Increasing economic activities in developing countries result in more energy and consumption demand, which generally leads to environmental degradation. As environmental awareness has grown, the idea of sustainable development has become widespread. The concept of sustainable development can be broadly defined as an attempt to reconcile growing environmental and social concerns (Hopwood et al., 2005). To preserve the ecological balance, it is very important to meet people’s socioeconomic needs. Sustainability plays a crucial role in geotechnical engineering. To be sustainable, an engineering system should be effective, dependable, robust, and adaptable. Efficiency means minimizing the engineered system’s resource use, cost, and environmental impact. As an important part of the construction industry, geotechnical engineering has the potential to have a major impact on sustainable development. Geotechnical works can be responsible for large movements of soil, associated energy consumption, and significant use of natural and man-made materials. Geotechnical engineers have a major impact on the natural environment and water resources by modifying the earth’s surface, altering soil properties, and dealing with contamination. They are often involved in the selection of sites for major infrastructure works, transport services, and buildings, which can have a significant impact on the social and economic aspects of the project (Basu et al., 2013).
Recent research studies on geosustainability are mostly based on the common notions of sustainability, such as recycling, reuse, and use of alternate materials, technologies, and resources. One of the areas in which geotechnical research is making great strides is the use of alternative, environmentally friendly materials in geotechnical construction and the reuse of waste materials (Jastrzębska and Łupieżowiec, 2023). Such materials are, for example, recycled aggregates, that is, reprocessed aggregates derived from demolition or construction waste that contribute to a circular economy by repurposing reclaimed material and waste. Recycled concrete aggregate (RCA) is a promising economic and environmental substitution for natural aggregates (Gabryś et al., 2023a). European countries have been seeking to employ RCA in new concrete structures since the early 1980s (Behera et al., 2014). More than 40 years ago, research began on the characteristics of RCA. RCA manufacturing provides sustainability for concrete waste and encourages recycling rather than landfill. In addition, it focuses on the lack of natural aggregates, minimizes the need for them, and ultimately allows the preservation of the natural course aggregates mined in the open pit. Despite the cost, these and many other benefits have led to an increase in interest in the production and use of RCA in design (Makul et. al., 2021).
In modeling the behavior of RCA when used as a geotechnical material, stiffness at small strains is a key property. Material small-strain stiffness is a critical parameter for predicting deformations of geostructures and soil–structure interaction problems. The stiffness of geomaterials, including the shear modulus (
Piezoelectric transducer applications for natural soils were originally introduced by Lawrence in the 60s (1963), and from then on, the experimental setting as well as the methods of signal interpretation (e.g., Wang et al., 2017) have improved. Some attempts to apply the BE technique to study the stiffness of recycled materials have been recently made. In particular, the authors studied the shear wave velocity in the time and frequency domains (TD and FD, respectively) of pure coarse concrete aggregate from concrete curbs from the demolition of roads and its modified versions, that is, mixtures with 15% rubber chips by RCA volume (Gabryś et al., 2017). The results showed that tire chips significantly decrease the
The current study examined the small-strain shear moduli of compacted, saturated, and isotopically consolidated mixtures of anthropogenic materials. Here, fine recycled concrete aggregates (fRCAs) with varying fine fraction (FF) contents (0% ≤ FF ≤ 30%, within increments of 5%) were selected as the test material. During evaluation of the use of the
BEs comprise two thin piezoceramic plates that are rigidly attached to a central metal plate. Due to the properties of the piezoceramic material, when excited by an input voltage, one piezoceramic plate stretches and the other shortens. This acts like a transmitter, producing a mechanical stimulus. When a bending element is mechanically excited, it produces an electrical output and acts as a signal receiver. From the theory of wave propagation, and assuming that the mass of soil is semi-infinite, homogeneous, and elastic, the modulus of elasticity in small strains can be calculated from the following equation (1):
To determine
The
The TD-based approach involves the problem of the “near-field effect,” which often results in significant errors in travel time measurement because of the bias in the early part of the output signal. Since there is still some uncertainty about how much the near-field effect will distort the output signal in a given state, the experimenter is forced to rely on trial and error to arrive at an appropriate delay. Meanwhile, the FD-based approach has the advantage of minimizing human error when measuring. However, it has been found that the resulting travel times are, in general, highly scattered and are smaller than those obtained by the TD approach. Thus, the difficulty in estimating the appropriate travel time remains. The meaning of the resulting output signal is not understood well enough theoretically to solve the problem (Ogino, 2019).
The laboratory tests presented in this paper were carried out in the GDS Instruments (a division of Global Digital Systems Ltd) GDS RC apparatus on the compounds of crushed concrete waste originating from a demolition site in Warsaw, Poland. The test material was provided by the capital’s supplier, directly in a shredded form. The recycled concrete delivered consisted of well-graded aggregate, which included gravel, sand, and silt-sized grains. Therefore, further crushing of the coarser gravel fraction as well as sieving were conducted before testing, and sand-sized or sand with soil-sized aggregates was the focus of this study. Six different compositions with the specimen codes M1, M2, M3, M4, M5, and M6 were studied and the details of these compositions are given below:
M1, M2, M3, M4 – a group of mixtures in air-dry conditions, with a grain size of 0/2 and an FF content of RCA of 0%, 10%, 20%, and 30%, respectively; M5, and M6 – a group of moisturized mixtures with a grain size of 0/2 and an FF content of RCA of 5% and 15%, respectively.
The grading curves for all the materials are given in Fig. 1, together with an image of the parent concrete aggregate. Table 1 gives a summary of the physical properties of the tested RCA. Based on the physical characteristics and the particle size distribution curves, the blends were classified as poorly graded fine SANDS (M1, M2, and M5 blends) (FF ≤ 10%), poorly graded SANDS with silt (M6 blend) (FF = 15%), and well-graded SANDS with silt (FF > 15%) according to the unified soil classification system specified in, for example, PN-EN ISO 14688-2:2018-05. The samples were produced by evenly mixing two materials, that is, pure fRCA and the calculated amount of FF for this fRCA. A detailed description of the mixes’ preparation and a report from other tests carried out on this material can be found in Gabryś et al. (2023b).
Fundamental physical properties of RCA in this work.
1 | M1 | 0.063–2.0 | 0.2 | 2.61 | Dry tamping | 0.61 | 16.29 |
2 | M2 | 0.02–1.0 | 0.16 | 2.61 | Dry tamping | 0.63 | 16.08 |
3 | M3 | 0.015–0.8 | 0.14 | 2.61 | Dry tamping | 0.72 | 15.43 |
4 | M4 | 0.015–0.6 | 0.21 | 2.61 | Dry tamping | 0.81 | 14.62 |
5 | M5 | 0.02–2.0 | 0.21 | 2.61 | Moist tamping | 0.74 | 14.96 |
6 | M6 | 0.02–1.0 | 0.18 | 2.61 | Moist tamping | 0.71 | 15.00 |
The GDS RC apparatus, an example of the fixed-free type of Stokoe RC system with BE inserts implemented, was used in the present study. A schematic of the testing system is available in many publications like He et al. (2018) and Gabryś et al. (2023b). The RC system adopted by the authors houses a specimen with dimensions of 14 cm in height and 7 cm in diameter. A range of isotropic pressures from 90 to 270 kPa was applied to each specimen. Both the top cap and the base were fitted with piezo-inserts. They protrude 3 mm out of the platen. The width and thickness of BEs are 11 and 1 mm, respectively. The BE test setup is shown in Fig. 2. When configured to the BE mode, one of the piezo-inserts triggers shear waves (S-waves) and the other acts as a receiver (Fig. 3), while in the extender element (EE) mode, the roles of sender and receiver are exchanged to generate and receive primary waves (P-waves) (Lings and Greening, 2001).
Compacted specimens were prepared in a two-piece metal split mold of four layers of similar heights from the oven-dried RCA. For the last two mixtures, that is, M5 and M6, RCA was moistened to a value close to the optimum moisture content (OMC). OMCs were set at 13.5% and 16% for the M5 and M6 mixes, respectively (Gabryś et al., 2023b). To compact the specimens, a lightweight rod was used, targeting a relative density (
The appeal of the BE technique lies in its apparent simplicity, both in performing the test and in interpreting the results. However, over the years, several researchers have demonstrated several potential (inherent and induced) sources of error involved in both BE testing and interpretation. Among these, the most common are near-field effects, wave interferences at the rigid boundaries, specimen geometry, transducer resonance, overshooting, electric noise, and grounding/shielding issues (Viana da Foncesa et al., 2009).
To avoid some of these errors, it is first recommended to comply with several technical requirements and boundary conditions (Lee and Santamarina, 2005). These include properly shielded and grounded, properly connected and encapsulated sensors, leakproof connections, and an environment free of noise. There are also other aspects involved, particularly spatial factors, including the orientation of BE, the wave being reflected off the edges and sides of the sample, and the relative distance between the transmitter and the receiver. Poor contact between BE and the soil, leading to poor coupling, especially at low confining pressures, and overshoot, as BE changes its dominant mode shape at high frequencies and the response becomes complex, are further issues that require consideration.
The input excitation frequency may greatly affect the results of BE tests because of the near-field effects and noise. The near-field component of the wave causes the transmitted wave to be distorted at its origin point, and the wave starts with a downward or upward deflection as described and presented in Ingale et al. (2017). As the shear wave propagates, two different wave signals are produced. One component of this signal wave propagates in the longitudinal direction and the other component propagates as a shear wave, which makes it difficult to distinguish the S-wave propagation time due to the early arrival of the P-wave component. This effect is more prominent when the transmitter and the receiver come closer. To minimize the near-field effect, some authors have suggested using higher input frequencies and wavelength ratios (Wang et al., 2007). The wavelength ratio (
The graphical representation of the test results for the M1 and M4 blend and all applied input signal frequencies for this mix is presented in Fig. 4a and b. Similar plots can be obtained for all other mixtures and pressures. The wavelength ratio (
The latest research on natural soils and their compounds indicates that an excitation frequency of the input signal (
Fig. 4a indicates that at
In Fig. 5, the small-strain modulus
Various methods have been proposed over the years for interpreting the received signals, ranging from the simplest, based on direct observation of the waveforms and measurement of the time interval between the starting points, to more sophisticated techniques supported by signal processing and spectrum analysis tools. Both TD and FD methods were initially taken into account for signal interpretation in BE testing of RCA. A short description of the main principles and applications of each method is given.
The first group of methods is based on the observation of the transmitted and received wave signals and the calculation of their difference as the travel time in the soil sample. With this technique, the arrival time of the shear wave is affected by the near field, the influence of compression wave signals, and other electrical noise and reflections. As a result, it is often difficult to read the arrival time (Sas et al., 2016). Viggiani and Atkinson (1995) compared the initial arrival of the received signal at different possible locations. Fig. 6 gives an example of different potential points of the resulting output signal. Point
Based on the same assumptions as above, Viggiani and Atkinson (1995) suggested using a cross-correlation (CC) function, which measures how two signals are correlated. CC of a low-frequency impulse and the response produce a time displacement, which can be used as the time of flight of the wave between the two points (Mohsin and Airey, 2003). Such a technique is strictly applicable to signals of the same amplitude, requiring frequencies of both waves of the same magnitude (Santamarina and Fam, 1997). However, it is not clear what, if any, benefit is there in using CC to match a single-pulse input signal and a more complex output signal.
The second group of methods calculates the cross spectrum of the transmitted and the received waves, producing the relations of the amplitude and phase angle with the frequency axis. The arrival time is then computed from the phase spectrum slope. As the frequency characteristics of the input and output waveforms are used, this is sometimes called the FD method (Yamashita et al., 2007).
Both groups of procedures require an experienced researcher with a proper knowledge of how to interpret the correlated result. There is also a subjective aspect to this testing technique.
Moreover, the same method used by the same researcher on the same test apparatus can provide different levels of interpretation reliability, simply because of the different properties of the soil. In the subsequent section of the article, only travel time interpretation issues for the RCA specimens will be examined.
In Fig. 7, the results of the small-strain shear modulus (
As presented in Fig. 7, regardless of the excitation frequency, all procedures gave satisfactorily close values for all the tests. In this regard, it should be noted that the input signals
By contrast, the BE tests carried out by the authors, but on natural Warsaw glacial quartz sands (Gabryś et al., 2021), have eliminated this technique, giving preference to the
All tested fRCA compounds were analyzed in a similar way to Fig. 7, and the conclusions also apply to other mixtures.
In the next order, the
The obtained experimental results are in agreement with those of other researchers, for example, He et al. (2018), dealing with the stiffness of recycled composite aggregate. They also concluded that the material with finer fractions has lower stiffness in comparison to the coarser fractions. When the soil contains a certain amount of smaller and softer (or weaker) material, the contact response is more likely to be dominated to some degree by the softest component. The surface contact behavior, in selected anthropogenic aggregates, might be changed (dropped) drastically compared to the pure fRCA, significantly reducing the wave propagation velocity and thus the mixture stiffness characteristics. In the present study, this drop in the particle contact response and, therefore, lower shear moduli only happened at higher pressures. At low pressures, the effect of the fines was not obvious.
Regarding the studied fRCAs, as an example of anthropogenic aggregates, the following conclusions can be drawn.
Both aspects of selecting the frequency of the test and the method of interpretation of the results appropriate for a particular anthropogenic soil are of great importance. When using the BE test method, it is crucial to be aware of all the factors that may affect the results obtained. In the case of fRCA mixtures, the received signal, at low input frequencies (i.e., ≤5 kHz), was affected by the near-field effect. In contrast, at higher frequencies, approximately >14 kHz, the noise levels appeared to be increasing, causing some problems in determining the travel time of the S-wave. Therefore, to obtain a representative value of the small-strain shear modulus of the fRCA blend, BE measurements should be conducted at intermediate input frequencies; for this study, frequencies equal to 10.0 and 12.5 kHz were chosen. Concerning the differences in the travel time interpretation methods, for the selected anthropogenic aggregates, the peak-to-peak method was chosen to identify the travel time and, consequently, estimate the initial shear modulus of the mixtures. It would be worthwhile to consider using artificial intelligence techniques in the future, for example, artificial neural network (ANN) (Hajian and Bayat, 2022), to make the selection of a technique for interpreting test results easier and more efficient. Artificial intelligence techniques are useful for predicting laboratory results without having to pay for more experiments, but a sufficient experimental dataset is required. The small-strain shear modulus of the fRCA compounds from the RC and BE tests was found to be in good agreement, despite differences in the test procedures themselves. The presence of the FF content in the studied mixtures may have softened the particle contact response, which led, especially at higher pressures, to a decrease in the initial stiffness characteristics. At a low pressure, the impact of fine particles was not evident.