6.2 Field Testing: Concrete, Soils, and Aggregates

Field Testing: Concrete, Soils, and Aggregates

Field testing is the physical execution of Quality Control and Quality Assurance on the job site. It provides empirical data to verify that the materials being placed meet the structural and durability requirements of the contract documents. For the PE Construction exam, you must be familiar with the standard ASTM procedures, the purpose of each test, and how to interpret the results.

Concrete Field Testing

Fresh concrete must be tested upon delivery to the site and before it is placed in the forms. These tests verify the mix design's properties and ensure long-term performance. Standard practice dictates that tests are performed on the first truck of the day and subsequently at a specified frequency (e.g., every 50 to 150 cubic yards) or whenever test cylinders are cast.

Slump Test (ASTM C143)

The slump test measures the workability, consistency, and fluidity of fresh concrete. A standard cone (12 inches high, 8 inches wide at the base, and 4 inches wide at the top) is filled with concrete in three equal layers by volume, with each layer rodded 25 times. The cone is then lifted vertically, and the downward settling (slump) of the concrete is measured to the nearest 1/4 inch.

• High Slump: Indicates a very fluid mix. If the high slump is caused by excess water (rather than water-reducing admixtures), the water-cement ratio is compromised, which will significantly reduce the compressive strength and increase shrinkage cracking.
• Low Slump: Indicates a stiff mix, which may be difficult to place and consolidate, leading to honeycombing or voids around dense rebar.

Air Content Test (ASTM C231 and C173)

Air entrainment is essential for concrete exposed to freeze-thaw cycles. Microscopic air bubbles act as pressure relief valves when water inside the concrete freezes and expands.

• Pressure Method (ASTM C231): Uses an air meter to apply pressure to a sample of concrete. The change in volume indicates the air content. It is only suitable for relatively dense aggregates.
• Volumetric Method (ASTM C173): Uses a roll-a-meter where the concrete is agitated with water and isopropyl alcohol. It is required for lightweight aggregates where the pressure method would yield inaccurate results by compressing the air inside the aggregate pores.

Temperature (ASTM C1064) and Cylinder Fabrication (ASTM C31)

Concrete temperature must be monitored because high temperatures accelerate the hydration reaction (causing premature setting and cracking), while low temperatures delay strength gain. Typical specifications limit placement temperatures to between 50°F and 90°F.

Test cylinders (typically 4x8 inches or 6x12 inches) are cast in the field (ASTM C31) for later compressive strength testing in the laboratory (ASTM C39) at 7 and 28 days. Cylinders must be stored on a level surface, protected from vibration, and kept within specific temperature and moisture ranges during initial field curing before being transported to the lab.

Soil Field Testing

Proper soil compaction is fundamental to the stability of foundations, roads, and earthworks. Field testing verifies that the contractor has achieved the required density. The goal is to measure the in-place field dry density and compare it to the maximum laboratory dry density to calculate the percent relative compaction.

Sand Cone Test (ASTM D1556)

The sand cone test is a direct, destructive method for determining the in-place density of soil. A small hole is dug in the compacted fill, and the excavated soil is carefully saved and weighed. To find the volume of the hole, a standardized sand of known density is poured into the hole using a calibrated cone apparatus. By knowing the weight of the sand used and its density, the exact volume of the hole is calculated. The field wet density is then the weight of the excavated soil divided by the volume of the hole. A sample of the soil is tested for moisture content to convert the wet density to dry density.

Nuclear Density Gauge (ASTM D6938)

The nuclear gauge is the most common method used today due to its speed and non-destructive nature. It uses radioactive isotopes to measure both the wet density and the moisture content of the soil simultaneously.

• Direct Transmission: A probe containing a radiation source is lowered into a pre-drilled hole in the soil, and a detector on the surface measures the radiation that passes through the soil. This provides the density.
• Backscatter: Used primarily for asphalt or thin lifts of soil, the source and detector remain on the surface, and radiation scattered back from the material is measured. The gauge provides rapid results, allowing the contractor to immediately know if more compaction passes are needed. However, the gauge requires special licensing, strict safety protocols, and calibration against direct methods (like the sand cone) for specific soil types.

Aggregate Field Testing

Aggregates make up the vast majority of concrete and asphalt volumes, and they form the structural base for roadways. Field testing ensures they meet grading and moisture requirements.

Sieve Analysis (Gradation)

Aggregates are passed through a series of standard sieves to determine the particle size distribution. A well-graded aggregate contains a good mix of large, medium, and small particles, which minimizes void space and requires less binder (cement or asphalt). A poorly graded (or uniform) aggregate has particles of mostly the same size.

Moisture Content

The moisture content of aggregates stored on site must be monitored, especially before batching concrete. Aggregates contain varying levels of water. If they are wet, they will contribute free water to the concrete mix, requiring the batch plant to reduce the amount of added mix water to maintain the correct water-cement ratio. Conversely, if aggregates are bone dry, they will absorb water from the mix, reducing workability.

Understanding these tests, their applications, and the interpretation of their results is fundamental for a PE in construction managing project quality.