Temperature distributions in concrete box girders due to heat of concrete hydration

Based on heat transfer theory,a two-dimensional complex exponential function was used to compute heat of concrete hydration.A concrete box girder consisting of a single box with two cells used on Harbin Songpu Bridge was measured on site.The two coefficients in the complex exponential function were determined to best fit the field measured data.ABAQUS program was used to simulate the heat transfer and determine the temperature distribution in the concrete box girders during concrete setting.The calculated temperature distribution in the box girders were compared with the field measured data and good agreement was observed.The temperature distribution and gradient in the entire box section,webs and bottom slab were analyzed using the measured and calculated results during the course of concrete hydration.

Hydration Heat Evolution of Cement and Its Relation With Setting Time

In order to veritably measure the first peak of hydration heat evolution that has been illustrated important in indicating cement behavior in early hydration, an improved way of water addition into cement in isothermally calorimetric experiment is put forward. The experimental results indicated that: the magnitude of first peak of heat evolution varies from sample to sample, correlation between heat evolution during first peak of heat evolution and initial (as well as final) setting time is unsatisfactory when samples are not classified; while groups of sample classified based on strength grade represent satisfactory correlations, which indicating the existence of close relation between hydration heat evolution in much earlier hydration age and setting property of cement in rather later age. Importance of first peak in hydration heat evolution for understanding cement setting property and reasons for sample classification are also discussed in this paper.

Preparation of New Cementitious System using Fly Ash and Dehydrated Autoclaved Aerated Concrete

We experimentally studied the interaction between pozzolanic material（fly ash） and dehydrated autoclaved aerated concrete（DAAC）. The DAAC powder was obtained by grinding aerated concrete waste to particles fi ner than 75μm and was then heated to temperatures up to 900 ℃. New cementitious material was prepared by proportioning fly ash and DAAC, named as AF. X-ray diffraction（XRD） was employed to identify the crystalline phases of DAAC before and after rehydration. The hydration process of AF was analyzed by the heat of hydration and non-evaporable water content（Wn）. The experimental results show that the highest reactivity of DAAC can be obtained by calcining the powder at 700 ℃ and the dehydrated products are mainly β-C2 S and CaO. The cumulative heat of hydration and Wn was found to be strongly dependent on the replacement level of fl y ash, increasing the replacement level of fl y ash lowered them in AF. The strength contribution rates on pozzolanic effect of fl y ash in AF are always negative, showing a contrary tendency of that of cement-fl y ash system.

Simulation Analysis on Mass Concrete Temperature Field of Lock Floor Layered Pouring

Based on the background of navigation lock structure engineering in Changsha Integrated Hub, this study used Parametric Design Language （APDL） compilation command on the t＇mite element program ANSAYS platforms to simulate the temperature field of the layered pouring process about the lower lock head. The temperature contour map and the change laws of temperature field with time in each different levels of the floor were obtained. And compared with the actual instrument measurement data, the feasibility of the simulation analysis was concluded. Then, this study optimized the pouring process, obtained the suitable methods of layered pouring and put forward the measures to reduce the concrete temperature crack.

Thermal and Stress Analysis of Early Age Concrete for Spread Footing

The early age performance of spread footing, especially the growth of cracks, is deeply influenced by the heat of hydration of cement. In this paper, 3D finite element method(FEM)models are set up to analyze the temperature distribution and thermal stresses of the spread footing during the first seven days after concrete placement. The mechanical properties of early age concrete are calculated, which are further used in the FEM models. The possibilities of crack growth are estimated by the method of crack index. The crack indexes of quite a number of points are very close to the allowable limit of 1.0 during the last three days. It is also indicated that the influence of foundation ring on the thermal stresses of concrete can be neglected.

Preparation for Retarding and High Early Strength Concrete

The primary objective of this research was to determine optimum dosage of mixing concrete containing plasticizers and fly ash, consistent with desirable structural grade concrete properties. Factorial tests were also conducted to investigate the four main factors： water-cementing materials ratio, water content, content of superplasticizers （SP） and fly ash content. It was found that the requirement for setting time played the dominant role in shrinkage and anti-cracking, and fly ash played a critical role in workability and reducing heat of hydration but showed insignificant effects on slump, early strength and initial setting time of concrete.

Effects of Rice Straw Fibres on the Mechanical Properties and Hydration of Cements

The purpose of this research is to study the effects of rice straw fibres (RSF) on the setting time,heat of hydration,hydration products (XRD),mechanical properties for cements. Three types of the cements (P.I.52.5,P.O.42.5,sulphate aluminium) are used in this test. Results show that with the addition of RSF,the setting times for these three kinds of cements are delayed. According to P.O.42.5 cement,The max temperatures of exothermic peak (Tmax) decrease,exothermic peak times (tmax) are delayed and the mechanical strengths generally drop as RSF concentration is increased. XRD test shows that the RSF hinder the formation of Ca(OH)2 phase and hydration of cement (C3S,C2S,C4AF),which resulted in the delay of setting time and hydration reaction.

The effect of SiO2 nanoparticles derived from hydrothermal solutions on the performance of portland cement based materials

The nanoparticles of SiO2 were used in cement systems to modify the rheological behavior, to enhance the reactivity of supplementary cementitious materials, and also to improve the strength and durability. In this research, low-cost nano-SiO2 particles from natural hydrothermal solutions obtained by membrane ultrafiltration and, optionally, by cryochemical vacuum sublimation drying, were evaluated in portland cement based systems. The SiO2-rich solutions were obtained from the wells of Mutnovsky geothermal power station （Far East of Russia）. The constant nano-SiO2 dosage of 0.25% （as a solid material by weight of cementitious materials） was used to compare the cement systems with different nanoparticles against a reference mortar and a commercially available nano-SiO2. Nanoparticles were characterized by X-Ray Diffraction （XRD）, BET Surface Area, Scanning Electron Microscope （SEM） and Fourier Transform Infrared （FTIR） spectroscopy techniques. It was demonstrated that the addition of polycarboxylate ether superplasticizer and the dispersion treatment using an ultrasound processor can be used to facilitate the distribution ofnano-SiOz particles in the mixing water. The effect ofnano-SiO2 particles in portland cement mortars was investigated by evaluating the flow, heat of hydration and compressive strength development. It was demonstrated that the use ofnano- SiO2 particles can reduce the segregation and improve strength properties.

Exothermic process of cast-in-place pile foundation and its thermal agitation of the frozen ground under a long dry bridge on the Qinghai-Tibet Railway

A number of dry bridges have been built to substitute for the roadbed on the Qinghai-Tibet Railway,China.The aim of this study was to investigate the exothermic process of cast-in-place (CIP) pile foundation of a dry bridge and its harm to the stability of nearby frozen ground.We present 3D heat conduction functions of a concrete pile and of frozen ground with related boundaries.Our analysis is based on the theory of heat conduction and the exponent law describing the adiabatic temperature rise caused by hydration heat.Results under continuous and initial conditions were combined to establish a finite element model of a CIP pile-frozen ground system for a dry bridge under actual field conditions in cold regions.Numerical results indicated that the process could effectively simulate the exothermic process of CIP pile foundation.Thermal disturbance to frozen ground under a long dry bridge caused by the casting temperature and hydration heat of CIP piles was substantial and long-lasting.The simulated thermal analysis results agreed with field measurements and some significant rules relating to the problem were deduced and conclusions reached.