In the past few years, the author has often participated in the survey and evaluation of various concrete structures in Singapore and Malaysia. Adopt advanced non-destructive testing methods, including shock echo test, impulse response test and radar measurement. In this article, the author briefly introduces the application of these methods. Discussed the investigation methods and analysis used in different cases. Also emphasized the test influencing factors. Here are two case studies on the assessment of the quality of bridge deck concrete and the inspection of the post-tensioned prestressed tendon duct voids in the prestressed slab.

Keywords: concrete, advanced non-destructive testing (NDT) method, evaluation

In Singapore and Malaysia, the use of shock echo, impulse response and radar as diagnostic tools for evaluating concrete is relatively new. This method requires skilled operators and experienced interpretation, which is very lacking in this area. Therefore, people's understanding of these technologies and their applications is limited. More work needs to be done to increase the use and appreciation of these technologies, which are currently considered new and advanced. Some of the main attractions of these technologies are their non-destructive nature, and the test requires only an accessible surface. With advanced computer software that helps analyze the signal in detail, these technologies are finding their own way for users who may not be familiar with the technology. Although any excessive simplification of analysis tends to increase the application of tools, it is likely to lead to misuse or misunderstanding of the method. Despite this, the region’s awareness of such tests is increasing, especially among non-destructive testing experts, partly because of the published literature. One of them is ACI 228.2R[1].

In the past few years, the author has used these methods to conduct various investigations and evaluations of concrete structures in Singapore and Malaysia. These include: determining the thickness of walls or slabs and steel reinforcement profile; evaluating the concrete integrity of slab structures, namely slabs, walls, bridge decks, etc.; investigating water seepage problems in basements and water tank walls; detecting post-tensioned reinforcement of prestressed slabs, beams, etc. The gap in the pipe.

1.1 Shock echo test Shock echo test is a method based on the non-destructive evaluation of concrete using stress (acoustic) waves generated by impacts that propagate through the structure and are reflected by internal defects and external surfaces. A small steel ball is used to generate the impact. The procedure for measuring the thickness of concrete slabs using the shock echo method is given in ASTM C1383 [2]

Applications of shock echo technology include: determining the thickness and defects of plate-like structural members, such as plates and walls; detecting defects in beams, columns, and hollow cylindrical structural members; evaluating the bonding quality of the covering layer; and detecting the posterior tension tendon duct The gap and so on.

One of its main advantages is that it can detect the location of the defect and the depth of the defect. The widely published comprehensive research done by the developers of this technology has increased the popularity of this technology. The resulting guide [3] promoted its commercial application. However, an experienced operator is still required.

1.2 Impulse response test The impulse response method is a well-known deep foundation evaluation method. Its most recent application is the evaluation of superstructure concrete members. The impulse response method uses a low strain hammer impact to send a stress wave through the test element. The impact force signal from the built-in force sensor of the impact hammer and the velocity response from the geophone (speed sensor) are processed by the on-site computer.

The typical application of impulse response testing is to evaluate the condition of large concrete structural components, such as floors, sidewalks, bridge decks, walls, storage tanks, etc. Impulse response testing provides a quick way to find defective areas, such as poor concrete consolidation, poor floor support and cavities, and delamination caused by corrosion of steel bars.

Robustness, fast output and good repeatability of test results are the main advantages of this test method. However, there are not many literatures about applying this method to concrete superstructure members at present. Similarly, although this test method has broad potential in engineering applications, its developers have only published a few case studies.

1.3 Radar Survey Radar (acronym for Radio Detection and Ranging) is similar to pulse-echo technology that uses electromagnetic waves (radio waves or microwaves). In civil engineering applications, the inspection depth is relatively shallow, and only short pulse electromagnetic waves (microwaves) are used. Therefore, this technology is often referred to as short-pulse radar, pulse radar or ground penetrating radar (GPR).

Radar measurements have been successfully used to locate steel bars and post-tensioned prestressed tendons, estimate the thickness of slabs, walls, and road surfaces, and recently have also been used to locate and identify concrete anomalies and deterioration (ie, moisture changes, delamination, honeycomb or cracks).

The most attractive feature of this technology is that it can scan a large area under investigation in a short time; it is highly sensitive to underground moisture and embedded metals; it can detect metal and non-metal objects.

2.1 Determination of wall or floor thickness and steel bar profile Generally, it is necessary to investigate the basement wall or bottom plate to check the uniformity of the thickness, profile and void ratio of the steel bar in the concrete. This is common in situations where the formwork or steel is suspected of moving or shifting. When assessing the steel profile in the wall, radar measurements have been used to supplement the coverage gauge measurements. On the other hand, impact echo testing has been used to assess the degree of change in wall thickness and void fraction in concrete.

For thickness measurement of plate-like structures (walls or slabs), it is recommended to use the following formula for calculation [2), (3]:-

In the formula, T——the thickness of the plate structure; Cp——the longitudinal wave velocity, which can be measured according to ASTM C1383; f-the frequency of the P-wave thickness mode of the plate obtained from the amplitude spectrum.

In practice, it would be more convenient and accurate if the P-wave velocity Cp can be measured in an area of ​​known thickness (acoustic concrete area).

2.2 Evaluation of concrete integrity of slab structures Advanced non-destructive testing technology has been widely used to evaluate the integrity of concrete structural components, such as continuous walls, reinforced concrete silo walls, bridge box girder walls, roof slabs, bridge decks, etc. Testing is usually required to check for possible problems such as honeycombs, bulging, weak areas, or high porosity caused by heavy rain pollution during the casting process.

Generally speaking, if the affected area to be tested is large, the impulse response test is first used to locate the relevant weak area for further analysis. The impulse response test is usually carried out in a grid pattern of 0.5m or smaller. The measurement parameter of the impulse response test, that is, the average mobility, can be plotted on the layout of the grid interval. Fluidity is defined as the surface velocity divided by the applied force. The average mobility is defined as the average of the mobility in the frequency range, that is, 1 Hz to 1 kHz, which is related to the thickness and elastic properties of the test member. Poor consolidation and honeycomb structure lead to an increase in average fluidity [4]. Since the average fluidity is a function of the elastic properties of the test member, higher fluidity may also indicate lower concrete strength.

Subsequently, a shock echo test was carried out to evaluate the concrete condition in the selected weaker area for verification. Shock echo testing is usually performed on a smaller grid pattern, that is, 200 mm. The determination of the honeycomb or gap is based on the analysis of the amplitude and frequency spectrum distribution. The typical test spectrum obtained from the shock echo test of acoustic concrete exhibits a single large amplitude corresponding to the P-wave thickness frequency (domain frequency). However, for concrete with subsurface anomalies, similar amplitudes will appear at lower frequencies than the domain frequency (usually accompanied by some amplitudes at higher frequencies). Shock echo test is a point test method, which provides test data in a small area under test.

This article introduces a case study of evaluating the quality of bridge deck concrete.

2.3 Investigation of the voids in the post-tensioned stress bars in the plates and beams The causes of voids in the ribs may be blockages, improper grouting procedures, problems with grouting materials and negligence in construction. Insufficient grouting may not fully protect the tendon from corrosion, thereby reducing durability.

Radar measurements were initially used to locate the tendon profile with high accuracy. This is followed by a shock echo test along the tendon ducts usually at 0.5m intervals. The result of the shock echo test depends on the analysis of the recorded spectrum. The intact concrete slab element exhibits a single large amplitude (domain frequency) corresponding to the thickness frequency. Similarly, for a fully grouted tendon, the domain frequency is at a slightly lower thickness frequency, and a second smaller amplitude peak can be observed due to the presence of tendon bundles. The cavity in the tendon is usually represented by the peak amplitude in the higher frequency range, and is accompanied by a shift in the domain frequency. However, if the pipe is partially grouted or the pipe is relatively deep compared to its size, the signal may not be as pronounced or large as the signal produced by a completely empty pipe.

This article presents a case study for investigating voids in post-tensioned pipes with prestressed slabs.

2.4 Relevance to Intrusive Testing Although the techniques discussed above are considered advanced in the region, intrusive testing is almost always required to verify analysis and findings. This is because, first of all, this method can only provide qualitative or indirect information about the status of test members. Secondly, for the region, perhaps even more important is the lack of confidence in the accuracy of industry interpretations. This is understandable because of the lack of use and understanding of these technologies.

Physical investigation can take the form of core taking and core sample extraction, cutting off a small part of concrete, drilling and fiberscope inspection. The concrete core sample is best to extract the abnormality of the concrete member from the location where the non-destructive test results indicate the existence. These core materials can be subjected to further tests such as visual and microscopic inspections and compressive strength tests to obtain physical conditions and performance.

Before starting a full investigation using these tools, it is also a good practice to conduct early verification during the investigation process to assess the suitability and reliability of the selected NDT method.

Information about the structural member to be investigated is necessary, which will help in testing and interpreting the test results. The following information should be collected:-

Case 1: Concrete quality assessment of bridge deck During the construction of a prestressed segmental bridge, it is reported that the bars slip during the prestressing process. Cracks were observed on the concrete surface near the area of ​​the force block. The 28-day concrete cube strength is lower than the design requirement. Therefore, the concrete quality of the affected part is still questionable. The affected area is approximately 4.0mx 4.0m.

The impulse response test, shock echo test, concrete core sample extraction and laboratory test, including compressive strength test, chemical composition analysis and petrographic inspection are proposed to evaluate the quality of concrete in the bridge deck. For comparison purposes, "good" areas where no damage was reported were also selected for testing. The bridge deck test area is close to the roadside and can be treated as a cantilever plate with a thickness ranging from 225mm to 900mm. The tendon ducts are evenly distributed at a distance of 800 mm (center to center), perpendicular to the curb.

An impulse response test is first performed on the bridge deck to locate anomalies for further investigation. In this case, the impulse response test is performed on a grid with an interval of 800 mm (parallel to the side of the road and between the ribs) and 300 mm (perpendicular to the side of the road). The analysis is based on a review of the average mobility and mobility slope for each test point. Selected areas with higher average liquidity and liquidity slope. Figure 3 shows the impulse response test chart of the affected area of ​​the bridge deck section (liquidity x liquidity slope diagram).

The average liquidity value of the affected area is much higher than the average liquidity value of the "good" area. This shows that the quality of the concrete in the suspected area is not as good as the "good" area. The compressive strength test results of the extracted core samples (described below) confirm these findings.

For verification purposes, shock echo tests were performed on the same grid points. The extraction of concrete core samples is based on the superimposed results of impulse response and shock echo tests.

At one of the test points (near the tendon stress block), the core sample taken showed a large void at a depth of about 30 mm from the deck surface. The average liquidity of this test point is significantly higher than other test points. At another test point, the core sample showed a honeycomb shape under the reinforcement layer (see Figure 4), which is not suitable for compressive strength testing. Similarly, the shock echo test at this time showed that there were some abnormalities. Figure 5 shows the shock echo test signal at this time.

Based on the concrete core strength results, the estimated average in-situ cubic strength (EICS) ranges from 14.5N/mm2 to 30.0N/mm2, the average EICS of the "affected" area is 23.5N/mm2, and the average of the "affected" area The EICS "good" area is 52.0 N/mm2, ranging from 47.5N/mm2 to 56.0N/mm2.

According to the results of impulse response and shock echo testing, the distribution of subsurface anomalies (voids, delaminations, and honeycombs) in concrete is usually localized and limited to two test points. Intrusive investigations, such as coring core samples for visual inspection, confirmed two non-destructive testing results. In addition to non-destructive testing results, extensive core compressive strength tests have shown that the cast-in-place concrete strength of deck concrete is generally lower than the required concrete grade.

Case 2: Detection of post-tensioned pipe voids in prestressed slabs. The shock echo test can be used to detect post-tensioned pipe voids in prestressed slabs and beams. The successful outcome of this application depends on several factors, such as the geometry and shape of the test member, the location and size of the tendon in the test member. For example, if the steel pipe is located in the flange of a typical I-beam, it is difficult to detect the void in the steel pipe. Similarly, if a small tendon tube is located in a relatively deep part of the slab or beam, detection is also difficult.

According to reports, during the construction of an industrial building, the steel bundle pipes were not grouted in accordance with the prescribed procedures. Therefore, it is suspected that there may be a gap in the tendon duct.

The building is a 10-story reinforced concrete structure with post-tensioned pre-stressed concrete slabs and beams. The thickness of the prestressed slab is 270mm, the steel pipes are uniformly distributed in one direction, and are supported by prestressed beams. The spacing between tendon ducts is approximately 700 mm (center to center). The oval tendon duct measures approximately 70 mm (width) by 20 mm (height) and contains four tendons with a diameter of 13 mm. The depth of the reinforced pipe (from the surface of the slab) varies from 20 mm near the supporting beam to 220 mm in the middle of the span. Figure 6 shows a typical tendon section.

A preliminary inspection was carried out using a fiberscope to check whether there is a gap in the tendon through some existing grout outlet hoses. Inspection revealed voids or partially filled grout in some tendons. These tendon ducts were selected for further detailed investigation. Figure 7 shows an internal view of a tendon. Then the shock echo test was performed on the surface of the board along the selected tendon at 0.5m intervals.

The thickness of the board is 270 mm. The recorded domain frequency is 6.3 kHz. According to equation (1), the P wave velocity of concrete is 3544 m/s.

The expected frequency of the P-wave reflection due to the gap in the tendon can be calculated by the following equation [3]:

Among them, d——the depth of the tendon tube from the surface;

Cp——P wave velocity;

In this investigation, it was found that a tendon was severely affected by poor grouting work. Figure 8 shows the shock echo test signal recorded at test point 7 at a distance of 3200 mm from the outlet hose (for information on the tendon duct profile and test points, see Figure 6). At this time, the depth of the tendon is 140mm. The expected frequency from the empty tendon duct is calculated to be 12.2 kHz (using equation (2)). In the recorded spectrum, the domain frequency moved down from 6.3 kHz to 5.4 kHz, and a significant peak was observed at 12.2 kHz, indicating the existence of a gap.

For quality control, shock echo tests were performed on some randomly selected tendon tubes without defect reports.

Some more advanced non-destructive testing techniques, such as impulse echo, impulse response and radar, have been increasingly used in Singapore and Malaysia. So far, although these technologies have limited applications here, they have been successfully used to study specific problems. This contributes to the user-friendliness of the new device to a certain extent, but in other respects, it is used with caution through skilled explanations. However, verification tests such as intrusive tests are almost always inevitable. In addition, the combination of technologies usually optimizes the results and analysis. This will ultimately greatly increase the credibility of these relatively new technologies in the region. However, efforts to raise awareness and appreciation need to be strengthened to further promote the acceptance of these effective tools. Nevertheless, the proficiency of tests and the development of expert use and interpretation should receive the same attention.