Research And Application On Volume Stability Of Gradient Structure Concrete

With the acceleration of urbanization, the problem of land resource shortage have become more and more serious, and the exploitation of the space underground has been an inevitable current. Due to the worse service environment and the longer service life of concrete structure in Seaport Engineering, it requires high durability. And the most important thing of its durability study should be volume stabilization of concrete. But, considering that there are few researches on volume stabilization of gradient structure concrete (hereinafter referred to as "GSC") nowadays, this paper according to Wuhan Yangtze River Tunnel Project and National 863 Project (Structure Design of highly anti-resistant, long life and large diameter tunnel segment material and Its Application), the graded structure concrete based on volume stabilization were designed, studied and applied in this thesis. The main achievements are listed as follows:1. Design idea and method of GSC based on volume stabilization was put forward for the first timeOn the basis of capillary tension theory and crystallization expansion mechanism, the Physical Model of volume deformation was build. And by mechanism analysis on destruction of concrete volume deformation, controlling technique of expansion and theory of crack refinement and self-healing were put forward which will provide theoretical basis for preparation of high volume stability concrete combined with the existing theory of shrinkage-compensating, one after another hydration and thermal stability.Leading the conception of gradient structure into the concrete design, this thesis put forward method and principle of gradient structure design which should require the protective layer of concrete has the characteristics of high crack resistance, self repair and compactness for its shelter function, and require the structure level matching to curing system for its structural support function. As a result, structural system of GSC was established.2. A new kind of anti-cracking and self-repairing additive (hereinafter referred to as "ACSH") was developed by single-factor test, using small molecule polyether and ultra-fine powder material containing calcareous and sialic resource as inhibitory component of deformation, organic hybrid fiber and "block" type repairing material with in-situ reaction as anti-cracking and self- repairing component.As compared with reference concrete with 25% fly ash, the result showed that:â‘ Mechanical PropertiesCompressive strength, flexural strength of the concrete mixed with 25% ACSH were increased by 119% and 144% at 12 hours, and enhanced by 7.5% and 9% at 28 days respectively under the natural curing conditions in autumn.â‘¡Volume StabilizationThe shrinkage was declined by 67%, 31% and 23% at 1, 28 and 90 days respectively. Moreover its expansion value in boiling case at 80℃was decreased by 25% at 14 days.â‘¢Cracking SensitivityThe cracking time was prolonged by 26%, and its cracking width was declined by 50%.â‘£Self-Repairing AbilityThe strength recovery ratio was increased from 0.72 to 0.92, and it was enhanced by 25%.3. The mechanical property, specific creep and shrinkage deformation of meso-defect interface transition zone free cement based material were investigated against protective layer of GSC. And the regularity of their growth with ages was analyzed, as follows:â‘ Elastic ModulusAccording to Mosley model, experimental results were regressively analyzed to reach a regularity of elastic modulus growth with ages, and the regularity is E_t=E₂₈[0.40+0.19×log(t)] (t≤28) (R²=0.96264), E_t=1.019 E₂₈ (t＞28).â‘¡Split StrengthConducting regressive analysis of experimental results by ACI C209, the regularity of split strength growth with ages is f_sp=6.92t/(1.7+t) (R²=0.98374).â‘¢Early Specific CreepAccording to hyperbolic model proposed by ACI (1978) to conduct regressive analysis of experimental results, the regularity of early creep growth with ages is C_t=38.7t/(6.3+t)(R²=0.98934).â‘£Shrinkage DeformationAccording to hyperbolic model proposed by ACI (1978) to conduct regressively analyze experimental results, the regularity of shrinkage growth with ages isε=560.2t/(8.2+t)(R²=0.99561).4. According to steam-curing usually used in tunnel segment concrete, the temperature history in interior of steam curing concrete taking account of synergistic effects of hydration heat of cement and temperature of steam-curing were simulated. And effect of material constituents on stabilization of ettringite in structure level was investigated in this temperature history.The result indicated that:â‘ Length ChangesUnder the water environment at 40℃, for pure cement system and shrinkage compensating system mixed singly with UEA which were cured in simulated temperature history, their swelling strain stared increasing again at about 60 days after the stabilization of first rapid swelling. While the system mixed with mineral additive, such as slag, might inhabit obviously the later overexpansion. Moreover, the NS additive might accelerate early expansion, and also stabilize the later expansion, which were beneficial for volume stabilization of concrete.â‘¡XRD AnalysisFor pure cement system and shrinkage compensating system mixed singly with UEA which were cured in simulated temperature history at an early age, they were susceptible to crystal shape transformation between ettringite and calcium monosulphoaluminate (hereafter referred to AFm), which is to say, ettringite formed in the early hydration period decomposed to AFm when it had experienced elevated temperature, and following ettringite decomposed by high temperature was re-crystallized during cooling; While they were cured in simulated temperature history after sealing, ettringite decomposed during elevated temperature was not re-crystallized during cooling.Moreover the NS additive might enhance the stabilization of ettringite and passivate the temperature effect of crystal shape transformation.â‘¢SEM AnalysisFor pure cement system and shrinkage compensating system mixed singly with UEA, the needle-like expansive ettringite crystals were formed. While in cement system mixed with mineral additive the large plate-like non-expansive ettringite crystals were formed.Moreover, adopting sealed curing, the leaf-like AFm crystals were found in pore of pure cement system during cooling, but the needle-like ettringite wasn't.5. Anti-cracking under multiple factor action and volume stabilization of GSC was appraised, and it was applied to Wuhan Yangtze River Tunnel Project.â‘ The matching of volume deformation between protective layer and structural level of GSC was first studied, and it was estimated by interface mechanics. Moreover the calculating result indicated that the optimized GSC had good compatibility of volume deformation, and it didn't cause the inconsistency damage.â‘¡Due that most concrete was exposed to environment from a single side, in this thesis the shrinkage deformation of concrete in underground engineering with High-Pressure and Rich Water after dehydration was estimated by a single side exposed dry environment, at the same time, the swelling deformation of its concrete after High-Pressure and Rich Water was also estimated by a single side exposed groundwater environment. The test result showed that as compared with pure cement system, the shrinkage of GSC under the condition that the single side was exposed to dry environment was declined by 26%, 30% respectively at 28, 90days, at the same time, the swelling of GSC, in which the single side was exposed to groundwater environment was declined by 60% at 44 weeks. While as compared with cement system mixed with 25% slag, the shrinkage was declined by 12%, 17% respectively at 28, 90days, and the swelling was decreased by 60% at 44 weeks. This indicated that FGSC could inhabit the long term volume deformation, and benefit for its volume stabilization.â‘¢by the restrained slab test, specimen was adopted to fulfill the comparative test for single factor evaluation on anti-cracking Performance under the stable environment (temperature 30±1℃, relative humidity 50±5%, wind speed 2m/s).The results showed that the visible crock appeared in GSC stayed in this environment for 14 days, but the visible crack with 0.2mm width appeared in 6 hours for cement system mixed with 25% slag.â‘£the segment prepared according to 1:5 dimension scales was adopted to fulfill the comparative test for multi-factor evaluation on anti-cracking performance under the combined action of underground and temperature cycle, additionally damage degree of micro-crack caused by underground and temperature cycle was characterized by the change rate of ultrasonic velocity and electric flux.The results showed that by means of change rate of ultrasonic velocity, the average damage degree of GSC system was 0.18, but the cement system mixed with 25% was 0.38. Moreover, the electric flux of GSC system was increased from 149 Q to 252 Q, and its average damage degree was 0.70, while the cement system mixed with 25% was increased from 1267Q to 2728Q, and its average damage degree was 1.15.It indicated that GSC system had a better anti-cracking performance⑤The segment prepared according to 1:1 dimension scales using GSC system was applied to the Wuhan Yangtze River Tunnel Project, and drying shrinkage ofthe segment concrete was 195, 245 respectively at 28, 90 days.6. Comparing and analyzing some measuring means of volume deformation for cement-based materials at home and abroad, a kind of online monitor for monitoring volume deformation of cement-based materials in multiple environments and its method was developed.â‘ Use environment simulation technology to simulate the engineering environment such as temperature, humidity, and gas intensity, etc.â‘¡Use micro-distance measuring technology of laser and eddy current to enhance the serviceable range of this development such as high temperature, high relative humidity and dust environment.â‘¢Use the multi-channel design and auto-control method to gather experiment data automatically, monitor on-line, at the same time, decline the measuring cost, enhance the service efficiency.â‘£By means of the modified mould, it needn't to break the mould or move the specimen during measuring process, which could avoid external disturbances. Meanwhile, it could prevent gauge heads from leaping and leaning when the specimen was molded and thereby the testing accuracy could be enhanced.⑤This testing method and the equipment is also suitable for measuring deformation of materials such as glass, ceramics, stone materials, walling materials and etc.