Research On Basic Principle And Design Method Of UHPC Strengthening For In-service Cracking Orthotropic Steel Decks

The orthotropic steel deck(OSD),as an integral part of the steel bridge,has two critical diseases due to its low stiffness: fatigue cracking and asphalt pavement damage.The development and use of steel-ultra high performance concrete(UHPC)lightweight composite deck(LWCD)has effectively solved the problems,but it is only used for new construction or in-service OSDs with slight cracks.However,numerous steel bridges were constructed in the 1990s in China’s transportation hubs and have remained in service for more than 20 years.Many penetrating and invisible cracks are difficult to identify and repair completely.In this case,it is difficult to apply the LWCD because steel deck plates with significant cracking issues lose protection for the UHPC layer,resulting in a failure of crack resistance at the bottom surface of the UHPC to meet the requirements.In this paper,a novel UHPC structure was proposed to strengthen in-service cracked OSDs.In this structure,transverse steel strips with studs are employed to strengthen the bottom surface of the UHPC layer.Combined with the original steel bridge deck,a steel strip-UHPC composite bridge deck is formed.Under the premise of ensuring the safety of the dead load of the main beam,the fatigue stress in the OSDs can be greatly reduced,and the expansion of existing cracks can be effectively inhibited.Meanwhile,the crack resistance of the bottom surface of the UHPC layer can be greatly improved without repairing the original cracks in the OSDs.Consequently,the problem of OSDs with significant cracking issues can be resolved successfully.The main contribution and innovation are as follows:(1)A modified steel strip-UHPC composite deck(SUCD)was proposed.By means of the sub-model technology of ANSYS,a finite element analysis(FEA)was performed according to the actual situation of penetrating cracks in the steel deck plates in order to analyze the stress of the OSDs with significant fatigue cracking,and the bottom surface of the UHPC layer.The SUCD improves the fatigue performance significantly by reducing the stress distribution of the fatigue details in the rib-deck weld by 78.8%～86.4%,as indicated by the results.However,when the steel deck plates with significant cracking issues lose protection for the UHPC layer,the transverse tensile stress at the bottom of the UHPC layer approaches 12.9 MPa.To improve the crack strength of the UHPC layer,a new structure of the UHPC layer strengthened with steel strips was proposed.(2)The static flexural behavior of the SUCD was investigated.The transverse bending static tests under the sagging moment were conducted to obtain the influences of primary parameters on the cracking characteristics of the UHPC layer.To be specific,the test parameters were set as width of the steel strip and thickness of the UHPC layer.The test results showed that the steel strips significantly improved the crack resistance of the bottom surface of the UHPC layer.The average crack spacing of the UHPC layer was decreased effectively by enlarging the width of steel strips,significantly inhibiting the spread of cracks width.The longitudinal flexural performance of SUCD was investigated,and failure of OSDs was shown to be the control index that led to SUCD failure.The problem of Force concentration occurred at the joint surface of the steel strips edge and the UHPC layer was discussed.Based on the section balance theory,the calculating theory and method of the ultimate bearing capacity were proposed.The calculation approach of ultimate bearing capacity agreed with the observed data.(3)The calculation theory for the crack width of UHPC layer in SUCD was established,upon which the development of crack width in the UHPC layer was predicted.Based on the phenomena of slip and strain uniformity between the steel strips and the UHPC layer,it is found that the cracking of the UHPC layer in SUCD has no direct connection with the slip of steel strips,and the cracking mechanism of the UHPC layer is revealed.A method for calculating the crack width in SUCD is presented based on the principle of crack width of UHPC structure without reinforcement in French UHPC Code(NF P 18-710)and the assumption that steel strips are exclusively utilized to strengthen the UHPC layer.The average crack spacing correlation coefficient in the recommended formula is rectified,and a calculation method for the tensile strain of the bottom UHPC layer is suggested.(4)The transverse bending fatigue behavior of the UHPC layer in SUCD was investigated,and the S-N curve of SUCD fatigue life evaluation was generated.Four specimens were constructed with the stress ratio "S" as the test parameter under different stress amplitudes to evaluate the fatigue properties of the SUCD,such as stiffness,fracture propagation,and residual strength.The current fatigue life evaluation method was used to directly analyze the fatigue life of four specimens,and the results were quite divergent.The fatigue endurance limit of the SUCD was identified when the maximum crack width of the UHPC layer reached 0.05 mm.The linear regression approach was used to fit the S-N curve for fatigue evaluation of the SUCD,and the correction coefficient of determination(Adj.R-square)was 0.99.Therefore,the curve fitting results possess high precision.(5)In the specific case of the practical application project of Yichang Yangtze River Highway Bridge,a full-scale model test was carried out,which included seven U ribs and two diaphragms.The effectiveness of SUCD in preventing the further spread of fatigue cracks in the original OSD was proven by comparing the fatigue performance of OSDs before and after strengthening by SUCD.When the load cycle was accumulated to 2 million times under the load amplitude of 1.0 Fr with Fr =120kN as the base load,no crack was discovered in the fatigue details.When the load amplitude was increased to 1.8 Fr and the load cycles were accumulated to 3.15 million times,fatigue cracks formed near the cutouts.The crack propagation rate was 0.61 mm/ 10000 times.After the SUCD was formed by casting UHPC,the existing crack only extended 1.2 mm after 2 million load cycles,and the crack growth rate decreased to 0.0095 mm/10000 times,a decrease of 98.4%.