Health monitoring of silica fume based concrete structures
RESEARCH ARTICLE
Present Address:
A
Krishna
Kumar
Maurya
1
Emailkkmaurya@nitsikkim.ac.in
Prasant
Kumar
1
Emailb200013@nitsikkim.ac.in
Fanindra
Kumar
1
Emailb200011@nitsikkim.ac.in
Prahlad
Kumar
1
Emailb200012@nitsikkim.ac.in
1
Civil Engineering
National Institute of Technology Sikkim
Ravangla
INDIA
Krishna Kumar Maurya
1 .
Prasant Kumar 1 . Fanindra Kumar1 . Prahlad Kumar1
Received: / Acceptance:
Abstract
A
Structural Health Monitoring (SHM) technique is employed for health assessment and characterization of materials of various infrastructural systems. The EMI technique used in SHM relies on the electromechanical coupling between a piezoelectric sensor and the structure. The strength of concrete based structures is the core concern throughout the globe. The application of supplementary cementitious material, viz., silica fume in the concrete matrix, can be a potential solution to enhance the strength of concrete structures through hydraulic and pozzolanic activity. Thus, the monitoring of strength developed in concrete systems due to the incorporation of silica fume using destructive and a non-destructive technique becomes significant. This research is primarily motivated by the health monitoring of silica fume-based concrete structures using destructive and non-destructive techniques. The concrete cube specimens were cast with the incorporation of silica fume with the replacement of cement by 0%, 5%, and 10%, respectively. Initially, the non-destructive tests viz., ultrasonic pulse velocity meter and rebound hammer, were conducted to the cubes after the 7, 14, and 28 days of curing. Further, compressive test (destructive) was performed with the cubes to determine their compressive strength after 7, 14 and 28 days respectively using compression testing machine. Furthermore, the assessed properties of silica fume-based concrete cubes have been considered for the numerical evaluation using a finite element-based ANSYS environment. For the numerical evaluations, the concrete cubes and the PZT sensors were modeled. The EMI technique is applied to extract the conductance and susceptance signatures for the modeled concrete cubes at 7, 14, and 28 days of properties consideration. The statistical indices viz. Root Mean Square Deviation (RMSD) and Mean Absolute Percentage Deviation (MAPD) were considered for the quantification of developed strength using the extracted conductance data on different days. The results showed that the quality of silica fume-based concrete is excellent and has higher surface hardness as compared to normal concrete. The compressive strength of silica fume was enhanced by 4.39% and 14.29% at 5% after 28 days of curing, respectively. Further, the extracted signatures showed shifting towards the right side, indicating more strength than normal concrete. The calculated values of RMSD and MAPD increased each day, indicating higher strength of silica fume-based concrete. The research is useful for introduction of new concept for monitoring of real-life structural systems.Keywords
Structural Health Monitoring. Electro-Mechanical Impedance Technique. PZT Sensor. Silica fume-based concrete cubes. Statistical indices
🖂 Krishna Kumar Maurya
kkmaurya@nitsikkim.ac.in
Prasant Kumar
b200013@nitsikkim.ac.in
Fanindra Kumar
b200011@nitsikkim.ac.in
Prahlad Kumar
b200012@nitsikkim.ac.in
1 National Institute of Technology Sikkim, Civil Engineering, Ravangla, INDIA
Introduction
Cement is a widely used construction material globally due to its versatility, sustainability, and cost effectiveness and its production has increased drastically since mid of nineteenth century. Cement production globally has grown rapidly over the past decades, producing more than four billion tonnes of cement every year (Andrew 2019). Cement manufacturing emits approximately 0.9 tons of CO2 per ton (Hasanbeigi et al. 2010). Which accounts for about 5–8% of global carbon emissions and ranks as second largest CO2 emitting source (Kajaste and Hurme 2016; Mikulcic et al. 2016).
A
Silica fume is a pozzolanic material, the production of silicon and ferro silicon alloys in electric arc furnaces can be a good partial substitute for cement in the concrete mix. Silica fume improves the cement concrete matrix through its filler and pozzolanic properties. The ultrafine particles of silica accelerate early hydration and it promotes rapid C-S-H gel formation, which leads to a denser microstructure. The presence of silica also shows higher early heat release due to faster hydration as compared to plain cement. In addition, silica fume consumes Ca(OH)2 and reduces calcium-silicate ratio of C-S-H significantly, forming additional polymerized and stronger C-S-H structure resulting in improved durability and strength (Muller et al. 2015).Several attempts in the past have been made to increase the strength of concrete using silica fume as a partial substitute of cement. Incorporation of silica fume in normal concrete mix results in improvement of 28 days compressive and tensile strength significantly by replacing cement with silica fume at optimum range of 5–10% by weight (Bhanja and Sengupta 2005). Natural pozzolan along with silica fume combination can be a method for the production high strength concrete up to the range of 69–85 MPa at 28 days with medium workability using 15% pozzolan and 15% silica fume in the concrete mix (Shannag 2000). Silica fume, 10% by weight in cement concrete greatly improves the strength and durability of recycled aggregate concrete at later ages, but it slightly reduces early age strength (Çakır and Sofyanlı 2015). It has also been seen that flexural strength can also be increased with silica fume, but it does not improve the splitting tensile strength of concrete. Secant modulus of concrete also increases with increase in silica fume content (Siddique 2011). Replacing cement with 10% silica fume in concrete mix, the compressive strength increases approximately 10–17% at varying water cement ratios (Duval and Kadri 1998). Higher replacement of cement with silica fume about 40% also exhibits more compressive strength compared to regular concrete. Water cement ratios and percentage of silica fume content effects the strength of concrete (Babu and Prakash 1995). The compressive strength of concrete increases significantly about 5–67% by inclusion of silica fume, but no significant changes in flexural strength is seen (Srivastava et al. 2012). Polypropylene fibre combined with silica fume shows remarkable increase in compressive strength of concrete due to fibre dispersion by silica fume in the concrete mix (Nili and Afroughsabet 2012). The combination of silica fume with polypropylene fibre showed decrease in expansion and increase in drying shrinkage, this effect pronounces more with increase in silica fume content (Toutanji 1999). With the combination of 15% silica fume and 5% ground pumice by weight in the concrete mix, high strength concrete can be produced (Saridemir 2013). High performance concrete can be produced by combining silica fume with moderate molecular weight and high carboxylic density polymer (Harichane et al. 2023). The addition of silica fume in production of high strength concrete does not affect the total shrinkage if added in small proportions however, increasing the amount of silica fume decreases the autogenous shrinkage (Mazloom et al. 2004). With two percent silica fume content geopolymer based concrete showed the highest compressive strength and densest microstructure (Das et al. 2020). Silica fume with 0.75% steel fibre in concrete improves impact resistance, increases ultimate load carrying capacity but reduces its workability (Mastali and Dalvand 2016). The use of silica fume in concrete mix also increases workability and improves fresh properties of self-compacting concrete (Sasanipour et al. 2019). The improvement of strength of concrete with added silica fume can be related to SiO2 content of silica fume (Boddy et al. 2003).
In completed structures where destructive testing is not possible, rebound rammer test and the ultrasonic pulse velocity test are examples of non-destructive tests, which evaluate concrete strength and assess the material without causing any damage. Among the two types of hammer tests- namely type N and Q, a strong correlation exists between the compressive strength estimated by the hammer test Q value and the velocity of an ultrasonic pulse (Hidayat et al. 2024). Additionally, using combined NDT methods drastically reduces error than using independent testing methods. The SonReb is one such method that combines two NDT techniques- rebound hammer and UPV. When compared against the reference values from destructive tests, SonReb shows higher accuracy (96–100%) and outperforms both these methods individually (Ivanchev 2022). Structural defects like carbonation, corrosion, debonding, delamination, moisture-related damage can be effectively diagnosed using NDT techniques integrated with SHM. Using viable repair practices, the durability and longevity of existing structures can be improved directly (Bhirud and Makwana 2024).
Nowadays, the use of embedded piezoelectric sensors and electro-mechanical impedance technique (EMI) has further advanced the continuous, real-time health monitoring of concrete. The stress–strain behaviour of concrete can be effectively monitored under compression with the help of piezoelectric lead zirconate titanate and piezoelectric cement sensors by applying electromechanical-impedance–based methodology (Pan and Guan 2022). Additional advances in the field of active sensing show a single, reliable, non-dimensional hydration parameter derived from the EMI response of embedded PZT sensors to accurately monitor the early-age hydration of concrete (Talakokula et al. 2018).
The baseline sensitivity problem in EMI-based SHM can be approached by using the baseline-free EMI resonance method which enables tracking of concrete stiffness independent of the sensor, vital for detecting deterioration (Sha and Zhu 2024). Further, the development of a baseline-free, movable-sensor resonance-based approach has progressed the EMI technique by scanning large areas and detecting small, near-surface delamination (Sha and Zhu 2025). Emerging applications across civil, mechanical, aerospace, and biomedical engineering, show that smart materials and EMI techniques not only enhance current structural health monitoring practices but also open new research directions in energy harvesting and intelligent infrastructure (Maurya et al., 2020). The strength of in-situ concrete elements can be determined effectively using EMI technique involving cheap PZT patches as a sensor material (Tawie and Lee 2010). The remaining life of in-situ manhole can be predicted with great accuracy using EMI technique (Singh et al. 2023). By using EMI technique with modelled PZT patches, it is observed that bacterial concrete beam has higher dynamic load carrying capacity, additionally the concrete heals the damage or crack without any human involvement (Maurya et al., 2023). The early age development of bond between concrete and steel rebar can be effectively monitored using EMI technique (Tawie and Lee 2010). The EMI signature is sensitive to strength development at early age and the signature gradually shifts towards right direction with increase in curing age indicating strength gain (Shin and Oh 2009). It has also been seen that the early age strength of recycled aggregate concrete can be monitored by EMI technique using embedded smart aggregates, and conductance resonant frequency (CRF) proved to be reliable indicator for the test (Li et al. 2021). Using EMI signals with clustering can be an effective way to detect early-stage damage in near surface mounted fibre reinforced polymer strengthened concrete (Perera et al. 2019). Piezoelectric cement sensor is more effective than PZT sensor for monitoring the strength of cementitious material (Pan and Huang 2020). Using two PZT sensors on a different side of a rebar, the initiation, progress and the direction of incoming corrosion on the rebar can be detected effectively (Ahmadi et al. 2021). The application of EMI technique is an effective method to monitor bacterial concrete, and it can be a significant method to monitor bacterial concrete structures (Maurya et al. 2024). The use of EMI technique using PZT patch as a sensor combined with global dynamic technique gives excellent monitoring results, as both techniques complements each other reasonably well (Shanker et al. 2011).
From the past studies it is evident that inclusion of silica fume in the concrete mix, enhances the strength, stiffness and homogeneity of concrete significantly. The increase in performance can be evaluated using destructive and non-destructive testing methods. The trend and effectiveness of EMI technique is becoming more prominent in determining the structural integrity of structures. This study aims to evaluate the strength of silica-fume based cement concrete under loaded condition, using EMI technique through the use of PZT patches attached to the concrete cubes. The compressive strength of cubes has been estimated for 7, 14 and 28 days using PZT patches, respectively and further by destructive method. Furthermore, finite element-based model has been prepared and analyzed using the assessed properties of silica fume based concrete cubes on ANSYS software. Thus, the investigation is significant for development of novel concept for real-life structural systems.
Materials and Methodology
Silica fume
Silica fume, a byproduct of the electrometallurgy industry during the production of silicon and ferrosilicon alloys, is highly valued for its pozzolanic properties due to its high amorphous silica content. This material is approximately 40% cheaper than Portland cement, offering a cost-effective alternative in various applications. Globally, around one million tons of silica fume are produced annually. Utilizing industrial wastes like silica fume for soil improvement not only reduces environmental pollution but also provides significant economic benefits. Silica fume typically contains 85–95% SiO2, along with small amounts of iron oxide, aluminium oxide, calcium oxide, sulphur oxide, and sodium oxide. Its carbon content does not exceed 2%, and the loss on ignition ranges from 1.5% to 3%, with a specific gravity between 2.20 and 2.30, respectively. Silica fume particles are extremely small, the mean primary size of particles ranges from 0.1 to 0.2 µm, where 95% of the particles are finer than 1 µm. That contains 85%–95% silica (SiO2). The particles are also spherical in shape. Often several individual particles can be fused together to form small agglomerates. The high surface area of silica fume particles significantly influences their reactivity, playing a crucial role in enhancing the physical and chemical properties of concrete. The addition of silica fume benefits concrete in two primary ways. First, the tiny particles physically reduce the void space within the cement matrix. Second, as an effective pozzolan, silica fume chemically reacts to form calcium silicate hydrate (C-S-H), thereby increasing the compressive strength of the concrete. Table 1 shows the typical physical properties of silica fume.
The chemical composition of silica fume varies based on the raw materials used in the furnace. Generally, the silica content in silica fume exceeds 85%. It contains relatively low amounts of other oxides such as alumina, ferric oxide, calcium oxide, and alkalis. Magnesium oxide is also present in low quantities, and the carbon content ranges from 0.5% to 1.5%, typically staying below 2%. The chemical composition of silica fume samples is given in Table 2.
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Table 1. Typical physical properties of silica fume (IS: 15388–2003)
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Table 2. Chemical composition of cement and silica fume.
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Cement
Portland Pozzolana Cement conforming to IS: 1489–2015 Part 1 has been used for the concrete mix. Different tests have been performed for the cement viz. fineness test, normal consistency, initial and final setting time and compressive strength test. Table 3 shows the different tests results obtained and permissible values according of Indian standard.
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Table 3. Properties of PPC cement
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Aggregates
Aggregates constitute about 60–75% of the total volume of concrete and has a great influence in concrete strength, durability and workability. Therefore, proper testing of aggregates becomes necessary. Various test like sieve analysis, specific gravity, water absorption has been performed as per IS: 2386–1963 (part I, III). The obtained tests results for fine aggregates and coarse aggregates are shown in Table 4 and Table 5 respectively.
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Table 4. Properties of fine aggregates
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Table 5. Properties of coarse aggregates
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Concrete mix design
In this study M30 grade of concrete has been designed, with silica fume content not exceeding 10%. The water-cement ratio is kept 0.4 for all samples. Three samples of cubes have been produced, (i) control cube (CC), concrete without silica fume, (ii) SF-5, concrete with 5% silica fume as replacement of cement and (iii) SF-10, concrete with 10% silica fume. The proportions of materials for single cube are shown in Table 6.
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Table 6 Proportion of materials for a single cube (in kg).
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Electro-mechanical impedance technique
The EMI technique is a high frequency non-destructive evaluation method used for real time monitoring of structural health. The EMI technique relies on electro-mechanical coupling between piezoelectric transducer and the host structure. Any change in structural parameters such as deformation, mass, stiffness, or cracks changes the mechanical impedance, which is reflected directly in the electrical impedance of the piezoelectric patch (Soh and Bhalla 2005). The EMI technique typically operates in the 30–400 kHz range. A piezoceramic lead zirconate titanate (PZT) patch is bonded with the structure using epoxy adhesive. The patch is then excited using LCR meter or impedance analyzer. The device records the electro-mechanical admittance consisting of both conductance and susceptance. These responses then act as characteristic signatures of structural condition (Liang et al. 1994; Soh and Bhalla 2005). The deviation of these signatures from the baseline signatures indicates structural deterioration. When a harmonic voltage is applied to the PZT patch, it undergoes expansion and contraction due to converse effect of piezoelectric materials, the surrounding structure responds dynamically to the vibration. These changes in signatures are reflected in the electrical output, which makes the EMI technique extremely sensitive to local anomalies (Park et al. 2003). Piezoelectric materials exhibit both direct and converse effect. The constitutive equations of direct and converse piezoelectric behaviour, describe the coupling between stress, strain, electric displacement and electric field.
1
2
Equations (1) represents the direct effect and Eq. (2) represents the converse effect of the PZT patch. These equations can be expressed in matrix or tensor form.
3
A
In Eq. (3), D represents electric displacement vector (C/m2), S is strain tensor, E is external electric field (V/m) and T is strain tensor (N/m2). For analytical modelling, the PZT patch is considered small compared to the host structure, with negligible stiffness and mass. Under alternating excitation, the patch behaves like a thin vibrating bar. Typically, the dimensions of PZT patches vary from 5 mm to 15 mm in length and width, and 0.1 mm to 3 mm in thickness, which are examined for structural analysis. The shape of the commonly used PZT patch is depicted in Fig. 1 while Fig. 2 illustrates the three orientations of the piezoelectric sheet identified as the 1, 2, and 3 axes. Figure 3 shows the complete setup of PZT patch embedded with host structure and Fig. 4 depicts an electro-mechanical model replacing the structure with two equivalent mechanical impedances, Z. The combined system's complex electro-mechanical admittance, Y can be calculated accordingly using Eq. 4 (Liang et al. 1994; Bhalla and Soh 2004).4
Where,
is the piezoelectric strain coefficient of the PZT material,
the complex Young’s modules under constant electric field,
the complex electric permittivity at constant stress,
the mechanical impedance of the PZT patch,
angular frequency and k the wave number.
Fig. 2
A piezoelectric material sheet with conventional 1, 2 and 3 axes.
Fig. 3
PZT patch bonded to structure under electric excitation.
Fig. 4
Interaction model of PZT patch and host structure.
Numerical evaluation
In the numerical analysis, a three-dimensional finite element model of a concrete cube integrated with a surface-bonded PZT patch was developed using ANSYS software to simulate the electro-mechanical behaviour. The concrete cube was modelled as a linear elastic or quasi-brittle material depending on the required accuracy, while the PZT patch was defined using coupled-field elements capable of representing both electrical and mechanical responses. Figure 5 shows the prepared ANSYS model of concrete cube embedded with PZT patch. Appropriate material properties given in Table 7 such as elastic modulus, possion’s ratio, density, piezoelectric constants, and dielectric coefficients were assigned to capture electro-mechanical coupling. The interaction between the PZT patch and the concrete substrate was defined using bonded contact elements to ensure full strain transfer during excitations. A harmonic analysis was performed by applying an alternating voltage to the PZT patch over a defined frequency range, enabling the computation of admittance signatures. These simulated signatures provide insight into the dynamic characteristics of the concrete-PZT system and allow comparison with experimental EMI response. The numerical model thus serves as a controlled environment to study stiffness changes, damage sensitivity, and the influence of boundary conditions on the EMI technique, supporting the interpretation of experimental results.
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Table 7. Modelling parameters
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  A Figure 5. Finite element model of cube attached to PZT patch on ANSYS. |
Statistical indices
Statistical indices are numerical measures used to analyze, summarize, and interpret data, providing a systematic approach to understanding variations and trends within datasets. In civil engineering, these indices are crucial for evaluating material properties, assessing construction quality, and optimizing design processes. They offer a quantitative basis for comparing experimental data with theoretical predictions, ensuring that engineering practices meet safety, reliability, and performance standards. Two commonly used statistical indices are Root Mean Square Deviation (RMSD) and Mean Absolute Percentage Deviation (MAPD).
The RMSD is a statistical measure used to quantify the average magnitude of differences between predicted and observed values. In the context of ANSYS APDL modelling analysis, RMSD was employed to compare conductance data corresponding to frequency obtained from simulations of control concrete cubes and silica fume-based concrete cubes. Specifically, when assessing thousand conductance data points across various frequencies, RMSD provides a single value that encapsulates the overall deviation between the two sets of conductance values. The conductance data, which indicates how well the concrete conducts electricity at different frequencies, is influenced by the concrete's composition, including the presence of additives like silica fume. The RMSD index is as shown in Eq. (5).
5
Where, G1i = Partially replaced concrete cube’s conductance at ith measurement point and Goi = control concrete cube’s conductance at ith measurement point.
After the simulation, thousand conductance value of partially replaced silica fume based concrete cube’s corresponding to frequency range of 30–400 kHz has been obtained which is termed as ‘G1i’. Similarly, thousand conductance value of control concrete cube’s corresponding to frequency range of 30–400 kHz has been obtained which is termed as ‘Goi’.
Further, the MAPD is a statistical measure used to quantify the average absolute percentage error between observed and predicted values, providing a normalized assessment of model accuracy relative to the magnitude of the observed data. In the context of ANSYS APDL modelling analysis, MAPD can be utilized to compare conductance data corresponding to frequency obtained from simulations of control concrete cubes and silica fume-based concrete cubes.
By examining thousand conductance data points across various frequencies, MAPD offers a comprehensive view of the relative differences between the two sets of conductance values. Conductance, which reflects the concrete's ability to conduct electricity at different frequencies, can vary with the inclusion of materials like silica fume. The MAPD index is as shown in Eq. (6).
MAPD (%) =
(6)
Where, G1i = Partially replaced concrete cube’s conductance at ith measurement point and Goi = control concrete cube’s conductance at ith measurement point.
After the simulation, thousand conductance values were obtained for concrete cubes with partial silica fume replacement, corresponding to a frequency range of 30–400 kHz. These values are referred to as ‘G1i’. Similarly, for control concrete cubes, thousand conductance values were obtained within the same frequency range, termed ‘Goi’.
Non-destructive tests
Non-destructive tests are performed to evaluate the in-situ quality and integrity of concrete structure without damaging the elements to be evaluated. These techniques help to estimate the mechanical properties, detect internal flaws, monitor deterioration, keeping the structural health in check during its lifetime. In this research ultrasonic pulse velocity test and rebound hammer test has been performed on the cube specimens to determine their internal and surface properties. For ultrasonic pulse velocity test, electro acoustical transducer conforming to IS: 13311 − 1992, has been used to generate the acoustical signal. The frequency of waves produced ranges from 20–150 kHz. This test has been performed in two ways, the direct and semi
  Figure 6. Direct UPV Test
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direct based on the position of receivers. In direct test the receivers are place on opposite faces of the cube and the receivers are placed on adjacent faces for semi direct test. Figure 6 shows the placement of UPV meter prove in opposite faces of concrete cube for direct test, similarly the proves are placed at adjacent faces of cube for semi direct test shown in Fig. 7.
   A Figure 7. Semi-direct UPV Test |
To ensure acoustical coupling epoxy resin is applied on the surface of concrete cubes. The quality of concrete depends upon the velocity of sound travelling through them. Higher the velocity better the quality of concrete and lower velocity suggests presence of defects or low-density concrete. IS: 13311 − 1992 has provided a classification of concrete based on velocity of sound passing through concrete, shown in Table 8. The tests were performed for all three types of cube sample viz. CC-0, SF-5 and SF-10 respectively. Testing was done to obtain results for 7, 14 and 28 days of curing of concrete cubes. Rebound hammer test as per IS 13311:1992 (Part 2) has been performed to assess the surface hardness and indirectly estimate the compressive strength of concrete cube specimens. This test operates on the principle of elastic rebound, where the spring-driven plunger impacts the concrete surface, and the rebound speed is recorded as an indication of surface hardness. Higher rebound number means harder and stronger concrete surface.
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Table 8. Ultra pulse sonic velocity vs concrete quality grading (IS: 13311–1992 part I)
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Destructive test
For destructive test, compression test of proposed specimen namely CC-0, SF-5 and SF-10 has been performed to determine their compressive strength using compression testing machine in accordance with IS: 516–2021. As per IS 516 the cube sizes of 150 mm*150 mm*150 mm have been cast and cured for period of 28 days. Figure 8 shows the complete setup for compression test performed in the laboratory.
  A Figure 8. Compression test on CTM. |
Results
Ultrasonic pulse velocity test
The UPV test results indicates consistent increase in quality of concrete with increase in silica fume content and curing age. Figure 9 and Fig. 10 illustrates the UPV test results for direct and semi-direct tests respectively, for concrete mixes incorporating different percentages of silica fume. A clear trend can be seen on both figures that the pulse velocity gradually increases with curing age, and mix containing silica fume performs better than the control mix. Among the samples, control mix shows lower pulse velocity and mix containing 10% silica fume (SF-10) shows the highest values across all ages. The improvement becomes more noticeable at 28 days of curing, suggesting that pozzolanic reaction of silica fume strengthens the internal structure of concrete over time by reducing the internal voids.
Overall, both direct and semi direct testing method reflect similar behavior which confirms that silica fume contributes significantly to more denser and uniform concrete matrix, leading to better internal quality and durability.
Fig. 9
Ultra-sonic pulse velocity variation for direct test.
Fig. 10
Ultra-sonic pulse velocity variation for semi-direct test.
Rebound hammer test
Rebound hammer test was conducted for all mixes at different curing ages. Figure 11 shows the results of rebound hammer test conducted on mixes containing 0%, 5% and 10% silica fume at 7, 14 and 28 days, respectively. The results indicate the progressive increase in surface hardness with higher level of cement replacement.
Fig. 11
Surface hardness in MPa.
Compressive strength
Figure 12. Compressive strength of concrete cubes at different ages.
The compressive strength of CC-0, SF-5 and SF-10, specimens have been determined for 7, 14 and 28 days, respectively. Figure 12 represents the compressive strength of proposed mixes at curing ages of 7, 14 and 28 days. As shown in Fig. 12, the compressive strength increases gradually with curing ages for all mixes, and sample containing silica fume showed more strength than the control mix. The maximum strength is obtained at 28 days, where silica fume-based mixes exceed control mix, indicating denser and more refined microstructure due to pozzolanic reaction.
Fig. 13
7 days conductance vs frequency with different cement replacements.
Numerical evaluation of concrete cubes
As shown in Fig. 13, the 7 days conductance signature exhibits multiple resonance peak distributed along the frequency test. The peaks for SF-5 and SF-10 mixes are comparatively higher than the control mix (CC-0), indicating increased stiffness at early age due to pozzolanic reaction due the silica fume. Similarly, Fig. 14 shows the 7 days susceptance response with upward trend acress frequency. The response of SF-10 remains most stable and elevated, indicating superior mechanical impedance at 7 days. The 14 days conductance signatures shown in Fig. 15 indicates a clear separation between mixes compared to 7 day results. The resonating peaks of SF-10 becomes more pronounced and significantly sharper, while CC-0 shows mild fluctuations. The increased peak amplitudes indicates noticeable stiffness gain from progressive pozzolanic reaction from silica fume. In Fig. 16, the susceptance curve rises more steeply for specimens containing silica fume. The response becomes more linear and stable, signifying more strength gain with curing age. The conductance results shown in Fig. 17 show well defined resonance peaks with larger amplitude for SD-5 and SF-10 compared to CC-0. The frequency amplitude peaks are much larger comared to 7 days and 14 days peaks indicating significant increase in stiffness. The SF-10 mix shows the highest peak, proving maximum material densification. As illustrated in Fig. 18, the susceptance response at 28 days show consistently increasing slope, which indicates maturity and stabilization of internal mocrostructure. The EMI results clearly demonstrate that silica fume significantly enhances stiffness and microstructural integrity, with 10% silica fume content showing the best performance.
Fig. 12
7 days conductance vs frequency with different cement
Fig. 14
7 days susceptance vs frequency with different cement replacements.
Fig. 15
14 days conductance vs frequency with different cement replacements
Fig. 16
14 days susceptance vs frequency with different cement replacements.
Fig. 17
28 days conductance vs frequency with different cement replacements.
Fig. 18
28 days susceptance vs frequency with different cement replacements.
Quantification of strength using statistical indices
As presented in Fig. 19 the RMSD values progressively increase from 0.13 at 7 days to 0.15 at 14 days, reaching a maximum of 0.20 at 28 days. After 28 days of curing, the RMSD value for 5% cement replacement increased by 20%. Similarly, the 10% cement replacement exhibited an even higher RMSD increase of 31% after the same curing period. These results suggest that incorporating silica fume at these replacement levels significantly affects concrete's consistency and quality over time.
Fig. 19
RMSD variation of conductance signature of control cube and 5% cement replaced cube.
The RMSD values revealed a significant increase in deviation for both replacement levels. Notably, the RMSD values for the 5% and 10% cement replacements with silica fume showed a drastic increase, indicating substantial changes in the concrete's properties. As presented in Fig. 20, the MAPD initially records a value of 0.91 at 7 days, reduces to 0.69 at 14 days, and then increases significantly to 1.08 at 28 days. The MAPD values over different curing periods demonstrates the evolving impact of SF on the concrete's strength development. Initially, at 7 days, the variability is higher, which can be attributed to the initial phase of the reaction not fully compensating for the reduced cement content. For 14 days, the reaction becomes more pronounced, reducing the deviation and indicating a more consistent strength gain.
Fig. 20
MAPD variation of conductance signature of control cube and 10% cement replacement cube.
At 28 days, although there is a slight increase in deviation compared to 14 days, the overall variability remains low, suggesting that SF continues to positively influence the concrete's performance. These values indicate the importance of the curing period when evaluating the performance of concrete with SF. The reduction in MAPD values from 7 to 14 days underscores the effectiveness of SF in improving the strength and consistency of concrete over time.
Initial period for 7 Days, the moderate MAPD value suggests that while the early age strength is developing, there is some variability likely due to the complex interactions between cement hydration and the SF’s reaction term for 14 days, the reduced MAPD value at 14 days indicates that the concrete properties are becoming more consistent as the reactions of SF enhance the microstructure of the concrete, leading to more uniform strength development. Long-term for 28 days, the increase in MAPD at 28 days may imply that the variability in the mix properties is influenced by prolonged curing and the diverse nature of activity over time.
Overall, it can be summarized that, RMSD values with respect to 5% and 10% cement replacement with silica fume is 20% and 31% respectively, after 28 days curing of concrete cubes. The MAPD values with respect to 5% and 10% cement replacement with SF is 84% and 108% for 28 days cured concrete cube.
The strength of silica fume-based concrete has been quantified in terms of RMSD and MAPD indices. It was observed that the value of RMSD increases with increase in the percentage silica fume that indicates strength development is higher in silica fume-based concrete as compared to the normal concrete. However, the MAPD results does not show any specific pattern. Thus, The RMSD index is better for the quantification of strength as compared to the MAPD.
Discussions
The experimental, non-destructive and numerical evaluations conducted in this study together confirms that the inclusion of silica fume in cement concrete significantly enhances the mechanical properties and stiffness of concrete. The increase in pulse velocity observed for SF-5 and SF-10 mixes at all curing ages indicates a denser concrete matrix, consistent with earlier findings on silica fume pozzolanic and filler effects on concrete (Bhanja and Sengupta 2002; Muller et al. 2015). The rebound hammer tests results also showed higher surface hardness for concrete containing silica fume which supports previous studies reporting increased surface density and reduced porosity. In this study compressive strength results showed highest value at 10% cement replacement, which aligns with the established literature which identifies 5–15% cement replacement as optimal range (Mazloom et al. 2004; Çakır and Sofyanlı 2015).
For the EMI based numerical analysis, the rightward shift in conductance and susceptance signatures for mixes containing silica fume, along with sharper resonance peaks, indicates higher stiffness which has been confirmed in earlier EMI studies linking frequency to strength gain (Shin and Oh 2009; Tawie and Lee 2010). The increase in RMSD values with silica fume content and curing age further quantified this improvement, consistent with previous observations that RMSD is a sensitive indicator of increased stiffness (Talakokula et al. 2018). The MAPD showed inconsistent values, showing that RMSD is more reliable for strength quantification. Overall, the different testing approaches concludes that silica fume helps produce a denser, stronger, and stiffer concrete and the EMI technique can effectively detect these improvements without damaging the specimen.
Conclusions
This study shows that partly replacement of cement by silica fume enhances the mechanical and microstructural performance of concrete, which are confirmed by destructive, non-destructive and numerical evaluations. The steady increase of pulse velocity with increase in silica fume content for all ages, indicates silica fume contributes to homogeneous and denser internal mix. The rebound hammer test results further highlighted the improvement of surface hardness by pozzolanic reaction and dense filler effect. The NDT results are supported by destructive test results where 10% cement replacement showed highest improvement level at 28 days of curing period, reaffirming that 5–15% replacement range exhibits maximum strength development. Further, the EMI based numerical analysis supported these trends. The rightward shift of conductance and susceptance signatures confirmed that inclusion of silica fume at optimum quality increases the stiffness of the concrete, showing maximum value at 10% replacement for this study. Statistical indices result also support these results, particularly RMSD index provided a clear quantitative measure of strength development difference between modified mix and control mix, with RMSD value increasing steadily with curing age. As for MAPD index, it showed inconsistent results proving it less reliable than RMSD index for this case.
Overall, the results suggest that replacing cement with silica fume at optimum quantity of 5–15% leads to a denser, stronger and stiffer concrete. The EMI technique supported by numerical evaluations, NDT and destructive test showed that stiffness development can be determined effectively without damaging the structure. Thus, the investigation is significant for the implementation to the real-life structural systems.
Nomenclature
CC control cube
SF silica fume
CTM compression testing machine,
EMI electro mechanical impedance,
IS Indian standard,
LCR inductance, capacitance and resistance,
NDE non-destructive evaluation,
PZT piezo-ceramic lead zirconate titanate,
RMSD root mean square deviation,
SHM structural health monitoring,
c damping constant,
D electric displacement,
F harmonic force,
m mass,
S strain,
u harmonic velocity,
w width of PZT,
x real part of effective impedance,
y imaginary part of effective impedance,
ν poisson’s ratio,
a subscript- active,
p subscript-passive,
eff subscript-effective,
1, 2 subscript, direction of deformations,
3 subscript, direction of electric field application,
Electronic Supplementary Material
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Acknowledgement
The author would like to gratefully thank the department of civil engineering and structural engineering lab of the National Institute of Technology Sikkim, Civil Engineering, Ravangla, India.
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Funding
There is no specific funding received by author(s) for this study.
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Conflicts of InterestThe authors declare that they have no conflicts of interest to report regarding the present study.
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Author contribution
Author contribution Krishna Kumar Maurya: Writing - original draft, editing and supervision. Prasant Kumar: writing-draft. Fanindra Kumar: Writing and corrections. Prahlad Kumar: editing.
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