some h y d r a u l i c means, Any f e a t u r e s -that a r ezonsidesed improvements on t h e a p p a r a t u s used by o t h e r s w i l l be incop-
p o r a t e d , I n t h e event t h a t a f a i r l y l a r g e program
of
t e s t s i s decided upon t h e n a l a r g e number of s i m i l a r a p p a r a t u s w i l l be r e q u i r e d , I n t h i s r e s p e c t t h e e x t e n t of t h e experimental work may be r e s t r i c t e d byt k s a v a i l a b l e space having t h e d e s i r e d c o n t r o l o v e r t h e temperature and humidity t e s t c o n d i t i o n s .
Some thought and e x p e r i m e n t a t i o n w i l l a l s o have t o be devoted t o t h e s e l e c t i o n of s u i t a b l e s t r a i n measuring d e v i c e s . The correspon- dence i n d i c a t e s t h a t a number of d i f f e r e n t t y p e s a r e being used but t h e ~ e i s no i n d i c a t i o n of t h e i r r e l a t i v e m e r i t s .
T I E R O D S 7
S T E E L END CYLINDf R R A M CYLINDER P L A T E S
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F I G U R E I A P P A R A T U S FOR A P P L Y I N G L O A D
U.S. ARMY CORPS O F ENGINEERS.
3 OR 4 SPRINGS 7 LOAD A P P L I E D BY H Y D R A U L I C R A M & MAINTAINED BY y , , , . r r .
-
CYLINDER CYLINDER C Y L I N D E R R A M I I I I L .-
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I I I 1 / . - - _ - -F I G U R E 2 A P P A R A T U S FOR A P P L Y I N G L O A D P O R T L A N D C E M E N T A S S O C I A T I O N . S T E E L END E E L B A L L T E N S I O N R I N G S 4 S P R I N G S F I G U R E3
A P P A R A T U S FOR A P P L Y I N G LOAD B R I T I S H BUILDING R E S E A R C H S T A T I O N .?k
A L L FIGURES SCHEMATIC ONLY.F R A M E
L
F I G U R E4
f B E A M 1 bl W +
BEAM P I V O T ----r,-
D E A D A P P A R A T U SFOR
A P P L Y I N G L O A D C.S.I.R.O. D I V I S I O N O F BUILDING RESEARCH,
A U S T R A L I A .END BLOCKS MADE OF 2 C O N C R E T E T E S T SHORT I BEAM S E C T I O N S CYLINDERS
