Creep Coefficient Research Articles

Proposição de parâmetros para durabilidade de estruturas de concreto armado com vida útil de projeto entre 50 e 100 anos no Brasil

Abstract The main system of a construction is the structure. Its replacement is most of the times unfeasible and its repair or demolition generates waste that is often difficult to recycle, reuse or dispose of. In this way, structures with longer design service life (DSL) will generate lower environmental impacts, in addition to being financially more interesting for their users. Reinforced concrete is one of the most used types of structure, and commonly suffers with attacks of chloride ions and carbon dioxide that can facilitate the corrosion of the reinforcement. Concrete structures also suffer effects from the passage of time, as probability of accidental load increase, creep and shrinkage. The objective of this study was to determine durability and time effect parameters for DSLs between 50 and 100 years. The durability study was conducted through a review of reference studies, a selection of DSL models based on characteristic forms of environmental aggressiveness and comparison with international standards, using DSLs between 50 and 100 years and the following parameters: w/c ratio, compression strength, minimum cement usage and minimum cover. The time effect study considered Brazilian standards and their probability for accidental loads, creep, shrinkage and variations of the compressive strength, using DSLs between 50 and 100 years. The durability results were compiled in a table with practical recommended dimensional parameters. Despite some proposed parameters being higher or lower than standard values, the differences in performance were accounted through other parameters in order to maintain safety levels and to obtain minimum cover thicknesses. Variable vertical loads presented increments of 4.29% for 75 years and 7.22% for 100 years and wind velocity demonstrated a variation of 5.08% increase at 75 years and 9.84% at 100 years. Compression strength of concrete, creep coefficient and specific shrinkage deformation did not present significant variations. FBM: conceptualization, formal analysis, methodology, writing; BFT e FLB: data curation, formal analysis.

The Impact of Using Different Types of Soft Soils Treated by Stone Columns on Creep Behavior

The stone column technique is an effective method to increase the strength of soft cohesive soil, which results in a reduction in foundation settlement and an increase in bearing capacity. The topic of restraining creep settlement through the use of stone columns techniques has gained increasing attention and consideration; because stone columns are widely used to treat soft soil deposits, caution should be applied in estimating creep settlement. We discovered a reversible relation between shear parameters and the creep settlement in floating stone columns; while, in case of end-bearing stone columns shows a direct positive relation between shear parameters and the creep settlement, and the creep settlement began at the primary settlement. The shear parameters affected the improvement factor (n) of creep settlement in both floating and end-bearing stone columns. The standard creep coefficient’s n values in floating and end-bearing conditions were more significant than the low creep coefficient’s n values in forwarded geometric conditions. The stress in both floating and end-bearing stone columns was increasing and uniformly distributed along the length of the floating stone column and in the case of end-bearing stone column was limited to the stiffness layer; the maximum vertical stress was in the central point of the embankment. The embankment’s maximum horizontal displacement occurred on the edge.

Fracture behavior study of self-healing bacterial concrete

Presently, the construction sector expects quicker strength attainment in concrete so that the works can be finished on or before the schedule. This mandate is solved by high early strength cement and the usage of a low water/cement ratio. However, the above process results in a large amount of thermal and drying shrinkage, modulus of elasticity, and lower creep coefficients. Further, with a large amount of cement, concrete shows larger cracking behavior because of amplified thermal shrinkage and drying shrinkage. Therefore, high early strength concrete enhances the risk of cracking compared to moderate strength concrete. To decrease the effect of initial cracks and to repair the generated cracks in the concrete, several methods have been used in the industry. Self-healing bacterial concrete is one such method. Preliminary investigations have shown that using bacterial self-healing concrete helps us in increasing the mechanical properties of concrete along with healing the cracks up to a width of 0.3 mm. In this research, an effort is made to comprehend the effect of bacterial self-healing concrete on the fracture behavior of reinforced concrete beams. To understand this, several parameters such as beam deflection, load-carrying capacity, cracking pattern, energy absorption capacity, ductility factor, and ultimate moment carrying capacity, were checked with M25 grade concrete with fly ash and silica fume. Different parameters confirm that the load-carrying capacity of self-healing concrete increased up to 21% with a change in fracture behavior and deflection with a greater number of cracks and less width. Further ductility of beam increased up to 45%, and with a minimum of increased energy absorption of 100%. This shows that bacterial concrete is a sustainable alternative for the future construction industry as it extends the durability of the structure.

Effect of Dynamic Creep Coefficients and External Load on Hunting Velocity in a Railway Vehicle

Effect of Dynamic Creep Coefficients and External Load on Hunting Velocity in a Railway Vehicle

Experimental assessment and modelling of effective tensile elastic modulus in high performance concrete at early age

This paper focuses on the age-adjusted effective elastic modulus (Ea(t,t0)) in high performance concrete subjected to sustained tensile loading conditions at early age. First, the existing approaches to determining Ea(t,t0) are discussed, underlining their limitations. Second, a novel experimental approach is put forward to capture Ea(t,t0) using an advanced Temperature Stress Testing Machine and a unique direct tensile test setup. In this approach, both pure elastic modulus (E(t)) and Ea(t,t0) can be directly measured, whereas, in existing methods, the predictions are based on empirically determined values/models of ageing and creep coefficients. A unique set of test data obtained based on the proposed approach is presented to assess the early-age evolution of Ea(t,t0) and its key influencing factors. Such obtained experimental values are used to compute the evolutions of the reduction factors (k(t,t0)) simply by obtaining the ratio between experimentally determined values of E(t) and Ea(t,t0). The applicability of existing approaches for predicting k(t,t0) is evaluated, and the causes for discrepancies between experimental values and predictions are discussed. Finally, an empirical model is proposed in this paper to quantify the k(t,t0) profiles. It is shown that the proposed model is convenient, easily adaptable for different types of concrete without the need for an extensive test database, and yet realistically reflects the nonlinearity of k(t,t0) profiles at early ages.

Modeling of nonlinear viscoelastic creep of a polyethylene pipeline during water hammer

ABSTRACT In this research, the nonlinear behavior of a viscoelastic pipe during water hammer is investigated. To this end, the creep coefficients of the multiple element Kelvin–Voigt model have been determined for different flow rates and then they are incorporated to derive a holistic relation for the creep compliance. A linear interpolation scheme has been adopted to arrive at the holistic generalized Kelvin–Voigt model as a function of flow rates. The proposed relation is then applied in the established mathematical model and corresponding numerical solution to predict pressure fluctuations. The proposed approach in the application of the creep functions could capture the nonlinear material behavior during transients. Specifically, it revealed a higher creep behavior for higher loading rates (Joukowsky pressures) which was consistent with the experimental observations. Several numerical and experimental results were analyzed and investigated and to demonstrate the performance of the proposed approach.

Prestress loss model for ultra-high performance concrete

Accurate estimation of prestress losses is a critical aspect of the design of prestressed structural elements. Ultra-high performance concrete (UHPC) exhibits distinct creep and shrinkage behaviors, but a review of the literature revealed a lack of data and a limited set of predictive models for creep and shrinkage behaviors of UHPC-class materials. To address this gap, this study collected data, developed new equations, and proposed data-driven models to predict creep and shrinkage behaviors of UHPC-class materials. The predictive models adjust the ultimate creep and shrinkage of UHPCs based on compressive strength, age, and maturity of the material at loading. Results were compared to measured creep and shrinkage of a suite of commercially available UHPC-class materials. The main objectives of this research were as follow: (1) develop data-driven models to predict ultimate creep coefficients and shrinkage strains of UHPC-class materials for different service conditions; (2) examine the current AASHTO LRFD equations for creep and shrinkage of conventional concrete and determine the applicability of parameters in the equations for UHPC-class materials; (3) compare the predictive models with measured data, AASHTO LRFD equations, and existing European recommendations for UHPC-class materials; and (4) examine the applicability of the proposed loss model by comparing the predictions to the behaviors of seven full-scale pretensioned UHPC girders.

Effect of fiber content and stress–strength ratio on the creep of basalt fiber–reinforced alkali‐activated slag concrete

AbstractAlkali‐activated materials are increasingly being used in practical applications as potential alternatives to Portland cement. If alkali‐activated slag (AAS) is to be applied to prestressed engineering and large‐size structures, it is necessary to study the creep performance of AAS. Existing studies have found that the creep of AAS concrete is larger than that of ordinary Portland cement (OPC) concrete, and adding fiber is an effective way to overcome this defect. In this study, we investigated the creep of basalt fiber–reinforced AAS (FRAAS) concrete for fiber contents ranging from 0% to 0.9% (by volume) at a stress–strength ratio of 0–0.6. After analyzing the experimental results, it was found that the basalt FRAAS concrete had no obvious linear creep range. It was observed that with an increase in the fiber content, the inhibition effect of fiber on creep became stronger initially and then weaker. The inhibitory effect of fiber on creep was strengthened by the increase in the stress–strength ratio. The creep coefficient showed no obvious change when the fiber content was in the range of 0%–0.9%. Based on the existing OPC concrete creep models, a method for predicting the creep of basalt FRAAS concrete was proposed.

Effect of nanosilica on drying shrinkage and creep properties of cement concrete

The microstructure and time dependent properties of nanosilica (nS) added high performance concrete (nS-HPC) were investigated, and a comparison of these properties with those of microsilica (mS) added high performance (mS-HPC) concrete and a reference concrete (RefCon) are presented. 3% colloidal nS and 7.5% powder mS were used to make nS-HPC and mS-HPC, respectively. The scanning electron microscopic image of the 90 days’ nS-HPC revealed that the quantity of Ca(OH)2 present was almost negligible and the concrete attained a finer and compact microstructure with finer C-S-H as compared to that of other two concretes. The creep and drying shrinkage of the nS-HPC were found to be higher than those of RefCon and mS-HPC. However, the observed drying shrinkage of all the concrete mixes was found to be conforming with the estimates made from Indian Road Congress (IRC):112-2011 model, while on the other hand, the creep coefficients of mS-HPC and nS-HPC was found to be higher than the corresponding estimated creep coefficients, and the same were found to be higher by 13.3% and 18.2%, respectively, at 100 days. The increase in drying shrinkage and creep of both the high-performance concretes (HPC) than that of RefCon may be attributed to higher amounts of gel water present in the finer C-S-H produced due to pozzolanic action. The results from the study indicates suitability of nS-HPC for construction of bridge structures. Copyright © 2017 VBRI Press.

MODEL OF POROUS MATERIALS BY RHEOLOGICAL-DYNAMICAL ANALOGY USING THE PRINCIPLES OF MASS AND ENERGY CONSERVATION

<jats:p>It is assumed that the porous material is based on the principle of conservation of mass and the principle of conservation of energy. The validity of both principles relies on experimental observations. Experimental results of different metals were used to compare the Poisson ratio as a function of porosity. A comparison is made between the theory of percolation and the model proposed in this paper based on the rheological-dynamic analogy (RDA). The results show that there is an excellent agreement between percolation theory and the RDA model. Finally, a new relationship between the creep coefficient and porosity was proved for all analyzed metals by defining their damage variable in the range of measurable porosities.</jats:p>

Random Field Reliability Analysis for Time-Dependent Behaviour of Soft Soils Considering Spatial Variability of Elastic Visco-Plastic Parameters

• K-L expansion method was adopted to generate random field with different spatial correlation lengths • A critical spatial correlation length existed for predicting time-dependent settlement of soft soils • A single random variable analysis is not always the most conservative analysis in predicting settlement of soft soils • It is essential to perform random field analysis to reduce the risk of the design to be applied in construction. Low embankment strategy is one of the effective methods to control time-dependent settlement of soft soils in infrastructure construction projects. Spatial variability of soil characteristics is a crucial factor, affecting the reliability of predictions of the long-term settlement in soft soils. In this paper, the time-dependent behaviour of soft soils is analysed incorporating spatial variability of elastic visco-plastic model parameters. Standard Gaussian random fields for correlated elastic-plastic model parameter ( λ / V ) and the initial creep coefficient ( ψ 0 / V ) are generated adopting Karhunen-Loeve expansion method based on the spectral decomposition of correlation function into eigenvalues and eigenfunctions. Then the generated random fields are incorporated in the proposed non-linear elastic visco-plastic (EVP) creep model. The impacts of spatially variable elastic visco-plastic model parameters (i.e. ψ 0 / V and λ / V ) on long-term settlement predictions are evaluated through random field analysis (RF) with different spatial correlation lengths, and results are then compared to a single random variable (SRV) analysis. The probability of failure (PF) is calculated adopting RF and SRV analysis to determine the critical spatial correlation length, resulted in a maximum probability of failure. This study can be employed by design engineers to determine the critical spatial correlation length for safe design in the absence of adequate data to determine the exact spatial correlation length. The results also confirm that SRV analysis is not always the most conservative analysis in predicting time-dependent settlement of soft soils; and it is essential to perform RF analysis considering the spatial correlation length to reduce the risk and increase the reliability of the design to be applied in construction.

Fatigue damage in bending of reinforced concrete frames using non-destructive tests for dynamic strength of the cylinder

PurposeThe purpose of this paper is to contribute to the solution of the fatigue damage problem of reinforced concrete frames in bending.Design/methodology/approachThe problem of fatigue damage is formulated based on the rheological–dynamical analogy, including a scalar damage variable to address the reduction of stiffness in strain softening. The modal analysis is used by the finite element method for the determination of modal parameters and resonance stability of the selected frame cross-section. The objectivity of the presented method is verified by numerical examples, predicting the ductility in bending of the frame whose basic mechanical properties were obtained by non-destructive testing systems.FindingsThe modal analysis in the frame of the finite element method is suitable for the determination of modal parameters and resonance stability of the selected frame cross-section. It is recommended that the modulus of elasticity be determined by non-destructive methods, e.g. from the acoustic response.Originality/valueThe paper presents a novel method of solving the ductility in bending taking into account both the creep coefficient and the aging coefficient. The rheological-dynamical analogy (RDA) method uses the resonant method to find material properties. The characterization of the structural damping via the damping ratio is original and effective.

A Practical Design Method for Reducing Postconstruction Settlement of Highway Subgrade Induced by Soil Creep

The postconstruction settlement of the bridge approach is usually uneven, which could create a bump in the roadway. Indeed, this is a typical situation at the end of the bridge approach and requires a solution. One of the main causes of postconstruction settlement is the creep of soil. This paper is aimed at generalizing a new design method for controlling highway postconstruction settlement by replacing subgrade with expanded polystyrene (EPS). In the new method, the creep coefficient can be calculated based on the Yin-Graham EVP model. Thus, the relationship between the overloading ratio (OLR) and overconsolidation ratio (OCR) is obtained. The new method involves five steps: (a) determine the creep coefficient based on the relationship between the creep coefficient and over consolidation ratio, (b) divide the ground into a suitable number of sublayers, (c) select groups of different overloading ratios and then calculate the average values of the additional stress and overconsolidation ratio for each sublayer under different OLRs, (d) calculate the postconstruction settlement under different OLRs, and (e) determine the replacement capacity for different sections. This method can be used for quantitative design according to different requirements of postconstruction settlement of foundation. Taking Huzhou Avenue as an example, the case study illustrates the calculation process of the new method in detail.

Long-term performance of glulam-lightweight concrete composite beams with screw connections

This paper presents the results of long-term shear tests on glulam-concrete screw connections and long-term bending tests on glulam-concrete composite beams. The influence of using lightweight aggregate concrete (LWAC) instead of normal concrete and the environmental conditions on the long-term performance of both shear connections and composite beams are evaluated. The experimental results show that the one-year creep coefficient of the screw connection using LWAC is about 1.2 times that of the screw connection using normal concrete, and the one-year creep coefficient of the screw connection tested in the outdoor environment is about 2.5 times that of the screw connection tested in the indoor environment. The overall growth trend of the long-term deflection of the composite beams using LWAC is quite similar with that of the composite beam using normal concrete. The composite beam with lower degree of composite action has more significant increase in the long-term deflection. The deflection of the composite beam tested outdoors increases much faster than that of the composite beam tested indoors. The superposition method of long-term deformation for TCC beams is verified by the experimental results and proved to predict the long-term deformation of TCC beams with good accuracy.

Numerical Prediction of Creep Rupture Life of Ex-Service and As-Received Grade 91 Steel at 873 K

Infallible creep rupture life prediction of high temperature steel needs long hours of robust testing over a domain of stress and temperature. A substantial amount of effort has been made to develop alternative methods to reduce the time and cost of testing. This study presents a finite element analysis coupled with a ductility based damage model to predict creep rupture time under the influence of multiaxial stress state of ex-service and as-received Grade 91 steel at 873 K. Three notched bar samples with different acuity ratios of 2.28, 3.0 and 4.56 are modelled in commercial Finite Element (FE) software, ABAQUS v6.14 in order to induce different stress state levels at notch throat area and investigate its effect on rupture time. The strain-based ductility exhaustion damage approach is employed to quantify the damage state. The multiaxial ductility of the material that is required to determine the damage state is estimated using triaxiality-ductility Cock and Ashby relation. Further reduction of the ductility due to the different creep mechanisms over a short and long time is also accounted for in the prediction. To simulate the different material conditions: ex-service and as-received material, different creep coefficients (A) have been assigned in the numerical modelling. In the case of ex-service material, using mean best fit data of minimum creep strain rate gives a good life prediction, while for new material, the lower bound creep coefficient should be employed to yield a comparable result with experimental data. It is also notable that ex-service material deforms faster than as-received material at the same stress level. Moreover, higher acuity provokes damage to concentrate on the small area around the notch, which initiates higher rupture life expectancy. It also found out that, the stress triaxiality and the equivalent creep strain influence the location of damage initiation around the notch area.

Experimental and numerical study of long-term behaviour of timber-timber composite (TTC) connections

Long-term sustained load (push-out) tests under an indoor uncontrolled environment were conducted on twelve specimens (in seven groups/types) of Timber-timber composite (TTC) joints with up to two identical specimens of each TTC joint type. The TTC joints were fabricated by connecting either Laminated veneer lumber (LVL) or Glued laminated timber (GLT) joists to Cross laminated timber (CLT) slab panels. Types of joists (LVL and GLT), size of coach screw shear connectors, type of shear connection (screws with or without adhesive) between the joists and CLT slab thickness and orientation (i.e., lengthwise, or crosswise) with respect to the loading direction were the main variables in the experimental program. The laboratory tests revealed that the creep coefficient after 420 days is in the range of 1.05 to 3.90 and the CLT slab orientation has minor and CLT thickness has major influence on the creep coefficient of TTC joints (for a given service load over loading capacity ratio). A creep model was calibrated against the experimental slip-time curves and the model was used to predict the creep coefficient for the 50-year design life of the structure. It was shown that more than 50% of the creep in the TTC joints occur in the first year of loading. Additionally, a 3D coupled Finite element (FE) model implemented as user defined subroutines in ABAQUS/Standard was validated against the long-term test results and accordingly input parameters for coupled long-term FE analysis of the LVL-CLT and GLT-CLT joints (with screw shear connectors) are recommend. The validated FE model was used to further confirm effect of the CLT panel thickness and orientation on the long-term slip-time and creep coefficient of the TTC joints.

Improving Wheelset Stability of Railway Vehicles by Using an H∞/LPV Active Wheelset System

The complexity of railway vehicle structures has been part of an evolutionary process for almost two hundred years. Challenges such as increased weight, increased maintenance, higher costs and energy consumption have become common. The vision for future railway vehicles is to reduce complexity, hence enable simpler structures and reduce maintenance and cost, and of course various research challenges arise from this. In fact, a number of papers in the railway engineering literature have presented practical ways to control steering of railway vehicles to improve performance. The model of the railway wheelset is highly nonlinear, mainly due to the nature of the wheelset structure and the related wheel-rail contact forces involved during operation. In this paper, the simplest design in terms of retrofitting, the actuated solid-axle wheelset is considered, we investigate actively controlled wheelsets from a Linear Parameter Varying (LPV) control aspect. We use the grid-based LPV approach to synthesize the H∞ / LPV controller, which is self-scheduled by the forward velocity, as well as the longitudinal and lateral creep coefficients. The aim of the controller is to reduce the lateral displacement and yaw angle of the wheelset. Simulation results show that the proposed controller ensures the achievement of the above targets in the considered frequency domain up to 100 rad/s.

Experimental investigation on the rheological behavior of timber in longitudinal and transverse compression

Understanding the rheological behavior of timber is vital for evaluating the long-term performance of timber structures. In this study, short-term experiments were first conducted to obtain the short-term mechanical properties of timber including yield strength and elastic modulus in both longitudinal and transverse compression. Subsequently, creep tests were carried out to capture the rheological behavior of timber over 360 days. Different loading types of compression and stress levels were chosen in the creep tests. Environmental temperature and relative humidity, moisture content, environmental strain and creep strain were monitored. The coefficients of environmental deformation and irrecoverable mechano-sorptive creep were obtained in longitudinal and transverse directions. Various creep models of timber were calibrated against the experimental data. Weighted nonlinear regression was adopted to better simulate the secondary creep behavior. The creep coefficients over 50 years were estimated using the calibrated models, which were compared with values from existing national standards and research. The experimental results demonstrate that the limit of linear creep in longitudinal compression was bigger than that in transverse compression. Also, the transverse timber creep was much bigger than the longitudinal creep.

Experimental Evaluation of Shrinkage, Creep and Prestress Losses in Lightweight Aggregate Concrete with Sintered Fly Ash.

The paper presents the experimental results of shrinkage, creep, and prestress loss in concrete with lightweight aggregate obtained by sintering of fly ash. Two concrete mixtures with different proportions of components were tested. Concrete with a density of 1810 and 1820 kg/m3, and a 28-day strength of 56.9 and 58.4 MPa was obtained. Shrinkage and creep were tested on 150 × 250 × 1000 mm3 beams. Creep was tested under prestressing load for 539 days and concrete shrinkage for 900 days. The measurement results were compared with the calculations carried out according to the Eurocode 2 as well as with the results of other research. A very low creep coefficient and lower shrinkage in relation to the calculation results and the results of other research were found. It was also revealed that there is a clear correlation between shrinkage and creep, and the amount of water in the concrete. The value of the creep coefficient during the load holding period was 0.610 and 0.537, which is 56.0 and 49.3% of the value determined from the standard. The prestressing losses in the analyzed period amounted to an average of 13.0%. Based on the obtained test results, it was found that the tested lightweight aggregate concrete is well suited for prestressed concrete structures. Shrinkage was not greater than that calculated for normal weight concrete of a similar strength class, which will not result in increased loss of prestress. Low creep guarantees low deflection increments over time.

Compressive Creep Behavior of Cellulose Fiber Reinforced Concrete

The effect of cellulose fiber on the long-term compressive creep performance of concrete was clarified. For this, the long-term compressive creep properties of four cellulose fiber reinforced concrete (CFRC) groups with contents of 0, 0.9 kg/m3, 1.1 kg/m3, and 1.3 kg/m3 were investigated using a spring creep tester. Loading ages of 7 days, 14 days, and 28 days, and a loading stress ratio of 0.40 were employed. Furthermore, the creep model of CFRC was established. The results revealed that the creep of concrete at three loading ages decreases by 16%, 12%, and 17%, respectively, after adding cellulose fiber to the concrete. With increasing loading age, the difference of creep degree of CFRC with different cellulose fiber content decreased gradually. Furthermore, the range of specific creep decreased from 8 × 10−6 / MPa for a 7 day loading age to 2 × 10−6 / MPa for a 28 day loading age. The change regulation of CFRC creep coefficient was similar to that of the specific creep. The influence of the cellulose fiber on the creep performance of the concrete was reflected in two aspects. On the one hand, the internal curing effect of cellulose fiber can optimize the microstructure of concrete and reduce the creep deformation. On the other hand, the fiber increases the water migration channel and defects of interface transition zone, which will increase the creep deformation. The power exponential function model for CFRC creep provided a good description of the relationship between creep and loading age and holding time. This model can be used to predict the long-term creep of CFRC under different loading ages.