Concrete Age Research Articles

Prediction of Compressive Strength Behavior of Ground Bottom Ash Concrete by an Artificial Neural Network

The objective of this work was to examine the compressive strength behavior of ground bottom ash (GBA) concrete by using an artificial neural network. Four input parameters, specifically, the water‐to‐binder ratio (WB), percentage replacement of GBA (PR), median particle size of GBA (PS), and age of concrete (AC), were considered for this prediction. The results indicated that all four considered parameters affect the strength development of concrete, and GBA with a high fineness can act as a good pozzolanic material. The optimal ANN model had an architecture with two hidden layers, with six neurons in the first hidden layer and one neuron in the second hidden layer. The proposed ANN‐based explicit equation represented a highly accurate predictive model, for which the statistical values of R2 were higher than 0.996. Moreover, the compressive strength behavior determined using the optimal ANN model closely followed the trend lines and surface plots of the experimental results.

Strength Comparison of Accelerated Cured & Normal Cured Fly-Ash Based Concrete

The most important property of concrete is its compressive strength, which is carried out after 28-days of proper curing of concrete. This test is affected by other factors like the condition of curing, water to cement ratio, method of transportation, handling of the concrete, extent of vibrations and quality of the ingredients of mix proportion. This research study is an attempt to develop a simple mathematical model, by using linear regression analysis to estimate the 28-day fc’ (Compressive Strength) of concrete from the test results carried out at early age. This simple linear equation develops a relationship of 28.5 hours. These results show that most of the predicted values of compressive strength, calculated via equations, lie within permissible range difference for compressive strength achieved by experimental method, which is clear indication of credibility of the equations obtained for compressive strength at different age of concrete. The results show that compressive strength of concrete increases with the increase in content of FA (Fly Ash) upto 30% replacement, and the compressive strength of the concrete starts decreasing beyond 30% FA substitution. This argument is totally in line with all the literature carried out for this research.

Characteristics of granite dust concrete with M−sand as replacement of fine aggregate composites

Concrete is the most widely used man-made material in existence. It is second to water as the most-consumed resource on the planet. But, while cement as it is the key ingredient in concrete which has shaped much of our built environment, it also has a massive carbon footprint which leads to emission of greenhouse gases. Granite dust is eco-friendly to the nature and used as an innovative alternative to cement which can be a part of developing high strength concrete. Due to quick growth in the structural industry the necessity for the sand is increased broadly, causing deficiency of suitable river sand. The main aim of this paper is to strengthen the concrete with the fractional replacement of cement by granite dust and replacing the river sand with manufactured sand. The current study is carried out by fractional replacement of cement with granite dust in varying proportions of 0%, 5%, 10%, 15%, and 20% by weight of the cement and fine aggregate is replaced with 50% of manufactured sand for M30 grade of concrete. The specimens of cubes, prisms and cylinders are cast to test for each proportion. The hardened concrete tests such as compressive strength, split tensile strength, flexural strength are conducted. The samples are tested at 7 and 28 days age of concrete. It has been observed that the concrete strength enhances significantly after the use of granite dust.

Strength Comparison of Accelerated Cured & Normal Cured Fly-Ash Based Concrete

The most important property of concrete is its compressive strength, which is carried out after 28-days of proper curing of concrete. This test is affected by other factors like the condition of curing, water to cement ratio, method of transportation, handling of the concrete, extent of vibrations and quality of the ingredients of mix proportion. This research study is an attempt to develop a simple mathematical model, by using linear regression analysis to estimate the 28-day fc’ (Compressive Strength) of concrete from the test results carried out at early age. This simple linear equation develops a relationship of 28.5 hours. These results show that most of the predicted values of compressive strength, calculated via equations, lie within permissible range difference for compressive strength achieved by experimental method, which is clear indication of credibility of the equations obtained for compressive strength at different age of concrete. The results show that compressive strength of concrete increases with the increase in content of FA (Fly Ash) upto 30% replacement, and the compressive strength of the concrete starts decreasing beyond 30% FA substitution. This argument is totally in line with all the literature carried out for this research.

Preliminary study on early compressive strength of foam concrete using Ordinary Portland Cement (OPC) and Portland Composite Cement (PCC)

The purpose of this study is to determine the initial characteristics of foam concrete mixtures. This performance, using Ordinary Portland Cement (OPC) and Portland Composite Cement (PPC) were examined for its density and compressive strength on day 1, 3 and 7. In addition, the production process was initiated by formulating a mix of foam solvent, simultaneously creating the mortar, with the use of OPC or PPC, followed by the individual integration of a foaming solvent. Furthermore, after density evaluation, a specimen is produced with the cylinder cast, in a diameter of about 150 mm, and length of 300 mm, containing individual water and atmosphere curing. The density obtained was higher than the outcome of air curing tests, which decreased continuously with the concrete age. Furthermore, the compressive strength measured was relatively lower in water curing testing, using PCC, and on day 7it w was up to 3.46 MPa, or 73.5%, which seems to be lower than the record obtained with the use of OPC, which was about 3.94 MPa. At similar age, the value obtained for air curing with PCC reached 4.52 MPa or 96.0%, exhibiting a lesser value than OPC (4.71 MPa).

Early Age Concrete Strength Monitoring Using Power Spectral Density and Wavelet Packet Analysis

Concrete is a complex building material. Under normal curing conditions, concrete strength shows a nonlinear development process at an early age (1∼28 d). In the first few days after the completion of pouring, the strength of concrete increases slowly. Subsequently, the strength of concrete increases rapidly, reaching about 90% of its age strength. Finally, its strength gradually stabilizes. This paper introduces the experiment of 28‐day concrete age strength monitoring based on embedded piezoelectric smart aggregate (SA). Two piezoelectric SAs were embedded in a concrete‐filled GFRP (glass fiber reinforced polymer) tube column, one of which emitted a sinusoidal sweep signal and the other SA received the signal. With the hydration reaction of concrete, the stress wave would be significantly different when passing through concrete, and the received signal is changing constantly. Through power spectral density and wavelet packet energy analysis, the monitoring signal of concrete age within 28 days was analyzed. The experimental results show that the wavelet packet energy and power spectral density of the sensor monitoring signal show a nonlinear growth trend with time during the concrete target age. It can be divided into three stages, and the fifth day and the fourteenth day are the demarcation point of energy growth. And the trend of energy change corresponds well with the change of actual concrete strength and age. Comparing and analyzing the received signal energy of the sensor and the power spectral density function of the stress wave signal of the concrete specimen, the trend of the amplitude in the natural frequency domain is found to be the same in the three stages.

Prediction of differential drying shrinkage of airport concrete pavement slabs

ABSTRACT In this study, strain gages were installed at the centre, edge, and corner of concrete pavement slabs of Incheon International Airport based on the slab depth, whereas thermometers were positioned at the slab centre to measure the slab behaviour according to the temperature change. A three-dimensional finite element model simulating the airport concrete pavement was generated, and the analysis was conducted by using the measured temperature and material properties as functions of the concrete age and slab depth as its input variables. The difference between the measured strain and predicted thermal strain was calculated as the drying shrinkage because the finite element analysis did not consider the moisture evaporation but rather only the temperature change. The calculated drying shrinkage was large near the top of the slab from where the moisture evaporated, and the differential drying shrinkage between the top and bottom of the slab increased until the concrete age of nearly one month. Consequently, an upward curling developed owing to the differential drying shrinkage. The upward curling was the most obvious at the slab corner because the position was more exposed to ambient air and less constrained than at other positions.

Evaluation of Concrete Quality for Reinforced Concrete Frame Members Exposed to Natural Fires at Early Ages Using in situ Test Methods

Background: In recent years, numerous reinforced concrete buildings have been constructed in Iraq and some developing countries. Fire accidents occur in wooden formwork especially during the summer season due to problems associated with the construction procedure and on-site management. And fires can erupt during the construction stage of buildings. Introduction: Generally, if fire accidents occur in a wooden formwork, in these situations the concrete in the field is at an early age (i.e., “young”). The internal structure and chemical composition of early-age concrete are different from that of the carrier due to incomplete hydration at an early age. This study aimed to evaluate the strength of reinforced concrete frames at early ages when exposed to natural fire. The evaluation of existing buildings is an important issue that involves researchers and engineers in many countries. Methods: The experimental program consisted of constructing three full-scale reinforced concrete frames that are then exposed to natural fire. When the concrete age reaches three and five days by firing its formwork, the concrete quality of the structure was evaluated via ultrasonic pulse velocity as a nondestructive measurement. Core test was used as a destructive technique to implement a relationship between compressive strength and nondestructive measurements Results: The results showed that the frame exposed to natural fire early was generally more affected than the other frame, and its compressive strength was reduced close to 33%. The results also showed that the ultrasonic pulse velocity test for the structural elements was smaller than those of the core test Conclusion: It can be concluded that the current assessment methodology must be evaluated to provide practical suggestions that can enhance the reliability of assessing the in situ strength of existing concrete structures by nondestructive tests and cores.

Principle of the overlay deformations in the theory of creep

The aim of the research is to justify in the non-linear statement the overlay principle of fraction creep deformation, known in the linear creep theory as Bolzmann’s principle of superposition. Methods. In contrast to the traditional approach the material of constructive elements is considered as an union of its links with statistical disturbed strength. The model of structural strength allows the deduction of rheological equations. In loading process so called structural stresses of capable to resist links are considered. Results. The modification Bolzmann’s principle of superposition for fraction creep deformations is proposed. This permits its applicability also under non-linearly dependence of deformations on stresses. In according to concept of the statistical distribution of the strengths of links and linear dependence of determinations on structural stresses the rheological of mechanical statement is reduced. This equation implies the suitable on relation problems the linear integral equation. The relation of structural strength of material with its energy of entirety and with the experimentally known independency of specific to strength deformation on age of concrete is showed. The correct interpretations of certain known mechanical state equations for concrete are represented.

Assessing the freezing process of early age concrete by resistivity method

In order to prevent damage to concrete due to early age freezing, it is very important to monitor the microstructure formation process and strength development of early-age concrete in winter construction. A resistivity method is presented in this study to assess the freezing process of early age concrete at different negative ambient temperatures. The results show that the freezing temperature leads to a sharp rise in resistivity of concrete. The evolution of resistivity can be divided into three stages, which are cooling and nucleuses formation stage, rapid crystallization stage and crystallization finish stage. In addition, some characteristic values, such as blocking time t1, maximum ice content time t2 and freezing rate α, are extracted from the resistivity-time relationship curve and further clarify the freezing process of concrete. There is a good log-linear relationship between resistivity and compressive strength, which can be used to predict the extent of frost damage to concrete. Moreover, the monitoring results of outdoor concrete resistivity confirm that this method is suitable for the evaluation of concrete frost damage on site during winter construction.

Comparison of Concrete Creep-Induced Structural Prestress Loss Calculated by Codes from Different Countries

Creep is an inherent property of concrete materials, and an accurate prediction of creep-induced prestress is critical in designing a prestressed concrete structure. The comparison of codes or specifications of different countries on concrete structures was performed in this study. A basic model of prestress loss caused by creep was first deduced from a simple engineering model, and then the derived model was compared with the calculations obtained by different codes. Results show that the overall fundamentals and considerations of these codes are similar. Among these codes, the formulas specified by the Chinese Codes, European Code (DD ENV 1992-1-1), and AASHTO Code (AASHTO LRFDUS-5-M) were similar to the concrete aging formula, while the ACI Code (ACI 209R-92), Australian Code (ACI 209R-92), and Canadian Code (CSA-S6-06) depended more on empirical coefficients obtained via experiments. In the calculation example, the value of creep coefficient directly affected the calculating result of prestress loss. By comparing of the creep models adopted in Code for Design of Concrete Structures (GB50010-2010), European Code (DD ENV 1992-1-1), ACI Code (ACI 209R-92), and Railway Code (TB10002.3-2005). The effects of input parameters on calculation of creep coefficient were discussed in details. This study revealed the appropriate conditions using different codes.

Nonequilibrium processes of power and enviromental resistance of reinforced concrete structures to progressive collapse

The problems of calculating the parameters of non-equilibrium power and environmental resistance of reinforced concrete elements of a structural system under special effects are considered. A static-dynamic diagram of the deformation of corrosion-damaged concrete during overlimit impact caused by the sudden removal of a supporting structure is constructed. The section of this diagram is described by the analytical dependence for change in the strength of loaded and corrosion-damaged concrete from time, which takes into account, on the one hand, the process of the increasing strength of “healthy” concrete in time (theory of concrete aging), and on the other, the process of the degradation of an aggressive environment on concrete. A section of the dynamic deformation diagram is described by the Kelvin-Voigt model in a version of the theory of plasticity of concrete G.A. Genius, which corresponds to the conditions of deformation of concrete as an elastic-viscous-plastic material. The dynamic tensile strength of corrosion-damaged concrete is taken into account by the coefficient of increase in the dynamic strength of the material based on the two-element model of its deformation.

Effect of premature loading on punching resistance of reinforced concrete flat slabs

Premature loading of reinforced concrete flat slabs in multi storey buildings during construction may occur after shuttering removal and loading slabs earlier than usual to meet project time targets. Some case studies showed failure of flat slabs, which were prematurely loaded during the construction process before it reaches its full characteristic strength (at 28 days), which was used in structural design. This research aims to address this problem through experimental testing and design application according to current building codes. Eight specimens with dimensions of 1100 * 1100 mm and a total thickness of 120 mm were experimentally tested to study the effect of concrete age and actual compressive strength at loading on the punching shear capacity of reinforced concrete slabs. All specimens were supported by a square column with dimensions of 150 × 150 mm and loaded at the four corners with a span of 1050 mm. Accelerating admixture was used in three studied specimens to achieve higher concrete compressive strength at early ages compared to their companions of normal concrete without these admixtures. It was found that increasing concrete compressive strength of slab from 25 N/mm2 to 35 Nlmm2 (40 % increase) for normal concrete, without early admixture, improved punching shear capacity by 26 %, while increasing it to 45 N/mm2 (80 % increase) improved punching shear capacity by 49 % when the specimens were loaded 7 days after casting. In addition, using an accelerating admixture increased early concrete compressive strength, which improved punching shear capacity of reinforced concrete slab over that without accelerating admixture by 31 % and 29 % after 7 days and 14 days, respectively. According to inclusion of reinforcement ratio, BS8110 - 97 and Eurocode-2004 (EC2) design codes showed the most accurate prediction of punching shear capacity at 28 days, while ACI-2014 and ECP 203-2018 seemed conservative as their equations do not take steel ratio into consideration. At early ages, BS8110 - 97 and Eurocode-2004 (EC2) design equations did not provide accurate prediction of punching shear capacity while ACI and ECP provided reliable equations. It is recommended to use the actual compressive strength of concrete at early age (7 days for example) for calculating punching shear resistance of flat slabs prior to shuttering removal to prevent any premature loading.

Experimental study on bond properties between early-age concrete and deformed steel bars

In this study, the bond properties of deformed steel bars in early-age concrete under monotonic loading was experimentally studied by concentric pull-out tests of specimens with six concrete ages and three concrete strength grades. The mechanisms of bond failure between concrete and deformed steel bars under monotonic loading were analyzed. Based on the mechanisms of bond failure, it was possible to determine a five-segment bond stress-slip constitutive model with folded lines. Moreover, the effects of concrete age and strength grade on the characteristic parameters of the bond stress-slip curves were discussed. The results indicated that for specimens with the same concrete strength grade, the bond stress at each characteristic point and the bond stiffness of each characteristic stage of the bond stress-slip curve present a trend of escalation, whereas the corresponding slip shows a downward trend with increasing concrete age. Moreover, with an increasing concrete strength grade, the bond stress at a characteristic point and the bond stiffness of the characteristic stage increased, whereas the slip corresponding to the bond stress decreased for specimens with the same concrete age. According to the test results, an empirical bond strength model and a five-segment bond stress–slip model of deformed steel bars in early-age concrete were proposed, in consideration of the concrete splitting tensile strength. Thereafter, the accuracy of the proposed models was verified by comparing the proposed models with the test results in this study and those of existing models.

A load induced thermal strain (LITS) semi-empirical model for plain and steel fiber reinforced concrete subjected to uniaxial compressive load

A Load Induced Thermal Strain (LITS) semi-empirical model for plain and steel fiber reinforced concrete is proposed, assuming concrete as a heterogeneous 2-phase (matrix + aggregates) material. The effects of dehydration creep and the geomechanical properties decay of the coarse aggregates are taken into account separately in the model by independent parameters calibrated on the basis of the main results on plain concrete available in the scientific literature. The role of the coarse aggregates and the cement content are investigated. It was noticed that the amount of coarse aggregates is directly proportional to LITS, with a logarithmic variation. A comparison between the results of the model with experimental tests in literature, considering different types of concretes (conventional, high strength and ultra-high performance), demonstrated the reliability of the proposed model. Moreover, the model was compared with experimental tests in uniaxial compression on Steel Fiber Reinforced Concrete (SFRC) cylinders with 11-years-old, showing a reasonable approximation. The effect of concrete aging to LITS is also investigated. It is demonstrated that concrete aging is responsible for the reduction of LITS. A reduction factor β(t0) was defined on the basis of a set of experimental values ranging between 28 days and 11 years.

High‐performance modeling of concrete ageing

AbstractLong‐term behaviour of concrete structural elements is very important for evaluation of its health and serviceability range. The phenomena that must be considered are complex and lead to coupled multiphysics formulations. Such formulations are difficult not only from physical perspective, but also from computational perspective.In this contribution attention to computational efficiency and effective implementation is payed. Presented model for concrete ageing is based on microprestress‐solidification (MPS) theory of Bazant [1], Kunzel's model for heat and moisture transport [2] and Mazars model for damage [3]. Ageing linear viscoelastic response, which is immanent to MPS theory and concrete creep, leads to ordinary differetial equation for internal variables solved for every quadrature/nodal point. Numerical structure of the finite element discretisation is examined. Few simplifications on physical model lead to a very efficient linear algebra problem for which standard preconditioned Krylov solvers are reviewed. In parallel, weak and strong scaling tests are performed. All results are produced within open‐source finite element framework FEniCS [4].These models are usually a basis for more involved thermo‐hygro‐chemo‐mechanical (THCM) models with migrating chemical species. It is anticipated, that presented results will help practitioners or other structural engineerers with the choice of suitable and efficient methods for long‐term concrete modeling.

THE OWN STRESSES INFLUENCE ON SCALE EFFECT IN CONCRETE

The experimental technique and the experiment results on the influence of own stresses unevenly distributed over crosssection on materials strength and on scale effect are presented. The reasons for the occurrence and distribution of suchinfluence are analyzed. Based on the experiments results on fragile duralumin samples, the inevitability of influencing onscale effect in concrete during compression stresses, which are caused by unevenly shrinkage over cross section was proved.Possible reasons for the different (sometimes opposing) results of experimental studies of various authors on scale effect inconcrete in compression are explained. The influence of own stresses unevenly distributed over cross section on scale effectin concrete during compression, depending on samples size and concrete age, is analyzed in detail

A new test method to characterize the pressure-dependent shear behavior of fresh concrete

The development of formwork pressure is essentially affected by the rheological behavior of fresh concrete, while the shear behavior itself might be influenced by the magnitude of normal pressure. A test set-up was developed in which the fresh mortar and concrete can be sheared in a pressure cell by a vane as a part of a concrete rheometer (ICAR). The concrete is directly loaded in the pressure cell by a hydraulic jack. Preliminary shear tests were conducted on concretes with moderate and high flowability at constant water/binder-ratio of 0.4 with the focus on the static yield stress. The additional pressure was applied up to 240 kPa. The consistency class of the concrete F4, F5 and F6 was varied by choosing the paste volume to 300, 315 and 330 l/m3, respectively. The results show the significant influence of applied pressure on the measured yield torque or yield stress in early concrete age especially for concretes with moderate flowability (F4) or with relatively high volume fraction of aggregates respectively. However, a significant increase of yield resistance by applied pressure was identified for all concretes after 90 min concrete age.

Assessment of GFRP bond behaviour for the design of sustainable reinforced seawater concrete structures

Freshwater is an increasingly scarce resource. Its use in the production of concrete is one important contributor to its fast depletion. Alternatively, the use of seawater for reinforced concrete production, combined with the use of glass fibre reinforced polymers (GFRP) as reinforcement, may represent an interesting solution to increase concrete sustainability. The objective of this paper is to explore this promising solution through the development of concrete compositions with seawater. Additionally, it is also an objective to assess the bond behaviour between GFRP rods and concrete, in a design perspective. The influence of concrete age, rod diameter and anchorage length were also investigated. An analytical model capable of determining the local bond stress-slip laws was used. Results demonstrated that the use of seawater had no relevant effects on neither concrete mechanical properties nor bond behaviour.

Experimental study on interior relative humidity development in early-age concrete mixed with shrinkage-reducing and expansive admixtures

Early age concrete may crack due to volumetric shrinkage caused by a decrease in the interior relative humidity (RH) in the self-desiccation process. Shrinkage-reducing admixtures (SRAs) and expansive admixtures (EAs) are utilized to mitigate the volumetric shrinkage. Using a self-developed wireless and automatic relative-humidity monitoring system, a series of experiments for the measurement of the RH within concrete specimens was conducted. A systematic analysis was conducted in the aspects of the setting time, critical time and interior RH development in early-age concrete with different dosages of the polymer-type SRA and CaO-type EA. On the basis, a critical time model and a two-stage RH development model considering the effects of the water to binder ratio, SRA and EA dosages were established. Both of the models are in good agreement with the experimental data.