Rebound Index Research Articles

Concrete Compressive Strength Prediction Using Combined Non-Destructive Methods: A Calibration Procedure Using Preexisting Conversion Models Based on Gaussian Process Regression

Non-destructive testing (NDT) techniques are crucial in making informed decisions about reconstructing or repairing building structures. The SonReb method, a combination of the rebound hammer (RH) and the ultrasonic pulse velocity (UPV) tests, is widely used for this purpose. To evaluate the compressive strength, CS, of the concrete under investigation, the ultrasonic pulse velocity Vp and the rebound index R must be mapped to the compressive strength CS using a suitable conversion model, the identification of which requires supplementing the NDT measurements with destructive-type measurements (DT) on a relatively large number of concrete cores. An approach notably indicated in all cases where the minimization of the number of cores is essential is to employ a pre-existing conversion model, i.e., a model derived from previous studies conducted in the literature, which must be appropriately calibrated. In this paper, we investigate the performance of Gaussian process regression (GPR) in calibrating the pre-existing SonReb conversion models, exploiting their ability to handle nonlinearity and uncertainties. The numerical results obtained using experimental data collected from the literature show that GPR calibration is very effective, outperforming, in most cases, the standard multiplicative and additive techniques used to calibrate the SonReb models.

Experimental evaluation of stress history effect on compressibility characteristics of lime-stabilized expansive soils

ABSTRACT Lime stabilization has long been identified as an effective method for reducing the swelling potential of expansive soils. Despite numerous studies evaluating the volumetric behaviour of lime-stabilized expansive soils, the influence of stress history variations on deformation characteristics of these soils has not been adequately explored. This study specifically evaluates the impact of stress history conditions on compressibility characteristics of lime-stabilized expansive soils, with special attention to the rebound index at different loading stages. In this regard, one-dimensional consolidation-swell tests were conducted on samples containing different lime percentages (3% to 12%) under various stress history conditions. To include different stress history conditions, overconsolidation ratio (OCR), seating pressure, and loading step were varied during the induction of stress history in the samples. Results showed that OCR and seating pressure significantly influenced both the rebound (increased up to 118%) and compression indices (decreased up to 53%), while the loading step had a negligible impact on the compressibility properties of stabilized soils. Additionally, when comparing the relative importance of investigated parameters, it was revealed that, OCR had the highest influence on the rebound index (up to 118.0% increase), while the compression index was most significantly affected by lime percentage (up to 61.4% decrease).

Mechanical properties of ignimbrites of Tenerife Island employed as building stone and their correlation with some physical properties

The aim of this research work was to determine and analyse the mechanical properties of a group of four varieties of ignimbrites, which are differentiated according to their colour: brown, violet, grey and green. These ignimbrites are mined in the southern part of Tenerife (Canary Islands) and are widely used as building stone. Mechanical characterisation was carried out by determining: ultrasound propagation velocity, uniaxial compressive strength, flexural strength under concentrated load, point load strength, Schmidt hammer rebound number and rupture energy. To evaluate the influence of the anisotropy planes and the degree of correlation between the different directions, all tests were done in more than one direction, except for the rupture energy. A simple regression analysis incorporating the physical properties available in the literature is included, with special emphasis on those with the highest correlation coefficient. Finally, a multiple regression analysis is performed to find the best fitting equations to predict the compressive strength. Of the four varieties studied, the green ignimbrite proved to be the one with the highest mechanical strength, with some properties three times higher than those obtained for the brown ignimbrite and almost twice as high as those obtained for the violet ignimbrite and the grey ignimbrite, the latter varieties having a similar behaviour to each other. In most cases, the behaviour of the ignimbrites studied is consistent with their lithology, but some properties of the green ignimbrite exceed the range reported in the literature, thus providing a new reference for this lithotype. Simple regression analysis revealed strong correlations between uniaxial compressive strength and water absorption (r = −0.89) and point load strength (r = 0.85). Flexural strength showed very strong correlation with ultrasonic velocity (r = 0.91) and point load strength (r = 0.95). Applying the multiple regression method, the best estimators of compressive strength found were a model with four independent variables; apparent density, open porosity, water absorption and Schmidt hammer rebound index (R2 = 0.91) and a model with five variables, adding point load resistance (R2 = 0.93).

Non-destructive strength estimation of vintage clay bricks based on rebound hardness in architectural heritage buildings

Assessment of masonry structures by rebound hammer is easy to be performed and does not require special preparations. The operating principle of the rebound hammer makes possible to collect direct information about the damping properties of the tested substrate near the surface that can be related to the compressive strength. In the technical literature multiple applications of the rebound hammers are reported for field testing of masonry, but there is no consensus about the reliability of the strength estimation of vintage clay bricks based on rebound hammer testing. Aims of this research were to collect detailed information about the inherent (local) variability of the rebound index based on a comprehensive statistical analysis and to compare the variability differences between different rebound hammers (L-type and N-type) for vintage clay bricks, similarly to that has already been demonstrated for concrete in the technical literature. Further aim of the research was to clarify if there is a direct relationship between the colour, the compressive strength and the surface hardness of vintage clay bricks. The research also targeted to refine the strength estimation proposals available in the literature and to clarify the potential influencing parameters for such relationships.

Rheological analysis of wet-mix shotcrete with various viscosity-modifiying admixtures

Shotcrete must be suitable for the process of transport (pumping) and application (spraying) on site. Therefore, it is necessary to obtain a suitable consistency and rheology for its placement. This paper evaluates the influence of various viscosity modifying admixtures (VMAs) on the rheology and thixotropy for application in wet-mix shotcrete. Six VMAs were used and classified into three groups, according to their composition: additives based on silica, phyllosilicates, and polymeric additives. These rheology modifiers were studied in depth in mortars, and the values of the yield stress (τo) and plastic viscosity (μ) of the material were obtained, as well as the thixotropy (hysteresis areas), which represents the energy required for the structural recovery of the fluid. To obtain these parameters, the fluid was tested under dynamic state with a laboratory rheometer, applying a ramp of shear rates. Additionally, the consistency in mortars was determined by obtaining the flow table diameter on the flow table test. This evaluation was carried out in mortars with different contents of superplasticizer (SP). All this information made it possible to evaluate the influence of SP in combination with each of the VMAs, obtaining a workability box, determining the hysteresis areas and verifying which combinations obtained a rheological behaviour superior to the control mix (without VMAs). The results described were correlated with the rebound index obtained in the shotcrete mixes performed on-site. The thixotropy in mortars and the values of the rebound rate on site led to the most accurate correlation that allowed the selection of the most effective VMAs for use in shotcrete. Lastly, both combined results (laboratory and on-site) permitted a process that facilitated analysis of the wet-mix shotcrete to be designed and optimised.

Non-destructive density-corrected estimation of the elastic modulus of slag-cement self-compacting concrete containing recycled aggregate

Non-destructive tests that cause no damage to concrete components can be used in rehabilitation works to determine concrete mechanical properties, such as the modulus of elasticity. In this paper, models are presented to predict the modulus of elasticity of Self-Compacting Concrete (SCC) with Recycled Aggregate (RA) and slag cement through the hammer rebound index and ultrasonic pulse velocity. Simple- and multiple-regression models were developed to estimate the modulus of elasticity according to the monotonic relation between variables shown by Spearman correlations. In these models, the modulus of elasticity was always inversely proportional to the square root of the non-destructive property under consideration, and the hardened density raised to the power of 2.5 as correction factor always increased estimation accuracy and robustness. The multiple-regression model with density correction yielded the most precise estimations of the elastic modulus with deviations below ±10% and ±20% in 82% and 94% of cases, respectively.

Hammer rebound index as an overall-mechanical-quality indicator of self-compacting concrete containing recycled concrete aggregate

The hammer rebound index has traditionally been solely and exclusively used to estimate the compressive strength of vibrated concrete. Its use has recently been extended to the prediction of the compressive strength of Self-Compacting Concrete (SCC) and concrete produced with Recycled Concrete Aggregate (RCA). The conventional use of the hammer-rebound test is further developed in this paper, by demonstrating how it can be used to estimate the overall mechanical behavior of SCC containing RCA. To do so, nine SCC mixes with different contents and fractions (coarse, fine, and powder) of RCA are analyzed. Following a simple-linear-regression validation and property standardization procedure, the hammer rebound index is then expressed as a linear combination of four mechanical properties, adjusted through a multiple regression. The hammer rebound index is therefore expressed as a weighting of both the mean value of compressive behavior (arithmetic mean of compressive strength and modulus of elasticity) amounting to a weight of 72.8%, and the mean value of bending-tensile behavior (arithmetic mean of splitting tensile strength and flexural strength) amounting to a weight of 27.2%. The hammer rebound index can therefore be construed as an overall-mechanical-quality performance indicator of the SCC containing RCA, which can also yield predictions of every mechanical property. In this way, the application of the hammer rebound index could likewise be of use in rehabilitation, pathology, and health-monitoring works where a full characterization of the mechanical performance of SCC with RCA is required, facilitating the use of SCC with RCA in real structures.

Influence of soil physical and chemical properties on mechanical characteristics under different cultivation durations with Mollisols

The degree of soil compaction is affected not only by the agricultural machinery but also by the soil physical, chemical, and mechanical properties. We hypothesized that the changes in the soil physical (clay content) and chemical properties (soil organic matter, SOM) due to long-term tillage operations would cause corresponding variations in the soil mechanics properties. The in-situ undisturbed soil samples were firstly adopted to detect soil properties as affected by long-term conventional tillage of Mollisol (black soil), and explore the relationship between soil physico-chemical properties and mechanical properties with 4 different cultivation duration croplands (0, 17, 30, and 40 yr). Then, artificial modified soil samples were further used to verify this relationship and confirm the predominate factor influencing compression-rebound behavior at 3 levels of SOM and clay contents, respectively, at 2 levels of gravimetric water contents. The compression index (Cc) and rebound index (Cr) were determined using a fast oedometer test. We found that with cultivation duration extending, the clay content and soil bulk density increased, while the SOM, Cc and Cr gradually decreased. The laboratory simulation tests also showed that the compressibility was higher when SOM or clay content was higher; while the soil resilient capacity was lower when the SOM was lower or clay content was higher. In conclusion, the decrease of SOM and the increase of clay content due to long-term cultivation could increase the risk of soil compaction and weaken its recovery capacity. SOM is predominant in influencing the soil mechanical properties relative to clay content. Maintaining and/or increasing SOM may improve the mechanical properties of arable Mollisols.

Consolidation Properties of Soil/Modified Bentonite Backfill in Salt Solution

The consolidation properties of soil-bentonite (SB) backfills containing 20% of the weight of sodium-hexametaphosphate-modified calcium bentonite (SHMP-20CaB) and prepared with 0 mM to 1000 mM calcium chloride (CaCl2) solution were evaluated. The results indicated that both consolidation stress and CaCl2 concentration had significant effects on the consolidation behaviors of the SHMP-20CaB backfill. In general, an increase in consolidation stress resulted in a decrease in the void ratio (e) and in the coefficient of volume change (mv), alongside an increase in the coefficient of consolidation (cv). The increased CaCl2 concentration yielded a dropped void ratio, and a nonlinear decreased compression index (Cc) and rebound index (Cs), respectively, from 0.18 to 0.13 and from 0.022 to 0.010, and a nonlinear increase cv. A threshold CaCl2 concentration of 100 mM was observed at the inflection points of the Cc, Cs, and cv. In contrast, the mv was insensitive to the CaCl2 solutions. The deterioration in e, Cc, and Cs and the increase in cv were the result of a compressed diffuse double layer of the bentonite by the CaCl2 solution, which thus exerted certain negative effects on the consolidation behaviors of the SHMP-20CaB backfill.

The Performance of Empirical Laws for Rebound Hammer Tests on Concrete Structures

The assessment of concrete compressive strength plays a key role in the analysis of the seismic vulnerability of existing buildings. However, the adoption of classical destructive tests is usually limited by their invasiveness, cost and time needed for the execution. Thus, in order to overcome these limits and allow investigations to be extended to a large number of points, the use of the rebound hammer test is investigated here with a detailed analysis of the effects on the accuracy of the strength assessment related to the choice of the conversion model relating rebound index to compressive strength. The analysis has been performed by comparing several empirical laws calibrated with data acquired in an experimental investigation of an existing concrete building. The relationships between the coefficients of the examined conversion models are then established, with the aim of reducing the unknowns in the calibration procedure. Furthermore, the influence of the coefficients of variation of concrete strength and rebound index on the results of the calibration procedure has been analyzed, thereby supporting the assessment of the accuracy of the concrete strength.

Prediction of Undisturbed Clay Rebound Index Based on Soil Microstructure Parameters and PSO-SVM Model

The rebound index Cr is an important design parameter in engineering construction, and its determination is cumbersome and susceptible to errors. Explaining the macroscopic rebound characteristic parameter Cr from the perspective of microscopic mechanism is an important research area that is addressed in this study. In this paper, the different soil parameters, including the Cr parameter and the physical parameters (void ratio er liquid limit water content WL, and plasticity index Ip), have been determined through experiments for the undisturbed clay of Chongming East Shoal (CES), Shanghai. Further, scanning electron microscopy (SEM) imaging was used to analyze the microstructural features. Through SEM, the grey correlation degree, the average abundance of structural units ACp and the average equivalent diameter of pores ADv were determined as the soil microstructure parameters with the most significant correlation with Cr. The predictive analysis model of Cr was then carried out combined with the PSO-SVM algorithm. In order to evaluate the influence of microscopic parameters of soil on the prediction accuracy, four sets of input parameter combinations were used. The results indicate the high prediction accuracy of the developed model. Sensitivity analysis was also carried out, which showed that the sensitivity of Cr to ACp and ADv was significantly lower than e; however, the difference from wL and Ip was small, indicating that it is imperative to consider microscopic parameters while predicting Cr. This study, thus, provides a basis and method for predicting the rebound index of soil from the microstructure perspective.

Modification of mechanical properties of Shanghai clayey soil with expanded polystyrene

Abstract In the present study, the resistance characteristics of Shanghai clayey soil-expanded polystyrene (EPS) mixture have been explored, using unconfined compressive and consolidation and rebound tests. EPS beads were mixed homogenously with clay soil in accordance with different mass ratios of beads to soil, that is, 0, 0.02, and 0.03%. Four particle sizes of EPS were used in the mixtures, that is, 0.5, 1, 3, and 7 mm, to reconstruct the samples. The experimental results indicated that (1) under the condition of unconfined compressive test, with the increase of EPS particle size, the compressive strength of reinforced soil increases significantly, but the compressive strength of reinforced soil decreases compared with the control sample, and it also decreases with high content compared to low EPS content; (2) under the consolidation and rebound test, compression index and rebound index of reinforced soil increased obviously compared with the control sample, but the compression modulus of reinforced soil decreases significantly; (3) in addition, the ductility of reinforced soil decreases with the increase of EPS particle size but increases compared with the control sample. At the same time, the stiffness of reinforced soil is much decreased compared with the control sample. Finally, the cause of deformation characteristics of reinforced soil was explained based on the feature of EPS material and the interaction between soil particles and particles of EPS beads.

Forecasting the Compressibility Parameters of Gypseous Soil using Artificial Neural Networks

To ensure safe design of structures against settlement, it is necessary to determine the compressibility parameters of the underneath soil especially compression and rebound indices. In this paper, an approach to forecast the compressibility parameters of gypseous soils based on index parameters was developed using Artificial Neural Networks technique. Two equations were developed to estimate compression and rebound indices using back propagation algorithm to train multi-layer perceptron, in which good agreements were achieved. The input parameters used were: the depth, gypsum content, liquid limit, plastic limit, plasticity index, passing sieve No.200, dry unit weight, water content and initial void ratio. Two output parameters were determined including compression index and rebound index. A parametric study was also conducted to investigate the generalization and robustness of both models. The findings indicate that both models were reliable within the range of utilized data. It was found that gypsum content has the highest effect on the compressibility index followed by water content, plasticity index, dry unit weight and plastic limit, while other parameters have lower effect. The gypsum content has the highest effect again on the rebound index followed by passing sieve No.200, initial void ratio, plastic limit and plasticity index, while other parameters have lower effect.

Comparative study on the successive impact behavior of composites in ship structures

Composite materials are categorized to be highly sensitive to low-velocity impact events. This feature is considered as a serious limitation for their application in engineering. Therefore, understanding impact energy absorption is critical in improving composite material damage tolerance and especially under successive impacts. This work was dedicated to an experimental investigation that aims to study and compare the energy absorption ability and damage behavior of PVC-foam sandwich and GFRP laminated composites under multiple impacts occurring at small energy levels. For this purpose, low-velocity impact repeated tests were carried out until total absorption of the impact initial energy was reached. A relative energy absorption index and a rebound index were proposed in order to assess energy absorption capacity. The results indicated that, directly after the first impact, the sandwich composite formed from two 4mm laminated skins absorbed 80% of the initial impact energy, in comparison to approximately 60% for 8mm laminated composite. This performance of sandwich composite is attributed to the damping ability of the core. Also, the impact velocity rebound rate of this composite was found to be higher than that of laminates. However, impact damage is greater in composite sandwiches than in laminates.

Geomechanical behavior evolution of the rock mass involved in the Arteara rock avalanche, Gran Canaria, Canary Islands, Spain

In Arteara (Canary Island), a Holocene rock avalanche comprises accumulation of large reddish blocks which cover the Fataga ravine. This course, is entrenched into the Phonolitic Formation, an alternating sequence of lava flows and ignimbrites. The avalanche defines an elongated deposit of variable thickness. A low friction angle was deduced, which is related to an easily weathered bedrock favorable to the rolling of the blocks. The movement would have been a dry granular flow with a component of saltation at the head and of turbulent flow at the intermediate and distal areas. The deposit varies widely in size and is structured in bands of blocks with a polymodal distribution and low selection. The geomechanical properties of the rocks involved vary substantially in each block and along a longitudinal profile of the deposit. Schmidt Hammer rebound measured in 233 blocks show a polymodal dispersion. Some facies have been differentiated in the blocks, not only by their appearance, but also by their rebound index (R). The different hardness reflects the differences in density and porosity. The hardness zoning shows the differential weathering of the blocks, which depends on the rock anisotropy and the flow turbulence, which determines the influence of abrasion and punching of the blocks. The rebound shows a direct correlation with the bulk density and an inverse correlation with the distance to the source area.

UCS field estimation of intact rock using the Schmidt hammer: A new empirical approach

In the present work we discuss the results of a number of Schmidt hammer tests (total number of impacts N > 2,400) that were performed in situ on rock outcrops of different lithology (marl, calcareous marl, limestone, sandstone, quartz sandstone and rhyolite) that occur in Italy. Firstly, a specific field procedure to choose the reference value of the rebound index adopted to calculate UCS of intact rock is suggested. A relationship between L and N hammer rebound index values (RL and RN, respectively) is subsequently assessed. Considering the experimental datasets provided by a Schmidt hammer construction company and other research available in literature, a new exponential equation for the correlation between RL and UCS of intact rock has been derived. Considering the here-proposed RL-RN relationship, a new exponential correlation between RN and UCS has also been defined. The newly proposed procedure and relationships were successfully utilised to determine the intact rock strength of different rocks. The calculated UCS values are very similar when using both types of Schmidt hammer (L and N) and are generally in line with previous determinations from experimental data available in literature.

Impacts of organic matter and loading methods on one-dimensional compression behavior of calcareous sand

To explore the effects of organic matter (OM) and loading methods on compression and particle breakage behaviors of calcareous sand (CS), a set of one-dimensional (1D) compression experiments were completed with two loading methods of constant rate of strain and incremental loading. Particle size distribution (PSD) analysis and three-dimensional (3D) modelling tests were performed to assess particle breakage degree and OM distribution form. Test results demonstrated that there existed an OM content of 25% in which the total vertical strain (TVS) in loading process declined first and then grew after that. While in unloading process, TVS decreased linearly from 16.1% to 4.2% approximately when OM content increased from 0 to 100%. In addition, there existed two loading stress turning points of 800 and 1600 kPa corresponding to vertical strain variation for CS-OM (A) and CS-OM (B) mixtures, respectively. The rebound index of mixtures increased from 0.002 to 0.107 when OM content increased to 100% which was determined collectively by the percentage of PCS and OM. OM can restrain particle breakage of CS to some degree and relative breakage ratio decreased from 0.24 to 0.03. Under the same condition, the relative breakage ratio (0.24) of CS-OM (B) mixtures with IL loading method were larger than that of CS-OM (A) mixtures or CRS loading method. Moreover, SEM and 3D models of CS-OM mixtures suggested that the OM are flocculent and distributed in surface or pores of CS particles.

ASSESSMENT OF THE COMPRESSIVE STRENGTH OF CONCRETE BY THE SONREB TESTS: CASE OF THE POLE MOSQUE OF ORAN

The non-destructive evaluation of concrete is a vital part and an active and interesting field of research which can be a fundamental step in the process of heritageization of contemporary architecture, especially for the new large mosques in Algeria. The present study consists of the assessment of the concrete structure of the pole mosque of Oran by applying a protocol of non-destructive testing of the compressive strength based on the combination of two tests of the SonReb approach; ultrasonic pulse velocity (UPV) and rebound index (RN). This method not only makes it possible to check the physical state of the structure, but also to collect the information necessary to predict the possibility of use in the future. The results of the experimental tests obtained indicate a very acceptable quality of the concrete of the pole mosque of Oran.

Some Geotechnical Properties of Clay Soil Enhanced with Silica Fume

Engineering structures found in, or on, some clay soil types may be subject to different damages due to the problematic nature and unfavorited properties of these soils. The unfavorited properties of these soils include shrink and swell changes, high settlement, and low bearing capacity. These soils have high sensitivity to moisture change. The way to reduce or overcome the unfavorited properties of problematic soils is the treatment or enhancement of these soils using different mechanical or chemical methods. In the present paper, silica fume selected as a soil stabilizer material, the effect of different contents of this material on the geotechnical properties of expansive clay soil has been experimentally investigated. Tests included soil specific gravity, soil plasticity, soil compaction, and soil compressibility. It was noted that silica fume, with different contents, decreased the specific gravity, and compaction density, while the compaction water content and Atterberg limits increased. The consolidation parameters of the expansive clay soil were affected by adding the silica fume. The silica fume reduced the consolidation parameters values of the clay soil. At low content silica fume, less than 10%, a very slight reduction can be seen especially for the rebound index value. The final findings of this paper appeared that the high settlement of clay soil can be reduced with the presence of silica fume.

One-dimensional compression feature and particle crushability behavior of dry calcareous sand considering fine-grained soil content and relative compaction

The particle breakage of porous and angular calcareous sand (CS) at the high-stress level is essential on compression feature but less quantified. In this research, a series of compression experiments and laser particle size analyzer tests were conducted under different ending pressures considering relative compaction and fine-grained soil (FS) content to investigate compression behavior and particle breakage of CS. The results indicate that the rebound curves are almost horizontal with a rebound index are between 0.0021 and 0.0054, and the FS content (smaller than 10%) has a little impact on resilience behavior. The irreversible plastic deformation is the main part of compression deformation. Besides, there exist two pressure thresholds of 400 kPa and 100 kPa corresponding to void ratio change trend when FS contents are smaller than 10% and reach 10%, respectively. Moreover, with FS contents increase from 0 to 10%, the moving distance of grading curves to the right becomes shorter gradually which suggests that the quantities of particle crushability decrease. FS can relieve stress concentration among CS that leading to less particle crushability. The combination of these factors including FS content, relative compaction, and axial loading has a complex effect on the relative crushability ratio. Both FS content and relative compaction can restrain particle crushability while axial loading could promote particle crushing. In addition, the relative crushability ratio has a negative linear relation with FS content and relative compaction. But, the relation between particle crushability ratios with axial loading can be described as a logarithmic function well. However, when relative compaction increases to a certain degree of 0.7, the influence of pressure on the relative crushability ratio is weakened. Similarly, the effect of relative compaction on the void ratio is weakened when the FS content is increased. Furthermore, there is a good negative linear correlation between the void ratio and relative crushability ratio.