Continuous strength measurements of cement pastes and concretes by the ultrasonic wave reflection method
Introduction
Compressive strength testing is a commonly utilized early-age test to characterize cementitious materials [1]. Strength evolution is a key parameter in construction, and consequently, all product development or quality control operations make extensive use of destructive compression and tensile strength tests. Due to the random and heterogeneous nature of the cement-based pastes, mortars, and concretes, an average of at least 3 specimens at a given age is typically required to obtain an acceptable precision of compressive strength results, often cited as the multi-laboratory coefficient of variation of about 7% for mortar cube testing according to the ASTMC109 standard test method [2]. The process of preparing specimens for testing by ASTMC109 requires mixing, casting, curing, and destructive testing. This process is labor intensive and cannot be easily automated. Replacing this method of testing with one that does not require this type of sample preparation, even on a partial basis, would represent a gain in material testing efficiency, as well as a reduction in material and labor costs.
Ultrasonic methods are used in industrial application to measure the evolution of the elastic modulus of a cementitious materials over time [3]. Three techniques are commonly used: (i) compression sound wave propagation through the concrete [3], [4], [5], (ii) speed of the surface wave at the interface between concrete and air [6], (iii) and wave reflection at the interface between the concrete and a wave guide [7], [8], [9], [10]. The three techniques measure the acoustic properties of the materials of interest, which are related to their mechanical properties. The attenuation of the ultrasonic wave through the material may be used to estimatethe evolution of the shear or bulk modulus of material, respectively G and K. Acoustic impedance measurements of shear waves have been successfully used to monitor the flocculation and setting times of cement paste [11], [12]. [8], estimated the compressive strengths of concretes with aggregate volume fractions from 50% to 70% using ultrasonic wave reflection techniques. Results indicate the reflection loss coefficient is sensitive to cement hydration and, after calibration, the reflection loss change may be used to predict concrete strength at early ages.
This study estimates the strength of concrete, with aggregate volume fractions ranging from 10% to 70%, using ultrasonic wave reflection techniques. Accelerating admixtures are added to the concrete assess the ability of this technique to estimate the strength of samples with a rapidly changing compressive strength. A custom-built ultrasonic device is used to measure the reflection loss coefficient of a reflected wave generated at the interface of a waveguide and a hydrating cementitious material. The shear modulus of the sample is estimated from this measurement, which is related to the elastic modulus and, ultimately, the strength. Compressive strengths estimated by this method are compared to traditional quasi-static compressive strength measurements to assess the suitability of replacing these measurements with non-destructive assessments of strength.
Section snippets
Mixture proportions
Both samples of cement paste and concrete, containing aggregates up to 70% by volume, are evaluated in this study. An ASTMC150 Type III ordinary portland cement (OPC) is used to limit the impact of the temperature increase during curing on the hydration kinetics of the samples [13]. Cement pastes were prepared using three non-commercial accelerators: two alkali-free sulfoaluminate suspensions, called accelerators A1 and A2, and a sodium silicate-based accelerator. The cement was mixed with a
Results
The results compare compressive strengths measured by quasi-static compression to compressive strengths estimated by Eq. (5) . The comparison is made by plotting as a function of and determining a model which fits the data using linear least squares regression. The results address the case of cement paste samples first and then address the case of concrete samples with two aggregate shapes at four volume fractions.
Conclusions
The compressive strength of concrete containing rounded and crushed aggregates at volume fractions from 10% to 70% was measured in quasi-static compression and estimated from the reflection of an ultrasonic wave. When the sample contains heterogeneity larger than the wavelengths of the ultrasonic waves, the strength of the sample is estimated by computing the composite shear modulus of the sample using the Hashin-Shtrikan lower bound model, which is used to compute the elastic modulus and,
Conflict of interest
The authors declare no conflict of interest.
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