Research outline

Design, Construction, and Maintenance of Earthquake-Resistant Dams:
Seismic performance evaluation for dams

Figure-1 Example of computer analysis simulating seismic behavior of a cracked concrete gravity dam

Photo-1 Shaking table test simulating seismic behavior of a cracked concrete gravity dam

Figure-2 Example of computer analysis for earthquake-induced settlement of a rockfill dam

Japan is one of the most earthquake-ridden countries in the world, and recent major earthquakes, including the Hyogo-ken Nambu Earthquake in 1995, the Mid Niigata Prefecture Earthquake in 2004, and the Iwate-Miyagi Nairiku Earthquake in 2008, have caused enormous damage to structures and lifelines. Given these conditions, there is an increasing need for large-scale structures such as dams to be earthquake resistant and have good seismic performance. Although no dam has been severely damaged by earthquakes in Japan as of yet, assessment of the seismic performance of dams in view of potential future large earthquakes is very important for the safe and secure national development. This section presents an example of seismic performance evaluation research of concrete gravity dams (dams built with concrete) and rockfill dams (dams built with rock and soil) based on the maximum credible earthquake motion in the future.

Ο Concrete gravity dam
While concrete gravity dams are very sturdy structures, strong earthquake motions may possibly crack the dam, and such a crack may eventually penetrate completely or almost completely through from the upstream face to the downstream face. Research is underway given these assumptions for the development of seismic performance evaluation method for concrete gravity dams, in which the behavior of a concrete block divided by a crack at the time of earthquake is estimated by model testing and computer analysis (Figure-1, Photo-1).

Ο Rockfill dam
Rockfill dams are built with rocks and soils, which makes them vulnerable to deformation such as sliding and settlement in the event of large earthquakes. Research is underway given these assumptions for the development of seismic performance evaluation method for rockfill dams in which testing of the strength and deformation properties of rocks and soils used for banking a dam and computer analysis are conducted to estimate the earthquake-induced deformation of a dam.(Figure-2).
The results of these studies are applied successively to actual aseismic capacity diagnosis for dams, contributing to safe and secure national development.

(Contact: Dam Structure Research Team)

Research on the Assessment of Composite Deterioration of Concrete Caused by Frost and Chloride Attack
Simplified inspection of concrete

Scaling                    Popout

3D scanner             Color varies
                             according to
                              peeling depth

Schematic diagram of surface scanning method

Inspection by the surface scanning method

In Japan, vast amounts of concrete structures have been built to date to develop the social infrastructure. Possible deterioration of durability of these structures due to aging can be a cause for concern going forward. Concrete in cold, snowy and northern regions such as Hokkaido in particular is subject to freezing and thawing effect during winter, which may cause cracks, scaling (surface peeling) and even popouts (conical peeling due to aggregate expansion). Coastal areas and regions where anti-icing agents are employed may be subject to chloride attack on top of deterioration due to frost damage. As for concrete structures in a saline environment, deterioration progresses further due to acceleration of degradation of the concrete itself and ease of the corrosion of rebars inside the concrete. The degree of concrete deterioration must be assessed to adequately maintain these structures.
Indices such as compressive strength, crack density, and concrete salt permeability are sometimes used to assess the degree of concrete deterioration. These methods of assessment, however, require the concrete to be destroyed and take time to conduct. The Materials Research Team is studying a non-destructive method of quick on-site deterioration assessment without the need to destroy any structures. This section describes the technology for using a 3D scanner to measure the depth of peeling due to concrete scaling and for inspecting frost damage deterioration by means of a surface scanning method using ultrasonic waves.
A 3D scanner is a device that uses laser beams to examine the irregularity of the concrete surface. Analyzing the scanned data allows for color-coding according to the peeling depth so that the data can be viewed as an image.
The research conducted up to this point has confirmed that the peeling depth is correlated to the visual assessment of the degree of deterioration and the technique can be used as a method of quantitatively evaluating the amount of peeling. In addition, this data allows efficient determination of the cover thickness of the concrete, which has been reduced due to scaling deterioration, to the reinforcing.
In the surface scanning method, an oscillator and a receiver are applied to the concrete surface to measure the time taken by the ultrasonic waves generated from the oscillator to reach the receiver through the concrete. Ultrasonic waves characteristically travel through harder objects faster, which means it takes longer for the waves to travel through deteriorated concrete, which has damaged sections, than it does through sound concrete. Analysis of this data has facilitated the assessment of the state of deterioration (depth and loosening of the concrete structure) inside the concrete due to frost damage and other factors. Going forward, we will confirm whether these non-destructive test methods are effective as methods of assessment of composite deterioration due to frost or chloride attack in order to continue with research aimed at practical application.

(Contact: Materials Research Team, CERI)