Manual Sampling Procedures for Aggregate Base Testing

Manual Sampling from Stockpiles (Fine Aggregate)

When dealing with fine aggregates stored in large stockpiles, technicians cannot simply take material from the surface. The surface material is often subject to segregation, moisture loss, or contamination from the environment.

To obtain a valid sample from a stockpile, the following steps must be taken:

1. Remove the outer layer: The technician must first clear away the weathered or outer layer of the stockpile to access the fresh material beneath.
2. Use Tube Increments: Samples are obtained by inserting a sampling tube into the "shipping face" (the exposed side) of the stockpile.
3. Randomization: The tube must be inserted at random locations to avoid bias.
4. Height Requirements: To ensure the sample is taken from the core of the material, it must be taken at a minimum height of 3 feet from the surrounding grade.
5. Minimum Increments: A single field sample must consist of a minimum of four tube increments.

Sampling from Transportation Units

Material being delivered to a job site via trucks or other transportation units requires a different approach. Because the material is contained within a moving unit, the technician must use a method that captures the diversity of the load.

The standard procedure for manual sampling from a transportation unit involves trench excavation:

• Excavation: The technician must excavate a minimum of three increments across the unit.
• Trench Dimensions: The trench must be level and maintain a specific size: 0.3 meters (approximately 1 foot) in both width and depth.
• Increment Collection: Within this trench, the technician obtains at least three increments from approximately equally spaced points. This is done by pushing a shovel downward into the material.

By spreading the increments across the width and depth of the unit, the technician minimizes the risk of sampling only a segregated portion of the load (such as only the larger stones that may have settled at the bottom).

Roadway Bases and Subbases

Sampling from a roadway that has already been spread but not yet compacted requires precision to avoid contaminating the sample with the underlying soil or subgrade.

When sampling from a placed base or subbase, follow these protocols:

• Random Selection: Sample units must be selected using a random method to ensure the data represents the entire stretch of road.
• Timing: Sampling must occur after the material is placed but before compaction begins.
• Full Depth Sampling: All increments must be taken from the full depth of the layer. It is vital to take care to exclude any underlying material (the subgrade) from the sample.
• Documentation: The technician must clearly mark the areas from which each sample increment was removed to maintain a record of the sampling locations.

By adhering to these specific increment and excavation requirements, technicians ensure that the aggregate base testing is accurate, providing a reliable foundation for all subsequent construction activities.