ACI Materials Journal/January-February 2012 109 Title no. 109-M12 ACI MATERIALS JOURNAL TECHNICAL PAPER ACI Materials Journal, V. 109, No. 1, January-February 2012. MS No. M-2011-051 received February 21, 2011, and reviewed under Institute publication policies. Copyright © 2012, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the November-December 2012 ACI Materials Journal if the discussion is received by August 1, 2012. Setting Time Measurement Using Ultrasonic Wave Reflection by Chul-Woo Chung, Prannoy Suraneni, John S. Popovics, and Leslie J. Struble sponded to initial set measured using ASTM C403/C403M. Others determined initial and final set times using the inflec- tion and inversion points measured with a low-impedance polymethyl methacrylate (PMMA) buffer using either S-waves 9 or P-waves. 10 The objective of this paper is to further explore S-wave UWR as a tool for monitoring the very early stiffening and setting processes of cement paste. The reflection coefficient is controlled by the relative difference in acoustic imped- ance between buffer and cement paste, and the effect of buffer material on the determination of setting time was studied by using buffer materials whose acoustic imped- ance values ranged from 1 to 25 MRayls. Then, using a very low-impedance buffer (1 MRayl), the stiffening and setting behaviors of cement pastes with various water-cement ratio (w/c) values were characterized. Methods are proposed to determine initial and final set using S-wave UWR values, and these set times are compared to corresponding values measured using penetration resistance. This work was part of the PhD thesis of the first author 11 and some results were presented in an earlier paper. 12 RESEARCH SIGNIFICANCE UWR monitors setting and strength development of cement paste and concrete. Most of the past research used an ultrasonic buffer material that has significantly higher acoustic impedance compared to that of the fresh cement paste. This research uses a buffer whose acoustic impedance is very close to that of cement paste, providing measure- ments to monitor the setting process with higher sensitivity. The testing setup of this study provides an improved method of monitoring stiffening and setting of cement and concrete starting at very early ages by eliminating the laborious work required for the current standard penetration resistance test. BACKGROUND If the properties of the buffer are known, then proper- ties of the sample (cement paste in this study) can be computed by manipulating the well-known normal-inci- dence reflection formula r P B i P B A Z Z R A Z Z - = = + (1) Ultrasonic shear wave reflection was used to investigate setting times of cement pastes by measuring the reflection coefficient at the interface between hydrating cement pastes of varying water- cement ratio (w/c) and an ultrasonic buffer material. Several different buffer materials were employed, and the choice of buffer was seen to strongly affect measurement sensitivity; high-impact polystyrene showed the highest sensitivity to setting processes because it had the lowest acoustic impedance value. The results show that ultrasonic shear-wave reflection can be used success- fully to monitor early setting processes of cement paste with good sensitivity when such a low impedance buffer is employed. Criteria are proposed to define set times, and the resulting initial and final set times agreed broadly with those determined using the standard penetration resistance test. Keywords: buffer; cement paste; setting; stiffening; ultrasonic shear wave reflection. INTRODUCTION The term “setting” refers to the progressive increase in stiffness of fresh concrete in which there is a gradual transition from fluid to solid. Generally, initial set is defined as the time when the concrete no longer exhibits fluid properties and final set the time when the concrete begins to develop some measureable level of strength. The setting process is important because the characteristics of concrete during this process usually determine its workability, finish- ability, and load-carrying capacity at early ages. 1 Penetration testing methods are normally used for the measurement of setting times, but these have certain defi- ciencies. There are two common standard methods, namely, ASTM C191 (the Vicat test) 2 for cement paste and ASTM C403/ C403M (the Proctor test) 3 for concrete. The Vicat test uses only one needle, whereas the Proctor test uses various- sized needles that allow for effective measurement across a wide range of fluidity. For this reason, the Vicat test can indicate only the time of setting, whereas only the Proctor test can monitor gradual changes in stiffness prior to initial set. Data from the Proctor test have been seen to correlate better with rheological data, making it the preferred method even for monitoring cement paste. 4 When testing concrete using ASTM C403/C403M, one must first sieve the concrete to remove the coarse aggregate particles. Therefore, the method has several drawbacks: it does not test concrete directly, the procedure is laborious and time-consuming, measurements are made at widely separated times, and the initial set and final set criteria in the standard are empirical and subjective. 5,6 A more recent approach for determining set times, and the focus of this paper, is ultrasonic wave reflection (UWR). Most applications of this nondestructive technique have focused on strength development, but a few studies explored setting. For example, Öztürk et al. 7 and Rapoport et al. 8 reported that the initial decrease in the S-wave reflection coefficient, measured using a high-impedance buffer, corre-