THE INFLUENCE OF FINES CONTENT AND PLASTICITY ON THE STRENGTH AND PERMEABILITY OF AGGREGATE FOR BASE COURSE MATERIAL Bambang Ismanto SISWOSOEBROTHO Department of Civil Engineering Bandung Institute of Technology Jalan Ganesha No. 10; Bandung 40132 INDONESIA Fax : +62-22-2502350 E-mail : bis@trans.si.itb.ac.id Pamudji WIDODO Department of Civil Engineering Bandung Institute of Technology Jalan Ganesha No. 10; Bandung 40132 INDONESIA Fax : +62-22-2502350 E-mail : pamudji@trans.si.itb.ac.id Erwandy AUGUSTA Program on Highway and Development Bandung Institute of Technology Jalan Ganesha No. 10; Bandung 40132 INDONESIA Fax : +62-22-2534167 E-mail : stjr@trans.si.itb.ac.id Abstract : The quantity and type of fines have a major influence on the performance of an unbound aggregate road-base. The Indonesian specification for aggregate base Class-A permits a fines content in the range of 0% to 8%; the quantity of fines is controlled by limiting the maximum values of Plasticity Index (PI) and Liquid Limit (LL) to 6% and 25%, respectively. This paper describes an investigation into the influence of fines content and type on the strength and permeability of aggregate base Class-A. Fines content investigated were 0, 4, 8, 12 and 16%; the fines had PI values of 0%, 9.47% and 24.09%. Results result indicates that PI is the most effective parameter for controlling the strength of the mix as it appears to be independent of fines type. Significant reduction in permeability occurs when 4% fines is added. Key Words : Fines, Road-base, Plasticity, Permeability, CBR 1. INTRODUCTION A flexible pavement is a multilayer system comprising of a surface layer, a road base layer and a sub-base layer installed on a compacted sub-grade. Each layer has particular functions and characteristics. Of these layers, the road-base layer is the main structural component of the pavement. It plays a major role in spreading the load imposed at he surface so that the stresses transmitted to the sub-base and sub-grade do not exceed the strength of these layers. The layer therefore must possess high stiffness and strength. Materials used in road-base construction include cemented and bituminous material and unbound granular material. An unbound granular road-base derives its high stability from particle interlock and inter-particle friction. Aggregate grading and properties are of particle importance in this type of construction. Drainage capability, as indicated by permeability, is also an important consideration when designing an unbound granular base. When stability is the primary criterion, a dense continuously-graded mix is preferred. However, this grading may conflict with the permeability requirement. 845
For a particular grading, there is a molding moisture content, the optimum moisture content, at which maximum dry density and hence stability is achieved. In the majority of grading specification, limits are placed on the percentage of fines, i.e. material passing the No. 200 (75 µm) sieve. In addition a maximum value is specified for Plasticity Index (PI). In the Standard National Indonesia (SNI) specification for Class A unbound granular road-base, the amount passing sieve No. 200 may range from 0 to 8%; the Plasticity Index should not exceed 6%. The amount of material finer than 75 µm can be expected to influence the dry density, strength and permeability of unbound granular base; the plasticity of the fines is likely to influence strength characteristic. This paper describes the influence of fines content and plasticity on the strength, compaction and permeability characteristics of SNI Class A road-base material. 2. UNBOUND AGGREGATE BASE COURSE 2.1 Aggregate Base Course Aggregate is the largest single material used in highway construction and its properties are important for the quality of the pavement. The design of the aggregate mixture to ensure high stability is an important aspect of pavement design. The stability of an unbound granular base course derives mainly from particle interlock and surface friction and gradation of particle size distribution is therefore an important characteristic for strength determination. 2.2 Plasticity of Fines In the design of an unbound aggregate mixture, among the first activities is an investigation of the plasticity of the fines because the physical properties of the binder soil have a great effect on stability. Activity (Krebs and Walker, 1971) Sieve Sizes (mm) Figure 1. Classification Chart for Swelling Potential The effect of the plasticity of the binder on the triaxial strength of gravel with varying amounts of material passing the No. 30 sieve shows that when the percent of material passing 846
the No. 30 sieve is relatively low, plasticity has very little effect on strength. However, as the amount of material passing the No. 30 sieve is increased, plasticity has an increased effect (Yoder and Witczak, 1975). Table 1. Classification of Clays Base on Activity Value Activity Classification < 0.75 Inactive clays 0.75 1.25 Normal clays > 1.25 Active clays The activity of a soil is defined as the ratio of the Plasticity Index to the percentage by weight of soil less than 0.002 mm in effective diameter. Activity values for clays range from about 0.4 for kaolinite of soils may be conveniently classified into three groups as shown on the Table 1. Activity is also used to assess swelling potential as shown on Figure 1. 2.3 Strength of Unbound Aggregate Base Course A base course is defined as the layer of material that lies immediately below the pavement surfacing. Base courses constructed of stone fragments, slag and soil-aggregate mixture lie close to the surface; hence, they must possess high resistance to deformation in order to withstand the high pressure imposed upon them. The functions of a base curse are prevention of pumping, drainage, prevention of volume change of sub-grade, increased structural capacity and expedition of construction. To accomplish these functions high density and stability are required. An aggregate with little or no fines content (Figure 2a) gains stability from grain-to-grain contact. An aggregate that contains no fines usually has a relatively low density but is pervious and not frost susceptible. This material is however difficult to handle during construction because of its non-cohesive nature. a. Aggregates with no fines b. Aggregate with sufficient fines c. Aggregate with great amount of fines (Yoder and Witczak, 1975) Figure 2. Physical States of Soil Aggregates Mixture An aggregate that contains sufficient fines to fill all voids between the aggregate grains will still gain its strength from grain-to-grain contact but has increased shear resistance (Figure 2b). Its density is high and its permeability is low. This material moderately difficult to compact but is ideal from the standpoint of stability. As shown in Figure 2c, material that contains a great amount of fines has no grain-to-grain contact and the aggregate merely 847
float in the soil. Its density is low; it is practically impervious and it is frost susceptible. In addition, the stability of this type of material is greatly affected by adverse water conditions. Paradoxically, the material at times is quite easy to handle during construction and compacts quite readily (Yoder and Witczak, 1975) The stability of soil-aggregate mix used in flexible pavement construction is normally determined by the CBR test. The test provides for compacting the soil in a cylindrical mold and soaking the sample for 4 days with an imposed load roughly equivalent to that which would be applied by a prototype pavement. The compaction simulates construction and the soaking simulates a water content adjustment roughly equivalent to that which would occur if the water table is 2 ft below the base formation. 3. BASE COURSE MIXTURE PREPARATION Base course Class A is a layer composed of coarse aggregate, fine aggregate and fines, i.e. material finer than 75 µm. The coarse aggregate fraction is material retained on the 4.75 mm sieve and is required to consist of hard, durable particles or fragments of crushed rock or gravel; the fine aggregate fraction is material passing the 4.75 mm sieve and retained on sieve No. 200 and consists of natural or crushed sand and fine mineral particles Table 2. Class A Base Course Gradation Sieve Size Grading Band Median mm % Passing % Passing 2 50 100 100 1 25 65 90 82.5 3/8 9.5 40 60 50 No. 4 4.75 25 45 35 No. 10 1.19 12 30 21 No. 40 0.425 6 16 11 No. 200 0.075 0-8 4 Percentage Passing Sieve Sizes (mm) Figure 3. SNI Specification for Base Course Class A 848
In this investigation, the coarse and fine aggregate used in the preparation of samples was 100% crushed rock; for the fines (material passing No. 200), materials having different Plasticity Index values were used. Crushed rock fines (Banjaran provided material having 0% Plasticity Index). As discussed before that materials having a PI values in the range of 6% - 25% were obtained after sampling and characterizing samples from nine locations. Samples of base course Class A were prepared based on the median of the SNI specification as shown on Table 2 and Figure 3. Mixed were prepared with 0%, 4%, 8%, 12% and 16% material passing sieve No. 200, i.e. finer than 75 µm. The gradations obtained are shown in Figure 4. A total of 15 mixture types were investigated, i.e. 5 fines contents and 3 values of Plasticity Index. Mixes are labeled as A, B and C in order of increasing Plasticity Index and 1 to 5 in order of increasing fines content. Percentage Passing Sieve Sizes (mm) Figure 4. Particle Size Distribution of Grading Investigated 4. RESULTS AND DISCUSSIONS One objective of the investigation was to determine the effect of the plasticity of the fines on the strength and permeability characteristics of unbound road-base Class A. Because of time constraints only three values of plasticity were investigated, i.e. soil with a PI value of 0% and 2 others in the range of 6 to 25%. Soil was sampled at a number of locations in Kabupaten Bandung (Bandung Municipality) and soils with the appropriate plasticity values were selected for the investigation. The tests were done in the Highway Laboratory and Soil Mechanics Laboratory of Bandung Institute of Technology. 4.1 Properties of Natural Soils The soil property of particular interest was the Plasticity Index, montmorillonite content was also determined on selected soils to avoid soils that have a high swelling potential. To determine the montmorillonite content, the methylene blue test was performed (done outside 849
of Bandung Institute of Technology and helped by Material and Chemical Laboratory of the Department of Energy and Mineralogy Resources). After field investigations at nine locations, the Plasticity Index of the soil at the nine locations and the montmorillonite content of the soil at 4 locations were determined and the results are shown on Table 3 and Table 4. Table 3. PL, LL and PI Values of Soils Samples No Soil Location LL(%) PL (%) PI (%) 1 Cipatik-1 41.50 22.40 19.10 2 Cipatik-2 28.80 19.22 9.58 3 Cipatik-3 44.90 27.30 17.60 4 Cipatik-4 51.30 29.64 21.76 5 Cimareme 31.10 21.63 9.47 6 Cipakem 54.20 30.21 24.09 7 Cipatey 34.58 21.52 13.06 8 Ciruum 47.50 27.23 20.27 9 Gn. Harikukun 38.00 18.49 19.51 Table 4. Montmorillonite Content of Selected Soils Soil Location Montmorillonite Content (%) Cimareme 15 Cipakem 30 Ciruum 20 Gn. Harikukun 20 The main criterion for soil selection was PI and the objective was to select 2 soils with widely different PI values but with acceptable montmorillonite content. Based on the Plasticity Chart developed by Casagrande, Cimareme soil is classified as an inorganic clay of medium plasticity and the Cipakem sol is classified as an inorganic clay of high plasticity and according the activity, soils from Cimareme and Cipakem are inactive clays. Table 5. Gradation of Mixtures Investigated Sieve Size (mm) 1 2 Percentage of Passing Type of Gradation 3 4 5 50 100 100 100 100 100 25 77. 77.5 77.5 77.5 77.5 9.5 50.0 50.0 50.0 50.5 50.0 4.75 35.0 35.0 35.0 35.0 35.0 1.19 21.0 21.0 21.0 21.0 21.0 0.425 11.0 11. 11.0 15.0 19.0 0.075 0.0 4.0 12.0 12.0 16.0 4.2 Classification of the Aggregate Mixtures The grading curve of the mixtures investigated are shown in Figure 4 and the grading investigated are summarized in Table 5, again the number of 1 to 5 are represented the fine 850
content of 0%, 4%, 8%, 12% and 16%. Classification of the mixtures investigated on the basis of the AASHTO system is given in Table 6. Mixture Type A B C % Passing No. 10 Table 6. Properties and Classification of Mixtures % Passing No. 40 % Passing No. 200 App. SG LL (%) PL (%) PI (%) AASHTO Classification A1 21.00 11.00 0 2.78 - - - A-1-a A2 21.00 11.00 4 2.78 - - - A-1-a A3 21.00 11.00 8 2.77 - - - A-1-a A4 21.00 15.00 12 2.77 - - - A-1-a A5 21.00 19.00 16 2.77 - - - A-1-a B2 21.00 11.00 4 2.77 22.5 17.86 4.64 A-1-a B3 21.00 11.00 8 2.76 24.0 18.69 5.31 A-1-a B4 21.00 15.00 12 2.75 25.0 18.65 6.35 A-2-4 B5 21.00 19.00 16 2.75 25.5 18.27 7.23 A-2-4 C2 21.00 11.00 4 2.77 22.0 16.43 5.57 A-1-a C3 21.00 11.00 8 2.76 24.0 17.69 6.31 A-2-4 C4 21.00 15.00 12 2.75 28.0 18.80 9.20 A-2-4 C5 21.00 19.00 16 2.74 30.0 17.36 12.64 A-2-4 SNI specifies that the Plasticity Index of the aggregate base course should not exceed 6%. Based on the Atterberg limits test results, value of Plasticity Index determine for mixtures Type B4, B5, C3, C4 and C5 are 6.35%, 7.23%, 6.31%, 9.20% and 12.64% respectively. These values exceed the maximum value of PI, the other mixtures are acceptable Plasticity Index (%) Fines Content (%) Figure 5. Effect of Quantity and Type of Fines on the Plasticity Index The results of the LL test gave values for mixtures Type B4, B5, C4 and C5 of 25%, 25.5%, 28% and 30% respectively. These exceed of the maximum value of 25% specified by standard but the other mixtures satisfy this requirement. 851
Figure 6. Effect of Quantity and Type of Fines on the Specific Gravity of the Combined Aggregate Fractions in the Mixtures 4.3 Compaction Test The maximum dry density of each mixture at the optimum water content was determined using the modified ASHTO compaction procedure. Material was compacted in the 6 in. diameter mold in five approximately equal layers to give a total compacted depth of about 5 in. Each layer was compacted by 56 uniformly distributed blows of a 10-lb hammer dropping freely from a height of 18 in. The influence of the fines content on the maximum dry density is shown on Figure 7 for mixture containing fines Type A, B and C, Max Dry Density (gr/cc) App. Spec. Gravity (gr/cc) Fines Content (%) Fines Content (%) Figure 7. Results of Modified Compaction Test on Unbound Aggregate Base The modified compaction test results shown in Figure 7 show that maximum dry density of aggregate base mixture Type A, i.e. material with non-plastic crushed stone fines, increases up to 8% fines content and then decreases with increased amount of fines. For mixtures 852
containing plastic fines, mixtures Type B and C, maximum dry density is obtained at a fines content of 4%. At this fines content the maximum dry densities for mixtures Type A and Type C (high PI) are almost identical; the highest dry density is achieved by mixture B (medium PI). As fines content is increased above 4%, maximum dry density reduces. In the case of the mixture containing fines of high plasticity the reduction is most evident when of the mixture containing fines of medium plasticity, increasing the fines from 12% to 16% causes the most significant reduction in the maximum dry density. 4.4 CBR Test This test is used in determining the bearing capacity of unbound pavement layers. The test is useful for evaluating sub-grade soil, sub-base and road-base course material containing only a small amount of material retained on the 19.0 mm sieve. Samples of soil-aggregate mixture for the CBR test were prepared at optimum moisture content and soaked for 9 hours before the test was carried out. The influence of fines content on the maximum dry density is shown on Figure 7 for mixtures containing fins type A, B and C. In all cases, the CBR value at 0.2 penetration exceeded the value at 0.1 penetration. The influence of the fines content on the soaked CBR value of the samples compacted at maximum density is shown on Figure 8. CBR (%) Fines Content (%) Figure 8. Results of California Bearing Ratio Test on Unbound Aggregate Base Variation in soaked CBR with fines content follows a similar pattern to tat observed for maximum dry density. In the case of mixture Type A (non-plastic fines) the CBR value peaks at a fines content of 8%; in the case of mixtures Type B and C (medium and high plasticity fines, respectively) maximum CBR is achieved at a fines content of 5%. A minimum soaked CBR value of 80% is specified by standard and the mixture with 0% fines content does not meet this specification. The mixtures with non-plastic fines meet the specification up to a fines content of 16%, the maximum fines content investigated. The mixture with fines of medium plasticity meets the specification at fines contents of 4% and 8% while the mixture 853
containing highly plastic fines just meets the specification at a fines content of 4%. The CBR of the mixture with highly plastic fines reduces very significantly as fines content is increased to 16%. 4.5 Permeability Test The permeability of a soil is a measure of its capacity to allow the passage of fluid through the soil. Procedure for the measurement of the permeability of a soil in the laboratory are of two types, Constant Head and Falling Head Tests. In this investigation the falling head method was used. Preparation of samples used the modified compaction procedure. The diameter of the mold was 6 in. and the sample was compacted in 5 approximately equal layers by applying 56 uniformly distributed blows of a 4.54 kg hammer dropping freely from a height of a 8 in. to each layer. Table 7. Summary of Test Results for the Unbound Aggregate Base Mixtures Investigated A B C A1 0.0 2.067 5.87 NP - 75.55 9.64E-5 A2 4.0 2.182 6.44 NP - 95.58 3.69E-5 A3 8.0 2.208 6.78 NP - 102.17 2.08E-5 A4 12.0 2.201 7.34 NP - 97.73 1.36E-5 A5 16.0 2.165 8.08 NP - 88.86 1.26E-5 B2 4.0 2.212 7.52 4.64 18.56 102.22 1.19E-5 B3 8.0 2.192 7.78 5.31 42.48 95.07 4.64E-6 B4 12.0 2.163 8.14 6.35 76.20 76.44 2.05E-6 B5 16.0 2.070 9.86 7.23 115.68 64.77 1.25E-6 C2 4.0 2.175 7.90 5.57 22.28 84.22 8.11E-6 C3 8.0 2.090 9.76 6.31 50.48 73.27 2.67E-6 C4 12.0 2.060 10.58 9.20 110.42 33.85 1.00E-6 C5 16.0 2.029 11.98 12.64 202.24 19.901 2.59E-7 The degree of permeability of Type A material (non-plastic fines) is classified as medium over the range of fines contents investigated and has a good drainage characteristics. Type B material (medium plastic fines) is also indicated to have a medium degree of permeability and good drainage characteristics over the range in fines content investigated. However at fines content of 12% and 16%, drainage characteristics are close to the boundary between good and poor. In the case of Type C material (highly plastic fines), the drainage characteristics of material containing 4% and 8% fines can be described as good although material with 8% fines is close to the boundary between good and poor. Material with 12% fines is on the boundary between good and poor while the drainage characteristics of material containing 16% fines fall into the poor category. Looking at the criteria for classification of permeability, all of the Type A materials and material Type B1 can be classified as having medium permeability; material Types B2, B3 and B4 and Type C1, C2 and C3 have low permeability and Type C4 is on the borderline between low and very low. Selection of the mixture should consider minimum soaked CBR value. However if the coefficient of permeability is also a criterion, mixture Type C2 with 4% of highly plastic fines and mixtures Type B2 and B3 with 4% and 8% medium plasticity fines are also acceptable. 854
Coefficient of Permeability (m/sec) Fines Content (%) Figure 9. Results of Permeability Test on Unbound Aggregate Base 5. CONCLUSIONS In this investigation, mixture of aggregate base Class A containing 0, 4, 8, 12 and 16% fines were investigated. The fines were of 3 type, i.e. non-plastic fines and fines with PI values of 9.47% (medium plasticity) and 24.09% (high plasticity). The conclusions reached are summarized as follows : a. Mixture containing 12% and 16% medium plasticity fines and 8%, 12% and 1% high plasticity fines have values of PI that exceed the 6% maximum specified by Indonesian standard. b. The maximum LL value of 25% specified by the Indonesian standard is exceeded by the mixture containing 16% medium plasticity fines and 12% and 16% high plasticity fines c. A peak value of maximum dry density is evident as the fines content of the mixture is increased. In the case of the mixture containing non-plastic fines, maximum dry density has a peak value at 8% fines content. In the case of mixtures containing plastic fines, maximum dry density peaks at a fines content of 4%. d. The variation in soaked CBR with increase in fines content follows a pattern similar to that observed for maximum dry density. Mixture containing non-plastic fines has a peak CBR value at 8% fines, the CBR of mixtures containing plastic fines peaks at 4% fines. e. The introduction of 4% fines to the mixture causes a very significant reduction in the permeability; there is a less dramatic reduction in permeability with further increase in the amount of fines. At any fines is considerably more permeable than the mixture made with plastic fines. 855
REFERENCES Augusta E. (2000) The Influence of Fines Content and Plasticity on the Strength and Permeability of Aggregate Base Class A, Thesis, Master Program on Highway and Development, Bandung Institute of Technology IRE Reseach Report (1989) A Road Materials Inventory for West Java, Ministry of Public Works, Agency for Research and Development, Institute of Road Engineer, Bandung Krebs, R.D. and Walker, R.D. (1971) Highway Materials, McGraw-Hill Book Company, New York, USA Suaryana, N (1999) Tinjauan Penyebab Kerusakan Lapis Pondasi, Departemen Pekerjaan Umum, Badan Penelitian dan Pengembangan PU, Pusat Penelitian dan Pengembangan Jalan, Bandung Yoder, E.J. and Witczak, M.W. (1975) Principles of Pavement Design, 2 nd edition, John Wiley & Son, Inc, New York, USA 856