Reasons for joint setting
The upper layer of concrete is composed of concrete panels of certain thickness, which has the nature of thermal expansion and contraction. Due to the change in temperature throughout the year, the concrete slab will produce different degrees of expansion and contraction. And in a day and night households, the daytime temperature rises, the top surface of the concrete slab temperature is higher than the bottom surface, this humidity slope difference will cause the middle of the panel protrusion. At night the temperature decreases, the top surface of the concrete slab temperature is lower than the bottom surface, will make the plate around and corner corner buckling. These deformations will be subject to the frictional resistance and bonding between the panel and the foundation, as well as the self-weight of the plate and wheel load, etc. constraints. Resulting in excessive stresses in the panel, causing fracture or arching damage to the panel. Cracks caused by warpage, then in the crack after the occurrence of the two plates divided by the body is not yet completely separated, but also has the ability to transfer the load, if the plate body temperature decreases uniformly caused by shrinkage, it will make the two plates pulled apart, thus losing the ability to transfer the load.
In order to avoid these defects, cement concrete pavement had to set up many joints in the longitudinal and transverse directions. The whole pavement is divided into many slabs. The joints of cement concrete pavement can be divided into two categories: longitudinal joints and transverse joints. The joints parallel to the center line of the route are called longitudinal joints, and the joints perpendicular to the route are called transverse joints.
The joint design should be able to
- control the location of cracks caused by shrinkage stress and warpage stress;
- provide sufficient load transfer through the joints;
- prevent hard debris from falling into the joints gap.
the longitudinal joints and their structure
Longitudinal shrinkage joints
When the width of a paving is greater than 4.5m, additional longitudinal indentation joints should be provided. The longitudinal indentation joints can be false joints plus tie rod type, and its structure is shown in Figure 2-8-7. The tie bar is set to prevent the slab from lateral displacement to enlarge the gap, and the tie bar should be set at 1/2 of the slab thickness; the slot door set at the upper part of the indented joints should be cut with a seam cutter or vibrated with wooden strips when the concrete is poured and reaches a certain compressive strength (6.0~12.0MPa for crushed concrete and 9.0~12.0MPa for gravel concrete) in general. The depth of the slot door should be moderate, too shallow, the strength of the concrete section weakened enough, thus not ensuring that later cracking occurs in the joint position; too deep, the irregular fracture surface area is too small, the joint load transfer capacity is reduced. According to experience, the depth of the slot is generally 1/4-1/5 of the plate thickness, and the width of the slot is as narrow as possible according to the construction conditions, usually 3~8mm.
Longitudinal construction joints
Due to construction conditions and other reasons, when the paving width is less than the width of the pavement needs to be divided into two or more pouring, then the longitudinal construction joints should be set. The longitudinal construction joints can be divided into two forms: flat joints and enterprise joints according to their construction: generally flat joints, and should be set in the center of the plate thickness ties to prevent the joints from opening and the plate up and down misalignment. According to the practical experience at home and abroad, the enterprise seam is easy to produce damage.
The reasons are
- The tongue size is too large, which reduces the strength of the joint and may lead to damage of the tongue.
- large and deep entrenches, leakage in the pouring of concrete, tongue and angles deformation, demolition difficulties, vibration, often causing early damage to the entrenches, and sometimes even vibration bad entrenches, need to be repaired. These damage to the subtle lines in the Yin enterprise mouth, under the action of repeated load. Local stress concentration, resulting in the development of cracks until the damage. In addition, the construction is more troublesome.
the horizontal joints and their structure
Transverse joints are generally divided into transverse shrinkage joints, expansion joints and transverse construction joints.
At the expansion joint, the concrete panel is completely disconnected, so it is also called a true joint. The purpose of setting expansion joints is to provide elongation margin for the expansion of concrete slab, so as to avoid excessive thermal stress. The expansion joint must run through to the end, with vertical walls: the width of the joint is 2.0-2.5 cm, and a force transmission bar is set in the center of the slab thickness. More than half of the force-bearing rod should be coated with asphalt or plastic sleeve, and a small sleeve of 10cm in length should be added, leaving a gap of 3cm between the bottom of the sleeve and the head of the force-bearing rod (filled with yarn). Its lower part is equipped with jointing board (wooden board coated with asphalt), and the upper 3~4m range is filled with caulking material for sealing. The expansion joint at the junction with the structure or at the intersection with other highways, when it is impossible to set the transmission bar, can be used edge reinforcement type or thick edge type.
Setting up expansion joints brings inconvenience to construction. At the same time, due to the construction of improperly set transmission rod (not correctly positioned) or bad sealing joints and other reasons. The concrete slab at the expansion joint is often cracked and other diseases. Use of experience and observation data show that the expansion joint spacing shorter sections (100m below), in the use of the process often appears to increase the gap between the expansion joints, so that relying on the role of aggregate inlay lock joints load-transfer capacity is greatly reduced; and because the caulking material is difficult to maintain its effectiveness, gravel and other debris will be easy to fall into the seam, resulting in the joint area of the concrete in the expansion is blocked when the cracking damage, or arching. Less (increase the spacing of expansion joints) or not (only in the intersection with the structure) expansion joints, on the one hand, to facilitate the construction, on the other hand, to restrain the displacement of the plate. Reduce the gap between the joints, so that the load-transfer capacity increased, the wrong platform, cracking and arching and other diseases reduced. Therefore, expansion joints are only set up at the following occasions: adjacent to bridges or other fixed structures; at the junction with flexible pavement; at the change of slab thickness; at the mouth of tunnels; at the change of longitudinal slope of small radius flat curves and concave vertical curves. Expansion joints at adjacent structures should be set at least two according to the construction temperature. In addition to the above-mentioned positions, the expansion joints should be set up as little as possible, and the spacing can be determined according to the construction temperature, the expansion of concrete aggregate and combined with local experience.
Ties are shaped steel bars set on the longitudinal joints to prevent lateral displacement of the slab. Tie bar should use rebar, located in the middle of the slab
It should be located in the center of the thickness of the slab and should be rust-proofed within 10cm of the middle of the tie.
The purpose of setting force-transmitting rod is to ensure the load-transferring capacity of the joints and the levelness of the road surface, and to prevent the generation of diseases such as misalignment.
The force transmission rod is mainly used for transverse joints, using light round steel reinforcement. Due to the different roles played by expansion joints and contraction joints, the size should be different, with the former being larger. For the force-bearing cuttings located at the shrinkage joints, half of their length plus 5cm should be coated with asphalt or plastic sleeve, and the asphalt-coated end should be staggered in the adjacent plate; for the force-bearing rods located at the expansion joints, a set of sleeve should be added at the asphalt-coated end, leaving a gap of 3cm inside, filled with yarn or foam. The set end should be staggered in the adjacent plate amount of its outermost transmission rod from the joint or free edge of the distance is generally 15 ~ 25cm.
the cement concrete pavement and the connection of the structure
With the concrete pavement connection parts are asphalt pavement, bridges, culverts and channels, etc., the connection parts are different from – general road sections. These parts of the concrete pavement often occur jump phenomenon, seriously affect the speed and comfort and service life of the road. The reason is multifaceted, mainly due to the differences in these parts of the settlement.
The principle of prevention and treatment, one is to reduce the amount of settlement after the completion of the foundation of these parts; the second is to strengthen and improve the overall durability of the pavement.
Concrete pavement and asphalt road and meet
Where the concrete pavement and asphalt pavement meet, damage occurs first at one end of the asphalt pavement and then at one end of the concrete pavement because the asphalt pavement has difficulty holding up against the horizontal thrust at the end of the concrete panel. In addition, out of the different settlement between asphalt pavement and concrete pavement, the joint becomes uneven and causes jumping. Therefore, for highways and primary roads, when the concrete pavement meets the asphalt pavement, a concrete plate with a length of 3m should be buried under the asphalt pavement surface, and the thickness of this plate at the end where the concrete pavement meets is the same as that of the concrete panel, and the other end is not less than 15cm. For other grades of highway. Due to the low speed of car travel, the traffic volume is not large, cement concrete pavement and asphalt pavement connection, can use concrete precast block transition or trail connection.
Concrete pavement and bridge connection
Concrete pavement and bridge connection, according to the road grade, use requirements and local experience to choose the following or other appropriate measures.
In each grade of highway, especially in the high grade of highway, should be set up bridge head lap board. Between the lap board and the concrete road surface using reinforced concrete panel transition, its length is not less than 5 m. The joint between the lap board and the reinforced concrete panel should be set up to transfer the rod. Expansion joints should be set between reinforced concrete panels and concrete panels. When intersecting with the bridge at an oblique angle, the reinforced concrete panel should be reinforced with reinforcing steel mesh in the acute angle part. The reinforced concrete panel shall be executed in accordance with the relevant provisions of reinforced concrete pavement. Tie plate and reinforced concrete panel at the junction with the tie, its size, spacing and concrete pavement and asphalt pavement intersection set the same tie. For the lower level of highway, or as a transitional measure of high-grade highway, the bridge can be paved with a section of precast concrete blocks or asphalt pavement, to be settled and stable, and then pave the concrete pavement. When the bridge head with a lap, its length is not less than 5m; when the end of the bridge head with a lap, its length is not less than 8m.
Structures across the road
In order to prevent the road surface above the crossing structures such as culverts and so on from lateral cracks, misplaced and jumping, the concrete panels on the top and both sides of the structure within the appropriate range should be reinforced with steel mesh, or reinforced concrete panels.
For box-shaped structures, when the distance d from the item surface to the bottom of the plate is less than 80cm, the top surface and the concrete panels within 6m on each side are reinforced with steel mesh. When d is less than 30cm or embedded in the grass-roots level, a double layer of reinforcing steel mesh should be used. The reinforcing steel mesh is laid at 1/3~1/4 of the slab thickness from the bottom and top of the slab respectively. The reinforcement is 10~12mm in diameter, with longitudinal reinforcement spacing
If the thickness of the grass-roots level on the top surface of the structure is less than 10cm, the grass-roots leveling is changed to concrete; when d is 30~80cm, a single-layer reinforcement network can be used, and the reinforcement network is laid at 1/3~1/4 of the slab thickness from the top of the slab, and the reinforcement is made of 8~10mm diameter bright round steel bars, with the spacing of 10~15cm longitudinal bars and 20~30cm horizontal bars. At the thickness of the slab, the reinforcement is made of 10-20mm diameter bright round bars with 10cm spacing between longitudinal bars and 20-30cm spacing between transverse bars.
Concrete panels under the edge of the longitudinal and transverse free edge of the foundation, when there is a possibility of large deformation, it is appropriate to add reinforcing steel in the edge of the plate, the corner with hairpin-shaped reinforcing steel or reinforcing steel network.
- Reinforcement at the edge of the slab
Concrete panel edge reinforcement, generally choose two 12 ~ 16mm rebar, arranged in the lower part of the plate, from the bottom of the plate for 1/4 of the plate thickness, and should not be less than 5cm, the spacing is generally 10cm, the ends of the reinforcement should be bent upward. The minimum thickness of the protective layer of reinforcement should not be less than 5cm.
- Corner reinforcement
Corner reinforcement part of the reinforcement, can choose 2 diameter 12 ~ 16mm rebar, arranged in the upper part of the plate, from the top of the plate should not be less than 5cm, from the edge of the plate for 10cm. plate angle is less than 90 °, can also use a double reinforcement reinforcing steel network reinforcement, reinforcing steel can be selected diameter 6mm, arranged in the upper and lower part of the plate, from the top and bottom of the plate to 5 ~ 10c is appropriate. The minimum thickness of the protective layer of reinforcement should not be less than 5cm.
Transverse shrinkage joints
Transverse shrinkage joints are set up to reduce the shrinkage stress and temperature warpage stress of concrete. Generally, the form of false joints is used, and there is no transmission bar. But on the highway of extra heavy traffic, due to the repeated action of the load and axial load, the load transfer capacity of the joints is rapidly reduced, and the phenomenon of misalignment appears, so it is appropriate to add the transfer bar: on the highway of other levels of traffic, adjacent to the expansion joints or the free end of the shrinkage joints, the gap will be gradually opened with the repeated expansion of the adjacent expansion joints or the free concrete panel of the road surface, in order to ensure the load transfer capacity of these joints, in the adjacent expansion joints or the road surface In order to ensure the load transfer capacity of these joints, it is advisable to add force transmission rods in the three shrinkage joints adjacent to the expansion joints or the free end of the road surface. The groove set in the upper part of the shrinkage joint is cut after the concrete is poured. The width is 3~8mm and the depth is 1/4~1/5 of the thickness of the slab, and the groove is filled with the seam material to prevent the penetration of moisture and the embedding of impurities.