Study Of Sea Sand UHPC And Its Application In Utility Tunnel

The river sand is exhausted rapidly,which creates a serious imbalance between supply and demand of engineering construction.The sea sand is rich in China coastal regions;therefore,the replacement of river sand by sea sand in engineering is possible to relieve the stress on supply of construction sand.However,chloride-rich salt in the sea sand is likely to bring about the corrosion of steel bars.Conventional desalination method of sea sand needs a large consumption of freshwater and a high cost,which cannot satisfy the rapid constructions in island-type cities.Utilization of sea sand in ultra-high performance concrete(UHPC)which is a new cementitious composite material remarked by ultra-high strength and high density,in theory,is possible to prevent the internal rebar from contacting with the external water and oxygen,and to avoid the erosion by the chloride ion;thus,sea sand UHPC shows a high potential in engineering construction.In this paper,the material property,durability and micro structure of sea sand UHPC were investigated firstly;then,the feasibility of applying sea sand UHPC to the utility tunnel was discussed;in the end,the trial design with the analysis of life cycle cost and the construction performance were carried out.The main conclusions were listed as follows:In comparison to the river sand,sea sand UHPC and C40 concretes were prepared,and the tests of mechanical properties,electric flux,electrochemical performance of steel corrosion and pore structure were carried out.The results show that the 28d compressive strengths of sea sand UHPC and C40 concretes were closed to those using river sand(diflference<5%).With the increase of chlorine salt content,the resistances to chloride ion permeability in UHPC and C40 concretes were gradually reduced,but the decrease magnitude of UHPC was weaker than C40 concrete,which means the anti-permeability performance of UHPC is more superior.The main pore size of C40 concrete was between 20 nm and 200 nm,while the pore size of UHPC was primarily less than 20 nm,which represents that UHPC had a better pore size distribution and compactness than C40 concrete.Contrary to the corrosion state of steel bars in C40 concrete prepared by sea sand,the steel bars in the sea sand UHPC were in the passive level from 7 days,which indicates that using of sea sand in UHPC has little effect on the corrosion of steel bar.The trial designs of utility tunnel made of river sand UHPC and sea sand UHPC with 2%steel fiber were carried out The results show that the utility tunnel with river sand UHPC was possible to meet the regulatory requirements;comparing to the C40 concrete utility tunnel,its own weight could be reduced by 45%,but the cost was increased by 68%,therefore,the bad economic influence of river sand UHPC might be make it inapplicable.The trial design of sea sand UHPC utility tunnel also satisfied the specification,however,it was presented that its cost firstly decreased and then increased with the section thickness increased.The relationship between cost and section thickness was proposed,showing that the suitable thickness of sea sand UHPC corridors which had lowest cost was between 17.5 cm and 18 cm.Compared to the C40 concrete utility tunnel,the life cycle cost of the optimized sea sand UHPC utility tunnel was reduced by 38%;the number of joints in the precast segment for each kilometer was decreased by half;in the same time,the maintenance period could be reduced by 66%,and the construction time was saved by 53.50.As previously mentioned,it was found that the sea sand UHPC utility tunnel has a more considerable advantage and a broader application prospect than C40 concrete utility tunnel in the level of materials(material property,durability,microstructure)and structure(structural force,economic performance and construction performance).