Construction technology of prestressed anti-corrosion and anti-floating bolt

1 Project overview

In a high-rise building, there are two single towers above the ground, each with 18 floors; the first floor is outdoors, and the roof of the basement is covered with soil 1m for greening land: 3 basement floors underground, the depth relative to the outdoor ground is about 12m: the column distance of the basement is at the owner's request, They are 10xl0m and 12xl0m respectively; in the preliminary design, the foundation adopts bored piles; the floor adopts beam-slab structure, and the thickness of the slab is 800mm. According to the survey report, the soil condition of the site is described as follows from top to bottom: (1) Artificial filling layer: the average thickness is 2.0m. (2) Silt layer: the average buried depth of the top surface is 2.0m, and the average thickness is 2.5m. (3) Silty clay layer: the average buried depth of the top surface is 4.5m, and the average thickness is 12.5m. (4) Strongly weathered rock formation: the average buried depth of the top surface is 17.0m, and the average thickness is 1.0m. (5) Moderately weathered rock formation: the average buried depth of the top surface is 18.0m, and the average thickness is 4.0m. (6) Micro-weathered rock formation: the average buried depth of the top surface is 22.0m, and the average thickness is not less than 6.0m. The groundwater level of the site where the building is located is relatively shallow, and the anti-floating water level of the groundwater is taken to be the same as the outdoor ground, and the floor anti-floating plays a controlling role. If the beam-slab structure is used directly to resist buoyancy, considering the beneficial effect of the self-weight of the bottom plate, the design value is taken as: 12xl0x1.2-0.8x24x0.8=129KN/m0. Taking the calculation and analysis of the 10xl0m standard floor as an example, the design theory of the 12xl0m is the same, so it will not be repeated here. Using C30 concrete and HRB335 steel bars, the diameter of the gluten bars is calculated to be 16mm and the spacing is 100mm. The crack width is 0.195mm: the bottom rib diameter is 20mm, and the spacing is 100mm.

2 Calculation of anchor rods

The damage of the anchor rod mainly includes: the damage of the bearing capacity of the rod body and the damage of the pull-out resistance. In the design process of the bolt, the interaction between the bolt and the bottom plate should also be considered, and the actual internal force of the bolt should be calculated according to the principle of deformation coordination. At the same time, anchor rods should also be arranged in consideration of the actual force of the bottom plate when the deformation is coordinated. In the design of anchor bolts, the reinforcement of the anchor bolts is usually assumed first, and then checked and adjusted to meet the design requirements. The deformation of the anchor rod and the bottom plate should also meet the corresponding limit values. Since there are many softwares available for design reference, it greatly reduces the amount of calculation for our designers. The following will discuss one by one in combination with the actual situation of this project.

(1) For the convenience of calculation, first assume that the standard value of the axial tension of the anchor rod is required: Nak=240KN. The failure conditions of the bearing capacity of the anchor rod body are:

γ0×γq×Nak≤ε2×fy×As

γo--Importance coefficient of slope engineering

γq--Load sub-item factor

ε2--The tensile working condition coefficient of the anchor bar

fv--the design value of the tensile strength of the anchor bar

Bei U: As≥γ×γq×Nak/(ε2×fy)=lx1.3×240×1000/(0.69×300)=1507mm

Use HRB335 grade steel bar, the area of 3d28 steel bar is As=1847mm2.

(2) The anti-pullout failure form of the anchor bolt is divided into: the anchorage failure between the anchor body and the formation, and the anchorage failure between the anchor reinforcement and the anchor mortar.

If the anchorage body diameter D=200mm is adopted, the anchorage length between the anchorage body and the formation needs to meet:

Lat≥Nak/(ε1×π×D×frb)=240×1000/(1×3.14×200×220)=1.17m (Formula 1)

ε1--the working condition coefficient of the bond between the anchor and the formation

frb--characteristic value of bond strength between formation and anchor

The anchoring length between the anchor bar and the anchoring mortar needs to meet:

La≥γο×γq×Nak/(ε3×n×π×d×ε4×fb)=1×1.3×240/(0.6×3×3.14×28×0.85×2.4 )=2.03m (Formula 2)

ε3--coefficient of working condition of steel bar and mortar bonding

n--the number of single-bundle anchor bars

d - the diameter of the anchor bar

84--reduction factor of characteristic value of bonding strength between steel bar and mortar

fb - characteristic value of bond strength between steel bar and mortar

It should be noted here that the code proposes that the design and calculation length of the anchorage section must meet the upper and lower limits in combination with actual engineering tests, design safety and structural requirements of the anchorage section. The length of the anchorage section of the bolt shall be calculated according to formula 1 and formula 2, and the larger value shall be taken. At the same time, the anchorage length of the rock bolt should not be less than 3m, and should not be greater than 45D and 6.5m. According to the above calculation results of this project, the value of the anchorage length should be La=3m.

(3) The above discussion assumes the standard value of the axial tension of the anchor: Nak: 240KN, but when the deformation is coordinated, under the joint force of the bottom plate and the anchor, what is the actual force of the anchor in the field of academic research? need to be calculated. Due to the beneficial effect of the anchor rod, it can be considered to reduce the thickness of the plate to adjust the force of the plate and the anchor rod, so that the anchor rod can share more force. Assuming that after setting the anchor rod thickness, the original plate thickness is adjusted to 500mm, and considering the beneficial influence of the self-weight of the bottom plate, the anti-floating design value is:

12×10×1.2-O. 5×24×0.8=134.4KN/m2.

The equilibrium equation is given by the deformation coordination condition:

△wxFw-△m×Fm=Fm/K (Formula 3)

Where: K=△s×Es/L (Formula 4)

Fm - the anti-buoyancy provided by the anchor

Fw--design value of water buoyancy per unit area

△w--deformation of the panel at the corresponding position under the unit uniformly distributed load

△m--The deformation of the corresponding position of the panel under the unit concentrated load

K--Anchor Rod 0°

Es - elastic modulus of reinforcement

L --- The length of the unbonded free section of the anchor rod

Arrange formula 3 and formula 4 to get: Fm=△W×Fw/(△m+l/K) (Formula 5)

Using some finite element analysis software here, it can be easily concluded that a total of 6 bolts are needed, and each bolt can provide a resistance of 233KN. This value shows that the standard value of the axial tensile force of the original assumption is more reasonable. At the same time, the steel bars needed for the bottom plate can be calculated. The diameter of the gluten bar is 12mm, and the spacing is 100mm, which is basically structural reinforcement; the diameter of the bottom bar is 22mm, and the spacing is 100mm. Roughly calculated and compared, the cost of the bottom plate and anchor rod at this time can save about 10% of the cost when only the bottom plate is used for anti-floating. The plate gluten is only required for structural configuration, indicating that there is still room for plate thickness reduction. Reduce the thickness of the plate to reduce the stiffness of the bottom plate, allowing the anchor to share more of the buoyancy effect. In this way, there is room for further saving of project cost, so this discussion will not continue to state.

(4) The calculation of the previous anchors is calculated based on the assumption that all the anchors are concentrated in the mid-span of the floor, but in actual engineering, in order for each anchor to fully play its role, it is necessary to maintain a certain distance between the anchors . At the same time, when using finite element software to calculate, we can see that the deformation of the mid-span of the bottom plate is distributed in a circle. In this project, regular hexagons are arranged in the mid-span of the slab, and the distance between anchor rods is 1.2m. The actual position of the anchor rod can be arranged in the plate with springs of equal stiffness, and then analyzed and calculated by finite element analysis software. It can be seen that each anchor rod can play its role well. (5) For joint deformation of the anchor rod and the bottom plate, the anchor rod should be put into a specific model for calculation. The project uses finite element calculation software to calculate that the maximum local deformation of the bottom plate is 5.7mm, which is far less than the deformation limit requirement.

3 Construction technology of anti-floating anchor

3.1 Measurement stakeout

① Carry out stakeout according to the foundation layout drawing, determine the position of the anchor rod, make a record during the measurement and setting out process, and report to the supervisor for review after the inspection is correct, and the construction can only be carried out after the review is correct.

② Set recheck points outside the anti-floating design range, clearly marked, convenient for measurement plan and inspection, and at the same time ensure convenient retest during construction and ensure the accuracy of hole positions.

③ The hole position stakeout point guarantees that the deviation is less than 20mm.

④ Clean up the obstacles in the construction area and investigate the relevant pipelines in advance to ensure the smooth progress of the construction.

3.2 Drilling and cleaning

3.2.1 Measurement positioning

During the construction stakeout, the anchor bolt measurement and positioning has a direct impact on the accuracy and quality of subsequent construction. It is required to stake out according to the unified number. According to the requirements of the plan, use the total station to measure the anchor bolt hole position in the construction area, mark it with the steel bar head, and Arrange personnel to review. The pile position error is controlled by this method, that is, the deviation of the hole position should be less than 20mm after the survey. During the construction process, if the position of the anchor rod needs to be moved under special circumstances, it must be reviewed and agreed by the owner and the design unit, and the construction can only be carried out after a comprehensive inspection of the shifted position.

3.2.2 Drilling rig in place

Before the drilling rig construction, the personnel need to level, straighten and stabilize the down-the-hole drilling rig, and adjust the rotating machine before opening the hole to ensure that the rotating rod is vertical. The diameter of the anchor hole is 150mm, and the deviation of the drilling depth is controlled within 1% of the design depth, not exceeding 500mm, and the deviation of the anchor hole is less than 5%, so as to ensure that the depth meets the design standard. Before the anchor hole is drilled, arrange personnel to conduct a comprehensive inspection of the drill bit size and equipment performance to ensure the accuracy of the drill hole diameter and the quality of the drill hole, and control the center of the anchor hole to avoid deviation. If the anchor hole deviates, the drilling must be stopped immediately, and deviation correction measures should be taken to ensure that the hole is formed again.

3.2.3 Cleaning the anchor hole

After the anchor hole is formed, connect the well flushing pipe to the air compressor, put it into the hole, and repeatedly flush the inside up and down until the air in the hole returns, and the hand is stretched into the hole without dust, so as to avoid the sediment in the hole from affecting the follow-up construction. After clearing the hole, use the detector to detect the depth, diameter and inclination of the hole, reasonably control the error, and maintain the hole well to prevent the slag from flowing into the hole.

3.3 Anchor production and lowering

According to the design drawings, the material is cut according to the design size. Before assembly, the oil stain and film rust on the surface of the steel bar should be removed. The length of the anchor rod should be the design length plus the length of the anchor section. After the bolt is manufactured and before the anchor is lowered, the grouting pipe needs to be inspected comprehensively to ensure that there are no bad conditions such as blockage and rupture. It is also necessary to ensure that the interface is firm and has passed the inspection and acceptance of the supervisor. In the process of inserting the anchor bar, it is necessary to avoid bending and twisting of the anchor bar, and to ensure that the angle of the rod body and the drilling angle have a good consistency. In the specific anchoring process, if the rod body cannot reach the bottom of the hole, it is necessary to pull out the rod body, drill the hole again and then anchor again. It is strictly forbidden to use mechanical equipment to press into the hole. The anchor bar and the grouting pipe are tied together and put into the drill hole together. Normally, the distance between the end of the grouting pipe and the bottom of the hole should be 100mm. The insertion depth of the anchor bar in the hole should not be lower than 95% of the length of the anchor bar, nor should it be too deep to ensure that the exposed length is reasonable. At the same time, the thickness of the concrete protective layer of the main bar body should not be less than 50mm.

3.4 Grouting

When preparing the grout required during the construction of the prestressed anti-floating anchor, the water-cement ratio should be 0.4 to 0.5, and the cement used should be P.C32.5 grade cement. Before grouting, construction personnel should stir the cement slurry evenly, then use the grouting pump to transport clean water to the anchor hole, dredge the internal pipeline and then grout. It is forbidden to pull out the grouting pipe before grouting, so as not to affect the compactness of the grouting at the bottom of the anti-floating anchor. The technique used for grouting is "underwater concrete pouring method". When the grout in the anchor hole is full and the filling coefficient exceeds 1.2, stop grouting. During the grouting period, the construction personnel should carry out the grouting operation continuously, and lift the grouting pipe while grouting, and the lifting height must be lower than the cement slurry surface in the anchor hole.

After the first grouting, the initial setting strength of the grouting body is 5MPa, and then the second high-pressure grouting is carried out. The raw material used for grouting is pure cement slurry, and external admixtures with expansion and early strength effects can be added if necessary. For the anchoring section of the basement, the grouting method can be "anchor hole bottom return grouting method". The construction personnel can directly insert the grouting pipe into the area about 50cm away from the bottom of the anchor hole, and then start the grouting pump to transfer the cement mortar into the anchor hole end. After grouting, the cement grout will fill up from the bottom of the anchor hole, expel the air in the anchor hole, and squeeze out the first grout directly from the hole. After the initial setting of the cement grout, the grout is replenished until the cement grout in the anchor hole is full and substantial. It should be noted that when making test blocks for prestressed anti-floating anchors during construction, three sets of test blocks should be made for every 20 anchors. The size of the test blocks is 70.7mm×70.7mm×70.7mm, and the compressive strength for 28d.

3.5 tension and lock

Clean the top of the prestressed anti-floating anchor, and then perform tension locking after the strength of the anchor reaches 100% of the design value. The tensioning method is the "secondary tensioning method". When tensioning for the first time, a single windlass can be used to apply about 10% to 20% of the tension value. During the tensioning period, the steel strand should be tightened to stabilize it into a pressing plate and an anchor seat. After the second stretching, use 8 bundles of steel strands to lock all anchors synchronously. The specific stretching steps include: install the pile head device→primary pre-tensioning→adjust the top of the anchor bar→secondary stretching→locking and sealing the anchor. During the tension period, the construction personnel's lifting rate per minute should be less than 10% of the design stress, and the unloading rate should be less than 20% of the design stress.

When threading the anchor, the construction personnel need to strip off the PVC casing of the steel strand in advance, then clean the oil on the steel strand with cotton yarn, sort out the center of the steel strand and put it into the anchor plate, respectively according to the bottom and middle , to perform tensioning. After the first tension of each anchor cable is completed, the construction personnel can carry out the second tension. Each tension load is about 0.25, 0.5, 0.75, 1, and 1.5 times the design tension of the anchor cable in the basement, and the tension reaches the last level of load. Finally, it should be stabilized for 10 to 20 minutes, and for other load levels, it should be stabilized for 5 minutes. When the tension is stable, the construction personnel should record the elongation of each steel strand and each level separately, and lock and seal the anchor when the prestress of the anchor bolt is not reduced. The way to seal the anchor is to cut off the remaining steel strands with a grinding wheel cutter, and then seal the anchor head with a φ180×16 matching steel backing plate.

3.6 Pull-out test

The acceptance test shall be carried out after the anchor bolt construction reaches the strength, and the number of anchor bolts for the acceptance test shall be 5% of the total number, and shall not be less than 6. The specific acceptance test shall be carried out in accordance with the relevant provisions of Appendix H of "Technical Standards for Anti-floating of Construction Engineering" (JGJ476-2019).

Requirements for non-destructive testing of bolt anchoring quality: The anchor bolts of this project should be inspected in accordance with JGJ/T182-2009 "Anchor Bolt Quality Non-destructive Testing Regulations"; the inspection content should include the length of the bolt body and the detection of anchorage density; It is detected by acoustic reflection method. The number of inspections should not be less than 10% of the total number of bolts, and each batch should not be less than 20.

After the basic test of the bolt is completed, the design parameters such as the designed bearing capacity of the bolt and the depth of penetration into the rock must be reviewed and updated.

3.7 Bolt maintenance and testing

In order to ensure the construction quality, 15 days of maintenance is required. During the maintenance process, the exposed anchor head steel bars need to be well protected to prevent them from being touched by external forces. After the maintenance is completed, the bolt drawing acceptance is required to grasp the specific construction quality.