|NDT.net - February 2000, Vol. 5 No. 02|
- International Symposium on NDT Contribution to|
the Infrastructure Safety Systems, 1999 NOV 22-26 Torres,
published by UFSM, Santa Maria, RS, Brazil
|TABLE OF CONTENTS|
In addition, improved performance of concrete structures requires better material processing techniques and early detection of defects. Poor service performance is often a result of improper material processing. Defects are often introduced during casting and measuring the 28-day compressive strength is often too late to remedy the situation. Early development of damage in concrete structures is also related to insufficient strength development prior to loading the structure. An NDE technique that monitors the setting and hardening process of portland cement concrete is presented. The technique is based on measuring the ultrasonic wave reflection factor (WRF) between the hardening concrete and a steel mold. This method can be used to assess the in situ properties of concrete at very early ages so that corrective measures can be taken.
KEYWORDS: Concrete, Nondestructive Evaluation, Attenuation, Wave propagation, Ultrasonic
Concrete gains strength over time from a chemical reaction between cement and water, known as hydration. Typically, concrete gains its design strength in 28 days. The gain of strength is monitored in the laboratory by testing standard cylinders in compression. The development of strength that is obtained under controlled laboratory conditions, however, does not reflect the true in-situ strength gain in concrete. In order to evaluate the in-situ strength gain, cores are often removed from the structure, which can result in damage to the structure. Also, techniques based on compressive strength measurement do not produce reliable results at early ages, during the initial setting and hardening of concrete. Improper assessment of early strength gain often results in early damage in concrete structures. A nondestructive technique for monitoring setting and hardening of concrete has been developed recently [Ozturk et al. 1999]. The experimental procedure is based on high-frequency ultrasonic measurements and monitoring the wave reflection factor (WRF) at the interface between steel and concrete. Preliminary studies have shown good correlation between the measured trends of WRF, elastic modulus and the hydration process of cement in the first 48 hours.
Damage in hardened concrete often takes the form of distinct large cracks that extend significant distances within the structure. Techniques that can detect, localize and characterize this damage in a non-destructive fashion, that is using NDE techniques, are of great interest to practicing structural engineers [Rens et al., 1998]. The utility of most existing concrete NDE procedures is based on the fact that opposite and parallel sides of the structure are accessible for through-thickness measurements. However some concrete structures, such as pavements, do not allow access to opposite surfaces. Efforts to modify many of the through-thickness techniques for application to only one surface have been shown to be unsatisfactory. One-sided and accurate NDE techniques for concrete structures are therefore desired. Researchers at Northwestern University have developed several one-sided NDE techniques for concrete structures based on wave attenuation that have been demonstrated to be sensitive to the presence of damage in concrete.
|Fig 1: Schematic Representation of the WRF setup.|
When concrete is placed in the mold, it is in a plastic state that resembles a fluid. According to wave mechanics, a sound wave travelling through metal that is incident upon a steel-water interface is entirely reflected. Thus, at early ages most of the wave energy is reflected and the amplitude of the received wave is large. As the concrete stiffens, more of the wave energy is transmitted through the concrete and less is reflected at the interface. The process of wave reflection can be quantified using a wave reflection factor (WRF) that defines the ratio of the amount of incident wave energy that is reflected from an interface between two materials. A plot of the typical variation of the wave reflection factor as a function of time is shown in Figure 2(a). The WRF is measured continuously after casting up to 48 hours. The temperature is also monitored throughout the hydration process using a disposable thermocouple inserted in the concrete. The temperature profile for the same concrete mixture is also shown in Figure 2(a).
|Fig 2: (a) Typical WRF response and temperature change measured during the first 36 hours after casting (b) Dynamic modulus as a function of time.|
It can be seen in Figure 2 that significant changes in the early response of the WRF coincide with distinctive stage of hydration indicated by the temperature change. After five hours, the concrete begins to stiffen noticeably. This corresponds with the end of induction period. The exothermic reaction starts and a stable cement matrix begins to coalesce. There is a noticeable kink in the WRF response at this time and the temperature begins to increase. This point is labeled as point A in the graph. Pt. A is shown in Figure 3 to correlate well with the set time for concrete containing various admixtures, determined using the pin penetration tests [ASTM C403]. After A, there is a steady, almost linear decrease in the WRF.
|Fig 3: Correlation of set time measured using ASTM C 403 and Point A on the WRF response.|
The WRF response typically exhibits a second distinctive kink, labeled Pt. B on the graph. After Pt. B there is a decrease in the slope of the WRF curve. In preliminary studies performed at the NSF Center for ACBM, the dynamic modulus of concrete determined after Pt. A was shown to exhibit a similar kink at a time corresponding to Pt. B in the WRF response. The dynamic modulus of concrete was determined using standard 100mm x 200mm cylinder following a procedure proposed by Subramaniam et al. . The dynamic modulus as a function of time after casting is plotted in figure 2 (b). This indicates that the observed trends in the WRF are owing to the change in the mechanical properties of concrete. Also, the abrupt change in the behavior at Pt. B suggests a change in the mechanism that effects property development in concrete. Hence one can surmise that Pt. B approximately represents the point in time that marks complete transformation of the fluid state to a sold state. Further hydration in after Pt. B is accomplished through migration of water through the solid phase and hence there is a decrease in the rate of reaction indicated by the reduction in the slope of the WRF after Pt. B.
The results from the WRF can hence be schematically represented as shown in Figure 4. Pt. A represents the setting time when the fluid concrete starts to stiffen. Pt. B corresponds with the time when the concrete is completely solid. The region between Pts. A and B is a transition region where the fluid concrete is converted to a solid from by the products of the hydration reaction. Further decrease in the WRF after B is due to the continuing hydration process that results in the strength gain.
|Fig 4: Schematic representation of setting process monitored by the ultrasonic wave reflection method.|
The hardware used to measure one-sided wave signal transmission consists of a controlled impact-based stress wave source, two receiving accelerometers, a digital oscilloscope, and a personal computer. The two receivers are located on the surface of the test specimen along a line with the source, away from the impact site. Two stress wave sources and two receivers are placed on the surface of the specimen along a line that straddles the crack, as shown in Figure 5. Transient stress waves which are generated by the source propagate along the surface of the specimen, first passing Receiver 1 and then Receiver 2. The stress wave source is a DC powered solenoid with a spring-loaded steel shaft; the shaft striker moves perpendicularly to the surface of the specimen. Miniature accelerometers are used as contact receivers at locations B and C. The accelerometers are coupled directly to the surface of the test specimen with a thin layer of wax.
|Fig 5: Experimental setup for wave transmission measurement.|
When the impact source is applied to location A, the propagating waves are detected by the two accelerometers (locations B and C successively), and are sent to separate channels of an oscilloscope. A typical signal detected by an accelerometer is shown in Figure 6. Only the direct surface-bounded wave components and the first L-wave reflection from the opposing side of the specimen are captured within the time window. The digital data are transferred to a personal computer with the GPIB interface and the time domain signal is then and transformed into the frequency domain with the FFT algorithm. Next, the impact source is applied at location D and the entire data collection procedure is repeated. As a result of the data collection, a total of four signals are obtained. Further manipulations of the four frequency spectra follow, as described below.
|Fig 6: Arrival features of a typical signal received by an accelerometer.|
Theoretical basis of transmission measurements
In the frequency domain, we can represent a stress wave signal sent by the source at location A and received by the nearest accelerometer at location B as a simple product of terms
|VAB = SA dAB RB||(1)|
where VAB is the FFT of the captured time domain signal, SA the generating response term, RB the receiving response term, and dAB the signal transmission function between locations A and B [Achenbach et al., 1992]. Similarly, the stress wave signal sent by the same impact event at location A and received by the far accelerometer at location C is given by
|VAC = SA dAB dBC RC.||(2)|
The Si and Ri terms contain undesired variability caused by variation of impact events, accelerometer coupling, etc. This variability masks the desired transmission response between the two accelerometers dBC. Thus, we are interested in determining dBC by eliminating the Ri, Si and extraneous dij terms. This can be accomplished by collecting another complimentary set of stress wave signals which is sent from the other side of the receiving accelerometer pair along the same line; that is, the stress wave source is moved to location D. The signals sent from D and received at C and B are expressed as VDC and VDB respectively, using the same convention as before. Thus four signals are obtained: VAB, VAC, VDB and VDC. Simple manipulation of the Vij terms results in an expression for the transmission between locations B and C
|dBC (f) = (VAC VDB / VAB VDC)0.5||(3)|
Equation (3) is valid assuming that the material is globally isotropic and that the coupled accelerometers at locations B and C have negligible effects on the passing surface waves and that the contribution of other waves (e.g. direct and reflected L-waves) is negligible. These assumptions have been demonstrated to be correct for this experimental set-up [Popovics et al., 1998b]. dBC is a function of frequency and can be visualized as the ratio of the amplitude of the signal from the far accelerometer to that of the near accelerometer. Thus, a transmission value of 1 indicates no amplitude loss (complete transmission) as the wave propagates between points B and C, whereas a value of 0 indicates complete signal amplitude loss (no transmission). In the case of a point source of stress waves dBC values should be less than 1, even for a perfectly transmitting material, since a considerable signal amplitude loss will result from beam spreading.
The sensitivity of dBC measurements to the depth of surface-breaking cracks in concrete is demonstrated first. Transmission measurements were performed on a free 10 cm thick concrete slab under three different path conditions between the accelerometers at B and C: across an undamaged path, across a 1 cm deep notch cut into the surface, and across the same notch after the slab was subjected to flexure until a crack emanating from the notch propagated several cm into the slab. Figure 7 shows the obtained signal transmission curves within a frequency range of 0 to 200 kHz for the three cases. Reliable results could not be obtained for higher frequencies. Clearly, dBC suffers a severe reduction in value for nearly all frequencies when the surface waves pass across the notch. A further reduction in all frequencies is noted when cracking is introduced additionally to the surface wave path. Thus, dBC is sensitive to the presence of near-surface damage in concrete. Further, this technique has been successfully applied to monitor healing of cracks [Aldea et al 1999] and by Popovics et al.  to monitor the depth and propagation of a surface breaking crack.
|Fig 7: Experimentally obtained surface wave transmission from a 102mm thick concrete slab with varying damage conditions.|
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