Research On Combined Permanent Formwork System Of Ultra-high Performance Concrete

In coastal concrete engineering,the problem of insufficient service life of concrete structures caused by chloride ion erosion is very serious,especially in tidal zone and wave splash zone,due to wave scouring and dry and wet cycle,the problem is more prominent.In order to ensure the service life of coastal water conservancy projects,the technical idea of making ultra-high performance concrete(UHPC for short)into permanent formwork to protect the concrete structure in tidal area and wave splash area is put forward.Considering the technical feasibility and economy,the design of the combined UHPC permanent template system was carried out,and the design,checking and test verification of the UHPC permanent template were carried out,and the joint filler of UHPC was developed.The main conclusions are as follows:(1)The combined UHPC permanent formwork system consists of a peripheral structure of UHPC permanent formwork,and the interior is filled with ordinary concrete;Through the design of the permanent formwork seam and the filling agent in the later period,the anti-chloride permeability of the seam can be guaranteed.UHPC permanent formwork is constructed by the existing climbing process,which is only used as the envelope structure and does not bear the structural load.While keeping the size of the original ordinary concrete structure unchanged,a layer of UHPC permanent formwork is added to the outer layer to improve the durability of the structure.(2)Referring to the existing steel template size,the UHPC permanent template size is 3m× 2.4m × 0.04 m,with 15 mm thick rib around the template and three vertical ribs in the middle of the template.The bearing capacity of the normal section is calculated and reinforced,and then the crack control and deflection are checked.The results are in line with the requirements of the code.The UHPC permanent template full-size specimen designed bears a load of 30 k N/m2,and its maximum deformation is 4.0mm.No cracks appear on the permanent template surface,which meets the actual use requirements.(3)Based on UHPC,UHPC filler with higher bond strength and smaller shrinkage can be prepared by adding additives such as expansion agent and cellulose ether;The expansion agent can not only reduce the shrinkage of the filler,but also produce hydration products to fill the inner part of the filler and improve the compactness.Cellulose ether improves the water retention of caulking agent,makes the hydration reaction at the bond interface fully proceed,and enhances the bond strength.Defoaming agent improves the compactness and strength of joint filler.Dispersible latex powder significantly improves the bonding strength of the filler.The additive content of the preferred filler is 6% of expansion agent + 0.1% of cellulose ether + 0.2% of defoamer + 2% of rubber powder.The fluidity during use should be controlled between 160mm-180 mm.(4)When the combined UHPC permanent template is used as the external column template,the ribbed permanent template is adopted to better solve the bonding problem between the permanent template and the post-pouring concrete,so as to ensure that the permanent template and the internal concrete form a whole and bear the force together.The cracking load is similar to the ultimate load compared with the casting column with the same bearing section.(5)When designing composite UHPC permanent template components,design should be carried out according to the original parameters of the structure;During construction,the UHPC permanent template shall be installed in the tidal area and the wave splash area;When the mold is removed,the steel formwork shall be removed and the permanent formwork shall be retained.When the volume of the internal concrete tends to be stable,the filling agent shall be carried out.