预应力混凝土轨枕的裂缝及结构耐久性（Cracks and structural durability of prestressed concrete sleepers）Crack and durability of prestressed concrete sleeperIntroductionChina prestressed concrete sleepers from research, production and application, which lasted forty years, the yield of more than billion, laying mileage totaled seventy thousand km, compared with other countries in the world adopt concrete sleepers, a large proportion of the number of. The use of more than thirty years, there have been a lot of damage to the concrete sleeper, relates to the traffic safety, which is often considered the damage should sleeper for operation line. But to change the new sleeper, in addition to the re manufacturing, but also for handling, transport and pillow work, on the line? Not only need to spend a lot of manpower and material resources, sometimes affect traffic, the cost is often a sleeper itself cost four to five times.In thirty years, and the damage for the sleeper, in addition to a small amount is due to mechanical loading and unloading, cause traffic accidents and other damage, most of it is due to the cracks of various kinds, worried about the influence of the sleeper bearing capacity by replacement.Sleeper as a kind of prestressed concrete structure, crack is difficult to avoid, so research causes cracks in prestressed concrete sleeper and its harm, how to prevent and control cracks, to improve the durability of concrete sleepers, prolong the service life of the sleeper, will be very important.The types and performance of L concrete sleeper cracks1.1 types of cracks in concrete sleeper1.1.1 rail vertical transverse crack (crack under rail positive moment)The cracks in the sleeper on both sides of the lower part, under normal circumstances, the crack width is small, below 0.1mm, the length does not exceed the neutral axis. On 1981 69, 69 bar chord sleeper survey results show that the sleeper at the rail joint, the proportion of the vertical fractures under the rail is 60%; and the large waist rail rail vertical crack reinforcement ratio, 69 chord 69 respectively 37% and 20%.Vertical 1.1.2 pillow (horizontal) cracks (positive moment and negative bending cracks in a pillow)The type 69 sleeper on 1981 (1971 - 1976), the positive moment reinforcement 69 sleeper crack were 34% and 11% (different sections), 69 string pillow cracks were 36% and 13% (sections). The width and length of cracks than the rail pillow in some serious cracks, cracks in positive and negative bending moment together to form circumferential cracks with multiple circumferential crack individual sleeper. The 1991 Survey of S - 2 (1986 production) sleeper, vertical fractures accounted for about 23.7% of the pillow sleeper crack investigation, the rail joint sleeper, vertical fracture ratio higher pillow, accounted for more than 63%.1.1.3 top surface of sleeper bolt hole longitudinal cracksThis kind of crack usually from bolt hole as the starting point to the middle and end of the sleeper extension, some has been split into the end, causing splitting, severe crack width of 35mm. The type 69 sleeper in 1975 and the 1981 survey, the number of bolt holes along the longitudinal cracks with different manufacturers and different sections and different. Along the longitudinal nail hole percentage of the total number of sleeper sleeper survey were 0%, 18%, 20%, 40%, 48% etc. The 1991 Survey of type II sleeper 193889 root damage sleeper 18682, damage rate was 9.6%, the top surface of the longitudinal crack along the bolt hole total damage 33.8% sleeper. In short, the top surface of the bolt hole along the longitudinal sleeper crack is the most common for the prestressed concrete sleeper, sleeper not only have several years of operation, there are roadside spare sleeper, sleeper factory has not even; not only domestic sleepers, even abroad, there are sleeper. For example, 19751976 annual production and laid the Tanzania Zambia Railway Prestressed Concrete sleepers, the Tanzania Zambia railway in 1977 inspection found that the number of prestressed concrete sleepers along the top surface of the bolt hole crack up to 20%.1.1.4 top surface of sleeper bolt hole (transverse direction parallel to the rail)Type 69 and type II sleeper pillow has this kind of cracks. From the survey results, the majority of transverse direction and train running direction, one-way running section appeared in the double track railway.1.1.5 sleeper end longitudinal crackThe longitudinal cracks appear in some of the sleeper end top surface and a bottom surface, some are found in two sides of the end,Roughly parallel and reinforced (Gang Si).The central longitudinal cracks of 1.1.6 sleeperThe longitudinal cracks occurred in the middle of the sleeper top and side, parallel to the direction of steel, crack length is 30110mm, the crack width is about 0.53mm, the maximum can reach 5mm.1.1.7 crackingCrack top surface o