Comparison of in-situ strengths and quality tests of concrete

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Comparison of in-situ strengths and quality tests of concrete

Lewandowski, R.; National Research Council of Canada. Division of Building

Research

CANADA INSTITUTE

FOR SClElVTlFlC AND TECHNICAL

INFORMATION

I

NSTITUT CANADl El\

DE L'INFORMATION SClENTlFlQUE

ET TECHlVlQUE

N R C / C N R TT

-

1857 TECHNICAL TRANSLATION TRADUCTION TECHNIQUE R. LEWANDOWSKI

COMPARlSON OF IN-SITU STRENGTHS

AND QUALITY

TESTS

( OF CONCRETE )

BETONSTEIN-ZEITUNG, 3 7 (8): 4 7 7

-

481, 1971

T R A N S L A T E D BY

/

T R A D U C T I O N DE

D.A. SlNCLAlR

THIS IS THE TWO HUNDRED A N 3 TWENTY-SECOND I N THE SERIES OF TRANSLATIONS PREPARED FOR THE DIVISION OF BUILDING RESEARCH

TRADUCTION NUMERO 222 DE LA s ~ R I E P R ~ P A R ~ E POUR LA DIVISION DES RECHERCHES EN B ~ T I M E N T

OTTAWA

National Research

Conseil national

PREFACE I n s t u d i e s o f t h e s a f e t y of r e i n f o r c e d c o n c r e t e s t r u c t u r e s t h r e e a s p e c t s of c o n c r e t e s t r e n g t h a r e r e l e v a n t : t h e s t r e n g t h of c o n c r e t e s p e c i f i e d , t h e s t r e n g t h and v a r i - a b i l i t y of c o n c r e t e s u p p l i e d , and t h e s t r e n g t h and v a r i - a b i l i t y o f t h e c o n c r e t e i n t h e a c t u a l hardened s t r u c t u r e . Information on t h e l a s t item i s meagre, b u t i s needed

p a r t i c u l a r l y f o r r e s e a r c h s t u d i e s . T h i s p a p e r i s a v a l u a b l e c o n t r i b u t i o n toward f i l l i n g t h i s need. The D i v i s i o n h e r e r e c o r d s i t s t h a n k s t o D . A . S i n c l a i r f o r t r a n s l a t i n g t h i s p a p e r , and t o W . G . Plewes of t h i s D i v i s i o n , who checked t h e t r a n s l a t i o n f o r t e c h n i c a l a c c u r a c y . Ottawa A p r i l 1976 C . B . Crawford D i r e c t o r D i v i s i o n o f B u i l d i n g Research

NATIONAL RESEARCH COUNCIL OF CANADA

CONSEIL NATIONAL DE RECHERCHES DU CANADA

TECHNICAL TRANSLATION

1857

TRADUCTION TECHNIQUE

Comparison of in-situ strengths and quality tests (of

concrete)

(Vergleich von Bauwerkfest

igkeiten und ~ u t e ~ r u f u n ~ e n )

Author/Auteur:

K .

Lewandowski

Reference/Rgfgrence:

Betonstein-Zeitung, 37 (8):

477

-

481, 1971

Translator/Traducteur: D. A. Sinclair

Canada Institute for

Institut canadien de

Scientific and Technical

Ifinformation

scientifique et

Informat

ion

technique

Ottawa, Canada

KIA OS2

COMPARISON OF IN-SITU STRENGTHS AND QUALITY TESTS

(OF CONCRETE)

1. I n t r o d u c t i o n

DIN 1 0 4 5 ' ~ ) and DIN 1 0 4 8 ' ~ ) c o n s t i t u t e t h e b a s i s f o r a p p r a i s i n g c o n c r e t e q u a l i t y . Accordingly, t h e s t r e n g t h of 28-day-old t e s t cubes of t h e same m a t e r i a l a s t h e f i n i s h e d c o n c r e t e work s e r v e s a s a y a r d s t i c k . Many b u i l d i n g c o n t r a c t o r s a r g u e t h a t when t h e cubes a t t a i n t h e r e q u i r e d c o n c r e t e s t r e n g t h t h e d e s i r e d performance i s a s s u r e d , provided no v i s i b l e d e f e c t s appear i n t h e c o n s t r u c t i o n .

Of c o n s i d e r a b l y g r e a t e r i n t e r e s t t o t h e e n g i n e e r t h a n t h e s t r e n g t h of t h e t e s t cube a t t h e d e s i g n i n g and e s t i m a t i n g s t a g e , might b e t o know what i n - s i t u s t r e n g t h s a r e normally a v a i l a b l e o r can b e expected from c o n c r e t e , s t e e l c o n c r e t e and

p r e - s t r e s s e d c o n c r e t e s t r u c t u r e s , r e s p e c t i v e l y . However, l i t t l e i s known about t h i s m a t t e r and, moreover, views d i f f e r a s t o how s t r e n g t h v a l u e s a s c e r t a i n e d i n s i t u a r e t o b e a p p r a i s e d (4

,

5)

The q u e s t i o n of whether t h e compressive s t r e n g t h of a specimen produced and s t o r e d i n compliance w i t h t h e s t a n d a r d s r e p r e s e n t s t h e s t r e n g t h of a s t r u c t u r a l p a r t c o n s t r u c t e d from t h e same c o n c r e t e , and i f s o t o what e x t e n t , i s almost a s o l d a s c o n c r e t e and s t e e l - r e i n f o r c e d c o n c r e t e c o n s t r u c t i o n i t s e l f ( 6 ) . Many

l a b o r a t o r y t e s t s of t h e compressive s t r e n g t h and s t r a i n behaviour of c o n c r e t e have indeed been c a r r i e d o u t , a s w e l l a s t e s t s on r e i n f o r c e d and u n r e i n f o r c e d components produced from them, and on t h e b a s i s o f t h e knowledge t h u s o b t a i n e d

methods of c a l c u l a t i o n and d e s i g n s p e c i f i c a t i o n s have been developed. N e v e r t h e l e s s , o u r knowledge o f t h e a c t u a l c o n d i t i o n s p r e s e n t i n f i n i s h e d c o n s t r u c t i o n work i s s t i l l v e r y l i m i t e d . A l l we can s a y i s t h a t under o r d i n a r y c o n d i t i o n s , i . e . , i f a s t r u c t u r e has been e r e c t e d i n c o n f o r m i t y w i t h o b l i g a t o r y t e c h n i c a l r e q u i r e m e n t s , t h e margins of s a f e t y i n h e r e n t i n t h e d e s i g n s p e c i f i c a t i o n s w i l l p r o v i d e s u f f i c i e n t p r o t e c t i o n a g a i n s t p r e s e n t i n a c c u r a c i e s .

l l i t h e r t o o n l y one c o n t r i b u t i o n h a s appeared i n which cube s t r e n g t h s a r e compared with i n - s i t u s t r e n g t h s . Moreover, t h i s can s c a r c e l y b e r e g a r d e d a s r e p r e s e n t a t i v e of c o n c r e t e c o n s t r u c t i o n i n g e n e r a l , s i n c e it d e a l s w i t h a l a r g e - s c a l e mass c o n c r c t e p r o j e c t i n s t a l l e d a s a p r o t o t y p e ( 7 y 8 ) . Otherwise, a l l we have a r e r e s u l t s on b u i l d i n g s where damage i s s u s p e c t e d ( 9 ) , and t h e r e s u l t s o f

l a b o r a t o r y t e s t s i n which a t t e m p t s have been made t o s i m u l a t e i n - s i t u c o n d i t i o n s (10,11,12)

Thus, f i n a l c o n c l u s i o n s on t h e s u b j e c t o f s t r u c t u r a l s t r e n g t h appear p r e - m a t u r e , and I t . . . complete c l a r i f i c a t i o n of t h i s problem w i l l have t o a w a i t t h e c o n c l u s i o n of many, p o s s i b l y l e s s i m p r e s s i v e , b u t undoubtedly e s s e n t i a l

i n v e s t i g a t i o n s " (13)

2 . T e s t Procedure and R e s u l t s

By s t u d y i n g t y p i c a l s t e e l - r e i n f o r c e d and p r e - s t r e s s e d c o n c r e t e i t was hoped t o g a i n a more p r e c i s e d e f i n i t i o n of t h e c o n d i t i o n s p r e v a i l i n g on normal c o n s t r u c t i o n s i t e s . The t e s t s were c a r r i e d o u t by t h e a u t h o r a s p a r t of a more e x t e n s i v e

s t u d y ' I 4 ) i n t h e c o u r s e of h i s a c t i v i t y a t t h e I n s t i t u t e f o r B u i l d i n g M a t e r i a l S c i e n c e and S t e e l Concrete C o n s t r u c t i o n of t h e Technische ~ n i v e r s i t a t Braunschweig. The chosen method of i n - s i t u i n v e s t i g a t i o n was i n a l l c a s e s t h e t a k i n g o f d r i l l c o r e samples i n t h e manner shown i n F i g u r e 1 . The advantage of t h i s " d e s t r u c t i v e " method, compared w i t h a l l n o n d e s t r u c t i v e o n e s , e . g . , Reference 15, i s t h a t it p r o v i d e s d i r e c t i n f o r m a t i o n on t h e i n t e r n a l s t r u c t u r e of t h e m a t e r i a l and

immediately r e v e a l s p r o c e s s i n g d e f e c t s ( s e e F i g u r e 2 ) . The compressive s t r e n g t h i s determined d i r e c t l y from c o n c r e t e specimens t a k e n from i n - s i t u s t r u c t u r e s . There i s no need f o r t h e i n t e r v e n t i o n of a more o r l e s s a c c u r a t e c a l i b r a t i o n c u r v e of t h e kind r e q u i r e d i n n o n d e s t r u c t i v e t e s t i n g . The a c c u r a c y of t h e

s t r e n g t h v a l u e s o b t a i n e d from specimens t a k e n " d e s t r u c t i v e l y " i s l i m i t e d o n l y by e r r o r s of t h e same o r d e r of magnitude a s t h o s e which r e g u l a r l y occur i n t e s t s o f c o n c r e t e compressive s t r e n g t h .

2 . 1 R e s u l t s of r o u t i n e i n v e s t i g a t i o n s

I t i s t h e t a s k of a l l b u i l d i n g m a t e r i a l t e s t i n g i n s t i t u t i o n s t o u n d e r t a k e r o u t i n e q u a l i t y t e s t s on c o n c r e t e cube specimens on b e h a l f of a p p l i c a n t s from every s e c t o r o f t h e c o n s t r u c t i o n i n d u s t r y . I n a d d i t i o n , i n - s i t u i n v e s t i g a t i o n s o f s t r u c t u r e s a r e c a r r i e d o u t when complaints a r e r e c e i v e d about components o r when no q u a l i t y r e c o r d s have been provided f o r t h e i n s t a l l e d c o n c r e t e . I n s p e c t i o n o f t h e voluminous d a t a which have accumulated from such r o u t i n e t e s t i n g over

C o n s t r u c t i o n of t h e Technische ~ n i v e r s i t a t Braunschweig a l l o w s s e v e r a l c o n c l u s i o n s t o be drawn about q u a l i t y c o n d i t i o n s i n c o n c r e t e c o n s t r u c t i o n .

From t h e r e s u l t s o f t h e q u a l i t y t e s t s a c c o r d i n g t o DIN 1048 we can t a k e s e v e r a l f e a t u r e s which a r e c h a r a c t e r i s t i c o f c e r t a i n g r a d e s of c o n c r e t e and t y p e s o f s t r u c t u r e . The a n a l y s i s o f about 9,600 complete t e s t c e r t i f i c a t e s from t h e y e a r s 1964 t o 1967 r e v e a l e d t h e f o l l o w i n g :

1. Regardless of t h e g r a d e of c o n c r e t e , t h e y e a r of c o n s t r u c t i o n and t h e t y p e

of s t r u c t u r e , t h e d i s t r i b u t i o n of t h e v a l u e s c o r r e s p o n d s , i n good a p p r o x i m a t i o n , t o a s t a n d a r d d i s t r i b u t i o n which can b e c l e a r l y d e s c r i b e d w i t h s u f f i c i e n t

accuracy by a mean v a l u e f o r random samples $w28 (=

3

and a s t a n d a r d d e v i a t i o n

S .

2 . The mean v a l u e s and s c a t t e r do n o t d i f f e r from y e a r t o y e a r ; t h u s , t h e r e i s no evidence of an e f f e c t of t h e s t a t e of t h e c o n s t r u c t i o n market on t h e c a r e of e x e c u t i o n . 3 . The a n t i c i p a t e d v a l u e f o r B 225 i s somewhere i n t h e v i c i n i t y of 350 kp/cm2, f o r B 300, 400 kp/cm2 and f o r B 450, 500 kp/cm2. I n o t h e r words, t h e r a t i o o f cube s t r e n g t h t o t h e o r e t i c a l s t r e n g t h d e c r e a s e s w i t h i n c r e a s i n g c o n c r e t e g r a d e . T h i s i s e v i d e n t l y independent of t h e t y p e of c o n s t r u c t i o n ( F i g u r e 3 ) . 4 . The s c a t t e r d e c r e a s e s somewhat w i t h i n c r e a s i n g c o n c r e t e g r a d e .

5 . The cubes can be expected t o f a l l s h o r t o f t h e t h e o r e t i c a l s t r e n g t h v a l u e s i n about 10 t o 15% o f a l l c a s e s .

During t h e same o b s e r v a t i o n p e r i o d a t o t a l o f about 1,350 c o r e samples were t a k e n from 200 outwardly a c c e p t a b l e s t r u c t u r e s of known t h e o r e t i c a l c o n c r e t e gra.de f o r v e r i f i c a t i o n o f t h e i n - s i t u c o n c r e t e compressive s t r e n g t h . The c o r e t e s t s were i n g e n e r a l m o t i v a t e d by n e g a t i v e c r i t e r i a , e . g . , t h e a b s e n c e of t e s t

c u b e s . A l l t h e specimens t a k e n were more t h a n two months o l d . These b u i l d i n g

s i t e s , which were n o t s a t i s f a c t o r y i n e v e r y r e s p e c t , r e v e a l e d t h e f o l l o w i n g ( F i g u r e 4 ) :

c a s e s , n e v e r t h e l e s s t h e s t r u c t u r a l s t r e n g t h s , on t h e a v e r a g e , were only 70 t o 80% of t h e t h e o r e t i c a l c o n c r e t e g r a d e , a l t h o u g h t h e specimen ages were o v e r f o u r weeks.

2 . I n many c a s e s t h e mean i n - s i t u s t r e n g t h reached o n l y 50% o r l e s s of t h e t h e o r e t i c a l s t r e n g t h .

3 . Both f o r i n d i v i d u a l s t r u c t u r e s and f o r groups r e p r e s e n t i n g d i f f e r e n t t y p e s of s t r u c t u r e s t h e s t a n d a r d d e v i a t i o n s were c o n s i d e r a b l y g r e a t e r t h a n t h o s e o b t a i n e d from t h e t e s t c u b e s , and g r e a t l y exceeded a n y t h i n g t h a t could b e c a l l e d an " a c c e p t a b l e t t s c a t t e r . The l a r g e s t a n d a r d d e v i a t i o n s of r o a d s might have been due t o w e a t h e r i n g .

4 . Although, i n g e n e r a l , l e s s t h a n 50% o f t h e specimens a t t a i n e d t h e t h e o r e t i c a l v a l u e s , t h e c o n d i t i o n s found i n b r i d g e s , roadways and s i n g l e - f a m i l y houses were somewhat b e t t e r . I n t h e c a s e of b r i d g e s t h i s could be a t t r i b u t e d t o

s m a l l e r s c a t t e r s , and i n t h e c a s e o f r o a d s and houses t o l a r g e r e x c e s s s t r e n g t h v a l u e s .

2 . 2 Comparative i n - s i t u t e s t s

To h e l p c l a r i f y t h e q u e s t i o n of t h e e x t e n t t o which t r a d i t i o n a l cube t e s t s a r e a measure o f i n - s i t u s t r e n g t h and t h e r e l a t i o n s h i p of t h e l a t t e r t o t h e

t h e o r e t i c a l s t r e n g t h we i n v e s t i g a t e d a t o t a l o f 24 b u i l d i n g s i t e s and p r e f a b r i c a t i o n p l a n t s . The s t r u c t u r e s and s t r u c t u r a l p a r t s involved met normal s t a n d a r d s and

were not s u b j e c t t o c o m p l a i n t s . The p r o j e c t s i n c l u d e d b r i d g e s a s w e l l a s h i g h - r i s e b u i l d i n g s and t a l l e n g i n e e r i n g s t r u c t u r e s . The b u i l d i n g e n t e r p r i s e s ranged from s m a l l b r i c k y a r d s t o l a r g e c o n s t r u c t i o n f i r m s d e a l i n g predominantly i n p r e - s t r e s s e d c o n c r e t e c o n s t r u c t i o n . The manufacture and s t o r a g e of cube specimens was c a r r i e d o u t i n t h e manner normally a p p l i e d by each e n t e r p r i s e . Core samples were t a k e n a t a g e s of two t o t h r e e weeks by members o f t h e I n s t i t u t e . A l l

specimens were t e s t e d i n t h e I n s t i t u t e a t an age of 28 d a y s .

Comparison of t h e s t a n d a r d d e v i a t i o n s o b t a i n e d a t t h e b u i l d i n g s i t e s w i t h t h e c r i t e r i a l a i d down by ~ i i s c h ( I 7 ) o r Conrad ( I 8 ) f o r cube t e s t s shows ( F i g u r e 5) t h a t i n g e n e r a l t h e s e l e c t i o n of b u i l d i n g s i t e s o r c o n s t r u c t i o n f i r m s was n o t u n t y p i c a l . The t e s t cube s c a t t e r s o b t a i n e d f i t t h e g e n e r a l s t a n d a r d ; s i m i l a r l y , t h e cube r e s u l t s from t h e s e s t r u c t u r e s conform t o t h e v a l u e s from t h e r o u t i n e

i n v e s t i g a t i o n s mentioned above. A s e x p e c t e d , t h e i n - s i t u s t r e n g t h v a l u e s a r e somewhat l e s s f a v o u r a b l e : whereas o n l y 25% of t h e cube v a l u e s f a l l o u t s i d e t h e " a c c e p t a b l e " r a n g e , 33% of t h e d r i l l c o r e v a l u e s do s o .

F i g u r e 6 shows t h e r e l a t i o n s h i p o b t a i n e d between t h e i n - s i t u s t r e n g t h B

Bau

and t h e cube s t r e n g t h

Bw.

L i n e a r i t y t e s t s i n accordance w i t h Reference 16 do

n o t r u l e out a r e c t i l i n e a r r e l a t i o n s h i p between f3 and BBau; t h e c o r r e l a t i o n

W

w i t h r = 0.93 i s v e r y c l o s e . The r e g r e s s i o n c a l c u l a t i o n g i v e s

(]n:ll, = 17 t 0,81

.

p,,

a s a f u n c t i o n a l r e l a t i o n s h i p .

T h i s e x p r e s s i o n can b e approximated by

The graph i n d i c a t e s t h a t t h e approximation i s v e r y good and l i e s w i t h i n t h e c o n f i d e n c e i n t e r v a l determined by t h e r e g r e s s i o n c a l c u l a t i o n . The r e l a t i o n s between t h e t h e o r e t i c a l c o n c r e t e B g r a d e s on t h e one hand and t h e s t r e n g t h s of

t h e cube specimens f3 o r t h e c o r e specimens f3 on t h e o t h e r , a r e shown i n

w Bau

T a b l e I . The lowest f r a c t i l e i s from a b r i d g e c o n s t r u c t i o n s i t e where t h e e x e c u t i o n l e f t much t o b e d e s i r e d and where even t h e s t a n d a r d d e v i a t i o n s of t h e t e s t cubes a c c o r d i n g t o Reference 17 l e a d t o a poor a s s e s s e m e n t .

By assembling t h e d a t a i n l a r g e r groups a c c o r d i n g t o c o n c r e t e g r a d e s , t y p e s o f s t r u c t u r e s and methods of p r o d u c t i o n , a number of common f e a t u r e s and more g e n e r a l l y v a l i d p o i n t s of view were developed:

1. Both f o r cubes and c o r e specimens t h e r a t i o of specimen t o t h e o r e t i c a l

s t r e n g t h d e c r e a s e s w i t h i n c r e a s i n g c o n c r e t e g r a d e ( F i g u r e 7 ) . The d e c r e a s e i s more r a p i d f o r t h e i n - s i t u s t r e n g t h s . T h i s c o r r e s p o n d s t o t h e tendency noted i n t h e r o u t i n e t e s t s a c c o r d i n g t o 2 . 1 .

2 . Within any s t r e n g t h group t h e d i f f e r e n c e s between s t r u c t u r a l p a r t s and t y p e s of c o n s t r u c t i o n produced under d i f f e r e n t manufacturing and p o s t h a r d e n i n g c o n d i t i o n s could b e r e g a r d e d a s s i g n i f i c a n t i n some c a s e s , b u t were n o t v e r y important a s a whole.

3 . Compressive cube s t r e n g t h s averaged about 37% above t h e d e s i r e d compressive

As a r u l e , i n s t r u c t u r e s e r e c t e d with normal c a r e t h e i n - s i t u s t r e n g t h r e a c h e s t h e t h e o r e t i c a l v a l u e . I f t h e e x e c u t i o n i s l e s s c o n s c i e n t i o u s , a s suggested by t h e absence of q u a l i t y c o n t r o l s i n t h e c a s e s d e a l t w i t h i n S e c t i o n 2 . 1 , t h i s r e l a t i o n s h i p a p p e a r s much l e s s f a v o u r a b l e .

4 . The 5% f r a c t i l e s of t h e i n - s i t u s t r e n g t h s a r e , i n t h e normal c a s e , about 20% t o 30% below t h e t h e o r e t i c a l s t r e n g t h . A t l e s s w e l l - r u n b u i l d i n g s i t e s , however, s u b s t a n t i a l l y l e s s f a v o u r a b l e v a l u e s may b e e x p e c t e d .

5 . As f a r a s t h e i n - s i t u s t r e n g t h v a l u e s a r e concerned t h e b r i d g e s t r u c t u r e which was deemed g e n e r a l l y e x c e l l e n t showed no advantage whatever over t h e o t h e r t y p e s o f c o n s t r u c t i o n ( F i g u r e 8 ) .

3 . Conclusions

I f t h e r e s u l t s of t h e i n v e s t i g a t i o n a r e a p p l i e d t o t h e c o n c r e t e q u a l i t y c l a s s i f i c a t i o n o f DIN 1045 E (yellow f o r m ) ( 2 ) , we g e t t h e c o n d i t i o n s p r e s e n t e d i n Table 11. I t i s c l e a r from t h i s t h a t t h e t e s t cubes a s a whole c o n t i n u e t o meet f u t u r e r e q u i r e m e n t s . The s p e c i f i c a t i o n s f o r i n - s i t u s t r e n g t h s ( D I N 1045 E , S e c t i o n 7 . 3 . 7 ) do n o t appear u n r e a s o n a b l e e i t h e r and seem a l t o g e t h e r a t t a i n a b l e . However, i n o r d e r t o avoid diagrammatic c o n s t r u c t i o n s from t h i s s e c t i o n , it

should b e r e f o r m u l a t e d and d i r e c t e d t o t h e d e t e r m i n a t i o n of a f r a c t i l e . Something l i k e t h e f o l l o w i n g f o r m u l a t i o n can a c c o r d i n g l y b e d e r i v e d from t h e t e s t r e s u l t s .

I n t h e i n - s i t u c o n c r e t e s t r e n g t h a n a l y s i s by t h e t e s t i n g of d r i l l c o r e s t h e compressive s t r e n g t h of a specimen a t l e a s t 20 days o l d must n o t , on t h e a v e r a g e , f a l l below t h e v a l u e s o f T a b l e I , column 3 ( 6 ) For c o n c r e t e s of g r a d e s Rn

Wm

450 and Bn 550 i t must r e a c h 0 . 9 t i m e s t h i s v a l u e . Otherwise, a s u f f i c i e n t l y l a r g e number of samples must b e t a k e n s o t h a t t h e 5% f r a c t i l e s of t h e i n - s i t u s t r e n g t h can be d e t e r m i n e d . The 5% f r a c t i l e s of t h e d r i l l c o r e s t r e n g t h s must n o t f a l l below 0.7 t i m e s t h e v a l u e s of T a b l e I , column 3 , and f o r c o n c r e t e of g r a d e s Bn 450 and Bn 550 t h e y must correspond t o 0.65 t i m e s t h i s v a l u e .

Taking t h e age of t h e c o n c r e t e i n t o account by c o n v e r s i o n f a c t o r s , a s

h i t h e r t o provided f o r i n S e c t i o n 7 . 3 . 7 , seems p o i n t l e s s . G e n e r a l l y , t h e c o n d i t i o n s of s e t t i n g of t h e i n - s i t u c o n c r e t e a s w e l l a s t h e c u r v e o f s e t t i n g o f t h e cement employed a r e n o t known i n s u f f i c i e n t d e t a i l , s o t h a t t h e i n t r o d u c t i o n of t i m e

f a c t o r s w i l l c o n s t i t u t e o n l y a vague e s t i m a t e . For most c a s e s it ought t o b e s u f f i c i e n t t o d e m o n s t r a t e from t h e d r i l l c o r e s t r e n g t h v a l u e s whether t h e s t a b i l i t y and u t i l i t y of a b u i l d i n g a r e a s s u r e d a t t h e t i m e of t h e i n s p e c t i o n .

The numerical s t r e n g t h r e q u i r e d f o r proof of s t a b i l i t y i s g i v e n i n D I N

1045 E f o r a l l c o n c r e t e s a s

BR

= 0 . 7

' ~ 2 8

.

I t was found h e r e t h a t t h e r a t i o a _BS = i n - s i t u s t r e n g t h : t h e o r e t i c a l s t r e n g t h , d e c r e a s e s w i t h i n c r e a s i n g c o n c r e t e g r a d e , and t h i s h o l d s f o r t h e mean v a l u e s and f o r t h e f r a c t i l e s . Hence, r e t a i n i n g t h e same f a c t o r o f 0.7

' ~ 2 8 f o r a l l c o n c r e t e g r a d e s a p p e a r s q u e s t i o n a b l e .

T a b l e I11 shows t h e c o n d i t i o n s . I t i s s u g g e s t e d t h a t f o r c o n c r e t e s o f h i g h e r g r a d e , i . e . , Bn 350 and h i g h e r , t h e v a l u e

BR

= 0 . 6

' ~ 2 8 b e chosen s o a s t o

r e t a i n an a d e q u a t e margin o f s a f e t y between t h e numerical s t r e n g t h

BR

and t h e 5% f r a c t i l e of t h e i n - s i t u s t r e n g t h and keep it a p p r o x i m a t e l y e q u a l f o r a l l c o n c r e t e s .

The s t r u c t u r e s and m a n u f a c t u r e r s s t u d i e d i n g e n e r a l met o r d i n a r y s t a n d a r d s N e v e r t h e l e s s we cannot simply assume t h a t t h e s e r e s u l t s a r e r e p r e s e n t a t i v e of a l l c o n c r e t e and s t e e l c o n c r e t e c o n s t r u c t i o n . They may r e f l e c t o n l y p a r t of i t

I t would therefore be d e s i r a b l e t o make s i m i l a r i n v e s t i g a t i o n s i n o t h e r p l a c e s and on o t h e r t y p e s of s t r u c t u r e s s o t h a t t h e g e n e r a l v a l i d i t y o f t h e r e s u l t s p r e s e n t e d h e r e may b e v e r i f i e d a t l e a s t on a spot-check b a s i s .

In c o n c l u s i o n , we wish t o thank t h e d i r e c t o r s and a l l t h e co-workers of t h e I n s t i t u t e f o r M a t e r i a l S c i e n c e and S t e e l C o n c r e t e , Technische ~ n i v e r s i t a t

Braunschweig, who were involved i n t h e e x e c u t i o n and e v a l u a t i o n of t h e i n v e s t i g a t i o n f o r t h e i r v a l u a b l e s u p p o r t .

Summarv

Investigations were carried out on struc- tures and components a t more than 20 construction sites and storage yards of precasting factories meeting normal standard requirements with regard to accuracy in production and placement of concrete. I t resulted thcrrfrom that in most of the structures investigated 1hc structural strongths after 28 days attained on an average the thcorctical concrete quality; fractile values wcrc below by about 20 to 3OD/0. I t has been explained that the specification required in DIN 1045E with regard to structural strengths can, with some accuracy, Ijc maintained without any difficulty, b u t that it should ~uitably be completed by stating a fractile. On the basis

of the test results it was recommended to modify the h l ~ ~ i l ' i c d strc,ngth ol' higher co1lcrc-t~ qualit1c.s stated in I l I N 1045 i?: its a constant value (11: 0.7 . (;\v,,.

References

[I] DIN 1045 - Bauwerke a u s Stahlbeton - Ausgabe No-

vember 1959

[Z] DIN 1045 - Beton- und Stahlbetonbau, Bemessung u n d

Ausfilhrung - Entwurf MBrz 1968

131 DIN I048 - Bestlmmungen fllr Betonprllfungen be1 Aur-

fiihrung von Bauwerken a u s Beton u n d Stahlbeton -

Kreuzausgabe F e b r u a r 1957

141 Reiter. E. L.: GewPhrlelstung von Betongllten. Die Bau-

wirtschaft, 1967. H. 40, S. 928-929

[5] Fuchs, G.: Zur GewPhrleistung von Betongllten. Dle Bau-

wirtschaft, 1968, H. 10, S. 244-245; Erwlderunn zu [I].

(61 Berndt, 0.; P r e ~ ' s s , E.: Versuche zum Verglelch d e r WUr-

felfestigkeit des Betons zu d e r Im B a u w e r k erzlelten Fe- stizkeit. Deutscher Ausschufl fllr Eisenbeton. H e f t 36, Ber- l i n 1915.

( I ] Iienzel, J . ; Grube. H.: Festigkeitsuntersuchungen a n Bau- wer-ksbeton und zugeh6rlgen Giltewilrfeln. Der Baulnge- riieur. 41 (1966). H. 12, S. 487-491

1.91 Mehmel, A , : Uber elnlge Grundlagen des modernen Mrs-

slvbaus. D e r Raulngenieur, 42 (1967), H 3, S. 17-43

191 Welnhold, J.; Meycr, H-G.: Stl.eutrng d e r Betondruckfe-

stlgkelt im Bauwerk. Beton- u n d Stahlbetonbau, 1961, H. I ,

s. 200-201

(101 Petersons. N.: S t r e n g t h of Concrete In Flnlshed S t r u c - tures. Kungl. Tekniska Hegskolans Handlingar N r . 232

(1964). Stockholm

[ll] Campbel R. H.; Tobln. R. E.: Core a n d Cyllnder ~ t r e n ~ t h i of Natural a n d Lightweight Concrete. ACI J o u r -

nal. V. 64. No. 4. Aortl 1967. S. 190--195

~ l i e m , D: L.: c o n c r e t e

nal. V 65. March 1968. 9. Structures. ACI J o u r -

113) ~ c w l c k i , ' 8 . : Dle Bezlehungen zwlcchen d e r Festlgkelt

des Betons in Konstruktlonen. d e r Berechnungsfestlgkeit und d e r Kontrollfestlgkelt a n ~ r o b e k b r p e r n u n t e r Berllck- sirhtigung d e r Streuung. H a u p t r e f e r a t a u s d e m Kolloqulum ..Beton" in Dresden, 17.--19. 6. 1968, veranstaltet , v o n d e r TU

Dresden und d e r Deutschen Bauakademie Bellln

(I¶] 1.cwandowskl. R.: Beurteilung von Bauwerksiestlgkelten a n Hand von Gutewilrfeln und -bohrproben. Schrlften- relhe d e r Institute f u r Konstruktiven I n g e n i e u r b a u d e r

Technischen Universltiit Braunschweig: Heft 3 a u s d e m In-

stitut f u r Baustoffkunde u n d Stahlbetonbau. Dlrektor

Prof. Dr.-Ing. K. Kordina. Werner-Verlag. Dllsseldorf 1971

1151 DIN 4240 - Kugelschlagprllfung von Beton mlt dlchtem

Gefiige, Richtlinien f u r die A n w e n d u n g - Ausgabe April

1962

[I61 Heinhold, J.; Gaede, K-W.: Ingenieur-Statlstlk. R. Olden-

bourg-Verlag, Milnchen-Wien 1964

L171 Rusch, H.: Zur statistischen QualitPtskontrolle d e s Be- tons. Materialprilfung, 6 (1964). H. ll, S. 387-394 1181 Conrad, D.: Beitrag zur Beurteilung d e r GleichmaBigkelt

von Beton. beton, 1966, H: 5. S. 202-205

Table I

R e l a t i o n between t h e o r e t i c a l c o n c r e t e g r a d e B and cube and i n - s i t u s t r e n g t h s

Table I1

Comparison of

DIN

1045

E

grades with test results

Table I 1 1

Relation between numerical strength

B

R and the 5% fractile of the in-situ strength

Fig. 1

DIN 1045 E

Sampling of a drill core from a column head with a wet core-drilling machine

flwm 200 300 400 500 En 150 250 350 450 Cubes - x -295 415 - 5 1 0

-

585 Drill cores X0.05 - 1 9 0 285 - 3 8 0

-

470 - X - 2 6 0 345 - 4 1 0

-

460 X0,05 - 1 7 0 215 - 2 7 5

-

325

F i g . 2

Concrete d r i l l cores. from a motorway b r i d g e . L e f t : w e l l compacted c o n c r e t e of high

compressive s t r e n g t h ;

c e n t r e : c o n c e n t r a t i o n of s h r i n k a g e c a v i t i e s s u g g e s t i n g compaction d e f i c i e n c i e s ; r i g h t : v e r y bad compaction which was i n v i s i b l e

on t h e o u t s i d e of t h e s t r u c t u r e 0 I00 200 300 100 500 600 lob Compressive strength [ L ; P I ~ ~ ~ F i g . 3 R e s u l t s of more t h a n 9600 c o n c r e t e q u a l i t y t e s t s on cubes of 20 cm a r r i s l e n g t h according t o D I N 1048; p e r i o d o f r e c o r d : 1964 through 1967; r e p r e s e n t a t i o n i n t h e p r o b a b i l i t y g r i d

Cum. freq. [ x l

0 roo 100 JOO 100 500 600 700 800 M O

Compressive strength [ k * l c d l

F i g . 4

Findings of subsequent t e s t s c a r r i e d o u t on more t h a n

200 s t r u c t u r e s by means of d r i l l c o r e s ; age of t e s t specimens > 2 months; r e p r e s e n t a t i o n i n t h e p r o b a b i l i t y g r i d Std. deviation s [kplcm2] I I I I m 100 m coo J 800

Compressive strength of cubes Ckplcrn7

o Placed horizontally Foundations 4 Placed vertically F i g . 5 Comparison o f s t a n d a r d d e v i a t i o n s s o f q u a l i t y c u b e s , dctcrmincd on t h e s t r u c t u r e s i n v e s t i g a t e d , with t h e c r i t e r i a a c c o r d i n g t o Conrad [I81

I n - s i t u s t r e n g t h O,[lrptcm2] Compressive s t r e n g t h of c u b e s p20 [ k p l c 4 W 1 8 F i g . 6 R e l a t i o n , d e t e r m i n e d a t 24 c o n s t r u c t i o n s i t e s , between s t r u c t u r a l s t r e n g t h

Bsau

and c o m p r e s s i v e s t r e n g t h of q u a l i t y c u b e s B W a,.,, = P,/e a , , =&A T h e o r e t i c a l q u a l i t y B F i g . 7 l t e l n t i o n s , d e t e r m i n e d by way of t e s t s , between t l l c o r c t i c a l c o n c r e t e q u a l i t y B and c o m p r e s s i v e s t r e n g t h o f q u a l i t y c u b e s B and o f s t r u c t u r a l c o n c r e t e

BBau

W

----

5 % fractile c o n f i d e n c e range F i g . 8

Comparison of s t r u c t u r a l s t r e n g t h

BBau

and t h e o r e t i c a l c o n c r e t e q u a l i t y B . C l a s s i f i c a t i o n