|NDT.net May 2003, Vol. 8 No.05|
Tests were performed to compare the accuracy between Direct Method and Indirect Method of Ultrasonic Pulse Velocity Method (UPV) in detecting the location of defects and determining its depth during the early age concrete. Specimens of five reinforced concrete (RC) slabs of grade 40 with a fabricated void at a known location were used and tested at day 3, 7, 14 and 28. The results obtained were compared to determine the accuracy of the two methods. While both methods were able to detect the location defects in specimens during the early age with accuracy of 100%, Indirect Methods was able to detect depth of defect (location inside specimens) with accuracy ranging from 60% -99%. Therefore, test results indicate that both methods can be used to assess the in-situ properties of concrete or for quality control on site as soon as after the removal of formwork.
non destructive testing, reinforced concrete, Ultrasonic Pulse Velocity, Direct Method, Indirect Method, defects and accuracy
Ultrasonic Pulse Velocity (UPV) is a non destructive technique involve measuring the speed of sound through materials in order to predict material strength, calculate low-strain elastic modulus and/or to detect the presence of internal flaws such as cracking, voids, honeycomb, decay and other damage.The technique is applicable where intrusive (destructive) testing is not desirable and can be applied to concrete, ceramics, stone and timber. The main strength of the method is in finding general changes in condition such as areas of weak concrete in a generally sound structure. Absolute measurements should be treated with caution. At the same time, the UPV technique is not always practicable in testing sound concrete. Especially in investigation of crack depth, it is ineffective if the crack is water filled. The performance is also often poor in very rough surfaces. Sometimes good contact requires the use of a coupling gel between the transducers and the structure. This may be aesthetically unacceptable on some structures. Figure 1 shows the leading portable UPV test instruments.
|Fig 1: PUNDIT.|
The development of ultrasonic techniques for non-metallic construction materials has lagged behind developments of sophisticated imaging systems in medicine and highly accurate measuring and testing systems for metals. A number of new systems are becoming available which utilize digital technology. Other related developed systems with more convenient features and advanced technologies are being introduced. For example the UK1401 is a simple to operate hand-held meter, which measures the velocity of sound propagation through concrete or masonry without a need for a prepared surface or coupling agent. This can be used to determine approximate strength, porosity and fracturing of rock while also able to search for near-surface defects. Another example of a model is the A1220 low frequency ultrasonic flaw detector. It is designed for inspecting concrete and masonry and can identify foreign inclusions cavities and cracks while able to determine approximate strength and the thickness of material.
In achieving the required strength for concrete, it is needed to specify a proper mix design with appropriate mix proportions of water, cement, fine aggregate and coarse aggregate for the trial mix. Hence, for the concrete in this context with strength grade 40, several mix designs need to be analyzed before coming up with a most suitable mix design. This is to configure the proportional content of the concrete, which could also affect the pulse velocity. Description of the proportional materials in the concrete mix is as shown in Table 1.
|2.||Fine Aggregate||Uncrushed River Sand|
|3.||Coarse Aggregate||Crushed 20mm Size|
|4.||Water||Clean Tap Water|
|Table 1: Description of materials in concrete mix proportions.|
Three cubes of grade 40 (with the dimension of 150mm x 150mm x 150mm) were casted in the specified dimension moulds on the same day in a concrete laboratory. In prior to cube tests, slump test was carried out for every trial mixes to ensure the mix is with optimal moisture content. The cubes were then tested (crushed) on the 28th day of casting to ensure the selected design was appropriate with the grade (compressive strength). The results of the concrete cube tests for grade 40 of the mix design is as in Table 2. The mix design with proportional amount of water, cement, sand and coarse aggregate was then confirmed for the concrete grade. Table 3 below is the summary of the concrete mix proportions for concrete grade 40.
|Mix Design||Weight, W (g)||Load, F (kN)||Compressive Strength (N/mm2)|
Cube (G 40)|
(Area = 22500mm2)
|Table 2: The results of the cube tests for grade 40 of concrete mix design.|
|Cement||Fine Aggregate (Sand)||Coarse Aggregate (20mm)||Water|
|5.0 kg||9.5 kg||13.0 kg||2.5 kg|
|Table 3: Concrete Mix Proportions For RC Grade 40 Slab (600x500x150)|
After finalizing on the proportions of the desired concrete grade, concrete mix of grade 40 was then ordered from a selected and approved concrete plant by using the tabulated mix design provided by the plant. Cube tests were carried out as soon as the concrete was poured. A total of 12 cubes for concrete grade 40 were needed for casting so that three different cubes can be used on day 3, 7, 14, and 28 respectively.
|Fig 2: Styrofoam (fabricated void).|
Concrete was then casted in the prepared slab moulds (600 x 500 x 150 mm). A total of 5 reinforced concrete slabs of grade 40 specimens with a fabricated void were needed to be casted for the use on every test at specified days. After the casting of the slabs, regular surface curing to prevent plastic shrinkage (excessive loss of surface moisture), which may cause pre-hardening cracks was introduced.
i) Cube Test Procedures
Concrete were casted into cube moulds (with 150 x 150 x 150 mm dimension), where the internal surfaces were lubricated with oil in prior. Concrete was filled in three layers. Every layer was compacted with 35 blows. The cubes were then left for 24 hours to be hardened. The moulds were dismantled and the hardened cubes were then submerged into water filled tank. Three cubes were taken out to be tested at day 3, 7, 14, and 28. The cubes were then weighed and crushed in a compressive strength test equipment to get the compressive strength. The compressive strength for grade 40 was checked to ensure it is appropriate with the concrete grade requirements.
ii) Slump Test Procedures
Concrete was filled into a slump test cone by three layers. Every layer was compacted with 35 blows. The cone was then slowly taken out by leaving the concrete slump a side and the difference of the built height with the original height of the cone was measured. The allowable range for the concrete grade 40 is between 75mm and 125mm.
iii) Ultrasonic Pulse Velocity testings
The measurement of the velocity of ultrasonic pulses as a means of testing materials was originally developed for assessing the quality and condition of concrete and the PUNDIT will undoubtedly be used predominately for this purpose. Figure 1 shows the picture of the equipment. In most of the applications it is necessary to measure the pulse velocity to a high degree of accuracy since relatively small changes in pulse velocity usually reflect relatively large changes in the condition of the concrete. For this reason it is important that care be taken to obtain the highest possible accuracy of both the transit time and the path length measurements since the pulse velocity measurement depends on both of these. Accuracy of transit time measurement can only be assured if good acoustic coupling between the transducer face and the concrete surface can be achieved. For a concrete surface formed by casting against steel or smooth timber shuttering, good coupling can readily be obtained if the surface is free from dust and grit and covered with a light or medium grease or suitable couplant. A wet surface presents no problem. If the surface is moderately rough, stiffer grease should be used but very rough surfaces require more elaborate preparation.
a) Direct Method
When an ultrasonic pulse traveling through concrete meets a concrete-air interface, there is a negligible transmission of energy across this interface so that any air-filled crack or void lying directly between the transducers will obstruct the direct beam of ultrasound when the void has a projected area larger than the area of the transducer faces. The first pulse to arrive at the receiving transducer will have been diffracted around the periphery of the defect and the transit time will be longer than in similar concrete with no defect.
The arrangement for direct method is as shown in Figure 3, where it requires access to two surfaces. The transmitting and receiving transducers are placed on opposite surfaces of the concrete slab. This will give maximum sensitivity and provide a well-defined path length.
|Fig 3: Void Detections using the Direct Method.|
b) Indirect Method
Performing UPV testing requires access to two surfaces, unless indirect (surface transmission) testing is to be done. Though indirect arrangement is least satisfactory upon sensitivity and defined path length, but it is more commonly used since direct method is not possible to use at most of the time. Figure 4 shows the indirect method for detecting void. The void depth can be estimated using the following equation:
Where Vd is the pulse velocity in the defect concrete (km/s),Vs is the pulse velocity in the sound concrete (km/s) and t is the depth of the defect (mm), x0 is the distance at which the change of slope occurs (mm).
|Fig 4: Void Detections using the Indirect Method.|
In a UPV test, a piezoceramic source is electrically pulsed to generate ultrasonic waves, which travel in the structural element, and are sensed by the matching receiver on the opposite side of the test member. The waveform at the receiver is recorded (including the signal transmission start time) by the PC-based system. Knowing the travel distance and travel time, the ultrasonic compression wave velocity is then calculated. After the receiver output is recorded by the PC data acquisition system, the data can be analyzed. Three parameters are used in the interpretation of data:
|Fig 5: Example Void Depth Determination by the Indirect Method.|
In defect areas, the compression wave velocity is slower than in sound areas. In some defect areas, such as honeycomb, the compression wave velocity may be almost the same as in sound areas, but distortion of the signal (filtering of high frequencies) may be used as an indication of honeycomb defect. In addition, defect areas such as honeycomb will generally result in lower amplitude signals.
i) Locations of void in slab specimens
a) Direct Method
UPV Test using Direct Method can easily identify the location of the void in slabs. The void can be detected when the travelling time of the Ultrasonic Pulse shows the highest reading among the rest. The locations of the void in the slabs on different days are spotted with the underlined readings of the Ultrasonic Pulse travelling time. The detected location of the void using Direct Method is shown in Table 4.6 below.
|Slab||Location (mm)||Location, Xo (mm)|
|Day 3||Day 7||Day 14||Day 28|
|Table 4: Detected locations of the void using Direct Method|
The detected locations of the void using Direct Method in Slab 1, 2, 3 and 5 are found at the distance of 200mm, while for Slab 4 it is at distance of 300mm. The determined defect location is then compared with the actual defect location. It showed exact similarity with the actual one for all the slabs and days. Therefore the accuracy for determining the defect location using Direct Method is 100%.
ii) Indirect Method
Unlike Direct Method, UPV Test using Indirect Method in identifying the location of the void is done by chart visualization. Charts below show the detected locations of the void in every concrete slab grade 40.
|Fig 6: Example of Detected Location, X0 of the Void using Indirect Method for Slab 1.|
|Slab 1G40||Depth= 53mm|
|Slab 2G40||Depth= 50mm|
|Slab 3G40||Depth= 67mm|
|Slab 4G40||Depth= 60mm|
|Slab 5G40||Depth= 40mm|
|Table 5: Accuracy Of Concrete Slabs Grade 40|
From the Table 5, the accuracy of Slab 3 is not consistent where it does not show any progress to the maturity period. This is definitely not of an appropriate data (could be due to some technical errors during data recording). Therefore the rest four reliable slabs are only will be taken into consideration for further analysis.
|Fig 7: Accuracy of Slab Grade 40 with Concrete Age (Indirect Method).|
During experimentation, there were several errors that were detected.
1) Inconsistent data output in ELE PUNDIT
Usually after using PUNDIT for around half an hour, the results offset a lot from the original results. For example, a normal reading of 20 ++ microseconds will offset to 60 - 130 microsecond at the very same day. Besides that, very often the results will not remain constant. The results will lessen and drops until the point of zero.
2) Concrete not properly vibrated
Due to the unavailability of a concrete vibrator, most of the concrete slabs are not properly compacted and vibrated. This has to be done manually and upon removal of formwork, some honeycombs were detected on most of the slabs. To remedy the situation, the honeycombs are filled with cement mortar. In one of the case, two slabs had to be rejected due to many honeycombs in the slabs.
3) Rained during concreting
On the day of concreting, it rained. Though only a slight drizzle, it changes the amount of water/cement ratio in the concrete. This affects the strength of concrete to some extent. But having said so, cube tests results of concrete grade were of satisfactory results.
The analysis shows that the accuracy of Ultrasonic Pulse Velocity Test does affected by the concrete age. Where as it matures, the accuracy of UPV Test increases. Apart from that, in comparison between Direct Method and Indirect Method, though direct method shows convenient and satisfactory upon sensitivity for determining the location of the defect but the ability to determine the depth of the concrete slab is not possible and it is also not suitable to use at most of the time since it requires access to two surfaces. Therefore in determination of both depth and location of deteriorations in any concrete slab, there is only Indirect Method would be possible though it is least satisfactory upon sensitivity and defined path length. In general, Ultrasonic Pulse Velocity Method showed better accuracy ranging from 60% to 99% respectively to the ages from day 3 till day 28 (full strength).
The authors wish to extend their deepest gratitude to Universiti Malaysia Sarawak (Unimas) for the Short Term Grant in supporting this research.
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