This article explains how to inspect cement-based materials without contacting them during the curing process. The ultrasonic excitation is produced by a small amount of ablation generated by a short-pulsed Nd:YAG-laser. On the reverse side of the sample, detection is performed interferometrically by a laser vibrometer.
A reproducible sound excitation was achieved by matching the suitable pulsed laser beam parameters to the material under study. This enables the interferometric measurement of the ultrasonic pulse velocity, vp, along with the velocity amplitudes of the compressive wave.
Characterization of Cement-Based Materials with Laser-Ultrasonics
Knowledge of the curing process for fresh cement-based materials is critical for quality control, material research, and the practical planning and delivery of construction projects.
The ultrasonic pulse velocity (vp), the first amplitude of the longitudinal wave, and the transmitted frequency content can be utilized to explain the material properties. Cement-based materials demonstrate a crucial damping effect on ultrasonic waves along with low pulse velocity immediately after mixing.
Throughout the curing process, the signal amplitudes and ultrasonic pulse velocities constantly increase. The ultrasonic pulse velocity is dependant on the admixtures, additives, and cements used, but also on the grain distribution, air pore content, and water/cement (w/c) ratio.
Laser-ultrasonic inspection is the preferential method for many specific applications because objects can be investigated without harm or contact and at a standoff distance of several meters.
Pulsed lasers enable a contactless excitation of shear, surface, and longitudinal waves precisely on the exposed surfaces. The ensuing velocity amplitudes are assessed in a contactless manner through the use of a laser vibrometer employing heterodyne detection and the Doppler effect.
The principles of the interferometric detection of ultrasonic waves and laser-induced excitation are outlined in the following paragraphs, with respect to the particular experimental setup that allows the investigation of cement mortars and pastes.
Demonstrated in Figure 1 is the principle hardware configuration. A Q-switched, solid-state Nd:YAG-laser with a fundamental wavelength of 1064 nm is utilized for laser ultrasonic generation, as shown in Figure 2. A scanning laser vibrometer is employed to characterize the excited ultrasonic waves, which detects the acoustic displacements utilizing a heterodyne interferometer.
A sample mold with transparent walls was created specifically for this application to enable laser-induced excitation and the detection of through-transmission ultrasonic waves in hardening and setting mixtures (Figure 3).
Two measuring grids were created and aligned opposite to each other, comprising of 27 measuring points for detection and excitation. The acquisition and evaluation of data was controlled by an algorithm enacted in LabVIEW.
This algorithm provides the automated detection of the compressive wave onset and additional signal parameters, for example the signal-to-noise ratio and the first amplitude of the longitudinal wave.
Figure 1. Laser ultrasonic inspection configuration and setup.
Figure 2. Laboratory setup includes: Excitation laser with scanner (right, background); positioning stage with sample (center); Scanning LaserVibrometer (left, foreground).
Figure 3. Testing mold for laser-based ultrasonic evaluation.
Experimental Investigations and Results
Sustained investigations have been performed on cement-based materials throughout the hydration process. Moreover, the local distribution of the ultrasonic pulse velocity, vp, has been established at precise times after mixing.
A range of cement mortars and pastes were investigated. Two different w/c ratios were applied in regards to the cement pastes. The mortar samples had a range of PCE-based superplasticizer content and various grain size distributions.
Figure 4 outlines the development of the ultrasonic pulse velocity for two cement pastes under changes to the w/c ratio. A measuring point in the center of the specimens was selected. The laser ultrasonic measuring sensitivity is adequate for a comprehensive transmission of these strongly absorbing systems.
Throughout the curing process, both experimental mixtures display an increase in the ultrasonic pulse. The cement paste with a lower w/c ratio exhibits a comparably stronger increase in velocity.
Contrastingly, an increase in the amount of water will result in a much smaller and delayed increase. In Figure 5, the evolution of the first amplitude of the longitudinal wave is demonstrated as the surface displacement on the detection side.
The cement paste containing a lower w/c ratio has a steeper and less delayed increase in comparison with the paste containing the higher w/c ratio. This parameter enables a consistent assessment of hydration kinetics. The effect of shrinkage processes on the coupling conditions becomes transparent as the microstructure continues to evolve.
Figure 4. Ultrasonic pulse velocity versus hydration time for cement pastes with various w/c ratios.
Figure 5. Development of longitudinal wave first amplitude for cement pastes with various w/c ratios.
Local Distribution of Elastic Parameters
The distribution of the ultrasonic pulse velocity of a cement paste containing a w/c ratio of 0.31 at various points of time after the start of the hydration is shown in Figure 6. Similar sound velocities were ascertained over the entire specimen at every point in time.
As such, it is probable that there are no changes in the mixture composition or in the hydration process and that the specimen is similar (Figure 7, right). The comparison of the ultrasonic pulse velocities with time enables the assessment of the hydration kinetics.
The distribution of the ultrasonic pulse velocity of a mortar sample at five hours and 24 hours post mixing is outlined in Figure 8. There are extensive variations in the ultrasonic pulse velocities over the test piece’s height.
Higher sound velocities are achieved in the bottom part of the specimen which can be justified by a higher aggregate content. This is because of inadequate mortar sedimentation stability. Prior to stiffening, a settling of the coarse components of the aggregate occurs and there is an enrichment of fine mortar in the upper part.
This is verified by the visual examination (Figure 7, left). Therefore, utilizing the laser-based ultrasonic transmission technique provides a non-harmful investigation of local structure variations vertical to the through-transmission direction.
Figure 6. Distribution of ultrasonic pulse velocity over a sample of cement paste. The narrow distribution in velocity throughout the sample at any given time point shows that the sample is homogeneous.
Figure 7. Mortar M1 with sedimentation phenomena (left) and homogeneous mortar M2 (right) with the heights of the measuring points.
Figure 8. Spatial distribution of ultrasonic pulse velocity in a sample of mortar. The trend from lower velocities at the top of the sample to higher velocities at the bottom shows that the sample is inhomogeneous.
The experimental results utilizing laser ultrasonic inspection on cement mortars and pastes demonstrate that ultrasonic through-transmission can be employed as a non-harmful, contactless investigation of the hardening and setting of building materials.
The time-dependent and local changes of the ultrasound parameters can be employed in order to outline the evolution of the material properties. The ultrasonic pulse velocity and the velocity amplitude of the longitudinal wave enable a continual assessment of the hydration kinetics.
It is viable to outline local variations of the mixture composition of the investigated cement mortars and pastes vertical to the transmission direction by utilizing the ultrasonic pulse velocity.
This can be employed to assess the sedimentation stability of these types of mixtures. The experimental setup outlined here provides the detection of vibration velocity limited to the out-of-plane direction.
Additional investigations applying a 3-D Scanning Vibrometer which analyzes the total displacement vector (out-of-plane along with in-plane) have uncovered that the velocity of propagation of transverse vibration components (shear waves) can be acquired at the same time during the hydration.
This is an intriguing approach that enables a more complete evaluation of the evolution of the material’s elastic parameters together with Poisson’s ratio and Young’s modulus.
This information has been sourced, reviewed and adapted from materials provided by Polytec.
For more information on this source, please visit Polytec.