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Infrastructure Technology Institute

Assuring More Durable Concrete:
An Interview with Jianmin Qu

Jianmin Qu is a professor and the department chair of Civil and Environmental Engineering at Northwestern. His research focuses on areas of applied mechanics and mechanics of materials. Prof. Qu joined Northwestern from Georgia Tech in the fall of 2009. In addition to his work with ITI, he has conducted research sponsored by the Defense Advanced Research Projects Agency, Air Force Research Laboratory, Motorola, Ford Motor Company, MCMD Consortium, and the National Science Foundation Packaging Research Center at Georgia Tech.

You and your students are working on a new method to evaluate alkali-silica reactivity (ASR) for aggregates used in concrete production. What is ASR and why is it a problem for the nation’s infrastructure?

ASR is a destructive process that occurs when chemicals in cement interact with some aggregates in ways that cause swelling, cracking, and eventually reduced performance and durability. Aggregates are a key component of concrete, and tens of billions of metric tons of aggregates are consumed each year for this purpose. Knowing the alkali reactivity of aggregates is essential for managing and mitigating ASR induced damage. Current tests of ASR can take several weeks. We’re looking for a much faster test. In technical terms ASR happens in cement-based materials such as mortars and concretes, when the hydroxyl ions in the highly alkaline pore solution attack the siloxane groups (Si-O-Si) of siliceous mineral components in the aggregates. Hydroxyl ions together with alkali metal cations (sodium or potassium) bind with siliceous species derived from the reactive minerals to form a crosslinked alkali-silica gel. The alkali-silica gel swells in the presence of moisture.

Expansion of this gel can lead to cracking, the cracks can grow and eventually coalesce, threatening the strength and reducing the service life of the concrete structure.

How is ASR currently evaluated? (i.e., what are the common methods in practice now?)

Currently, several standard test methods are used in practice in different parts of the world. In the US, the most commonly used testing methods are the accelerated mortar bar test, the concrete prism test, and the accelerated concrete prism test. These tests all rely on measuring the expansion of concrete samples under exposure to an alkali environment. This means that the tests take a long time ranging from weeks to years. This becomes a serious impediment to the construction process.

Testing is required because the alkali reactivity of aggregates can vary significantly from source to source and sometimes even within a single source. In addition, with the growing use of higher alkali supplementary cementitious materials to develop advanced properties in concrete, screening aggregates for alkali reactivity is more critical than ever.Therefore there is a real need for more reliable, low cost and rapid test methods to assess aggregate alkali reactivity.

What are the advantages of the nonlinear ultrasonic techniques you’re investigating?

In this project, we are developing a nondestructive testing method to assess the alkali reactivity of aggregates. The method is based on the mixing of nonlinear ultrasonic waves. In the test, a piezoelectric transducer attached to the sample induces two ultrasonic waves of different frequencies into a concrete sample with possible ASR damage. As the ultrasonic waves propagate through the concrete sample, they mix in the presence of the alkali gel, a byproduct of ASR damage. The mixing produces a resonant wave that can then be detected by another sensor attached to the sample. The magnitude of this resonant wave is called the acoustic nonlinearity parameter. Since the wave mixing is induced by the ASR damage, our hypothesis was that this acoustic nonlinearity parameter is directly related to ASR damage in the concrete sample.

Preliminary results from the first 8 months of our project clearly show that, indeed, the acoustic nonlinearity parameter is very well correlated to the degree of ASR damage in the sample. Furthermore, almost all existing methods require standardized sample size and geometry, because they measure the sample expansion caused by ASR damage. This prevents the field application of these methods to existing concrete structures. In contract, the acoustic nonlinearity parameter measured in our ultrasonic test characterizes the intrinsic state of ASR damage, independent of the sample size or geometry. This unique capability enables the application of our ultrasonic method to existing concrete structures. By attaching transmitters and sensors to an existing structure, ultrasonic measurements can be conducted to assess the ASR damage without damaging the structure.

What is the potential for widespread future deployment of nonlinear ultrasonic evaluation for ASR? What would the impacts be?

This new method detects the earlier stages of ASR damage. Therefore, screening the aggregates for alkali reactivity can be performed in much shorter duration than the existing technology. Considering the large quantity of aggregates used, substantial shortening of test duration of this test would mean tremendous saving for the construction industry. Further, the new ultrasonic method Could be developed into a portable tool for field tests. This would produce a huge impact on the maintenance of civil infrastructures because it permits in-the-field, nondestructive assessment of ASR damage.

You have over 20 years of experience in use of ultrasound for nondestructive evaluation of various materials, including cement and concrete. What do you think the role of ultrasonic evaluation in civil engineering will be 20 years from now?

Ultrasound has been widely used for nondestructive evaluation (NDE)
in different industries for many decades. One of the earlier adopters of ultrasonic NDE methods is the medical community. Today, ultrasonic scan is commonly used for diagnosis of various diseases. In aerospace engineering, ultrasonic NDE is one of the standard methods for routine inspection of air frames and engine components. Even in civil engineering, ultrasonic NDE is being used widely to detect corrosion damage in oil pipelines and for structural health monitoring. An example of such applications is the ITIsupported work on bridge condition monitoring being conducted by Northwestern Professors Achenbach and Krishnaswamy.

It is always difficult to predict how a technology will evolve over time. However, I do see that ultrasonic NDE is going through a transition from a diagnostic tool to a prognostic tool. To put it simply, most of the existing ultrasonic NDE methods nowadays mainly detect (diagnose) the abnormality (cracks, delamination, etc.) in the materials or structures being monitored. However, these techniques are typically unable to estimate (prognosticate) the remaining service life of the structure. The next technology breakthrough in ultrasonic NDE would be the prognostic capabilities. I believe that nonlinear ultrasonic NDE, such as the one we are developing now, is one step in this direction.

You earned your Ph.D. in Theoretical and Applied Mechanics at Northwestern. How does it feel to return to campus as a professor?

Qu: The Chinese have a saying, “Leaves eventually fall under the tree.” That is how I felt upon my returning to my alma mater. NU had provided me with a great education that enabled me to enjoy an exciting academic career. I owe NU a debt of gratitude. Since being an engineering professor for the last 20 years at Georgia Tech has not made me wealthy enough to give millions to NU, but I figure the best way to pay my debt to NU is to work for it.