Massive Concrete Structures Research Articles

Data-Based Modeling of Early-Age Concrete Mechanical Behavior for Structural Calculation

AbstractWhen considering the early-age behavior of massive concrete structures, it is important to accurately reproduce the evolutions of the mechanical properties and their evolutions as a functio...

Predicting thermal performance of a mass concrete foundation – A field monitoring case study

High-temperature differentials in a mass concrete structure pose great risks of temperature-induced stresses and cracking. Prior knowledge of temperature development within such a structure is essential. In this context, this paper presents a case study in which the construction of a mass concrete bridge foundation in Iowa, USA was investigated and a computer program, ConcreteWorks (CW), was used to predict its overall thermal performance with an aim to prevent thermal cracking. The properties of mass concrete mixes, required as CW inputs, were measured through isothermal and semi-adiabatic calorimetry tests. The temperature development profile, temperature differential, maturity, and compressive strength of the mixes were predicted and compared with those measured through the real-time monitoring of the bridge foundation. It was observed that CW predictions match well with their corresponding measured values. Three locations, core, top, and the face nearest to the core of the foundation, were found to be critical points for high temperature differentials. A sensitivity analysis, analyzing the effects of various mass concrete parameters, is also presented. The results provided clear insights into the temperature development of concrete with complex material compositions and environmental conditions. CW is a useful tool in developing thermal control plan for mass concrete projects.

Research on the Reasonable Spacing of Post-casting Belt of 100m Long Inverted-T Shape Concrete Structures on Soft Foundation

Setting up post-casting belt is one of the most important measures for temperature control and crack prevention of mass concrete, but current specifications have not given unified and clear rules on the spacing of post-casting belt. In this paper, a 100m long inverted-T shape concrete structure on soft foundation in Yangtze River Delta region is taken as the research object. The temperature and stress field of concrete during construction under different conditions are simulated by finite element method, and bottom plate’s and gate pier’s recommended spacing of post-casting belt is obtained when poured in different seasons. This will be of guiding significance to the construction of such mass concrete structure.

Influence of the incorporation of phase change materials on temperature development in mortar at early ages: Experiments and numerical simulation

The present work is focused on a thorough experimental campaign to assess the thermo-physical properties and performance of mortar containing phase change materials (PCMs) with peak melting temperature of 34 °C. This selected melting temperature was targeted to reduce the peak temperature developed in mortar at early ages, induced by heat of hydration of cement. Indeed, 34 °C is well above normal casting temperatures, yet well below peak temperatures normally attained in massive concrete structures: therefore, this choice would represent a somewhat generalizable choice for attenuating peak temperatures. Due to experimental constraints, the tests conducted herein were performed on mortar (smaller size specimens are possible). However, the methods and findings may easily be extrapolated to concrete.New preparation and incorporation protocols have been considered for using PCMs into mortar: (i) grated pristine PCM to be dispersed into mixtures; and (ii) macro capsule core of PCM. Three PCM volume fractions of 0, 10 and 20% using the two above mentioned different strategies of incorporation into the mortar were used to prepare a total of five mortar test prototypes. Furthermore, mechanical tests were then carried out on companion specimens to evaluate the effect of incorporating PCM on the compressive and flexural strengths.The addition of grated PCM (i.e. shredded) in mortar led to enhancement of volumetric heat capacity while maintaining acceptable structural strength. The results revealed that the addition of PCM reduces density, thermal conductivity, and mechanical strength. In respect to the strength reduction, such percentages of incorporation were found to be small enough (<50%) as to not compromise the use of such mixtures for structural application.As expected the tested PCM mortar specimens had a lower peak temperature, as compared to that of the reference specimens (mortar without PCM). Indeed, the 20% dispersed PCM mixture allowed peak temperature to decrease from ∼52 °C to ∼44.8 °C.Complementarily, numerical models were developed and calibrated using the experimental data. The comparison between numerical simulation (using DIANA software) and experimental results revealed a very good agreement of the temperature profiles in the early ages. This allows to validate the capacity of simulating temperature evolution within mortar containing PCM, as well as, to understand the contribution of PCM to reduce the temperature rise coupled with the cement hydration phenomenon. It also represents a baseline methodology for scientists and practitioners to perform parametric analyses for other situations.

ТРЕЩИНОСТОЙКОСТЬ МАССИВНЫХ БЕТОННЫХ КОНСТРУКЦИЙ В СТРОИТЕЛЬНЫЙ ПЕРИОД С УЧЕТОМ ТЕМПЕРАТУРНЫХ ВОЗДЕЙСТВИЙ

Concrete is a fundamental material of modern construction due to its reliability, the ability to harden in air and under water, the ability to build structures of any shape and sizes. In particular, it is used in the construction of advanced structures and their parts: nuclear power plants, large hydraulic struc-tures, bridges, massive foundations of multi-storey buildings, etc. This paper is devoted to relevant objective in the field of construction of buildings and structures with a high level of responsibility, the study of thermal crack resistance of massive concrete structures, taking into account the temperature effects during the building period. The calculations are given of the thermal crack resistance of the foundation plate of the reactor department of Leningrad NPP-2 under continuous concreting with one block at full height. It is highlighted that the use of the simplest calculation model, which does not con-sider fluctuations in air temperature during the building period, leads to a significant error of the sur-face insulation thickness that is optimal from the point of view of providing the thermal crack re-sistance of a massive concrete structure.

Numerical study on crack thermal resistance effect on thermo-mechanical coupled behavior of concrete structure at room temperature

Cracking behavior in massive concrete structure is inescapable. Cracks would impair the thermal conduction ability of concrete and indirectly induce temperature redistributions, which, in this study, is termed “Crack Thermal Resistance (CTR) effect”. In order to evaluate the influence of CTR on massive concrete structure, a specific thermo-mechanical (TM) coupled model at room temperature, featuring with the evolution of concrete thermal conductivity during material fracture, is proposed based on the thermodynamic principle and concrete damage plastic model. This coupled model is coded as subroutines in ABAQUS and the implementation validated by an uniaxial cyclic test of a single element and a double edge notched specimen (DENS) test. In the DENS numerical test, it is observed that CTR effect can cause heat flux turbulences around cracking area, leading to redistributions of temperature. As a consequence, the stress field and crack opening change around the cracking area as well. In the end, this model is applied to investigate the influence of CTR on the TM coupled behavior of a gravity dam under a long term operation period thermal load. The comparison between the simulations with and without CTR effect demonstrates that (a) CTR effect has a negative effect on structure interior crack propagation, because thermal resistances near dam toe protect the dam from the downstream environment temperature shock; (b) CTR effect can induce redistributions of stress field around the crack area, but the difference in stress field does not affect the compressive stress state on dam-foundation interface.

Temperature Control Measurement of Bridge Foundation Concrete based on the Optical Fiber Sensing Technology

Temperature monitoring is an important component of structural health monitoring of mass concrete after pouring. This study investigates temperature measurements of mass concrete based on Brillouin optical time-domain analysis (BOTDA). First, the fundamental principle of using BOTDA-based distributed optical fiber sensing (DOFS) technology to measure temperatures is provided. Then, a temperature-measuring system is introduced. Finally, a case study on an engineering structure was conducted to explore issues relating to the practical application of BOTDA-based DOFS technology, including optical fiber cable laying and protection, measurement and results analysis. In addition, point temperature sensors and precision thermometers were used to measure the temperatures at several points along the optical fiber cable and the ambient temperature, respectively. The measurement results were compared with those measured by the optical fiber cable. Our study shows that BOTDA-based DOFS technology can be used to take accurate distributed temperature measurements of mass concrete structures. This temperature-measuring method provides an effective means for measuring the temperature of mass concrete structures.

Prevention of crack formation in massive concrete at an early age by cooling pipe system

Cooling pipe system (CPS) in concrete arrays plays an important role to control the temperature changes in massive concrete such as dams, foundation bridges, etc. The application of CPS in massive concrete has also been studied by some researchers but only in positive qualities point of view of the CPS. The present study presents the results of an investigation of thermal behavior regarding crack formation due to the hydration heat in massive concrete structures. To reduce the thermal load and improve the reliability of protection against cracks in massive concrete, the temperature changes, particularly the temperature field and thermal stress, in such concrete with the presence of CPS were simulated and calculated with the help of the advanced computer programs. In addition, both the temperature field and the cracking index in the massive concrete with the CPS’s temperature of 15 °C were determined. As the results, the cracking index over time was greater than the allowable value of 1.5, which indicates that the risk of cracking did not occur when the CPS was applied in the massive concrete.

MASS CONCRETE CONSTRUCTION USING SELF-COMPACTING MORTAR

Risk of thermal cracking is a well-known problem in mass concrete structures. One of the steps in mitigating this problem is to keep the initial placement temperature of concrete as low as possible. From this standpoint, Preplaced Aggregate Concrete (PAC) is advantageous because the friction among the coarse aggregate particles during the mixing operation causes the initial temperature of concrete to rise. Making PAC by the conventional method of grout injection requires special equipment and skilled workers. An alternative method using a self-compacting mortar for grouting preplaced aggregate was investigated in this study. Using different concrete compositions made with locally available cementitious materials and aggregate fractions, massive concrete cubes, each 1 m. were prepared. The specimens were tested for thermal and mechanical properties. In addition to low temperature rise that is suitable for mass concrete, the PAC method produced concrete specimens with compressive strength and modulus of elasticity values comparable to conventional concrete.

Influence of additives on hydration heat of mass concrete

In this paper, the development of early hydration heat and the performance of large temperature difference of mass concrete structures are studied. Five mass concrete samples with two strength grades and five mixing ratios were made. The size of samples is 1000mm∗1000mm∗1400mm. Temperature sensors are embedded in the component while the components are constructed. Temperature of early hydration heat temperature rise and instant heating on one side are tested by the sensors. It is found that the early hydration heat temperature rise of self-made crack resistance concrete with internal curing agent or shrinkage reducing agent is lower than that of common commercial concrete. The results of single-side instantaneous temperature rise test show that the temperature changes least on the heating surface of concrete and most in the middle position.

Simulation and Modelling in Cracks Prevention of Early Age Concrete

Cracks often arise in mass concrete structures, due to the thermal stress and low tensile strength of early age concrete. To prevent the undesired thermal stress induced crack, controlling the temperature of concrete has been considered as an effective approach. In this paper, a temperature controlling measure evaluation system (TCMES) is proposed, which includes distributed fiber optic temperature monitoring, prediction of temperature and stress fields, and concrete crack risk evaluation. We first experimentally monitor the temperature evolution of the concrete using the distributed fiber optic temperature sensing. Then thermal parameters of in-situ concrete are retrieved by performing back-analysis. Subsequently, the concrete temperature field and thermal stress field can be predicted from the retrieved parameters and the experimental parameters of concrete. Under the concrete crack risk evaluation principles, temperature controlling measures for different stages are proposed. Our analysis indicates that the proposed system is an effective approach to prevent cracks of early age concrete.

Analytical solution for temperature field of nonmetal cooling pipe embedded in mass concrete

Non-metal cooling water pipe is widely used in temperature control of mass concrete structures. However, the previous calculation method for water pipe cooling contained a Bessel function, which makes the calculation very cumbersome. In this paper, the theoretical solution and simplification of cooling of non-metal pipes, compared with the cooling of metal pipes, are studied. It is found that the form of the theoretical solution obtained by the separation variable method can be simplified to an exponential function form. A new formula is presented herein for the theoretical solution of the temperature field in pipe cooling. For non-dimensional parameters, the numerical values of these parameters for the formula are given, which are convenient for direct calculation.

Optimization of concrete mix design to account for strength and hydration heat in massive concrete structures

In the case of massive concrete structures, the heat generated by cement hydration may cause cracking due to thermal strains. The mix design of the concrete used for such structures has to take account of mechanical properties and generated temperatures. Using experimental design principles, the hydration heat and the development of compressive strength are measured in order to determine how the composition of concrete and the presence of SCM influence the characteristics of concrete and to create a mix design protocol. This protocol can help to determine which mix design minimizes the hydration temperature for a given compressive strength.

Long-tern field test and numerical simulation of foamed polyurethane insulation on concrete dam in severely cold region

Abstract The environmental temperature is the key factor for cracking failure of mass concrete structures in severely cold regions. In this paper, field test and numerical calculation of long term insulation are carried out to prevent thermal cracks, and ensure the durability and safety of concrete arch dam. Firstly, an equivalent simulation element for concrete-polyurethane composite structure and its calculation method is proposed, based on the equivalence idea of heat emission coefficient for concrete layer and surface insulation layers with different thickness. Secondly, field tests are demonstrated on an 87.5 m height concrete gravity-arch dam, named Xianghongdian dam, from January 1, 2001 to May 30, 2017. Distributions laws, change laws of temperature field and horizontal displacement of the dam, before and after setting polyurethane insulation layer, are compared deeply. Then, the insulation effects of different thicknesses of foamed polyurethane layer are investigated upon the establishment of a FEM model of a concrete arch dam in northwest China. Consequently, stress fields under different broken conditions of polyurethane insulation layer with different typical temperature loads in severely cold region are calculated and analyzed. The results show that the polyurethane insulation layer generally decreases the temperature change range of concrete dam body over 30% in all conditions and even 90% at some locations. Accordingly, the horizontal displacement component, caused by dam temperature, decreases greatly from 10.80 mm to 2.34 mm and its contribution ratio to yearly horizontal displacement decreases from 75.11% to 36.23%. The insulation layer broken would result in a sharp increase of tensile stress of the corresponding area, with the combination influences of low temperature and cold wave. Under small broken condition, the tensile stress increases from 3.69 MPa to 7.57 MPa among broken area; while under large broken condition, the tensile stress increases to 8.68 MPa, which exceeds the maximum tensile stress of concrete material. In this case, a cracking failure of concrete structure would happen. As the dam concrete is an insulation material in some degree, the stress filed doesn’t change a lot in areas the insulation layer is unbroken.

Structural performance of steel-concrete sandwich beams with carbon nanofiber reinforcement

Cementitious materials such as concrete are typically characterised as quasi-brittle with low tensile strength and low strain capacity, which hence affect the long-term durability of the structure. One of the most important issues in designing and maintaining massive concrete structures like offshore and nuclear power plants is concrete cracking, which is due to the low tensile strength of concrete. This can destroy the structural aesthetic and lead to deterioration of the structure.The addition of fibers to concrete has been proven to be a good mean to control its crack behaviour and maintain its ductility in tension. Further, since the discovery of carbon nanotubes/fibers (CNT/CNF), they have been also considered as efficient fibers for construction materials such as concrete.This study presents the structural performance of steel-concrete (SC) elements with a fiber reinforced concrete (FRC) core using both single and hybrid fibers (i.e. consisting of two types of fibers). For this study carbon nanofibers, and steel fibers which are conventionally used in practice, are used for the FRC. Static tests were conducted on eight SC beams with different concrete types. The paper reports on the experimental results obtained from four-point flexural loading of the SC beams. The study shows considerable improvement for both the strength and ductility of the tested specimens. The research laid the groundwork for additional in-depth studies on using carbon nanofiber reinforced concrete within structural members.

Actual Temperature Evolution of Thick Raft Concrete Foundations and Cracking Risk Analysis

Heat of the hydration-induced temperature evolution of a 3.30 m thick raft concrete foundation for wind turbines at the early ages was monitored in situ through a temperature sensor testing system. The temperature variation patterns and risk of cracking were studied. Finite element analysis (FEA) conducted on the temperature fields determined the lower thickness threshold requiring temperature control. A comprehensive temperature control approach suitable for thick raft foundations was proposed based on a practical engineering project. Temperature monitoring and analysis results showed that the early temperature field evolution featured two characteristic phases: heat accumulation and heat release. A remarkable temperature gradient was observed along the vertical direction of the foundation. The maximum temperature difference between the concrete core and the top surface was approximately 35°C, indicating a risk of cracking. The accuracy of the FEA was ensured by adopting the concrete heat generation rate obtained from the adiabatic temperature rise test. A further FEA performed on foundations with various thicknesses demonstrated that a thicker foundation corresponded to a higher vertical temperature gradient. Moreover, a raft thickness larger than 2.50 m corresponded to a maximum temperature difference between the concrete core and the surface higher than 25°C, above which cracking prevention measures should be taken. Field test results proved the applicability of a suite of temperature feedback regulation measures proposed herein, including layered pouring, thermal insulation, and in situ real-time temperature monitoring, to thick raft mass concrete structures with relatively small volumes. Good control of temperature difference was achieved using this approach.

Finite element simulation of prevention thermal cracking in mass concrete

The mass concrete structures play a very important role in civil engineering. The cracking of concrete is regarded as one of the biggest engineering problems. Therefore, it is very necessary for the cracking of mass concrete to do the control analysis. Some factors should be considered in mass concrete crack control analysis, mainly including the heat releases model of concrete, the mechanical model to the concrete, the process of temperature control in the pipe model. Differential evolution algorithm and equivalent algorithm are adopted to solve the coefficient of adiabatic temperature and cool water effect. In the paper, stress field calculation, back analysis calculations, and cooling pipe processing create secondary development based on the ABAQUS software platform. The second development of the code is used to reasonably solve the problem with one actual aqueduct in hydraulic engineering.

Application of distributed temperature sensing for cracking control of mass concrete

This paper presents a framework of cracking control for a mass concrete structure in a reservoir project, by taking advantage of Distributed Temperature Sensing (DTS) system. The DTS system in this project, mainly consisting of an Optical Backscatter Reflectometer (OBR) interrogation unit and a single fibre-optic cable, was deployed to measure and monitor concrete temperatures of an intake tower in block placement. The temperature measurements in concrete blocks were utilized as the fundamental data to determine the thermal properties of the cast-in-situ concretes, through the inverse analysis method based on temperature simulation. Based on thermal stress simulation using the thermal properties, cracking risks of each concrete block were predicted and evaluated under the modes of temperature control associated with the time-varying construction and ambient conditions, for appropriate modes before and after the concrete placement. The framework of cracking control, developed by integrating the DTS measurement with the temperature forecast and the cracking prediction based on thermal stress simulation and cracking risk evaluation, improves the efficiency of temperature regulation and cracking control in mass concrete construction, and can be further integrated into the intelligent construction management of concrete projects.

Influence of steel and polypropylene fibers on cracking due to heat of hydration in mass concrete structures

Steel and polypropylene fibers result in a significant enhancement in strength, ductility, and toughness of concrete. This paper investigates experimentally and numerically the effect of these fibers on heat of hydration and early‐age cracking in mass concrete structures. Experimental investigations were conducted on evolution of strength, Young's modulus, and adiabatic heat rise in fly ash concrete mixes with steel and polypropylene fibers. Concrete mixes with ordinary Portland cement, 40% fly ash replacing the cement, 40% fly ash with 0.3 and 0.5% steel fibers, and 40% fly ash with 0.3 and 0.5% propylene fibers were investigated. The evolution of compressive and tensile strength, modulus of elasticity, and adiabatic heat rise using semi‐adiabatic calorimeter for all mixes was measured. The evolution of temperature at several locations in two full‐scale mass concrete blocks with normal concrete and fly ash concrete was measured. A multiphysics‐based finite element simulation of the two field mass concrete blocks was carried out using the software DIANA FEA BV (South Holland, Netherlands), which predicted the heat generation, temperature field, stresses developed in the blocks, and the associated cracking with good accuracy. The effect of steel and polypropylene fibers on heat, temperature, and stress generation in mass concrete blocks was investigated using the experimentally obtained parameters. The polypropylene fibers result in significant performance enhancement for early‐age cracking compared with the steel fibers.

Experimental and theoretical internal forced convection investigation on air pipe cooling of large-dimension RC walls

Air pipe cooling is an emerging technique in dealing with the hydration heat and thermal induced cracking of massive concrete structures. In order to investigate the influence of air pipe cooling on temperature distribution in large-dimension concrete walls, in situ experiments of heat transfer coefficient for internal forced convection were conducted on one experimental wall of 3.6 m × 3.6 m × 0.8 m in dimension with properly embedded corrugated pipes. The relationship between average inlet air velocity and average heat transfer coefficient for internal forced convection was then obtained and fitted to a proposed formula. In addition, air cooling experiments were performed on another three experimental walls to monitor the temperature variations of internal concrete. Meanwhile, finite element (FE) thermal analysis with the proposed formula was carried out and compared with the results of air cooling experiments to verify the accuracy of the proposed FE method. As the comparison results show, the calculated temperature curves are in good agreement with the tested data, with an average deviation of 0.07 °C, 0.13 °C and 0.19 °C under average inlet velocity u¯in of 3.78 m/s, 8.12 m/s and 11.64 m/s, respectively. It indicates that the FE analysis with the proposed heat transfer coefficient formula for internal forced convection is effective in estimating concrete temperature variations, providing a reliable fundamental approach for further thermo-mechanical coupling analysis and ventilation design in practical engineering projects.