ROAD RESEARCH LABORATORY

Ministry of Transport

RRL Report LR 293

RESEARCH ON LOW-GRADE AND UNSOUND AGGREGATES

by

J.R. Hosking, M.Sc., A.C.S.M., A.I.M.M. and LoW. Tubey

Materials and Tropical Sections

Road Research Laboratory

Crowthorne, Berkshire

1969

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

CONTENTS

Page

Abstract i

i. Introduction 2

2. The assessment of low grade aggregates 2

2.1 The modified aggregate-impact test 5

3. The detection of unstable aggregates 7

3.1 Petrographic examination 7

3.2 Soundness tests 9

3.2.1 Tests on U.K aggregates 9

3.2~2 Tests on aggregates from Mauritius 9

3.2.5 Tests on soft limestones from Jamaica 9

3.2.4 The sodium-sulphate soundness test 9

3.2.5 Discussion of the sodium-sulphate soundness test IO

3.5 Other tests 12

5.5.i Modifications of standard aggregates tests 12

3.3.2 Aggregate shrinkage 12

5.3.3 Chemical tests 12

5.3.4 Field tests and soils tests 13

4. Quarry practice 13

4.1 stripping overburden and scalping 13

5. The use of substandard aggregates and the manufacture of of synthetic aggregates 14

5.1 Brick aggregates 14

5.2 Burnt soil and burnt shale aggregates 17

5.5 Sand-cement and sand-lime aggregates 17

5.4 Other synthetic aggregates 17

6. Conclusions 17

7. Acknowledgements 19

8. References 19

9. Appendix I. Petrographic examinations of some dolerites from Basutoland

9.1 Sample 2101

9.2 Sample 2105

9.3 Sample 2104

9.4 Sample 2105

21

21

22

22

22

10.

11.

9.5 Sample 2106

9.6 Sample 2107

9.7 Sample 2108

Appendix 2. Assessment of the soundness o£ two aggregates from Mauritius 25

10.1 Introduction 25

10.2 Examination and testing 24

10.2.1 Description of the samples 24

10.2.2 Sodium sulphate soundness tests 24

10.2.5 Modified aggregate-impact tests 26

10.5 Discussion of results 26

I0.4 Conclusions 27

Appendix 3 Assessment of two unsound aggregates from Great Britain 27

ii. 1 Introduction 27

11.2 Petrographic descriptions 28

11.5 Standard British roadstone tests 28

11.4 Modifications of aggregate-impact test 28

11.5 Sodium sulphate soundness tests 29

11.6 Conclusions 50

Page

22

25

23

(~) CROWN COPYRIGHT 1969 Reproduced by permission of the Controller of HMSO. Extracts fronl the'text may be reproduced, except for commercial purposes, provided the source is acknowledged.

RESEARCH ON LOW-GRADE AND UNSOUND AGGREGATES

ABSTRACT

This Report reviews research at the Road Research Laboratory and elsewhere on the use of low grade and unsound aggregates in road making. The major problems are to establish appropriate tests and criteria for both low-grade and potentially unsound aggregates for use in any of the pavement layers, to make the best use of local aggregates and to develop synthetic aggregates.

Themain conclusions are as follows:

i. Low-grade, but otherwise sound, aggregates may be satisfactorily a s s e s s e d by means of the modified aggregate-impact and ten-per-cent-fines tests on water-saturated samples.

Experience to date s u g g e s t s that for un- surfaced roads, for bases on more lightly trafficked roads and for sub-bases, useful test

limits would be:-

Modified aggregate-impact value - 40 maximum

Ten-per-cent-fines value - 5 minimum

2. No really satisfactory test has yet been de- signed which will detect all types of unsound aggregates. The sodium-sulphate soundness test would probably be satisfactory if it were not for its poor reproducibility and lengthy procedure.

3. Point counts of secondary minerals and modified impact tests on water-soaked and on boiled samples have proved satisfactory for parti- cular rock types.

4. Good quarry practice is likely to improve the quality of aggregates from many existing sources which are at present described as being of low grade or as yielding unsound material.

5. B r i ck a g g r e g a t e s made by t h e h e a t t r e a t m e n t o f c l a y s , m a r l s and s h a l e s may p r o v i d e m a t e r i a l i n r e g i o n s l a c k i n g i n n a t u r a l a g g r e g a t e .

Suggestions are given for further research.

i. INTRODUCT ION

Where good q u a l i t y r o a d s t o n e i s no t a v a i l a b l e a t an economic p r i c e , road engineers often need to utilize local aggregates of relatively low quality. This need is particularly acute in many developing countries, where there is often no alternative source of material. The use of low-grade aggre- gates has led to many failures, both in the U.K. and overseas, an increas- ing number of which can be attributed to the use of rocks which apparently decompose after being laid in the road.

The present Report gives an account of the work that has already been carried out on this subject at the Road Research Laboratory, together with a review of a cross-section of the work that has been carried out overseas. A study of all this work suggests that there are four major problems, which are as follows:-

i. Establishing criteria for low-grade, but otherwise sound, aggregates.

. Establishing tests to detect those apparently sound aggregates which disintegrate in service.

3. Making the best use of local materials by improved quarry practice.

. Finding methods of using aggregates which would normally be unsatis- factory in service and the manufacture of synthetic aggregates.

An account of each of these problems is given in subsequent sections of this Report.

2. THE ASSESSMENT OF LOW-GRADE AGGREGATES

The British Standard aggregate-crushing and impact tests I are employed for assessing the strength of normal roadmaking aggregates in the United King- dom. Because of occasional anomalous test results with weaker aggregates, research has been carried out at the Road Research Laboratory2 which has led to the development of the ten-per-cent-fines test for the testing of weaker aggregates. This test has been included in B.S.812 "Methods for sampling and testing of mineral aggregates, sands and fillers!' since 19601 .

Subsequent work carried out by Irwin 3 at the Road Research Laboratory showed that some materials gave significantly poorer test results when they were tested in a water-saturated state. The results of these tests, which were carried out on a range of low-grade aggregates from overseas, are given in Table I.

Further work published in 19634 , was carried out on a range of U.K. low-grade argillaceous and gritty rocks of known performance in unsurfaced roads and sub-bases. Samples of these rocks were subjected to a wide range of tests in order to establish a suitable test procedure and test criteria. The tests included modified aggregate-impact (see 2.1) and ten-per-cent-fines tests on dried and water-saturated samples, and sodium-su!phate soundness tests. From the results, reproduced in Table 2,

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it was concluded that the modified aggregate-impact and ten-per-cent-fines tests on water-saturated samples gave results which showed good correlation with engineers' experience with these materials. It was also concluded that for unsurfaced roads, for bases on more lightly trafficked roads and for sub-bases, useful tentative test limits would be 40 maximum and 5 minimum for these two tests respectively. Apart from unstable aggregates (Section 3) and aggregates that show a large reduction in strength on soaking, there has been no subsequent evidence which suggests that these test limits need amending.

Of these two tests, the modified impact test is rather more easily carried out and gives results a little more quickly; it also has the advantages of lower cost and portability of apparatus.

Further evidence of the need to test some aggregates in the water- saturated state is given in RRL Report No.215, which includes comparative modified aggregate-impact and ten-per-cent-fines test values obtained on a range of dry and saturated soft Jamaican limestones. These results are given in Table 3 together with similar results obtained with four other weak Jamaican aggregates.

Croft 6 has also reported the results of a number of engineering and modified engineering tests for the evaluation of shale aggregates. The aggregate-impact test and a "bail-mill test" were applied to samples of shale which had been pre-treated by soaking, tumbling, boiling with sodium carbonate and by submission to cycles of wetting and drying. He found that soft kaolinitic and micaceous shales suffered breakdown more readily than indurated shales.

The frost susceptibility of aggregates is of importance in temperate and cold climates. Croney and Jacobs7 have studied the frost suscepti- bility of soils and road materials in Great Britain. Details of a frost susceptibility test are given in their report, which includes details of the relationship between the frost susceptibility of different materials (cohesive soils, non-cohesive soils, chalks, limestones, granites, burnt colliery shales, slags and pulverized fuel ashes) and classification data (such as liquid and plastic limits, particle size distribution or satura- tion moisture content of the aggregate).

2.1 The modified aggregate-impact test

Samples for the water-saturated form of this test are prepared by soaking in distilled water (or rain water) for 24 hours and blotting the free moisture from the surface with a damp towel immediately before the test. After the test the sample must be oven-dried (12 hours at lOO°C to llOOC) before sieving out the fines.

The modified test is very similar to the standard test described in detail in B.S.812 I, the only difference being that the number of blows of the hammer (normally 15) is limited to a number which will yield between 5 and 20 per cent fines; the "modified aggregate-impact value" is then obtained by multiplying the percentage fines by i5 and dividing the product by the number of blows. If for example, S blows yielded 12 per

5

TABLE 3

Engineering properties of soft limestones and weak "shales" from Jamaica

Physical properties

Sample No. Ten-per-cent-

fines value

Dry

LIMESTONES:-

1712 14.2

1713 8.8

1714 2.4

1715 10.4

1716 3.0

1717 4.6

1712 A 12.9

1712 C 12.1

1712 E 18.4

1712 G 13.5

1713 A 7.2

1713 B 6.5

1713 C 10.2

1713 D 10.4

W e t

ii .0

6.7

1.9

i0.0

2.4

4.7

9 . 7

9.5

14.5

10.1

5.4

5.0

7.8

8.5

3.0

5 .4

2 .4

2 .4

M o d i f i e d a g g r e g a t e -

impac t v a l u e

Dry

25

35

135

33

116

72

Wet

"SHALES" / :-

1721B 9.6

1721D 9.4

1721F 12.7

1721G 8.7

20

53

133

34

138

85

Water absorption ( p e r c e n t )

5.4

6.2

12.0

4.4

I0.9

7.3

- 28 5.2

25 34 4.8

- 25 5.1

26 33 6.4

48 66 6 .0

- 69 6.3

36 40 7.0

34 36 5.3

- 99 3.5

79 88 2 .0

54 129 5 .6

74 123 6.3

S p e c i f i c gravity

2.23

2.24

1.96

2.37

1.96

2 .20

2.26

2.16

2.25

2.23

2.23

2.21

2.24

2.28

2.49

2 .54

2.35

2.28

Sodium- sulphate soundness (per cent)

23

34

40/60*

6 /25*

58

36

13

6

8

55/29*

37

30

37

33

79

65

i00

99

* Repea t t e s t s

/ These m a t e r i a l s were known l o c a l l y as s h a l e s , bu t were n o t p e t r o g r a p h i c a l l y i d e n t i f i e d .

c e n t f i n e s ; t h e " m o d i f i e d a g g r e g a t e - i m p a c t v a l u e " wou ld be (12 x 1 5 ) / 5 -- 36,

The m o d i f i e d t e s t has t h e a d v a n t a g e o v e r t h e s t a n d a r d i m p a c t t e s t t h a t i t can d i s c r i m i n a t e be tween d i f f e r e n t weak a g g r e g a t e s w h i c h would pack down t o o t i g h t l y i n t h e c y l i n d e r d u r i n g t h e s t a n d a r d t e s t t o e n a b l e v a l i d c o m p a r i s o n s t o be made.

3. THE DETECTION OF UNSTABLE AGGREGATES

Reports of apparently sound aggregates which rapidly decompose after being laid in the road have been received from many countries. These include Australia, Basutoland, Fiji, Great Britain, Kenya, Mauritius, South Africa and the United States of America. Decomposition during storage in stock- piles has also been reported. All of the rocks subject to this trouble have been of a basic nature and all have given satisfactory results when subjected to the usual strength tests. The problem of detecting aggre- gates which behave in this way has been studied in several countries.

Solutions have been s o u g h t using:

I. Petrographic examination

2. Soundness tests

3. Other tests

Another ~ossible solution, studied by Weinert, Clauss, Winterbach and Van Der Merwe = in South Africa has been to sub-divide a region according to climatic factors such as the evapo-transpiration rate.

3.1 Petrographic examination

The problem of unstable aggregates in surfacings and road bases was first studied by Scott 8 and other workers in the U.S.A. but more recently the problem has been receiving considerable attention in South Africa where Weinert and his co-workers 9,10, found a link between the soundness of the rocks (dolerites) used in road bases and the number of secondary minerals observed in a 'point count' analysis of petrographic sections of the rocks concerned. In this technique a device is attached to the normal microscope stage which traverses the thin section of the rock (20-30 microns thick) linearly across the microscope at predetermined intervals. A grid-like network of the sections is examined by means of a series of traverses and the mineral appearing at the intersections of the cross-wires identified and recorded on a series of classified counters. Normally about one thousand points are examined in each thin section. Weinert found that, for those materials with a satisfactory service record, less than thirty per cent of the mafic minerals (dark coloured minerals, usually of ferro-magnesian composition) were identifiably altered, but for those prone to rapid deterioration, more than thirty per cent of the mafic minerals were altered. Using this technique he has been able to predict

troubles from th is cause for all except borderline materials with_between thirty and thirty-three per cent. Tkese were sometimes satisfactory and sometimes not.

During a petrographic examination at the Road Research Laboratory of a range of weathered doleritic gravels from Basutoland (Appendix i), a limited number of point counts of altered mafic minerals were made. The results (Table 4) show that the alteration was very extensive and that there were wide differences between different petrological sections from the same sample. Even so the point count method is considered adequate for givinga first indication of the degree of weathering.

Similar problems have been met in Australia where Ryan of Ready-mix Concrete (Australia) Ltd. has had a number of samples of volcanic breccia examined by Weinert who found that the criteria which had proved so useful in South Africa did not hold true with the unsound Australian breccia.

During the work on the Basutoland aggregates differences in the void (inter-granular spaces) patterns were noticed but not studied. It is possibly relevant that a failure in some of the Portland stone used to repair St. Paul's Cathedral in Londonll was found to be associated with the void pattern of the ~ unsound stone which permitted the contaminated air and rain to penetrate the stone and cause it to disintegrate. For this study both microscopic and soil-suction techniques were employed. Less porous stone from a different part of the quarry has been used to replace the faulty material and has given subsequent satisfactory service.

The nature o£ the bond between the grains (or crystals) of the rock is a third possibly relevant factor which could usefully be investigated by petrographic methods.

TABLE 4

Results of a petrographic examination of some dolerites from Basutoland

Laboratory Sample No.

2104

2107

Petrographic Data

Percentage of secondary minerals counted

Ran ge

30-SI

54-58

2105

2101

2106

2108

2103

Mean I

33-70

32-91

69-72

52-96

65-98

36

56

61

62

65

69

79

No. of slides examined

4

3

5

4

5

5

5

3.2 Soundness tests

The inability of normal mechanical tests to indicate potentially unsound material has led some engineers to make use Of simulated weathering tests such as the sodium-sulphate soundness test 12,13 In these tests a sample of the aggregate is subjected to 24-hour cycles of immersion in a saturated solution of sodium sulphate, draining, oven-drying and cooling. The growth of crystals in interstices in the aggregate exerts high pressures which will cause the disintegration of weaker materials. A fuller account of the version of this test which has been used for the purposes of this Report is given in 3.2.4 below.

3.2.1 Tests on U.K aggregates. In the course of two Road Research Laboratory full-scale experiments, 14'15, two U.K. aggregates were found to fail prematurely as a result of decomposition in the road surface. At one site olivine-dolerite chippings wore abnormally rapidly and at the other a heavy loss of olivine-basalt chippings was observed. Samples of these two aggregates, which were subjected to a range of tests (Appendix 3) at the Road Research Laboratory, gave sodium-sulphate soundness test values of 30 and 32 respectively, when tested by the method described in sub- section 3.2.4 below. These results compare with values of between 0 and 2 which are obtained with normal roadstones of comparable strength. The appearance of these two materials after test was very similar to that of ~:~: sample of the olivine dolerite that had been exposed for 12 months to atmospheric weathering in a tray on the roof of the laboratory at Harmonds- worth, Middlesex. The sodium-sulphate soundness test was also employed in the Road Research Laboratory investigation of argillaceous and gritty rocks described in Section 2; the results (reproduced in Table 2) showed some evidence of correlation with service behaviour and indicated that only the very lowest grade materials give values greater than 20.

5.2.2 Tests on aggregates from Mauritius. Failure of the runway at : Plaisance Airport, Mauritius, was investigated in 1965. The failure was r thought to be due to the decomposition of one of the two types of basic rock used in the construction. The second type of rock gave a satisfactory performance even though it was apparently softer. The sodium-sulphate soundness test was found to be able to distinguish the sound from the unsound material. A full account of the investigation is given in Appendix 2.

3.2.3 Tests on soft limestones from Jamaica. A study of the petrology of a number of soft limestones from Jamaica b was made at the Road Research Laboratory and the results were related to their engineering properties as determined in the laboratory. Sodium-sulphate soundness tests were made on 14 of these materials using the method described in Sub-section 3.2.4 below. The results are reproduced in Table 5, where they are compared with other physical properties. These results showed correlation between the sodium-sulphate soundness and ten-per,cent-fines tests, the correlation coefficient of -0.75 on the 14 pairs of results being signifi- cant at a 1 per cent probability level.

3.2.4 The sodium-sul/0hate soundness test. The variant of the sodium- sulphate soundness test that has been used at the Road Research Laboratory

is as follows:-

I. Sixty chippings gradedbetween 19 mm and 12,5 mm ~-in and ~-in) test sieves were immersed in a saturated solution of sodium sulphate for 18 hours at room temperature [20oc).

. The solution was drained from the chippings which were then dried in an oven for 4 hours at a temperature of lO0OC to llO°C.

3. The d r i e d c h i p p i n g s were c o o l e d a t room t e m p e r a t u r e f o r 2 h o u r s .

. 14 further cycles of steps 1 to 3 above were carried out on the sample.

. The chippings were finally washed in water, dried and sieved on a 9.5 mm (3/8-in) test sieve.

. The s o d i u m - s u l p h a t e soundness v a l u e was e x p r e s s e d as t h e p e r c e n t a g e o f m a t e r i a l by w e i g h t which p a s s e d t h e 9.5 mm ( 3 / 8 - i n ) t e s t s i e v e .

5.2.5 Discussion of the sodium-sulphate soundness test. The investi- gations described in the Sub-sections 5.2.1 to 3.2.3 above all suggest that the sodium-sulphate soundness test can be used to discriminate between sound and unsound rocks, but further investigations are necessary to establish whether it is applicable to all types of rock. Two serious disadvantages of the test were also made apparent; these were the length of time required by the test (about 3 weeks) and the poor reproducibility. The length of test might be reduced by reducing the number of cycles of treatment. The results obtained at the end of different numbers of cycles is given in Table 5 for some of the materials tested, and these suggest that 5 cycles might be adequate for the purpose. The poor reproducibility is very apparent in the results given in Table 3. It has been suggested that the variety of forms into which sodium sulphate may crystallize may contribute to the poor reproducibility of the test, and that magnesium sulphate, which crystallizes in only one form, would be a better alternative. Magnesium sulphate is, indeed, an alternative that is given in the ASTM 12 test procedure, and any further research should look into the possible advantages of this form of the test. Weinert I0 has also pointed out the deficiencies of the sodium-sulphate soundness test and states that it should be used only for comparisons within batches of materials and that an absolute meaning shouldnot be attached to a test result.

i0

TABLE 5

Sodium-sulphate soundness values after different ~umhers of test czcles

3 c y c i e s I

% O r d e r of

Loss M e r i t I I

1721 F 7O 1

1721 G 58 2

19 34 4 1721 B 38 3

1721 D 29 5

1716 13 i0

1712 G 15 7

18 13 9

1714 12 II

1713 C 3.7 16

1713 A 17 6

1717 16 8

1713 D 4.3 14

1713 B I0 12

22 2.2 19

1712 A 6.5 13

15 2.4 18

1712 E i. 4 20

1715 3.2 17

1712 C 4.1 15

14 0.3 22

ii O. 3 24

5 0 .3 23

6 0.4 21

3 0.I 26

1 O. 1 25

2 0.0 27

6 . cyc les

O r d e r % o f

Loss Merit

I

97 1

77 2

52 3

48 4

44 5

18 ii

36 6

23 8

16 12

8.9 14

25 7

20 i0

12 13

2O 9

7.1 16

7.1 15

4.5 17

3.8 19

3 . 3 20

4.0 18

1.0 21

I 0 . 4 24

0 . 8 -22

0.5 23

O. 1 26

0.2 25

0.i 27

9 cyc les 12 cyc les 15 cyc les

% " Orde r ' % "- Order ' % Order of of of

Loss Loss Loss Merit Merit Merit

I I I I I

LO0 1 I00 1 100 1

94 2 96 2 99 2

78 3 88 3 89 3

66 4 75 4 79 4

55 5 58 5 65 5

46 6 52 7 58 6

40 7 53 6 55 7

31 I 9 42 8 44 8 i

30 i I0 34 i0 40 9

21 14 22 14 37 i0 !

34 8 35 9 37 ii

26 ii 53 ii 36 12

25 12 27 12 33 13

23 13 24 13 30 14

9.1 16 13 15 20 15

I0 15 Ii 16 13 16

5.0 19 9.4 17 i0 17

5.2 17 4.8 19 7.9 18

5.1 18 5.9 18 6.1 19

4.2 20 4.7 20 5.6 20

i.i 21 1.8 21 2.1 21

0.6 23 i.i 22 1.5 22

1.0 22 i.I 23 1.3 23

0.8 24 0.9 24 0.9 24

0.2 26 O. 3 25 0.5 25

0.3 25 0.i 26 0.2 26

0.0 27 0.0 27 0.0 27

11

3.3 Other tests

5.3.1 Modifications of standard aggregates tests. Although none of the standard or existing modifications of the standard aggregate test pro- cedures described in Section 2 above was able to discriminate between stable and unstable aggregates, further work at Road Research Laboratory has suggested that other modifications may prove more satisfactory. The work described in Appendix 2 on the two stones from Mauritius showed that prolonged soaking (up to 20 days) before carrying out a modified impact test resulted in a considerable reduction in the strength of the apparently stronger unsound rock. Further tests showed that similar results were more quickly obtained by boiling the sample in water for 24 hours before testing. The sound (but weaker) rock was unaffected by this treatment. Further work carried out at the Road Research Laboratory by Hitch on the two unsound aggregates from Great Britain (Appendix 3) was disappointing in that it showed no reduction in the strength of samples either after boiling in water for 24 hours or after soaking in water for 21 days. Additional samples were then treated for 24 hours in pressure-oxidation bombs at a temperature and pressure of 145oc and 280 kN/m 2 (40 ib/in 2) respectively, still without any loss in strength.

Although this limited amount of work was not conclusive, extension of these methods or other modifications of standard tests (such as have been made by Croft 6, see Section 2) may provide a satisfactory test pro- cedure for distinguishing unsound rocks.

3.3.2 Aggregate shrinkage. Workers at the Building Research Station (B.R.S.) have found that there is good correlation between the drying shrinkage of concrete and the drying shrinkage of the coarse aggregate from which it is made. 16 One of the two unsound rocks referred to in Section 5.2.1 was one of those which have been investigated by B.R.S., who have found that it has a high drying-shrinkage. It is possible that there is a connection between the properties of shrinkage and unsoundness, the unsoundness being caused by disintegration following the internal stressing which would be set up by cycles of wetting and drying. In addition to actual disintegration of the aggregate itself, failure of roadmaking mixtures (concrete and "black-top") could result from differen- tial expansion of the binder and an aggregate with a high drying-shrinkage.

Because high drying-shrinkage has been frequently encountered with concretes made from dolerite aggregate, much of the work (29 out of 45 quarries) by B.R.S. was carried out on this group of rocks. Aggregate drying-shrinkages of between zero and 0.065 per cent were recorded for the dolerite group, with a mean of 0.027 per cent, and considerable varia- tion was obtained within individual quarries.- Fairly good correlation was found between the percentage shrinkage and the water absorption of these doleritic rocks, but the correlation broke down when extended to cover a widerrange of rock types. Further research into possible correlation between unsoundness and shrinkage could well be profitable.

17 3.3.3 Chemical tests. Apart from chemical analysis, pH tests, tests with buffer solutions, I0 flocculation tests,lO and the possible oxidation/ hydration that could take place in the soaking and boiling tests (3.3.1 above),

12

few a t t e m p t s have been made t o make u se o f c h e m i c a l t e s t s i n t h e d e t e c t i o n o f unsound n a t u r a l r o c k s . A number o f c h e m i c a l t e s t s a r e u s e d , however , f o r d e t e c t i n g u n s o u n d n e s s i n b l a s t f u r n a c e s l a g l S . These employ c h e m i c a l a n a l y s e s , s o a k i n g i n w a t e r and t h e m i c r o s c o p i c e x a m i n a t i o n o f e t c h e d p o l i s h e d s u r f a c e s . F u r t h e r r e s e a r c h c o u l d u s e - fully be directed at establishing a suitable chemical method of detect- ing unsound natural aggregates. Three possible lines for research are:-

I. Treatment of aggregates or powdered rock with chemical reagents.

2. Treatment with reagents followed by mechanical testing.

3. Treatment with reagents of thin sections or polished surfaces, with subsequent microscopic examination.

3.3.4. Field tests and soils tests. The best solution of the problems" of detecting unsound aggregates would be the finding of a simple test which could be quickly carried out in the field by inexpert staff. With this object in view, C.B.R. and plasticity tests 19 have been carried out by Clauss 17 on samples of sound and unsound aggregates, but the results were disappointing.

4. QUARRY PRACTICE

In addition to being able to select aggregates from a wide range of geo- logical formations, producers of aggregates in the U.K. have the advantages of many years of experience and ready access, through their trade federa- tions, to the results of the latest research. Most U.K. quarries are long: established sources which have the additional advantage that quarrying wil~l have extended well beyond the zone of weathered rock which covers many rock formations, so that there is ready access to the sound rock.

Aggregate producers in many developing countries may have none of these advantages and, to make matters worse, the weathered rock zone in tropical regions is usually much more extensive than in the U.K.

Although there are undoubtedly many regions which lack any strong rock and others where unsoundness is a major problem, it is likely that adoption of good quarry practice would materially improve the aggregate from many supposedly low-grade sources. Of the large number of factors under the control of the quarrymaster, two have a profound effect on the quality of the product; these are the stripping of overburden and scalp- ing.

4.1 Stripping overburden and scalping

In the majority of quarries the fresh rock is capped by an over- burden of partially decomposed rock and surface soil, and the line of demarcation between fresh and decomposed rock is rarely clear-cut. In addition, decomposition maypenetrate into cracks and joints in the sound

13

rock, which may also be traversed by veins of rock of inferior quality. The overburden can usually be eliminated if it is stripped back well in advance of quarrying operations, but in some cases this is not sufficient and it is necessary to resort to hand-sorting at the quarry face. De- composed or inferior rock may still find its way into the crushers, but it can usually be eliminated after the first stage of crushing by scalp- ing, that is, removing the fines (which will normally contain the greater part of the inferior rock) by passing the crushed stone over a screen. The most suitable aperture size for the screen depends on the nature and amount of inferior rock and differs from one quarry to another; the size is usually in the range 25 mm ( 1 in) to IO0 mm (4 in) and is commonly 50 mm (2 in). If a considerable amount of clay is present, washing of the aggregates may also be required; in gravel pits it is nearly always necessary.

S. THE USE OF SUBSTANDARD AGGREGATES AND THE MANUFACTURE OF SYNTHETIC AGGREGATES

In a d d i t i o n t o t he p rob lems o f a s s e s s i n g low-grade aggregates and d e t e c t - i ng unsound rock , a t h i r d p rob lem a r i s e s in r e g i o n s where no s a t i s f a c t o r y a g g r e g a t e s e x i s t . In such r e g i o n s i t i s n e c e s s a r y to f i n d t he b e s t way in which s u b - s t a n d a r d a g g r e g a t e s can be used and to c o n s i d e r t he p o s s i - b i l i t i e s of manufacturing synthetic aggregates.

Research has been carried out on the utilization o£ sub-standard aggregates, under a wearing course, by cement or lime stabilization and on the stabilization of sands with bitumen. Work has also been carried out in the field of manufacturing synthetic roadstones. Although the object of this work has been the manufacture of high-quality aggregates with special properties such as good polishing resistance20, 21 similar methods could be employed to manufacture general-purpose aggregates at a much lower cost. A number of different types of synthetic aggregates are described in Sections 5.1, 5.2, and 5.5 below.

5.1 Br ick a g g r e g a t e s

One o f the most w i d e s p r e a d and l e a s t expens ive types o f s y n t h e t i c a g g r e g a t e i s t h a t m a n u f a c t u r e d from b r i c k s made by f i r i n g v a r i o u s c l a y s , mar l s and s h a l e s . The r e s u l t s o f s t r e n g t h t e s t s on a range o f U.K. b u i l d i n g and e n g i n e e r i n g b r i c k s i s g iven in Table 6. These r e s u l t s , which were o b t a i n e d on whole b r i c k s , a re d i f f i c u l t t o i n t e r p r e t in te rms o f a g g r e g a t e s t r e n g t h s , bu t s u g g e s t t h a t many would be s u i t a b l e f o r r o a d - m a k i n g . C o n f i r m a t i o n i s g iven in t h a t t he i n t e r n a l roads o f a l a rge b r i c k works in L a n c a s h i r e , which a r e paved wi th a c i d - r e s i s t i n g b r i c k s (bo th as s e t t s and as c r u s h e d a g g r e g a t e ) have g iven s a t i s f a c t o r y s e r v i c e f o r many y e a r s and t h a t b r i c k s are s t i l l used e x t e n s i v e l y in Hol land f o r road s u r f a c i n g .

A number o f c r u s h e d - b r i c k a g g r e g a t e s have been t e s t e d in t he cour se o£ s e v e r a l r e s e a r c h p r o j e c t s a t t he Road Research Labora to ry and the r e s u l t s a r e summarized in Table 7; t h e s e r e s u l t s a l s o sugges t t h a t many o f our e x i s t i n g b r i c k s would p r o v i d e s a t i s f a c t o r y roadmaking a g g r e g a t e .

14

TABLE 6

The crushing strength of a number of types of brick

BRICK TYPE

Red

Tividale blue

Yellow-faced baked

Stourbridge Fire clay

London stock

Sand-lime

Gault Clay

Hand-made red facing

Fletton

Fareham red

Red Marl, N. Wales

Staffordshire blue (normal)

Facing

Red Marl, Lancs

Red or Brindle engineering

Acid-resisting

Staffordshire blue (best)

Vitrified Paving brick

CRUSHING STRENGTH*

KN/m 2 Ibf/in 2

5,540

9,580

7,000- iO,OOO

ii,0OO

Ii,0OO- 16,000

17,OOO

13,6OO- 18,2OO

804

1,390

1,010- 1,450

1,600

1,600- 2,300

2,500

1,980- 2,640

24,100

27,000

39,000

54,000

3,500

3,900

5,600

7,800

54,000- 88,000

62,000- 89,O00

iO2,OOO

64,0OO-107,000

Ii0,O00

170,000

186,O00

7,800-12,800

9,000-13,OOO

14,800

9,300-15,6OO

16,000

25,000

27,000

* Tested in accordance with B.S. 125724 or by similar methods.

IS

Material

TABLE 7

Test results of a range o f brick a g g r e g a t e s

C r u s h i n g s t r e n g t h (ibf/in 2)

Building bricks:-

Clay brick 1 Clay brick 2 Clay brick 3 Sandy brick 1 Sandy brick 2 Crushed mortar

Engineering bricks:-

Accrington Reds Staffordshire Blue Southwater I

Refractory fire-clay bricks

"Nettle i" i 10,5OO "Dougall" I 2,500 "Nettle 2" 26,100 "Mossite" "Glenboig AI" "Glenboig G50" "LPD" "PRL"

Natural roadstones +

Artificial Sranite Limestone ~ii groups

A g g r e g a t e c r u s h i n g

v a l u e

41 42 52 49 57 35

28 20 24 19

Ten~per- cent -fines

value

6.3 5.5 4.2 2.5 2.0 4.7

A g g r e g a t e impact value

26 29 30

42* 138" 147" 33* 38*

27 19 23 19

A g g r e g a t e a b r a s i o n

v a l u e

5 19 12 14 42 78 8 9

8.3 4.8

13.7 5.7

Polished stone value

55

67 63 58 75 82 90 62 65

53 53 39 52

* Modified test as described in text

+ Average o£ group, ex Road Note No.2425

16

In I n d i a and P a k i s t a n , where b r i c k m a / t i n g i s a n e s t a b l i s h e d c o t t a g e i n d u s t r y , b r i c k s a r e a l s o u s e d s u c c e s s f u l l y as s e t t s , and when c r u s h e d as a g g r e g a t e . Ahmad 22 has r e p o r t e d on t h e u s e s o f c r u s h e d b r i c k a g g r e g a t e i n s u b - b a s e s , b a s e s and a s p h a l t i c c o n c r e t e i n P a k i s t a n , and o f t h e s u p e r i o r i t y o f c r u s h e d b r i c k s o v e r whole b r i c k s f o r t h e s e p u r p o s e s .

Clays, marls or shales must exist in many regions lacking in good roadstone and some of these would almost certainly provide satisfactory aggregates after pressing into bricks and firing in kilns at a suitable t e m p e r a t u r e .

5.2 Burnt-soil and burnt-shale aggregates

Grainger 23 has shown that low-grade aggregates can be made by burning the highly plastic black cotton soil which occurs in Nigeria. Best results were obtained at temperatures between 500 and 700oc when aggregate impact and aggregate abrasion resistances were similar to those of a weak U.K. limestone. Similar results have been obtained by burning clays in Guyana and India but the cost of adequate control has proved to be excessively high. In the U.K. low-grade aggregates include burnt colliery shale, which is better than the unburnt material.

5 .5 Sand-cemen t and s a n d - l i m e a g g r e g a t e s

In d e s e r t a r e a s where sand s o i l s p r e d o m i n a t e and good a g g r e g a t e s a r e l a c k i n g , s u i t a b l e s y n t h e t i c a g g r e g a t e s m i g h t be p r o d u c e d f rom s a n d - c e m e n t o r s a n d - l i m e m o r t a r s .

5.4 Other synthetic a g g r e g a t e s

"Synopal" is a proprietary white synthetic aggregate that is made by specially treating a glass manufactured from limestone, dolomite and silica sand.

Metallurgical slags and aggregates made from industrial wastes such as pulverised fuel ash are used extensively as roadmaking aggregates, but are not likely to be available in developing countries.

Further processes which might prove satisfactory, are the fusing or sintering of rock fines either on their own or with a suitable flux, and the bonding of rock fines or sands with silicates.

6. CONCLUSIONS

The main conclusions are as follows:-

1. The re i s some e v i d e n c e t h a t l o w - g r a d e , b u t o t h e r w i s e s o u n d , a g g r e g a t e s may be assessed satisfactorily by means of the modified aggregate impact and ten-per-cent-fines tests on water-saturated samples.

E x p e r i e n c e t o d a t e s u g g e s t s t h a t f o r u n s u r f a c e d r o a d s , f o r b a s e s on more l i g h t l y t r a f f i c k e d r o a d s and f o r s u b - b a s e s u s e f u l t e s t l i m i t s would b e : -

17

Modified aggregate-~mpsct value 40maximum

Ten-per-cent-fines value 5 minimum

2. No really satisfactory test has yet been devised whick will detect all types of unsound aggregates. The sodium-sulphate soundness test would probably be satisfactory if it were not for its poor reproducibility and lengthy procedure. It is likely that this test will also detect frost susceptible aggregates.

3. Three methods of detecting unsound rock have shown promise for particular rock types.

These a r e : -

(i) Point counts of secondary minerais in petrographic sections using a modified petrological microscope.

(ii) Modified impact tests on samples that have been soaked in water for 21 days.

(iii) Modified impact tests on samples that have been boiled in water for 24 hours.

4. Improved quarry practice is likely to improve the quality of aggre- gates from many existing sources which are at present described as being of low-grade or unsound material.

5. Br ick a g g r e g a t e s made by t h e h e a t t r e a t m e n t o f c l a y s , mar ls and s h a l e s and a g g r e g a t e s made from sand-cement and sand- l ime m i x t u r e s may p r o v i d e s u i t a b l e m a t e r i a l i n r e g i o n s l a c k i n g i n n a t u r a l a g g r e g a t e s .

6. Further research is required in the following fields:-

(i) Studying the mechanism of weathering of a range of rock types under different climatic conditions.

( i i ) Developing a simple and speedy soundness test that is universal in application; or, if this proves impossible, devising appropriate criteria for different classes of unsound rock suitable for the different pavement layers.

[iii) Finding methods of using sub-standard aggregates.

[iv) Manufacturing inexpensive general-purpose synthetic aggregates from the local materials found inregions which lack better- quality aggregates.

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7. ACKNOWLEDGEMENTS

U s e h a s b e e n m a d e i n t h i s i n v e s t i g a t i o n o f u n p u b l i s h e d , w o r k . c a r r i e d o u t

a t the Road Research Laboratory by F.A. Shergold, L.S. Hitch, D.C. Cawsey and D. Newill.

This Report was prepared in the Materials Section of the Construction Division.

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8 . REFERENCES

BRITISH STANDARDS INSTITUTION. British Standard No. 812: 1967. Sampling and testing of mineral aggregates, sands and fillers. London, 1967 [British Standards Institution).

SHERGOLD, F.A. and J.R. HOSKING. A new method of evaluating the strength of roadstone, with particular reference to the weaker types used in road bases. Rds. and Rd. Constr., 1959, 37 [438), 1964-7.

IRWIN, M.J. A laboratory investigation of the resistance to crushing of some tropical aggregates. Department of Scientific andlndustrial Research, Road Research Laboratory Note No. RN/3489/MJI. May 1959 (Unpublished). Harmondsworth.

SHERGOLD, F.A. and J.R. HOSKING. Investigation of test procedures for argillaceous and gritty rocks in relation to breakdown under traffic. Rds. and Rd. Constr., 1963, 41._, (492) 376-8.

TUBEY, L.W. and P.J. BEAVEN. A study of the petrology of some soft limestones from Jamaica in relation to their engineering properties. Ministr Z of Transport. R.R.L. Report No. 21. Harmondsworth, 1966 [Road Research Laboratory).

CROFT, J.B. An investigation into the methods of selecting shales for pavement construction.Australian Road Research Board. Proc. 1966, Vol.3, Part 2, 1260-83. Melbourne, Australia.

CRONEY, D. and J.C. JACOBS. The frost susceptibility of soils and road materials. Ministr~ of Transport, R.R.L. Report No. LR 90 Crowthorne, 1967 (Road Research Laboratory).

SCOTT, L.E. Secondary minerals in rock as a cause for pavement and base failure. Proc. High~. Res. Bd., 1955, 34, 412-7.

WEINERT, H.H., CLAUSS, K.A., W_INTERBACH, D.J. arLd D.B. VAN DER MERWE. Determination of the soundness of weathered basic igneous rocks (dolerites), used in road foundations. National Inat. Rd. Res., RS/6/60; (Unpublished) Pretoria, S. Africa.

WEINERT, H.H. Basic igneous rocks in road foundations, Council for Sci. and Ind. Res. Report No. 218, [681), 1-47 Pretoria, S. Africa, 1964.

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II.

12.

13.

14.

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18.

19.

20.

21.

22.

23.

HONEYBOURNE, D.B. and P.B. HARRIS. The structure of porous building stone and its relation to weathering behaviour. Szmposium of the Colston Research Society. March 24-27, 1958. (Vol. X), 343-65. London, 1958. (Butterworth Scientific Publications Ltd.

AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM Standard Part I0. Concrete and mineral aggregates. Philadelphia, 1967, (American Society for testing and materials).

BRITISH STANDARDS INSTITUTION. British Standard No. B.S. 1438: 1948. Media for biological percolating filters. London, 1948 (British Standards Institution).

BROWN, J.R. An experiment comparing the performance of roadstones used as chippings in rolled asphalt. Mini str)rof Transport RRL Report No. LR 63 Crowthorne, 1967 (Road Research Laboratory).

HOSKING, J.R. An experiment comparing the performance of road- stones in different bituminous surfacings: A. 30 Blackbushe, Hamp- shire. Ministry of Transport RRL Report No. LR 81. Crowthorne, 1967. (Road Research Laboratory).

SNOWDON, L.C. and A.C. EDWARDS. The moisture movement of natural aggregate and its effect on concrete. Mag. of Concrete Res. July, 1962. 14, C41), 109-116.

CLAUSS, K.A. The pH of fresh and weathered dolerite as in indica- tion of decomposition and stabilization requirements. 4th Regl. Conf. for Africa on Soil Mechanics and Fdn. Eng. Pretoria, S. Africa.

BRITISH STANDARDS INSTITUTION. British Standard No. 1047:1952. Air-cooled blast-furnace slag coarse aggregate for concrete, London, 1952 (British Standards Institution).

BRITISH STANDARDS INSTITUTION. British Standard No. 1377:1961 Methods of testing soils for civil engineering purposes. London, 1961. (British Standards Institution).

JAMES, J.G. and J.R. HOSKING. Improvements in or relating to artificial roadstone. Provisional British Patent Application No. 38630/67. August, 1967.

HOSKING, J.R. Synthetic polishing-resistant aggregates: gritty aggregates. Ministry of. Transport, RRL Report (in the course of preparation).

AHMAD, M.S. Crushed brick in road construction - (Roads Maintenance Circle) Bahawalpur, Pakistan. Undated.

GRAINGER, G.D. A study of burnt clay as a roadmaking aggregate. Estratto da: Recherches et essais sur lea structures en terre cuite. Symposium RILEM - Milan, 1962. 1965 Rome.

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24.

25.

26.

BRITISH STANDARDS INSTITUTION. British Standard No. 1257: 1945. Methods of testing clay building bricks. London, 1945 CBritish Standards Institution).

ROAD RESEARCH LABORATORY. Road Note No. 24. Roadstone Test data presented in Tabular form. London, 1959, (H.M. Stationery Office).

BRITISH STANDARDS INSTITUTION. British Standard No. 594:1961 Specification for rolled asphalt [hot process). London, 1961 (British Standards Institution).

9. APPENDIX 1

Petrographic examinations of some dolerites from Basutoland

A petrographic examination of samples of unsound "as dug" materials from Basutoland was made in an attempt to relate differences in their engineer- ing properties to their petrography. Special attention was paid to the degree of decomposition, as measured by the percentage of secondary mine- rals observed in their thin sections.

Descriptions of the individual samples are given in the following paragraphs (9.1 to 9.7). The feldspars were labradoritic; although chemically fresh some were cracked and disintegrated. In the severely weathered material the feldspars had been completely pseudomorphed. The type of cloudiness which is common in altered granitic materials was rarely seen. The pyroxene minerals were the most altered and appeared to be the first to weather, those exhibiting the strongest colour in the thin sections appearing to be the most susceptible.

Mechanical weathering was apparent in two samples (Samples 2105, 2104). Sample 2106 contained parallel banding of fine clay-like material travers- ing the cracks which suggests the action of percolating water, and the freshness of the metallic grains in the sections suggests that the clay like material came from an external source rather than from within the nodule.

9.1 Sample 2101

4 sections of 25 mm - 19 mm (i in - ~ in-) "as dug" material. Secondary minerals: average 62 per cent; range 32-91 per cent.

An ophitic dolerite, with the pyroxenes averaging about 0.4 mm in size. The feldspars were predominantly tabular and unaltered. In one section they were of larger size and much twinned, but plexuses of smaller feldspars with only rare twinning were more usual. Alteration was mainly in the pyroxenes particularly where they contained included feldspar and was often concentrated in the feldspar plexuses where it was interstitial to the feldspar. In the more altered sections there was reduction in the apparent grain size, and in the most altered material only relics of the original minerals and a few pseudomorphs (mostly after feldspars) were discernible.

21

9.2 Sample 2103

5 s e c t i o n s o f 25 mm - 19 mm (1 in - ~ in ) "as dug" m a t e r i a l . S e c o n d a r y m i n e r a l s : a v e r a g e 79 p e r c e n t ; r ange 65-98 p e r c e n t .

M i c r o - g a b b r o i c i n t e x t u r e w i t h no e u h e d r a l f e l d s p a r and on ly f a i n t l y c o l o u r e d p y r o x e n e s . O l i v i n e abundan t and u n a l t e r e d e x c e p t around t h e g r a i n b o u n d a r i e s and a l o n g f i n e , s l e n d e r c r a c k s . R e l a t i v e l y c o a r s e - t e x t u r e d w i t h g r a i n s g e n e r a l l y abou t 1 mm and some up t o 2 mm in s i z e . Some o f t h e s e c t i o n s r i c h i n opaque m e t a l l i c m i n e r a l s w i t h some corona r e a c t i o n r i n g s i n t h e more w e a t h e r e d m a t e r i a l . A l t e r a t i o n was most e x t e n s i v e i n t h e p y r o x e n e s , bu t a l s o o c c u r r i n g a long g r a i n b o u n d a r i e s , w i t h t h e f e r r o - m a g n e s i a n s t e n d i n g t o s e g r e g a t e i n t o m e t a l l i c g r a i n s f o rming a d e n d r i t i c c r a c k p a t t e r n .

9 .3 Sample 2104

4 s e c t i o n s o f 25 mm - 19 mm (1 in - ~ in ) "as dug" m a t e r i a l S e c o n d a r y m i n e r a l s : a v e r a g e 36 p e r c e n t ; range 30-51 p e r c e n t .

Most s e c t i o n s c o n t a i n e d c o a r s e - g r a i n e d twinned and zoned e u h e d r a l f e l d s p a r s o f abou t 1 .5 mm i n s i z e and were a l s o r i c h i n f e r r o - m a g n e s i a n m i n e r a l s . The m e t a l l i c g r a i n s (some o f which appea red t o be p r i m a r y ) were v a r i a b l y d i s t r i b u t e d . One o f t he s l i d e s c o n t a i n e d p a t c h e s o f a f i n e r m a t e r i a l which was a l m o s t c o m p l e t e l y a l t e r e d . There was some S c h i l l e r i z a t i o n , and e x t e n s i v e c r a c k i n g a c r o s s t he g r a i n b o u n d a r i e s was a n o t i c e a b l e f e a t u r e . A l t e r a t i o n was m a i n l y i n t h e pyroxenes and p a t c h e s o f f i n e m a t r i x , b u t t he l a r g e r c r a c k s a l s o c o n t a i n e d some a l t e r e d m a t e r i a l .

9.4 Sample 2105

5 s e c t i o n s o f 25 ram - 19 mm (1 in - ~ in) "as dug" m a t e r i a l . S e c o n d a r y m i n e r a l s : a v e r a g e 61 p e r c e n t ; range 35-70 p e r c e n t .

The p y r o x e n e g r a i n s a v e r a g e d about 0 .6 mmand showed o p h i t i c t e x t u r e . Some s e c t i o n s w e r e ~ i c h e r in f e l d s p a r than o t h e r s and some c o n t a i n e d r a r e , smal l o l i v i n e c r y s t a l s . G e n e r a l l y r i c h in meta l g r a i n s . Some S c h i l l e r i z a t i o n and e x t e n s i v e c r a c k i n g gave an i m p r e s s i o n of mechanical disintegration especially in the feldspars. There was some indication of a little secondary quartz. Alteration was most severe in, but not exclusive to, the pyroxenes which were very pale and variably altered. The feldspars, although chemically fresh, wore severely cracked and some pseudomorphs were observed in the most altered slide.

9.5 Sample 2106

3 sections of 25 mm - 19 mm (i in - ~ in) "as dug" material. Secondaryminerals: average 65 per cent; range 59~72 per cent.

0 p h i t i c t e x t u r e w i t h abundant f e l d s p a r . Grain s i z e g e n e r a l l y abou t 0 . 3 m m w i t h some c o a r s e r m a t e r i a l i n one s e c t i o n . Less b a s i c t h a n t h e o t h e r samples w i t h t r a c e s o f p o s s i b l y p r i m a r y q u a r t z . Pyroxenes were

22

mostly altered, and the unaltered feldspars were disintegrated. A dark clay material appeared to be penetrating into the material in bands.

9.6 Sample 2107

3 sections of 25 mm - 19 mm (i in - ~ in) "as dug" material. Secondary minerals: average 56 per cent; range 54-58 per cent.

Variably textured material. The feldspars mostly seen as elongated idiomorphic crystals up td 1.6 mm in length showing both Carlsbad and repeated twinning and often zoned. The feldspars were fresh but cracked, and in one section had ragged irregular boundaries, whereas in another they appeared as an eutectic intergrowth. One slide contained a little altering biotite and a single heavily altered olivine crystal. (Olivine was not identified in the other sections). The alteration was patchy and mostly in the pyroxenes.

9 . 7 Sample 210.8,

5 sections 25 mm - 19 mm (i in - ~ in) "as dug" material. Secondary minerals: average 69 per cent; range 52-9.6 per cent.

Similar in texture to Sample 2101 but slightly richer in metallic grains and slightly more altered, with the alteration again mostly in the pyroxenes especially those around the fine feldspars of the plexuses.

I0. APPENDIX 2

Assessment of the soundness of two aggregates from Mauritius

i0.I Introduction _.

Parts of the runway surfacing at Plaisance Airport, Mauritius, ravelled badly and pitted within 18 months of construction. The surfacing was a rolled asphalt to B.S. 594, 26 and the failure appeared to be due to the decomposition of the aggregate. The aggregate had been obtained from Candos Quarry, and had been chosen on the basis of aggregate crushing tests to be the best available aggregate on the island. Subsequent tests by the Resident Engineer using the sodium-sulphate soundness test on this aggregate and crushed field boulders showed that the aggregate from Candos Quarry broke down after cycles of immersion in sodium-sulphate solution followed by drying, whereas the aggregate from the field boulders did not. This Appendix describes an investigation of samples of the two materials which were sent to t h e Road R e s e a r c h L a b o r a t o r y .

The investigation, which had the object of devising a simple and quick test capable of indicating the difference between the performance of these two aggregates, was carried out in two parts. These consisted of a close visual examination and an evaluation of their soundness by means of the modified impact test used in conjunction with pretreatments by soaking and boiling in water.

23

10.2 Examination and testing

10.2. i. Description of the samples. The sample from Candos quarry (Laboratory Sample No. 2504) contained five large pieces between i00 mm and 400 mm (4 in and 15 in) in size and of differing colour together with a quantity of 12.5 mm - 9.5 mm (~ in - ~ in)chippings. The texture of the hand specimens varied from fine-grained with individual crystals not distinguishable to more coarsely crystalline material containing darker phenocrysts about 3mm (~ in) in size. (Plates i, 2, and 3). These phenocrysts showed signs of alteration, were often cracked and had small areas plucked out which were partly infilled with a brownish powder. Alteration was less visible in the finer textured material, but spots of a similar brown powder were observed in it.

The f i e l d b o u l d e r s a m p l e ( L a b o r a t o r y sample No. 2505) c o n s i s t e d o f two p i e c e s o f b o u l d e r 300 and 450 mm (12 and 18 i n ) i n s i z e and a q u a n t i t y o f 12 .5 mm (3 i n ) t o d u s t c r u s h e d m a t e r i a l . The hand s p e c i m e n s were o f v e r y f i n e t e x t u r e ( s e e P l a t e 4 ) , s t e e l g r e y i n c o l o u r w i t h o u t d i s t i n g u i s h - a b l e crystals, and, except for a variable vesicularity, without distinguish- able sub-divisions. They were similar in appearance to a fine-grained slag and contained no decomposed or other material infilling the vesicles. Fractured vesicle sides were fresh and unaltered.

10.2.2 Sodium-sulphate soundness tests. The sodium-sulphate soundness test used by the consulting en~iReers consisted of cycles of 19 hours immersion in a solution of 4OOg sodium sulphate in 1 1 of water, followed by a half-hour draining , four hours' oven-drying at llOOC and a half-hour cooling. At the end of e:ach cycle the samples were inspected, and the material retained on a sieve of aperture size one smaller* than the one which retained the whole sample before test was weighed. The samples were generally subjected to 20 cycles at a rate of five a week. Sample M.A.2 was subjected to 40 cycles.

The r e s u l t s a r e e x p r e s s e d as a p e r c e n t a g e , by w e i g h t o f t h e w h o l e s a m p l e , wh ich p a s s e s t h e s e p a r a t i n g s i e v e , and a r e g i v e n i n T a b l e 8.

*In the series 19 ram, 12.5 mm, 9.5 ram, 6.3 ram, 4.8 mm (2 in, ~ in, ~ in, I in, 3/16 in).

24

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10.2.3 Modified aggregate-impaqt tests. The soakingprocedure was to immerse sufficient chippings for duplicate tests in a beaker of distilled water, shake to remove entrapped air and stand at room temperature (20o C) for the designated period. The boiling procedure was to boil the sample immersed in water in a simple hot extractor fitted with a refluxing condenser. The samples were then surface-dried and the modified impact test carried out as described in Section 2.1. The results of these tests are given in Table 9 and Fig. i.

TABLE 9

R e s u l t s o f m o d i f i e d a g g r e g a t e - i m p a c t t e s t s

Sample

Pretreatment

Dry s t r e n g t h Normal procedure

After 24 hrs soaking

After 6 days soaking

After 20 days soaking

24 h r s s o a k i n g + 48 h r s a i r d r y i n g + 8 h r s oven d r y i n g

24 h r s b o i l i n g i n water

No. of blows

6

Candos stone

% f i n e s fo rmed d u p l i c a t e

r e s u l t s

8.9 9.0

8.3 8.3

9 . 6 9 . 8

8.5 8.4

Modified impact value

19

21

24

32

21

No. of b 1 ows

4

7.5 9.2

30

Field boulder stone

% fines formed Modifie duplicate impact results value

15 .S

11.3

i0.O

11.7

9.5 7.0 9.1

i0 ~ 9 18.1

6 ii .2

5 9.7

6 11.9

6 I0.5

27

28

29

29

25

10.3 Discussion of results

Both series of tests show a similar pattern in that although the dry strengt| of the Candos stone was superior to that of the field boulder material, the Cando~ stone disintegrated severely under cycles of the sodium-sulphate soundness test indicating a potentially weak aggregate. The field boulders, apart from selecte( weak samples, showed little sign of disintegration.

Close visual examination confirmed that the Candos stone was in fact weather( a~d contained decomposition minerals, whereas the field boulder material was unweathered and fresh.

26

The results given in Table 9 and Fig. I show that, whereas the field boulder material was not significantly affected by any of the treatments, the Candos material was significantly affected both hy prolonged soaking and by boiling. It was observed that some of the Candos material after boiling had spots of greyish powder visible in the surface and when hot was soft and readily crushed between the fingers, although this effect was less pronounced in the aggregate when it had cooled. Table 9 suggests that drying after soaking for 24 hours does not lead to the sample recovering its initial strength, but the differences are small and may not be significant. With the Candos stone, boiling for 24 hours reduced the modified impact value by an amount equivalent to that produced by between 16 and 18 days' immersion. The data in Fig. l suggests that the Candos stone is unusual in that the reduction of strength is continuing at an almost constant rate even after 20 days' soaking. ~ Limited ex- perience with weak British aggregates had suggested that 24 hours' soaking was adequate to show any reduction in strength thatwas likely to occur.

i0. 4 Conclusions

From the visual examination and test results it was concluded that:-

(I) The Candos stone was variable and highly weathered in its natural state. Despite its seemingly sound appearance and high dry strength it was potentially weaker than the vesicular but fresh field boulders.

(2) The use of the modified impact procedure, in conjunction with pretreatment by soaking in water, gave results similar to the sodium- sulphate soundness test; but, like the sodium-sulphatesoundness test, it has the disadvantage of taking a long time (three to four weeks).

(3) Pretreatment by boiling in water prior to testing showed promise as a means of detecting potentially unsound material and gave results similar to the sodium-sulphate soundness test in only two days.

ii. APPENDIX 3

Assessment of two unsound aggregates from Great Britain

11.1 I n t r o d u c t i o n

In t h e c o u r s e o f two o f t h e Road Resea rch L a b o r a t o r y s f u l l - s c a l e road e x p e r i m e n t s , two a g g r e g a t e s were found t o be unsound . 14,15 Both o f t h e s e a g g r e g a t e s w e a t h e r e d a f t e r t h e y had been l a i d i n t h e road s u r f a c e . A l though such b e h a v i o u r i s r a r e in Grea t B r i t a i n , i t has f r e q u e n t l y been r e p o r t e d from a number o f o v e r s e a s r e g i o n s .

In this Appendix is described the work that was carried out at the Road Research Laboratory with the object of finding a method by which these unsound materials may be distinguished from sound aggregates.

27

11.2 Petrographic descriptions

Both o f t h e unsound a g g r e g a t e s c o n s i s t e d o f b a s i c rock i n t he B a s a l t r o a d s t o n e g roup . One was an o l i v i n e - b a s a l t and came from D e r b y s h i r e ; t h e o t h e r was an o l i v i n e - d o l e r i t e from R e n f r e w s h i r e . I t i s n o t e w o r t h y t h a t a l l w e l l - a u t h e n t i c a t e d examples o f unsound rocks i n o v e r s e a s r e g i o n s have been o f a b a s i c rock t y p e .

ii.3 Standard British roadstone tests

The results of a range of standard British roadstone tests on both of these materials are given in Table iO, and these' are compared with some of the other roadstones which gave a satisfactory performance in the same experiments. These tests were carried out on samples taken at the time of the laying of the two experiments, and gave no indication of the proneness to weathering of these apparently sound aggregates.

TABLE I0

The results of standard roadstone tests on two unsound aggregates from Great Britain

Source

Unsound 'A'

Unsound 'B'

Sound 'A'

Sound 'B'

Sound 'C'

Sound 'D'

Sound 'E'

Sound 'F'

Rock type

Olivine-basalt

Olivine-dolerite

Gritstone

Breccia

Granite

S l a g

G r i t s t one

C a l c i n e d b a u x i t e

Aggrega te c r u s h i n g

v a l u e

18

iS

20

22

22

24

19

33

Aggrega te abras ion

va lue

7

13

iO

4

4

8

12

3

Aggregate impact value

19

16

22

23

24

25

20

31

11.4 Modifications of L aggregate impact test

Modified aggregate impact tests were carried out on samples of one of the two unsound aggregates; further tests were then carried out on samples which had been subjected to the following pretreatments:-

(i) Soaking in distilled water for 24 hours.

(ii) Soaking in distilled water for 21 days.

28

(iii)

(iv)

Boiling in distilled water for 24 hours in a hot extractor fitted with a refluxing condenser.

Boiling in distilledowater for 24 hours at 145°C under a pressure of 280 kN/m ~ (40 ib/in 2) in pressure oxidation bombs.

The r e s u l t s o f t h e s e t e s t s (Table 11) show t h a t t he a g g r e g a t e was n o t weakened by any o f t h e s e p r e - t r e a t m e n t s .

TABLE 11

R e s u l t s o f m o d i f i e d impac t t e s t s on unsound a g g r e g a t e 'B '

M o d i f i c a t i o n o f t h e t e s t A g g r e g a t e impac t v a l u e

Standard

Modified (material as supplied)

Modified - oven dried

Modified - 24 hours soaking

Modified - 21 days soaking

Modified - 24 hours boiling

Modified - 24 hours in oxidation bomb

16

16

17

15

16

16

16

ll.S Sodium-sulphate soundness tests

Duplicate samples of the two unsound a g g r e g a t e s were s u b j e c t e d to t h e s o d i u m - s u l p h a t e soundness t e s t ( S e c t i o n 3 . 2 . 4 ) , b u t 12.5 - 9 .5 mm (3 - ] in) a g g r e g a t e was t e s t e d i n s t e a d o f the 19 - 12.5 mm s i z e used f o r the e a r l i e r t e s t s , 4 The s e p a r a t i o n o f f i n e m a t e r i a l was t h e r e f o r e made on a 6 . 3 mm (¼ in) test sieve instead of the 9.5 mm (~ in) size.

In T a b l e 12 the r e s u l t s o f the t e s t s a r e compared w i th t h o s e o b t a i n e d f o r a number o f sound r o c k s , wi th s i m i l a r m o d i f i e d impac t t e s t v a l u e s . These r e s u l t s showed t h a t t h e s o d i u m - s u l p h a t e s o u n d n e s s t e s t was a b l e t o d i s t i n g u i s h t h e s e two unsound rocks and t h a t t h e r e was good ag reemen t be tween t h e d u p l i c a t e t e s t s . Examina t ion o f t h e samples o f a g g r e g a t e s a f t e r t e s t showed c o n s i d e r a b l e v a r i a t i o n be tween i n d i v i d u a l c h i p p i n g s and p i t t i n g was a p p a r e n t i n b o t h s amp le s .

29

TABLE 12

Results of sodium-sulphate soundness tests on unsound aggregates 'A' and 'B'

Material

Unsound a g g r e g a t e 'A'

Unsound a g g r e g a t e 'B'

Sound aggregate 'G'

Sound aggregate 'H'

Sound aggregate 'I'

Sound aggregate 'J'

Sound aggregate 'K'

Sodium-sul

ist sample

23

30

)hate soundness va lue

2nd sample

21

30

Mean

22

30

0

0

1

1

1

11.6 Conclusions

It was concluded that, of the tests studied, only the sodium-sulphate soundness test was able to distinguish the unsound rock.

30

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ABSTRACT

Research on low-grade and unsound aggregate: J.R. HOSKING, M.Sc., A.C.S.M., A.I.M.M., and L.W. TUBEY: Ministry of Transport, RRL Report LR 293: Crowthorne, 1969 (Road Research Laboratory). This Report reviews research at the R.R.L. and elsewhere on the use of low grade and unsound aggregates in road making. The major problems are to establish appropriate tests and criteria for both low-grade and potentially unsound aggregates for use in any of the pavement layers, to make the best use of local aggregates and to develop synthetic aggregates.

The main conclusions are as follows:

. Low-grade, but otherwise sound, aggregates may be satisfactorily assessed by means of the modified aggre- gate impact and ten per cent fines tests on water-saturated samples.

Experience to date suggests that for unsurfaced roads, for bases on more lightly trafficked roads and for sub-bases, useful test limits would be:-

Modified aggregate impact value - 40 maximum

Ten-per-cent fines value - 5 minimum

. No really satisfactory test has yet been designed which will detect all types of unsound aggregates. The sodium- sulphate soundness test would probably be satisfactory if it were not for its poor reproducibility and lengthy procedure.

3. Point counts of secondary minerals and modified impact tests on water-soaked and on boiled samples have proved satisfactory for particular rock types.

4. Good quarry practice is likely to improve the quality of aggregates from many existing sources which are at pre- sent described as being of low grade or as yielding unsound material.

. Brick aggregates made by the heat treatment of clays, marlsand shales may provide material in regions lacking in natural aggregate.

Suggestions are given for further work.