CALCINED BAUXITE AND OTHER ARTIFICIAL, POLISH-RESISTANT ... · 2.4 ~ Other bauxites and highly...
Transcript of CALCINED BAUXITE AND OTHER ARTIFICIAL, POLISH-RESISTANT ... · 2.4 ~ Other bauxites and highly...
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ROAD RESEARCH LABORATORY
Minis t ry of Transport
RRL REPORT LR84
- ~ . . ~
CALCINED BAUXITE AND OTHER ARTIFICIAL, POLISH-RESISTANT, ROADSTONES
by
J. G. James
CROWTHORNE
ROAD RESEARCH LABORATORY
1967
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CONTENTS
Abs t r ac t
1. Introduction
2. Calcined bauxite
2 . 1 2 . 2 2 . 3
2 .4
The Colnbrook By-pass Exper iment The Blackbushe Exper iment The d i sadvan tages of calcined Demera ra
bauxite Other bauxi tes and c lays
3. P o l i s h - r e s i s t a n t stones: some theoret ical models
3.1 3 . 2 3, 3.
3 .4
3 .5
E x t r e m e l y hard ma te r i a l s Cong lomera t ions o f s m a l l hard pa r t i c l e s D i s p e r s i o n s of hard p a r t i c l e s in a sof te r
m a t r i x M a t e r i a l s which f rac tu re in an i r r egu la r
manner V e s i c u l a r m a t e r i a l s
4. Natura l roads tones
4.1 Poss ib l e improvement by heating
5. Ar t i f i c i a l roads tones
5.1 C e r a m i c s 5.2 G l a s s e s 5.3 Waste p roduc t s 5 .4 Soluble s i l i c a t e s 5 .5 Hydraul ic cemen t s 5 .6 P l a s t i c s
6. Conclus ions
7. R e f e r e n c e s
Page
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7 8
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12 12 14 14 14 15
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r a • c t CROWN COPYRIGHT 1967 Ext s f rom the text may be reproduced
provided the source is acknowledged
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Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.
This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
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CALCINED BAUXITE AND OTHER ARTIFICIAL, POLISH-RESISTANT, ROADSTONES
ABSTRACT
Calcined bauxite f rom Guyana has been found to have an exceptionally high r e s i s t ance to pol ishing under t r a f f i c . It is, however, expensive and only l imi ted suppl ies of road - stone s izes are avai lable . This Repor t s u m m a r i z e s the r e su l t s obtained f rom road t r i a l s with it and d i s c u s s e s methods whereby other aggrega te s might be p roduced with s imi l a r o r still be t t e r p rope r t i e s at l e s s cost .
1. INTRODUCTION
As traffic continues to inc rease in volume and speed it is e s s e n t i a l to ensure that roads a re adequately r e s i s t a n t to skidding. One development toward this end has been the product ion of specia l bituminous binders which a re designed to erode away in a cont ro l led manner ensuring that the aggregate r e m a i n s exposed thus mainta in ing a des i rable depth of surface texture. Neve r the l e s s , it is equal ly important to ensure that the aggregate (which m a k e s up the g r e a t e r par t of any surfacing) is of a type :which does not pol i sh unduly under the action of t raff ic .
The dependence of the r e s i s t ance to skidding of a road su r face on the polishin~ c h a r a c t e r i s t i c s of the roads tone was s tudied by Shergold et al i and appara tus was devised which enabled the degree to which a given stone would polish On the road to be a s s e s s e d in the labora tory 2. The resul tant 'Pol ished Stone Value ' (P. S. V. ) was determined for a very la rge number of roads tones and Fig , 1 reproduces the resul t s .
Simultaneously Giles, Sabey et al we re inves t iga t ing the relat ionship between skidding accidents and the s l i p p e r i n e s s of road surfaces and a r r ived at the conclusion that Certain l i m i t s were d e s i r a b l e for the 'Sideway' Force Coefficient ' (S. F. C. ) on roads if the r i s k of skidding was to be reduced to an acceptably low level 3, 4.
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F r 0 m a c o r r e l a t i o n of t h e s e two l i ne s of r e s e a r c h the f ac t e m e r g e d t h a t few r o a d s t o n e s a r e a v a i l a b l e which will enab le the d e s i r a b l e S. F . C. s to be m a i n t a i n e d t h r o u g h o u t the y e a r at the m o s t d i f f icul t s i t e s i . e . o n s h a r p bends , s t e e p g r a d i e n t s , e tc . 5, 6.
M e a n w h i l e , it e m e r g e d tha t a c e r t a i n g r a d e of bauxi te , which h a d b e e n h e a t e d to a t e m p e r a t u r e of abou t 1 , 6 0 0 o c , and which w a s n o r m a l l y s o l d a s an a g g r e g a t e f o r r e f r a c t o r y f i r e b r i c k s , had a P. S .V . h i g h e r t h a n tha t of any n a t u r a l r o a d s t o n e ye t t e s t e d . It i s r e l a t i v e l y expens ive a n d s u p p l i e s a r e l i m i t e d but it i s now thought tha t new m a t e r i a l s wi th s i m i l a r o r even b e t t e r c h a r a c t e r i s t i c s migh t be m a d e in l a r g e q u a n t i t i e s a t l o w e r c o s t .
T h i s R e p o r t d e s c r i b e s the w o r k tha t h a s b e e n done with ca l c ined baux i t e , s u g g e s t s l i n e s w h i c h m i g h t be fo l lowed to p r o d u c e a l t e r n a t i v e s , a n d g i v e s the r e s u l t s of s o m e e x p l o r a t o r y work .
2. C A L C I N E D BAUXITE
2 . 1 The C o l n b r o o k B y - p a s s e x p e r i m e n t
In 1959 t r i a l a r e a s of s u r f a c e d r e s s i n g w e r e la id on the C o l n b r o o k B y - p a s s ( T r u n k Road A. 4) u s ing epoxy r e s i n a s the b i n d e r a n d v a r i o u s 1 / 8 in g r i t s a s the a g g r e g a t e . B e c a u s e the r e s i n had a g l a s s y t e x t u r e and w a s not e x p e c t e d to w e a t h e r away , it was c o n s i d e r e d d e s i r a b l e to u s e a g g r e g a t e s w h i c h w e r e a s p o l i s h - r e s i s t a n t a s p o s s i b l e . T w e l v e a g g r e g a t e s w e r e c o m p a r e d , s ix of which w e r e n a t u r a l r o a d s t o n e s and s i x w e r e m o r e expens iv e a r t i f i c i a l m a t e r i a l s s u c h a s s i l i c o n c a r b i d e a n d c o r u n d u m . One o f the a r t i f i c i a l a g g r e g a t e s s e l e c t e d f o r the t r i a l w a s D e m e r a r a baux i t e which had been c a l c i n e d to a h i g h t e m p e r a t u r e : it w a s only one q u a r t e r the p r i c e of c o r u n d u m but it w a s hoped tha t i t s h igh ly a l u m i n o u s cons t i t u t i on might give it s i m i l a r p r o p e r t i e s .
The s k i d - r e s i s t a n c e of the t r i a l a r e a s w a s m e a s u r e d r e g u l a r l y wi th the p e n d u l u m a p p a r a t u s and an i n t e r i m r e p o r t on the r e s u l t s i s s u e d in 1960 7. The e x p e r i m e n t cont inued f o r 4 y e a r s (unt i l t he r o a d w a s r e s u r f a c e d by the County a s p a r t of i t s r e g u l a r ~ m a i n t e n a n c e p r o g r a m m e ) and s o m e of the r e s u l t s ob ta ined o v e r t h i s p e r i o d a r e shown g r a p h i c a l l y in F i g . 2a . The c a l c i n e d baux i te w a s c o n s i d e r a b l y b e t t e r t h a n the b e s t n a t u r a l r o a d s t o n e u s e d in the t r i a l d e s p i t e the f a c t t h a t t h e l a t t e r had b e e n d e l i b e r a t e l y chosen to be r e p r e s e n t a t i v e of t he b e s t a v a i l a b l e . The P. S .V . of c a l c ined baux i t e
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P . _ J
was m e a s u r e d and found to be 75*, h i g h e r than tha t Of any n a t u r a l l y o c c u r r i n g s tone yet t e s t ed .
It was dec ided to obtain c o n f i r m a t o r y e v i d e n c e of i t s r o a d p e r f o r m a n c e in m o r e convent iona l t y p e s of s u r f a c i n g .
2 .2 The B lackbushe E x p e r i m e n t
In 1962 an e x p e r i m e n t was begun on T r u n k Road A. 30 a t B lackbushe , H a m p s h i r e 8 to c o m p a r e the p e r f o r m a n c e of a n u m b e r of r oads tone§ in b i t uminous s u r f a c i n g m i x t u r e s and in t a r a n d b i t u m e n s u r f a c e d r e s s i n g s : c a l c ined bauxi te w a s inc luded . The t e s t s e c t i o n s in th i s e x p e r i m e n t w e r e d e l i b e r a t e l y m a d e long enough to p e r m i t S . F . C . m e a s u r e m e n t s to be m a d e a s wel l a s p e n d u l u m m e a s u r e m e n t s . The r e s u l t s of the S. F . C . m e a s u r e m e n t s a r e shown in F i g s . 2b and 2c. Again the ca l c ined bauxi te h a s p roved to be c o n s i d e r a b l y b e t t e r t han the bes t n a t u r a l r o a d s t o n e .
2 .3 The d i s a d v a n t a g e s of D e m e r a r a c a l c i n e d b a u x i t e
The c r u s h i n g s t r e n g t h of the D e m e r a r a c a l c i n e d b a u x i t e = used in t h e s e e x p e r i m e n t s is r e l a t i v e l y low. It w a s f e a r e d at f i r s t tha t th i s migh t c a u s e it to w e a r away p r e m a t u r e l y but f r o m the e x p e r i e n c e ga ined so f a r th i s would not s e e m to be so. It h a s a v e r y good A g g r e g a t e A b r a s i o n Value and t h i s a p p a r e n t l y c o m p e n s a t e s f o r • the low c r u s h i n g s t r e n g t h to s o m e ex ten t in r e s i s t i n g t r a f f i c w e a r . However , it does have two d i s a d v a n t a g e s wh ich l i m i t i t s adop t i on g e n e r a l l y ; t he se a r e i t s high cos t r e l a t i v e to tha t of n a t u r a l s tone a n d i ts l imi t ed ava i l ab i l i t y .
The r a w bauxi te , a f o r m of c l ay , i s m i n e d and c a l c i n e d in Guyana: i t is then shipped a c r o s s the A t l a n t i c and i s a v a i l a b l e a t the d o c k s he re , a t about £15 p e r ton,, a s a m i x t u r e o f s m a l l l u m p s a n d d u s t , mainly the l a t t e r . A f t e r g r a d i n g by i n t e r m e d i a t e a g e n t s the p r i c e of s i n g l e - s i z e d m a t e r i a l r i s e s to about £30 p e r ton. The r e a s o n s f o r th is a r e p a r t l y the handl ing c o s t s and p a r t l y t he v e r y s m a l l y i e ld of m a t e r i a l in the s i z e s r e q u i r e d f o r r o a d w o r k i . e . 3 / 8 in and l a r g e r .
Mos t of the P. S .V . m e a s u r e m e n t s in t h i s w o r k w e r e m a d e b e f o r e 1965. In tha t y e a r the me thod of t e s t w a s a m e n d e d 2 w i th the r e s u l t tha t c u r r e n t v a l u e s a r e about 10 p e r c e n t l o w e r t h a n b e f o r e . Throughout th i s R e p o r t the v a l u e s h a v e b e e n a d j u s t e d a n d the r e v i s e d 1965 equ iva l en t s q u o t e d
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2 . 4 ~ O t h e r b a u x i t e s and h igh ly a l u m i n o u s c l a y s
The D e m e r a r a baux i t e , c a l c i n e d to about 1, 600°C, is p r o d u c e d a s an a g g r e g a t e f o r r e f r a c t o r y work and f o r tha t p u r p o s e the v e r y h igh a l u m i n a con ten t and the low a m o u n t s of d e l e t e r i o u s c o n s t i t u e n t s a r e c a r e f u l l y c o n t r o l l e d . F o r r o a d s t o n e p u r p o s e s it i s p o s s i b l e t h a t l e s s p u r e f o r m s of b a u x i t e might be adequa t e . Al though t h e r e a r e m a n y c h e a p e r b a u x i t e s and f i r e c l a y s on the m a r k e t , none of t h e m is a v a i l a b l e in a h ighly c a l c i n e d f o r m su i t ab le fo r i m m e d i a t e t e s t i n g a s r o a d s t o n e . Some of t h e s e g r a d e s , i m p o r t e d into the Uni ted K i n g d o m f o r a l u m i n i u m e x t r a c t i o n , h a v e high i ron c o n t e n t s and o t h e r s h a v e h igh s i l i c a c o n t e n t s ( see T a b l e 1). They cos t b e t w e e n £6 and £10 p e r ton.
A s a r e s u l t of s o m e e n q u i r i e s a few s a m p l e s of c a l c i n e d c l a y s w e r e o b t a i n e d and t e s t e d but , a s c a n be seen f r o m the r e s u l t s g iven in T a b l e 2, no th ing of v a l u e e m e r g e d . F u r t h e r w o r k c o v e r i n g a w ide r r a n g e of m a t e r i a l s , t e m p e r a t u r e s and t i m e s of hea t ing i s n e e d e d to e s t a b l i s h w h e t h e r good p o l i s h - r e s i s t a n t s t ones of adequa t e s t r e n g t h c a n be o b t a i n e d f r o m o t h e r b a u x i t e s o r c l a y s b y s i m p l e hea t ing .
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TABLE 1
Typical ana lyses of bauxite and o ther c l ay s
Type
Bauxite Bauxite Bauxite Bauxite Bauxite Bauxitic f i rec lay
Chinaclay F i rec lay F i rec lay F i rec lay F i r ec l ay F i r ec l ay Marl Ganis ter Ganis te r Dinas Quartzi te
Dis t r i c t of or igin
Guyana (Demerara)
Alumina
67
Constituents
Silica
3
I ron oxide
1.5
W a t e r
27 .5
O t h e r s
F r a n c e ( l ) Ghana N. I re land France(2)
Glenboig(1) St. Austel l Glenboig(2) Derby Ches te r Leeds Dowlais Staffordshire Sheffield La i s t e rdyke Glamorgan U.S.A.
60 57 52 52
40 40 34 36 28 20 21
16-30
1.5 1 1
.
1.5 i0 27
40 46 47 52 59 71 67
63-76 89 97 98 98
24 13
8 0 . 5
3 m
2 2 3 1 .5 1 .5 3 3 1 1 1
12 27 25 17.5
13 14 15 10 I0 6 7
5-I0
I 1.5 5 3
4
2
1.5 3 . 5 1-3 4 0 .5
Note: This Table was compiled f rom in format ion given by
Fox 9, Knibbs 10, Searle 11 and individua ! ;Suppliers
/ "
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TABLE 2
R e s u l t s of po l i sh ing t e s t s on some ca lc ined c l ays
• Type
Bauxi te (Guyana)
Bauxi te (Guyana)
Bauxi te (Ghana)
C h i n a c l a y (St. Aus te l l )
Baux i t i c f i r e c l a y (Glenboig)
F i r e c l a y (Glenboig)
A l u m i n i u m r e f i n e r y was t e
Stated t e m p e r a t u r e
of c a l c i n a t i o n oc
1, 600
1, 300
1, 550
Not known
1, 400
1, 320
1, 200
Stated typ ica l composi t ion
A lumina
88
88
75
43
40
35
8
Si l ica
7
7
2
54
50
57
Iron Oxide
2
2
20
4
3
21
Others
3
3
3
2
6
5
68 (Mainly CaO)
Polished stone value
70-78*
Too weak to test
Too weak to tes t
41
51
44
55
* Note: It i s p o s s i b l e to so r t D e m e r a r a Calcined bauxite into f r ac t ions with n o t i c e a b l y d i f f e r en t appea rance . Tes t ing these d i f ferent f r a c t i o n s p roduced the range of r e s u l t s quoted. The P. S.V. of the m i x e d m a t e r i a l a s suppl ied i s about 75
3. POLISH-RESISTANT STONES : SOME THEORETICAL MODELS
The d i s c o v e r y that an agg rega t e p roduced for the r e f r a c t o r y indus t ry is of va lue as a n o n - p o l i s h i n g m a t e r i a l in ano ther indus t ry unde r l ines the r e c u r r e n t ques t ion , "What a r e the f a c t o r s which cause some m a t e r i a l s to r e m a i n n o n - s k i d ? " . If they could be i so la ted then it should be p o s s i b l e to p r o d u c e d e l i b e r a t e l y s tones with the d e s i r e d c h a r a c t e r i s t i c s .
F r o m c e r t a i n known e x a m p l e s it i s p o s s i b l e to outline some idea l ized t y p e s of m a t e r i a l which would be l i k e l y to p e r f o r m well. F ive of these a r e i l l u s t r a t e d in F ig . 3.
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3.1 E x t r e m e l y h a r d m a t e r i a l s ( F i g . 3 . 1 )
The s i m p l e s t concep t of a n o n - p o l i s h i n g s tone i s a n a n g u l a r shape with a s h a r p point u p p e r m o s t c o n s i s t i n g of a u n i f o r m m a t e r i a l of e x t r e m e h a r d n e s s . In th i s con tex t the w o r d h a r d i s u s e d in the s e n s e e x p r e s s e d by M o h ' s Sca le : it m u s t not be c o n f u s e d with h igh r e s i s t a n c e to c r u s h i n g which i s an unconnec ted p r o p e r t y . F o r i n s t a n c e , m a t e r i a l s such a s fused a l u m i n a and s i l i con c a r b i d e c o n s i s t of a m a s s of c r y s t a l s which a r e indiv idual ly v e r y h a r d but a r e w e a k l y bonded t o g e t h e r and thus have a l ow c r u s h i n g s t r e n g t h . On the o t h e r hand s o m e s t o n e s wi th high c r u s h i n g s t r eng th , such a s h o r n f e l s , m a y p o l i s h r e a d i l y on the road .
Table 3 l i s t s a n u m b e r of m i n e r a l s and t h e i r h a r d n e s s on Moh ' s Sca le t o g e t h e r with t h e i r Knoop H a r d n e s s . The Knoop T e s t invo lves indenta t ion by a s p e c i a l l y cut d i a m o n d and e n a b l e s m o r e a c c u r a t e g r a d u a t i o n to be obta ined .
The h a r d e s t known m a t e r i a l is d i a m o n d . Undoubtedl .y, d i a m o n d s with su i t ab ly cut f a c e t s c o r r e c t l y o r i e n t a t e d would m a k e v e r y good r o a d s t o n e s if they w e r e r e a d i l y a v a i l a b l e in l a r g e s i z e s . The next h a r d e s t m a t e r i a l s in c o m m o n u s e a r e the a r t i f i c i a l a b r a s i v e s m a d e in an e l e c t r i c f u r n a c e , s i l i con c a r b i d e and a l u m i n i u m oxide (corundum) . C o m p a r e d with d i amond they a r e r e l a t i v e l y c h e a p a t about £100 p e r ton but they a r e s t i l l too e x p e n s i v e f o r r o a d u s e . T h e y have a n o t h e r d i s a d v a n t a g e , in c o m m o n wi th d i a m o n d , t ha t t h e y a r e only n o r m a l l y a v a i l a b l e in g r a i n s i z e s up to abou t 1 / 8 in.
F u r t h e r down the s c a l e c o m e the c h e a p e r a b r a s i v e s such a s e m e r y (a f o r m of c o r u n d u m con ta in ing i ron) and g a r n e t w h i c h o c c u r n a t u r a l l y in some p a r t s of the wor ld . T h e i r c o s t i s s t i l l high, h o w e v e r , c o m p a r e d to that of r o a d s t o n e s and they a r e a l s o only a v a i l a b l e i n s m a l l g r a i n s i ze s .
F ina l ly we c o m e to the m i n e r a l s of wh ich m o s t n a t u r a l r o a d s t o n e s a r e c o m p o s e d . T h e s e a r e not g e n e r a l l y h a r d enough to m a i n t a i n s h a r p e d g e s u n d e r the po l i sh ing wh ich o c c u r s on the r o a d .
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TABLE 3
H a r d n e s s of v a r i o u s a b r a s i v e s and m i n e r a l s
M i n e r a l
D i a m o n d Boron c a r b i d e S i l i con c a r b i d e A l u m i n i u m oxide ( co rundum) T u n g s t e n c a r b i d e Topaz S i l l i m a n i t e C o r d i e r i t e Q u a r t z G a r n e t g roup O l i v i n e g roup Ep ido t e g roup C h a l c e d o n y F e l d s p a r ( o r t h o c l a s e ) P y r o x e n e group A m p h i b o l e g roup Apa t i t e Ze01 i t e s F l u o r i t e D o l o m i t e C a l c i t e G y p s u m T a l c
H a r d n e s s
M o h ' s Scale
10
9
8 7 .5
7 - 7 . 5 7
6 . 5 - 7 . 5 6 . 5 - 7
6-7 6
6 - 6 . 5 5-7 4 - 6 . 5
5 3 . 5 - 5 . 5
4 3 . 5 - 4
3 2 1
I~mop n u m b e r
7, 000 2 ,900 2 ,500 2 ,000 1, 880
1, 400-1, 500
800-900
600-700
500
200
130 5O 2O
Note: T h i s T a b l e was c o m p i l e d f r o m i n f o r m a t i o n g iven by T h i b a u l t and N y q u i s t 12, K n i l l l 3 and Gi l l and Spence 14
3 . 2 C o n g l o m e r a t i o n s of s m a l l h a r d p a r t i c l e s (Fig. 3 .2 )
R e f e r e n c e h a s a l r e a d y been m a d e to two m i n e r a l s of t h i s type , f u s e d a l u m i n a and s i l i c o n c a r b i d e . As ob ta ined f r o m the e l e c t r i c f u r n a c e t h e y c o n s i s t of m a s s e s of s m a l l c r y s t a l s but they a r e i n s u f f i c i e n t l y c o h e r e n t fo r u s e a s r o a d s t o n e s . When u s e d a s a b r a s i v e s t h e y a r e b r o k e n down into s m a l l p a r t i c l e s which a r e bonded toge the r w i th s y n t h e t i c r e s i n s to m a k e g r i n d s t o n e s . Subs tan t ia l q u a n t i t i e s of b r o k e n g r i n d s t o n e s a r e a v a i l a b l e and w e r e t r i e d a s an a g g r e g a t e in the C o l n b r o o k B y - p a s s e x p e r i m e n t . However , the r e s i n o u s bond was too weak and the m a t e r i a l r a p i d l y w o r e away .
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N a t u r a l l y o c c u r r i n g e x a m p l e s which m a y be c i t e d a r e s a n d s t o n e s and g r i t s t o n e s . In t h e s e the ind iv idua l p a r t i c l e s c o n s i s t of m i n e r a l s of i n t e r m e d i a t e h a r d n e s s , such a s q u a r t z , which a r e bonded t o g e t h e r by a so f t e r s i l i c eous o r c a l c a r e o u s m a t e r i a l . Q u a r t z i s not a s h a r d a s the a b r a s i v e s , but p r o v i d i n g the bond i s weak enough to a l low s low a t t r i t i on to o c c u r u n d e r t r a f f i c , such s t o n e s c a n m a i n t a i n a h a r s h t e x t u r e b e c a u s e of the cont inua l e x p o s u r e of f r e s h p a r t i c l e s . C l e a r l y , howeve r , the bonding m e d i u m m u s t be n e i t h e r too w e a k (or the s tone wil l a b r a d e too r ap id ly ) no r too s t r o n g (or the s tone wil l po l i sh ) . As is to be expec ted , s a n d s t o n e s and g r i t s t o n e s a r e found to r a n g e f r o m one e x t r e m e to the o t h e r 15. In the m i d d l e i s a g r o u p of good p o l i s h - r e s i s t a n t s t ones su i t ab l e fo r r o a d u s e but the n u m b e r of t y p e s i s s m a l l .
3 .3 D i s p e r s i o n s of h a r d p a r t i c l e s in a s o f t e r m a t r i x (F ig . 3 . 3 )
In th i s type the binding m a t e r i a l p r e d o m i n a t e s o v e r t he a b r a s i v e p a r t i c l e s . Idea l ly the p a r t i c l e s would be v e r y h a r d but , p rov id ing tha t the m o r t a r e roded away at a s low r a t e e x p o s i n g f r e s h p a r t i c l e s occas iona l l y , the g r i t would not need to be of e x t r e m e h a r d n e s s . I m p e r f e c t e x a m p l e s of th i s c l a s s a r e the r a g s t o n e s o r g r i t t y l i m e s t o n e s : The m a j o r i t y of l i m e s t o n e s po l i sh r e a d i l y but t h e r e a r e s o m e wh ich conta in a s m a l l p e r c e n t a g e of s i l i c e o u s g r i t (up to about 15 p e r cent ) and t h e s e have m a r k e d l y b e t t e r P. S. V. s ( see F ig . 1). On the o t h e r hand, in c o m m o n with m o s t l i m e s t o n e s they tend to have low a b r a s i o n r e s i s t a n c e and a r e not r e a l l y su i t ab le f o r h e a v y duty .
3 . 4 M a t e r i a l s which f r a c t u r e in an i r r e g u l a r m a n n e r (F ig . 3 . 4 )
Some s tones tend to f lake in such a m a n n e r t h a t t h e i r edges round off and they b e c o m e smoo th . F l i n t s , f o r i n s t a n c e , h a v e low P . S . V s . When f l in t s have been c a l c i n e d to a b o u t 1, 000°C, h o w e v e r , t h e i r cons t i tuen t s i l i ca u n d e r g o e s a c h a n g e f r o m a type w h i c h i s f o r p r a c t i c a l p u r p o s e s a m o r p h o u s to a c r y s t a l l i n e type and t h e y b e c o m e r idd led with i r r e g u l a r c r a c k s and weak p l a n e s . The P. S . V . r i s e s c o n s i d e r a b l y in the p r o c e s s (see F ig . 1). T h i s m a y be due in p a r t to a change in the h a r d n e s s of the s i l i c a but it i s p r o b a b l y a l s o due l a r g e l y to the new mode of f r a c t u r e which it adop t s , the f o r m e r conchoida l f lak ing giving way to i r r e g u l a r c r y s t a l l i n e b r e a k s . The c r u s h i n g s t r e n g t h is a d v e r s e l y a f f ec t ed but not s e v e r e l y enough to p r e v e n t i t s use as a r o a d s t o n e .
3 . 5 V e s i c u l a r m a t e r i a l s (Fig. 3 .5 )
V e s i c u l a r r o c k s a r e t h o s e c o n t a i n i n g n u m e r o u s s m a l l c a v i t i e s and m a y be exempl i f i ed by c e r t a i n b l a s t - f u r n a c e s l a g s . The p r e s e n c e of a high p r o p o r t i o n of v e s i c u l a r s l ag in a b a t c h o f t en i m p r o v e s i t s P . S . V . but r e d u c e s i t s c r u s h i n g s t r e n g t h . In p r a c t i c e , the l a t t e r d i s a d v a n t a g e ou twe ighs the i m p r o v e m e n t and t h e r e h a s b e e n a t endency for v e s i c u l a r s l a g s to d i s a p p e a r f r o m the r o a d s t o n e f i e ld in r e c e n t y e a r s .
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4. NATURAL ROADSTONE S
Most n a t u r a l l y o c c u r r i n g roads tones conta in a mix tu re of s eve ra l m i n e r a l s and m a y be c l a s s i f i e d as compounds of types 3.2 and 3 .3 . Unfo r tuna te ly the m i n e r a l s of which they a re composed tend to be of too s i m i l a r h a r d n e s s to each other for d i f f e ren t i a l wear to take place , o r of too f ine a g r a i n s ize for such wea r to produce a h a r s h texture , o r of i n s u f f i c i e n t h a r d n e s s to r e s i s t po l i sh ing by road dust and t y r e s . A de t a i l ed p e t r o g r a p h i c a l examina t ion of na tu ra l rocks with high P. S. Vs. was m a d e by Kni l l 13.
Befo re t u r n i n g to a r t i f i c i a l s tones it i s worth cons ide r ing one way in w h i c h n a t u r a l s tones might be m o d i f i e d to improve the i r p o l i s h r e s i s t a n c e .
4.1 P o s s i b l e i m p r o v e m e n t by heat ing
The change obta ined in f l in t by ca lc in ing has a l r eady been noted. The P. S .V. r i s e s f r o m about 35 to 55 although the c rush ing s t r eng th d rops . It i s p robab le that mos t rocks will undergo a s i m i l a r change with hea t al though g e n e r a l l y to a l e s s e r extent.
An inves t i ga t i on into the effect of heat on the c rush ing s t r e n g t h of r o c k s was made at the L a b o r a t o r y some y e a r s ago as pa r t of a study on a g g r e g a t e - d r y i n g p r o c e s s e s 16. Table 4 r ep roduces some of the r e s u l t s and it can be seen that a l l of the rocks tes ted were a f f ec t ed by hea t ing to 1 ,000°C for two minu tes .
It i s p roposed to c a r r y out f u r t h e r work to see what the c o r r e s p o n d i n g ef fec t is on the P. S. V.
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TABLE 4
Resu l t s of c rush ing and impact t e s t s on a g g r e g a t e s before and a f te r hea t ing to 1 ,000°C for 2 m i n u t e s
Aggregate type
C r u s h i n g : t e s t r esu l t ~
Imp'act - t es t re sult*
Before Af te r Before Af te r
6 .7 37 .4 19..6 16.1 16.5
Hornfels 5.9 Granite 23.3 Fl int g rave l 7 .4 Slag 19.1 Quar tz -do le r i t e 13.5
10. 12.
7. 9.
11. 10.
7.
37. 15. 21. 19.
1 4. 4 21. 9 9. 7 13. 0 I I .
6 7 7 2 0
11.3 14.2
1 8 . 6 19 .0 17.5 15.2
Basalt Sandstone Granophyre Ol iv ine-do le r i t e L imes tone Quartzi te
4 11. 0 13. 9 18. 2 20. 7 23. 9 14.
4 6 9 8 2 1
8 . 3 12.7
8.3 8 .8
10.3 13 .0
The tes t s were c a r r i e d out in a c c o r d a n c e with the me thods desc r ibed in B.S. 812. The v a l u e s r e p r e s e n t the p e r c e n t a g e
1 of f ines formed when a s tandard sample of ~ inch ch ipp ings is c rushed or impac ted under s t andard condi t ions : thus the h igher the value the weaker the stone
5. ARTIFICIAL ROADSTONES
In setting out to manufac tu re a non-pol i sh ing stone the mos t p romis ing s ta r t ing-poin t would appear to l ie be tween Types 3 .2 and 3.3. For the g r a n u l a r p a r t i c l e s in e i t he r type t h e r e i s a var ie ty of ab ras ive g r i t s ava i lab le the most a t t r a c t i v e of which i s probably the compara t ive ly plentiful , s m a l l - s i z e d , c a l c ined bauxi te . Because the gri t is l ike ly to be more expens ive than the m a t r i x or bonding medium, Type 3.3 (in which the l a t t e r p r e d o m i n a t e s ) i s p r e f e r r e d . The r e m a i n i n g dec is ion i s the choice of the bes t m a t r i x . It must be cheap, ea s i l y compounded with the gr i t , and have a s trength such that it w e a r s away u n d e r t ra f f ic at the op t imum ra te . Several poss ib le m a t e r i a l s a re d i s c u s s e d below and e x a m i n a t i o n of some of them has a l r e ady begun.
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5.1 C e r a m i c s
C e r a m i c p r o c e s s e s a r e one obvious choice. Clay is p l en t i fu l and cheap and, by va ry ing the heating p r o c e s s and the added f luxes , it i s p o s s i b l e to p roduce m a t e r i a l s ranging f r o m weak porous b r i c k s to dense p o r c e l a i n s with a c ru sh ing s t rength of 50, 000 l b s / sq. in. By us ing c h i n a c l a y s white aggrega te s can be produced which have addi t ional advan tages in some s i tua t ions .
At the beg inning of th is work it was r e a l i s e d that f i r e b r i c k s have a somewhat s i m i l a r compos i t ion to the theore t i ca l ideal: they a r e a m i x t u r e of v a r i a b l e p a r t i c l e s and f requent ly contain calc ined baux i te . Samples were obtained f rom va r ious sou rces and tested with the r e s u l t s shown in Table 5.
As one would expect with b r i ck s , the c rush ing s t rengths tend to be low, i but some v e r y high P. S. Vs. were found which conf i rm the va lue of t h i s l ine of r e s e a r c h . No fu r the r work is being c a r r i e d out on f i r e b r i c k s because of the high p r i c e of the bes t examples (up to £40 p e r ton) and the r e l a t i v e l y low s t rengths , but r e s e a r c h is p lanned on spec i a l l y compounded m i x t u r e s .
5 .2 G l a s s e s
F o r a n u m b e r of y e a r s now an a r t i f i c i a l aggregate cons is t ing of a f o r m of d e v i t r i f i e d g l a s s has been marke ted . Th i s ma te r i a l , "Synopal" , i s p roduced p r i m a r i l y as a white aggregate for use where l igh t colour i s r equ i r ed . Samples tes ted at the Labora to ry have had an ind i f f e r en t P. S.V. (40 to 49) but it i s p robab le that by adding a sui table a b r a s i v e gr i t to it du r ing manufac tu re th i s could be r emed ied . The m a n u f a c t u r i n g p r o c e s s is fu l ly d e s c r i b e d in the i n v e n t o r ' s patent 17.
Synopal i s of g rea t i n t e r e s t a s it has demons t ra t ed that the produc t ion of a r e a s o n a b l y p r i c e d roads tone i s a c o m m e r c i a l p r a c t i c a l i t y . Although the manu fac tu r e involves a double heating p r o c e s s and the p lant cos t s £500, 0 0 0 1 8 , s eve r a l p l an t s have been bu i l t and the aggrega te i s sold e x - w o r k s at about £6 p e r ton.
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TABLE 5
Resu l t s of t es t s for pol ishing, c rush ing s trength, and abras ion r e s i s t ance on some c rushed f i r e b r i c k s
Type of br ick
t t , t
, , , !
t , , t
, t , !
Mossi te br ick
Silica br ick (unidentified) Scottish f i r ec l ay br ick ("Nettle 1")
" " " ("Dougall") Sil ica br ick (Pearson Kaolith 1500) Silica br ick (Morgan HT Chinaclay) Scottish f i r ec l ay br ick ("Nettle 2") Bauxite br ick (Stein 70)
" " (Stein 73) (Stein 84P) (Alumantine 55) (Alumantine 63) (Alumantine 72)
Calcined D e m e r a r a bauxite for compar i son
Po l i shed Stone Value
72 67 63 60 58 58 83 78 87 84 88 87 72
75
C rush ing Strength
( l b s / s q . in)
8 ,800 10, 500 2 ,500 8,500 5, 500
26,100 10, 000 13,000
1 1 , 0 0 0
Aggrega te Abrasion:
Value (lowest bes t )
21 5
19 12
8 12 12
6 8
36 38
5 14
4
Note: For compar i son of these va lues with those of na tu ra l roads tones see Fig. 1 and R. R . L . Road Note No. 24 15
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5 .3 W a s t e P r o d u c t s
T h e r e a r e m a n y w a s t e m a t e r i a l s or cheap b y - p r o d u c t s f r o m min ing and o the r i n d u s t r i a l p r o c e s s e s which migh t be t r e a t e d to m a k e su i t ab l e m a t r i c e s .
M a t e r i a l s of ou t s tand ing po ten t i a l a r e the s l a g s ob ta ined f r o m m e t a l - e x t r a c t i o n p r o c e s s e s which a r e p r o d u c e d in an a l r e a d y m o l t e n f o r m and would t hus r e q u i r e no f u r t h e r heat ing .
L iqu id s l a g s a r e not ob ta ined f r o m the e x t r a c t i o n of a l u m i n i u m , but m i l l i o n s of tons of w a s t e in the f o r m of a c a l c a r e o u s mud a r e ob ta ined each y e a r and a r e at p r e s e n t dumped. Until r e c e n t l y a s i m i l a r s i tua t ion ex i s t ed with the fly a s h obta ined f r o m l a r g e m o d e r n f u r n a c e s , but s e v e r a l u s e s f o r th is have now been found, one of which is in the m a n u f a c t u r e of l igh tweight a g g r e g a t e s . Mos t m a t e r i a l s of th i s type a r e c a p a b l e of being s i n t e r e d to f o r m a g g r e g a t e s and, given su i t ab le t r e a t m e n t migh t wel l y i e l d s u c c e s s f u l r o a d s t o n e s .
5 . 4 Soluble s i l i c a t e s
One of the f i r s t u s e s f o r sod ium s i l i ca t e ( " w a t e r g l a s s " ) when it w a s c o m m e r c i a l l y deve loped in the e a r l y n ine teen th c e n t u r y w a s in the m a n u f a c t u r e of a r t i f i c i a l s a n d s t o n e s fo r bui lding p u r p o s e s . The a r t c o n s i s t e d of m ix ing su i t ab le f ine a g g r e g a t e s and f i l l e r s with ~he s i l i c a t e and then c o n v e r t i n g the l a t t e r to a w a t e r insoluble m a t e r i a l by the u s e of v a r i o u s a g e n t s a n d / o r heat . Soluble s i l i c a t e s w e r e l a t e r o u s t e d a s b i n d e r s fo r a r t i f i c i a l bui ld ing s t ones by P o r t l a n d c e m e n t which w a s m o r e r e l i a b l e . Sodium s i l i ca te , howeve r , is a c h e a p m a t e r i a l a n d m a y j u s t i f y r e - e x a m i n a t i o n a s it p r o v i d e s a p r o c e s s not involv ing h igh t e m p e r a t u r e s ; t h i s m e a n s tha t if co lou red a g g r e g a t e s w e r e r e q u i r e d c h e a p e r p i g m e n t s than those r e q u i r e d fo r c e r a m i c p r o c e s s e s cot/ld be u s e d . F u r t h e r m o r e , with p r o c e s s e s which can be c a r r i e d out at t e m p e r a t u r e s be low the m e l t i n g point of g l a s s ,
~ t m i g h t be p o s s i b l e to i n c o r p o r a t e ba l lo t in i in the m i x t u r e and thus p r o d u c e a r e t r o r e f l e c t i v e a g g r e g a t e .
5 . 5 H y d r a u l i c c e m e n t s
A r t i f i c i a l bu i ld ing s t o n e s a r e s t i l l m a n u f a c t u r e d with P o r t l a n d c e m e n t a s the b inding m e d i u m . It is doubtful , however , a r o a d s t o n e could be p r o d u c e d by th i s m e a n s which would have a d e q u a t e s t r e n g t h and a b r a s i o n r e s i s t a n c e .
if
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5.6 Plastics
Pigmented p l a s t i c s have been used to make g r a n u l e s fo r rol l ing into the sur face of asphal t (19). The cost of p l a s t i c s (£100- £500 pe r ton) is too high to make them the b a s i s of r o a d s t o n e s fo r more widespread use. However, if in the fu ture l a rge quan t i t i e s of was te p l a s t i c s become ava i l ab le at lower p r i c e s , they would have to be recons idered .
6. CONCLUSIONS
D e m e r a r a bauxite ca lc ined to about 1 ,600oc has been found to have a P. S.V. be t te r than that of any n a t u r a l l y - o c c u r r i n g roads tone so f a r tested. However, it i s r e l a t i ve ly expensive , p a r t i c l e s l a r g e r than 1 i n a re in short supply, and it has a r e l a t i v e l y low c r u s h i n g strength.
Cons idera t ion of the f ac to r s which affect the P. S.V. l e a d s to the conclusion that it should be pos s ib l e to manu fac tu r e a g g r e g a t e s with s t i l l h igher P. S. Vs . , with h i g h e r s t r eng th s if n e c e s s a r y , probably at a lower p r i c e . The most p r o m i s i n g app roach i s to incorporate a suitable h a r d gr i t in a somewhat so f te r m a t r i x . Work has a l r eady begun to inves t iga te the i nco rpo ra t ion of f ine c a l c i n e d - bauxite gr i t into va r ious m a t r i c e s such a s b l a s t - f u r n a c e s lag and c e r a m i c s .
It i s a l so poss ib le that the P. S.V. of na tu ra l r o a d s t o n e s m i g h t be improved by heat t r e a t m e n t . Work ha s been s t a r t ed to see if a s ignif icant improvemen t can be ach ieved without p roduc ing any harmfu l s ide -e f fec t s such as grea t r educ t ion in c r u s h i n g s t rength .
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.
.
3.(a)
(b)
.
.
.
.
.
,
10.
7. REFERENCES
MACLEAN D. J . and F. A. SHERGOLD. The polishing of r0adstone in re la t ion to the res i s tance to skidding of bituminous road surfacings;~ Road Research Labora tory Technical Paper No. 43, H . M . S . O . , London 1958
BRITISH STANDARDS INSTITUTION. Bri t ish Standard 812:1960. Methods for sampling and test ing of mineral aggregates ; and Amendment No. 3 Revised determinat ion of the Polished Stone Value; London, 1965, Bri t i sh Standards Institution.
ROAD RESEARCH LABORATORY. Research on road safety (Chapter 14, The road surface); H . M . S . O . , London, 1963
GILES, C. G. The skidding res i s tance of roads and the r equ i r emen t s of modern t raff ic; Proc. Inst. Civ. Engrs, 6, 216-249
1957,
SHERGOLD, F. A. Summary of information on the S. F. C. va lues achieved with s tones of known P. S. V. , Road Research Labora to ry Technical Note No. 22, !965, unpublished.
HOPKINS, L. C. The p rob lems of providing aggregates for road surfacing m a t e r i a l s which have good non-skid proper t ies ; Journ. Inst. Highw. Engrs , 1959, 6, 203-212.
HOSKING, J. R. An es t imat ion of the requi red and available tonnages of roadstone for Category A and B sites; Road Research Labora to ry Technical Note No. 31, 1965 (unpublished).
JAMES, J . G. A s m a l l - s c a l e road experime nt to compare va r ious aggrega te s for use in r e s in -based surface dress ings : Road Resea rch Labora to ry Resea rch Note No. RN/3843, 1960 (unpublished).
HOSKING, J. R. An exper iment comparing the per formance of roads tones in d i f ferent bi tuminous surfacings; A. 30 Blackbushe, Hampshi re (An In te r im Report); Road Research Laboratory Report in the p r e s s .
FOX, C. Bauxite; Crosby Lockwood, London 1927.
KNIBBS, N. V. S. Indust r ia l u s e s of bauxite: Benn & Sons Ltd. , London, 1928.
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11.
12.
13.
14.
15.
16.
17 ̧"
18.
19.
SEARLE, A. B. Re f r ac to ry m a t e r i a l s ; Griff in & Co. L t d . , London, 1950 (3rd edition).
THIBAULT, N. W. and HELEN L. NYQUIST. The m e a s u r e d Knoop Hardness of hard subs tances ; T r a n s . Amer . Soc. Meta ls , 1947, 38, 271-330.
KNILL, DIANE C. Pe t rog raph i ca l a s p e c t s of the pol i sh ing of na tura l roads tones ; J . appl C h e m , 1960, 10 (pt. 1), 28-35
GILL, R. M. and G. SPENCE. Wear r e s i s t a n t c e r a m i c s ; New Scient is t , 1962, 29 I , 574-577
HOSKING, J. R. Roadstone t e s t data in t abu la r fo rm; Road R e s e a r c h Labora to ry Road Note No. 24, H . M . S . O . London 1959.
SHERGOLD, F. A. The effect of high t e m p e r a t u r e on the s t rength of roadmaking aggrega tes ; Rds. and Rd. Cons t r . 1953, 31 (366), 161-163.
KROYER, K. K. Br i t i sh Patent No.795787, Pa t en t Office, London, 1958.
JORGENSON, S. F. Syntet iske v e j m a t e r i a l e r (Synthetic r o a d ma te r i a l s ) ; Ingeniorens Ugeblad, 1962, 32 (August 11), 1 6 - 1 7 .
FARBWERKE HOECHST AG., B r i t i sh Pa ten t No. 975223, Patent Office, London, 1964.
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5
0 10
5
0
F (Stag) .(Calcined bauxite) I ~ Artificial group
F I i i i Basalt group
5 F (Normal f l i n t )
0
(Calcined f l int)
I I ~ I ' ' Flint group
sl-.. 0 I i r / / / / i A , I Gabbro group
~ \ \ \ \ \ \ ~ l l l l l / J k \ \ \ \ \ ~ I
I i I
1 Granite group
J Gritstone group
Ul I I e 0 o.
E 10 f "6 9
~ I I ,~ 0 E
z s F 0 I . . . . . . . "~'/"}'}'}~---'~',,'i I I I I I I Hornfels group
15 [- . (Normal l imestones)
( Gr i t ty l imestones) V / f / f / A ~ J f / f i 4
I I I
1of 5
0
1 Limestone group
I Porphyry group
0 i , r . . / / /H ,~ . . . . . . . . . . , FY / / / / } ~ \ \ \ ' ~E / / / / / / J I i i Quartzite group 25 30 35 40 45 50 55 60 65 70 ?5
Polished stone value
Fig1 DISTRIBUTION OF POLISHED STONE VALUES IN DIFFERENT GROUPS OF ROCK
Note: this table is largely based on Fig. 14 of Technical Paper 43 published in 1958. Since 1%5, however, the method of determining the P.S.V. has been modified and values obtained by the new method are I0 per cent lower than formerly.
as def in i t ive.
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o .
(/)
(/3
90, L I I I I I
7 0 1 ~ ~ -- ~ . , . . , . t , , . ~ - " i Calcined bauxite
5ol1 . ' ( ~ ~ ~ ' ~ 7 ~ ' ~ ] 6 Nat ura l roads tones , F V ~ X ~ ~ (/Gritstone best,
: , , J \Li:estone:orstJ
0 1 2 3 4 5 6 7 (June t959)
A g e ( y e a r s ) (a)Epoxy-resin surface dressings with variousl/s-in~'aggregates
0-9 O) u
"~ 0.7 U
u
o 0.5 o
~ 0.3
Trunk Road A6 Colnbrook By-pass .... . ~ . .
I I I .I I %
"11,~ ~. : - C a l c i n e d bauxite
NoturQ roo ,on.s (Gr i tst0ne best, \G ranophy re wors t , /
I I I I I I 0 1 2 3- 4 5 6
(sept 1962) Age (years)
(b)Tar surface dressings with various 3/8-in. aggrega.tes " Trunk RoadA30 at Blackbushe
C
u
0 u
u
b ~ , 0.5 o
" 0
b~ 0-3
0.9~ I ( ( ( . i
0.? .
6 Natural roadstones Gr i ts tone best, '~ Granophyre worst, , /
I I I I I I 0 1 2 3 z, S 6 ?
(Sept 1962) . A g e ( y e a r s ) i
(c)Ro[[ed asphalts with vorious-t/2-in.aggregates ro[led into the s u r f a c e
Trunk Rood A30 at B[ackbushe
Fig.2. SKID-RESISTANCE AND SIDEWAY FORCE COEFFICIENT MEASUREMENTS ON VARIOUS SURFACINGS MADE WITH
CALCINED BAUXITE AND OTHER AGGREGATES
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i i i !iii i!i! i i i i i ! i~
3.1 Very hard m a t e r i a l s
3.2 C o n g l o m e r a t i o n s of s m a l l ha rd p a r t i c l e s .
. . - ~
• (~..-~!~.. • . . ~ , ~ . ~ . ' - - . . . . . . . . ~ : . ~
. . . . . o~'-'-" ~': , 1..,,,,)" • .F. ":. "..... C~C~..:..~. .~...: ='. :':i~:~
~i~ ..... ~ ............................. ~..: : .~ ".,-'.~ , ; :- ~ " ~ , ~ - , , . L : ' : : , ~ : ~ : - ' : ~
. . . . . . . . . : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : , : , . ~,~. ~ % . ~ ' / ~ . .~:"1 ' : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : . . . . . . . . . . . . . . . . . . . . . . . . . . ::::::::::::::::::::::::::: - - " " ~ ' ~ ' ~ . : ' . : : . . . 1 " " ' ~ 1 : : " : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : . ~ [ ° • . . ~ , . . . • ~, - . ===========================================
3.3 D ispers ions of hard pa r t i c l es in a s o f t e r m a t r i x .
3.4 M a t e r i a l s w h i c h f r ac tu re in an i r r e g u l a r a n g u l a r manner•
iijiiiii iiiiU iiji iii!i iiii i iiiiiiiiiiiiiiii ............ i i! 3.5 Vesicular materials.
Fig.3 . SOME THEORETICAL TYPES OF POLISH-RESISTANT ROAOSTONE
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