Quarterly Journal of Engineering Geology, 27, 25-38. 0481-2085/94 $07.00 © 1994 The Geological Society

Aggregate properties and resources of granitic rocks for use in concrete in Hong Kong

T. Y. Irfan

Geotechnical Engineering Office, Civil Engineering Building, 101 Princess Margaret Road, Kowloon, Hang Kong

Abstract Introduction

Granitic rocks are the major source of crushed fine and coarse aggregates for use in concrete in Hang Kong. The existing quarries are situated in a variety of granite types. Although some quarries produce coarse aggregates with better proper- ties, most are considered unsuitable for special purposes.

Laboratory testing of the various granite types present in the Territory indicates that fine-grained granites have superior mechanical and physical properties in comparison with other granite types and hence are potentially suitable for many special concrete purposes.

Granitic rocks, which are widespread in Hang Kong, form the major source of local crushed rock aggre- gates. These are primarily used in concrete for the construction industry, with a small amount of granitic aggregates utilized in bituminous materials for roads. Crushed rock fines comprise about 40% of the total aggregate production. This is due mainly to the lack of land sources of sand in Hang Kong. With the recent

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~ CRETACEOUS TERT'ARY l OEVON'ANPERMIAN SEDIMENTARY ROCKS S4EDIM~NTARY R)OCKS

FIG. 1. Simplified geological map showing the location of sampling stations, quarnes and rock crushing plants.

26 T.Y. IRFAN

discovery of significant reserves of high-quality quartz sands which may be dredged from shallow coastal waters (Carbray 1988), this scenario may change in the future.

Until ten years ago, all aggregates used in Hong Kong were produced locally from granite quarries and other conveniently located rock processing sites not far from the urban centres. A rapidly growing population and economy has resulted in intensive building and construction activity in recent years which, together with increasing environmental controls and costs, has resulted in the import of processed stone from neighbouring Chinese sources (Earle 1990). In addition to granitic aggregates, a significant amount of volcanic rock aggregates is being imported into Hong Kong (Gilbert & Irfan 1990). The import of aggregates from the quarries in the neighbouring Shenzhen region and the outlying islands of China has already increased dramatically over the last few years and has reached about 46% of the total aggregates used in Hong Kong in 1991.

Although volcanic rocks of Jurassic age (mainly pyroclastic tufts and lavas of acidic composition and rhyolite dykes) are widespread in Hong Kong, they

have not been exploited as a local aggregate source. Granites are likely to continue to be the major source for crushed rock aggregate in the foreseeable future in view of well-documented problems elsewhere in the world regarding the suitability of volcanic rock aggregates for concrete (Dolar-Mantuani 1983) and particularly with regard to concerns about the possible alkali reactivity of these rocks.

Local aggregates are currently provided from the following three sources (Fig. 1):

• contract quarries • rock crushing sites • site crushers

With the closure of the last remaining government direct labour quarry in 1990, the primary source of local aggregates at the end of 1991 was the seven contract quarries (Fig. 2). A total of 9.1 million tonnes of crushed rock aggregates was produced from the contract quarries in 1991, with only a small amount coming from a major rock crushing site and two former quarry sites that have been incorporated into site formation projects, which include rights to quarry stone. The site crushers are located at public and

FIG. 2. Two contract quarries along Anderson Road.

GRANITIC AGGREGATES, HONG KONG 27

private construction sites which are granted permission to produce either rockfill or aggregate from the rock excavated within the sites for use solely within the projects.

In addition to use as crushed rock aggregate, granite has been used as the basic raw material of the construction industry since the early days of Hong Kong in the 19th century. It was originally used as dressed stone for buildings and gravity retaining walls, dam walls and bridges (Davis 1965). The current uses include armourstone and pell mell for marine embank- ments and sea-walls, crushed rock filter and fill materials, ornamental stone on buildings and ballast in railroads. The majority of the ornamental granitic stones which are widely used in the interiors and exteriors of many buildings are imported into Hong Kong from various parts of the world, mainly Italy and China.

Published test results on the properties of quarry- produced granitic rock aggregates in Hong Kong are limited and confined to a few generalized ranges of values (Hindley 1984; Brand, Maunder & Massey 1984; Choy, Earle, Irfan & Burnett 1987). Although it is known that some quarries produce aggregates with better properties, most are considered unsuitable for special purposes because of their rather high crushing and 10% fines values and poor particle shapes related to their generally coarse-grained nature. However, no systematic laboratory testing had been carried out on the aggregate properties of various granitic rock types present in the Territory (classified by the Hong Kong Geological Survey in terms of grain size and compo- sition). Hence the suitability of particular granitic rock types for various specific aggregate uses had not been researched in any detail.

A laboratory testing programme was initiated by the Geotechnical Engineering Office (formerly the Geo- technical Control Office) in the mid-1980s in order to provide the industry and the government with reliable and high-quality test data concerning the index and aggregate properties and possible uses of each of the Territory's major rock types (Irfan, Cipullo, Burnett & Nash 1992).

This paper presents and discusses the results of the in-house testing programme on the granitic rocks in the context of their geological setting. The results are compared with the routine test values available for quarry-produced aggregates and the typical acceptance limits in international usage. Suitability of various granitic rock types for use in concrete is assessed mainly from the material properties.

Geological setting of the granites and related rocks

Granites and granodiorite occur in many parts of

Hong Kong (Fig. 1) as major intrusions, roughly circular in shape and varying from 3 km to 15 km in diameter. They are of Upper Jurassic to Lower Cretaceous age and in certain areas intrude volcanic rocks of slightly older age. The granites are classified and mapped by the Hong Kong Geological Survey according to their grain size in four basic types (Strange & Shaw 1986):

• fine-grained granite (grain size of less than 2 mm) • fine- to medium-grained granite (about 2 mm) • medium-grained granite (2 to 6 mm) • coarse-grained granite (over 6 mm)

The granites display cross-cutting and intrusional relationships, particularly near the edges of the plutons since they are of slightly different ages. In addition, they show evidence of alteration and modification by younger intrusions. Hydrothermal alteration (e.g. kaolinization) and a small amount of mineralization are particularly common along major shear zones, as reported by Choy et al. (1987). Fine-grained rocks of acidic composition (e.g. rhyolite) also occur through- out the Territory as small intrusions and as dykes and sills of various thickness, usually cutting across granitic and volcanic rocks.

The granites generally have equigranular texture, but inequigranular and megacrystic varieties are also common, particularly in rocks modified by later intrusions. They consist of alkali and plagioclase feldspars occurring in about equal amounts and quartz (usually 30 to 40% of the rock). A small amount of biotite, usually less than 5%, is also present. The granites are dominantly fine-to-medium and medium- grained and deeply weathered, with rock suitable for aggregate production (i.e. mass weathering grades I and II) generally occurring at a depth of 30 m or more.

Quartz-syenites and monzonites (quartz-deficient acidic rocks) are also present in the Territory.

Laboratory testing and characterization of granitic rocks as aggregates

Sampling and sample preparation

Laboratory test samples were collected as blocks of rock from a number of working and disused quarries as well as from cuttings in various granite types throughout the Territory. The sampling locations are shown in Fig. 1. A number of 75 mm diameter cores were drilled from the block samples for testing of rock index properties and strength determination. The remainder of the samples were broken into smaller pieces by sledgehammer and rock breaker. Standard size (10-14mm) and nominal 20mm size aggregates

28 T . Y . IRFAN

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were prepared from the smaller pieces using a laboratory jaw crusher.

Testing methods

A number of rock index determinations were made on the cores including specific gravity, bulk density, water absorption, porosity and sonic velocity, in accordance with the International Society of Rock Mechanics recommended methods (Anon 1978). The strength of the rock was determined on irregular lumps using the point load strength test (Anon 1985) and the Schmidt hammer test was carried out on block samples for a limited number of rock types.

The aggregate tests undertaken included aggregate crushing value (ACV), aggregate impact value (AIV), 10% fines value (10% FV), water absorption and flakiness and elongation indices (I F, IE)" These were carried out on standard size aggregates in accordance with BS812 (BSI 1975). Los Angeles abrasion value (LAAV) was determined on nominal 20mm size aggregates in accordance with ASTM 131-81 (ASTM 1981) and aggregate petrography was determined from thin sections of the rock cores.

In addition to the laboratory testing programme, test data available in routine test reports from a number of government and contract quarries was compiled in order to evaluate the properties of aggregates from these quarries and to provide comparative data for the laboratory-crushed material.

Classification and characterization

There are a number of classifications and descriptive schemes used to characterize aggregates. For this work the CADAM system recommended by the Geological Society Working Party on Aggregates (Collis & Fox 1985) was adopted to describe and classify the aggregates. A more detailed evaluation of aggregates in terms of petrographical properties was also carried out using specially prepared data sheets. A summary of the petrological properties of various granite types tested is given in Table 1.

Rock index and aggregate properties of granites

Rock index properties

The results of the physical and mechanical index tests are given in Table 2. These show that all granites tested are very strong rocks in the fresh state with point load strengths (PLS) of over 5.6MPa (or

uniaxial compressive strengths over 130 MPa using a conversion factor of 24). They have low effective porosities (less than 1.5%) and hence very low water absorption values of less than 0.5%. The sonic velocities determined on intact cores range between 3600 and 5300 m/s in comparison with the denser finer grained volcanic rocks in Hong Kong where velocities of over 5000m/s were obtained (Gilbert & Irfan 1990).

There are no discernible differences between the density, water absorption and porosity properties of the various granite types tested. Nevertheless, the finer grained granites (i.e. fine- and fine to medium-grained types) gave generally lower seismic velocities but higher strengths in comparison with the coarser grained varieties (Table 2).

Aggregate properties of laboratory- crushed material

The results show that there is no regular pattern of increase or decrease in the mechanical properties of granites with increase in grain size (Table 3). However, it is clear that the aggregate properties of the fine- grained granites are superior to other granite types. They have the lowest range of ACVs and the highest 10% FVs in comparison with other granite types. The aggregate properties of the megacrystic granite which has a fine- to medium-grained fabric are similar to the fine-grained granites.

The mechanical properties of the monzonite aggre- gates are also very similar to those of the fine-grained granites in terms of ACV, AIV and LAAV (Table 3), in spite of the rock being deficient in quartz (Table 1).

Plots of PLS against ACV (Fig. 3) and LAAV (Fig. 4) show that the mechanical properties are directly related to the rock strength, which is itself a function of porosity, density, grain size, microfracturing and mineralogical composition. Figure 5 shows a plot of ACV against the sonic velocity.

In terms of physical properties, the fine-grained granites produced the most flaky aggregates (Table 3). There does not seem to be any relationship between the granite type and the elongation index, except that all the medium-grained granites tested had values in the lower range of I~ results (Table 3).

No chemical tests (e.g. soundness tests) were carried out as the rocks were fresh. Chemical reactivities (e.g. silica-alkali reactivity) were not determined as there has been no record of reactivity in concrete using granitic aggregates in Hong Kong. However, in view of the presence of strained quartz in some of the granites and microcrystalline quartz in the very fine-grained varieties, the possibility of alkali reactivity cannot be discarded if other conditions for such a reaction exist as explained by Dolar-Mantuani (1983). Recently, West (1991) and French (1991) reported that there is

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G R A N I T I C A G G R E G A T E S , H O N G K O N G 31

little observational or experimental evidence that strained quartz gives rise to alkali-silica reaction in concrete.

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• F i n e - g r a i n e d • C o a r s e - g r a i n e d • Fine to medium-gra ined o Negacrys t i c A Ned ium-g ra ined + Nonzon i te

FiG. 3. Aggregate Crushing Value versus Point Load Strength for laboratory-crushed granite aggregates.

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Point Load Strength (MPa)

FIG. 4. Los Angeles Abrasion Value versus Point Load Strength for laboratory-crushed granite aggregates.

Properties of quarry-produced aggregates

A summary of the test results on standard size aggregates produced by the quarries in Hong Kong is given in Table 4. Selected tests have been carried out on samples of aggregate collected from the stockpiles of all quarries, on a more or less regular basis since 1972.

Until 1977 both the ACVs and AIVs were deter- mined but since then only the latter tests have been undertaken. The 10% fines value tests have been carried out since 1979.

The main granitic rock types underlying each quarry site are indicated in Table 4. The aggregates produced by quarries located mainly in the fine-grained granites have the lowest mean ACV and AIV (16 to 26) and the highest 10% FV (170 to 215 kN) although some scatter from the mean occurs (not shown in the table). There is a general rise in the mean ACV or AIV values with increase in grain size of the rock. In spite of noticeable differences amongst the aggregate strength properties, there does not seem to be a significant variation in the relative densities of the rock types (2.59 to 2.62 Mg/m 3) tested from each of the quarries (Table 4).

There is an improvement of I F values in recent years for almost all quarries, the mean results fall in a narrow band of I F = 11 to 18 for the post-1982 years, irrespective of the rock type. The similarity and improvement of the flakiness values are largely the result of the quarry operators exercising better control of the aggregate shapes in response to the construction industry's demands for the production of better quality concrete. Elongation values have also improved in recent years (Table 4). The individual I E values vary widely, however, not only between the quarries but also for the same quarry (not shown in Table 4) from year to year.

Relationships between aggregate properties

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Sonic Veloci ty (m/s)

FIG. 5. Aggregate Crushing Value versus Sonic Velocity for laboratory-crushed granite aggregates.

Laboratory-produced aggregates

The aggregate impact values are plotted against ACV, LAAV, 10% FV, I F and I E in Figs 6 to 9. A reasonably good linear relationship can be seen between ACV and AIV (Fig. 6), similar to those reported in Collis & Fox 0985) for a number of rock types. The variation in aggregate impact and crushing values within each rock group is attributed to the influence of both aggregate particle shape (Ramsay 1965) and geological properties such as composition, grain size, texture and degree of alteration (Dhir, Ramsay & Balfour 1971), as well as to the character- istics of the testing apparatus used. Figure 9 shows that

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GRANITIC AGGREGATES, HONG KONG 33

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FIG. 6. Aggregate Impact Value versus Aggregate Crushing Value for laboratory-crushed granite aggregates.

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FIG. 7. Aggregate Impact Value versus Los Angeles Abrasion Value for laboratory-crushed granite aggregates.

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10% Fines Value (kNI

FIG. 8. Aggregate Impact Value versus 10% Fines Value for laboratory-crushed granite aggregates.

AIV increases with decreasing I F. This is in contrast to the result for granitic rocks in Collis & Fox (1985), which shows an increase in AIV with increasing I F. In addition, marked differences in AIV versus I F curves are reported by Collis & Fox (1985) for fine-grained (less than 0.2mm in" grain size) and coarse-grained igneous rocks (over 2mm). There are insufficient

results to confirm this finding for Hong Kong granites, but in general the fine-grained granites with a grain size of less than 1 mm have lower AIV and higher I E values than the coarser grained (over 1 mm) rocks. The relationships between AIV and I E and between I F and I E (not shown) are rather poor.

The contrasting relationship between AIV and I F for Hong Kong granites may in part be due to the rather high aggregate impact values of between 23 and 31 (Table 3). It is reported by Ramsay, Dhir & Spence (1977) that when AIV exceeds 26, the sensitivity of the test may be impaired due to the filling of pore spaces between particles with crushed fines. The 10% FV and the modified AIV are more sensitive to I F than the standard impact values.

Figure 8 is a plot of AIV versus 10% FV. Although there were too few test results to determine a rela- tionship between these values it is apparent that the 10% FV increases linearly with decreasing AIV. This is also confirmed by the more comprehensive mean values of the quarry-crushed materials plotted in Fig. 11.

The Los Angeles abrasion value test provides an estimation of surface wear, particularly for aggregates used in bituminous surfaces (ASTM 1976). However, the test has been criticized for a number of reasons (Fookes 1991), in particular it does not simulate the actual type of deterioration to which an aggregate is subject in service. Figure 7 shows a good linear relationship between AIV and LAAV, with the fine- grained granites clearly plotting in the lower range of the values. The LAAV values are not usually deter- mined on quarry aggregates in Hong Kong as they are mainly used in concrete.

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F lak iness Index (%1

FIG. 9. Aggregate Impact Value versus Flakiness Index for laboratory-crushed granite aggregates.

Quarry-produced aggregates

The mean test values determined on quarry aggregates from different sites are plotted against each other in Figs 10 to 13, averaged over a number of years as

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Meon Aggregate Crushing Value(%)

Quarries LT kam Tel PTW Pok Tung Wan MB Mount Butler TST Tai Sheung Tok SO Shek 0 AR3 Pioneer, Anderson Rd OH Diamond Hil l CKL Cha Kwo Ling TH Turret Hill

Dominant Granite Type • Fine-grained • Medium-grained • Fine to medium-grained

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Mean Flak iness Index (%)

FIG. 10. Mean Aggregate Impact Value versus mean Aggre- FIG. 13. Mean 10% Fines Value versus mean Flakiness Index gate Crushing Value for quarry-produced granite aggregates, for quarry-produced granite aggregates.

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FIG. 11. Mean Aggregate Impact Value versus mean 10% Fines Value for quarry-produced granite aggregates.

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indicated in Table 4. The bulk production from a quarry may include some discoloured (weathered) material from the slightly and moderately weathered rock mass as well as the fresh rock grades, in contrast to the almost fresh granite aggregates produced in the laboratory under more controlled conditions. The approximate boundary between suitable and unsuitable material for granitic rocks occurs between the partially discoloured and completely discoloured rock (Baynes, Dearman & Irfan 1978; Fookes 1980).

Figure 10 shows that the relationship between mean ACV and AIV of the quarry-produced aggregates is very similar to that of the laboratory-produced aggregate (ACV = 1.1 x AIV). A good linear relation- ship exists between AIV and 10% FV (Fig. 11) but the relationships are poor between I F and AIV or 10% FV, except for the fine-grained granites (Figs 12 and 13). The general trends in AIV versus I F curves of the quarry aggregates are in broad agreement with those curves given in Collis & Fox (1985), but the absolute values are rather different for the fine- and coarse- grained granites.

Discussion on suitability of various granitic rock types as concrete aggregate

- - . - - F i n e - g r a i n e d : AIVm : 0.7IFm " 8.6, r 2 = 0.6

C o a r s e - g r a i n e d :

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FIG. 12. Mean Aggregate Impact Value versus mean Flaki- ness Index for quarry-produced granite aggregates.

The suitability of a rock type from a particular locality for use as concrete aggregate depends on the mechan- ical and physical properties of the aggregates pro- duced, as well as the rock mass properties such as joint spacing, weathering state and overburden thickness. More important, however, is the influence of environ-

G R A N I T I C A G G R E G A T E S , H O N G K O N G

TABLE 5. Typical acceptance values for concrete aggregates

35

Aggregate property Acceptance value Authority

Aggregate crushing value

Aggregate impact value

Los Angeles abrasion value

10% fines value

Flakiness index

Elongation index

Water absorption

Max 202 Higginbottom (1976) Max 224,5 Fookes (1984) Max 30' Higginbottom (1976) Max 2& ,5 Fookes (1984) Max 256 BS 882:1983 Max 302 BS 882:1983 Max 402 ASTM C 131-81 Max 45 ~ BS 882:1983 Max 305 AS 2758.1 (1985) Max 354 AS 2758.1 (1985) Max 402.4.5 Fookes (1984) Max 403 AS 2758.1 (1985)

Min 50 ~ BS 882:1983 Min 1002 BS 882:1983 Min 1506 BS 882 : 1983 Min 100 Hong Kong (1992)

Max 35 BS 882:1983 Max 254,5 Fookes (1984) Max 35 Hong Kong (1992) Max 304,5 Fookes (1984) Max 30 t Higginbottom (1976)

Max 1.54,5 Fookes (1984) Max 0.8 Hong Kong (1992)

1, general use; 2, wearing surfaces; 3, protected concrete; 4, moderate exposure; 5, severe exposure; 6, heavy duty concrete. The limiting values suggested by Fookes (1984) are for Far East aggregates to be used for structural concrete under exposed conditions.

mental, operational and haulage factors. In terms of the mechnical and physical properties, the suitability of rock for a particular use is usually decided by limiting values specified in various standards and specifications. However, both the test methods adopted and their limiting values vary considerably from country to country depending on application, climate, availability of materials, exposure conditions, etc. In Hong Kong, British Standards are generally followed in every construction field including the materials industry, with amendments to suit the local climatic and soil conditions. The limiting values specified for concrete aggregates (Government of Hong Kong 1992) are generally those given in BS 882 and other related standards (Table 5).

Hong Kong has a humid monsoonal tropical climate, in striking contrast to the cooler temperate climatic conditions in the United Kingdom. It has also an open coastal location and although there is a marked summer rainy season and a winter dry season the humidity is generally high in excess of 70% throughout the year. Another significant difference between Hong Kong and the UK is that the rocks are deeply weathered; very fresh rock is mainly found only at a significant depth below surface. Usually, a transitional zone of slightly to moderately weathered rock exists above the fresh rock with suitable material occurring

as corestones surrounded by marginally suitable to unsuitable weathered rock and soil-like material. Clearly a significant proportion of weathered rock with poorer properties will inevitably be present in the aggregates produced from these zones. A mixture of strong and weak aggregate is likely to lead to problems both in design and in controlling difficulties in the mix as the product will not be reliably constant.

Based on the above factors, Fookes (1984) suggested more stringent specifications for aggregates to be used in concrete production in humid tropical areas. For example, for exposed conditions, Fookes recommended that the water absorption should be less than 1.5%, AIV less than 20%, ACV less than 22% and 10% FV more than 80 KN, in contrast to the limiting values of 3%, 30%, 30%, 50kN respectively recommended by the British or other standards (Table 5).

The suitability of various Hong Kong granitic rock types for concrete aggregate uses is discussed below, based mainly on the mechanical and physical proper- ties of both laboratory-crushed and quarry-crushed aggregates. In the case of monzonites, syenites and granodiorites, which have never been exploited as aggregate source rocks in Hong Kong, the assessment of their suitability for aggregate is based on their geological properties, supplemented by limited labora- tory test results in the case of monzonites.

36 T.Y. IRFAN

Monzonite and syenite

The mechanical properties, based on limited testing, indicate that the monzonite or syenite may be suitable for aggregate production, as the test are well within the acceptable limits for general concrete purposes (Tables 3 and 5). Nevertheless, the aggregates from these rocks are expected to have lower resistance to abrasion due to their lower free silica contents (less than 10%). Furthermore, the samples of monzonite tested did not have the typical anisotropy due to alignment of large feldspar megacrysts shown by other intrusions in the Territory.

Although the mechanical properties are favourable, the limited sill or dyke-like occurrence of the monzo- nites and quartz-syenites and the frequent presence of a thick weathered overburden on the larger intrusions would generally make these rocks uneconomic as an aggregate source.

outcrops of megacrystic rock can be found in all four main granite types.

The mechanical and physical properties of the laboratory-crushed aggregates from the coarse-grained granite are similar or even slightly better than the fine- to-medium and medium-grained types commonly quar- ried in Hong Kong (Table 3). The mechanical strength properties of the megacrystic variety are also almost as good as the fine-grained granite. The limited results obtained indicate these two granite types would be suitable for most aggregate uses. However, it is considered that they are at the high end of the mechanical strength values which would normally be expected from such coarse-grained rocks if they were quarried. The dark colour and the highly variable texture may make the megacrystic variety rather unattractive for use in concrete and in addition, it weathers relatively rapidly on site once the rock is completely discoloured.

Granodiorite

Granodiorite is very similar to other granitic rocks in terms of mineralogical composition except for the higher content of biotite and amphibole and domi- nance of plagioclase over alkali feldspar. As the strength and rock index properties of the granodiorite in Hong Kong are very similar to those for the granites (Irfan & Powell 1985), it is expected that the aggregates from the granodiorite would have similar properties to granites.

Granodiorites have been quarried in other parts of the world as aggregate sources. However, in Hong Kong the granodiorite outcrops are deeply weathered in the hilly areas or are covered by thick superficial deposits and weathered mantle in the low-lying areas. The oldest intrusive rock type in the Territory, it has been affected by alteration, mineralization and shearing as a result of later intruding granites. These processes have resulted in rapid changes in the physical and mechanical properties of the rock, hence the aggregate properties are likely to vary within short distances in a potential quarry site. These factors together with its generally dark colour and variable texture, would make granodiorite a less desirable source rock for use in concrete in comparison to the light coloured granites.

Coarse-grained and megacrystic granites

None of the working or disused quarries is in the coarse-grained granite. The megacrystic granite, which has a very variable texture, occurs as small outcrops m the Pok Tung Wan Quarry, Lamma Island, but large

Medium- and fine-to-medium-grained granites

The test results on the laboratory-crushed aggregates from these two types of granite are very similar, with most values being at the boundary or slightly within the acceptable limits for general use (Tables 3 and 5). Although the fine-to-medium-grained granite is signifi- cantly stronger than the medium-grained granite (Table 2), this is not reflected in the mechanical test results (Table 3). As explained previously, this may be due to both types of granites having high crushing or impact values such that the test method may become insensitive to variations in the mechanical properties.

The 10% Fines Value test, being a more discrimi- nant test for determining the aggregate strength, gave values well within the acceptable limits for the medium-grained granite and in accordance with the intact strength of the rock.

The ACVs or AIVs of the aggregates produced by the quarries operating in the fine-to-medium-grained and the medium-grained granites are very similar to the test results obtained on artificially crushed equiv- alents. They were either at the boundary or just within the limiting values for acceptance in the early years of the quarry operations. Since 1978, the impact values from these quarries have shown slight improvements, which are also apparent in their shape indices (Table 4).

Whilst AIVs or ACVs of quarry-produced aggre- gates do not differ significantly for either granite, the 10% Fines Values show clear variations, being 125 to 145kN for the fine-to-medium-grained and 105 to 125 kN for the medium-grained granites. These results indicate that both types of granites would be suitable for most concrete uses including wearing surfaces.

G R A N I T I C AGGREGATES, HONG KONG 37

Although the medium-grained granites have higher (less superior) crushing values, the corresponding flaking indices are generally significantly better (lower) than for the finer grained variety, at least in the early years of the quarry operations. In the latter years, however, the results are very similar for both types of granite.

Fine-grained granite

Low ACVs and LAAVs and very high 10% Fines Values of the laboratory-crushed aggregates indicate that the fine-grained granites are suitable for most aggregate uses (Tables 3 and 5). However, their shape indices are rather high, and close to or slightly above the acceptable limits recommended by BS 882 and Fookes (1984).

The impact or crushing values of the aggregates produced by three quarries (Mount Butler, Lam Tei and Pok Tung Wan) situated in dominantly fine- grained granite were rather high (in excess of 24) in the early years of their operation. This may be partly due to extraction of the less strong, more weathered, rock from the higher levels and partly to the higher flakiness and elongation indices of the aggregates produced. The granite in the three quarry sites is not entirely fine grained; coarser-grained varieties also occur. Inclusion of the latter in the aggregate mixes would also lead to higher impact or crushing values. Nevertheless, all three quarries have shown significant improvements in their mechanical values in the later years of their operations. With high 10% Fines Values of over 180kN, the aggregates produced by these quarries would be very suitable for many special uses such as concrete to be placed in exposed conditions (Table 5). The shape indices have also improved over the years and in spite of their fine grain size, in recent years both the flakiness and elongation indices have been very similar to those of the coarser grained granites.

The LAAVs of the fresh fine-grained granite are lower than those for any of the other granites tested in the laboratory study and are within the acceptable limits for all concrete uses (Tables 3 and 5).

Conclusions

The existing quarries in Hong Kong are situated in a variety of granite types and almost all of the quarries encounter more than one type of granite. The laboratory testing indicated that in their fresh to slightly discoloured state all granite types are suitable for general concrete purposes. Other acidic igneous rocks such as monzonites, syenites and granodiorites also appear to be potentially suitable as far as their

mechanical properties are concerned, although the limited testing was only carried out on the monzonite.

The fine-grained granite has superior mechanical properties compared to the other granite types and is therefore potentially suitable for many special concrete purposes. This is also reflected in the mean properties of the aggregates produced from the quarries situated dominantly in the fine-grained granite.

Good relationships exist between some intact rock and crushed aggregate properties and also between some mechanical and physical aggregate properties. These relationships, which are based on limited testing, can be used to assess one property from another. Additional testing would be required to be confident of the representative relationships, however.

The shape indices, as well as the mechanical properties of the quarry aggregates, have improved significantly over the last ten years. This is due in part to the quarry operators exercising better control of the aggregate shapes in response to the construction industry's demands for higher quality concrete in recent years.

Silica-alkali reactivity has not been recorded in Hong Kong for concrete structures where granitic rocks have been used. However, the possibility of such a reaction cannot be discarded in view of the presence of a small amount of microcrystalline quartz in the fine- and fine-to-medium-grained granites. Further investigation is required on this aspect.

ACKNOWLEDGEMENTS. This paper is published with the permission of Director of Civil Engineering of Hong Kong Government. The contributions made by the technical and professional staff of the Geotechnical Engineering Office and the Public Works Central Laboratory in sampling and laboratory testing are acknowledged and the author is grateful to Messrs S. T. Gilbert, Q. G. Earle and J. M. Massey for helpful comments on an earlier draft.

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Received 9 November 1992: revised typescript accepted 6 October 1993