Short answer: A Water Bound Macadam (WBM) road uses three coordinated materials: specified-size crushed coarse aggregate, screenings that fill the voids between the larger stones, and binding material used where the design requires a finer filler. They are not interchangeable. Buy each material against the approved layer, compacted thickness, grading and test limits—not against a supplier’s generic product name.
IRC:19-2005 provides the core WBM criteria, but your contract, drawings, road authority and currently adopted specification control the purchase. Confirm those documents before releasing a bulk order.
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What WBM is and why the material roles matter
Water Bound Macadam is a mechanically interlocked layer. Large stones are spread on a prepared foundation and compacted; smaller screenings are worked into the voids; water assists the packing and grouting; a fine binding material may then be added to fill remaining spaces and reduce ravelling.
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IRC:19-2005, clause 2.1.1, defines it this way: “Water Bound Macadam (WBM) shall consist of clean, crushed coarse aggregates mechanically interlocked by rolling, and voids thereof filled with screening and binding material with the assistance of water, laid on a prepared subgrade, sub-base, base or existing pavement as the case may be.”
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WBM may serve as a sub-base, a base beneath bituminous surfacing, or a surfacing course. The required stone quality, grading, plasticity and layer thickness change with that function. A material that is acceptable in a lower layer can be unsuitable for a surfacing course.
The three materials at a glance
| Material | Job in the layer | What to specify | Key acceptance checks |
|---|---|---|---|
| Coarse aggregate (stone metal) | Forms the load-bearing skeleton and locks together under rolling | Approved material source, grading and compacted layer thickness | Cleanliness, durability, particle shape, grading and representative test results |
| Screenings | Fills the spaces between the rolled coarse aggregate | Compatible source, Type A or Type B size and required gradation | Gradation, plasticity and fines; material must be dry enough for spreading and brooming |
| Binding material | Fine filler that helps grout remaining voids and limit ravelling where required | Fine-particle requirement and plasticity range for the course | Passing 425-micron sieve and plasticity-index result |
Coarse aggregate: the stone metal you order
Specify the stone by function, not by nominal size alone
Ask the quarry or supplier for the exact WBM grading identified in the approved design. The aggregate should be hard, durable and free from flat, elongated, soft, disintegrated, dirty or otherwise deleterious particles. A delivery labelled only “road stone,” “90 mm metal” or a similar trade description does not establish that it matches the required grading.
Permitted aggregate types in IRC:19-2005 include crushed or broken stone, crushed slag, overburnt brick (Jhama) metal, and suitable naturally occurring kankar or laterite. Each alternative has conditions of use. Crushable materials are generally restricted to lower pavement layers, so obtain written approval before substituting one for conventional crushed stone.
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Durability and shape limits
The code permits either the Los Angeles (LA) abrasion test or the aggregate impact test as the durability criterion. Your contract must identify the adopted method and limit. Water-softening materials such as brick metal, kankar and laterite should be assessed by an impact test under wet conditions.
| Intended use | Maximum LA abrasion | Maximum aggregate impact value | Flakiness limit |
|---|---|---|---|
| WBM sub-base | 50% | 40% | Not specified in the cited limit for this use |
| Base with bituminous surfacing | 40% | 30% | 20% |
| Surfacing course | 40% | 30% | 15% |
The flakiness limits apply where the code’s provision covers crushed or broken stone and slag. Do not apply the table blindly to a different aggregate type or a newer local specification; have the engineer confirm the governing test and limit.
Grading must match compacted thickness
IRC:19-2005 gives these grading examples. They are specification references, not permission to choose a grading without design confirmation.
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| Grading | Stone-size range | Associated compacted layer | Use noted by the code |
|---|---|---|---|
| Grading 1 | 90–45 mm | 100 mm | Sub-base only |
| Grading 2 | 63–45 mm | 75 mm | Use where the approved design calls for it |
| Grading 3 | 53–22.4 mm | 75 mm | Use where the approved design calls for it |
The relationship is practical: the stone skeleton has to suit the layer being compacted. If you order a larger grading for a thinner course, the layer may not compact or interlock as intended; if you order a smaller grading for a design calling for larger metal, the void structure and quantity of screenings will change.
Screenings: the material that fills the stone skeleton
Screenings are spread after the coarse aggregate has been rolled. Dry rolling and brooming work them into the voids. They should generally be produced from the same parent material as the coarse aggregate so that the particles are compatible in hardness and behaviour.
Permitted alternative screening materials
IRC:19-2005 permits predominantly non-plastic kankar, moorum or gravel other than river-borne rounded aggregate when all of these conditions are met:
- Liquid limit is below 20.
- Plasticity index is below 6.
- No more than 10% passes the 75-micron sieve.
Rounded river gravel is not an automatic substitute: its particle shape can prevent the mechanical interlock expected from crushed screenings. Have the proposed source and gradation approved before delivery.
Type A and Type B screenings
| Screening type | Nominal size identified by IRC:19-2005 | Associated coarse-aggregate grading |
|---|---|---|
| Type A | 13.2 mm | Grading 1 |
| Type B | 11.2 mm | Grading 3 |
| Type A or Type B | 13.2 mm or 11.2 mm | Grading 2 |
The adopted specification may add a detailed sieve curve or select a different designation. Include the complete gradation in the purchase order rather than relying on the words “Type A” or “Type B” alone.
Screenings must be spread uniformly and must not be visibly segregated. IRC:19-2005 says damp or wet screenings should not be used, because moisture prevents controlled spreading and interferes with the dry-rolling and brooming operation.
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Binding material: a separate fine filler
Binding material is not another name for screenings. It is a finer, plasticity-controlled filler applied when the voids need additional grouting or when the course requires extra resistance to ravelling.
Particle-size and plasticity requirements
IRC:19-2005 requires binding material to pass 100% through a 425-micron sieve. Its plasticity index (PI) depends on the course:
- Surfacing: PI 4–8.
- Sub-base or base with bituminous surfacing: PI below 6.
Limestone dust or kankar nodules may be used where locally available, subject to the applicable requirements and approval. Request a plasticity result for the actual source, not a generic statement that the material is “stone dust.”
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When binding material may be reduced or omitted
Binding material may be unnecessary when crushable moorum or gravel screenings themselves provide the required filling action. There is an important qualification for surfacing: if those screenings have a PI below 4, IRC:19-2005 calls for a small quantity of binding material with PI 4–6. The engineer should confirm the decision for the particular source, layer and adopted specification.
How to write a purchase specification that suppliers can quote
Use a separate line item for coarse aggregate, screenings and binding material. A clear order prevents a supplier from treating the three materials as one blended product.
- Identify the layer and thickness. State whether the material is for sub-base, base beneath bituminous surfacing or surfacing, and give the required compacted thickness.
- Name the grading. State Grading 1, 2 or 3 only when that designation is in the approved design, and include the corresponding sieve requirements in the order.
- State the quality limits. Include the adopted LA abrasion or impact limit, flakiness limit where applicable, cleanliness requirement and any wet-impact requirement for water-softening materials.
- Describe the screenings separately. Identify Type A or Type B (or the local equivalent), the required gradation and whether the source must match the coarse aggregate.
- Describe binding material separately. Require 100% passing the 425-micron sieve and the PI range applicable to the course.
- Require representative documentation. Ask for test certificates that identify the quarry or stockpile, sampling date, test method, test result and volume represented. A certificate without a traceable sample is not proof that every truck complies.
- Set delivery controls. State how loads will be measured, where they will be tipped, how contaminated or segregated loads will be quarantined, and who can approve a substitution.
- Obtain approval before scaling up. Have the engineer approve the source and a representative sample before committing to the full quantity.
A useful order description is explicit about the function, for example: WBM base aggregate, approved Grading 2, for a 75 mm compacted layer; screenings compatible with that grading; binding material only where the approved method statement requires it. The exact wording and limits must come from the project documents.
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How much material is needed?
Quantities depend on the area, compacted thickness, grading, void structure, wastage allowance and the project’s specification tables. Start with the bill of quantities and the approved cross-sections. Do not convert a guide quantity into a final takeoff without checking the contract.
Indicative binding-material quantities from IRC:19-2005
| Compacted layer | Course | Binding material guide quantity |
|---|---|---|
| 75 mm | Sub-base or base | 0.06–0.09 m³ per 10 m² |
| 75 mm | Surfacing | 0.10–0.15 m³ per 10 m² |
| 100 mm | Sub-base | 0.08–0.10 m³ per 10 m² |
These are construction-estimation guides, not guaranteed consumption rates. Actual use changes with the coarse aggregate’s voids, the effectiveness of rolling and brooming, the moisture condition and whether the screenings themselves perform part of the binding function.
CPWD Specifications 2009 also contains quantity tables for coarse aggregate, screenings and binding material at specified thicknesses and areas. Use those tables only as an additional estimating reference; the project bill of quantities and current adopted specification govern payment and procurement.
A practical takeoff sequence
- Measure the plan area of each WBM course separately.
- Confirm the compacted thickness and grading for each area.
- Apply the project’s quantity table or approved rate for coarse aggregate and screenings.
- Apply the relevant binding-material guide only where the design calls for binding material.
- Add only the wastage or bulking allowance authorised by the contract, and record whether quantities are measured loose, stacked or compacted.
What tests should you ask for when buying road aggregate?
The IRC quality-control handbook suggests minimum frequencies, but it was published in 1984. Treat these as a baseline for planning tests and confirm the current contract frequency with the engineer.
| Test | Suggested minimum frequency in the 1984 IRC handbook | What it verifies |
|---|---|---|
| LA abrasion or aggregate impact | One test per 200 m³ | Resistance to wear or impact against the adopted durability limit |
| Aggregate and screenings grading | One test per 100 m³ | Particle-size distribution and compliance with the approved grading |
| Flakiness | One test per 200 m³ | Particle shape where the applicable limit covers the material |
| Binding-material plasticity | One test per 25 m³ | PI compliance for the course and source |
The same handbook calls for regular control of grade, camber, thickness and surface finish. Testing is not a substitute for visual inspection: a load can have a passing certificate and still arrive contaminated, segregated or from a different stockpile.
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- Match the delivery ticket to the approved source, material designation, grading and intended layer.
- Check that the load is clean and free of soil, organic matter, clay lumps and other visible contamination.
- Look for segregation, excessive flat or elongated particles and unexpected moisture in screenings.
- Verify the delivered quantity using the contract’s measurement method and record the truck, stockpile or batch identity.
- Compare the certificate’s sample date, test method and represented volume with the actual delivery.
- Keep suspect loads separate and notify the engineer before incorporation.
- Do not infer compliance from a supplier’s product name, a previous project or an undated certificate.
How the materials work together during construction
The construction sequence explains why substitutions are risky.
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- Prepare the foundation. Confirm line, level, drainage and a firm, properly compacted subgrade or underlying course.
- Place coarse aggregate. Spread the approved grading uniformly to the loose depth needed for the specified compacted thickness, then roll to establish interlock.
- Apply screenings progressively. Spread dry screenings in controlled applications while continuing dry rolling and brooming so particles enter the voids instead of forming a loose surface blanket.
- Add water carefully. Sprinkle enough water to assist grouting and continue sweeping and rolling until the stone is bonded. Excess water can weaken or damage the base and the subgrade.
- Apply binding material if required. Spread it in thin layers, grout the remaining voids and continue rolling until the surface is tight and stable.
- Protect the finished layer. Keep traffic off the WBM until it has set. If a bituminous surfacing will follow, wait until the WBM is completely dry.
IRC:19-2005 requires uniform spreading and prohibits segregation. The crew should not compensate for an incorrect grading by dumping extra fines or water; stop and resolve the material problem with the engineer.
Foundation and drainage compatibility
Even compliant aggregate cannot rescue a poor foundation. The code advises against placing WBM directly on silty or clayey subgrade without a suitable intervening granular layer. Confirm that the foundation can drain and support rolling before stockpiled materials are released for placement.
Placement over an existing bituminous surface also needs treatment for bond and drainage. IRC:19-2005 generally advises avoiding direct placement without suitable preparation. The project method statement should define cleaning, bond treatment, levels and any corrective layer.
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When more than one quarry or screening source is available, compare them in this order:
- Specification fit: Can the source consistently supply the exact grading for the required compacted thickness?
- Test compliance: Do representative results meet the applicable abrasion or impact, flakiness, grading, liquid-limit, fines and PI requirements?
- Material compatibility: Are the screenings made from, or demonstrably compatible with, the coarse aggregate? Will the proposed binding material suit the course?
- Delivery condition: Can the supplier keep loads clean, dry where required, correctly identified and free from segregation?
- Logistics and total delivered cost: Compare haul distance, loading, unloading, stockpile space, rejected-load risk and the contract budget—not just the quoted price per tonne.
Local prices, supplier rankings and availability change by project and were not established by the cited standards. Make the commercial comparison only after technical compliance has been demonstrated.
Common buyer mistakes and the correction
- Ordering by nominal stone size alone. Correct it by naming the grading, layer function and compacted thickness.
- Calling screenings and binding material the same thing. Keep separate line items and separate gradation or plasticity requirements.
- Assuming binding material is always necessary. Check whether the approved screenings already provide the filling action and whether the course imposes a PI requirement.
- Using guide quantities as the final takeoff. Reconcile them with the BOQ, cross-sections, measurement rules and approved wastage.
- Accepting a generic test certificate. Check sample identity, representativeness, date, method and the limits in the current contract.
- Treating old IRC frequencies as universally current. Confirm the testing schedule in the governing project specification and local road-authority requirements.
- Allowing traffic or bituminous surfacing too soon. Protect the layer until it sets, and wait for complete drying before the bituminous course.
Final pre-order checklist
- Approved drawings identify the WBM course, grading and compacted thickness.
- The adopted specification edition and local authority amendments are recorded.
- Coarse aggregate, screenings and binding material are described as separate materials.
- Durability, flakiness, grading, liquid-limit, fines and PI limits are written into the purchase request where applicable.
- The supplier has submitted representative, traceable test results.
- Source approval and a delivery inspection process are in place.
- Quantity calculations distinguish compacted layer requirements from loose delivery volumes.
- Foundation drainage, rolling sequence, water control and protection of the completed layer are included in the method statement.
Buyer’s decision: Release a WBM order only when the material designation, layer function, test limits, quantity basis and acceptance process are explicit. The stone creates the structure, screenings fill its working voids and binding material finishes the grout where the course requires it; performance depends on the three being specified and installed as one system.
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