Analysis on the causes and prevention of early dam

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Analysis on the causes of early damage of cement concrete bridge deck pavement and prevention measures

at present, the bridge deck pavement of expressway bridge projects in Hebei Province generally adopts composite bridge deck pavement, that is, the lower layer adopts cement concrete pavement, and the upper layer is asphalt concrete pavement. The bridge deck pavement is a part of the direct action of the wheels, which is to prevent the wheels from directly wearing the carriageway slab, protect the main beam from rain erosion, and distribute the concentrated load of the vehicle wheel weight; The deck pavement shall have a certain strength and meet the requirements of crack resistance, impact resistance and wear resistance. Although the weight of cement concrete bridge deck pavement is 10kg, the requirement of direct participation in the force is not considered in the design process, but it plays an important role in the load distribution, so its quality cannot be ignored. The following mainly analyzes the situation of concrete cracks from the perspective of construction design, finds out the main reasons for the premature cracking of cement concrete bridge deck pavement, and discusses the prevention measures for the early damage of bridge deck pavement

2 cause analysis of premature damage of cement concrete bridge deck pavement

2 1. The construction thickness of bridge deck pavement is insufficient

in the process of beam and slab construction, the quality assurance system of the construction unit and the supervision unit is not perfect, and the inspection of all links of acceptance is not strict, resulting in the formed beam and slab size is too high; And the elevation control of the support is not strict, which is higher than the design elevation; Either the inclination of the beam and slab is too large, or the adjustment of the longitudinal and transverse slopes of the bridge deck and other factors may cause the height of the top surface of the beam and slab to be higher than the design value, so the thickness of the bridge deck pavement is locally too thin. The design is generally 8 ~ 12 cm, and the specification requirements should not be less than 8 cm

however, due to the above reasons, the local thickness of the bridge deck pavement is insufficient, and its stiffness and wear-resistant function are greatly reduced, resulting in the early damage of the bridge deck pavement

2. 2. The bridge deck pavement is poorly combined with the concrete on the surface of the main beam and cannot form a whole.

before the bridge deck pavement construction, the top of the beam and slab is not thoroughly cleaned, the beam and slab chiseling and brushing effect is poor, the laitance, mud, oil stain and loose concrete on the surface are not cleaned, and the surface of the main beam is not chiseled or the density and depth of chiseling are not enough, which seriously affects the bonding force between the bridge deck pavement and the concrete on the top of the beam and destroys the integrity of the cement concrete, Under the repeated action of vehicle load, the bridge deck pavement is prone to cracking and loosening

2. 3. Deformation and inaccurate positioning of reinforcement in bridge deck pavement

the general design requirements of reinforcement in bridge deck pavement are 3:7, that is, at the middle and upper part. However, in the actual construction process, the positioning of reinforcement is inaccurate, and the supporting measures adopted are not in place, or pasted on the top surface of the beam, or at the lower part; And in the process of pouring cement concrete, because it can't withstand the influence of factors such as the artificial trampling of construction personnel and the rolling of transportation machines and tools, the reinforcement is seriously deformed, which seriously weakens the ability of the reinforcement to bear the load, especially the bridge deck pavement in the negative bending moment area, which is more prone to early damage such as bridge deck cracks

2. 4. The design section of beam and slab is too small, and the pre camber in the middle of beam and slab is too large

in the process of beam and slab design, in order to meet the economic requirements, especially the T-beam, the section is getting smaller and smaller, so the stiffness ratio is affected, the pre camber is very large, and the deflection is also very large in the process of operation. In this way, if the beam and slab are placed according to the specification requirements, the thickness in the middle of the beam span cannot be guaranteed under the condition of ensuring the pavement thickness at the support; At the same time, due to the large dispersion of concrete and the uneven application of prestress during the construction process, the midspan camber of beams and slabs produced in the same batch is different

2. 5. The strength of waterproof concrete is too low

due to the fact that the strength of beams and slabs is different from that of paved waterproof concrete, for example, the concrete strength design grade of many bridge deck pavement is C30, while the concrete strength of beams and slabs is C40 and C50. Under the action of vehicle load, the stress state of bridge deck pavement concrete is very complex according to the media reports in Volgograd, and it is easy to cause damage due to its low strength; At the same time, during the construction process, insufficient attention was paid to the construction quality of bridge deck pavement concrete, the quality of raw materials used was poor, and the sand rate was too high. Some scientific research institutions and material experts have been committed to the research on the subject of stress corrosion cracking, the lax control of water cement ratio, the poor grading control of sand and gravel, the insufficient impermeability strength, the poor workability of concrete mixtures, and the insufficient vibration during the construction process, resulting in honeycomb Too many pores and other defects cause its strength to be reduced and its durability to be insufficient, which destroys the integrity of the pavement layer and reduces its ability to resist cracking, impact, bending and wear

2. 6. The curing measures are not in place, and the concrete shrinkage is affected.

if the bridge deck pavement is constructed in windy weather, or the full coverage and full wetting curing are not carried out in time after the construction is completed, or just after the construction is completed, it is considered that early cracks are prone to occur, and the crack depth is mostly between 5 and 20 mm, and the construction quality will be greatly affected. At present, there is a widespread phenomenon of ignoring concrete curing, which further promotes the development of temperature shrinkage and dry shrinkage cracks, resulting in premature damage of the bridge deck

at present, the bridge deck pavement mostly adopts the pumping concrete process. In order to meet the requirements of pumping concrete with large slump, the value is generally 12 ~ 16 cm. In addition to adding additives, the amount of cement and water cement ratio are also increased, so that the dry shrinkage of concrete will be greatly increased. At the same time, when the amount of cement is large, the hydration heat increases, causing the temperature difference between the carriageway slab and the bridge deck pavement to produce deformation constraints. Because the tensile strength of concrete at the initial stage of hardening is small, the tensile stress produced by several shrinkage and cold shrinkage exceeds its tensile strength, which will lead to cracks in the interior and surface of concrete

2. 7. Improper treatment of construction joints

bridge deck pavement should strive to reduce the number of construction joints, and those requiring construction joints should also be set at the top of piers and abutments. When the bridge deck is not wide, it is better to cast it in full width at one time, and the dividing strip can be used as the interface if the bridge deck is wide. However, many construction units set up construction joints at will when paving the bridge deck, and the treatment of construction joints is also improper (such as not chiseling according to the specifications). In addition, the interval time in the process of pouring concrete is too long (generally not more than 1 h). The limited edition "early bird" reward package will be returned to the initial supporters. A set of drilling machine version shred buddy, and the construction joint is not set as required. All these seriously affect the continuity and integrity of concrete

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