A Surface Scanning Method for Frost Damage Inspection and Diagnosis in Concrete
Photo 1 Problems in diagnosing
frost damage to concrete
Photo 2 Surface scanning for
frost damage diagnosis
Figure 1 Flow of frost damage assessment
using the surface scanning method,
and verification of the reliability
of assessment
(Data are shown in an orange-colored
space when the region of true values of
relative dynamic modulus of elasticity
is adequately estimated.)
Table 1 Frost damage inspection and diagnosis
using the surface scanning method
Frost is a cause of damage to concrete structures in cold regions. Frost damage is a physical phenomenon in which moisture in the concrete undergoes freeze-thaw cycles that promote cracking and gradually reduce the concrete's compactness.
The severity of frost damage is typically assessed by using cores from sections where such damage is suspected. The ultrasonic propagation velocity in the cores is determined by applying an ultrasonic oscillator to one side and an ultrasonic receiver to the other side of each core, and the measured velocity is used to evaluate the depth and severity of frost damage. Core sampling, however, involves the following problems (Photo 1).
(1) Drilling can cause damage to concrete members
or reinforcement.
(2) Considerable cost, time and labor are required for
a wide-reaching survey.
(3) Reasonably determining the sections from which to
take core samples is not easy, because damage to
the interior concrete is not visible.
Accordingly, the application of a surface scanning method in which an ultrasonic oscillator and a receiver are used to estimate the thickness of the deteriorated layer from the surface to frost damage inspection and diagnosis was attempted in this study for the purpose of establishing a technology to enable the simple, quick, nondestructive diagnosis of frost damage severity within the scope of normal everyday management (Photo 2).
The results confirmed that the surface scanning method enables the region of true values of relative dynamic elastic modulus, an indicator of frost damage, to be estimated with a probability of at least 90% (Figure 1). The relative dynamic elastic modulus of concrete free of frost damage is defined as 100, and the modulus decreases with increases in frost damage severity.
The findings were compiled into the "Frost Damage Inspection and/or Diagnosis Manual of Concrete with Surface Scanning Method" in October 2016. The "Highway Design Guidelines of the Hokkaido Development Bureau (Reference Material C, 2nd Edition, 3rd Collection)" explicitly state that the maintenance of concrete structures that are likely to be affected by frost damage should comply with the "Manual for Inspecting and Controlling Structures in Which Frost Damage is Suspected" (Supervised by CERI). This manual refers to the "Frost Damage Inspection and/or Diagnosis Manual of Concrete with Surface Scanning Method" as a guide that gives a detailed explanation about the use of surface scanning for inspecting and diagnosing frost damage.
The "The Frost Damage Inspection and/or Diagnosis Manual of Concrete with Surface Scanning Method" is downloadable for free at the following address:
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The surface scanning method was used, for example, in the inspections of highway bridges described below. These inspections were conducted to determine the priorities of detailed investigations.
Specifically, the bottom sections of 208 highway bridges on 9 national highways (Routes A~I) in Hokkaido were inspected with the surface scanning method. Table 1 shows the number of bridges where the lower limit of the region of true values of relative dynamic elastic modulus (Figure 1) near steel reinforcements 10cm below the surface was smaller than the allowable value. Although the current allowable value is 60%, the value of 50% was used in the inspections because the ultrasonic propagation velocity in concrete immediately before the concrete is affected by frost damage is difficult to confirm and varies from 4.0km/h to 4.5km/h, and because the relative dynamic modulus of elasticity calculated by using the ultrasonic propagation velocity has a margin of error of plus or minus 10%. The percentage of bridges having a relative dynamic modulus of elasticity of 50% or less varies depending on the highway. The inspection results indicate that Routes B, E and D should be given priority for detailed investigations.
(Contact: Materials Research Team, Civil Engineering Research Institute for Cold Region)
Listening to the Sound of Sand in Water: Measuring the Amount of Bedload Sediment
Photo 1 Erosion at an upland field
in the snowmelt season
Photo 2 The hydrophone (lower left),
and the basic experiment for data analysis
(i.e., identifying the characteristics of
the sounds picked up by the hydrophone from
the impact of sand and stone of various sizes)
Background
At times, soil in upland fields is carried by rain or snowmelt water running over the surface of the fields. This phenomenon is called erosion (Photo 1). Eroded soil accumulates in drainage channels and rivers, where it disrupts water flow, or it is transported downstream to lakes, where it causes lake water quality to deteriorate. For measures against soil erosion and sediment runoff, the amount of sediment runoff needs to be investigated.
However, it is not easy to estimate sediment runoff, for the reasons explained below. Sediment transported in rivers and drainage channels is divided largely into two types. One is bedload sediment, which is transported on or near the riverbed; the other is suspended sediment, which is made up of relatively fine particles and is suspended in water at any depth from the water surface. The amount of suspended sediment at a given point in a drainage channel or a river is obtained by multiplying the flow rate by the sediment concentration. But the amount of bedload sediment cannot be calculated in the same manner, and it is impractical to directly measure the amount of bedload sediment. In erosion control engineering, work has been done toward the practical use of a hydrophone (i.e., an acoustic bedload sensor) for indirectly measuring the amount of bedload sediment, as explained below.
The Irrigation and Drainage Facilities Research Team of the Civil Engineering Research Institute for Cold Region used a hydrophone to measure the amount of bedload sediment. A paper on the measurement results (UNOKI Keiji, KOHIYAMA Masayuki, SUZUKI Takuro, and NAKAMURA Kazumasa: Observation of Sediment Yield by Acoustic Bedload Sensor and Turbidity Meter in Agricultural and Forest Watershed) was published in "Irrigation, Drainage and Rural Engineering Journal, No.299", of the Japanese Society of Irrigation, Drainage and Rural Engineering. Dr. Suzuki, one of the four authors of the aforementioned paper, is a researcher at the Forestry and Forest Products Research Institute and is an expert in hydrophones.
Study method
The study described in the paper was conducted for two years to measure the amount of bedload sediment discharged from a river basin area of 11.4km2 consisting of 63% forest, 25% farmland, and 12% wasteland and other. A hydrophone was used for measurement. The hydrophone was a microphone installed inside an iron pipe. At the bottom of a river or a drainage channel, the hydrophone was placed at a right angle to the direction of water flow (Photo 2). The microphone inside the hydrophone detected the sound of bedload sediment hitting the iron pipe. Variations in sound level were analyzed to estimate the sediment particle size and the amount of bedload sediment.
There is a settling pond for depositing sediment immediately downstream from where the hydrophone was placed. The pond is 60m long, up to 30m wide, and 1.8m deep. The amount of sand deposited in the pond was measured to verify the accuracy of the hydrophone measurements.
Study results
At the end of the two-year study, the amount of sediment estimated to have been transported to the settling pond was 2,068 tons (i.e., the sum of the measured amount of suspended sediment (1,910 tons) and the amount of bedload sediment calculated by using the hydrophone (158 tons)). The measured amount of sediment deposited in the settling pond plus the measured amount of sediment discharge from the pond was 2,263 tons, slightly greater than the estimated 2,068 tons. The bedload sediment (made up of soil particles larger than 2mm) that deposited in the settling pond weighed 160 tons, while the amount of bedload sediment estimated by using the hydrophone was 158 tons. These results helped to confirm that measurement with a hydrophone is useful for obtaining the amount of bedload sediment.
In the upland cropping area in the basin at the upper reaches of the Abashiri River where the study was conducted, it was found that the ratio of suspended sediment to bedload sediment was 92:8 at the end of the two-year study period. Before conducting the study, it was expected that bedload sediment would account for a higher proportion of the total settlement, because the upland cropping area includes a relatively large area of steeply sloped forest. The fact that the percentage of bedload sediment discharged from these sloped lands was as low as 8% suggests that the percentage of bedload sediment is lower in a river basin where the majority of land is farmland. Thus, it is inferred that the measurements of suspended sediment volume that were taken by researchers in the past without using a hydrophone in various river basins where farmland predominated can be used for approximating the amounts of discharged sediment including bedload sediment volume.
The paper on these study results was given the FY2017 Award for Excellence Paper by the Japanese Society of Irrigation, Drainage and Rural Engineering.
(Contact: Irrigation and Drainage Facilities Research Team, Civil Engineering Research Institute for Cold Region)