Self-restraint caused by differential shrinkage along the radius of a concrete ring has great driving effect on the restrained cracking in the traditional ring test. In order to estimate cracking potential of concrete under restrained shrinkage, i.e. steel ring restraint effect, an improved circular/elliptical ring test was proposed by pre-setting a 6 mm long real crack at the inner circumference of the concrete specimen. Meanwhile, a numerical study based on fracture theory was conducted to investigate the cracking mechanism of the improved ring test. By introducing the fictitious crack model, the crack initiation age and propagation process in the ring specimen were simulated using the initial fracture toughness-based cracking criterion. The driving forces of the shrinkage cracking, provided by self-restraint and steel ring restraint, were analyzed and the influence of the pre-set crack length on the cracking age was discussed. The results indicated that, different from the traditional ring test, the shrinkage crack firstly initiated from the pre-set crack and then unstably propagated toward the outer circumference of the concrete in the improved method. In the crack propagation process, the self-restraint provided cracking insistence rather than driving force on the shrinkage cracking due to the bending effect along the radial direction caused by non-uniform moisture distribution in concrete. Therefore, the driving force of the shrinkage cracking was totally provided by the steel ring restraint and would overcome the cracking resistances from concrete material and non-uniform shrinkage. Furthermore, the advantage of the elliptical geometry in increasing the restraining level was still available for the improved ring test with a pre-set crack, which could make the shrinkage cracking occur in advance and enhance the steel ring restraint.

在传统的环试验中，沿混凝土环半径收缩收缩引起的自约束对约束开裂具有很大的驱动作用。为了估计在收缩收缩即钢环约束作用下混凝土的开裂可能性，提出了一种改进的圆/椭圆环试验，方法是在混凝土试样的内周预先设定一个6mm长的真实裂纹。同时，基于断裂理论进行了数值研究，以研究改进环试验的开裂机理。通过引入虚拟裂纹模型，使用基于初始断裂韧度的裂纹准则模拟了环形试样的裂纹萌生年龄和扩展过程。自约束和钢环约束提供的收缩裂纹的驱动力，进行了分析，讨论了预设的裂纹长度对开裂时间的影响。结果表明，与传统的环试验不同，采用改进的方法，收缩裂纹首先从预设裂纹开始，然后不稳定地向混凝土的外圆周传播。在裂纹扩展过程中，由于水分在混凝土中分布不均匀而引起的沿径向的弯曲作用，自约束提供了对裂纹的坚持，而不是对收缩裂纹的驱动力。因此，收缩裂纹的驱动力完全由钢环约束提供，可以克服混凝土材料的抗裂性和不均匀的收缩。此外，