Study On Crack And Permeability Resistance Of Modified Carbon Fiber Reinforced Cement Based Materials

The basement of civil air defense needs to be designed with personnel residential areas,emergency rooms and material warehouses,and needs to meet the performance requirements of anti-corrosion,moisture resistance,and earthquake resistance,so the design of super large air defense basement is bound to face many problems.The early strain of the super long basement structure is obvious.The main reason that affects the concrete strain is the internal temperature change,which leads to the sharp increase of the temperature stress.When the tensile stress of the confined concrete exceeds the tensile strength of the concrete,the concrete will produce shrinkage cracks,which are easy to cause leakage and difficult to repair.Therefore,concrete crack control is very important for the quality of underground engineering.In this paper,a kind of modified carbon fiber is prepared,which is combined with expansion agent and waterproof agent to compensate shrinkage in the whole stage,reduce or even eliminate all kinds of shrinkage,and avoid cracking from the source,so as to improve the crack and permeability resistance of concrete.Firstly,the modified carbon fibers with nano-Si O₂and multi walled carbon nanotubes were prepared.The results of scanning electron microscopy(SEM)and transmission electron microscopy(TEM)showed that nano-Si O₂and multi walled carbon nanotubes were formed on the fiber surface,and the prepared nano-Si O₂particles are 40nm?150nm,and the outer diameter of the multi-walled carbon nanotubes are 20nm?50nm.The results of X-ray diffraction(XRD)analysis showed that the nano silica on the surface of the modified carbon fiber was not crystallized after microwave irradiation and still had activity.Fourier transform infrared spectroscopy(FTIR)analysis showed that nano-Si O₂and carbon fiber are connected by chemical bond.Through the improvement of process parameters,the best process parameters were obtained:ultrasonic time was 60min,tetraethyl orthosilicate(TEOS)dosage was 60ml,TEOS dropping rate was 3 seconds/drop,ammonia water dosage was 30ml,microwave radiation time was 10 seconds.Then,the modified carbon fiber is combined with expansive agent and waterproof agent to study the influence of the modified carbon fiber on the strength,shrinkage and impermeability of mortar.The results show that the composite of expansive agent,waterproof agent and modified carbon fiber has a significant effect on the performance of mortar.When the content of modified carbon fiber increases to 0.6vol.%,the mortar has the highest strength.Compared with group B,the flexural strength of mortar in group M-0.6CNT@NS@CF at 3 days increased from 5.7 MPa to 7.9MPa;the flexural strength of mortar in group M-0.6CNT@NS@CF at 28 days increased from 7.9MPa to 8.6 MPa.Compared with the group B,the early shrinkage of mortar in group M-0.6CNT@NS@CF at 1 day decreased from 86.973×10^-6to 26.205×10^-6,and the drying shrinkage at 90 days decreased from 1246.167×10^-6to 570.637×10^-6;at 28 days,the seepage height decreased from 9.95mm to 4.60mm.Through the analysis of non-evaporating water content,microscopic morphology observation and pore structure analysis,it is known that the addition of expansion agent increases the content of Ca O and Mg O in the cement.At the initial stage of hydration,Ca O and water generate Ca(OH)₂,the amount of calcium hydroxide crystal in the hydration product of cement increases,the volume of mortar expands and fills the pores,which makes the structure of cement paste compact;At the later stage of hydration,Mg O reacts with water to form Mg(OH)₂,which makes the volume of mortar expand slightly and the structure of cement paste become more dense,which improves the strength and anti crack and anti permeability of mortar.Most of the pores are filled by the active substances in the waterproofing agent through complexation precipitation,which leads to the decrease of porosity and the improvement of crack and permeability resistance of mortar.The nano-Si O₂on the surface of the modified carbon fiber reacts with Ca(OH)₂in the cement twice,which makes the CH grain refined,the number reduced and the structure more compact.The carbon nanotubes on the surface of the modified carbon fiber are bridged and lapped with the cement paste to form a network structure,which makes the bonding strength between the modified carbon fiber and the cement-based interface higher.Therefore,the strength and anti crack and anti permeability performance of the modified carbon fiber mortar are improved.