Technical tools for measuring the density of asphalt

In-place asphalt density is one of the most important measurements used to determine asphalt quality when paving a roadway. A small increase in in-place density can potentially lead to a significant increase in the service life of asphalt pavements. The industry relies on compaction to increase the density and lifespan of pavements, but what tools do we use to ensure that the compaction is done correctly?

Quality control in the field is always essential for the success of the work. In most states, contractors receive incentive or disincentive compensation (financial penalties), depending on the final in-place density of the asphalt on the roadway. In extreme cases of low density, contractors may be required to remove and replace new asphalt if it does not meet density specifications. Make sure you get and track those numbers to succeed with the right tools.

Traditionally, pavement quality was measured by coring the placed asphalt and bringing those samples back to a laboratory for evaluation. Not only does this take time, but it also has an impact on the quality of the pavement. Non-destructive testing (END) methods offer a good alternative to traditional testing; they reduce calibration-only coring and provide immediate test results.

There are three main types of equipment available for measuring density on the jobsite with an NDT method; nuclear gauges, non-nuclear gauges and georadar. There are benefits and limitations to each of these methods and it will be up to your crews comfort level with each technology and the specifications of the offering to determine which you should use.

Nuclear Asphalt Density Gauges

According to the Environmental Protection Agency (EPA), the use of nuclear gauges is a type of industrial radiography. Radiography is the use of radiation to produce an image on photographic film. This is similar to how a medical x-ray uses radiation to produce images of bones.

A nuclear density gauge measures the density of hot mix asphalt (HMA) in place using gamma radiation. Gamma rays are emitted from the source and interact with electrons in the material. By calculating the percentage of gamma rays returned to the source, the gauge can quickly and accurately measure the density and internal structure of the test material.

A nuclear density gauge offers the following key advantages over destructive (nuclei) density measurement:

  1. Portability. A person can easily carry a typical nuclear density gauge.
  2. Quick results. Most nuclear gauges allow one-minute and four-minute readings. These are much faster than typical densities obtained from cores which can take anywhere from days to weeks.
  3. Virtually non-destructive. Used in backscatter mode, the nuclear density gauge is completely non-destructive. Used in direct mode, the gauge requires only a small penetration into the finished carpet of less than 1 inch. in diameter and about 2 in. Deep.

There are two different test modes that a nuclear gauge can perform: forward transmission mode and backscatter mode.

In direct transmission mode, the sensor measures the amount of gamma radiation emitted from the source rod. The less radiation detected by the sensor, the higher the density of the material. This is why it is important to select the correct source rod penetration depth, as a shallower depth will allow more radiation to reach the sensor than if it were deeper in the same material.

In backscatter mode, the source is in the same plane as the sensor. The shielding inside the gauge means that the radiation emitted by the source must first be deflected by the material before reaching the sensor. This means that the greater the radiation detected by the sensor, the higher the density of the material.

Although these gauges have been widely used in the past, they use radioactive materials which can be hazardous to the user’s health unless proper precautions are taken. Users of this gauge should familiarize themselves with applicable safety procedures and government regulations. Some agencies require the use of personal monitoring devices such as a thermoluminescent dosimeter or film badge. There are also permits and training courses required for the operation of this equipment.

Effective instructions, as well as routine safety procedures such as source leak testing, recording and evaluation of personal monitoring device data, etc., are an integral part of the operation and storage of this gauge.

Non-Nuclear Asphalt Density Gauges

Just like nuclear gauges, non-nuclear devices are used in the same way to measure density – only without the radiation component. With increasing regulations and costs associated with the use of nuclear source materials, non-nuclear density gauges are growing in popularity.

Non-nuclear density gauges estimate pavement density by inferring the relative proportion of air-filled voids in asphalt from a measurement of dielectric permittivity. This means that these devices operate based on the principles of electrical impedance for a current flowing through the HMA material. These devices have many advantages in that they are able to provide density measurements very quickly and completely non-destructively, are easy to handle and are not subject to complicated regulations.

In general, non-nuclear devices have been advertised to provide significant savings over a nuclear gauge. The initial cost of the non-nuclear device is similar or slightly lower than that of a nuclear gauge (depending on the model), but the majority of savings are generated by eliminating the costs associated with licensing, training and maintenance.

Some concerns have however been raised regarding the use of non-nuclear gauges. Researchers expressed common themes including 1) insensitivity to actual changes in density, 2) poor correlations with other density measurements, and 3) the need for a better calibration procedure. The current consensus seems to be that while non-nuclear gauges offer a large number of practical advantages, their results are significantly affected by a number of factors that may need to be considered with future designs.

Ground penetrating radar to measure the density of asphalt

Ground Penetrating Radar (GPR) works by sending a small pulse of energy through a material and recording the strength and time required for any reflected signal to return. A series of pulses over a single area is called a sweep. Reflections are produced whenever the energy pulse enters a material with different electrical conduction properties or dielectric permittivity than the material it left. The strength or magnitude of the reflection is determined by the contrast of the dielectric constants and conductivities of the two materials.

GPR technology has been used most successfully to determine pavement layer thickness, but has also been used to identify underground voids, assist in forensic investigations, locate utilities and reinforcement, assess the degradation of bridge decks and estimation of volumetric properties of asphalt pavements.

Recent research has shown that The GPR has the ability to perform 100% analysis of the new pavement and identify density defects. The method too provides significantly more data than traditional density testing and calibrated GPR dielectric values ​​correlate better with core density values ​​than standard nuclear and non-nuclear density gauges.

The collection process can also provide a continuous profile of existing road conditions. The GPR can utilize high-speed data collection at speeds of up to 50 mph, thus requiring less traffic control and resulting in greater safety compared to handheld devices.

The results for these devices have showed a very strong correlation between GPR-based air gap measurements and laboratory measurements, but there are also limitations with potential interference from ambient radio noise and antenna variations.

Achieving and measuring asphalt density will continue to be an ongoing quest for perfection. Whichever density verification method you choose, increasing asphalt densities testing will result in a higher quality pavement, allowing pavement contractors to quickly verify that the pavement meets the specified density, to reduce the over-compaction of the roadway and to better optimize the paving. treat.

About Debra D. Johnson

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