Abstract
Corrosion-induced concrete cover spalling represents severe deterioration of structural durability and has a considerably large impact on structural safety[1].However,compared to corrosion-induced cracking,fewer researches focus on this field.For predicting the concrete spalling,this paper proposes an analytical model based on mechanical analysis and experimental results.
Six concrete beams(180 mm×250 mm×1 200 mm)were casted and the dimensions as well as the reinforcement information are indicated in Figure 1.To accelerate the steel corrosion,the six beams were subjected to sustained loading coupled with wetting-drying cycles simultaneously.During the 4-day wetting cycle,the target corrosion area was wrapped by a sponge saturated with sodium chloride solution of 3.5%concentration and the sponge was unwrapped to let the oxygen into concrete during the 10-days drying cycles,as shown in Figure 2.After test terminated,a spalling test was conducted on the beams.The external force F was applied to the beams and the amplitude of the force F was recorded by the dynameter as shown in Figure 3,then the spalling bending moment was transferred to the spalling concrete by the steel rod and steel block,which can be calculated by Eq.(1)by choosing OF as pivot point.The crack patterns and the spalling concrete blocks were recorded subsequently as shown in Figure 4.
where MFa is the spalling bending moment;Fa is the force applied on the spalling concrete;La and Lm is the moment arm;Fe is the external force;Lr is the length of the rod.
Figure 1 Sample of a figure caption(unit:mm)
Figure 2 Accelerated corrosion method
Figure 3 Spalling test setup(https://www.daowen.com)
Figure 4 Example for cracking pattern and spalling concrete block
Followed by the spalling test,the geometrical criteria for concrete cover spalling was discussed,the mechanical interaction between the spalling concrete block and concrete beams were analyzed in order to establish the simplified spalling model as shown in Figure 5.The cohesive force in concrete,the cohesive force at the steel-concrete interface and the aggregate interlocking force were considered as the resisting force against spalling.The critical state for concrete spalling is defined as that,when the spalling bending moment MFa contributed by the external force exceeds the resisting bending moment MCon contributed by the resisting force,the spalling concrete block was separated from the main body of the concrete beams.All the bending moments were calculated by choosing OO'as the axis and the equation can be described as follows:
where M(tni)is the spalling bending moment contributed by the cohesive force at steel-concrete interface tni,M(tns)is the spalling bending moment contributed by the cohesive force on face CC'OO'tns,M(tnt)is the spalling bending moment contributed by the cohesive force on face AA'DD'tnt,and M(Vci)is the spalling bending moment contributed by the aggregate interlocking force on the side of the spalling concrete block Vci.
Figure 5 Mechanical analysis of spalling model
The bending moment M(tni)can be calculated based on the corrosion layer thickness[2],M(tns)and M(tnt)can be calculated based on the longitudinal crack widths[3]and M(Vci)can be calculated based on the transverse crack widths[4].The predicted value,resisting bending moment MCon and the tested value,spalling bending moment MFa are shown in Table 1 and a satisfied agreement is reached.It can be concluded that the proposed model predict the corrosioninduced spalling accurately.
Table 1 Comparison of predicted and tested values
