Cohesion refers to the binding force between the molecules on the interface between the adhesive and the bonded object.For a long time, the corrosion of reinforcementreinforced concreteThe influence of component cohesion has always been paid attention to by the engineering community, and its influence mainly focuses on the change of cohesion and bearing capacity.
Chinese name
Cohesion
Foreign name
Adhesive force
Type
Binding force
Definition
Intermolecular at the interface between adhesive and bonded object
The finite element method can be used to simulate the influence of reinforcement corrosion. One is to simulate the internal force caused by volume expansion when reinforcement corrosion occurs, and the other is to simulate the displacement when reinforcement corrosion occurs.From the temperature point of view, that is, a certain temperature is applied to the reinforcement to simulate the influence of its expansion process on the cohesion and bearing capacity of members, and the test results are compared and analyzed.[1]
For rock, the compressive strength σ c, tensile strength σ t, shear strength τ o and cohesive force C of rock are related as follows:
σ c=10 σ t (coefficient of σ t varies from 6 to 20)
σ c=5 τ o (coefficient of τ o varies from 6 to 20)
τ o=1.8 σ t (coefficient variation range of σ t is 6~20)
τ o=0.7C (coefficient of C varies from 6 to 20)[1]
Model establishment
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objective
(1) It is proved that the change trend of bond force and load value are close to the test results.[1]
(2) Using temperature expansion model to simulate reinforcement corrosionreinforced concreteThe influence of bond force and bearing capacity is effective and feasible, and avoids the adverse effect of deviation from the actual situation by applying uniform internal force and deformation to the reinforcement to simulate the corrosion effect. It is a supplement and improvement to the numerical analysis method of reinforcement corrosion in reinforced concrete.[1]
Unit selection division
Use axial symmetryfinite element analysis For the model, the axis of symmetry is taken on the centroid of the main reinforcement.The concrete is a circle with an inner radius of 7mm and an outer radius of 50mm.The diameter of the main reinforcement is 14 mm. The anchorage length of the reinforcement in the concrete is 10 times the diameter of the reinforcement, that is, 140 mm. The main reinforcement is a steel ring with an inner diameter of 5 mm and an outer diameter of 7 mm.The height of main reinforcement rib is 0.5mm, and the spacing between ribs is 7mm. The stirrup adopts the equivalent circular section area of rectangular section.Four point axisymmetric block element is adopted for concrete and stirrup, and two node axisymmetric shell element is adopted for main reinforcement and rib.The friction between reinforcement and concrete is ignored, but the section of main reinforcement rib is used as a supplement.ABAQUS program is used for analysis and finite element element division.[1]
Material properties
Concrete is considered as elastoplastic material, with elastic modulus E=34500MPa, Poisson's ratio ν=0.18, compressive strength fc=50MPa, tensile strength ft=4.25MPa, and plastic strain at failure taken as 1.4 × 10-3The softening caused by the change of shear stiffness shall be considered after the crack is generated.After assuming cracksTensile strength of concreteIs the linear loss and the strain is 1.2 × 10-3There is no tensile stress after.The ratio of concrete biaxial ultimate compressive strength to uniaxial compressive strength is 1.16. The stirrup is made of elastic material, the main reinforcement is made of elastoplastic material, and the elastic modulus E=2.06 × 10fiveMPa，Poisson's ratio ν=0.3, steel yield strength is 550MPa, tensile strength is 600MPa.[1]
Analysis steps
The numerical analysis consists of the loading stage before reinforcement corrosion, the loading stage after reinforcement corrosion and the loading stage after reinforcement corrosion. The failure process of the component under load is analyzed according to the instability analysis principle and the modified RIKS method, taking into account the influence of geometric nonlinear changes.[1]
The volume expansion of reinforcement due to corrosion of reinforcement is analyzed by using the volume expansion of main reinforcement element under the action of temperature, and the expansion coefficient of steel is the orthogonal expansion property under the action of temperature, that is, the circumferential expansion is considered while the longitudinal and radial expansion of reinforcement is ignored.[1]
If the original radius of reinforcement before corrosion is r0, the corrosion depth under temperature is X, the radius after expansion is r, and t is the expansion amount, then:
r-r0=t（1）[1]
Assume thatConcrete crackThe length is lcr, the width is w, and the corrosion depth X is derived from the equal volume of the components before and after the crack is generated:
X=t[1+lcr/（2r0）]（2）
In formula (2), expansion t is directly calculated by finite element method.[1]
Numerical stage analysis
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Loading stage
Load displacement curve at the loading end.It can be seen that when concrete is regarded as a nonlinear elastoplastic body, it will show obvious strain softening after splitting failure.[2]
Rust stage
It is the displacement of concrete unit under the corrosion state of reinforcement, which is basically consistent with the crack development position of the test.[2]
Loading after corrosion
In order to compare the influence of stirrup configuration, the analysis and calculation are carried out in case 1 and case 2 respectively.The relationship curve between the corrosion depth and the maximum load in both cases.It can be seen that there is an obvious downward trend at the beginning of corrosion, which is also the trend of most test results.The maximum cohesive force is greater than the maximum value before corrosion.At the same time, it can also be found that the two analysis curves are closer to the test curve in the middle.[2]
Importance of cohesion
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When the friction between asphalt layers or on the joint surface between asphalt layer and base course is much lower thanAsphalt mixtureIn itself, there will be weak links of insufficient shear strength at the interface between layers. When the pavement is subjected to a large horizontal shear force, shear displacement will easily occur, which will cause horizontal displacement, rutting, congestion and other diseases of the asphalt pavement.The bonding layer plays a vital role in the transmission of tensile stress and shear stress between asphalt layers. The insufficient bonding force between layers will lead to the interlayer pushing and the concentration of tensile stress at the bottom of the upper layer, which will accelerate the fatigue cracking and lead to the destruction of the entire pavement.To ensure thatasphalt pavementThe excellent road performance ofasphalt concreteThe bonding layer is laid between layers or treated to improve the bonding force between layers.[2]
Classification of tack coat
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1) Lower seal.When it is necessary to open traffic because the asphalt surface cannot be paved in time after paving the base course or the pavement with large gap and serious water seepage on the asphalt surface course of expressways and Class I highways mostly used in rainy and humid areas, the lower seal coat should be paved after spraying the priming oil.The asphalt asphalt aggregate ratio of the lower seal layer is 7.5%~13.5%, the stone consumption is 5.4kg/m2~8.1kg/m2, the stone specification is Grade II mineral aggregate, and the construction process is slurry seal machine mixing and paving.[2]
2) Tack coat.Before paving the upper layer of double-layer or three-layer hot mixed hot paved asphalt mixture pavement, the asphalt layer below it has been polluted. When paving the asphalt surface layer on the cement concrete pavement, the adhesive layer must be spread.The amount of asphalt is 0.3L/m2~0.5L/m2, the construction process is spraying, and no stone is needed.[2]
3) Prime coat.The graded gravel, graded crushed stone base course of asphalt pavement and the semi-rigid base course of cement, lime, fly ash and other inorganic material stabilized soil or granular materials must be sprayed with prime coat asphalt.The amount of asphalt is 0.7L/m2 ～ 1.1L/m2, the amount of stone is 2kg/m2 ～ 3kg/m2, the specification of stone is stone chips or sand, and the construction process is asphalt, stone chips and rolling.[2]
Stress analysis of bonding layer
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The role of the bonding layer is to bond the upper and lower asphalt surface layers, so that the load can be transferred from the upper asphalt layer to the lower asphalt layer without interlayer slip or delamination.According to the viewpoint stated in reference [1], when the test temperature is 60 ℃, the shear strength increases with the increase of asphalt content within a certain range. The reason is that after the asphalt mixture is compacted on the base surface, the asphalt mixture is in close contact with the base surface, and the shear strength is derived from the friction between the aggregate in the asphalt mixture and the base and the cohesion of the asphalt,As the amount of asphalt increases, the contact surface between asphalt mixture and base increases, so the shear strength increases.When the asphalt thickness of the tack coat exceeds the texture depth of the asphalt mixture, the adhesive force of the asphalt tack coat decreases. Due to the compaction effect, the redundant asphalt is squeezed out of the test piece, and the reduced level of the adhesive force tends to be stable.When the test temperature is 30 ℃, the viscosity of asphalt increases due to the decrease of temperature, so the adhesive force increases. However, with the increase of asphalt content, the excess asphalt is squeezed out, and the adhesive force tends to be stable.[2]
Cohesion and asphalt
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Road performance analysis
The construction technology of lower seal coat, adhesive layer and priming oil has a common feature, that is, they can improve the shear strength between rigid and flexible structural layers, but their shear strength is different.When the lower seal layer is used as the interlayer binder of asphalt pavement structure, the shear strength is the largest, and the effect is the best as the bonding transition layer.[3]
Generally, the compaction method can be used to simulate the engineering practice, which can be used to determine the reasonable amount of asphalt tack coat, and also to determine the shear strength of asphalt tack coat. However, the strength of the base material is highly required, otherwise the base material will be damaged when hit.The test temperature of the specimen is the key, low temperature has high shear strength, high temperature has low shear strength.Under the same conditions, the test piece formed by compaction method,cement concreteThe shear strength of the test piece on the base is higher than that of the test piece on the steel plate, which indicates that the roughness of the base has a greater impact on the shear strength than the strength of the base.Therefore, we should pay attention to improving the roughness of the substrate while improving its strength, so as to obtain a higher shear strength.[3]
Determination of asphalt materials
Asphalt has thermal viscosity, but it has the disadvantages of low softening point and easy to be softened by heat absorption.Due to the lack of high temperature deformation resistance of asphalt, the bearing capacity of pavement under high temperature environment is reduced, and rutting and other diseases are easy to form. Of course, when asphalt is placed in the surface layer, tire adhesion will also occur, which will cause the wheels to drift, causing potential safety hazards.The lower seal layer should be treated by layer laying method or slurry seal method.Emulsified asphalt orModified emulsified asphaltAs binder.The selection of priming oil is based on the type of base course, and the commonly used ones are liquid asphaltemulsified asphalt , coal tar pitch, etc.Liquid petroleum asphalt can be fast setting, medium setting and slow setting according to the purpose and place of use.It is better to use petroleum asphalt with high penetration. Before use, it is made by mixing kerosene or light diesel oil in the order of heating before adding diluent, and then mixing and diluting properly.The type of emulsified asphalt shall be selected according to the aggregate type and service conditions.Cationic emulsified asphalt can be used for all kinds of aggregates, and anionic emulsified asphalt can be used for alkaline aggregate.The demulsification speed and viscosity of emulsified asphalt should be selected according to the use and construction method.When coal tar pitch is used as the base course prime coat of various grades of highways, it should be T21 or T22, and mixed with road petroleum asphalt and emulsified asphalt to improve the permeability and connect with the base course as a whole.The tack coat oil should be quick cracking or medium cracking emulsified asphalt, modified emulsified asphalt, or fast and medium setting liquid petroleum asphalt.The type and amount of tack coat oil shall be determined by test according to the type of underlying layer.When a thin layer of large void drainage pavement is paved on the tack coat oil, the amount of tack coat oil should be increased to 0.6L/mtwo～1.0L/mtwo。When the asphalt layer is used as sealing layer, the sprayed tack coat oil should beModified asphaltOr modified emulsified asphalt, the dosage shall not be less than 1.0L/mtwo。[3]
Construction preparation
Before the construction of tack coat, the quality of base course or underlying asphalt layer shall be checked, and the asphalt layer shall not be paved if it does not meet the requirements.When the old asphalt pavement or underlying layer has been polluted, it must be cleaned or milled and the construction can be carried out after the pavement is dry.The construction shall be carried out in summer as far as possible, and the construction at night or in the early morning is not allowed when the mid air temperature is high.[3]
construction plan
According to the requirements of the specification, during the construction of the tack coat, the asphalt shall be sprayed into a mist and evenly distributed into a thin layer within the full width of the pavement.The tack coat oil should be spread on the same day. After the emulsified asphalt is demulsified and the water evaporates, or the diluent in the diluted asphalt is basically volatilized, the asphalt layer should be paved immediately.During prime coat asphalt construction, if the asphalt is heat modified asphalt, especiallySBS modified asphaltThe spraying shall be carried out at high temperature, and the construction quality is affected by the construction environment, temperature and the cleanliness of the base course.Otherwise, segregation, separation of water and asphalt, uneven spraying, and asphalt blocking the oil outlet of the distributor are likely to occur.The construction of prime coat asphalt requires high dryness of stone chips, and the stone chips used must be screened.At the same time, stone chips must be rolled in time after spreading to achieve satisfactory quality effect.The prime coat of semi-rigid base course with lower bearing course should be sprayed when the surface becomes slightly dry after the base course is rolled and formed, but has not hardened.In order to better enhance the bonding force of the prime coat, the prime coat asphalt shall be spread on the inorganic binder granular base course 1~2 days before the asphalt layer is paved.The effect of the lower seal is obviously superior to that of the tack coat and prime coat.It is a thin layer construction technology of fine asphalt concrete, which consists of graded stone, asphalt emulsion, additives and additives, and is fed, mixed and paved in proportion by slurry seal construction machinery,CohesivenessAnd sealing are particularly good.When paving the asphalt surface layer, the construction of asphalt pavement can be directly carried out. The lower seal layer will be further compacted under the high temperature of asphalt mixture and the vibration of the road roller, which really plays the role of bonding, compaction and waterproofing.[3]
Strength evaluation
The evaluation of the cohesion between asphalt layers is mainly through the experimental analysis of the shear strength of asphalt mixture.There is a direct shear instrument with simple principle and changeable parameters, which can also be used to conveniently test the road core and the cylinder specimen prepared in the laboratory.For this reason, a simple direct shear apparatus that can be operated on MTS is designed and manufactured.The spacing between the two shear sleeves of the instrument is adjustable, and MTS can be used to conveniently load in the mode of controlling strain or stress, and the load size and test temperature can also be adjusted.The instrument realizes shearing through the offset of the support under the shearing sleeve, which is easy to operate, and the test data is automatically collected and output by the software.[3]
The bonding strength of the bonding layer with water on the surface (simulated rainwater) is lower than that of the dry road section under the same conditions.The bonding strength of wet sections increases with time, but it can not reach the strength of dry sections under the same conditions.The amount of interlayer binder is 0.36L/m2, and the shear strength of wet sections is higher than that of wet sections with the amount of interlayer binder 0.09L/m2.[3]
Through the analysis of the main structural layers that have a significant impact on the cohesive force, the precautions for improving the cohesive force of asphalt are extracted from the aspects of asphalt cohesive layer structure, material characteristics, construction, etc., in order to guide the construction of asphalt pavement more comprehensively.[3]
Cohesion and glazed tile
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Glazed tile is a kind of facing tile widely used for wall decoration, which is generally inlaid with cheap cement mortar.However, due to the poor cohesion of cement mortar as the binder, the glaze inlaid on the wall is easy to fall off, which not only seriously affects the decorative effect of the wall, but also is troublesome to re inlay.Therefore, in order to prevent falling off, the bottom surface of glazed tile is generally grooved.Recently, a kind of glazed tile with stronger adhesion to cement mortar has been introduced overseas.[4]
It is reported that this glazed brick is made by using gypsum and glass chips as the main raw materials as the bottom layer, filling the brick blanks on it, pressing to form a composite body, drying, glazing and sintering.After sintering, the bottom layer is gypsum ceramic layer, and the main layer is ceramic layer. The thickness of the bottom layer is 1/10~1/3 of the total body thickness.Gypsum porcelain layer is not easily soluble in water, but the surface of gypsum recrystallized particles is not completely covered by glass, and still has a certain degree of water absorption.When the gypsum porcelain bottom absorbs the water in the mortar and dissolves anhydrite, it reacts with the cement in the mortar to generate hydrated calcium sulphoaluminate and hydrated calcium sulphoferrate acicular crystals, which are filled in the fine gap of the gypsum recrystallized particles.Therefore, the gypsum porcelain bottom can produce high cohesive force with cement mortar.If will be usedAnhydriteSpecial cement mortar for glaze with broken glass chip as base(Cement: sand=1:2, inlaid on the wall, cured in the air for 28 days, the bond strength reaches 22.6kg/cm2, while the common glazed tiles with grooved bottom are inlaid with the same mortar or organic binder, the bond strength is 12.2kg/cm2 and 24.9kg/cm2 respectively. When inlaid with cement mortar, the bond strength of glazed tiles with gypsum porcelain bottom is different from that of common glazed tilesExtremely small, if the bottom surface of gypsum porcelain is re grooved, the bonding strength will be much improved;And withOrganic binderIs there a small difference in the bond strength of ordinary glazed tiles?If the gypsum porcelain bottom is re grooved?The bonding strength of glazed tiles can be further improved.[4]
The appearance of this kind of glazed tile with high cohesive force solves the problem of low cohesive force in cement mortar inlay, effectively prevents the brick from falling off, and reduces the times of multiple inlay construction due to the falling off of glazed tile.[4]
