5. PAVEMNET MATERIALS - UNBOUND GRANULAR MATERIALS

Transcription

1 5. PAVEMNET MATERIALS - UNBOUND GRANULAR MATERIALS 5.1 Introduction Pavement design requires the efficient use of locally available materials if economically constructed roads are to be built. Pavement materials include: Granular materials (aggregates) which includes crushed rock aggregates obtained from hard rock sources, natural (pit-run) gravels, gravel-sand-soil mixtures either as dug or semi-processed (i.e. screening, crushing of oversized stones, mixing with other materials (mechanical stabilization) and other artificial or modified materials Binders surfacing binders such as bitumen and cement; binders used for stabilizing or modifying the properties of subgrade/subbase/base such as lime, cement, foam bitumen, etc. Granular materials (aggregates) make up the bulk (by volume and weight) of the pavement structure and are used in different layers of pavement structure. They may be used alone or in combination with various types of cementing materials. They provide a number of functions depending on the layer in which they are used. In general, they have to be stable and hard to carry the loads by traffic and construction equipment without failure, excessive deformation and other undue effects, they have to be able to resist wear due to abrasion by traffic and they have to be durable to resist undue environmental effects (like freezing and thawing, moisture variations (wetting and drying). The manner in which they do so depends on the inherent properties and qualities of the individual particles and on the means by which they are held together (i.e. interlocking, binders, or both). In Gravel Roads, soil-aggregates form the entire pavement structure; have to be well graded to furnish adequate stability (strength and stiffness) to carry traffic stresses, should possess adequate amount of fines and plasticity of fines to bind the coarse aggregates. Subbase aggregates in flexible pavements are specified mainly by their gradation:- to furnish adequate load-bearing capacity to carry construction traffic and further reduce traffic stresses on subgrade, prevent the intrusion of fine particles (filtration as required), improve the subsurface drainage characteristics of the roadway, etc. 82

2 Base layer aggregates should have such properties and be graded in such a manner that they have high stability (strength and stiffness), which is the factor of primary importance. Base layers may also be used for subsurface drainage. Base/Subbase Aggregates, under rigid pavements, are not specified mainly by their the loadbearing capacity, but emphasis is also placed on achieving a gradation which will prevent pumping of the subgrade or intrusion of frost-susceptible materials while at the same time improving the subsurface drainage characteristics of the roadway. In high-quality bituminous road surfacing, aggregates comprise of up to about 95 per cent of the weight of the surfacing. The surfacing aggregates should have adequate stability (as they are primarily responsible for any load-carrying capacity which the surfacing may have); must be resistant to abrasion and durable (resistant to adverse weather conditions). Although there are very many types of bituminous surfacing, in general, the ideal aggregates should have adequate strength and toughness, ability to crush into chunky particles, free from dust, unduly thin and elongated particles, and hydrophilic (water loving) characteristics, and should have particle size and gradation appropriate to the type of construction. These criteria are also important for concrete, particularly those relating to particle shape and size distribution, since they affect water requirements and workability of concrete mixes as well as other important concrete properties. A wide range of materials can be used as unbound base and subbase courses including crushed quarried rock, crushed and screened, mechanically stabilized, modified or naturally occurring as dug or pit run gravels. Their suitability for use depends primarily on the design traffic level of the pavement and climate. However, such materials must have a particle size distribution and particle shape which provide high mechanical stability and should contain sufficient fines (amount of material passing the mm sieve) to produce a dense material when compacted. The use of locally available materials is encouraged, particularly at low traffic volumes. Their use should be based on the results of performance studies and should incorporate any special design features which ensure their satisfactory performance. 83

3 5.2 Sources of Aggregates Sources of aggregates for use in pavement works include: Hard rock sources (crushed quarried rock) hard sound bed rock exposures that need blasting and crushing Naturally occurring gravels which includes alluvial deposits, and highly weathered and fractured residual formations (rippable or can be worked using earth moving machinery such as Dozers). These may be used as is (pit-run) or may need further processing to be suitable for use such as crushing over sized stones and screening and/or other modifications such as mechanical stabilization. The principal sources of road aggregates in Ethiopia include natural sand and gravel deposits, and crushed rock. Pulverized concrete and asphalt pavements and other recycled and waste materials are not used, but could be the future source of pavement materials. Crushed aggregates - Hard rocks are important sources of aggregates. There are different types of rocks, all composed of grains of crystalline minerals held together in a variety of ways. The Property of a rock depends upon the properties of its constituent minerals and nature of bond between them (i.e. composition, grain size and texture of the rock) which in turn depends on its mode of origin. Geologists classify rocks into three major types according to their mode of origin/formation. These are Igneous, Sedimentary, and Metamorphic rocks. Igneous rocks are formed from the cooling of molten material (magma). Some classifications/definitions related to igneous rocks are given below: Extrusive (volcanic) igneous rocks (Basalts, Rhyolite, Trachyte) which formed on or near the surface of the earth s crust due to rapid cooling of magma and hence rocks are fine grained (glassy or vitreous/like a glass (without crystal) or partly vitreous and partly crystalline (with small grain sizes)). Intrusive (Plutonic) igneous rocks (Granite, Gabbro) formed by the cooling of magma below the earth s surface, and hence are crystalline (the crystals may be big enough to be visible by the naked eye). Acidic Rocks igneous rocks with high silica (Sio 2 ) content > 63%. (granite, rhyolite) 84

4 Intermediate rocks igneous rocks with intermediate silica content, Sio 2 between 52% - 62%. (andesite, diorite) Basic rocks igneous rocks with low silica content, Sio 2 between 45 52%. (Basalt, gabbro). Generally speaking, igneous rocks with medium grain sized particles are preferable as source for aggregates. Coarse grained rocks (grain size >1.25mm) are liable to be brittle and to break down under compacting roller (not tough). While rocks with too fine grain sizes (<0.125mm) and if especially vesicular, the aggregates are liable to be brittle and splittery (flaky/elongated shapes). Acidic rocks tend to be vely charged and aggregates containing large amounts of feldspar and quartz in large crystals do not bind well with bitumen (bitumen has a slight ve charge). Basic rocks bind well with bitumen. Most common igneous rocks used for production of aggregate are basalt and granite which are both strong and resistant to wear. But one may need to check the bonding property of granitic aggregates with bitumen, which may need anti-stripping agent. Sedimentary rocks are formed from the solidification of chemical or mineral sediments deposited under ancient seas. They are usually layered since the original material was deposited in this manner. Sedimentary rocks may be siliceous rocks formed from disintegrated rock sediments transported by wind or water, re-deposited as sediments, then consolidated or cemented in to a new rock type (e.g. conglomerate (consolidated gravel), sand stone (consolidated sand), shales (consolidated mud/clay, rich in organic matter: silt stone or clay stone). Calcareous rocks rocks formed by chemical deposition of organic remains in water (Gypsum, chalk, Limestone). Indurated sediementary rocks (Fine grained well cemented sand stones, limestones) are used as aggregate sources. However, they are not entirely satisfactory as high quality aggregates due to their variable cementation (hence variable strength and durability), their tendency to polish (not suitable for surfacing aggregates) and risk of alkali aggregate reaction (concrete aggregates). Metamorphic rocks are igneous or sedimentary rocks that have been changed (metamorphosed) due to intense heat and pressure into new rocks by the recrystallization of their constituents (e.g. Quartzite, Gneiss, Schist, Slate, marble, etc.). Schist and slate are highly foliated rocks which are not desirable as they are fissile and liable to be crushed when compacted with rollers. Quartzite and gneisses can furnished good aggregates. The properties of aggregates produced in quarries from bedrock depend on the type of bedrock. Igneous and metamorphic rocks are usually very hard and make excellent aggregates for most purposes. Limestone and dolomite are quite common sedimentary rocks. They are softer than 85

5 igneous rocks, but are still acceptable as aggregates for most purposes. Shale, being composed of clay grains, is very weak and disintegrates easily when exposed to the weather and is a poor aggregate material. Natural sand and gravel pits have been used extensively as sources of road aggregates. Sand or gravel pit is first stripped of topsoil, vegetation, and other unsuitable material from the surface of the deposit to obtain pit run materials. The material obtained is loose, and is usually excavated with power shovels or front-end loaders. Often it is crushed, especially if there are cobbles or boulders in the deposit. The smaller sizes go through the crusher without change, whereas larger particles are broken down to the desired size. Crushed gravel, as this is called, is a high-quality aggregate used for many purposes. Sand or gravel deposits might be composed of many different types of mineral particles-such as limestone, sandstone, and granite--depending on the original bedrock source of the particles. Recycled material - the use of pulverized concrete from pavements, sidewalks, and buildings being demolished is growing in other countries both due to the increased cost of natural aggregates and the desire to recycle rather than landfill these materials. Recycled concrete is crushed, processed, and used as base material and in concrete and asphalt paving mixtures. Asphalt pavements can be recycled and reused in pavements. Pulverized asphalt mixtures are also used as aggregates in base courses, but the proportion may be limited to about 30-50% as the strength of the layer can be reduced due to the lubricating effect of the asphalt film on the particles. Aggregates produced from bedrock are obtained from quarries. Aggregate production involves extraction (blasting and breaking to pieces), crushing (reduction to size using compression/impact crushers) and screening. After stripping and opening the quarry, holes are drilled from the surface. Then dynamite is placed and detonated in these holes to break the rock into sizes that can be transported. The rock is then fed to crushers which reduce it (crush it) to the required sizes in various types of rock crushers. The aggregates are then screened to the various required sizes. 86

6 5.3 Aggregate tests Aggregates are obtained from different sources and consequently differ considerably in their constitutions; inevitably they differ also with regard to their engineering properties. The properties of aggregate that are important for road construction include its cleanliness (contamination with dust and other deleterious materials), particle size and shape, gradation, toughness - resistance to crushing, abradability - wearing/abrasion resistance, durability/soundness, specific gravity and water absorption, surface texture, tendency to polish, bonding property with bitumen. Aggregate tests are necessary to determine the suitability of the material for a specific use and to make sure that the required properties are consistently within specification limits. The following sub-sections discuss important tests of aggregates and their significance of application; detailed test procedures can be found in standard material testing manuals. Particle Size and Shape Gradation test: Gradation is the characteristic of aggregates on which perhaps the greatest stress is placed in specifications for highway bases, cement concretes, and asphalt mixes. Hence, gradation test, also called sieve analysis, screen analysis or mechanical analysis, is the most common test performed on aggregates to evaluate the suitability of the aggregate materials with respect to their grain size distribution for a specific use. Gradation is determined by separating the aggregates into portions, which are retained on a number of sieves or screens having specified openings, which are suitably graded from coarse to fine. The results obtained may be expressed either as total percentage passing or retained on each sieve or as the percentages retained between successive sieves. The theoretical maximum density of aggregates is obtained when the grain size distribution follow the Fuller maximum density equation of the form p = 100 d D n (5.1) in which, p is the percent passing sieve size "d", "D' represents the maximum aggregate size in the material, and n is a constant which varies between 0.45 and 0.5 for maximum density. The assumption in this relationship is that the voids between the larger particles are filled with still smaller particles, until the smallest voids are filled with a small amount of fines. Strength, or 87

7 resistance to shear failure, in road bases and other aggregate layers that carry load is increased greatly if the mixture is dense graded. The larger particles are in contact with each other, developing frictional resistance to shearing failure, and tightly bound together due to the interlocking effect of the smaller particles. When aggregate particles are to be bound together by cement or bitumen, a variation in the grading of an aggregate will result in a change in the amount of binder required to produce a material of given stability and quality. Proper aggregate grading contributes to the uniformity, workability and plasticity of the material as it is mixed. Often the fines content must be limited, because they are relatively weak, and require an excessive amount of binder to cover them. If fines are present as dust on larger particles, they weaken the bond between the cement and those particles. Fines in highway bases may lead to drainage and frost- heaving problems. Also, excessive amounts of fines may result in weak mixtures, as the large particles are not in contact with each other. The strength of the mixture would then depend only on friction between the small particles, which is much less than between large particles. In practice, the required gradation is not found naturally, particularly, if the aggregates are pit-run materials. In such cases, combining two or more aggregates of different sources satisfies the gradation requirement for a specific use. Aggregate Crushing Value (ACV) Test. Aggregate crushing test evaluates the resistance of aggregates against the gradually applied load. The test is used to evaluate the crushing strength of available supplies of rock, and in construction, to make sure that minimum specified values are maintained. The test is undertaken using a metal plunger to apply gradually a standard load of 400kN to a sample of the aggregate (10 14 mm) contained in a standard test mould. The amount of material passing 2.36 mm sieve in percentage of the total weight of the sample is referred to as the Aggregate Crushing value (ACV). Over the range of normal road making aggregates, ACVs vary from 5 percent for hard aggregates to 30 percent for weaker aggregates. For weaker aggregates than this, the same apparatus is used to evaluate the Ten Percent Fines value i.e. the load which produces 10 percent of fines passing 2.36 mm sieve. The value is obtained by interpolating of the percentage of fines produced over a range of test loads. Aggregate Impact Test. This test is a means of evaluating the resistance of aggregates to sudden impact loading. It is carried out by filling a steel test mould with a sample of aggregate (10 14 mm) and then the impact load applied is by dropping hammer at a height of 380 mm. The Aggregate Impact Value (AIV) is the percentage of fines passing 2.36 mm sieve after 15 blows. This test produces results that are normally about 105 per cent of the ACV and it can be used for 88

8 the same purposes. Both tests give results which are sufficiently repeatable and reproducible for contract specifications. Abrasion Test. Abrasion test is the test used to know how the aggregate is sufficiently hard to resist the abrasive effect of traffic over its service life. The most widely used abrasion test is the Los Angeles Abrasion Test which involves the use of a steel drum, revolving on horizontal axis, into which the test sample of chippings is loaded together with steel balls of 46.8 mm diameter. The Los Angeles Abrasion Value (LAV) is the percentage of fines passing the 1.7 mm sieve after a specified number of revolutions of the drum at specified speed. The drum is fitted with internal baffles causing the aggregate and the steel balls to be lifted and then fall as the drum revolves. The test therefore gives an indication of the impact strength in combination with the abrasion resistance of the aggregate. For bituminous surface dressings, chippings with an ACV less than 30 are desirable and the stronger they are the more durable will be the dressings. With premixed bituminous materials and with crushed stone bases, high mechanical strength, though useful, is not always of paramount importance. The repeatability and reproducibility of this test are satisfactory and appropriate for use in contract specifications. Soundness Test. This test procedure is useful in both survey and design for the evaluation of aggregates to resist disintegration due to weathering. A sample of aggregate is saturated in a solution of magnesium sulphate or sodium sulphate, and then removed and dried in an oven. This process is repeated for five cycles. On completion, the percentage lost gives the durability of the material. The test is not suitable for providing a single criterion for the susceptibility of aggregates to rapid weathering but it may find a place as part of the evaluation procedure of aggregates suspected of containing minerals that are weakened by chemical alteration. Specific Gravity and Water Absorption. The tests are likely to be used both in surveys of aggregate resources and in design, particularly in the interpretation of compaction tests and in the design of bituminous mixtures. They may also be used as part of quality control during construction, particularly when the survey has indicated that aggregate from the chosen source is subject to variations in density. The test procedure is simple and the tests are repeatable and reproducible. Most rocks absorb less than one per cent by weight of water and, up to this level, water absorption is of no great consequence. However, some rocks can absorb up to 4 percent of water. This suggests that the rock may be of low mechanical strength and will be difficult to dry and 89

9 heat during processing to make bituminous mixtures. Inadequate drying will cause difficulty in securing good adhesion between bitumen and stone, and in hot process mixtures, where the stone must be heated to about 180 o C, it causes a large waste of energy. In the tests, a 4 kilogram sample of the crushed rock of specific nominal size chippings is soaked in distilled water for 24 hours, weighed in water (W W ), surface dried and weighed in air (W S ). It is then oven dried at 105 o C for 24 hours and weighed again in air (W D ). The specific gravity and the water absorption are then obtained as follows: Specific gravity W W W D S D = 100 D w W W Water absorption (%) = (5.2) W D Shape Tests. Three mechanical measures of particle shape which may be included in the specifications for aggregates for road construction, are the flakiness index, elongation index and angularity number. The flakiness index of an aggregate is the percentage by weight of particles whose least thickness is less than three-fifths of their mean dimension. The mean dimension, as used in each instance, is the average of two adjacent sieve aperture sizes between which the particle being measured is retained by sieving. The elongation index of an aggregate is the percentage by weight of particles whose greatest length is greater than 1.8 times their mean dimension. The angularity number of an aggregate is the amount, to the nearest whole number, by which the percentage of voids exceeds 33 when an aggregate is compacted in a specified manner in a standardized metal cylinder. Use of the shape tests in specifications is based on the view that the shapes of the particles influence both the strength of aggregate particles and internal friction that can be developed in the aggregate mass. Since, other factors being equal, an aggregate composed of smooth rounded particles of a certain gradation will contain less voids than one of the same grading but composed of angular particles, the angularity of an aggregate can be reflected in terms of the volume of contained voids when the aggregate is compacted. Measurements show that the angularity number may range from zero for a material of highly rounded beach-gravel particles to 10 or more for newly crushed rock aggregate. 90

10 Blending aggregates To meet the gradation requirements of aggregates for particular uses in pavement construction, it is often necessary to blend two or more aggregates together. Charts and diagrams are available to do this blending, but the trial-and-error method is simpler and just about as fast as more complex methods. Consider two aggregates graded and designated as aggregate A and B, and let the specification limit be as given in Table 5-1. The use of the trail-and-error method for blending is then illustrated as follows: Table 5-1: Aggregate gradation to be combined to meet specification limits % Passing Sieve Aggregate A Aggregate B Specification Mid-point Combined aggregate 12.5 mm No No It is clear in Table 5-1 that all the material passing a No.10 sieve must come from aggregate B, i.e., approximately 48% which leaves 52 % for aggregate A. Or consider the retained percentage on No.10 sieve for alternative solution. All materials retained on No.10 must come from aggregate A, which is 52 % require in the specification, i.e. 52 % from A and 48% from B. In both cases, the proportion which best fits the specification limits can be satisfied. The following equation may be written to apply the procedure to any given sieve: aa + bb = T (5.3) where, A and B are percentages from aggregates A and B to be blended for satisfying the specification limits. a and b are the respective sieve analysis values for a given sieve X, expressed as a decimal fraction, and T is the sieve analysis value in the blended aggregate. The equation can be used for gradation expressions 91

11 1. Percentage retained on a given sieve, 2. Percentage passing on a given sieve, and 3. Percentage retained on two or more sieves. The result of this equation is used to proportion the 1 st trial blend for the trial-and-error method. The second and the subsequent blends are proportioned by observation until the specification is satisfied. In the above illustration, the equation can be written for the No.10 sieve, % passing, as: a A + b B = T p p p p p (5.4) in which the subscript p indicates the percentage passing. The known variables here are a p = 0, b p = 1, and T p = 48%, which implies that B = 48%. For percent retained, the equation can be written as: a A + b B = T (5.5) r r r r r in which the subscript r indicates the percentage retained. The known variables here are a r = 1, b r = 0, and T r = 52%, which implies that A = 52%. 92

12 Example: Three aggregates are to be blended to meet a specification. The aggregates, gradations, and the specification are given in Table 5-2. Table 5-2: Aggregate gradation and specification for the example Sieve size Aggregate A Aggregate B Aggregate C Specification Spec. Mid-point Combined gradation (1 st trial) Passing 12.5 mm mm mm mm mm μm μm μm μm Solution: Most of coarse aggregate will come from aggregate A and most of the fines will be obtained from aggregate C. To obtain a mixture that is approximately in the middle of the specification, we first use the equation and continue with more trials. The equation can be written to blend aggregate A, B, and C for retained on 9.5 mm sieve and passing 75 μm sieve as follows: aa + bb + cc = T (5.3) For retained materials on 9.5 mm sieve, the known variables are a r = 0.38, b r = 0, c r = 0 and T r = 20%, which implies that A = 53%. Similarly, for passing 75 μm, the known variables are a p = 0, b p = 0, c p = 0.18 and T p = 4.5%, which results C = 25%, and B = = 22. The first trial blend as seen in Table 5-2 is within the specification limit, but on the coarse side. Reducing the contribution of aggregate A and increasing B, or C or both for the second and the subsequent trials can result in a blend more close to the middle of the specification. 93

13 5.4 Unbound Base and Subbase Materials (ERA Pavement Design Manual Requirements) Unbound base and subbase courses in pavement structures are granular materials from sand or gravel deposits or crushed rock from quarries without admixtures. The required properties of these materials vary with the type of pavement and the depth of the material in the pavement structure. Different standard methods of design specify materials of construction differently considering the traffic load, locally available materials, and environmental conditions. The following describes the requirements set for different unbound pavement materials for base and subbase courses as specified in ERA pavement design manual (2002) Base course Graded crushed aggregate: This material is produced by crushing fresh, quarried rock usually termed a 'crusher-run', or alternatively the material may be separated by screening and recombined to produce a desired particle size distribution, as per the specifications. The rock used for crushed aggregates should be hard and durable. Laboratory and field experiences have shown that crushed particles have, in general, more stability than rounded materials due to primarily to added grain interlock. In addition, crushed materials possess high coefficient of permeability. Alternate gradation limits, depending on the local conditions for a particular project, are shown in Table 5-3. After crushing, the material should be angular in shape with a Flakiness Index of less than 35%, and preferably of less than 30%. In constructing a crushed stone base course, the aim should be to achieve maximum impermeability compatible with good compaction and high stability under traffic. 94

14 Table 5-3: Grading limits for graded crushed stone base course materials Test sieve (mm) Percentage by mass of total aggregate passing test sieve Nominal maximum particle size 37.5 mm 28 mm 20 mm Note 1. For paver-laid materials a lower fines content may be accepted. To ensure that the materials are sufficiently durable, they should satisfy the criteria given in Table 5-4.These are a minimum Ten Per Cent Fines Value (TFV) and limits on the maximum loss in strength following a period of 24 hours of soaking in water. Alternatively, if requirements expressed in terms of the results of the Aggregate Crushing Value (ACV) are used, the ACV should preferably be less than 25 and in any case less than 29. Other simpler tests e.g. the Aggregate Impact Test may be used in quality control testing provided a relationship between the results of the chosen test and the TFV has been determined. Unique relationships do not exist between the results of the various tests but good correlations can be established for individual material types and these need to be determined locally. The in situ dry density of the placed material should be a minimum of 98% of the maximum dry density obtained in the Heavy Compaction. The compacted thickness of each layer should not exceed 200 mm. Crushed stone base materials described above should have CBR values well in excess of 100 per cent, and fines passing mm sieve should be nonplastic. 95

15 Table 5-4: Mechanical strength requirements for crushed stone base defined by TFV Typical annual rainfall (Mm) Minimum 10% fines values (kn) Minimum ratio wet/dry Test (%) > < Requirements for natural gravels and weathered rocks: A wide range of materials including lateritic, calcareous and quartzitic gravels, river gravels, boulders and other transported gravels, or granular materials resulting from the weathering of rocks can be used successfully as base course materials. Table 5-5 contains three recommended particle size distributions for suitable materials corresponding to maximum nominal sizes of 37.5 mm, 20 mm and 10 mm. When the traffic is in excess of 1.5x10 6 ESA, only the two larger sizes should be considered. Table 5-5: Recommended particle size distributions for base course material Test sieve (mm) Percentage by mass of total aggregate passing test sieve Nominal maximum particle size 37.5 mm 20 mm 10 mm For materials whose stability decreases with breakdown, an aggregate hardness based on a minimum soaked TFV of 50 kn may be specified. The fines of these materials should preferably be nonplastic but should normally never exceed a PI of 6. If the PI approaches the upper limit of 6, it is desirable that the fines content be restricted to the lower end of the range. To ensure this, a maximum Plasticity Product (PP) of 60 is recommended or alternatively a maximum Plasticity Modulus (PM) of 90 where: 96

16 PP = PI x (percentage passing the mm sieve) PM = PI x (percentage passing the mm sieve) When used as a base course, the material should be compacted to a density equal to or greater than 98 per cent of the maximum dry density achieved in the Heavy Compaction. When compacted to this density in the laboratory, the material should have a minimum CBR of 80% after four days immersion in water. In low rainfall areas, typically with a mean annual rainfall of less than 500 mm, and where evaporation is high, moisture conditions beneath a well sealed surface are unlikely to rise above the optimum moisture content. In such conditions, high strengths (CBR>80 %) are likely to develop even when natural gravels containing a substantial amount of plastic fines are used. In these situations, for traffic loading within 0.7 million equivalent standard axles, the maximum allowable PI can be increased to 12 and the minimum soaked CBR criterion reduced to 60% at the expected field density. Rocks such as basalts, dolerites, and granular materials derived from their weathering, transportation or other alteration release undesirable plastic fines during construction or in service. The release of these minerals may lead to a consequent loss in bearing capacity and this is likely to worsen if water enters the pavement and lead to rapid and premature failure. The state of decomposition also affects their long-term durability when stabilized with lime or cement. When weathering is suspected, petrographic analysis to detect secondary (clay) minerals and soundness tests using sodium or magnesium sulphate should be carried to evaluate the durability of the materials. Indicative limits based on these tests are (a) a maximum secondary mineral content of 20%, (b) a maximum loss of 12 or 20% after 5 cycles in the sodium or magnesium sulphate tests respectively. Naturally occurring gravels which do not normally meet the normal specifications for base course materials have occasionally been used successfully. They include lateritic, calcareous and volcanic gravels. In general their use should be confined to the lower traffic roads. Laterite gravels with plasticity index in the range of 6-12 and plasticity modulus in the range of is recommended for use as base course material for of traffic volume up to 1.5 million equivalent standard axles. The values towards higher range are valid for semi-arid and arid areas of Ethiopia, i.e. with annual rainfall less than 500 mm. Cinder gravels can also be used as base course materials in lightly trafficked (below 0.7x10 6 ESA) surface dressed roads. 97

17 1.1.2 Sub-base course materials The sub-base is an important load spreading layer which enables traffic stresses to be reduced to acceptable levels on the subgrade. It also acts as a working platform for the construction of the upper pavement layers separating the subgrade and base course. Under special circumstances, it may serve as a filter or as a drainage layer. The selection of sub-base materials depends on the design function of the layer and the anticipated moisture regime, both in service and at construction. Bearing capacity: A minimum CBR of 30 per cent is required at the highest anticipated moisture content when compacted to the specified field density, usually a minimum of 95 per cent of the MDD achieved in the Heavy Compaction. Under conditions of good drainage and when the water table is not near the ground surface the field moisture content under a sealed pavement will be equal to or less than the optimum moisture (Light Compaction). In such conditions, the sub-base material should be tested in the laboratory in an unsaturated state. If saturation of the sub-base is likely, the bearing capacity should be determined on samples soaked in water for a period of four days. Materials which meet the recommendations of Table 5-6 and Table 5-7 will usually be found to have adequate bearing capacity. Use as a construction platform: In many circumstances the requirements of a sub-base are governed by its ability to support construction traffic without excessive deformation or ravelling. A high quality sub-base is therefore required where loading or climatic conditions during construction are severe. Suitable material should possess properties similar to those of a good surfacing material for unpaved roads. The material should be well graded and have a plasticity index at the lower end of the appropriate range for an ideal unpaved road wearing course under the prevailing climatic conditions. These considerations form the basis of the criteria given in Table 5-6 and Table 5-7. Material meeting the requirements for severe conditions will usually be of higher quality than the standard sub-base material. In the construction of low-volume roads, where cost savings at construction are particularly important, local experience is often invaluable and a wider range of materials may often be found to be acceptable. In Ethiopia, laterite is one of the widely available materials and can be used as a sub-base material. Laterite meeting the gradation requirements of Table 5-7 can be used for traffic levels up to 3x10 6 ESA provided the following criteria is satisfied: 98

18 Plasticity Index (%) < 25 Plasticity Modulus (PM) < 500 CBR (%) > 30 Table 5-6: Recommended plasticity characteristics for granular sub-bases Climate Typical Annual Rainfall Liquid Limit Plasticity Index Linear Shrinkage Moist tropical and wet >500mm <35 <6 <3 tropical Seasonally wet trop >500mm <45 <12 <6 Arid and semi-arid <500mm <55 <20 <10 Table 5-7: Typical particle size distribution for sub-bases Test Sieve (mm) Percentage by mass of total aggregate passing test sieve (%) Filter or separating layer: This may be required to protect a drainage layer from blockage by a finer material or to prevent migration of fines and the mixing of two layers. The two functions are similar except that for use as a filter the material needs to be capable of allowing drainage to take place and therefore the amount of material passing the mm sieve must be restricted. The following criteria should be used to evaluate a subbase as a separating or filter layer: a) The ratio D15(coarse layer) should be less than 5 D85(fine layer) where D15 is the sieve size through which 15% by weight of the material passes and D85 is the sieve size through which 85% passes. b) The ratio D50 (coarse layer) should be less than 25 D50 (fine layer) 99

19 For a filter to possess the required drainage characteristics a further requirement is: c) The ratio D15 (coarse layer) should lie between 5 and 40 D15 (fine layer) These criteria may be applied to the materials at both the base course/sub-base and the subbase/subgrade interfaces Selected subgrade materials and capping layers These materials are often required to provide sufficient cover on weak subgrades. They are used in the lower pavement layers as a substitute for a thick sub-base to reduce costs, and a cost comparison should be conducted to assess their cost effectiveness. The requirements are less strict than for sub-bases. A minimum CBR of 15 per cent is specified at the highest anticipated moisture content measured on samples compacted in the laboratory at the specified field density. This density is usually specified as a minimum of 95 per cent of the MDD in the Heavy Compaction. Recommended gradings or plasticity criteria are not given for these materials. However, it is desirable to select reasonably homogeneous materials since overall pavement behaviour is often enhanced by this. The selection of materials which show the least change in bearing capacity from dry to wet is also beneficial Gravel surface roads Gravel surface roads are generally roads which are constructed and maintained at low cost using locally available materials in the near vicinity of the site. Coarse well graded gravel is a very satisfactory material for constructing cheap all-weather roads. This type of construction is designed for AADT between 350 and 400 and when the weight of the individual vehicle is in the order of 10 ton. Beyond these, they often become not economical. At higher traffic the following problems such as surface pitting, the formation of transverse corrugation, high cost of replacing or grading, and dust may occur. The general soil-aggregate mixture used for constructing gravel roads should be stable (support the loads without detrimental deformation which is the function of particle size distribution and particle shape, density, and internal friction and cohesion), abrasion resistant, should shed a large 100

20 portion of the rain which falls on the surface, posses capillarity properties to replace the moisture lost by surface evaporation, and low-cost. Type 1 (Table 5-8) is recommended for gravel wearing course material in the new construction of roads having an AADT greater than 50 and for all routine and periodic maintenance activities. According to the Tanzanian Design Manual (1999), gravel wearing course for major roads require a minimum CBR of 25 %. Type 4 materials may be used in the new construction of roads having an AADT less than 50. Minor gravel roads (AADT design less than 20) which are normally community roads are usually unsurfaced (earth roads) and constructed by labor-based methods. However, for subgrade CBR values less than 5% and longitudinal gradients of greater than 6%, a gravel wearing course is recommended. Materials for gravel wearing course shall comply with the requirements for Type 4 material for new construction and Type 1 for maintenance activities. The CBR requirements may be reduced to 20% if other suitable material is not locally available. Table 5-8: Gradation requirements for gravel wear course (ERA, 2001) Test Sieve Percent(%) by mass of total aggregate passing test sieve Size(mm) Type 1 Type 2 Type 3 Type 4 Type 5 Type

21 Type 1: The grading of the gravel after placing and compaction shall be a smooth curve within and approximately parallel to the envelopes detailed in Table 5-8. The material shall have a LAV of not more than 50 at 500 revolutions. The material shall be compacted to a minimum in-situ density of 95% of the maximum dry density. The plasticity index should be not greater than 15 and not less than 8 for wet climatic zones and should be not greater than 20 and not less than 10 for dry climatic zones. The linear Shrinkage should be in a range of 3-10%. Type 2 & 3: These materials may be more rounded particles fulfilling the following: the Plasticity Index lies in a range of 5-12% in wet areas, and in any case less than 16% in other areas, a minimum crushing under traffic in percentage by weight of particles with at least one fractured face of 40%, the CBR should be in excess of 20 after 4 days of soaking at 95% of maximum dry density under Heavy Compaction. For very low traffic, the requirement may be relaxed to a CBR of 15. Type 4: This material gradation allows for larger size material and corresponds to the gradation of a base course material. The use of this gradation of materials is subject to the local experience and shall be used with PIs in a range of Type 5 & 6: These materials gradations are recommended for smaller size particles. They may be used if sanctioned by experience with plasticity characteristics as for material Type