Effects of the Water-Cement Ratio and the Molding Temperature on the Hydration Heat of Cement

The effects of the water-cement ratio and the molding temperature on the hydration heat of cement were investigated with semi-adiabatic calorimetry.The specimens were prepared with water-cement ratios of 0.31,0.38,and 0.45,and the molding temperature was specified at 10 and 20℃.The experimental results show that,as the water-binder ratio increases,the value of the second temperature peak on the temperature curve of the cement paste decreases,and the age at which the peak appears is delayed.The higher the water-cement ratio,the higher the hydration heat release in the early period of cement hydration,but this trend reverses in the late period.There are intersection points of the total hydration heat curve of the cement pastes under the influence of the water-cement ratio,and this law can be observed at both molding temperatures.With the increase in the molding temperature,the age of the second temperature peak on the temperature curve of the cement paste will advance,but the temperature peak will decrease.The higher the molding temperature,the earlier the acceleration period of the cement hydration began,and the larger the hydration heat of the cement in the early stage,but the smaller the total heat in the late period.A subsection function calculation model of the hydration heat,which was based on the existing models,was proposed in order to predict the heat of the hydration of the concrete.

Effects of Water-cement Ratio and Hydration Heat Regulating Materials on Hydration Process of Early-age Cementitious Materials

By means of low-field nuclear magnetic resonance(LF-NMR),the transverse relaxation time(T_(2))signals of physically bound water in cement paste were monitored to indicate water content change and characterize the early-age hydration process.With the curves of the T_(2)signals and hydration time obtained,the hydration process could be divided into four typical periods using the null points of the second derivative curve,and the influences of water-cement ratio(w/c)and hydration heat regulating materials(HHRM)on hydration process were analyzed.The experimental results showed that the hydration rate of pure cement paste in accelerated period presented a positive correlation with w/c.Compared to pure cement paste,the addition of HHRM extended all four periods,and led to a much faster hydration rate in initial period as well as a slower rate in accelerated period.Finally,according to the LFNMR test results,the early-age hydration model of cementitious materials was proposed considering w/c and HHRM content.

海上风电超低水胶比水泥基灌浆料的水化过程研究

基于水泥-硅灰-矿粉三元胶凝体系,制备了海上风电超低水胶比水泥基灌浆料,通过XRD和SEM分析了灌浆料不同龄期的水化产物与微观结构,并研究了其水化过程。结果表明:硅灰和矿粉共同促进了水泥的水化反应,且随着龄期的增加,促进作用增强;90 d时,在硅灰和矿粉的协同效应下,Ca(OH)_(2)晶体大幅度减少,从而减少了Ca(OH)_(2)自身晶体取向性对灌浆料造成的缺陷;水化反应生成大量低钙型C-S-H凝胶,形成排列紧密的层状构造,灌浆料内部结构致密,使其具有较高的抗压强度和耐久性能。

静停时间对路缘石性能的影响

蒸养制度是提高混凝土制品早期强度、提高其生产效率的重要技术手段。在进行蒸养前,需要进行静停养护以提高混凝土初始结构强度,抵抗混凝土在蒸养过程中的热胀影响。基于此,研究静停养护时间对超低水灰比高成型压力条件下干硬性混凝土路缘石性能的影响。结果表明,随着静停养护时间的延长,干硬性混凝土路缘石的出窑抗压强度先升高后下降,28 d抗压强度先升高后趋于稳定,吸水率和50次冻融循环质量损失率先下降后趋于稳定。在静停时间为4.5 h时,路缘石综合性能最佳。

Preparation and Optimization of Mix ratio for C50T Girder Manufactured Sand and Concrete

Considering actual construction conditions of Binchuan-Heqing Highway,this paper provides the C50 mix ratio conforming to engineering requirements by strictly controlling the quality of raw materials,optimizing the design of mix ratio scientifically,preparing superior C50 concrete 0 with manufactured sand,and optimizing the concrete mix ratio based on the adjustment of fly ash replacement,water-cement ratio,polycarboxylate-type water reducer mixing amount,sand ratio,etc.The result indicates that,the water-cement ratio has a great influence on the concrete strength,and if the ratio of coal ash is high in the binding material,the early compressive strength of the concrete will increase slowly.

Comprehensive Analysis of the Effects of Superplasticizer Variation on the Workability and Strength of Ready-Mix Concrete

This experimental study aims to examine the influence of many crucial parameters on the workability and compressive strength of Ready-Mix Concrete (RMC). The study utilized two distinct varieties of superplasticizers obtained from the local market. The fine aggregates utilized in this study were sourced from Sylhet sand, whereas the coarse aggregates were comprised of boulder crushed stone chips. The experimental procedures adhered to the requirements outlined by ASTM. A comprehensive investigation was conducted on a range of concrete compositions that used diverse chemical admixtures. The slump test was performed at regular intervals of 15 minutes until the slump value reached or fell below 3 cm after the mixing of the concrete. In the scenario involving two-stage admixture dosage, the second stage of admixture was introduced once the slump reached or dropped below 3 cm, following which the casting process was initiated. The process of curing concrete specimens consists of two distinct stages: the main stage and the final stage. Cylindrical specimens, with a diameter of 4 inches and a height of 8 inches, were manufactured for the purpose of evaluating their compressive strength at both 7 and 28 days. During the experimental trials, the water-cement (w/c) ratio was kept consistent, while different dosages of admixture were applied. The findings of the study indicate that the utilization of a two-stage dose of admixture resulted in enhanced and extended workability, along with higher strength of the concrete in comparison to specimens that did not incorporate any admixture. This research study enhances the comprehension of optimizing qualities of ready-mix concrete (RMC) by varying the superplasticizer, providing useful insights for the building sector.

Long term performance of recycled concrete beams with different water-cement ratio and recycled aggregate replacement rate

The purpose of this study is to reveal the service performance of recycled aggregate concrete(RAC)components for different values of water-cement ratio and replacement rate of recycled coarse aggregate(RCA).Generally,the concrete strength decreases with the increase of the replacement rate of RCA,in order to meet the strength requirements when changing the replacement rate of RCA,it is necessary to change the water-cement ratio at the same time.Therefore,the axial compressive strengths of prism with 25 mix proportions,the short-term mechanical properties and long-term deformation properties of reinforced concrete beams were tested respectively by changing water-cement ratio and RCA replacement rate.The bearing capacity and the strain nephogram of samples under different loads were obtained using the Digital Image Correlation(DIC)method,and a self-made gravity loading experimental device was used for long-term deformation investigation.Results showed that the damage pattern of RAC was the same as that of natural aggregate concrete(NAC),but the brittleness was more pronounced.The brittleness of concrete before failure can be reduced more effectively by adjusting the replacement rate of RCA than by adjusting the water-cement ratio.The water-cement ratio has an evident influence on the axial compressive strength and early creep of concrete,while the replacement rate of RCA has a remarkable effect on the long-term deformation of the concrete beams.

Experimental study on expansion and cracking properties of static cracking agents in different assembly states

Rolled static cracking agent(RSCA)can solve the intractable problem of traditional bulk static cracking agent(BSCA)in engineering applications.This paper innovatively studies the rational water-cement ratio of BSCA and the immersion soaking time of RSCA under the condition of controlling temperature.Through the expansion and cracking performance experiments,the development characteristics of expansion pressure,the cracking effect of the single-hole specimen and the performance of hole spraying prevention under the action of BSCA and RSCA were compared and analyzed.The results show that:(1)The volume growth rate of static cracking agent decreases with the increase of water-cement ratio,and the fluidity increases with the increase of water-cement ratio.The rational water-cement ratio for BSCA application is 0.3,and the rational immersion time of RSCA is 2-2.5 min;(2)Under the bore diameters of 30,35,40 and 45 mm,the expansion pressure of BSCA with a water-cement ratio of 0.3 is 38.2,52.3,61.5 and 68 MPa,and the expansion pressure of RSCA immersed in water for 2.5 min is 43.5,58.8,69.5 and 75.1 MPa,respectively.Among them,the development speed of expansion pressure of BSCA is higher than that of RSCA,and the arrival time of the peak expansion pressure of RSCA is 1.7 times that of BSCA;(3)The crack initiation speed of single-hole specimen under the action of RSCA is 10.3%lower than that under the action of BSCA,but the cracking speed of the former is 72.6%higher than that of the latter;(4)The hole spraying occurs in BSCA under the bore diameter of 50,55 and 60 mm,while the hole spraying occurs in RSCA under the bore diameter of 60 mm.In terms of bore diameter,the hole spraying prevention of the RSCA is better than that of BSCA.The research results enrich the static blasting technology and provide data support and theoretical reference for field application.

Experimental Study on the Expansion Behavior of Full Aggregate Steel Slag Concrete

To improve the utilization rate of steel slag,this paper investigates the expansion behavior of Full Aggregate Steel Slag Concrete(FASSC).Orthogonal experiment was performed to analyze the effects of sand ratio,water-cement ratio,steel slag sand content,replacement particle size of steel slag sand,coarse steel slag content,and replacement particle size of coarse steel slag on the expansion rate of FASSC.Test results demonstrated that the expansion rate of FASSC decreased with a gradual increase in the replacement particle size of steel slag sand or the coarse steel slag content.With a gradual increase in the water-cement ratio or the steel slag sand content,the expansion rate of FASSC first increased and then decreased.As the sand ratio increased,the expansion rate of FASSC first decreased and then increased.Among these,the steel slag sand content and the replacement particle size of steel slag sand significantly affected the expansion rate of FASSC.The mix proportions of SSC for self-stressing CFST and shrinkage compensating SSC were suggested according to the experimental research.Additionally,a formula for approximately predicting the expansion rate of FASSC was proposed based on the single factor weighted fitting.

Characterisation of Early Age Deformations in Cement Paste: Case of Chemical and Autogenous Shrinkage

Concrete is commonly seen as a durable and long-lasting construction material. However, the long-term performance of a concrete structure can be greatly compromised by early-age cracking. This work is an experimental contribution to study early age deformations of cement paste. Its aim is, firstly, to develop an experimental dispositive for assessing chemical and autogenous shrinkage, and secondly, to measure these volumetric deformations in cement paste. The setup was done following the gravimetric method of measurement, which exploits the Archimedes’ principle. It is made up of an electronic balance, a data accusation unit, a temperature control unit and a buoyancy bath. Investigations were done on Portland cement (CPA-CEM II) at the following W/C ratios: 0.25, 0.3, 0.35, 0.4 and 0.5. It was noticed that the water-cement ratio does not influence the magnitude of the chemical shrinkage in a significant manner but had a kinetic effect;a lower W/C induces a faster rate of chemical shrinkage. Autogenous shrinkage was discovered to be highly inversely proportional to the W/C and was also noticed to be in a function of chemical shrinkage within the first 2 to 4 hours when the paste was still liquid.

Preparation Process and Properties of Porous Ecological Concrete

By using waste bricks as aggregate,a kind of porous ecological concrete was prepared and the mixing technique and its properties were studied. Effects of the cement paste fluidity on workability of the porous ecological concrete were investigated,and influences of the water-cement ratio,the aggregate-cement ratio and the aggregate size on compressive strength and porosity of the porous ecological concrete were also discussed. Results show that for preparation of the porous ecological concrete,the aggregates should be enveloped with neat cement paste,and recommended fluidity of the cement paste is 200～235 mm. With the increasing of the aggregate-cement ratio,porosity of the porous ecological concrete increases while the 28 d compressive strength decreases. With increasing of the water-cement ratio,porosity of the porous ecological concrete decreases. And relationship between compressive strength and water-cement ratio of the porous ecological concrete does not follow Bolomey's formula. As a result of this study the porous ecological concrete is prepared,its porosity is 38.93%,the water absorption is 11.39% and the compressive strength is 1.14 MPa.

超低温冻融循环对砂浆性能的影响(英文)

水泥基材料超低温下的性能与其常温及低温下有很大差异。测量了不同砂浆的抗压抗折强度、相对动弹模量、相对质量损失率和含水率,研究了水融过程中吸水量、水灰比和最初含水率对砂浆抗超低温(-110℃)冻融性能的影响。结果表明:砂浆试样超低温冻融破坏比普通冻融破坏更加严重、迅速,且砂浆试样的抗折强度对超低温冻融更加敏感。试样在超低温冻融过程中逐渐从外界吸水,经过21次超低温冻融循环后,其吸水量甚至大于在真空饱水机中的吸水量。

低水胶比条件下水泥基复合材料的微结构形成机理

超高性能水泥基复合材料(UHPCC)由于其水胶比极低,水化进程明显与普通混凝土不同,故采用多种分析测试手段以期揭示其水化过程及微结构形成机理。结果表明,低水胶比条件下的UHPCC材料水化反应一直持续进行,随着养护龄期的延长,火山灰反应消耗了Ca(OH)2,孔隙率得以进一步降低(4%以下),界面得到强化。纳米力学性能测试分析表明,UHPCC材料存在大量未水化的水泥颗粒,且绝大部分水化产物为UHD C-S-H凝胶,未发现类似普通混凝土材料中的LD C-S-H凝胶,存在少量的HD C-S-H凝胶,且未水化水泥颗粒被水化产物UHD C-S-H凝胶紧密包围,不会对后期的耐久性能产生过多影响,因此UHPCC才显示出优异的力学性能和耐久性能。

Deformation field and crack analyses of concrete using digital image correlation method

The study on the deformation distribution and crack propagation of concrete under axial compression was conducted by the digital image correlation (DIC) method. The main parameter in this test is the water-cement (fT/C) ratio. The novel analysis process and numerical program for DIC method were established. The displacements and strains of coarse aggregate, and cement mortar and interface transition zone (ITZ) were obtained and verified by experimental results. It was found that the axial displacement distributed non-uniformly during the loading stage, and the axial displacements of ITZs and cement mortar were larger than that of coarse aggregates before the occurrence of macrocracks. The effect of W/C on the horizontal displacement was not obvious. Test results also showed that the transverse and shear deformation concentration areas (DCAs) were formed when stress reached 30%-40% of the peak stress. The transverse and shear DCAs crossed the cement mortar, and ITZs and coarse aggregates. However, the axial DCA mainly surrounded the coarse aggregate. Generally, the higher W/C was, the more size and number of DCAs were. The crack propagations of specimens varied with the variation of W/C. The micro-crack of concrete mainly initiated in the ITZs, irrespective of the W/C. The number and distribution range of cracks in concrete with high W/C were larger than those of cracks in specimen adopting low W/C. However, the value and width of eraeks in high W/C specimen were relatively small. The W/C had an obvious effect on the characteristics of concrete deterioration. Finally, the characteristics of crack was also evaluated by comparing the calculated results.