Research And Development Of The Performance Improvement Of Concrete Cover For The Constructions In Marine

The durability survey of buildings in the marine environment at home and abroad indicates: many concrete structures appeared premature destruction and deterioration in 20 to 30 years after being built, far from reaching the designed service life and had to be rehabilitated costly. The main reason for premature deterioration is steel corrosion caused by chloride. To prolong the service life of buildings in marine effectively and economically, the pivotal approach is to improve the performance of cover concrete. The research of this paper comes from the project of "The parameters and construction techniques research on the concrete structure durability design in the Southeast coastal railway" supported by the Ministry of Railway. In order to improve the impermeable and cracking resistance performance of the cover concrete, The high performance concrete with high impermeable and cracking resistance and its factors and the new high durable firm shelling Separating—Coupling Cover Structure (SCCS) which will provide the more credible and effective protection for the internal reinforcement bars are studied by adopting both new materials and construction techniques. The main work and achievements of this paper are as follows:1. Three curing methods are used for concrete curing which are wet cotton mats, a curing agent and plastic film. The strength, shrinkage, central temperature and chloride permeability under various curing methods have been studied. The effect of wet curing duration on the strength, impermeability and corrosion resistance for protecting internal reinforcement of plain and slag-cement concrete (in which 40 % cement was replaced by blast furnace slag) are studied by experiments including strength, permeability, half-cell potential and concrete cover cracking time in the accelerated corrosion test. The results show the strength and chloride impermeability of concrete cured with cotton mats is the highest, while concrete cured with curing agent shows the lowest strength and chloride impermeability. Concrete cured with curing agent starts shrinking earlier and more quickly than the others, while the early shrinkage and its developing speed for concrete cured with wet cotton mats is the lowest. For the central temperature, concrete cured with plastic film is the highest, and then that cured with curing agent. Concrete cured with wet cotton mats shows the lowest. Plastic film shows the best effect for heat preservation, which may be very useful for concrete cast in a cold environment. Longer wet curing duration is essential to achieve higher strength, durability and corrosion resistance characteristics especially for slag-cement concrete. The plain cement concrete is relatively more tolerant to inadequate curing. The 14 days wet curing or more is necessarily especially when constructions in the deleterious environment.2. The influences of different formwork materials including Wood (W), Bamboo (B), Steel (S), Plastic (P) and Controlled Permeability Cloth (CP) on the performance of the cover concrete are studied. The results show the cover concrete molded by CP is the densest and without any cracks on the surface. The strength and permeability of cover concrete molded by W, B, S and P are similar; however, those of cover concrete molded by PC are significantly improved. This improvement is mainly due to water seeping, air passing and wet curing action of CP on the fresh concrete.3. The Deep Penetrating Sealer (DPS) can increase the strength and impermeability of cover concrete effectively. The advisable spraying dosage range of DPS is 200 g/m²ï½ž300 g/m². The advisable schedule to spray DPS is during the 7～14 days after the concrete casting. If the cover concrete is in the surface dry saturated condition when spraying DPS, the effect will be better.4. The tidal zone of marine constructions used to be destroyed firstly. Polypropylene fibers (PPF) reinforced Silica Fume (SF)—high performance concrete is used to place the zone tentatively. The results show the addition of both PPF and SF significantly increase the concrete chloride resistance. The incorporation of fibers improves the performance of cracking resistance at the early age significantly, reduces the adverse influence of improper placement or consolidation and materials sedimentation on the homogeneity of concrete, however, just incorporating PPF has no obvious improvement on strength and impermeability and induce more porous fiber-matrix interfacial zone (FMIZ) by PPF which is detrimental to the chloride resistance, but the microstructures of both the matrix and the FMIZ in the concrete with PPF and SF are denser. Thus it is concluded that the significant increase of the chloride resistance of fly ash concrete due to the simultaneous addition of PPF and SF results from both the anti-cracking effect of PPF and the formation of denser microstructures of both the matrix and FMIZ by the high reactivity of silica fume.5. The new high durable firm shelling Separating—Coupling Cover Structure (SCCS)and its construction method have been designed based on the above reaearch. The materials of cover and the internal are dissimilated through the subtle Separating—Coupling steel meshwork. The concrete cover has been cast by Self—Compacting Polypropylene Fiber Reinforced Silica Fume High Performance Cement—based Materials (SPSM).The upper and lower layers of the construction can place SPSM directly; the Separating—Coupling steel meshwork is used to separate the flowing concrete when the lateral cover and internal are placed by SPSM and ordinary cement concrete at the same time, respectively. The loading transferred reinforcement bar can solve the problem of the lower interfacial bonding strength caused the Separating—Coupling steel meshwork more than sufficiently. The simulative tests show the mechanical and steel bar protection performance of SCCS is better. The SCCS can improve the performance of the cover concrete and prolong the service life of the constructions significantly more effectively and economically.6. The mixing proportations and technique of SPSM have been studied. The main performance index of SPSM: self—compacting, the compressive strength of 28th day is more than 55 MPa, the chloride diffusion coefficient of 90th day is less than 1.0×10^-12m²/s, the grade of anti-cracking is the first level and the shrinkage value of 90th day is less than 400s.