Research On Fracture Behavior Of Two-lift Concrete Pavement

Two-lift concrete pavement is a new type of pavement paved with two different kinds of fresh concrete(wet-on-wet).2LCP not only can use regenerate pavement materials as the bottom layer,which can reduce material costs and protect the environment,but also can reasonably choose top layer materials and improve surface functions.As a new type of pavement structure,2LCP has wide application prospects.Cement concrete pavement in frozen area,due to the comprehensive effect of freezethaw cycle and vehicle load,often suffers structural destruction within a short period,its service life is far less than the design life.For this reason,this paper proposes to adopt 2LCP structure to optimize the design of surface concrete so as to improve the structural safety and durability of pavement.The main study of this paper are as follows:Use the polypropylene fiber concrete as the top layer of 2LCP,the shrinkage,frost resistance and mechanical properties of portland cement concrete(PCC),polypropylene fiber concrete(PFRC)and two-lift concrete(2LC)has been studied.The results show that compared with PCC,the use of PFRC as the top layer of 2LC can effectively inhibit shrinkage,improve frost resistance and mechanical properties.The fatigue life of PCC,PFRC and 2LC with different degree of freeze-thaw damage was tested,and the fatigue equations of PCC,PFRC and 2LC considering freezw-thaw cycles were established.The results show that: the effect of freeze-thaw cycles will significantly reduce the fatigue life of concrete;compared to PCC,the fatigue life of PFRC and 2LC have different degrees of increase;compared with PCC,PFRC and 2LC can inhibit the attenuation of fatigue life under the freeze-thaw cycle attack.The fracture properties of PCC,PFRC and 2LC after freeze-thaw cycles were studied.The results show that: PFRC and 2LC with polypropylene fiber,the fracture toughness and fracture energy of PCC is improved,using 2LCP structure can effectively resistant the pavement from top-bottom cracking.Progressive damage model is established and verified based on the finite element model of three-point bending beam fracture,sensitivity analysis of main influence factors on the fracture parameters of 2LC was carried out.The results show that the fracture toughness and fracture energy do not have the size effect when the height of the beam is more than 200 mm,and the height ratio of the initial crack does not affect the fracture parameters;the increase of the thickness of the PFRC layer can effectively improve the fracture performance of the 2LC;the strength of the PCC layer has a great influence on the fracture parameters;the maximum diameter of the aggregate is 20mm;is best admixture of polypropylene fiber is 0.66‰.Stress analysis of 2LCP structure was carried out,and the influence of pavement structural parameters on the required PFRC thickness and tensile stress at the bottom of the slab was discussed.The results show that the improved PCC bottom layer strength,PFRC top layer strength and the bottom foundation modulus can reduce the PFRC top layer thickness;increase PFRC top layer strength and thickness of slab bottom,foundation modulus can decrease the tensile stress of slab bottom.The research in this paper shows that 2LCP is more tolerant to freeze-thaw than concrete pavement,and has higher fatigue life and stronger seams capacity.