Prestress Loss And Capacity Of Pre-tensioned Concrete Beams With Corrosive Cracks

Bridge,as a structure used by people and vehicles to cross obstacles,plays a key role in the normal operation of road traffic.With the rapid growth of social economy,bridge engineering has unprecedentedly developed,and most of these are prestressed concrete bridges.Pre-tensioned concrete bridge is widely used in small/medium span bridge due to its superiority of convenient construction and mass precasting.The pre-tensioned concrete bridge has a small size and a thin protective concrete cover.Without the protection of corrugated pipe,the prestressed strand would be easy to corrosive.Strand corrosion can cause concrete cracking,degradae the bond performance at the strand-concrete interface,lead to prestress loss,and deteriorate the capacity.The present study mainly focuses on the concrete cracking,prestress loss and flexural capacity of pre-tensioned concrete beams after strand corrosion.The main research works are as follows:(1)The relationship of corrosive crack width and filling proportion of corrosion products has been established.The effect of stirrups on corrosion-induced cracking has been investigated based on the accelerated corrosion test.The corrosion morphology of strand is observed after the accelerated corrosion test.The filling of strand corrosion products during concrete cracking is investigated.The effects of stirrups on the filling of corrosion products and concrete cracking are clarified.The filling extent of corrosion products varies with crack propagation.The rust filling extent increases with crack propagating until a critical width.Beyond the critical width,the rust filling extent keeps stable.The critical crack width decreases by 20%using stirrups.An empirical model for crack width is developed,incorporating the filling proportion of corrosion products and twisting shape of strand.The proposed model is verified by experimental data.The prediction of corrosion-induced crack is sensitive to the rust filling extent.The prediction model should rationally incorporate the filling effect of corrosion products.(2)The effect of prestress on corrosion-induced cracking has been clarified.Strand corrosion-induced concrete cracking under various prestress has been investigated experimentally.The expansion ratio of strand corrosion products is estimated based on the infrared spectroscopy and thermal gravimetry.Prestress can increase the proportion of iron oxide and decrease the proportion of iron hydroxide in corrosion products.These changes have a slight effect on the expansion ratio of strand corrosion products.Strand rust expansion-ratio in the electrochemically accelerated corrosion conditions is proposed as 2.78.The effects of prestress on critical time of cover cracking and crack widths are clarified.Prestress can accelerate the corrosion-induced cracking process.By varying prestress from 0 to 75%strand tensile strength,the critical time of cover cracking decreases by 22%and the crack propagation rate increases by 9%.(3)The analytical models based on elastic and fracture theory are proposed to predict prestressed concrete cracking induced by strand corrosion,incorporating the prestress and geometric properties of the strand.A reduction factor is employed to reflect the residual tangential stiffness in cracked concrete.The effects of parameters on corrosion-induced cracking are discussed,such as prestress,concrete cover,concrete tensile strength,strand diameter and rust-expansion ratio.Prestress has an adverse effect on corrosion-induced cracking.Varying prestress from zero to 75%strand tensile strength,the corrosion losses of micro-crack formation,cover cracking and 0.1 mm crack width decrease by 46%,44%and 9%,respectively.The critical corrosion losses at the three stages increase with increasing concrete tensile strength and concrete cover,and decrease with increasing rust-expansion ratio and strand diameter.(4)Strand corrosion expansion can lead to prestress loss.A novel model is proposed to predict the corrosion-induced prestress loss in pre-tensioned concrete beams.The coupling effects of concrete cracking and bond deterioration are incorporated into the model.The prestress loss in corroded strand could be evaluated based on the strain compatibility and force equilibrium equations.The effective prestress in eight corroded pre-tensioned concrete beams under various stress levels is explored by the four-point flexural test.The experimental results are employed to verify the proposed model.Prestress loss in corroded pre-tensioned concrete beams depends on corrosion level.Corrosion-induced concrete cracking would not degrade bond strength and effective prestress when the corrosion level is less than 6.6%.As corrosion further progresses,bond strength and effective prestress reduce monotonically,and then decrease to zero when the corrosion level reaches 34.0%.(5)An analytical model,incorporating the effects of strand cross-section reduction,material deterioration,concrete cracking and bond degradation,is developed to predict the flexural capacity of corroded prestressed concrete beams.Additionally,the effects of flexural cracks can be also included in the model.Corrosion-induced cracking and bond degradation have been evaluated based on the thick-walled cylinder theory.An equivalent bond strength concept is introduced to consider the effects of flexural cracks,which makes it possible to evaluate the bond force of corroded strand for the flexural capacity calculation.A compatibility coefficient is introduced to quantify the incompatible strain between corroded strand and concrete under the ultimate state.Considering the compatibility coefficient,the conventional strain-compatibility method can be modified to predict the flexural capacity of corroded prestressed concrete beams.The flexural capacity deterioration of prestressed concrete beams depends on corrosion degree.Strand corrosion less than 5.5%can lead to a slight decrement of flexural capacity.As corrosion progresses,the flexural capacity would exhibit a significant deterioration.