C30 Concrete Research Articles

Multiple Thermal Parameter Inversion for Concrete Dams Using an Integrated Surrogate Model

An efficient and accurate method for concrete thermal parameter inversion is essential to guarantee the reliable and prompt thermal analysis results of dams. Traditional inversion methods either suffer from low analysis efficiency or are limited in accuracy. Thus, this paper presents a method for multiple thermal parameter inversion based on an integrated surrogate model (ISM) and the Jaya algorithm. This method replaces finite element analysis with an ISM incorporating three machine learning algorithms, Kriging, support vector regression (SVR), and radial basis function (RBF), to describe the mapping relationship between thermal parameters and structure temperature responses. The input datasets for model training and testing are generated by a uniform design approach. Subsequently, a simple and efficient global optimization algorithm, Jaya, is used to identify the thermal parameters by minimizing the error between calculated and monitored temperatures. The effectiveness and practicality of this method are verified by applying monitored data of two strength grades of concrete in a dam. The verification results indicate that the proposed approach can obtain more accurate inversion results than the above individual models. Compared with these models, the inversion errors using ISM are reduced by 8.45%, 3.93% and 20.85%, respectively for C35 concrete, and by 6.53%, 23.82% and 44.43%, respectively for C40 concrete. Additionally, this approach maintains the powerful computational efficiency of surrogate-based optimization, and compared to the methods that directly invert using swarm intelligence algorithms, the analysis efficiency is improved by about 111.7 times.

3D finite-element analysis of innovative coconut palm stem shaped headed shear connectors

Headed studs are shear connectors in composite structures used at the adjoining face of a steel beam and a concrete slab. In this research, the conventional shape of a headed stud was restructured to resemble the shape of a coconut palm stem (royal palm) without a change in the overall material volume, with the aim of improving the shear strength of the composite connection. Six innovative shear connectors for composite structures, named coconut palm stem royal shaped headed stud shear connectors (CPSR studs), were examined. Abaqus/Explicit was used to model a push-out specimen and the finite-element model was successfully validated using published experimental results. The six different CPSR studs encased in three grades of concrete (C40, C50 and C60) were tested for shear strength, stiffness and load–slip performance. The results for the innovative CPSR studs were compared with those for uniform cross-section headed studs; when embedded in C40, C50 and C60 grade concrete, the results showed, respectively, 35–41%, 37–44% and 41–52% improvements in the ultimate strength of the shear connection. The improved shear strength capacity of the CPSR studs without a change in the overall volume of stud material means that fewer studs are required, leading to economic benefits.

Analysis of the compressive creep strain of the concretes with steel fibers: A holistic view in micro and macro scale

The aim of this paper is to evaluate the behavior of steel fiber reinforced concrete (SFRC), compared to concrete without fiber at the strength classes of 30 MPa and 50 MPa and subjected to compression creep. The volumetric contents of steel fibers were 0.38, 0.50, 0.64 and 0.77%. Test were carried out on concrete specimens at the age of 28 days, subjected to compressive strength from 30 to 40% of the estimated ultimate compressive strength, in controlled conditions of temperature and relative humidity. The duration of loads was 84 days. Thus, elastic deformation (ED) and specific creep (SC) values were measured. In addition, the theoretical SC was calculated based on Brazilian standards and compared to the experimental SC. As a result, no specimen failed under constant loads during the study time. The ED was higher for C30 concretes (about 35 × 10−6/MPa) than for C50 concretes (nearly 23 × 10−6/MPa), independent of the volume of fibers. Besides that, C30 concretes showed values of specific creep ranging of from 3.4 to 5.6 × 10−6/MPa and from 2.4 to 3.9 × 10−6/MPa for C50. This performance was justified by the microstructural differences in the cement matrices and their hydrated products, cracks, and porosity, among others. Moreover, there was a difference between the experimental and theoretical specific creep values. For the C30 series, these differences were 50% and for C50 were 20%. The volumetric fraction of steel fibers of 0.38% and 0.50% produced minors creep strain for C50 and C30, respectively. This reduction corresponded to 24.4% and 41.7% in comparison to reference concrete. On a macroscopic and mesoscopic scale, this optimum fiber content can be important for controlling of crack open, crack widths, crack propagation, and durability of concrete harsh environments. Finally, the positive impact on concrete durability properties was discussed in terms of scientific arguments.

Flexural behaviour of deconstructable composite beams

In order to promote the development of full life-cycle deconstructable steel–concrete composite beams, and to better understanding their flexural behaviour, an experimental study on the flexural performance of a novel deconstructable steel–concrete composite beam is carried out in this paper. With various connector (bolt) diameters (M20, M24) and material strengths (C40, C50 concrete) being incorporated, the failure mode, load–deflection response, load-slip response, strain distribution, ductility, characterised loading capacities and deconstructability are analysed thoroughly base on two-point loading tests. By comparing the test results with that specified in national standards for the flexural capacity and stiffness of the composite beams, new calculation formulae for this new deconstructable composite beams are put forward. By employing the shear-slip constitutive model proposed previously for shear connectors of such deconstructable composite beams, a finite element model is developed and further validate against experimental results. Subsequent parametric analyses are carried out, and the mechanism of shear connection is analysed accordingly. In addition, effects of different parameters (including sectional dimension, strength of concrete and degree of shear connection) on their flexural capacity, stiffness and interface slip are clarified.

Experimental study of the thermodynamic properties of high thermal conductivity energy pile

In this paper, high thermal conductivity concrete (containing 4% graphite and 0.6% steel fiber) was prepared as a pile material by a model test. A high thermal conductivity energy pile was produced. The temperature field distribution of the pile and soil, the additional stress of the pile, the soil pressure at the pile end, and the displacement of the pile top were analyzed. The results showed that compared with the thermal conductivity, compressive strength, and flexural strength of ordinary C30 concrete, those of the high thermal conductivity concrete were increased by 30%, 11%, and 9%, respectively. Under load, the additional stress of the pile was large in the middle and small at both ends. After three thermal cycles, the maximum additional tensile and compressive stresses of the pile were 0.48 MPa and −0.58 MPa, respectively. The maximum value of the soil pressure at the pile end decreased with an increase in the number of thermal cycles. Thermal cycles caused cumulative pile top settlement, with a final settlement of 0.26 mm. In practical engineering, the influence of long-term thermal cycles on the safety of buildings should be noted.

Experimental Study on the Influence of Humidity on Double-K Fracture Toughness and Fracture Energy of Concrete under Water Environment

Saturated concrete is significantly different from dry concrete in fracture mechanical properties. Using the wedge-splitting tensile method to research the rule of change in moisture content, double-K fracture toughness and fracture energy of three strength grades (C20, C30, and C40) of concrete immersed in a free water environment for 0 h, 2 h, 5 h, 24 h, and 120 h were studied in order to provide support for the safety evaluation of concrete structures in a water environment. The initial cracking fracture toughness of C20, C30, and C40 concrete in saturated state were, respectively, 29.6%, 23.2%, and 33.4% lower than that in dry state. The unstable fracture toughness of C20, C30, and C40 concrete in saturated state were, respectively, 22.7%, 23.9% and 33.8% lower than that in dry state. The fracture energy of C20, C30, and C40 concrete in saturated state are only 71.99%, 70.29%, and 66.11% of that in dry state, respectively. The initial cracking fracture toughness and unstable fracture toughness of concrete all show a linear, decreasing trend with an increase in moisture content. Before the crack initiation, the measured P–CMOD curve had an obvious linear elastic stage, stable expansion stage, and unstable expansion stage. The critical crack opening displacement gradually decreases with an increase in moisture content; the deformation capacity and toughness of concrete are shown to decrease. The humidity state should be fully considered when evaluating the fracture mechanical properties of concrete.

Performance and deterioration mechanism of concrete incorporated with corrosion-inhibiting admixtures under the coupling effect of composite salt and freeze-thaw cycles

The purpose of this study is to provide multi-functional, green, low-carbon and environmental-friendly corrosion-inhibiting admixtures (CIA) to alleviate the deterioration of reinforced concrete under the coupling effect of composite salt and freeze-thaw cycles (FTCs). The effects of different contents of CIA on the performance of C30 concrete, including mechanical behavior (mass loss rate (MLR), compressive strength loss rate (CSLR) and relative dynamic elastic modulus (RDEM)), chloride ion penetration of concrete, and the electrochemical performance of rebars in concrete, were investigated. Furthermore, the deterioration mechanism of reinforced concrete was probed in detail by evaluating the pore structure parameters of concrete and the microscopic morphology of concrete and rebars via low-field nuclear magnetic resonance (NMR) and scanning electron microscope (SEM), respectively. The results showed that the mechanical behavior of concrete with CIA was prominently superior to that of concrete without CIA, with MLR decreased by 51.13–78.60%, CSLR decreased by 40.11–84.93%, and RDEM increased by 29.89–56.27% after 100 FTCs. The average content of free chloride ion in the concrete containing CIA was decreased by 12.14–55.65% under 100 FTCs due to the addition of CIA reducing the transmission path and the percentage of much detrimental pores (decreased by 39.03–85.99% after 100 FTCs). The correlation between free chloride ion content and diffusion depth conformed to Fick's second law, showing that there was a linear positive correlation between chloride diffusion coefficient (D) and FTCs. In addition, the electrochemical performance of rebars in concrete with CIA was improved owing to the good protection of CIA to rebars. This study is of great significance for improving the durability of reinforced concrete in harsh environment.

Push-out test of headed stud connectors subjected to freeze-thaw cycles and artificial corrosion

To obtain the shear capacity calculation method and competition failure mechanism of headed stud connectors of steel–concrete composite structures under the influence of freeze–thaw cycles (FTCs) and artificial corrosion (AC), the material performance of concrete specimens after the FTCs and push-out tests of 36 headed stud connectors specimens were tested under the influence of FTCs and AC. The cubic compressive strength, dynamic elastic modulus, and mass loss rate of C50 concrete were obtained under different FTCs. Failure modes, load-slip curve, shear capacity, and shear stiffness of stud connectors were investigated based on push-out tests of miniaturized specimens. Based on the nonlinear fitting of test data, a formula for calculating the shear capacity reduction factor is proposed. Results show that the shear capacity and stiffness of stud connectors will deteriorate due to the deterioration of concrete performance caused by FTCs or the weakening of stud section caused by AC. The two failure modes of stud breakage or concrete cracking will change with the aggravation of FTCs and AC effect.

Effect of Different Expansive Agents on the Deformation Properties of Core Concrete in a Steel Tube with a Harsh Temperature History

The shrinkage of core concrete during construction is the key reason for the separation of steel pipes and core concrete. Utilizing expansive agents during cement hydration is one of the main techniques to prevent voids between steel pipes and core concrete and increase the structural stability of concrete-filled steel tubes. The expansion and hydration properties of CaO, MgO, and CaO + MgO composite expansive agents in C60 concrete under variable temperature conditions were investigated. The effects of the calcium-magnesium ratio and magnesium oxide activity on deformation are the main parameters to consider when designing composite expansive agents. The results showed that the expansion effect of CaO expansive agents was predominant in the heating stage (from 20.0 °C to 72.0 °C at 3 °C/h), while there was no expansion in the cooling stage (from 72.0 °C to 30.0 °C at 3 °C/d, and then to 20.0 °C at 0.7 °C/h); the expansion deformation in the cooling stage was mainly caused by the MgO expansive agent. With the increase in the active reaction time of MgO, the hydration of MgO in the heating stage of concrete decreased, and the expansion of MgO in the cooling stage increased. During the cooling stage, 120 s MgO and 220 s MgO resulted in continuous expansion, and the expansion curve did not converge, while 65 s MgO reacted with water to form brucite in large amounts, leading to its lower expansion deformation during the later cooling process. In summary, the CaO and 220 s MgO composite expansive agent in the appropriate dosage is suitable for compensating for the shrinkage of concrete in the case of a fast high-temperature rise and slow cooling rate. This work will guide the application of different types of CaO-MgO composite expansive agents in concrete-filled steel tube structures under harsh environmental conditions.

Effect of thermal fatigue on mechanical properties and microstructure of concrete in constant ambient humidity

The daily and annual temperature differences in mid and high latitudes areas vary greatly, which leads to thermal fatigue deterioration of concrete. The deterioration of concrete properties can also be generated by the cyclic change of humidity in concrete. In the experiment, the thermal fatigue tests of concrete with two strength grades were carried out at the temperature difference of 20 °C, 30 °C, and 40 °C (the initial temperature is 20 °C) under the condition of constant ambient humidity. The macroscopic properties change of concrete such as compressive strength and splitting tensile strength were measured. The microstructure changes of concrete were determined by ultrasonic nondestructive testing technique and mercury intrusion test. The results indicated that thermal fatigue has obvious deterioration effect on concrete. With the increase of temperature difference and cycle number, the strength of concrete decreased dramatically and the decrease of C40 concrete was greater than that of C25 concrete. The splitting tensile strength was more sensitive to thermal fatigue than compressive strength. The ultrasonic wave velocity gradually decreased and the damage factor increased, indicating the internal crack defects of concrete increased. At the same temperature difference, the porosity, total pore volume, average pore diameter, medium pore diameter, and the most probable pore diameter of concrete increased gradually with the increase of the cycle number, while the total pore surface area decreased. The pore structure showed the characteristics of coarsening and the trend of deterioration. The porosity of C40 concrete was less than that of C25 concrete, but the relative change value of porosity was larger, which reveals the internal reason for the strength damage of concrete under thermal fatigue at the microscopic level.

Effect of high pump pressure on chloride migration in ITZ of concrete

Concrete pumping tests with distances of 150, 300, 450, 600, and 750 m were conducted to study the influence of pumping on the compressive strength and the chloride migration of concrete. Long-distance pumping had a significant impact on the chloride migration coefficient of concrete, and it was observed that the Cl− migration coefficient of concrete decreased with an increase in the pumping distance. When the pumping distance was 750 m, the Cl− diffusion coefficient of C40 and C60 concrete decreased by 28.73 and 27.65%, respectively. The Backscatter (BSE) test was conducted, and the porosity and thickness of ITZ were determined following the threshold segmentation method. The porosity and thickness of ITZ decreased with an increase in the pumping distance. When pumped over a distance of 750 m for C40 and C60 concrete, the porosity of ITZ decreased by 59.78 and 58.57%, respectively, and the thickness of ITZ decreased by 26.43 and 25.96%, respectively. The effect of pumping on the porosity of ITZ was greater than that on the thickness of ITZ. Considering ITZ effect, dilution effect and tortuosity effect, the influence of pumping on chloride migration coefficient of concrete was analyzed and calculated quantitatively.

Seismic Fragility Assessment of RC Columns Exposed to the Freeze-Thaw Damage

Freeze–thaw damage is one of the primary causes deteriorating the seismic resistance of reinforced concrete (RC) structures. This paper proposed a freeze–thaw damage deterioration model for C30 concrete, and it can be employed to study the time-varying seismic performance of aging RC columns. Next, this study developed a seismic fragility analysis framework for deteriorating RC columns considering the effect of freeze–thaw damage. Considering the geometric parameters of the case-study bridge, the deterioration characteristics of material, and the uncertainties involved in structural modeling and ground motions, a probabilistic seismic fragility analysis on aging RC columns was conducted. The results indicate that the influence of freeze–thaw damage cannot be ignored in studying the seismic performance of aging RC structures. The seismic fragilities of deteriorating RC columns shown a nonlinear increase trend as the increased of freeze–thaw cycles and severity of the damage state. In the early stage of freeze–thaw cycles, the seismic fragilities of RC columns increased slowly. However, the closer to the later stage of freeze–thaw cycles, the more significant of the increase in the seismic fragilities of the columns.

Assessing Fresh Concrete Performance and Water Absorption Characteristics of Concrete with Partially Replaced Broken Tiles

A number of research seeking alternative materials to augment normal coarse aggregate to reduce overreliance on granite while saving the environment is on the rise. This study assessed the workability, water absorption and density of concrete with waste ceramic tiles as partial replacement for crushed granite coarse aggregate. Crushed waste ceramic tiles from ceramic manufacturing industries and construction sites were blended with crushed granite for concrete production. A mix ratio 1: 1.11: 2.72 (cement: sand: coarse) for nominal C30 concrete was produced with (0, 10, 20, 30, 40, 50 and 100) percent volume ceramic waste replacement for crushed granite at constant water-cement ratio of 0.5 to produce concrete cubes and cured for 7 and 28 days. Tests conducted revealed concrete workability and density decreased with increasing ceramic waste while water absorption increased with increasing ceramic waste but not above acceptable limits. However, use of waste ceramic tiles as partial replacement for crushed granite in concrete is encouraged, but not exceeding 20% maximum for structural concrete. This mode of recycling ceramic waste could positively promote green construction for sustained environment; hence, it should be encouraged with conditional limitation and further research of its use.

Assessing Structural Performance of Ceramic Waste as Partial Replacement of Coarse Aggregate on Properties of Concrete

The need to reduce the increasing demand for crushed granite as coarse aggregate as well as the need to safeguard the environment from degradation has given rise to various researches on alternative materials that can serve the same purpose while minimizing environment hazard. This study assessed the suitability of waste ceramic tiles as coarse aggregate in concrete and to ascertain its strength against normal crushed granite. Crushed waste ceramic tiles from ceramic manufacturing industries and construction sites were mixed with crushed granite stones as partial replacement for concrete. A mix ratio of 1: 1.11: 2.72 (cement: sand: stones) for nominal C30 concrete was produced with (0, 10, 20, 30, 40, 50 and 100) percent volume ceramic waste aggregate replacement for crushed granite at a constant water-cement ratio of 0.5. Concrete cubes of size 150mm x 150mm x 150mm were produced and tested for 7 days and 28 days compressive strength, density and water absorption. The results revealed the viability of waste ceramic tiles as partial replacement for crushed granite in concrete production, but the partial replacement should not exceed 20% as recommended maximum for structural concrete. This mode of recycling ceramic waste could positively sustain the environment.

Mechanical properties and acoustic emission analysis of desert sand concrete reinforced with steel fiber

In this study, to reduce the consumption of natural sand and improve the utilization rate of desert sand in western China, while preparing 14 groups of samples, desert sand is used to replace natural sand by the ratio of 20%, 40%, and 60%, and steel fiber is mixed with volume fraction 0.5%, 1.0%, 1.5%, and 2.0%. The mechanical properties of the specimens, including compressive strength, splitting tensile strength, and axial compressive strength were tested. Besides, the microstructures of the samples were analyzed by SEM, XRD, and acoustic emission detection technologies to identify the damage process. The results show that the desert sand can refine the microstructure and fill the pores, and it has good comprehensive properties at a 40% substitution rate. The compression properties of specimens are not apparently improved, but the tensile strength and deformation properties are significantly improved. The steel fiber with 1.5 vol% content behaves better, and the 28d compressive strength of the optimized group reaches 58.7 MPa. As a result, the polynomial fitting degree of total AE hits and stress level receives a more incredible goodness (R2) value than 0.96. The strength characteristics of steel fiber-desert sand concrete (SFDSC) can meet the demands of C40 concrete, and this research can provide a reference for engineers using desert sand in their designs.

Research on Impermeability of Underwater Non-Dispersible Concrete in Saline Soil.

The permeability of different strength grades of submerged non-dispersible concrete with different granulated slag admixtures in a saline soil environment simulated by different erosion solutions was investigated. The variation patterns of the chloride ion diffusion coefficient and pore characteristics were tested using NEL and MIP. The microscopic morphology of the specimens in different erosion environments and with slag doping was observed using SEM. The results showed that the impermeability of concrete in sulfate and complex salt environments was significantly reduced. The resistance of concrete to chloride ion penetration increased with the increase in strength grade, and the Cl- diffusion coefficient of C35 was 5-30% lower than those of C30 and C25 underwater non-dispersible concrete at 360 d. Meanwhile, the admixture of granulated blast-furnace slag optimized the pore size distribution and improved the matrix compactness and permeability.

Effect of strength grade on mechanical properties of concrete under dynamic load

AbstractThe uniaxial compression test and dynamic compression test of C30, C50, and C80 concrete were carried out. The effects of strength grade and strain rate () on dynamic mechanical properties of concrete were analyzed in terms of dynamic compressive strength, dynamic increase factor (DIF), dynamic peak strain, and dynamic peak toughness. And the strain rate sensitivity of mechanical indexes and DIF fitting model of concrete with different strength grades were explored. The results show that the dynamic compressive strength, DIF and dynamic peak toughness of concrete with different strength grades all have obvious strain rate effects. With the increase of strength grade of concrete, the dynamic peak strain of concrete decreases continuously, and the strain rate sensitivity of dynamic compressive strength and dynamic peak toughness of concrete increases continuously. The mechanical indexes of C80 concrete have the strongest sensitivity to the strain rate. The DIF empirical formula of Comite Euro‐International du Beton (CEB) cannot fit the DIF of C30 and C80 concrete effectively. There is a critical strain rate beyond which the strain rate effect of DIF is significantly enhanced. With the increase of strength grade of concrete, the critical strain rate of concrete decreases continuously. The piecewise linear fitting of DIF and of concrete with different strength grades shows good results.

Drying characteristics of concrete under surface low air pressure

The humidity change of C30, C50, and C80 concretes under surface low air pressure and atmospheric pressure were tested. Based on the mechanism of moisture transfer under low air pressure, a theoretical model of concrete humidity field considering the effect of pressure was established. The model was used to analyze the changes of concrete internal humidity field when the surface pressure changes. The results show that the relative humidity values of each concrete under low air pressure are lower than that of atmospheric pressure. And the effect is greater near the drying surface. The improved calculation model was used to calculate the humidity change of concrete under low air pressure. The calculated values were in good agreement with the experimental values. It can be used to study the internal humidity field of concrete under different pressure conditions.

Compressive properties and microscopic analysis of engineered cementitious composites after salt freezing

In order to study the compressive properties of Engineered Cementitious Composites (ECC) after salt freezing, the failure, compressive strength, and deformation capacity of cubic concrete specimens subjected to single-side freezing–thawing cycles considering the number of cycles, aggregate type Maowusu or Tengger Desert sands (MDS and TDS, respectively) and river sand (RS), and salt medium (NaCl or NaCl + Na2SO4). The results showed, In the presence of salt, the compression failure modes of all samples involved the formation of vertical cracks in the stress direction, but MDSECC and TDSECC showed better integrity than C30 concrete. With increasing freeze–thaw cycles, the strengths, slopes of the stress–strain curves, peak stress, and curve area of the desert sand specimens decreased, but the residual stress and ductility increased. Through regression analysis of the strength loss rate, the ECC lifetime was predicted, and using X-ray diffractometry and scanning electron microscopy, the effects of salt freezing on the mechanical properties of ECC were revealed.

Ship Matrix-Deck and Analysis of Its Mechanical Properties

Ship deck is generally made of steel materials. With the development of ship technology and the demand of ship lightweight design, a lightweight and modular ship deck is needed. In this paper, a ship deck is designed based on the form of matrix; The deck is mainly made of engineering plastics and filled with load-bearing materials. The overall strength of the material is analyzed by FEM simulation. According to the simulation results, the compressive strength of this grid composite structure is basically equivalent to that of C30 concrete, but the weight can be reduced by 30% compared with C30 concrete The matrix-deck has the characteristics of high strength and low density, and is suitable for occasions with high requirements for material strength and material quality.