NO.7 • DECEMBER 2008 - The Hong Kong Polytechnic University

Technology Transfer and Research Developments
技 術 轉 移 與 科 技 發 展
Nondestructive Evaluation Methods as Evaluation Tools of the Quality
of Reinforced Concrete Structures
無 損 檢 測 方 法 可 診 斷 鋼 筋 混 凝 土 結 構 的 質 量
The ability of a civil engineering and building structure to withstand
earthquake is dependent on its structural design and quality control during
construction. After the Sichuan earthquake, sub-standard constructions have
been accused as a major cause of a large number of structural collapses,
especially to schools which led to huge numbers of casualties. The alleged
sub-standard constructions included (1) deficiency in steel reinforcement
inside the concrete structures, and (2) poor concrete quality. There may be
a need to scientifically examine the quality of the building structures after the
buildings have been completed. Nondestructive evaluation (NDE) methods
can be used to visualizethe internal conditions and assess the quality
of large-scale reinforced concrete structures without damaging the already
built structures. Amongst the many NDE methods being used in the civil
engineering discipline, ground penetrating radar (GPR) and ultrasound pulse
velocity (UPV) are the two most effective methods in the case of Sichuan.
basic GPR system costs less than US$50,000; while a basic UPV system
costs about US$5,000; (5) user friendly: 1-2 day training is adequate for
personnel with a civil engineering background, and (6) portable: typically,
these systems weigh about a few kilograms which makes mobilization and
handling very convenient.
12
Figures 1
GPR reveals the arrangement
of steel reinforcements
embedded in a concrete wall.
Figures 2
The UPV set up
GPR transmits high-frequency electromagnetic (EM) wave into the internal
concrete structure via a small transmitting antenna placed on the concrete
surface. Anomalies (such as steel reinforcement) embedded in the reinforced
concrete reflect the EM wave back to the receiving antenna differently from
that of the surrounding concrete matrix. Using a portable computer, the
measured EM signal can be used to quantify the number and spacing of
steel reinforcement present in the concrete in real-time. Figure 1 shows the
typical results of such measurements of a reinforced concrete wall. We can
clearly seethe internal distribution and alignment of the steel reinforcements
(including quantity, spacing and depth) embedded 25mm beneath the
concrete surface.
The working principle (transmission and reflection) of UPV (Figure 2) is
somehow similar to that of GPR, except that its transmission is by sound
wave. Transmission of ultrasound in a material is highly correlated with its
density, which is a determining factor of the compressive strength of concrete.
Hence, this correlation may be used as a rough indicator of the quality of the
examined concrete. In general, a concrete with a UPV higher than 3500m/
s may be regarded as a good concrete.
The advantages of the above two NDE methods are: (1) reasonably
accurate; (2) totally nondestructive to the structure; (3) results are obtained
real-time and on-site during the evaluation such that sample collection and
associated laboratory tests are not required, typically, a general inspection
of a 3-storey building takes about 2 days; (4) relatively inexpensive: a
()
( )25
(,)
3,500
()()()
()
5
5,000
()
()
Professor C.S. Poon
潘 智 生 教 授
Email
cecspoon@polyu.edu.hk
Department of Civil and
Structural Engineering
08