Abstract

Alkali-silica reaction(ASR)and delayed ettringite formation(DEF)are both serious durability problems for concrete structures and surface map cracking is their common feature.The problems have been reported in several countries for the respective damage types and they were also observed occur simultaneously onsite with the effect of interaction.For ASR,the damage mechanism is due to the formation of ASR gel from the reaction of siliceous minerals in aggregates and alkali ions in cement.The damage is usually initiated at the aggregate particles and interfacial transition zone(ITZ).On the other hand,DEF is caused by the elevated temperature curing(normally over 70℃)of concrete.Under the high temperature,the formed ettringite will be decomposition and delayed formed after the temperature cooling down.The delayed ettringite formation can cause destructive pressure and cracks after the concrete is hardened.Both types of damage can cause the cracking of concrete inside,which is the reason of significant reduction in concrete strength.Due to the localization of expansion inside,the map cracking pattern is observed at the surface of concrete.However,the stress development with expansion inducing the initiation and propagation of cracks remains unclear since the cracks distribution inside is difficult to detect by experimentation.Also,the quantification of each expansion ef fect is hard to achieve by setting parameters,as well as the mechanical property evaluation of concrete under a couple of ASR and DEF ef fects[1].

Since the cracking is of great importance to understanding of the mechanical degradation and hardly to be obtained by experimentation,it is wise to choose a computer-aided tool to simulate the cracking behavior of concrete.In our previous studies,we found that the rigid body spring model(RBSM),a discrete analysis method,has large advantages in simulating the small deformation and cracks.The concept of RBSM was first proposed by Kawai[2].And then Nagai and his coworkers made their ef forts to develop the model from 2D to 3D and to simulate the mechanical performance from concrete materials to complicated reinforced concrete(RC)structures[3].It has been successfully used to study the frost damage and fatigue damage of concrete,the corrosion of rebar induced bond deterioration and the anchorage effect on beam-column joints and so on.After almost two decades of development,the RBSM can simulate the behavior of RC structures in three-dimensional and visualize the stress and deformation performance.But for the complicated durability problems,the cracking nature has not been paid much attention yet.To achieve a unified model,the RBSM can be further developed to study more comprehensive durability problems and to be a very useful tool for lifetime evaluation.

Therefore,in this study,the stress and crack development due to ASR and DEF are investigated based on a discrete analysis by using the RBSM.The initial strains are applied to the springs of ITZ or interface of mortar elements to introduce the ASR and DEF damage,respectively.Also,regarding the localization expansion in each damage,the percentage of reactive coarse aggregate and intensified expansion area are used for ASR and DEF simulation,respectively.Then the parametrical study on a coupled effect of ASR and DEF are conducted.As shown in Figure 1,the surface cracking behavior of specimens under the combined effects can be obtained.After the ASR/DEF damage,the compressive loading is simulated,the internal stress and cracking development of damaged concrete are also recorded,as presented in Figure 2.Besides,the confinement effect of the respective factor can be found from the compressive strength reduction tendency with consideration of different degree of ASR damage,as can be observed in Figure 3.The simulated results are significant to the accurate damage evaluation and clarification of the difference in reduction of mechanical properties of concrete suf fered from different durability problems.

Figure 1　Surface cracking pattern of concrete damaged by(a)ASR(b)DEF(c)ASR+DEF

Note:A-ASR expansion,D-DEF expansion,Number-the expansion ratio for the certain case.

Figure 2　Internal stress and cracking development of ASR,DEF,and ASR+DEF damaged concrete(middle cross-section)under compressive loading(the expansion of A0.8D0.8 is 1%)

Figure 3　Normalized compressive strength change with expansion for concrete specimen with a coupled effect of ASR and DEF