Study on Estimation for Compressive Strength of Concrete in Structure by Ultrasonic Method

Abstract

In this study, for the purpose of improvement of estimation accuracy for compressive strength of concrete by ultrasonic method, a measurement method for ultrasonic velocity and a strength estimation method for concrete were investigated. In the experiment with reinforced concrete members of actual size, the precision of proposed method for measuring ultrasonic velocity of inner concrete was confirmed to be high. Further, the estimated value of compressive strength of concrete in structure by proposed strength estimation method was close to the actual measurement value.

Variation in Compressive Strength of Concrete aross Thickness of Placed Layer

“…It can even be considered as negligible. The obtained results do not corroborate Stawiski’s theses [1,2,3], but confirm the results reported in [7,8,9,10].…”

“…The obtained compressive strength increments across the thickness of the placed layer of concrete do not corroborate Stawiski’s theses [1,2,3], but confirm the results reported in, [7,8,9]. Therefore, there can be agreement with Neville’s statement [10] that the slight increase in concrete compressive strength with depth below the top surface is a natural thing and need not to be taken into account in the evaluation of the strength of concrete in the structure.The concrete compressive strength determined on core samples only slightly depends on the depth of where the sample came from (provided the ingredients of the concrete do not segregate as it is being placed and compacted).The use of the ultrasonic method for testing concrete with point-contact exponential probes showed the variation in concrete strength along the height of the core sample could be quite accurately evaluated and areas of lower quality concrete could be indicated. This was mainly from the thick layer (approximately 30–40 mm) extending from the top edge and the thick layer (approximately 30–50 mm) extending from the bottom edge of the samples 01–06 taken perpendicularly to the upper plane of the element.…”

“…While in the lower part of the samples, it is due to the improper vibration of the placed layer of concrete mixture. The decrease in concrete strength at the side edge of the slab in the case of samples 07–12 can be due to the wall effect [9,10]. From the point of view of the assessment of the concrete structure, supplementary tests on the slab in the future should be carried out using ultrasonic tomography [20,21].…”

“…Ultimately, it was decided that this phenomenon in samples 01–06 was caused from the top by bleeding [17] and concrete sedimentation, and from the bottom by the improper vibration of the concrete by means of the immersion vibrator (the vibrator was not fully immersed in the freshly placed concrete). In samples 07–12, the fall in velocity C L of the longitudinal ultrasonic wave can be caused by the wall effect [9,10]. Figure 5 and Figure 6 show the structure of the concrete along the height of the samples taken perpendicularly to and parallel with the top surface of the slab.…”

“…Neville [9] found that the slight increase in concrete compressive strength below the top surface was a natural thing, but need not be taken into account. When testing a reinforced concrete wall and beam by means of the ultrasonic method, Watanabe, Hishikawa, Kamae and Namiki [10] found the compressive strength of concrete in samples taken from the lower part of the element was slightly higher than in samples taken from its upper part. They treated this as a natural thing which did not need to be taken into account.…”