1.Granular Sub-Base (GSB) Mix Design:
The Granular Sub-Base (GSB) layer is the lowest layer in road pavement, situated above the subgrade (compacted soil). Its purpose is to distribute the traffic load evenly to the subgrade and to provide drainage for the pavement.
Objectives of GSB Mix Design:
- To create a load-bearing foundation layer.
- To facilitate drainage and prevent water accumulation in the pavement.
- To prevent the intrusion of finer materials from the subgrade into the upper pavement layers.
Materials for GSB:
- Aggregates: Crushed stone, gravel, or natural sand.
- Gradation: Well-graded aggregates are essential for stability, and the material should meet the required gradation limits (as per local standards such as MORTH in India).
- Fines: A small amount of fines (particles passing through a 75-micron sieve) is allowed but should not exceed specified limits to maintain permeability.
GSB Mix Design Process:
- Aggregate Gradation: Properly graded aggregate material is essential for load distribution. The mix should be checked against standard gradation limits, such as those provided in MORTH (Ministry of Road Transport and Highways) or ASTM.
- Compaction Tests: Conduct compaction tests (like the Proctor Test) to determine the maximum dry density (MDD) and optimum moisture content (OMC). These values are important for achieving adequate compaction in the field.
- CBR (California Bearing Ratio): The CBR test is used to assess the strength of the sub-base material. Higher CBR values indicate better load-bearing capacity.
- Moisture Content: The material must maintain enough moisture to achieve proper compaction during construction.
2. Wet Mix Macadam (WMM) Mix Design:
Wet Mix Macadam (WMM) is a higher-quality, better-compacted road base than GSB. It consists of well-graded aggregates mixed with water to form a stable base course for the road. The WMM layer is usually placed above the GSB layer and provides a stronger structural foundation.
Objectives of WMM Mix Design:
- To achieve a high-strength road base capable of withstanding traffic loads.
- To improve the road’s overall durability and stability.
- To reduce the permeability of the road base, ensuring that water does not accumulate within the pavement layers.
Materials for WMM:
- Coarse Aggregates: Crushed stone or gravel with angular particles to ensure interlocking.
- Fines: Limited to a small percentage (similar to GSB) to maintain strength and compaction properties.
- Water: Sufficient water is added during mixing to help with compaction and binding of the aggregates.
WMM Mix Design Process:
- Aggregate Gradation: Well-graded aggregates are crucial. The gradation should conform to standards like MORTH or IRC (Indian Roads Congress) for WMM. The mix should have both coarse and fine aggregates.
- Optimum Moisture Content (OMC): Water content plays a crucial role in achieving maximum compaction. OMC is determined using a Proctor Test, similar to GSB.
- Compaction Tests: The material should be compacted to the Maximum Dry Density (MDD), which is determined in the lab.
- Strength Tests: Conduct CBR or plate load tests to assess the strength of the mix and its load-bearing capacity.
- Field Density Test: The in-situ density is checked using a sand replacement method or a nuclear density gauge to ensure compaction meets the design criteria.
Key Tests in GSB/WMM Mix Design:
- Aggregate Impact Value (AIV): Measures the toughness of the aggregate to withstand impact.
- Los Angeles Abrasion Test: Determines the hardness and wear resistance of aggregates.
- Water Absorption Test: Ensures the aggregate does not absorb too much water, which can affect the stability of the mix.
- Proctor Compaction Test: Determines the OMC and MDD to ensure proper compaction.
- CBR Test: Measures the strength of the mix in terms of its load-bearing capacity.
- Field Density Tests: To check the actual compaction achieved in the field.
