• Aucun résultat trouvé

Evaluation of test methods for alkali-aggregate reactivity

N/A
N/A
Protected

Academic year: 2021

Partager "Evaluation of test methods for alkali-aggregate reactivity"

Copied!
17
0
0

Texte intégral

(1)

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site

LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=ece430be-5fd0-4dbf-94b0-dd855ba4d1c6

https://publications-cnrc.canada.ca/fra/voir/objet/?id=ece430be-5fd0-4dbf-94b0-dd855ba4d1c6

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Evaluation of test methods for alkali-aggregate reactivity

(2)

-- - I

Ser

TIS.

National Research

Conseil national

N21d

\t)

Council Canada

no.

1236

de recherches Canada

c. 2

BLDG

EVALUATION OF TEST METHODS FOR ALKALI-AGGREGATE

REACTIVITY

*

by

P.E. Grattan-Bellew

Reprinted from

Danish Concrete Association

6th. International Conference

"Alkalis in Concrete: Research and Practice"

Technical University of Denmark

Copenhagen, 22-25 June 1983

Proceedings, p. 303

-

314

N R C

-

C I S T I

1

BLDG.

RES.

L I B R A R Y

84-

11-

. O 6

B I B L ~ O T H Z Q U E

Reth.

Bttim.

C N R C

-

I C I S T i

DBR Paper No. 1236

Division of Building Research

(3)

L e s msthodes d ' e s s a i d e l a r h c t i o n a l c a l i - g r a n u l a t comprennent

c e l l e s q u i p e r m e t t e n t d e d'eterminer l a r ' e a c t i v i t ' e p o t e n t i e l l e

d'un g r a n u l a r ( l ' e s s a i chimique e t l a m6t hode p'et r o g r a p h i q u e

propos'ee

pour d'eterminer l ' a n g l e d ' e x t i n c t i o n o n d u l a t o i r e du

q u a r t z )

e t c e l l e s q u i v i s e n t

2

mesurer l ' e x p a n s i m

d'un

g r a n u l a t d a n s l e b'eton (msthodes du p r i s m e d e b'eton, du b a r r e a u

d e m o r t i e r

e t

d e

l ' g p r o u v e t t e

c y l i n d r i q u e ) .

P l u s i e u r s

m o d i f i c a t i o n s

o n t

'et5

apport'ees

B

l a m'ethode

chimique,

1

ASTM

C289,

pour

p r e n d r e en

c o n s i d ' e r a t i m

l e s v a r i a t i o n s

r ' e g i o n a l e s d e s g r a n u l a t s .

le

a r i n r i n a l

A U e & u e s

d e

I

mesure d e l ' e x p a n s i f

e n

oeuvre.

Pour t e n t e r

Ye s

d 1 a c c S l ' e r a t i o n o n t

1

n

s

d a n s d e s s o l u t i o r

B

1 'a u t o c l a v a g e .

1

re

i n t e r p r ' e t ' e e s avec,

- -

---

- _

W

6lev'ees,

il

s e p r ~

6

n e s e p r o d u i r a

n

atmosphgrique

.

(4)

DANISH

,

'CONCRETE ASSOCIATION

DBF

ALKALIS IN CQMCRlETlE

Research

and

Practice

Paper reprinted from the

PROCEEDINGS

TECHNICAL UNIVERSITY

of Denmark

COPENHAGEN

22.-25.

J U N E

-

1983

(5)

EVALUATION OF TEST METHODS FOR ALKALI-AGGREGATE REACTIVITY P.E. Grattan-Bellew

D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research Council Canada Ottawa, Canada K1A OR6

ABSTRACT

T e s t methods f o r a l k a l i - a g g r e g a t e r e a c t i v i t y c a n be d i v i d e d i n t o t h o s e t h a t determine t h e p o t e n t i a l r e a c t i v i t y of an aggregate ( t h e chemical t e s t and t h e proposed p e t r o g r a p h i c method f o r determining t h e undulatory e x t i n c t i o n a n g l e of q u a r t z ) and t h o s e t h a t p u r p o r t t o measure t h e e x p a n s i 9 i t y of a n a g g r e g a t e i n c o n c r e t e ( c o n c r e t e prism, mortar b a r , and rock c y l i n d e r methods). S e v e r a l m o d i f i c a t i o n s have been developed f o r t h e chemical method, ASTM C289, t o t a k e i n t o account r e g i o n a l v a r i a t i o n s i n aggregates. The main disadvantage of t h e methods measuring expansion is t h a t they t a k e a long t i m e . To t r y t o overcome t h i s problem, v a r i o u s methods of a c c e l e r a t i o n have been proposed, v a r y i n g from immersion of t h e samples i n s a l t s o l u t i o n a t an e l e v a t e d temperature t o autoclaving. Autoclaving methods must be regarded w i t h some c a u t i o n , however, because a t e l e v a t e d temperatures and p r e s s u r e s hydrothermal r e a c t i o n s occur t h a t do n o t t a k e p l a c e a t atmospheric pressure.

KEY WORDS: E v a l u a t i o n , T e s t s , A l k a l i , Aggregate.

1. INTRODUCTION

The i d e a l method of determining t h e p o t e n t i a l a l k a l i e x p a n s i v i t y of a n

aggregate i n c o n c r e t e should be a p p l i c a b l e t o a l l rock t y p e s , should be r a p i d , t a k i n g no more t h a n a few days t o complete, and must g i v e r e l i a b l e r e s u l t s . The q u e s t f o r t h i s t e s t h a s k e p t a number of r e s e a r c h e r s busy f o r many y e a r s , b u t l i k e t h a t of t h e a l c h e m i s t s b e f o r e them i t w i l l n o t succeed. M v e r s i t y i n t h e composition, g r a i n s i z e , and p o r o s i t y , and t h e presence o r absence of secondary cementing m i n e r a l s i n r o c k s p r e d i c a t e s a g a i n s t t h e development of a u n i v e r s a l l y a p p l i c a b l e , r a p i d t e s t method. Some t e s t s , f o r example t h e c o n c r e t e prism method, a r e a p p l i c a b l e t o a l l rock t y p e s b u t may t a k e s o l o n g t h a t t h e s t r u c t u r e w i l l be b u i l t b e f o r e t h e t e s t i s complete. Other tests, l i k e t h e mortar b a r and t h e chemical methods, a r e s u i t a b l e f o r a g g r e g a t e s e x h i b i t i n g c l a s s i c a l a l k a l i - s i l i c a r e a c t i v i t y but not f o r r e a c t i v e c a r b o n a t e aggregates. A s a r e s u l t , a number of d i f f e r e n t t e s t methods have been, and continue t o be, developed. This review w i l l examine t h e e f f e c t i v e n e s s and l i m i t a t i o n s of e x i s t i n g and proposed t e s t methods.

2. CHEMICAL TEST

The chemical method, ASTM C289 111, was developed a s a r a p i d means of d i f f e r e n t i a t i n g between p o t e n t i a l l y r e a c t i v e and non-reactive s i l i c a - b e a r i n g aggregates. The c r i t e r i a f o r e v a l u a t i o n were developed from t h e r e s u l t s of p a r a l l e l s e t s of mortar b a r and chemical t e s t s on many a g g r e g a t e s from t h e United S t a t e s 121. The r o c k s t e s t e d included some c o n t a i n i n g o p a l , c h e r t o r chalcedony a s t h e r e a c t i v e component and o t h e r s i n which v o l c a n i c g l a s s was t h e r e a c t i v e component. A s t h e two main r e a c t i v e components, o p a l and v o l c a n i c g l a s s , c o n s i s t of c r y p t o c r y s t a l l i n e S i 0 2 and an a l u m i n o s i l i c a t e g l a s s , r e s p e c t i v e l y , i t i s s u r p r i s i n g t h a t one s e t of c r i t e r i a were found t o be a p p r o p r i a t e f o r e v a l u a t i o n of b o t h types.

The problem of o b t a i n i n g meaningful v a l u e s f o r t h e d i s s o l v e d s i l i c a (Sc) and t h e r e d u c t i o n i n a l k a l i n i t y (Rc) a r e complicated i f c a r b o n a t e s o r s u l p h a t e s , e.g., gypsum 131, a r e p r e s e n t i n t h e a g g r e g a t e because t h e y r e a c t w i t h NaOH t o

(6)

reduce its a l k a l i n i t y . The v a l u e s of Sc and Rc a r e a l s o very s u s c e p t i b l e t o o p e r a t o r influence. For example, i f t h e sample i s n o t properly s i z e d and a s i g n i f i c a n t amount of -150

um

m a t e r i a l is included i n t h e test, t h e value of Sc w i l l be markedly increased. Grinding q u a r t z t o s i z e s less t h a n about

100

u m

g r e a t l y i n c r e a s e s i t s s o l u b i l i t y (Figure 1).

The phenolphthalein i n d i c a t o r does n o t g i v e a s h a r p end p o i n t and t h e v a l u e of Rc determined is t h u s , t o some e x t e n t , dependent on t h e judgement of t h e operator. Soaie of t h e p r o b l e m i n h e r e n t i n t h e chemical test were d i s c u s s e d by Dent G l a s s e r and Kataoka 141. Their main conclusion i s t h a t t h e value of Rc, as c u r r e n t l y determined, i s r a t h e r meaningless s i n c e i t i n c l u d e s

reductions i n both OH- and t h e c o n c e n t r a t i o n of ~ a + i n t h e s o l u t i o n . They suggested t h a t t h e method should be a l t e r e d s o t h a t O H can be determined by a pH meter and ~ a + by a s p e c t r o s c o p i c method.

The q u e s t i o n a r i s e s a s t o t h e need t o d e t e r d n e t h e reduced a l k a l i n i t y , Rc;

i s determination of Sc alone s u f f i c i e n t f o r p r e d i c t i n g t h e p o t e n t i a l

r e a c t i v i t y of a n aggregate? To i n v e s t i g a t e t h i s , test r e s u l t s f o r a s e r i e s of q u a r t z sands containing varying amounts of opal 121 were p l o t t e d i n Figure 2; i t i s e v i d e n t t h a t expansion of mortar b a r s c o r r e l a t e s b e t t e r w i t h t h e f a c t o r Rc/Sc than with Sc alone, i n d i c a t i n g t h a t f o r t h e s e aggregates i t i s b e t t e r t o determine both Rc and Sc.

I t has been e s t a b l i s h e d t h a t t h e c r i t e r i a given i n C289 a r e n o t adequate f o r the e v a l u a t i o n of I c e l a n d i c sands containing volcanic g l a s s o r of Danish f l i n t g r a v e l s , f o r which l o w v a l u e s of t h e amount of d i s s o l v e d s i l i c a are i n d i c a t i v e of p o t e n t i a l r e a c t i v i t y . New c r i t e r i a were developed f o r t h e i r e v a l u a t i o n a s follows ( p e r s o n a l comnunication, Hakon Olaf sson) :

I c e l a n d i c sands:

non-reactive

-

Sc <I00 nsnol/L

p o t e n t i a l l y r e a c t i v e

-

Sc 100 t o 200 mmol/L r e a c t i v e

-

Sc s200 mmol/L Danish g r a v e l s : non r e a c t i v e

-

Sc <50 mmol/L p o t e n t i a l l y r e a c t i v e

-

Sc 50 t o 200 mmol/~ r e a c t i v e

-

Sc >200 mmol/L I n c o n t r a s t , i n B r i t a i n t h e Thames Valley f l i n t g r a v e l s y i e l d e d v a l u e s of Sc i n excess of 100 mmol/L y e t caused no problems i n concrete 131. It appears t h a t s e p a r a t e s e t s of c r i t e r i a need t o be developed on a r e g i o n a l b a s i s , even f o r e v a l u a t i o n of what a r e nominally t h e same rock types.

I n t h e late-expansive a g g r e g a t e s of t h e Malmesbury Formation i n t h e Cape Peninsula of South Africa, i n which s t r a i n e d q u a r t z is t h e r e a c t i v e component, t h e c r i t e r i a s p e c i f i e d i n C289 a r e e f f e c t i v e f o r d i f f e r e n t i a t i n g r e a c t i v e and non-reactive aggregates; but i n t h e Tygerberg Formation from t h e same r e g i o n lower v a l u e s of Sc were obtained from r e a c t i v e aggregates. The r e s u l t s of the chemical t e s t on t h e s e aggregates, reported by Brandt,

Oberholster and Westra 151, a r e r e p l o t t e d i n F i g u r e 3. It i s e v i d e n t t h a t t h e d i v i d i n g l i n e between r e a c t i v e and non-reactive aggregates must l i e a t a value of Sc between 35 and 50 nnnol/L.

I n S e r b i a t h e chemical method, ASTM G289, used by MitroviE and

~ u r i E I61 i n d i c a t e d t h a t t h e aggregates a r e p o t e n t i a l l y r e a c t i v e ( a s d i d t h e mortar bar t e s t ) , although

no

problems have been observed i n concrete. There a p p e a r s t o be some doubt concerning t h e v a l i d i t y of both methods f o r

e v a l u a t i n g t h e s e p a r t i c u l a r aggregates.

I n A u s t r a l i a , t h e c r i t e r i a s p e c i f i e d i n C289 a r e e v i d e n t l y s a t i s f a c t o r y ; they have been adopted

as

an A u s t r a l i a n standard, AS 1141 Section 39, 1954.

(7)

2.1 Discussion

The c h e d c a l test i s t h e only rapid, currently a v a i l a b l e method, t h a t has been thoroughly evaluated. With some m d i f i r a t i o n of t h e c r i t e r i a used t o d e f i n e r e a c t i v e aggregates, and i n co-njuaction w i t h a p e t r o g r a p h i c

examination, it can be used t o e v a l u a t e t h e p o t e n t i a l r e a c t i v i t y of a wide

range of s i l i c e o u s aggregates. Improvements might be made i n t h e a n a l y t i c a l procedures s o t h a t t h e r e d u c t i o n s i n

OH-

and i n ~ a + a r e determined s e p a r a t e l y . There is some evidence t h a t determination of d i s s o l v e d s i l i c a a l o n e i s

sufficient t o d i f f e r e n t i a t e between r e a c t i v e and non-reactive aggregates. Owing t o t h e wide d i v e r s i t y i n the conpositlo-n g r a i n s i z e and p e r m e a b i l i t y of rocks, it is most u n l i k e l y t h a t u n i v e r s a l l y a p p l i c a b l e c r i t e r i a can be developed t o d i f f e r e n t i a t e between r e a c t i v e and non-reactive aggregates; i n s t e a d , c r i t e r i a a p p l i c a b l e t o s p e c i f i e d rock t y p e s should be developed. 2.2 Modified chemical test

A d i f i e d c h e d c a l t e s t was developed f o r e v a l u a t i n g t h e p o t e n t i a l r e a c t i v i t y of a g g r e g a t e s i n she Schleswig-lblstein region of Germany. The aggregate i s d i v i d e d i n t o two s i z e f r a c t i o n s : 1-2 mm and 2-4 mm. These a r e d i g e s t e d f n NaOH s o l u t i o n f o r 60 d n a t BO'C. The samples are washed, d r i e d , and weight l o s s determined; t h i s r e p r e s e n t s t h e amount of s i l i c a d i s s o l v e d o u t by N&H and g i v e s , i n p r i n c i p l e , a measure of t h e p o t e n t i a l r e a c t i v i t y of t h e aggregate. The l i m i t a t i o n s of t h i s test have been d i s c u s s e d by Jensen, C h a t t e r j i , Christensen, Thaulow and Gudmndsson 171. It would be a p p l i c a b l e

only t o a g g r e g a t e s c o n t a i n i n g r e a c t i v e silica, i.e., opal, c h e r t o r chalcedapy and p o s s i b l y a l s o t o aggregate composed of v o l c a n i c g l a s s .

3. MORTAR

BAR

METHOD

The mortar b a r t e s t , ASTM C227-8 / 8 / , i s t h e most widely used method of e v a l u a t i n g t h e p o t e n t i a l a l k a l i - r e a c t i v i t y of aggregates. A number of v a r i a t i o n s i n t h e method of e v a l u a t i n g t h e t e s t r e s u l t s a r e i n u s e by d i f f e r e n t o r g a n i z a t i o n s . Despite t h e almost u n i v e r s a l acceptance of t h e mortar bar method, i t h a s s e v e r a l shortcomings: It u s u a l l y t a k e s from s i x months t o a y e a r t o complete, except w i t h some opal-bearing aggregates t h a t c a n be e v a l u a t e d i n about t h r e e months. Such l o n g l e a d times f r e q u e n t l y do not e x i s t before c o n s t r u c t i o n i s s t a r t e d , even i n major s t r u c t u r e s f o r which t h e d u r a b i l i t y of t h e c o n c r e t e i s of c r i t i c a l importance.

The e v a l u a t i o n of t h e r e a c t i v i t y of c o n c r e t e aggregate by t h e mortar b a r method assumes t h a t expansion of concrete i n t h e f i e l d c o r r e l a t e s w i t h expansion of test mortar bars. Although t h i s holds t r u e f o r some aggregates, t h e r e a r e exceptions. The carbonate aggregate from Kingston, Ontario, f o r example, shows much g r e a t e r expansion i n c o n c r e t e prisms than i n mortar b a r s , a s do late-expansive quartz-bearing aggregates such a s Malmesbury aggregate from t h e Cape Peninsula i n South A f r i c a and a greywacke from Nova S c o t i a , Canada 191 ( s e e Figure 4, #73-50).

3.1 I n t e r p r e t a t i o n of observed expansion

Recommended c r i t e r i a f o r e v a l u a t i o n of t h e r e s u l t s of t h e mortar b a r t e s t a r e given i n ASTM C33-82, Appendix XI.1.3

/lo/.

It must be s t r e s s e d t h a t t h e s e a r e recommendations, n o t a mandatory p a r t of t h e standard. Although they a r e f r e q u e n t l y i n t e r p r e t e d a s "limits," t h i s i s i n c o r r e c t .

According t o C33, expansions a r e t o be considered e x c e s s i v e i f they exceed 0.05% a t t h r e e months o r 0.10% a t s i x months; t h e three-month d a t a should only be used, however, when a six-month t e s t i s n o t possible. The a l k a l i c o n t e n t of t h e cement should be 0.6%, pr p r e f e r a b l y 0.8%. With t h e h i g h a l k a l i cements now commonly i n use, an a l k a l i l e v e l of 1.0% o r g r e a t e r would be more

(8)

a p p r o p r i a t e . The problem w i t h t h e s e recommended maximum p e r m i s s i b l e expansions i s t h a t they assume a f a s t r a t e of expansion w i t h a s h o r t i n i t i a t i o n period. A slowly expanding aggregate may show mortar b a r

expansions w e l l below t h e v a l u e s considered t o be d e l e t e r i o u s , according t o C33, but t h e continuing slow expansion w i l l e v e n t u a l l y l e a d t o e x c e s s i v e s t r e s s and cracking.

A t y p i c a l example of expansion of a mortar b a r made w i t h Malmesbury

aggregate and a cement c o n t a i n i n g 0.82% a l k a l i i s shown i n Figure 5 ( p e r s o n a l comnunication, R.E. O b e r h o l s t e r ) . T h i s a g g r e g a t e i s known t o cause

d e l e t e r i o u s expansion i n c o n c r e t e made w i t h a high a l k a l i cement. The problem of e v a l u a t i n g t h e r e s u l t s of expansion of mortar b a r s made w i t h l a t e e x p a n s i v e aggregates has a l s o been discussed by Kennerly e t a l . 1111 and by Sims 131. 3.2 Corps of Engineers

The Corps of Engineers c r i t e r i a f o r e v a l u a t i o n of t h z r e s u l t s of t h e mortar bar t e s t a r e p o s s i b l y more a p p r o p r i a t e than t h o s e recommended i n C33.

EM 1110-2-2000 1121 s t a t e s t h a t expansion of 0.10% o r more a t any a g e should be considered d e l e t e r i o u s . An expansion of 0.05% o r more a t s i x months may a l s o be considered d e l e t e r i o u s . When expansions c l o s e t o t h e s e l i m i t s a r e recorded, t h e t r e n d of t h e time-expansion curve should be taken i n t o account. T h i s i s i n l i n e w i t h t h e concept of determining t h e r a t e of expansion proposed by Grattan-Bellew 191. Cracking of mortar b a r s i s g e n e r a l l y observed when expansion exceeds about 0.04%; i f c r a c k i n g i s accepted a s evidence of d e l e t e r i o u s r e a c t i o n , expansions of l e s s than 0.10% must i n d i c a t e p o t e n t i a l a l k a l i - a g g r e g a t e r e a c t i v i t y . The problem i s t h a t a t p r e s e n t n o t enough information on t h e c o r r e l a t i o n between expansion of mortar b a r s and f i e l d c o n c r e t e e x i s t s t o s e t r e a l i s t i c s a f e l i m i t s of expansion f o r mortar bars. 3.3 Canadian Standards Association

The Canadian s t a n d a r d (CAN3.A23.1-M77, Appendix B3.4 1131) s t a t e s t h a t expansions i n excess of 0.04% a t any age a r e t o be considered d e l e t e r i o u s . It a l s o n o t e s t h a t t h e shape of t h e expansion curve i s o f t e n of importance i n making a judgement. The 0.04% l i m i t f o r expansion i s i n agreement w i t h f r e q u e n t o b s e r v a t i o n s of c r a c k i n g when t h i s l e v e l of expansion i s exceeded; t h e l i m i t may, however, be too r e s t r i c t i v e f o r some aggregates f o r which expansion may j u s t exceed t h i s v a l u e and t h e n l e v e l o f f .

3.4 Accelerated Danish mortar b a r t e s t

T h i s t e s t , d e s c r i b e d by Jensen e t a l . 171, i s an a c c e l e r a t e d v e r s i o n of t h e mortar bar method C227 and i s used f o r Danish f l i n t g r a v e l s . The mortar b a r s a r e made i n t h e u s u a l way and cured i n a f o g room a t 23OC f o r 27 days. They a r e then placed i n s a t u r a t e d NaCl s o l u t i o n a t 50°C and expansion i s monitored w i t h time. No l i m i t s have been proposed. T h i s method i s r e p o r t e d t o be s a t i s f a c t o r y f o r t h e aggregates t e s t e d i n Denmark, but t h e r e a r e no r e p o r t s of

i t s use elsewhere.

3.5 Chinese autoclaved mortar bar t e s t

T h i s i s a n o t h e r a c c e l e r a t e d mortar b a r t e s t . Mortar b a r s 1 x 1 x 4 cm a r e prepared and cured f o r one day i n a fog room. Following t h i s , they a r e steam cured a t 100°C f o r 4 h and then immersed i n 10% KOH s o l u t i o n and autoclaved a t 150°C f o r 6 h. Expansions a r e recorded a f t e r each phase of t h e c u r i n g cycle. A l a r g e number of a l k a l i - s i l i c a r e a c t i v e a g g r e g a t e s were t e s t e d i n t h i s way by Ming-shu, Su-fen and Shi-hua 1141, who concluded t h a t t h e method i s capable of d i f f e r e n t i a t i n g between r e a c t i v e and non-reactive aggregates. More t e s t i n g should be c a r r i e d out t o e v a l u a t e i t s e f f e c t i v e n e s s f o r a wider v a r i e t y of rock types. Hydrothermal t e s t s must, however, be regarded w i t h some c a u t i o n

(9)

because of the possibility that the observed expansion might be distorted by hydrothermally induced reactions that would not occur in concrete under normal conditions.

3.6 Discussion

No firm limit of expansion can be specified that is applicable to all rock types. Cracking of mortar bars is frequently observed when expansion exceeds about 0.04%, and this value might be considered indicative of potentially deleterious expansion. In some cases, however, this could result in

classification of some aggregates that perform satisfactorily in concrete as deleterious. It would be desirable to have a time limit for the mortar bar test; owing, however, to wide divergences in initiation periods and in rates of expansion it is not possible to establish a universally applicable time limit. It is possible that regionally applicable time-expansion limits such as those proposed in C33 may be established for some suites of aggregates. Accelerated tests have so far not been evaluated for a wide enough range of aggregates to permit conclusions to be drawn as to their applicability.

4. CONCRETE PRISM TEST

The concrete prism test (CSA A23.2-14A 1151) was developed in Canada for evaluating the expansivity of reactive dolomitic limestones that do not cause significant expansion in mortar bars. The test has also been used success- fully for late-expansive siliceous aggregates and limestones containing reactive silica. Similar tests have been used in New Zealand and

South Africa. The criteria for evaluation of the test results are

specified

in CSA A23.1.M77, Appendix B3.5. Linear expansions of more than about 0.03% indicate potentially deleterious expansion. Appendix

3.5

concludes that some aggregates expand beyond the three-month period and that, where possible, the test should be continued until expansion has virtually ceased. This is sound advice, but it may often not be practical since with some late-expansive aggregates it may take up to two years for expansion to taper off. The use of the rate method of evaluating expansivity of aggregates 191 may help in evaluating the expansivity of concrete prisms before expansion tapers off. It is very desirable to carry out a petrographic examination in conjunction with the concrete prism test so that the type of reactivity to be expected can be determined; this helps in the correct evaluation of early test results if it is not possible to wait until the expansion has tapered off.

The concrete prism method may be the most reliable test for all types of aggregate. In it, the aggregate is tested in the same size range and with the same cement:aggregate ratio that is used in field concrete. For some l a t e expansive siliceous aggregates and for carbonate reactive aggregates such as those from Kingston, Ontario, the concrete prism test is the only satisfactory method since these aggregates do not cause significant expansion in mortar bars. Expansion can be significantly accelerated by storage of the prisms at 38°C and 100% RH instead of at 23°C as specified.

A

few preliminary

experiments to try and accelerate expansion by immersion of the prisms in NaCl solution, after the method of the Danish accelerated mortar bar test, were not very successful, but more research is needed before any firm conclusions can be drawn.

5. CONCRETE CUBE TEST

This test was developed in Germany 1161, and cracking is the criterion used to determine whether a cement-aggregate combination is reactive. There are two versions of it. In the first, 30-cm cubes are made and expansion is

(10)

a c c e l e r a t e d by s t o r a g e i n a f o g room a t 40%. Other exposure c o n d i t i o n s a r e a l s o used, f o r example, s t o r a g e on t h e roof of t h e r e s e a r c h l a b o r a t o r y .

I n t h e second v e r s i o n of t h e test, 10-cm cubes a r e s t o r e d a t 65%

RH,

with t h e I

bottom 1 c m immersed i n water a t 20°C. I n some samples c o n t a i n i n g r e a c t i v e I a g g r e g a t e marked c r a c k i n g was observed j u s t above t h e w a t e r l i n e .

I n B r i t a i n , a t t h e Cement and Concrete Association, a modified v e r s i o n of

the 10-cm cube t e s t is being i n v e s t i g a t e d (D.W. Hobbs, personal communica-

~

t i o n ) . Cubes a r e immersed i n a water b a t h a t 23'C and crack formation i s

observed. A temperature of 23'C i s reported t o be t h e optimum f o r s o l u b i l i t y of s i l i c a .

I n South A f r i c a , 30-cm cubes a r e used a t outdoor exposure s i t e s where they may be submitted t o v a r i o u s moisture and weather exposure conditions. T e s t s involving t h e o b s e r v a t i o n of cracking have a major advantage over tests r e q u i r i n g a c c c u r a t e measurement of l e n g t h change i n t h a t no e l a b o r a t e a p p a r a t u s i s required. This means t h a t t h e c o n c r e t e cube t e s t could r e a d i l y be c a r r i e d o u t by p i t , quarry o r ready-mix operators. The disadvantage i s

t h a t u n l e s s r a p i d l y expanding aggregates a r e being e v a l u a t e d i t would t a k e a n unacceptably long time f o r v i s i b l e cracks t o develop. From l a b o r a t o r y

o b s e r v a t i o n s by t h e a u t h o r i t is known t h a t a c o n s i d e r a b l e amount of expansion can f r e q u e n t l y be observed before cracks become v i s i b l e t o t h e unaided eye.

6. ROCK CYLINDER METHOD

1

The rock c y l i n d e r method (ASTM C586-69 1171) was designed t o e v a l u a t e t h e p o t e n t i a l r e a c t i v i t y of a l k a l i r e a c t i v e carbonate aggregates, but it has a l s o been used w i t h varying degrees of success t o e v a l u a t e t h e r e a c t i v i t y of a l k a l i - s i l i c a r e a c t i v e aggregates /5, 10, 18, 191. Paragraph 9.3 of C586 s t a t e s t h a t expansive behaviour of aggregate (carbonate) i n c o n c r e t e i s q u a l i t a t i v e l y p r e d i c t e d by t h e r e s u l t s of t h e rock c y l i n d e r t e s t .

Grattan-Bellew 1191 found moderate c o r r e l a t i o n between expansion of c o n c r e t e made with s i x samples of Kingston aggregate and expansion of miniature rock prisms made w i t h t h e same aggregates. Much poorer c o r r e l a t i o n was observed f o r expansion of c o n c r e t e prisms and t h a t of miniature rock prisms made with late-expanding s i l i c e o u s aggregates. Kennerley e t a l . 1111 found t h a t some prisms of New Zealand greywacke expanded e x c e s s i v e l y i n NaOH, but t h a t t h e same rock caused no expansion i n concrete.

It i s concluded t h a t t h e rock c y l i n d e r t e s t , o r v a r i a t i o n s of i t , can only be used t o e v a l u a t e t h e r e a c t i v i t y of c e r t a i n rock types. I t s optimum u s e would probably be f o r e v a l u a t i n g t h e p o t e n t i a l e x p a n s i v i t y of i n d i v i d u a l hori- zons i n a quarry containing a rock type f o r which c o r r e l a t i o n of expansion of rock c y l i n d e r s and t h a t of c o n c r e t e prisms has previously been demonstrated.

6.1 Accelerated rock prism test

I

An a c c e l e r a t e d rock prism t e s t f o r e v a l u a t i o n of t h e p o t e n t i a l r e a c t i v i t y of carbonate aggregates has been developed by Kazimir 1201 and adopted a s Czechoslovakian s t a n d a r d CSM 72 1160. Rock prisms 1 x 1 x 3 cm a r e prepared and autoclaved i n NaOH s o l u t i o n a t 215OC and 2.1 MPa f o r 6 h. This t e s t

i s

r e p o r t e d t o be s u c c e s s f u l i n d i f f e r e n t i a t i n g between expansive and non- expansive carbonate aggregates. Some r e s e a r c h was conducted by t h e author on t h e a p p l i c a b i l i t y of t h i s t e s t t o late- expanding s i l i c e o u s aggregates, but i t

was discontinued a f t e r a f e l d s p a r - l i k e phase was found t o have formed i n t h e prisms. Expansions occurred, but a s r e a c t i o n s were t a k i n g p l a c e ( t h e s e do n o t occur a t one atmosphere) i t seemed most u n l i k e l y t h a t t h e observed expansions would c o r r e l a t e w i t h expansions of t h e same prisms under atmospheric

conditions. More r e s e a r c h should probably be done a t various temperatures and p r e s s u r e s t o see whether an a c c e l e r a t e d t e s t could be developed f o r l a t e - expansive s i l i c e o u s aggregates.

(11)

7. GEL PAT TEST

This test was developed in England as a rapid method of identifying

potentially deleterious minerals such as opal or chert in mortar samples.

It does not give any indication of the expansivity of the aggregate.

As reported by Sims 131, a smoothed, sawn surface of the mortar is prepared

and immersed face down in alkali for three days. Gel forms on the reactive

particles, which can thus be readily identified and counted to yield a measure

of the percentage of reactive component in the aggregate.

A somewhat similar test, CSN 72-1162 1211, is used in Czechoslovakia to

accelerate rim development around prisms of siliceous aggregates embedded in

mortar. It is useful for preliminary screening of potentially alkali-silica

reactive aggregates, but additional tests would

be

necessary to obtain a

measure of the expansivity of the aggregate in concrete.

8. PETROGRAPHIC EXAMINATION

Petrographic examination, ANSIIASTM C595 1221, has been changed from a

technique for determining the mineralogical composition of a rock to a

semi-quantitative method of determining the potential reactivity of certain

rock types by Dolar-Mantuani's development of a technique for determining the

undulatory extinction angle, UE, and relating it to the expansivity of

aggregate 1231. Following on earlier work 124-261, describing the development

of the technique for measuring strain in the quartz lattice by means of the

undulatory extinction angle, Dolar-Mantuani suggested that aggregates with

undulatory extinction angles of less than 15 deg may be characteristic*

nowreactive aggregates.

Recent unpublished results obtained by the present author show that some

South African aggregates (a quartzite from the Witwatersrand, a greywacke, and

a spotted slate from the Malmesbury Formation in the Cape Peninsula, all of

which cause excessive expansion in concrete) have UE angles of 36, 36 and

43 deg, respectively, indicating that there is some correlation between high

UE

angle and the reactivity of the aggregates. More research into this method

should be carried out. It has great potential as a rapid method of evaluating

the potential reactivity of quartz-bearing aggregates, particularly the late-

expansive types common in Precambrian terrains. It would be useful to conduct

a series of interlaboratory measurements of UE angles on reference samples so

that participants could adjust their measuring techniques to ensure results

consistent with those of other operators. In a limited test carried out in

Ottawa, a variation of 10 deg was found among measurements made by three

participants on the same quartz grains in a thin section. Such a wide

variation may have resulted because one or more of the participants measured

the extinction range, ER, rather than the undulatory extinction angle, UE, as

defined by DolarMantuani.

8.1

Use of scanning electron microscope

Use of the scanning electron microscope (SEM) to extend the scope of the

petrographic method by permitting characterization of texture and composition

of fine argillaceous rocks may, in due course, make it possible to distinguish

between expansive and non-expansive horizons in a particular deposit.

Figure 6 shows micrographs of two rocks from the Pittsbury quarry near

Kingston, Ontario. Figure 6a shows the highly expansive dolomitic limestone

characterized by isolated dolomite rhombs in a matrix of calcite; Figure 6b

shows a dolostone, which is non-expansive. This sample is composed almost

entirely of a more coarse grained dolomite. Electron probe analysis showed

that the Ca/Mg count rate ratio was different in the two samples

(12)

i n t h e r e a c t i v e dolomite; i t i s , t h e r e f o r e , a m e t a s t a b l e protodolomite 1271. The SEM h a s one g r e a t advantage over o t h e r techniques, (e.g., p e t r o g r a p h i c microscopy), t h a t i s , t h e speed w i t h which o b s e r v a t i o n and a n a l y s i s can be done; i n about h a l f a n hour a c h i p of rock can be coated, examined, and analysed by an expqrienced o p e r a t o r .

9. CONCLUSION

There i s a d e f i n i t e t r e n d towards m o d i f i c a t i o n of t h e s t a n d a r d ASTM t e s t s f o r use i n c e r t a i n r e g i o n s and f o r s p e c i f i e d types of aggregate. This h a s come about a s a r e s u l t of inadequacies i n t h e s t a n d a r d t e s t methods, e i t h e r because they f a i l t o p r e d i c t c o r r e c t l y t h e r e a c t i v i t y of an aggregate o r because t h e t e s t t a k e s t o o long t o complete. The t e s t methods have been modified i n two ways: expansion h a s been a c c c e l e r a t e d , a s i n t h e Chinese autoclaved mortar b a r t e s t ; t h e c r i t e r i a f o r t h e chemical t e s t , a s

recommended, have been modified t o p r e d i c t t h e r e a c t i v i t y of l o c a l a g g r e g a t e s more a c c u r a t e l y , a s i n I c e l a n d and Denmark where s p e c i f i c l i m i t s on t h e d i s s o l v e d s i l i c a have been proposed.

Modified c r i t e r i a f o r e x i s t i n g t e s t methods may be l i m i t e d t o s p e c i f i e d aggregates i n c e r t a i n regions. For example, t h e proposed Danish c r i t e r i a f o r e v a l u a t i n g t h e r e s u l t s of t h e chemical t e s t of f l i n t a g g r e g a t e s , t h a t

Sc values between 50 and 200 mmol/L a r e i n d i c a t i v e of p o t e n t i a l r e a c t i v i t y , would be q u i t e u n s u i t e d f o r t h e e v a l u a t i o n of Thames Valley f l i n t s , which a r e non-expansive i n c o n c r e t e d e s p i t e having Sc v a l u e s i n excess of

100 mmol/L.

There i s s t i l l a g r e a t need f o r b e t t e r , more r a p i d methods of diagnosing p o t e n t i a l l y r e a c t i v e aggregates. For reasons a l r e a d y o u t l i n e d t h e new t e s t s w i l l probably be developed f o r t h e e v a l u a t i o n of c e r t a i n a g g r e g a t e s i n s p e c i f i c regions. The most promising f o r f u t u r e development a r e t h e chemical method, i n conjunction w i t h improvements i n techniques f o r p e t r o g r a p h i c e v a l u a t i o n , and p o s s i b l y some type of a u t o c l a v e a c c e l e r a t i o n f o r mortar o r c o n c r e t e expansion t e s t s .

Acknowledgement

The a u t h o r i s indebted t o P.J. Lefebvre f o r h i s enthusiasm f o r t h e work and t h e p r e c i s i o n w i t h which he c a r r i e d o u t t h e experiments r e p o r t e d i n t h i s paper. T h i s i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g Research, National Research Council Canada, and i s published w i t h t h e approval of t h e D i r e c t o r of t h e Division.

References

/1/ ASTM C289-81, P o t e n t i a l R e a c t i v i t y of Aggregates (Chemical Method), Annual Book of ASTM Standards, P a r t 14, Concrete and Mineral Aggregates. American S o c i e t y f o r T e s t i n g and M a t e r i a l s , 1916 Race S t . , P h i l a d e l p h i a , PA 19103, U.S.A., 1982, 198-205.

/2/ MIELENZ

,

R.C., GREENE, K.T

.

and BENTON, E .J

.

,

Chemical T e s t f o r R e a c t i v i t y of Aggregates w i t h Cement A l k a l i s ; Chemical Processes i n Cement-Aggregate Reaction. J o u r n a l of t h e American Concrete I n s t i t u t e , 19(3), 1947, 193-221.

/ 3 /

EMS,

I., The A p p l i c a t i o n and R e l i a b i l i t y of Standard T e s t i n g Procedures f o r P o t e n t i a l A l k a l i R e a c t i v i t y . Proc., 5 t h I n t e r n a t i o n a l Conference on Alkali-Aggregate Reaction, Cape Town, S252/ 13, 1981, 12 p.

/4/ DENT GLASSER, L.S. and KATAOKA, N., Some Observations on t h e Rapid Chemical T e s t f o r P o t e n t i a l l y R e a c t i v e Aggregate. Cement and Concrete Research,

11,

1981, 191-196.

(13)

/5/ BRANDT, M.P., OBERHOLSTER, R.E. and WESTRA, W.B., A Contribution to the

Determination of the Potential Alkali Reactivity of Tygerberg Formation

Aggregates. Proc., 5th International Conference on Alkali-Aggregate

Reaction, Cape Town, S,252/11, 1981, 6 p.

I61 MITROVIC, N. and DUCIC, V., Prilog, Zbornik, 1 MK-a, 10, 1981, 195-204.

/7/ JENSEN, A.D., CHATTERJI, S., CHRISTENSEN, P., THAULOW, N. and

GUDMUNDSSON, H., Studies of Alkali-Silica Reaction

-

Part 1, A Comparison

of Two Accelerated Methods. Cement and Concrete Research,

12,

1982,

641-647.

I81 ASTM C227-81, Potential Alkali Reactivity of Cement-Aggregate

Combinations (Mortar Bar Method).

Annual Book of ASTM Standards,

Part 14. American Society for Testing and Materials, 1916 Race Street,

Philadelphia, PA 19103, U.S.A.,

153-158.

/9/ GRATTAN-BELLEW, P.E., A Review of Test Methods for Alkali Expansivity of

Concrete Aggregates. Proc., 5th International Conference on

Alkali-Aggregate Reaction, Cape Town, S25219, 1981, 12 p.

1101 ASTM C33-82, Appendix XI. Methods for Evaluating Potential Reactivity of

an Aggregate (Para. 1.2, Method C289).

Annual Book of ASTM Standards,

Part 14. American Society for Testing and Materials, 1916 Race Street,

Philadelphia, PA 19103, U.S.A.,

13-22.

1111 KENNERLEY, R.A., ST. JOHN, D.A. and SMITH, L.M., A Review of Thirty Years

of Investigation of the Alkali-Aggregate Reaction in New Zealand.

Proc., 5th International Conference on Alkali-Aggregate Reaction,

Cape Town, S252112, 1981, 9 p.

1121 ENGINEER MANUAL EM 1110-2-2000, Engineering and Design Standard Practice

for Concrete, (Appendix B, 3C).

Department of the Army, Office of the

Chief of Engineers, Washington, D.C. 20314, 1971.

1131 CSA CAN3-A23.1-M77, Appendix B3, Tests for Alkali-Aggregate Reactivity

(3.4, ASTM Standard C227, Potential Alkali Reactivity of Cement-Aggregate

Combinations (mortar bar method).

3.5, CSA Test Method A23.2-14A,

Alkali-Aggregate Reaction, National Standard of Canada)

:

CAN3 AZ3.2-7 7,

Concrete Materials and Methods of Concrete Construction, Methods of Test

for Concrete. Canadian Standards Association, Rexdale Boulevard,

Rexdale, Ontario, Canada, 1977, 122-123.

1141 MING-SHU, Tang, SU-FEN, Han, and SHI-HUA, Zhen, A Rapid Method for

Identification of Alkali Reactivity of Aggregate. Cement and Concrete

Research,

2,

1983, 417-422.

1151 CSA A23.2-14A, Alkali-Aggregate Reaction (Concrete Prism Test).

National

Standards of Canada, CAN3 A23.1-M77, CAN3 A23.2-M77, Concrete Materials

and Methods of Concrete Construction. Methods of Test for Concrete.

Canadian Standards Association, Rexdale Boulevard, Rexdale, Ontario,

Canada, 1977, p. 183-185.

1161 BONZEL, Von J. and DAHMS, J. ,

Alkalireaktion im Beton, Herstellung

Verwendung 23, 11, S4951500 und H.12, S5971554, 1973.

1171 ASTM C586-69, Standard Test Method for Potential Alkali Reactivity of

Carbonate Rocks for Concrete Aggregates (Rock Cylinder Method).

Annual

Book of ASTM Standards, Part 14, Concrete and Mineral Aggregates.

American Society for Testing and Materials, 1916 Race Street,

I

Philadelphia, PA 19103, U.S.A.,

1982, 357-360.

1181 DUNCAN, M.A.G.,

SWENSON, E.G., GILLOTT,

J.E. and FORAN, M.R., Alkali-

Aggregate Reaction in Nova Scotia, Part I, Summary of a 5-Year Study.

Cement and Concrete Research,

2,

1973, 55-70.

I191 GRATTAN-BELLEW, P.E., Evaluation of Miniature Rock Prism Test for

Determining the Potential Alkali-Expansivity of Aggregates. Cement and

Concrete Research,

11,

1981, 699-711.

1201 KAZIMIR, J., MoZnosE Skratenia Sk613ky Rozpinania Dolomitovy'ch ~ornl'n

s

Alkaliami, Staveb. Cas. 25, c s. Veda Bratislava, 1977, 413-432.

(14)

1211 CSN 72 1162, Stanovenie Odolnostri Prirodnc$ho ~tavebnkho

I9me6a Proti

~ilicifikich. MDT 691.2.620.1

679.682.1, Ceskoslovenskti Stgtna Norma

Schvhle~:

24.10. 1981.

1221 ANSIIASTM 295-79, Standard Reconmended Practice for Petrographic

Examination of Aggregates for Concrete. Annual Book of ASTM Standards,

Part 14, Concrete and Mineral Aggregates. American Society for Testing

and Materials, 1916 Race Street, Philadelphia, PA 19103, U.S.A.,

1982

218-229.

1231 DOLAR-MANTUANI, L.M.M., Undulatory Extinction in Quartz Used for

Identifying Potentially Alkali-Reactive Rocks. Proc., 5th International

Conference on Alkali-Aggregate Reaction, Cape Town, S252136, 1981, 6 p.

1241 DEHILLS, N.S. and CORVALAN, J., Undulatory Extinction in Quartz Grains of

Some Chilean Granitic Rocks of Different Ages. Geological Society of

America, Bulletin 75, 1964, 363-366.

1251 DOLAR-MANTUANI, L.M.M..

Petrographic Aspects on Siliceous Alkali-Reactive

Aggregates. P;OC.,

~ ~ ~ ~ o s i u m - o n ~ ~ l k a l i ~ ~ ~ ~ r e ~ a t e

Reaction, Reykjavik,

1975.

p.

87-100.

1261 M A ~ R ;

K., Presentation at Symposium on Alkali-Aggregate Reactivity.

Annual Meeting of Transportation Research Board, Washington, D.C., 1976.

/27/ SWENSON, E

.G. and GILLOTT, J. E

.

,

Alkali-Carbonate Rock React

ion. Highway

(15)

F i g u r e 1 E f f e c t of g r a i n s i z e of q u a r t z on the a o u a t af d i s s o l v e d s i l i c a determined by t h e chemical method, ASTM C289

1 0 0 0 5 0 0 300 2

-

=

aoa 0 4 8 1 2 16 20 24 % OPAL I N QUARTZ S A N D l l . l L , ! l ,

- b

-

-

1

-

\ \

-

-

F i g u r e 2 P l o t showing v a r i a t i o n i n Sc and Rc/Sc determined by t h e chemical t e s t , ASTM C289, and l e n g t h change of m o r t a r b a r s of q u a r t z sand c o n t a i n i n g v a r i o u s amounts of o p a l E 6 0

-

\

-

3 0 '\ 1 5 -

-

-

-

5 0 m200 " 1 4 0 0 ' t "6 0 0 " 800 1 0 0 0 MEAN G R A l N S 1ZE, ,LL m 2 5 0 1 1 1 1 1 1 1 1 1 2 0 0

-

1 5 0

-

INNOCUOUS 1 0 0

-

t

a DELETERIOUS

4

D I S S O L V E D S I L I C A S c , m m o l l l

Figure

3

R e s u l t s of chemical t e s t , ASTM C289, f o r s o w r e a c t i v e and non-

.

r e a c t i v e South A f r i c a n aggregates. Non-reactive: No ( n o r i t e ) and

WS ( q u a r t z sand)

(16)

.- . . . Y e ? - . ; - . .

.

- -. -.- -

=-

-

- - . . .. . - -

- .

. : .

..-

- - 5 -

-

The Fare Bridges

Construction site visited during the Conference Excursion

Trykt I Vejdlrektoratet K~benhavn

(17)

T h i s p a p e r ,

w h i l e b e i n g d i s t r i b u t e d i n

r e p r i n t form by t h e D i v i s i o n of B u i l d i n g

R e s e a r c h ,

remains t h e c o p y r i g h t of

t h e

o r i g i n a l p u b l i s h e r .

It s h o u l d n o t be

r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e

p e r m i s s i o n of t h e p u b l i s h e r .

A

l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from

t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o

t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of

B u i l d i n g R e s e a r c h ,

N a t i o n a l R e s e a r c h

C o u n c i l

of

C a n a d a ,

O t t a w a ,

O n t a r i o ,

KIA

0R6.

Figure

Figure  3  R e s u l t s   of  chemical  t e s t ,   ASTM  C289,  f o r   s o w   r e a c t i v e   and  non-

Références

Documents relatifs

The opposite is found for plagioclase with low, very homogeneous Sm/ Nd (except for Millbillillie, which can be explained by the presence of microinclusions of opaque or

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB..

To understand the thermal quenching of their multimodal emission, we develop a coupled rate-equation model that includes the effect of carrier thermal escape from a quantum dot to

/ La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.. For

The first laser pulse produces postpulse 1D align- ment of the most polarizable axis of the molecule, at which point the second pulse produces a torque about the most polarizable

/ La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur. For

If noise is a concern, one can increase the number of baseline types used to visit each antenna, so each phase calibration is sampled multiple times using multiple baseline

ELECTRIC LIGHTING ENERGY SAVINGS FOR AN ON/OFF PHOTOCELL CONTROL – A COMPARATIVE SIMULATION STUDY USING DOE2.1 AND DAYSIM..