Kishor Kumar

Prime Coat Construction Methodology – Part 2 This is Part 2 of the Prime Coat Construction Methodology series and is a continuation of Part 1. This part covers the remaining methodology from Sl. No. 12 to Sl. No. 29, including the application procedure, quality control requirements, inspection points, curing, acceptance criteria, common defects, preventive measures, and best construction practices as per MoRTH Specifications. 12. Tray Test Procedure The Tray Test is the most reliable field method used to verify the actual Prime Coat application rate delivered by the bitumen pressure distributor. Although the distributor may already be calibrated, the tray test confirms that the required quantity of SS-1 Grade Cationic Bitumen Emulsion is being uniformly applied under actual site conditions. The tray test shall be carried out before the commencement of spraying and thereafter at regular intervals during production, particularly after changing the spray pressure, vehicle speed, spray nozzles, or any major maintenance of the distributor. The measured application rate shall comply with the approved spray rate specified in the project Quality Assurance Plan (QAP) and MoRTH Clause 502. Purpose of Tray Test To verify the actual spray rate (kg/m²). To check uniformity of spray across the entire spray bar. To identify blocked or damaged spray nozzles. To confirm calibration of the pressure distributor. To ensure compliance with the approved application rate. Equipment Required Standard calibration trays (generally 300 mm × 300 mm) Electronic weighing balance (0.1 g accuracy) Measuring tape Marker or chalk Calculator Observation sheet Personal Protective Equipment (PPE) Step-by-Step Tray Test Procedure Ensure the pressure distributor has already been inspected and calibrated, and that all spray nozzles are clean, properly aligned, and free from blockage. Measure and record the dimensions of each calibration tray. The tray area shall be known accurately for calculating the application rate. Place the trays flat on the pavement surface at equal intervals across the full spray width. Normally, one tray is placed below each alternate nozzle or at locations approved by the Engineer. Ensure that the trays remain level and stable during spraying. Movement of trays may lead to inaccurate results. Operate the pressure distributor over the trays at the proposed production speed while maintaining constant pump pressure. After spraying, carefully collect each tray without spilling any emulsion. Determine the weight of emulsion collected in each tray using a calibrated electronic balance. Calculate the application rate for each tray using the prescribed formula. Compare the calculated spray rate of each tray with the specified application rate. The results should be reasonably uniform across the entire spray width. If significant variation exists between trays, inspect the spray nozzles, spray bar height, pump pressure, and vehicle speed before repeating the tray test. Formula Application Rate (kg/m²) = Collected Weight (kg) ÷ Tray Area (m²) Worked Example Parameter Value Tray Size 300 mm × 300 mm Tray Area 0.09 m² Collected Emulsion 22.5 g (0.0225 kg) Calculated Spray Rate 0.0225 ÷ 0.09 = 0.25 kg/m² Result Within Specified Limits ✔ Acceptance Criteria The average spray rate shall comply with the approved project specification. The difference between tray readings shall be minimal, indicating uniform spray distribution. No tray shall show excessive or insufficient emulsion due to blocked nozzles or incorrect spray overlap. If the average spray rate is outside the specified range, spraying shall not commence until corrective actions have been implemented. Quality Control Checklist ✔ Tray dimensions verified. ✔ Electronic balance calibrated. ✔ Trays placed across full spray width. ✔ Constant vehicle speed maintained. ✔ Constant pump pressure maintained. ✔ Spray rate calculated for each tray. ✔ Average application rate within specification. ✔ Calibration records maintained. Common Site Problems and Corrective Actions Observation Possible Cause Corrective Action One tray contains very little emulsion Blocked nozzle Clean or replace the nozzle. One tray contains excessive emulsion Damaged or oversized nozzle Replace the nozzle. All tray values are higher than specification Vehicle speed too low or pump pressure too high Increase vehicle speed or reduce pressure. All tray values are lower than specification Vehicle speed too high or pump pressure too low Reduce speed or increase pressure. Tray readings vary considerably Incorrect nozzle alignment or spray bar height Adjust spray bar height and nozzle angles, then repeat the tray test. Fresh Engineer Learning Point Many fresh engineers believe that the average spray rate alone is sufficient. In actual highway construction, the uniformity of individual tray readings is equally important. For example, an average of 0.25 kg/m² may appear acceptable, but if one side of the spray bar applies 0.35 kg/m² and the other side applies only 0.15 kg/m², the pavement will experience non-uniform bonding, leading to localized failures. Therefore, always review each tray reading, not just the average. Senior Site Engineer’s Practical Tip On major projects, experienced engineers conduct a tray test immediately before the first day’s production and again whenever there is a change in pump pressure, spray nozzles, vehicle speed, or emulsion source. Spending 15–20 minutes on a tray test can prevent hundreds of metres of improperly primed pavement and avoid expensive rework. 13. Setting Spray Bar Height The spray bar height is one of the most important factors affecting the uniformity of Prime Coat application. Even if the pressure distributor is properly calibrated and the vehicle speed is maintained correctly, an incorrect spray bar height can produce streaking, excessive overlap, untreated strips, or non-uniform application of the bitumen emulsion. The spray bar shall be adjusted so that the spray fans from adjacent nozzles overlap uniformly over the pavement surface. Proper overlap ensures that every part of the pavement receives the specified quantity of emulsion without excess or deficiency. Why Spray Bar Height is Important? Each nozzle produces a fan-shaped spray pattern. When the spray bar is set at the correct height, the spray fans overlap uniformly, resulting in a continuous and even application across the full pavement width. Incorrect spray bar height leads to uneven binder distribution, which adversely affects the bonding between pavement layers. Recommended Nozzle Arrangement Parameter Recommended Practice Nozzle Angle

Determination of Kinematic Viscosity of Bitumen The Kinematic Viscosity Test is conducted to determine the resistance of bitumen to flow under the influence of gravity at a specified temperature. The test is performed using a Glass Capillary Viscometer and is widely used for quality control of paving bitumen in highway construction projects. In major highway projects executed by leading EPC contractors, including L&T ECC Division, the test is carried out during material approval, source verification, quality assurance, and routine laboratory testing to ensure that the bitumen possesses the required flow characteristics for mixing, pumping, spraying, and compaction. Objective To determine the kinematic viscosity of bitumen for: Material Approval Quality Control Testing Bitumen Source Verification Highway Pavement Construction Batch Acceptance Testing Compliance with IS and IRC Specifications Apparatus Required Glass Capillary Viscometer (Cannon-Fenske or equivalent) Constant Temperature Bath Thermometer Vacuum or Suction Device Stopwatch (0.1 second accuracy) Heating Oven Beaker Cleaning Solvent Drying Oven Principle The test is based on measuring the time required for a fixed volume of bitumen to flow through a calibrated glass capillary viscometer under gravity at a specified constant temperature. The flow time is multiplied by the viscometer constant to obtain the Kinematic Viscosity, expressed in mm²/s (centistokes, cSt). Detailed Testing Procedure The following procedure is commonly adopted in highway material laboratories for determining the Kinematic Viscosity of Bitumen using a calibrated glass capillary viscometer in accordance with ASTM D2170 / IS requirements. Step 1 – Selection of Viscometer Select a clean, calibrated Glass Capillary Viscometer (Cannon-Fenske, BS U-Tube or equivalent) having a suitable viscometer constant for the expected viscosity range. Verify that the viscometer calibration certificate is valid. Ensure that the capillary tube is free from scratches, deposits and blockages. Record the viscometer identification number and calibration constant. Site Engineer’s Tip: Always use the same calibrated viscometer for duplicate observations whenever possible. Step 2 – Preparation of Bitumen Sample Heat the bitumen slowly in a clean container until it becomes sufficiently fluid for pouring. Use indirect heating wherever possible to avoid localized overheating. Stir the sample gently using a clean spatula until a uniform consistency is achieved. Do not allow smoke to appear from the sample. Remove visible air bubbles before transferring the sample into the viscometer. Practical Note: Excessive heating accelerates oxidation and permanently changes the viscosity of bitumen. Step 3 – Cleaning the Viscometer Wash the viscometer thoroughly using a suitable solvent. Flush with a volatile cleaning liquid. Dry the viscometer completely using filtered dry air. Ensure no moisture remains inside the capillary tube. Quality Check: Even a very small amount of solvent or moisture can significantly affect viscosity measurements. Step 4 – Charging the Viscometer Pour the heated bitumen carefully into the viscometer through the filling tube. Fill only up to the prescribed level recommended for that viscometer. Avoid trapping air bubbles. Clean any bitumen adhering to the outside surface. Step 5 – Conditioning in Constant Temperature Bath Suspend the viscometer vertically inside the constant temperature bath. Maintain the bath temperature at the specified test temperature (normally 135°C for paving bitumen). The temperature variation should not exceed ±0.1°C. Allow the specimen to remain in the bath for at least 20–30 minutes until complete thermal equilibrium is achieved. Laboratory Practice: Do not begin timing immediately after placing the viscometer in the bath. Wait until the entire sample reaches the test temperature. Step 6 – Raising the Bitumen Column Using a suction bulb or vacuum device, draw the bitumen above the upper timing mark. Ensure the meniscus is approximately 5 mm above the first timing mark. Release the suction smoothly. Allow the sample to flow freely under gravity. Step 7 – Timing the Flow Start the stopwatch when the leading edge of the meniscus crosses the upper timing mark. Observe the flowing bitumen carefully without disturbing the viscometer. Stop the stopwatch when the meniscus reaches the lower timing mark. Record the flow time to the nearest 0.1 second. Important: Timing errors of even one second can noticeably influence the calculated viscosity. Step 8 – Repeat the Determination Conduct at least two determinations on the same sample. The difference between duplicate observations should be within the permissible repeatability limits. If the difference exceeds the allowable limit, repeat the test using a fresh sample. Use the average value of acceptable observations for reporting. Step 9 – Calculation Calculate the kinematic viscosity using: Kinematic Viscosity (ν) = Viscometer Constant (C) × Flow Time (t) Where: ν = Kinematic Viscosity (mm²/s or cSt) C = Calibration Constant of Viscometer t = Average Flow Time (seconds) Step 10 – Reporting of Results Report the test temperature. Report the viscometer identification number. Record duplicate flow times. Mention the viscometer constant used. Report the calculated kinematic viscosity in mm²/s (cSt). State the applicable test standard (ASTM D2170 / IS Method). Record the date of testing and the name of the testing engineer. Best Practice Followed in EPC Highway Projects: The laboratory report should also include the bitumen grade (VG-10, VG-20, VG-30 or VG-40), batch number, source, test temperature, calibration details of the viscometer and observations for complete traceability. Observation Table Record all observations carefully. Duplicate determinations should be carried out, and the average flow time should be used for calculating the kinematic viscosity. Observation Symbol Unit Bitumen Grade – – Sample Identification – – Test Temperature T °C Viscometer Identification No. – – Viscometer Constant C mm²/s² Flow Time – Trial 1 t₁ Seconds Flow Time – Trial 2 t₂ Seconds Average Flow Time t Seconds Kinematic Viscosity ν mm²/s (cSt) Calculations Average Flow Time Average Flow Time (t) = (t₁ + t₂) / 2 Kinematic Viscosity ν = C × t Where: ν = Kinematic Viscosity (mm²/s or cSt) C = Viscometer Calibration Constant t = Average Flow Time (seconds) Example Calculation Given: Bitumen Grade = VG-30 Test Temperature = 135°C Viscometer Constant (C) = 1.20 mm²/s² Flow Time (Trial 1) = 398.5 s Flow Time (Trial 2) = 401.1 s Step 1 – Average Flow Time t =

Determination of Specific Gravity of Cement and Fly Ash The Specific Gravity Test is conducted to determine the relative density of cement and fly ash using the Le Chatelier Flask Method. The result is used in concrete mix design, pavement quality concrete (PQC), dry lean concrete (DLC), and quality control of cementitious materials. As followed in major highway projects, including EPC projects executed by leading contractors, the test is carried out during material approval, source verification, mix design preparation, and periodic quality control. Objective To determine the specific gravity (relative density) of cement and fly ash for use in: Concrete Mix Design PQC and DLC Mix Design Rigid Pavement Construction Material Approval Quality Control Volumetric Calculations Apparatus Required Le Chatelier Flask Kerosene or Naphtha (Non-reactive Liquid) Analytical Balance (0.01 g Accuracy) Glass Funnel Thermometer Spatula Dry Cloth Desiccator Principle The test is based on the liquid displacement principle. A known mass of cement or fly ash is introduced into a Le Chatelier flask containing kerosene. The increase in liquid level represents the volume occupied by the material. Since cement reacts chemically with water, kerosene or naphtha is used instead of water. The specific gravity is calculated by dividing the sample mass by the displaced liquid volume. Step-by-Step Test Procedure Step 1 – Preparation of Apparatus Ensure the Le Chatelier flask is clean, dry and free from dust. Check that the flask has no cracks or damage. Allow the flask to attain room temperature before testing. Record the laboratory temperature if required. Practical Note: Even a small amount of moisture inside the flask can affect the test result. Step 2 – Fill the Flask with Kerosene Fill the flask with clean kerosene up to the graduation between 0 and 1 ml. Avoid overfilling. Remove any visible air bubbles by gently rotating the flask. Step 3 – Record Initial Reading (V1) Place the flask on a level surface. Allow the liquid level to stabilize. Read the lower meniscus carefully at eye level. Record the initial reading as V1. Step 4 – Weigh the Sample Take approximately 64 g of dry cement or fly ash. Weigh the sample accurately using an analytical balance. Record the sample weight as W. Ensure the sample is dry and free from lumps. Step 5 – Add the Sample into the Flask Using a glass funnel, slowly introduce the sample into the flask. Add the material in small portions. Avoid spilling the sample. Prevent the material from sticking to the neck of the flask. Step 6 – Remove Entrapped Air Gently rotate the flask until all air bubbles disappear. Do not shake the flask vigorously. Ensure the sample is completely wetted by kerosene. Continue rotating until no air bubbles are visible. Field Practice: Normally, the flask is rotated for about 5–10 minutes until all entrapped air is removed. Air bubbles may reduce the calculated specific gravity. Step 7 – Record Final Reading (V2) Place the flask on a vibration-free table. Allow the liquid level to stabilize. Read the lower meniscus carefully. Record the final reading as V2. Observation Table Observation Symbol Initial Flask Reading V1 Mass of Cement / Fly Ash W Final Flask Reading V2 Volume Displaced (V2 − V1) Calculations Volume Displaced V = V2 − V1 Specific Gravity Specific Gravity = W / (V2 − V1) Where: W = Mass of Cement/Fly Ash (g) V2 − V1 = Volume Displaced (ml) Example Calculation Sample Weight (W) = 64 g Initial Reading (V1) = 0.8 ml Final Reading (V2) = 21.2 ml Volume Displaced 21.2 − 0.8 = 20.4 ml Specific Gravity 64 ÷ 20.4 = 3.14 Typical Specific Gravity Values Material Typical Specific Gravity OPC 3.10 – 3.16 PPC 2.90 – 3.15 PSC 2.85 – 3.00 Fly Ash 2.10 – 2.60 GGBS 2.80 – 2.95 Precautions Use only clean and dry apparatus. Use fresh and dry cement or fly ash. Do not use water as the displacement liquid. Remove all entrapped air before taking the final reading. Read the meniscus at eye level. Avoid vibration while taking readings. Clean the flask thoroughly after completing the test. Common Errors Error Effect on Result Moisture inside flask Incorrect volume measurement Entrapped air bubbles Lower calculated specific gravity Incorrect meniscus reading Measurement error Wet or lumpy sample Unreliable results Using water instead of kerosene Cement reacts with water, making the test invalid Engineering Significance Specific gravity is an important property of cementitious materials used in concrete mix design. Accurate values are essential for calculating material proportions, determining concrete density, and ensuring consistent quality. Regular testing also helps verify material conformity with manufacturer specifications and detect any changes due to moisture absorption, contamination, or storage conditions. Frequently Asked Questions (FAQs) What is the specific gravity of cement? Specific gravity of cement is the ratio of the density of cement to the density of water at the same temperature. It indicates how heavy cement particles are compared to water and is an important property used in concrete mix design. What is the typical specific gravity of Ordinary Portland Cement (OPC)? The typical specific gravity of Ordinary Portland Cement (OPC) ranges from 3.10 to 3.16. However, the actual value may vary slightly depending on the manufacturer and cement composition. What is the typical specific gravity of fly ash? The specific gravity of fly ash generally ranges between 2.10 and 2.60. The value depends on the source of coal, combustion process, and chemical composition of the fly ash. Why is the specific gravity test of cement important? The test is important because the specific gravity of cement is required for concrete mix design, calculation of absolute volume, quality control, and verification of cement properties before use in construction. Why is the specific gravity test of fly ash important? Determining the specific gravity of fly ash helps in designing concrete mixes accurately, calculating replacement quantities, and ensuring uniform quality of supplementary cementitious materials. Which method is used to determine the specific gravity of cement? The specific gravity of cement is determined using

PQC M40 Construction, QA/QC, Failures & Maintenance | Highway Engineering Guide PQC M40 Construction, QA/QC, Failures & Maintenance Guide 1. Overview of PQC Pavement Quality Concrete (PQC) is a rigid pavement layer designed to directly transfer traffic loads to the subgrade through slab action. It is widely used in National Highways for long service life (30–40 years). ✔ Key Advantage: High flexural strength, rut-free surface, excellent durability, and low maintenance requirement. 2. Step-by-Step PQC Construction Process 2.1 Subgrade & GSB Preparation The quality of Pavement Quality Concrete (PQC) primarily depends on the strength and uniformity of the foundation layers. Before laying Dry Lean Concrete (DLC), the completed subgrade and Granular Sub Base (GSB) shall be prepared, inspected, and approved to ensure adequate load-bearing capacity, proper drainage, and accurate pavement geometry. Setting Out: Establish centre line, edge lines, benchmarks, offsets, and finished levels using Total Station/Auto Level before commencing earthwork. Subgrade Preparation: Trim the prepared subgrade to the required line, level, camber, and cross fall. Remove loose soil, organic matter, soft pockets, and any unsuitable material encountered during trimming. Compaction: Compact the subgrade uniformly using suitable vibratory rollers until the specified density (minimum 97% of Maximum Dry Density as per project specifications) is achieved. Moisture content shall be maintained close to Optimum Moisture Content (OMC). Proof Rolling: Carry out proof rolling using a loaded dump truck or pneumatic tyred roller to identify weak, yielding, or pumping areas. Excavate and replace defective portions with approved material before proceeding. Drainage Arrangement: Ensure side drains and temporary drainage channels are functional so that no water stagnation occurs on the prepared subgrade during construction. GSB Laying: Spread Granular Sub Base (GSB) material in the specified layer thickness using a motor grader or sensor paver. Control segregation during unloading and maintain uniform thickness throughout the carriageway. GSB Compaction: Compact each GSB layer with vibratory rollers until the specified field density is achieved. Add water uniformly wherever required to maintain optimum moisture for effective compaction. Surface Finish: After compaction, the GSB surface shall be firm, dense, and free from ruts, loose aggregates, depressions, or segregated patches. Any irregularities shall be corrected before laying the next layer. Quality Control: Verify field density, moisture content, layer thickness, levels, cross fall, and longitudinal profile at the prescribed frequency. The completed GSB shall satisfy all tolerance limits before commencement of DLC. Final Approval: Clean the finished GSB surface thoroughly and obtain Engineer’s approval before laying Dry Lean Concrete (DLC). Traffic movement over the approved surface should be restricted to prevent damage or contamination. 2.2 Dry Lean Concrete (DLC) Layer Preparation Dry Lean Concrete (DLC) is the rigid foundation layer placed below Pavement Quality Concrete (PQC). It provides a stable, uniform, and non-erodible platform for PQC, distributes wheel loads to the sub-base, minimizes pumping, and facilitates the movement of paving equipment during concrete pavement construction. Surface Preparation: Before laying DLC, ensure the approved GSB surface is clean, compact, free from loose material, and maintained at the specified line, level, thickness, and cross fall. Any damaged or segregated portions shall be repaired. Mix Production: Produce DLC using a calibrated batching plant in accordance with the approved mix design. Control water content carefully to achieve the specified consistency without segregation. Transportation: Transport the concrete in transit mixers or dumpers without delay. Prevent contamination, excessive moisture loss, and segregation during transportation. Mechanical Laying: Spread the DLC uniformly using a sensor paver or mechanical paver to the specified thickness. Manual laying should be limited only to inaccessible or minor areas. Compaction & Finishing: Compact the laid concrete immediately using internal and surface vibrators integrated with the paver. Finish the surface with a screed to achieve the required level and smoothness without overworking the concrete. Level & Thickness Control: Continuously check layer thickness, longitudinal profile, cross fall, and surface levels during paving. Any deficiency beyond permissible tolerances shall be rectified immediately. Joint Construction: Construct longitudinal and transverse construction joints at planned locations. The edges of completed panels shall be neat, straight, and properly protected before adjacent concreting. Curing: Begin curing immediately after the surface has hardened sufficiently. Cure the DLC continuously for a minimum of 7 days using water or approved curing methods to prevent rapid moisture loss and shrinkage cracking. Quality Control: Verify thickness, density, compressive strength, line, level, and surface regularity at the prescribed frequency. Only approved DLC shall be accepted for subsequent PQC paving. Pre-PQC Inspection: Before laying PQC, inspect the DLC surface for cracks, honeycombing, loose particles, laitance, contamination, and ponding water. Clean the surface thoroughly and obtain Engineer’s approval prior to commencement of PQC. Site Engineering Tip: DLC acts as the structural foundation of rigid pavement. Poor compaction, inadequate curing, low thickness, or an uneven surface can lead to pumping, loss of support, slab cracking, faulting at joints, and premature failure of the PQC pavement. Investing time in proper DLC construction significantly improves the long-term performance and service life of the concrete pavement. 2.3 Separation Membrane A separation membrane is a polyethylene sheet laid over the finished DLC surface before placing Pavement Quality Concrete (PQC). It acts as a bond breaker between DLC and PQC, allowing the concrete slab to expand and contract freely due to temperature and moisture variations. Proper installation of the membrane minimizes frictional restraint, reduces shrinkage stresses, and improves the long-term performance of the rigid pavement. Surface Preparation: Before laying the membrane, ensure the DLC surface is clean, dry, smooth, and free from loose particles, laitance, sharp aggregates, oil, or standing water. Repair any damaged or uneven areas. Material: Use approved polyethylene (PE) sheet of 125–200 micron thickness (or as specified in the contract). The sheet shall be free from tears, punctures, folds, and manufacturing defects. Laying Procedure: Unroll the membrane carefully over the DLC surface without wrinkles or air pockets. Lay it continuously in the direction of paving and ensure complete coverage of the pavement width. Overlap: Provide a minimum 300 mm overlap between adjacent sheets. The overlaps shall remain flat and shall not create ridges

Stone Matrix Asphalt (SMA) – Complete Technical Guide as per IRC & MoRTH Stone Matrix Asphalt (SMA) is a gap-graded bituminous mixture designed to provide exceptional rutting resistance, durability, and long-term pavement performance under heavy traffic loading. It consists of a stone-on-stone aggregate skeleton, rich bituminous mortar, mineral filler, and stabilizing fibres that work together to create a highly stable and durable pavement surface. Originally developed in Germany, SMA has become one of the most preferred surfacing materials for national highways, expressways, urban arterial roads, intersections, climbing lanes, and other heavily trafficked pavements. In India, the requirements for SMA are specified under MoRTH Clause 515, which covers materials, mix design, production, laying, compaction, and quality control requirements. This comprehensive guide explains the composition, design philosophy, material requirements, construction methodology, special performance tests, and quality control procedures for Stone Matrix Asphalt in accordance with IRC and MoRTH specifications. MoRTH Clause 515.1 – Scope of Stone Matrix Asphalt (SMA) This clause covers the construction of fibre-stabilized Stone Matrix Asphalt (SMA) in single or multiple layers over a previously prepared bituminous bound surface. SMA may be used as either a wearing course or a binder/intermediate course depending upon the aggregate gradation and layer thickness requirements. Application of SMA Layers The SMA mixture consists of a gap-graded aggregate skeleton, rich bituminous mortar, mineral filler, and stabilizing fibres. The fibres help prevent binder drain down and ensure uniform distribution of the higher bitumen content characteristic of SMA mixes. SMA Type Maximum Aggregate Size Recommended Use Nominal Layer Thickness 13 mm SMA 13.2 mm Wearing Course 40–50 mm 19 mm SMA 19 mm Binder / Intermediate Course 45–75 mm Engineering Significance 13 mm SMA is primarily used as a wearing course due to its superior surface texture, skid resistance, and riding quality. 19 mm SMA is generally used as a binder or intermediate course where higher structural capacity and load distribution are required. The specified layer thickness ensures adequate aggregate interlock, proper compaction, and development of the stone-on-stone contact structure essential for SMA performance. SMA is particularly suitable for highways, expressways, intersections, climbing lanes, toll plazas, and other locations subjected to heavy traffic loading and rutting stresses. Practical Site Note Before laying SMA, the underlying bituminous layer should be structurally sound, properly compacted, clean, and provided with an approved tack coat to ensure adequate bond between layers. Site Engineer’s Checklist: Use 13 mm SMA for wearing course (40–50 mm thick). Use 19 mm SMA for binder/intermediate course (45–75 mm thick). Ensure fibre addition at the approved dosage. Verify layer thickness before compaction. Maintain paving and rolling temperatures as per JMF. Material Requirements for Stone Matrix Asphalt (SMA) The performance of Stone Matrix Asphalt (SMA) depends significantly on the quality of constituent materials. MoRTH Clause 515.2 specifies stringent requirements for bitumen, aggregates, mineral filler, and stabilizing additives to ensure the development of a durable stone-on-stone aggregate skeleton capable of resisting rutting and moisture damage under heavy traffic loading. 1. Bitumen The binder used in fibre-stabilized SMA shall be viscosity grade paving bitumen conforming to IS:73 or Modified Bitumen conforming to IS:15462 and IRC:SP:53. The selected binder grade shall be capable of satisfying all mix design requirements and shall conform to the requirements specified in MoRTH Table 500-2. Engineering Significance: SMA typically contains higher binder content than conventional Bituminous Concrete. Therefore, selection of an appropriate binder grade is critical for achieving durability, rut resistance, and resistance to binder drain down. 2. Coarse Aggregate Coarse aggregates shall consist of crushed rock retained on the 2.36 mm sieve. Aggregates shall be clean, hard, durable, cubical in shape, and free from dust, clay, organic matter, and other deleterious substances. Physical Requirements of Coarse Aggregates Property Test Method Requirement Cleanliness IS:2386 Part 1 < 2% passing 0.075 mm sieve Combined Flakiness & Elongation Index IS:2386 Part 1 < 30% Los Angeles Abrasion Value IS:2386 Part 4 < 25% Aggregate Impact Value IS:2386 Part 4 < 18% Polished Stone Value* IS:2386 Part 114 > 55% Soundness (Na₂SO₄) IS:2386 Part 5 < 12% Water Absorption IS:2386 Part 3 < 2% *Polished Stone Value requirement is not applicable for SMA used as binder/intermediate course. Engineering Significance: SMA derives its load carrying capacity primarily from stone-on-stone aggregate contact. Therefore, aggregate strength, durability, and shape requirements are more stringent than conventional dense graded mixes. 3. Fine Aggregate Fine aggregate passing the 2.36 mm sieve and retained on the 0.075 mm sieve shall consist entirely of crushed manufactured sand obtained from approved crushing operations. 100% crushed material Clean and durable particles Non-plastic in nature Free from organic and deleterious matter Sand Equivalent Value ≥ 50 Engineering Significance: Manufactured sand improves internal friction and stability of SMA compared to natural river sand. 4. Mineral Filler Mineral filler shall consist of finely divided mineral matter such as stone dust or hydrated lime. Use of fly ash as filler is not permitted. Grading Requirements of Mineral Filler IS Sieve Cumulative Passing (%) 0.600 mm 100 0.300 mm 95–100 0.075 mm 85–100 The Plasticity Index of filler shall not exceed 4. This requirement does not apply when hydrated lime is used. Special Requirement: If the SMA mixture fails the Moisture Susceptibility Test (AASHTO T 283), a minimum of 2% hydrated lime by total aggregate weight shall be incorporated. 5. Stabilizing Additive (Cellulose Fibre) Only pelletized cellulose fibres shall be used as stabilizing additives in SMA. The fibres prevent binder drain down and ensure uniform distribution of the rich bituminous mortar. Minimum Dosage 0.3% by weight of total mix (on loose fibre basis) Performance Requirement The fibre dosage shall be sufficient to ensure that binder drain down does not exceed 0.3% when tested in accordance with ASTM D6390. Cellulose Fibre Requirements Property Requirement Maximum Fibre Length 8 mm Ash Content Maximum 20% Oil Absorption More than 4 times fibre weight Moisture Content Less than 5% Site Engineer’s Check: Verify manufacturer’s certificate for fibre properties and ensure fibres are protected from moisture before use. SMA Mix Design Requirements After selection of suitable materials