# Gradation of Aggregate for Concrete According to AASHTO Standards
## Introduction
Aggregate gradation is a critical factor influencing the performance, durability, and workability of concrete mixtures. The American Association of State Highway and Transportation Officials (AASHTO) provides comprehensive guidelines for aggregate gradation to ensure optimal concrete properties. Proper gradation affects the packing density, water demand, strength, and resistance to segregation in concrete.
This article explores the principles of aggregate gradation as per AASHTO standards, discussing its importance, methods of evaluation, classification systems, and practical implications in concrete mix design. Additionally, it examines how deviations from recommended gradations can impact concrete performance..jpg)
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## 1. Importance of Aggregate Gradation
Aggregates constitute approximately 60-75% of the total volume of concrete. Their particle size distribution (gradation) influences:
– Workability: Well-graded aggregates reduce voids and improve flowability.
– Strength: Proper packing enhances compressive strength by minimizing weak zones.
– Durability: Dense gradations reduce permeability and increase resistance to freeze-thaw cycles.
– Economy: Optimized gradation reduces cement demand by improving particle interlock.
AASHTO M 43 (Standard Specification for Sizes of Aggregate for Road and Bridge Construction) defines grading requirements for coarse and fine aggregates used in concrete.
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## 2. Types of Aggregates Based on Gradation
(a) Fine Aggregates
Fine aggregates (sand) typically pass through a 4.75 mm (No. 4) sieve. AASHTO M 6 (Standard Specification for Fine Aggregate) specifies grading limits:
| Sieve Size | Percent Passing |
|————|——————|
| 9.5 mm (3/8″) | 100 |
| 4.75 mm (No. 4) | 95–100 |
| 2.36 mm (No. 8) | 80–100 |
| 1.18 mm (No. 16) | 50–85 |
| 600 µm (No. 30) | 25–60 |
| 300 µm (No. 50) | 10–30 |
| 150 µm (No. 100) | ≤10 |
(b) Coarse Aggregates
Coarse aggregates are retained on the No. 4 sieve (>4.75 mm). AASHTO M 80 (Standard Specification for Coarse Aggregate) provides grading