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| 題名: | 乾點式超音波儀量測混凝土表面之波速特性 |
| 作者: | 黃彥儒;Huang, Yan-ru |
| 貢獻者: | 土木工程學系 |
| 關鍵詞: | 超音波波速;表面裂縫 |
| 日期: | 2017-10-25 |
| 上傳時間: | 2018-01-16 10:12:38 (UTC+8) |
| 出版者: | 國立中央大學 |
| 摘要: | 乾點式超音波儀可用於檢測現地混凝土劣化程度，多數現地檢測
數據顯示，波速直方圖常有雙峰的現象。第一個峰值約略出現在波速
3600 m/s~4500 m/s 之間，第二個峰值則約略在波速2200 m/s~2600 m/s
之間。波速直方圖為何會有雙峰現象？其微觀機制為何？尚未見有文
獻進行探討。
為探究波速直方圖之雙峰現象，本文中進行一系列的物理模型試
驗，透過試驗觀察及理論分析，以建構波速模擬模式。在物理模型試
驗方面，採用壓克力版及混凝土版，以鋸片製作具有不同裂縫深度之
模型，移動探頭與裂縫間之相對位置，進行不同點位之外視波速量
測。量測結果顯示，當探頭遠離裂縫時，外視波速量測值略小於理論
值；但當探頭靠近裂縫時，外視波速量測值急遽下降，遠小於理論值。
波速急遽下降的量測區域，隨著裂縫深度的加深而擴大。其原因為當
探頭過於靠近裂縫時，裂縫可能變成波傳遞的障礙，使得量測外視波
速出現異常低，甚至無法量測的情形。從壓克力版與混凝土版之量測
波速，經正規化後，兩者相當吻合，顯示波速下降的現象與材料之關
連性不大。當探頭過於接近裂縫，出現異常低波速量測值的區域，本
文定義為盲區，盲區之量測正規化波速約為0.48～0.49。本文使用之
探頭距離為15 公分，當裂縫深度在4.5 公分以內，盲區範圍約略與裂縫深度相當，當裂縫深度大於6 公分，壓克力版及混凝土版全區均
為盲區。
當探頭接近裂縫時，裂縫形成波傳障礙，使得波傳遞的路徑增
加，由物理模型試驗的結果，可求得不同量測點位之距離差值（ΔL）。
物理模型的試驗結果顯示，壓克力版與混凝土版之距離差值與量測點
位的關係頗為接近，似乎與材料的關連性不大，而是探頭位置與裂縫
距離間之函數。
由物理模型試驗所求的距離差值（ΔL）固然可以考慮探頭靠近鋸
片製作人工裂縫的盲區效應，但現地裂縫寬度遠小於人工裂縫寬度，
且不若人工裂縫完全分離，如採用模型試驗所得之距離差值（ΔL），
恐有過度考慮盲區之效應。從現地超波速的量測數據顯示，探頭跨越
裂縫是導致外視波速下降的主因，但是部分跨裂縫的外視波速並無明
顯下降，其可能原因為裂縫屬淺層裂縫或癒合裂縫。故現地調查所得
之裂縫密度亦應予以折減。為了考量盲區效應及裂縫密度之折減，本
文引入盲區折減因子（α）及裂縫密度折減因子（β）。
模擬以網格法量測現地混凝土超音波速，需要完整混凝土之波
速、裂縫密度、裂縫深度，這些數據可從現地調查獲得。乾點式超音
波探頭距離固定為15 公分，裂隙密度乘以裂縫密度折減因子（β），
利用柏松分配方程式，可求得隨機放置探頭之波傳路徑。若探頭位於
ii
盲區內，則總路徑長為波傳路徑與距離差值乘以盲區折減因子（α）
之和。若探頭位於非盲區，則總路徑即為波傳路徑。總路徑長除以探
頭距離（15 公分）乘以完整材料波速即為外視波速，重複以上的程
序，可求得外視波速直方圖及累積機率密度函數。本文提出之α及β
折減因子具有其物理意義，但目前尚欠缺具體而有效的量測方法，本
文根據現地波速量測數據以擬合度最佳化求得α及β兩個折減因
子。從本文建構之模擬結果與現地波速累積曲線比較，兩者吻合度尚
佳。本文提出之模式可合理地模擬波速直方圖的雙峰現象，並解釋此
一現象之機制。
關鍵字：超音波波速、表面裂縫;A dry-point ultrasonic instrument can be used to detect the degree of
deterioration in concrete. Most data shows that wave velocity histograms
are often a bimodal distribution. The first peak will appear at a velocity of
3,600 m/s to 4,500 m/s, with the second peak at about 2,000 m/s to 2,600
m/s. Literatures, however, haven’t investigated how wave velocity
histograms appear or what their micro-mechanisms are.
In order to explore the bimodal distribution of wave velocity
histograms, a series of physical model tests were carried out in this paper.
Through experimental observation and theoretical analysis, a wave
velocity simulation model was constructed. In the physical models test, a
series of acrylic and concrete model with different depths of fracture
created by saw blade. Apparent wave velocities of various locations were
measured by moving the position of the probe. The measurement results
showed that when the probe was far from the cracks, the wave velocity
value was slightly lower than the theoretical value. When the probe was
near the crack, however, the wave velocity value dropped sharply and
was much lower than the theoretical value. The rapidly decreasing area
continued to expand in conjunction with the increasing depth of the
iv
cracks. The reason for this was that when the probe was too close to the
cracks, the wave velocity was unusually low or unable to be measured.
After regularization of the measured acrylic and concrete wave velocities,
the two were found to be quite consistent. This showed that the
phenomenon of wave velocity decline and material were unrelated. When
the probe was too close to the crack, there was an area of abnormally low
velocity measurement, which this paper defines as the “blind zone.” The
normalized wave velocity was about 0.48 ~ 0.49. In this paper, the
distance of the probe was 15 cm. When the crack depth was within 4.5
cm, the zone was about the same as the depth of the crack. When it was
more than 6 cm, they were all in the blind zone.
When the probe was close to the crack, the wave was harder to
transmit and therefore may have caused the wave path to increase. The
distance difference (ΔL) of the different measurement points may be
obtained from the results of the physical model test. The physical model
of the test results showed that they were close, which appears to indicate
that it was not related to the material, but rather the probe position and
fracture distance.
The distance difference from the physical model test may take into
v
account the blind effect, but the in-situ width of the crack was much
smaller than the artificial width. Furthermore, if the artificial cracks were
not completely separated and ΔL was used, there is the possibility of
excessive consideration of the effects of the blind zone. The measured
data from the wave velocity showed that the probe across the cracks was
the main cause of the decrease in wave velocity, but there was no
significant decrease in the wave velocity of the cross-cracks. A possible
reason was that the cracks were shallow or healing. The crack density
obtained from the current survey should also be reduced. In order to
consider the reduction of the blind zone effect and fracture density, this
paper introduces the dead zone reduction factor (α) and fracture density
reduction factor (β).
In order to simulate the grid method to measure the wave velocity of
concrete, the data for intact concrete wave velocity, crack density, and
crack depth can be obtained from a local survey. The distance from the
dry-point ultrasonic probe was fixed to 15 cm and the crack density was
multiplied by the crack density reduction factor (β). Using the Poisson
distribution equation, the wave path of the probe can be obtained
randomly. If the probe is in the blind zone, the total path length is the sum
vi
of the wave propagation path and the distance difference multiplied by
the blind zone reduction factor (α). If the probe is in a non-blind zone, the
total path is the wave path. The total path length divided by the probe
distance (15 cm) and multiplied by the complete material wave velocity
gives the apparent wave velocity. Repeat this procedure, and the apparent
wave velocity histogram and cumulative probability mass function may
be obtained. The α and β reduction factors proposed in this paper have
their physical meaning, but there is still no specific or effective
measurement method. According to the current wave velocity
measurement data, the α and β reduction factors will be found. From the
simulation results constructed in this paper and the wave velocity
accumulation curve of the current measurement, numerical solutions
showed a good agreement with the experiments. The model proposed in
this paper can reasonably simulate the bimodal phenomenon of the wave
velocity histogram and explain its mechanism.
Keywords：P-wave velocity、Surface crack |
| 顯示於類別: | [土木工程研究所] 博碩士論文
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