The Effect of Drill Rotational Speed on Drilling Resistance in Non-Destructive Testing of Concrete

Abstract

Drilling resistance (DR) measurement is a promising non-destructive technique for evaluating the mechanical properties of concrete. However, the reliability and repeatability of DR measurements are still limited by an insufficient understanding of how drill rotational speed influences the recorded drilling response. In addition, a systematic investigation of the influence of rotational speed on multiple drilling response parameters simultaneously is still lacking. This study investigates the relationship between imposed rotational speed and DR parameters—namely, rotational speed reduction, drilling force, and electrical power consumption—measured during controlled drilling tests in C30 and C50 concretes. A laboratory-developed DR testing methodology with constant feed rate and synchronized RPM, force, and power measurements was applied. Five nominal drilling speeds (in the range of 1400–2200 RPM) were examined. The results show clear, speed-dependent trends across all measurements. Strong correlations between nominal and in-hole rotational speeds were observed, while drilling force exhibited a nonlinear dependence on rotational speed. This study reveals distinct drilling behavioral signatures that differentiate concrete strength classes and clarify the mechanical origin of drilling-induced RPM reduction. The findings confirm that DR parameters, when analyzed collectively rather than individually, provide valuable diagnostic information and have strong potential for application in the non-destructive evaluation of concrete structures.

1. Introduction

Non-destructive testing (NDT) and semi-destructive testing are now essential for checking the condition and mechanical performance of concrete in places when coring is not possible, not desired, or needs to be kept to a minimum [1,2,3]. Drilling resistance (DR) has become a promising micro-invasive method for obtaining depth-resolved mechanical information that is very useful for evaluating heterogeneous materials like concrete [4,5,6]. DR measures how resistant a material is to penetration when drilling is performed in a controlled way. This is usually performed by measuring the force, torque, penetration rate, work, or energy use. This gives a direct indication of the material’s local stiffness, hardness, and microstructure [7]. Early studies of stone and heritage materials have shown that the method is sensitive to changes in microstructure and degradation processes. It may also be correlated with standard strength measures such as uniaxial compressive strength [8]. Up to now, DR has quickly grown from being used to obtain information concerning the mechanical properties of surface-treated stone and on mortars, masonry, and more recently, concrete [9,10,11]. In the evaluation of concrete, DR presents numerous advantages compared to traditional NDT methods, including rebound hammer (ReH) and ultrasonic pulse velocity (UPV): it is less affected by carbonation on the surface, can see depth, and can find localized flaws like voids or weak layers [7,10].

Recent advancements integrate DR with the prevalent SonReb (ReH + UPV) methodology, markedly enhancing prediction precision for in situ compressive strength [12]. Moreover, DR-derived parameters exhibit robust correlations with compressive strength (generally r ≈ 0.87–0.97), either competing with or enhancing other NDT techniques [7]. Even though these results are promising, the lack of standardization is still one of the burdens which needs to be faced for a wider use. One of the biggest problems is that the drilling process is very sensitive to operational and mechanical factors, such as the shape of the drill bit, how much wear it has, what thrust is applied, and—most importantly—how fast it spins (RPM). Güneş et al. pointed out that maintaining the rotational speed constant is crucial to make sure that DR measurements in concrete are accurate. Nevertheless, this is hard to achieve in practice due to the way that tools and materials interact and the restricted capabilities of the devices [12]. However, alternative approaches keep the force constant and measure the penetration rate [13]. Felicetti also showed that drilling mechanics have a big effect on torque, thrust, and bit work. This means that these factors cannot be seen as just material properties without likewise considering drill kinematics [14]. Key DR parameters analyzed in the literature are presented at the following

Table 1. Key parameters in DR.

Parameter	Description	Ref
Rotational Speed (RPM)	200–1500 RPM, influences cutting efficiency and heat generation.	[15,16,17,18]
Penetration Rate (mm/min or mm/s)	Axial feed rate; must remain constant for accurate resistance profiling (1–80 mm/min; 3–10 mm/min at 600 rpm for building stones).	[19,20,21,22,23]
Force/Depth Curve	Real-time graph; identifies voids, cracks, or heterogeneity (1–100 N).	[16]
Drilling Resistance (J/mm or s/cm)	Main output indicating material density/hardness.	[16]
Drilling Depth (mm)	Depth measurement provides consistent and interpretable data.	[15,16]

.

The current published research offers conflicting hypotheses concerning the impact of rotational speed on DR response largely due to differences in material heterogeneity and dominant material removal mechanisms. Research on concrete shows that the interaction between the drill bit and coarse aggregates, which are often harder than the cement paste around them, causes variable loading that is significantly influenced by RPM, affecting the rate of energy dissipation and penetration [7,12]. Experiments conducted on microbiologically corroded concrete show that RPM influences the detectability and resolution of subsurface anomalies, which indicates a more intricate relationship between kinematic settings and measurement fidelity [17]. These differing conclusions highlight the need for systematic research on the impact of drill speed specifically for concrete, a material that is fundamentally more heterogeneous than stone.

The objective of this study is to systematically examine the influence of nominal drill rotational speed on DR response data in C30 and C50 concrete through controlled drilling experiments and quantitative analysis. In particular, the study investigates how variations in rotational speed affect penetration behavior, recorded DR, and the stability and variability of measured response parameters. Despite the growing use of DR methods, the lack of a clear understanding of these speed-dependent effects represents a significant scientific gap. This gap limits the physical interpretation of DR measurements and reduces the reliability and repeatability of their practical application in the non-destructive evaluation of concrete. From an applied perspective, this uncertainty hinders the definition of the optimal drilling conditions required for stable, repeatable, and comparable DR measurements in practice.

2. Materials and Methods

To address the identified scientific and applied problems related to the influence of drill rotational speed on DR measurements, a structured experimental research program was designed. The program consisted of a series of controlled laboratory drilling tests performed at multiple nominal rotational speeds while maintaining constant and repeatable drilling conditions. Two concrete strength classes (C30 and C50) were selected to evaluate material-related effects. During drilling, key response parameters—rotational speed, axial drilling force, and electrical power consumption—were recorded simultaneously, enabling a comprehensive assessment of drilling resistance behavior, data stability, and speed-dependent effects. The statistical analysis was limited to descriptive measures (mean values and standard deviation) and regression analysis to quantify speed-dependent trends.

2.1. Concrete Characterisations

The mixtures of C30 and C50 concrete in detail are described in the authors’ previously published paper and are referred to the X0 exposure class according to EN 206 [7]. Both mixtures were designed in laboratory using the same components: a standard Portland cement (CEM I 42.5 N, Schwenk Ltd., Broceni, Latvia), natural sand, and crushed natural aggregates with maximum grain size of 11.2 mm, but with different water-to-cement ratios to obtain distinct mechanical behavior. The mixture C50 was proportioned with a W/C ratio of 0.67. After 28 days of curing, the average compressive strength of 150 mm cube specimens was 49.8 MPa, placing the mixture in the higher-strength category within the scope of this study. The corresponding dry density was 2241 kg/m³. The C30 mixture was prepared with W/C ratio of 0.80. At 28 days, the average compressive strength was 31.2 MPa, and the dry density averaged 2163 kg/m³. The use of C30 and C50 concrete types enables evaluation of DR behavior across a meaningful strength range while maintaining comparable composition and aggregate type. This selection allows the influence of drill rotational speed on drilling response parameters to be assessed under controlled conditions, while ensuring that the results remain relevant for practical non-destructive evaluation of existing concrete structures.

2.2. Drilling Test Device Setup

2.2.1. Drilling Test Device

A custom-designed drilling machine stand placed inside the loading frame of the Zwick Z100 (ZwickRoell, Ulm, Germany) was used to perform controlled DR tests on concrete specimens (Figure 1). The stand integrates a handheld rotary drill (Bosch GSB 13 RE, Bosch, Hangzhou, China) with a rigid mechanical frame that ensures stable and repeatable drilling under laboratory conditions. The drill was firmly mounted to the overhead crosshead, which guides its vertical motion and maintains alignment with the specimen surface during penetration. Drilling was performed using a 5 mm tungsten carbide masonry bit (Bosch CYL-9, Bosch, Hangzhou, China). The loading frame controlled the downward motion of the drill by imposing a constant feed rate, ensuring consistent drilling speed throughout each test. Concrete specimens with dimensions of 15 × 15 × 15 cm were positioned below the drill and clamped securely to prevent lateral movement or rotation. Two external sensing devices were incorporated to monitor the drill motor behavior during penetration. A non-contact optical tachometer (“Extech RPM250W Laser Tachometer”, Teledyne FLIR LLC, Hong Kong, China) was mounted near the drill chuck to continuously measure instantaneous rotational speed (RPM). In addition, an inline digital power meter (“PZEM-016 Energy Tester” (Ningbo Peizheng Electronic Technology Co., Ltd, Ningbo, China) was connected to the drill power supply to record real-time electrical input, enabling evaluation of the energetic demand associated with cutting, friction, and torque fluctuations.

2.2.2. Drilling Parameters

Drilling parameters are continuously monitored using a set of integrated sensors. The rotational speed (ω) of the drill bit is measured with a laser tachometer, while the feed rate (v) and penetration depth (ΔL) are recorded from the displacement system of the loading frame. Penetration depth is continuously measured to log the profile of resistance vs. depth [16,24]. Depth data are also used to identify layers or transitions in the material—for example, an abrupt increase in resistance at a certain depth could indicate encountering a harder aggregate or a different concrete lift/substrate [16]. Depth was limited by the device’s travel; most DR tests in concrete are performed to a depth of 50 mm or less, and in this research up to 30 mm drilling depth was evaluated [16,25]. The electrical power drawn by the drill motor (P) is logged using an inline power meter, and in selected test series the axial force (F) exerted on the bit is recorded to characterize resistance during drilling. These measurements enable a detailed assessment of the mechanical response of the concrete under different drilling speeds and feed conditions. Five nominal rotation speeds were selected for evaluation—1400, 1600, 1800, 2000 and 2200 RPM. A higher RPM (at a given feed) generally lowers the resistance force by reducing the depth of cut per revolution, whereas a lower RPM increases resistance (as more material is cut per revolution) [18]. Across all rotational speeds, the first 10 to 15 s, corresponding to the drill’s entry into the concrete surface and transition from free-running to loaded conditions, were removed from further evaluation of mean drilling values [13,18].

3. Results

3.1. Drilling Patterns

The individual drilling curves in Figure A1 (C30) and Figure A2 (C50) show consistent behavior across all three repetitions for each nominal drilling speed (see Appendix A of the manuscript). In both concretes, the rotational speed initially drops sharply during the first 10 to 15 s of penetration, after which the drill reaches a quasi-steady drilling regime. At lower nominal speeds (1400–1600 RPM), the drilling patterns exhibit stronger fluctuations, reflecting more variable interaction with aggregates and greater sensitivity to local material heterogeneity. As the nominal speed increases, the curves become smoother and more stable, indicating improved cutting continuity and reduced torque variation. A clear difference between the two concretes is visible: C30 concrete shows a larger overall reduction in in-depth rotational speed and more pronounced oscillations, whereas C50 concrete maintains higher rotational stability and exhibits smaller fluctuations throughout drilling. Despite these differences, all three repetitions within each speed group show similar curve shapes, confirming the repeatability of the drilling test procedure.

3.2. Rotational Speed

The average in-depth rotational speed during drilling in C30 concrete for five nominal rotational speeds, 1400, 1600, 1800, 2000, and 2200 RPM, are presented in Figure 2a. At 1400 RPM, the average rotational speed decreases to approximately 900–1000 RPM, where it remains with moderate fluctuations throughout the drilling time. Increasing the nominal speed to 1600 RPM results in an average drilling speed of approximately 1300–1400 RPM, with a relatively stable profile and occasional local variations. The 1800 RPM setting shows a smaller proportional reduction in speed, with the average drilling RPM stabilizing at 1500–1600 RPM for most of the test duration. At 2000 RPM, the operational drilling speed remains in the range of 1750–1850 RPM, showing only moderate fluctuations. The highest nominal speed, 2200 RPM, produces the highest in-depth rotational speeds, generally stabilizing at 1900–2000 RPM after the initial transient.

Figure 2b presents the average in-depth rotational speed during drilling in C50 concrete. The overall behavior closely resembles that observed for C30 concrete, with an initial reduction in rotational speed occurring within the first 5–10 s of drilling, followed by stabilization at a lower in-hole RPM level specific to each nominal speed. For all tested rotational speeds, the stabilized drilling RPM increases proportionally with the nominal setting and remains within comparable ranges to those observed in C30 concrete.

The average rotational speeds measured during drilling in C30 and C50 concretes excluding the first 15 s of the drilling are presented in Figure 3. Each data point represents the mean value obtained from three repeated drilling tests, with error bars indicating the standard deviation. For both concretes, the average drilling RPM increased linearly with the nominal rotational speed. Linear regression models show a strong relationship between imposed and actual rotational speed, with coefficients of determination of R² = 0.98 for C30 and R² = 0.97 for C50. Across all nominal speeds, the C50 concrete maintained a higher in-depth drilling RPM than C30. Although the differences are moderate in absolute terms, the trend is consistent across the entire range of speeds. The standard deviations were relatively small for both concretes, indicating the good repeatability of the drilling process and uniform material response within each concrete class.

3.3. Drilling Force Measurements

Figure 4 presents the average and maximum drilling forces measured. In both concretes, the average drilling force initially decreases as the rotational speed increases from 1400 to 1600–1800 RPM, after which it rises again at higher speeds. This produces a characteristic U-shaped dependence captured by the fitted parabolic trendlines. For C30 concrete (Figure 4a), the average drilling force ranges from 41 N at 1400 RPM down to a minimum of 23–24 N near 1800–2000 RPM, followed by an increase to 92 N at 2200 RPM. The maximum drilling force follows a similar pattern, ranging from 117 N at 1400 RPM, decreasing to 70–76 N at mid-range speeds, and increasing to 247 N at 2200 RPM. For C50 concrete (Figure 4b), the same trend is observed but at consistently higher force levels. Average drilling forces vary from 79 N at 1400 RPM to 86–109 N at mid-range speeds and increase to 192 N at 2200 RPM. Maximum forces range from 210 N at 1400 RPM to 163–217 N at 1800–2000 RPM and reach 357 N at 2200 RPM. The high coefficients of determination (R² ≈ 0.86–0.95) for both the average and maximum force curves indicate a strong parabolic relationship between force and drilling speed.

3.4. Power Consumption

The average and maximum electrical power consumed by the drilling machine during penetration into C30 and C50 concrete are presented in Figure 5. In both concretes, power consumption shows a clear increasing trend with higher rotational speed. For C30 concrete, the average power rises from 78 W at 1400 RPM to 148 W at 2000 RPM and reaches 164 W at 2200 RPM. Maximum power follows a similar trend, increasing from 96 W at 1400 RPM to 131 W at 2000 RPM and reaching 195 W at 2200 RPM. The linear trendline for the average values demonstrates a strong correlation with RPM (R² = 0.99), indicating that power consumption increases almost proportionally with drilling speed. The maximum power values fit well with a second-order polynomial curve (R² = 0.95), capturing the slightly nonlinear increase at higher speeds. For C50 concrete, the average power increases from 81 W at 1400 RPM to 138 W at 2000 RPM and reaches 156 W at 2200 RPM. Maximum power grows from 91 W to 124 W and finally to 174 W over the same speed range. As with C30, the average values follow an almost linear trend (R² = 0.99), while the maximum values exhibit a stronger nonlinear rise with speed (R² = 0.97).

4. Discussion

4.1. Drilling Patterns

The individual drilling patterns in C30 concrete (Figure A1) provide important insight into the mechanical interaction between the drill bit and the material microstructure. Although the nominal rotational speeds ranged from 1400 to 2200 RPM, the actual in-hole RPM was consistently lower due to torque demands imposed by the cutting process. This behavior is consistent with the fundamental description of DR, where the engagement of the bit with the material causes an immediate increase in torque demand and a reduction in rotational speed [26]. The initial rapid RPM reduction observed across all tests reflects the transition from free rotation to fully engaged drilling, during which the bit must overcome the concrete surface hardness, friction, and the formation of an initial cutting groove. This early-stage instability, commonly described as a “lock-in” phase in the DR literature, precedes stabilization [8]. The extent of the RPM drop depends on both the nominal speed and the mechanical properties of the C30 concrete. At lower nominal speeds (1400 and 1600 RPM), the reduction represents a large percentage of the initial speed—sometimes exceeding 40%—indicating that the drill operates near the lower end of its torque curve, where even moderate increases in resistance produce pronounced speed reductions. This agrees with observations that low ω combined with high instantaneous resistance results in unstable drilling behavior and stronger oscillations in DR traces [18]. Consequently, RPM traces at these speeds exhibit stronger oscillations caused by aggregate hardness variations and local microstructural heterogeneity. At intermediate and higher speeds (1800–2200 RPM), the rotational speed stabilizes more quickly and remains closer to the nominal value. This behavior aligns with the characteristics of electric drill motors, where higher initial rotational speeds provide a larger available torque reserve. Consequently, fluctuations arising from material heterogeneity—such as transitions between paste-rich zones and aggregate clusters—produce relatively smaller disturbances in the rotational speed. This is most evident in the 1800 and 2200 RPM tests, where the curves for the three repetitions nearly overlap for extended periods, demonstrating both improved drill stability and good material uniformity at the scale of the cutting process. The consistency among repetitions corroborates the findings in previous DR research that, when user-controlled variables such as speed and penetration rate are fixed, drilling patterns are highly reproducible despite local heterogeneity [16]. The improved repeatability at higher nominal speeds indicates that torque-to-speed conversion becomes less sensitive to small-scale irregularities, making higher RPM drilling potentially more suitable for diagnostic purposes in NDT. These observations emphasize that drilling rotational speed is not simply a passive measurement but an active indicator of the drill’s load state [8]. For C50 concrete, the data show that lower nominal speeds result in greater motor load sensitivity and larger RPM variability, while higher speeds mitigate local disturbances and generate more uniform drilling signatures. This aligns with findings that both material heterogeneity and drilling parameter selection (especially ω and PR/ω) influence the stability and diagnostic quality of drilling-based NDT measurements [18]. This has practical implications: selecting a higher initial rotational speed may reduce noise in drilling-based measurements and improve the repeatability of DR tests when used as an NDT method.

4.2. Rotational Speed Response to Different Concrete

The averaged drilling patterns reveal that, under identical nominal rotational speeds, C30 concrete consistently exhibits lower in-hole rotational speeds and larger RPM drops than C50 concrete. This behavior is evident across the entire tested speed range (1400–2200 RPM) and is confirmed by the averaged RPM values shown in Figure 2a,b. The observation indicates that the drill motor experiences a higher effective load and greater speed reduction when drilling C30 concrete, despite its lower compressive strength. At first glance, this behavior may appear counterintuitive, as higher-strength materials are often associated with greater DR. However, DR studies emphasize that rotational speed response is governed not only by material strength, but by the efficiency of material removal and the stability of the cutting process [8,16]. In this context, the weaker C30 concrete, characterized by higher porosity, lower stiffness, and less cohesive paste–aggregate bonding, promotes micro-crushing, frictional drag, and irregular debris formation during drilling. These mechanisms increase energy dissipation and torque instability, leading to a more pronounced reduction in rotational speed. In contrast, C50 concrete maintains higher in-hole RPM at all nominal speeds, despite requiring higher drilling forces. The denser microstructure and stronger aggregate–paste interfaces of C50 enable a cleaner fracture and chipping mechanism at the bit–material interface. This results in more stable torque transfer and reduced losses due to the vibration, grinding, or repeated crushing of loosened material. Similar behavior has been reported in the DR literature, where denser and less porous materials exhibit smoother drilling signatures and reduced RPM variability, even when absolute resistance is higher [8,16,26,27]. These findings demonstrate that rotational speed during drilling reflects the effectiveness and stability of material removal rather than compressive strength alone. The larger RPM drop observed in C30 indicates inefficient cutting dominated by frictional and crushing mechanisms, while the higher RPM maintained in C50 reflects more efficient chipping and stable drilling, despite higher absolute forces [18,27,28]. When interpreted alongside drilling force and power data, RPM behavior offers valuable complementary information for distinguishing between concretes of different strength classes and microstructural characteristics in drilling-based NDT [29,30,31].

4.3. Drilling Force Measurements

The drilling force results exhibit a clear parabolic dependence on the nominal drilling speed, with forces decreasing from 1400 to approximately 1600–1800 RPM and increasing again at higher speeds. This behavior reflects the combined influence of cutting mechanics, frictional resistance, and drill–aggregate interaction, which evolve nonlinearly with rotational speed [32]. Similar nonlinear force–speed relationships have been reported in drilling optimization studies [18]. At the lower nominal speeds (1400–1600 RPM), the drill bit operates in an inefficient cutting regime. Chip formation is slow, the bit interacts with the material in a more discontinuous manner, and frictional drag on the cutting edges is high. This aligns with findings indicating that weak or heterogeneous materials at low cutting speeds promote crushing-dominated removal, which increases force demand and torque spikes [27]. As the speed increases to the mid-range zone (1600–1800 RPM), cutting becomes smoother, chip evacuation improves, and the drill transitions into a more efficient cutting mechanism. This regime corresponds to the optimal PR/ω ratio described in DR theory, where reduced torque fluctuation leads to lower force demand [18].

At the higher speeds (2000–2200 RPM), the drilling force rises again, producing the upward branch of the parabola. This increase is driven by intensifying frictional and dynamic effects: higher rotational velocity increases the sliding friction between the bit and the material, raises the temperature at the cutting interface, and produces higher-frequency impacts with coarse aggregates [8].

Clear differences between the two concrete mixes are observed. C50 consistently exhibits higher drilling forces—both average and peak—at all nominal speeds. This reflects the higher compressive strength, denser microstructure, and stronger aggregate–paste bonding of C50, which collectively impose greater mechanical resistance on the drill bit. The positive correlation between drilling force/resistance and strength has been described in multiple studies, where higher-strength materials require significantly greater drilling force [16]. The force difference between C30 and C50 becomes more pronounced at higher speeds, where the harder aggregates in C50 amplify the impact forces and generate larger local spikes. By contrast, C30 produces lower overall forces but shows stronger fluctuations [26].

4.4. Power Consumption

The power consumption results demonstrate a clear and systematic increase in electrical demand with rising nominal rotational speed for both C30 and C50 concretes. This behavior is expected from the mechanics of rotary drilling, where the motor must supply additional torque to maintain a higher angular velocity under load. DR operational studies explain that increased DR directly increases the mechanical load on the motor, raising the power demand when the system attempts to maintain speed under torque stress [26]. At low speeds (1400–1600 RPM), the cutting process is relatively energy-efficient. As speed increases, both the frequency of bit–aggregate contacts and the shear rate in the cement paste grow, leading to higher frictional losses and a steady rise in power consumption. The intrinsic specific energy model shows that higher removal rates require proportionally greater energy input, consistent with the observed monotonic rise in electrical power with nominal RPM [18]. A notable distinction is observed between average and maximum power values. While average power increases nearly linearly with RPM in both concretes—indicating a stable mean resistance during drilling—the maximum power values follow a more pronounced, nonlinear, parabolic trend. This reflects short-term torque spikes caused by encounters with harder aggregate particles or localized increases in material density [16]. Such events require the motor to momentarily draw significantly higher power to maintain speed, particularly at elevated RPM where dynamic forces amplify.

Differences between the two concrete mixes are also evident. C30 concrete displays consistently higher maximum power consumption at high rotational speeds compared to C50, even though C50 is the stronger material. This counterintuitive behavior arises from the drilling stability: the weaker C30 matrix leads to more irregular material removal, localized crumbling, and sudden changes in cutting resistance, each of which triggers transient torque peaks. Similar behavior is reported in studies of weak or porous materials, where crushing-dominated removal produces higher instantaneous torque spikes and unstable energy demand [27]. By contrast, C50 concrete provides a more uniform and continuous cutting interface, allowing the drill to maintain speed with fewer fluctuations, despite its higher intrinsic strength.

The increasing gap between average and maximum power at higher speeds further highlights the effect of drilling dynamics. At low and mid-range speeds, both concretes show relatively narrow variations, indicating stable cutting. At 2000–2200 RPM, however, the magnitude of the power spikes grows sharply, especially in C30, suggesting a transition from predominantly cutting-dominated behavior to mixed cutting–impact behavior—an effect previously associated with high ω drilling in heterogeneous materials [8].

4.5. Combined Drilling Analysis

The comparative analysis of drilling performance parameters reveals clear and systematic trends reflecting the mechanical and structural differences between C30 and C50 concretes, as well as the influence of nominal drilling speed on DR and efficiency (

Table 2. Relationship between nominal drilling speed and drilling resistance indicators in C30 and C50 concrete.

Nominal RPM	C30/C50 Difference (%)			RPM Drop (%)		Force Ratio	Cutting Eff. (RPM/N)		Power Eff. (W/N)
	RPM	Force	Power	C30	C50		C30	C50	C30	C50
1400	2.5%	93%	4%	31.4%	29.7%	1.9	23.4	12.5	1.9	1.0
1600	2.9%	247%	–2%	19.9%	17.9%	3.5	42.7	12.7	3.2	0.9
1800	4.6%	274%	8%	14.6%	10.8%	3.7	66.8	18.7	4.5	1.3
2000	2.0%	354%	–5%	14.9%	13.2%	4.5	70.9	15.9	5.5	1.1
2200	2.4%	109%	–5%	15.1%	13.1%	2.1	20.3	10.0	1.8	0.8

). The relationships among RPM stability, drilling force, and power consumption confirm that DR is governed by a balance between cutting efficiency and material-induced torque fluctuations, which manifest differently in weaker versus stronger concretes. This aligns with prior DR theory establishing that cutting efficiency, torque stability, and power demand are interdependent indicators of material hardness and fracture behavior [8]. Across all nominal speeds, the difference between the nominal rotational speed and the actual in-depth RPM remains small (2–4.6%), indicating that the drill motor maintains speed effectively under load. However, the percentage RPM drop relative to the nominal speed is consistently higher for C30, particularly at lower speeds (31.4% at 1400 RPM vs. 29.7% for C50). This behavior is consistent with the documented effects of porosity, weaker cementation, and heterogeneous paste zones, all of which amplify load sensitivity and reduce rotational stability under drilling [27]. The force measurements show the strongest contrast between the two concretes. While C30 requires relatively low forces (23–92 N), C50 requires substantially higher forces (86–192 N) across all speeds. The resulting force ratio (C50/C30) ranges from 1.93 to 4.54, with the largest differences occurring in the mid-speed range (1600–2000 RPM). The drilling literature confirms that DR increases proportionally with strength, making force highly sensitive to strength differences [16]. The updated Δ force differences (up to +354%) further indicate that the drilling force is extremely sensitive to material strength, significantly more so than the power consumption or RPM behavior. Interestingly, the force ratio decreases at 2200 RPM, suggesting that at very high rotational speeds the bit begins to remove material in a more dynamic, brittle manner even in the stronger C50, reducing the relative discrepancy between concretes. The power consumption differences between C30 and C50 are comparatively smaller (–5% to +8%), indicating that the electrical load alone is not a strong discriminator between concrete strengths at identical nominal speeds. This behavior is consistent with DR theory, where drill motors compensate for increased cutting resistance by reducing the RPM [26].

The cutting efficiency parameters provide the most insightful link between drilling mechanics and material behavior. Cutting efficiency, expressed as RPM per Newton of drilling force (RPM/N), is consistently much higher in C30 concrete. For example, at 1800 RPM, C30 achieves 66.93 RPM/N compared to C50’s 18.69 RPM/N. This indicates that each unit of force produces significantly more rotational progress in C30 than in C50, confirming its lower resistance to penetration and easier material removal. This aligns with drilling reviews noting that weaker materials exhibit higher apparent efficiency [16]. At 2000 RPM, the efficiency peaks again for C30 (70.92 RPM/N), showing that mid-to-high speeds optimize the cutting performance in weaker materials. C50, however, maintains more stable but lower efficiencies, reflecting its stiffer and more homogeneous resistance to drilling. Similarly, power efficiency (W/N) is systematically higher in C30, indicating a lower energetic cost per unit of resistance. For instance, at 1800 RPM the power efficiency of C30 reaches 4.52 W/N, compared to 1.30 W/N for C50. The intrinsic specific energy model again supports this interpretation, as materials requiring lower energy per volume removed generate more favorable efficiency metrics [18].

The present study has several limitations that should be acknowledged. The experimental program was limited to two concrete strength classes and a single drill bit type, and all tests were conducted under controlled laboratory conditions. Consequently, the influence of additional factors such as different aggregate types, bit wear, alternative drill geometries, and field conditions was not addressed. While the selected setup ensured high repeatability and controlled comparison, further studies are required to assess the robustness of the proposed approach under practical on-site conditions.

4.6. Comparison with Previous Drilling Resistance Studies

A key difference between this study and much of the existing DR literature is the use of a smaller drill bit diameter (5 mm) compared to the 6–14 mm bits commonly reported (

Table 3. Comparison of drilling resistance test configurations and key response trends reported in previous studies and the present work.

Study	Concrete Strength (MPa)	Drill Bit Ø (mm)	Control Mode	Nominal RPM	Feed Rate	DR Response Metric	Reported DR Response	Reference
Karatosun et al.	4–39	n/d	Constant	n/d	n/d	Time-based (s/cm)	0.8–1.8 s/cm	[13]
Gunes et al.	4–40 (Dmax 5–22 mm)	8–12	Constant force	1100	n/d	Time-based (s/cm)	1.0–1.7 s/cm	[12]
Felicetti	50–60 (Dmax 16 mm)	6–14	Constant force	1040	n/d	Energy-based (J/cm)	1.0–1.5 J/cm	[10]
Lencis et al.	37–53 (Dmax 11.2 mm)	6	Variable	1800	20 mm/min	Speed-based (%)	20.2–31.6% RPM drop	[7]
This work	37–53 (Dmax 11.2 mm)	5	Variable	1400–2200	20 mm/min	Speed-based (%)	10.8–31.4% RPM drop

). The reduced diameter lowers the absolute cutting load but increases the sensitivity to local material properties, making the RPM, force, and power responses more responsive to changes in the concrete strength. Across previous studies, increases in compressive strength of approximately 10–20 MPa consistently result in higher drilling resistance, expressed either as longer penetration time, higher energy per depth, or greater RPM reduction. The present results show a comparable sensitivity: within the 37–53 MPa range, RPM reduction varies from about 10% to over 30%, confirming that rotational speed-based DR metrics are highly responsive to strength variations. The inclusion of power consumption further improves the diagnostic robustness, as power integrates both torque and speed and better represents the energetic cost of material removal. Compared with single-parameter DR indices, the combined analysis of RPM, force, and power provides a more stable and physically meaningful assessment of drilling resistance, enhancing the precision and applicability of DR for the non-destructive evaluation of concrete.

5. Conclusions

This study investigated the influence of drill rotational speed on drilling resistance (DR) measurements in concrete by analyzing tachometer feedback, drilling force, and electrical power consumption during controlled drilling tests in C30 and C50 concretes. The results demonstrate that each measured parameter captures a distinct aspect of the drill–material interaction and that their combined interpretation provides a robust basis for using drilling resistance as a non-destructive testing method for concrete. Based on the consistent experimental trends and repeatable response patterns, the following conclusions can be drawn:

• The rotational speed response was found to be strongly influenced by the nominal drilling speed and material characteristics. All tests exhibited an initial RPM reduction upon engagement with the concrete surface, followed by a stabilized drilling regime. Across all nominal speeds, C50 concrete maintained consistently higher in-depth rotational speeds than C30, indicating more stable torque transfer and more efficient material removal in the stronger, denser material. Conversely, C30 exhibited greater RPM reduction, reflecting the increased frictional losses, vibration, and energy dissipation associated with micro-crushing and material heterogeneity.
• Drilling force proved to be the most sensitive parameter for distinguishing between concrete strength classes. C50 systematically exhibited higher average and peak drilling forces than C30, reflecting its higher compressive strength, denser microstructure, and stronger aggregate–paste bonding. Power consumption complemented the force measurements by capturing the energetic efficiency: although the drilling forces in C30 were lower, the power demand remained comparable to C50 due to the stronger RPM reduction and higher torque instability. Both the average and maximum power displayed strong correlations with the nominal rotational speed (R² > 0.91).
• In contrast to many previous DR studies that focus on a single response parameter or rely primarily on empirical strength correlations, this work provides a systematic and integrated evaluation of rotational speed, drilling force, and power consumption across multiple drilling speeds and concrete strength classes. A key novel finding is that weaker concretes may exhibit greater rotational speed reduction despite lower drilling forces, due to inefficient material removal and increased energy dissipation. This observation clarifies the mechanical origin of drilling-induced RPM reduction and extends the current DR interpretations beyond strength-based explanations alone.
• Future research should expand the experimental scope to include a wider range of concrete classes, drill bit diameters and geometries, moisture conditions, and automated drilling systems, enabling statistical hypothesis testing and the development of predictive models linking DR parameters to concrete compressive strength with improved reliability.