Investigation of the Dynamic Characterization of Traditional and Modern Building Materials Using an Impact Excitation Test

Abstract

This study presents a comprehensive non-destructive evaluation of a broad range of construction materials using the impulse excitation of vibration (IEV) technique. Tested specimens included low- and normal-strength concrete, fiber-reinforced concrete (with basalt, polypropylene, and glass fibers), lime mortars (NHL-2 and -3.5), plaster, and clay bricks (light and dark). Compressive and flexural strength tests complemented dynamic resonance testing on the same samples to ensure full mechanical characterization. Flexural and torsional resonance frequencies were used to calculate dynamic elastic modulus, shear modulus, and Poisson’s ratio. Strong correlations were observed between dynamic elastic modulus and shear modulus, supporting the compatibility of dynamic results with the classical elasticity theory. Flexural frequencies were more sensitive to material differences than torsional ones. Fiber additives, particularly basalt and polypropylene, significantly improved dynamic stiffness, increasing the dynamic elastic modulus/compressive strength ratio by up to 23%. In contrast, normal-strength concrete exhibited limited stiffness improvement despite higher strength. These findings highlight the reliability of IEV in mechanical properties across diverse material types and provide comparative reference data for concrete and masonry applications.

1. Introduction

Understanding the mechanical behavior of building materials is crucial for structure’ safety, durability, and performance. This study investigates diverse traditional and modern construction materials through destructive and non-destructive testing methods. A comprehensive comparison of their physical and dynamic mechanical properties is presented to support material assessment and selection in structural applications. Although elastic modulus (E) is a key parameter in the mechanical characterization of construction materials, existing scientific literature on its evaluation remains relatively limited and fragmented. The absence of a standardized methodology for determining E has led to inconsistencies in reported values across different studies, making it difficult to perform reliable comparisons or draw general conclusions [1].

The determination of E is carried out using both static and dynamic testing methods. Among the static tests used are the compression test and the flexural test. The compression test measures the elastic behavior of a material under compression, allowing for the determination of the static elastic modulus (E_st). Compression testing is the most widely used method today, but there is still no consensus among researchers and regulations on how to perform these tests and their calculation methods. For example, it is possible to obtain the E_st value by obtaining the compressive strength (f_c)-strain graph of frequently studied concrete samples [2,3,4]. However, most researchers use their methodology [5] or calculate according to the values specified in the regulations [6,7,8]. ASTM C469/C469M [6] defines the chord E_st as the slope between the stress levels corresponding to axial strains of 0.00005 and 0.4 f_c. Similarly, TS 500 [8] defines the secant modulus at a stress level of 0.4 f_c. On the other hand, EN 12390-13 [7] specifies the chord modulus as the slope between stress levels of (1/10) f_c and (1/3) f_c. As can be seen, the regulations use different ranges and points. This may show similar results for relatively homogeneous materials. However, most building materials are composite materials (concrete, lime mortar, brick, etc.). The flexural test, on the other hand, analyzes the material’s response to bending to calculate E_st. The modulus derived in this way reflects the material’s response under flexural stresses (f_f). It may differ slightly from the modulus obtained via compression due to stress distribution, specimen geometry, and potential micro-cracking [9,10]. If we consider concrete as an example for this test method, ASTM C78 [11] and EN 12390-5 [12] regulations are used. These regulations have differences in the loading speed, dimensions of the specimens, and test setup. The formulations for E_st calculations of the samples under bending are also different. In addition, for this testing method, some researchers also calculate some E_st values using the Timoshenko beam theory and the current Digital image correlation (DIC) method [13]. With this method, the researchers stated that the E_st value can be calculated closer to reality.

Dynamic tests calculate E based on the material’s vibration behavior, commonly referred to as non-destructive testing methods. These dynamic methods provide faster results than static tests and are typically preferred for on-site and practical measurements. Ultrasonic pulse velocity (UPV) testing determines the dynamic elastic modulus (E_dyn) by measuring the velocity of ultrasonic waves propagating through the material. The material’s elastic properties, density, and internal discontinuities, such as voids or cracks, influence the propagation speed. According to EN 12504-4 [14] and ASTM C597 [15], UPV results can also be used as correlation models for estimating E_dyn, although accuracy depends on material type and internal structure. In other words, accuracy and measurement range can be quite limited [16,17]. The impulse excitation vibration (IEV) test involves applying a short-duration impulse to measure the material’s vibration frequency and compute E_dyn. EN 14146 [18] and ASTM E1876 [19] establish procedures for determining E_dyn by analyzing the natural resonance frequencies of prismatic specimens subjected to impulse excitation. EN 14146 [18] provides similar guidance, suggesting support locations based on the nodal point theory, ensuring the minimal energy loss and maximum accuracy of frequency detection. Despite minor procedural differences—such as specimen size tolerances and data analysis techniques (e.g., use of correction factors for Poisson’s ratio)—the two standards are functionally equivalent regarding the theoretical basis and expected outcomes.

In general, since there is no consensus on a consistent, applicable static testing methodology for building materials universally accepted by all researchers, civil engineers are directed to use E_dyn values [20,21]. As stated before, the IEV test technique stands out among this test method. Many researchers independently perform IEV tests on concrete, lime mortar, plaster, brick, and stone samples. Marques et al. [1] focused on evaluating the dynamic mechanical properties of cement and lime mortars using IEV. By measuring resonant frequencies, the researchers determined the E_dyn of various cement and lime mortar compositions. The findings highlight the effectiveness of IEV as a non-destructive method for assessing the mechanical behavior of lime-based materials. Ozdemir et al. [13] used the DIC and IEV methods in static tests in their study on lime mortars and different types of clay bricks. The authors found that the E_dyn values obtained from the IEV technique and the E_st values obtained using DIC were very closely related. Makoond et al. [22] studied the mechanical behavior of handmade clay brick and lime mortar samples using IEV and UPV techniques. Their results highlight the effectiveness of the IEV testing, particularly in detecting dynamic elastic properties in heterogeneous or low-density cement-based materials, where UPV may show limitations. The study emphasizes the potential of IEV methods for the improved non-destructive characterization of construction materials. Makoond et al. [23] examined the correlation between the dynamic and static values obtained from the previous study. The researchers shared that the E_st value should be approximately 87% of the E_dyn value. Zuo et al. [24] used the IEV tests to determine the mechanical properties of plaster. In their findings, the researchers highlighted the importance of the water content of the material during the test. Pimienta et al. [25] investigated the elastic properties of sandstones using both UPV and IEV techniques under dry and fluid-saturated conditions. Their study demonstrated the sensitivity of E_dyn values to fluid content and highlighted the complementary nature of UPV and IEV methods in evaluating the geo-mechanical behavior of rocks. Martínez et al. [26] compared the E_st and E_dyn values of carbonate rocks using the IEV technique. The authors found that structural features, such as fractures and voids, negatively affect the relationship between E_dyn and E_st. As a result, a new equation combining E_dyn and the spatial attenuation of compressive waves was proposed to predict E_st.

Unlike most previous studies that focus on either a single material type or solely on E_dyn values, this study presents a broad comparative analysis across a diverse set of modern (concrete specimens with varying strengths (low and normal); fiber-reinforced concretes containing glass, basalt, and polypropylene fibers) and traditional (lime mortars (NHL-2 and NHL-3.5), plaster, and different types of clay bricks (light and dark) commonly used in masonry structures) construction materials. Using the advantages of the IEV technique, such as its non-destructive nature, the ability to deliver more stable results, and its tendency to yield lower standard deviation values—when characterizing these materials contributes greatly to the accuracy and general validity of the data obtained. It combines dynamic and mechanical test results on identical specimens, allowing for a direct correlation of E_dyn, dynamic shear modulus (G_dyn), and dynamic Poisson’s ratio (ν_dyn) with f_c and f_f. Furthermore, including torsional (T) and flexural (F) frequency ratios and E_dyn evaluations provides a novel perspective on the efficiency and deformation behavior of various materials. This integrated approach has not been reported in this manner before. With this study, these deficiencies in the literature will be addressed, and comprehensive information about the characterization of building materials will be provided to researchers. The following section presents detailed information on the construction materials used in the study, including their physical descriptions and preparation procedures. It also outlines the experimental protocols employed, such as sample dimensions, support conditions, measurement points, instrumentation, dynamic parameters calculation procedures, and the IEV method’s testing standards.

2. Experimental Study

2.1. Materials and Properties

2.1.1. Concrete and Fibers

This section details the properties of the materials used in the experimental program and the procedures followed during testing. Experimental studies were conducted in the Materials and Mechanics Laboratory of the University of Oxford. As part of the study, prismatic concrete specimens with dimensions of 40 × 40 × 160 mm were cast to assess their compressive and flexural strengths and their dynamic mechanical properties. Each concrete composition was molded in standard metal molds. Three specimens were prepared for each concrete type. Initially, IEV and flexural strength tests were performed; subsequently, compressive strength tests were conducted on the remaining half-prisms from the flexural test procedure. Additionally, 50 × 50 mm cube specimens were also produced for each concrete mixture. However, these cubes were not used in the main experimental evaluation. Instead, they were employed solely to monitor compressive strength development during the curing process.

The concrete mixtures were prepared using Portland Cement II 32.5R, with river-sourced fine and coarse aggregates. The nominal maximum size of the sand was ≤4 mm, while that of the coarse aggregate ranged from 4 mm to 16 mm. The normal-strength (NS) concrete was designed to reach a 28-day compressive strength of 40 MPa, while low-strength (LS) concrete was targeted at 10 MPa. These variations were intended to investigate the influence of compressive strength on dynamic mechanical properties. The detailed mix proportions are presented in

Table 1. Mix proportions for LS and NS concrete (kg/m³).

Name of Specimen	Cement	Fine Aggregate	Coarse Aggregate	Water	Total
LS	260	855	708	260	2083
NS	400	1065	663	160	2288

. To obtain LS concrete, the water–cement ratio was chosen as 1, which is quite high. For NS concrete, this ratio was determined as 0.4. To improve workability and ensure adequate fiber dispersion, a polycarboxylate-based superplasticizer was added to NS and all fiber-reinforced mixtures at 1% of the cement weight. Before determining the water content for molding, a moisture analyzer (A&D MX-50) was used. The preparation and curing of all concrete specimens were carried out by EN 196-7 [27]. The concrete specimens were cured by sealing the mold in a plastic bag to ensure 95% (±5%) humidity for 7 days (demolding the specimens 3 days after casting them). Then, they were stored in laboratory conditions for 28 days.

Researchers commonly use fiber to improve the tensile strength and crack resistance of concrete [28]. Since different fibers (e.g., basalt, polypropylene, and glass) can significantly alter the internal microstructure and crack propagation behavior, their inclusion allows for a broader assessment of the IEV method’s sensitivity to composite material variations. In addition to NS concrete mixtures, fiber-reinforced concretes incorporating glass (NS-G), basalt (NS-B), and polypropylene (NS-P) fibers were also prepared to investigate the potential influence of fiber type on dynamic mechanical properties. Each fiber type was added at a fixed volumetric ratio of 1.5%, uniformly distributed within the NS mix. The goal of using fiber as efficiently as possible was to keep the concrete mix’s flexural strength and optimum workability. The definition of this ratio was informed by the experimental evidence gathered from earlier investigations [29,30,31]. The same aggregate and binder components used in the NS concrete mixtures were employed to ensure consistency.

Table 2. Mechanical and physical properties of the fibers used.

Properties	Fibers
	Basalt	Polypropylene	Glass
Specific gravity (kg/m3)	2600	910	2680
Length (mm)	12	6	12
Diameter (μm)	15	18	14
Aspect ratio	800	333	857
Tensile strength (MPa)	2900	650	1700
Elastic modulus (MPa)	85,000	3500	72,000

provides information on the mechanical and physical properties of the fiber utilized in the combination. Every specification was acquired from the producers. Fiber addition was expected to enhance energy dissipation and internal friction characteristics, particularly influencing torsional and flexural resonance behavior, which are crucial in dynamic modulus evaluations. Figure 1 illustrates selected photographs from various specimen production and preparation stages, providing a visual insight into the experimental workflow.

2.1.2. Lime Mortars

In this study, natural hydraulic lime-based mortars were prepared using NHL-2 and NHL-3.5 binders (St Astier), and Morestead pale-yellow river sand was used to investigate the effect of binder strength class on the dynamic properties. All mortar mixtures were produced using a constant binder-to-sand ratio of 1:2.5 by weight. As stated in [13], the target flow was generally achieved with water contents of approximately 19% for NHL 3.5 and 21% for NHL 2. Instrumental moisture analysis indicated a moisture content of 17.1% for NHL 3.5 and 18.8% for NHL 2 by weight before molding, suggesting approximately 2% water loss due to evaporation and absorption during mixing and testing. Before determining the water content for molding, a moisture analyzer (MX-50, A&D, Toshima City, Japan) was used. The workability control and curing procedures for the lime mortars were carried out by EN 1015-3 [32] and EN 1015-11 [33], respectively. The prismatic lime mortar specimens with dimensions of 40 × 40 × 160 mm were cast to assess f_c and f_f and their dynamic mechanical properties. After 72 h of initial setting, the mortar specimens were demolded and sealed in plastic bags for 96 h to ensure continued curing under humid conditions. Subsequently, all samples were stored under standard laboratory conditions until the testing date (100 days). In addition to three 40 × 40 × 160 mm prisms used for mechanical and dynamic testing, 50 × 50 mm cube specimens were also prepared. However, these cubes were solely used for periodic strength monitoring and were not included in the main experimental evaluation. The unit volume weight for NHL 2 mortar ready for testing was measured as 1674 kg/m³, and for NHL 3.5, it was measured as 1757 kg/m³. Selected images related to the production of lime mortars are presented in Figure 1.

2.1.3. Plaster

An extensive research program on historical plasters is underway at the University of Oxford Materials Laboratory. As part of this initiative, significant efforts have been devoted to the reproduction and mechanical characterization of plaster and jute-based materials commonly found in historic structures. The mechanical properties of plaster reported by Zuo et al. [24,34] were utilized as a reference in this study. In their experimental work, Zuo et al. used plasters supplied by Industrial Plaster Ltd., with a water-to-plaster ratio 0.7. Before molding, the moisture content was determined using a moisture analyzer (A&D MX-50). Prismatic specimens with dimensions of 40 × 40 × 160 mm were produced using three steel molds, following the procedures outlined in EN 13279-2 [35]. After casting, the samples were dried in an air convection oven at 45 ± 3 °C and relative humidity below 50% to prevent moisture retention and condensation. Once dried, they were stored in a desiccator until testing. The unit weight of the plaster at the time of testing was measured as 1090 kg/m³.

2.1.4. Clay Bricks

The mechanical properties of handmade clay bricks reported by Ozdemir et al. [13] were utilized as a reference in this study. The dimensions of the light and dark clay bricks used in this study are 210 × 100 × 65 mm ± 2 mm. The bricks used were frogless. Solid light multi-bricks (light orange/red) and dark multi-bricks (deep red/purple) manufactured by H.G. Matthews were tested. The bricks were hand molded and fired in gas ovens. Six bricks of each type were subjected to tests. Water absorption tests according to EN 772-21 [36] and initial rate of absorption (IRA) tests according to EN 772-11 [37] were performed on the bricks. Water absorption of the light and dark bricks by weight values were determined as 12.8% and 11.8%, respectively. IRA values were determined as 8.8 and 9.6 kg/m²min. It was seen that both brick types had a relatively high-water-absorption capacity and IRA values. Unit volume weights of the light and dark bricks were calculated as 1688 and 1670 kg/m³, respectively.

Capping (10 mm thick) was applied to the specimens’ lower and upper surfaces for pressure tests. For the specimens to be used for bending tests, a similar application of 10 mm width and 10 mm height was applied to the areas where the supports were in contact (2 lower surfaces and 1 upper surface). Blue Hawk fast-setting cement, characterized by a 7-day compressive strength of 50 MPa, was used in the capping application. Tests on these samples were performed at least 7 days after the capping application.

2.2. Experimental Methods

Within the scope of the experimental program, all test specimens were initially subjected to IEV tests. Subsequently, three-point bending tests were conducted on the same samples (three specimens per group). Compressive strength tests were performed on the remaining half-prisms obtained after flexural testing (six specimens per group) for concrete, lime mortar, and plaster. In the case of brick samples, compressive strength tests were conducted on three separate specimens per group. The experimental campaign yielded 90 IEV, 30 flexural, and 54 compressive strength test results.

The IEV technique was employed in this study to determine the dynamic mechanical properties of the specimens in a non-destructive manner. This method measures the natural frequencies of prismatic samples subjected to a light mechanical impulse under specific support conditions. The dimensions of the prismatic specimens (40 × 40 × 160 mm) and brick samples were suitable for IEV testing according to ASTM E1876 [19]. The tests were conducted using an excitation device (soft-tipped impact hammer) and a vibration measurement system (PDV-100 laser vibrometer, Polytec, Hörsching, Austria). By ASTM C215 [38], the specimens were supported on rubber bands placed along the nodal lines of the targeted vibration modes. While these standards provide a reliable and well-established framework, their application to brittle materials—such as lime mortars, plasters, and clay bricks—may introduce certain limitations. These include challenges in achieving uniform vibration modes and increased measurement variability due to microstructural inconsistencies. Repeated tests were conducted for each sample to minimize these effects, and statistical consistency was evaluated using coefficient of variation values. In addition, reflective tape was applied at the measurement point to enhance signal quality. Figure 2a illustrates the experimental setup for flexural and torsional tests. Figure 2b illustrates the experimental setup for a concrete specimen with polypropylene fiber. In Figure 2c, an example of a light brick that is ready for torsional testing is shown.

Following the mechanical impact, velocity signals were captured over a 10 s interval at a sampling rate of 24 kHz. The post-impact vibration response was processed using a Fast Fourier transform (FFT) to convert the time-domain signal into the frequency domain, allowing for the identification of dominant natural frequencies via spectral peaks. To ensure reliability, each specimen was tested thrice under identical conditions, and the resonance frequency results were averaged to obtain a representative value. The frequencies obtained from F and T modes were used to calculate E_dyn, G_dyn, and the dynamic Poisson’s ratio (ν_dyn) by ASTM E1876 [19]. The equations (Equations (1)–(3)) employed in these calculations are provided below:

$$E_{dyn}=0.9465\left[{\displaystyle \frac{\left(m{F}^2\right)}{b}}\right]\left({\displaystyle \frac{L^3}{d^3}}\right)T_1$$

(1)

$$G_{dyn}=\left[{\displaystyle \frac{\left(4Lm{T}^2\right)}{\left(bd\right)}}\right]\left[{\displaystyle \frac{B}{\left(1+A\right)}}\right]$$

(2)

$${\nu }_{dyn}=\left({\displaystyle \frac{E_{dyn}}{\left(2G_{dyn}\right)}}\right)-1$$

(3)

where m is the mass of the specimen (g), L is the length of the specimen (mm), b is the width of the specimen (mm), d is the thickness of the specimen (mm), and A and B are correction factors. These coefficients were determined based on the width-to-thickness ratio of the specimen (b/d), using the tables or empirical equations provided in Annex A3 of ASTM E1876 [19]. For specific aspect ratios, the values of these coefficients are presented directly in tabulated form, and linear interpolation is recommended for intermediate values. Since T₁ depends on ν_dyn, an iterative procedure is necessary. A starting Poisson’s ratio (ν_o) of 0.2 was assumed to initiate the iteration. The iteration was continued until a change of less than 1% was observed among the iteration results. The same correction factors (A, B, and T₁) and constants were applied consistently to ensure comparability across material groups.

Since the IEV tests are non-destructive, the same specimens were subsequently used for flexural testing. The test procedures followed were EN 12390-5 [12] for concrete prisms, EN 1015-11 [33] for lime mortar, EN 13279-2 [35] for plaster, and ASTM C67/C67M [39] for brick samples. Flexural tests on concrete, lime mortar, and brick specimens were conducted using an Instron 5582 universal testing machine under displacement control at a loading rate of 10 μm/s. Flexural tests on plaster were conducted using a Controls Automax machine in force control (50 N/s). Loading devices were used to ensure uniform loading and contact. The device features cylindrical aluminum components, which load and support the element. f_f was calculated using Equation (4) with the load (P) data obtained from the flexural test. The flexural test setup is presented in Figure 3a.

$$f_f=1.5\left(PL\right)/\left(b{d}^2\right)$$

(4)

Compressive strength (f_c) tests were conducted on concrete, lime mortar, plaster, and brick specimens to explore correlations between material parameters. The compression test setup is shown in Figure 3b. For concrete, EN 12390-3 [40] was followed using half-prism specimens previously subjected to flexural testing. Lime mortars were tested by EN 1015-11 [33], while plaster specimens were tested following EN 13279-2 [35] using broken half-prisms. For brick specimens, the compressive strength tests were carried out by EN 771-1+A1 [41]. Lime mortar tests were conducted using an Instron 5582 universal testing machine in displacement control (10 μm/s) while concrete, plaster, and brick tests were conducted using a Controls Automax machine in force control. The loading rates were applied 2000 N/s for concrete samples, 300 N/s for plaster samples, and 1250 N/s for brick samples.

This section presents the experimental framework, including material specifications, sample preparation, and test protocols. The subsequent section provides a comprehensive analysis of the test results, aiming to investigate interrelations among dynamic and mechanical properties across the evaluated material groups.

3. Test Results and Discussion

All experimental results obtained from unit weight measurements, mechanical tests (f_c and f_f), and dynamic tests (F and T frequencies) are summarized in

Table 3. Summary of destructive and non-destructive test results.

Sample		Unit Weight	fc	ff	F	T	νdyn	Edyn	Gdyn
Group	Name	(kg/m3)	(MPa)	(MPa)	(Hz)	(Hz)		(MPa)	(MPa)
Concrete	C-LS	2050 (2.83)	11.08 (5.44)	1.85 (10.75)	3927 (0.10)	5179 (0.85)	0.296 (10.00)	17,417 (0.58)	6721 (1.69)
	C-NS	2173 (5.90)	48.48 (4.17)	6.20 (11.12)	4437 (0.32)	6114 (0.14)	0.163 (4.48)	21,857 (0.71)	9397 (0.08)
	C-NS-B	2163 (3.71)	49.15 (4.46)	6.60 (9.32)	4794 (0.23)	6348 (0.12)	0.277 (1.57)	27,291 (0.51)	10,684 (0.24)
	C-NS-P	2102 (3.96)	43.75 (0.95)	6.68 (6.69)	4510 (0.65)	5942 (0.41)	0.302 (8.77)	22,570 (1.60)	8670 (0.83)
	C-NS-G	2090 (2.84)	56.28 (2.93)	6.14 (8.71)	4651 (0.10)	6221 (0.29)	0.232 (3.62)	23,257 (0.10)	9438 (0.58)
Lime Mortar	NHL-2	1674 (2.58)	4.07 (6.91)	1.21 (9.88)	2503 (2.61)	3482 (2.56)	0.154 (11.96)	5643 (3.36)	2447 (4.45)
	NHL-3.5	1757 (10.51)	4.17 (5.35)	1.25 (9.96)	2566 (1.04)	3563 (0.29)	0.157 (24.26)	6256 (4.28)	2704 (1.73)
Plaster	P-0.7	1090 (0.78)	8.23 (5.80)	3.86 (9.82)	3085 (0.58)	4184 (0.71)	0.260 (20.15)	5482 (4.38)	2274 (1.15)
Clay Brick	BL	1688 (0.66)	12.99 (16.4)	1.99 (19.93)	2191 (3.74)	2347 (2.35)	0.193 (31.07)	5598 (10.13)	2333 (9.91)
	BD	1670 (1.78)	19.76 (9.1)	2.36 (30.71)	2243 (2.40)	2390 (1.51)	0.215 (51.00)	6163 (7.90)	2504 (17.60)

. The calculated E_dyn, G_dyn, and ν_dyn values for each material group are also presented. The values in parentheses indicate the coefficient of variation (COV, %) for each corresponding parameter, reflecting the level of data dispersion within each group. The table comprises data from various material types, including concrete, lime mortar, plaster, and brick, representing a wide range of mechanical and dynamic performances. This diversity allows for a comparative analysis of how different material properties—such as strength and unit weight—relate to their dynamic response.

When Table 3 is examined, COV values calculated from the static tests are significantly higher than those from the dynamic tests. In particular, error rates in both static and dynamic tests performed on brick samples were observed to reach as high as 50%. Each brick sample may vary from one another due to its handmade nature, which introduces inherent inconsistencies in the geometry, density distribution, and internal defects. Even factors such as the firing temperature and the position of the bricks within the kiln can lead to noticeable differences in the microstructure and mechanical performance, further contributing to the variability among samples. These variations likely account for the high scatter observed in both test results.

Table 3 also shows that the 100 day compressive strengths of NHL-2 and NHL-3.5 mortar samples are remarkably close. While this result may initially seem unexpected considering the difference in binder classification, it can be explained by the use of an identical mix ratio (1:2.5) and similar curing regimes. Moreover, factors such as the maturity or storage age of the NHL binders may influence their reactivity. For example, a partially pre-carbonated NHL-2 binder may have developed higher early strength. Despite the similar compressive strength results, the E_dyn values of NHL-2 and NHL-3.5 mortars indicate notable differences in the mechanical behavior. NHL-3.5 specimens exhibited higher stiffness, consistent with their classification as a stronger hydraulic binder. However, their relatively lower deformation capacity than NHL-2 suggests that this increased rigidity may be accompanied by more brittle behavior. These findings emphasize the importance of evaluating both strength and stiffness parameters to fully characterize lime-based materials.

Figure 4 illustrates the FFT-based frequency response spectra for all tested materials under flexural and torsional excitation modes. The figure enables a comparative evaluation of dynamic behavior across different material groups, emphasizing the influence of material type, internal structure, and additive presence on resonant frequency characteristics.

The NS and its fiber-reinforced variants exhibit pronounced resonance peaks, with F generally higher than T counterparts. Among these, the NS-P displays the highest F frequency (~4800 Hz), suggesting a notable increase in stiffness due to the fiber bridging effect that delays crack propagation and enhances the elastic response. Adding glass fibers (NS-G-F) also resulted in relatively high F frequencies, albeit slightly lower than polypropylene, indicating moderate reinforcement efficiency. Conversely, NS-B exhibits lower peak frequencies (~4300 Hz) and a broader response, potentially due to increased internal damping or the heterogeneous dispersion of fibers. The plain NS concrete shows modest resonance (~4400 Hz). In comparison, LS has the lowest dominant frequency (~3900 Hz), reflecting its relatively lower stiffness and possibly higher porosity or weaker aggregate-matrix bonding.

Among the lime mortars, NHL-3.5 samples exhibit significantly higher frequencies in both F and T modes than NHL-2. This is consistent with the expected increase in strength and stiffness due to the larger hydraulic component in NHL-3.5, which facilitates faster and stronger pozzolanic reactions. The NHL-2 mortar, being weaker and more porous, present’s lower and broader peaks (~2500 Hz for F), indicating lower stiffness and increased energy dissipation. P-0.7 demonstrates relatively sharp and high intensity peaks at around ~3000 Hz for the flexural mode, which implies the material’s relatively stiff but brittle nature. The BL exhibits slightly higher peak frequencies among clay bricks than the BD, especially in T mode. This may initially seem counterintuitive; however, it can be attributed to the lower mass and potentially stiffer firing conditions of light bricks, leading to a more elastic structure. In contrast, dense bricks display dampened and lower frequency responses due to their compact microstructure and higher mass, likely caused by increased internal friction and micro-crack development.

The calculated ratios of T/F exhibited a distinct separation between material types. For concrete specimens, T/F values varied between 1.31 and 1.38, while lime mortars exhibited a slightly narrower but higher range of 1.38 to 1.39. Plaster showed a consistent ratio of approximately 1.35. These three material groups, when considered together, yielded an average T/F ratio of 1.35 with a coefficient of variation of 2.16%, indicating a relatively stable and material-invariant dynamic behavior in terms of mode coupling between bending and torsion. The relatively stable T/F ratio also supports the validity of using either mode to approximate dynamic elastic properties. In contrast, brick specimens exhibited significantly lower ratios, ranging from 1.06 to 1.07, suggesting a fundamentally different vibrational response. The marked deviation seen in brick specimens is consistent with prior studies that noted reduced torsional responses in masonry units due to their geometric discontinuities and non-uniform internal damping. The significantly lower T/F frequency ratio observed in brick specimens (1.06–1.07) compared to other materials (~1.35) is likely attributable to their heterogeneous internal structure and anisotropic behavior. Clay bricks often exhibit manufacturing-induced defects (especially handmade ones, like the one used in this study), directional micro-cracking, and inhomogeneous density distributions due to firing and compaction processes. Additionally, their geometric proportions and surface roughness may amplify flexural stiffness more than torsional stiffness, leading to a suppressed torsional resonance frequency and, consequently, a lower T/F ratio. The marked deviation seen in brick specimens is consistent with prior studies that have noted reduced torsional responses in masonry units.

The correlation between E_dyn and both F and T frequencies is presented in Figure 5a. A strong linear relationship was observed between E_dyn and the frequencies, particularly for the flexural mode (R² = 0.94), indicating that the dynamic modulus is highly sensitive to F frequency changes. The T frequency exhibited a slightly weaker but still significant correlation (R² = 0.87), which may be attributed to the greater influence of shear behavior and specimen geometry in T modes. The steeper slope of the flexural regression line suggests that materials with higher stiffness exhibit a more pronounced increase in flexural frequency than torsional. This relationship confirms the validity of non-destructive resonance-based methods to estimate the elastic properties of heterogeneous construction materials. Moreover, the consistency and strength of these correlations across diverse materials—ranging from lightweight plasters to dense concretes—underline the robustness of the E_dyn as a reliable mechanical descriptor derived from vibration-based testing.

Figure 5b illustrates the correlation between E_dyn and G_dyn for all tested materials. A strong linear relationship is observed, represented by the regression equation E_dyn = 2.4904 G_dyn with a high coefficient of determination, indicating that G_dyn explains 99.35% of the variability in E_dyn. The slope of the fitted line closely aligns with the theoretical elastic relationship E_dyn = 2 G_dyn (1 + ν_dyn), and the calculated slope (2.4904) corresponds to an average ν_dyn of approximately 0.245. This finding demonstrates that the material behaviors conform to the elastic theory and confirms the consistency of the experimental data. Moreover, these ν_dyn values agree with those reported in the literature for quasi-brittle materials, such as concrete, lime mortar, plaster, and brick, which typically range between 0.20 and 0.30. The average E_dyn and G_dyn values presented in Table 3 were reinserted into Equation (3) to assess the internal consistency of ν_dyn values across material groups. The resulting value was called ν_dyn,calc. A comparison between the measured ν_dyn and ν_dyn,calc derived from the average E_dyn and G_dyn values is presented in Figure 6a. While an excellent agreement (within ±0.01) was observed for concrete and NHL-2 samples, deviations were identified in other materials, as shown in Figure 6a. The difference between ν_dyn and ν_dyn,calc was found to be 31% for NHL-3.5, 27% for P-0.7, 4% for BL, and 7% for BD specimens. These deviations are attributed to the level of dispersion in F and T frequency measurements among specimens, which affects the accuracy of the averaged E_dyn and G_dyn values used in the theoretical calculation. As shown in Table 3, the COV for the calculated ν_dyn in these samples ranges between 20% and 51%, indicating a high degree of variability contributing to the observed inconsistency between measured and theoretical values.

Figure 6b demonstrates the E_dyn and the f_c correlation for all tested material groups, including concrete, lime mortar, plaster, and clay brick. The scatter plot reveals a generally increasing trend, indicating that materials with higher compressive strength exhibit greater dynamic stiffness. The fitted regression line, expressed as E_dyn = 487.15 f_c, shows a moderately strong linear relationship with a coefficient of determination (R² = 0.70), implying that ~70% of the variability in E_dyn can be explained by variations in f_c. Compressive strength values of 40 and above for concrete samples and correspondingly high E_dyn values (approximately 18,000–27,000 MPa) dominate the linear regression trend and shape the overall slope. The strong correlation observed in this group is primarily due to the homogeneity and low porosity of the concrete, resulting in stable elastic behaviors. Lime mortar specimens, on the other hand, are located in the lower region of the graph with low compressive strength (~4 MPa) and low E_dyn (~5000–6000 MPa). However, the data distribution within this group is more scattered, likely due to sensitivity to water/binder ratios typical of lime-based materials. While exhibiting compressive strengths comparable to lime mortars, plaster samples show even lower E_dyn values. This can be attributed to their higher porosity and lower binder content. The finer grain size and presence of micro-cracks in plaster likely contribute to the reduced dynamic stiffness observed in this group.

Figure 7 presents a comparative evaluation of two normalized parameters—E_dyn/f_c and f_f/f_c ratios—across different building material formulations. These normalized indices provide a valuable insight into the stiffness and tensile capacity efficiency relative to f_c for various materials. The E_dyn/f_c ratio, represented by the blue bars, indicates the stiffness per unit compressive strength. This ratio is notably highest in the C-LS, reaching values of ~1500, suggesting a disproportionately high elastic modulus relative to their compressive strength. The common feature of these materials is that they have low strength and very high initial stiffness. This behavior means that the materials remain rigid at the beginning of loading, but then begin to deform highly and lose their elasticity. Conversely, C-NS, C-NS-B, and CNS-P show E_dyn/f_c values (approximately in the range of 400–550), which implies that despite having higher strength, their elastic response is relatively modest. A more balanced E_dyn/f_c ratio was observed in C-NS compared to C-LS. This is attributed to the relatively more minor increase in f_f compared to f_c in C-LS, which reduced the f_f/f_c ratio. The addition of fibers significantly influenced the mechanical behavior of the specimens. In particular, the inclusion of basalt and polypropylene fibers had a pronounced effect on the initial stiffness (rather than f_c), resulting in a 14–23% increase in the E_dyn/f_c ratio. While basalt fiber contributed to a balanced enhancement in the f_f/f_c ratio, polypropylene fibers exerted a more pronounced effect on f_f than on f_c. As shown in Table 3, the glass fiber-reinforced specimens exhibited the highest compressive strength among all concrete groups. However, despite a 16% increase in compressive strength compared to the C-NS sample, the E_dyn/f_c ratio was lower, indicating that the improvement in initial stiffness was not proportional. This behavior also corresponds to the lowest f_f/f_c ratio (0.11) among the concrete specimens, suggesting that the high f_c in these samples was primarily due to the fiber reinforcement, with limited enhancement in flexural capacity.

Despite the partial increase in strength (2%) in lime mortars, the approximately 8% increase in the E_dyn/f_c ratio indicates a much better improvement in initial stiffness. This can be associated with the binder and water ratio used. In these samples, it is understood that there is no change in the f_f/f_c values, and the two mechanical properties change proportionally. In general, when lime mortars are evaluated compared to other materials, it is understood that they have higher initial stiffness and flexural capacity despite their very low axial carrying capacity. The f_f/f_c ratio, shown with red bars, reflects the relative flexural performance. This ratio is highest in the P-0.7 sample (0.47), indicating superior flexural performance compared to its f_c, which may stem from enhanced ductility and interfacial bonding within the matrix. When examining the ratios for clay bricks, it is observed that the BD specimens exhibit a 52% increase in f_c compared to the BL specimens; however, this increase is significantly more dominant than the corresponding improvement in initial stiffness. A similar trend is observed in terms of f_f. While the f_f/f_c ratio was 0.15 for BL specimens, it decreased to 0.12 for the BD specimens, indicating that the enhancement in f_f was much less pronounced than the gain in f_c. This divergence may be attributed to differences in the internal microstructure, compaction quality, or firing temperature during brick production, which can disproportionately enhance compressive strength without substantially improving the material’s ductility or stiffness. The relatively limited increase in flexural capacity suggests that while the BD bricks are stronger under axial loads, they may remain brittle and less resistant to bending or tensile stresses.

The combined interpretation of E_dyn/f_c and f_f/f_c indices allows the differentiation of materials not only in terms of absolute strength or stiffness, but also in their mechanical efficiency and balance between stiffness and ductility. These ratios enable a comparative assessment of different materials—especially when destructive testing is limited—and can support more informed decisions in material selection for structural retrofitting, restoration, or lightweight design scenarios. In other words, these ratios provide an insight not only into the strength or stiffness of a material, but also into its anticipated load–deformation response and potential failure mechanisms in structural applications.

4. Conclusions

This study aimed to evaluate the dynamic and mechanical properties of various traditional and modern construction materials (concretes with and without fibers, lime mortars, plasters, and clay bricks) using IEV technique. This method determined and critically analyzed key dynamic parameters, such as flexural and torsional natural frequencies, dynamic elastic modulus, dynamic shear modulus, and Poisson’s ratio. In addition to the dynamic evaluation, this study investigated the correlations between these dynamic properties and mechanical characteristics obtained from destructive tests commonly used in material characterization.

Building upon the dynamic and mechanical parameters evaluated in this study, the frequency variations observed across and within material groups provide valuable insights into their structural characteristics. Some crucial results obtained within the scope of this experimental research are mentioned below:

• Torsional-to-flexural frequency ratios (T/F) showed consistent values across all materials (average ≈ 1.35), except for clay bricks, which exhibited significantly lower ratios (≈1.06–1.07). This deviation is likely linked to their unique geometry, mass distribution, or anisotropic microstructure, suggesting material-specific behavior that should be considered in structural assessments.
• The steeper slope of the flexural regression line suggests that materials with higher stiffness exhibit a more pronounced increase in flexural frequency compared to torsional. This relationship confirms the validity of non-destructive resonance-based methods to estimate the elastic properties of heterogeneous construction materials.
• The results reveal a strong correlation between dynamic moduli and vibration frequencies, particularly with flexural frequency, indicating the reliability of non-destructive dynamic testing in estimating the stiffness of materials. A high linear relationship between E_dyn and G_dyn was observed (R² = 0.99), and the derived slope closely matched the theoretical elastic relationship, validating both the consistency of experimental data and the applicability of classical elasticity theory to semi-brittle materials.
• Furthermore, relationships between dynamic stiffness and compressive strength were analyzed. While E_dyn correlated moderately with f_c (R² = 0.70), material-specific trends revealed that increases in compressive strength did not always result in proportional enhancements in initial stiffness or flexural performance. Fiber-reinforced concretes, particularly those incorporating polypropylene and basalt fibers, demonstrated improved E_dyn/f_c and f_f/f_c ratios, confirming the positive impact of fibers on stiffness and ductility.

In conclusion, this study highlights the reliability and applicability of dynamic non-destructive testing methods for evaluating the mechanical behavior of various construction materials, including conventional and fiber-reinforced concretes, lime mortars, plasters, and fired clay bricks. The strong correlations observed between dynamic and mechanical parameters offer valuable insights into the characterization and comparison of both reinforced concrete and masonry materials. These findings provide a robust reference for future material selection, performance evaluation, and quality control processes in modern and traditional construction applications, especially when preserving sample integrity is required. Future studies may focus on expanding the material database and exploring the applicability of dynamic testing methods under different environmental or aging conditions to enhance predictive capabilities further. In addition, integrating finite element modeling (FEM) to simulate the vibration behavior of construction materials may provide deeper insights into the observed dynamic responses.