1. 1

In recent years, the analysis of broken piles of bored piles in several urban bridges shows that improper construction technology is the main reason for broken piles of bored piles. According to experience, underwater concrete pouring technology should be adopted when the rising speed of water seepage at the bottom and wall of the hole is >: 6mm/min. Only when the rising speed of seepage is less than or equal to 6 mm/min can ordinary concrete be used for construction. However, if ordinary concrete is used for pouring because of the sharp increase of water seepage, all concrete cast-in-place piles will be scrapped, resulting in economic and construction period losses. This is because in the process of concrete pouring, the water (free water) oozing from the shaft wall is immersed in the concrete mixture, and too much free water "erodes" the mortar, so that the cement slurry floats first, then the sand particles are dispersed, and finally the coarse particles are embedded in the hollow skeleton, becoming defective piles.

1.2 preventive measures

For the concrete of a small amount of water seepage in the excavated pile, ordinary concrete cannot be used for pouring, that is, the rising speed of water seepage at the bottom and wall of the hole >: 6mm/min, and the pile foundation must be poured as underwater concrete. At this time, the injection height should be

2 The geometric dimension error of prestressed concrete frame is beyond the allowable range.

2. 1 reason

2. 1. 1 template making reasons

Prestressed concrete beams should be prefabricated with frame steel formwork, but the following problems often appear in the production process.

The front and rear columns (commonly known as "drumsticks") of (1) frame are too short, so it is difficult to operate and easy to make mistakes.

(2) The frame spacing is too large, and some are100 cm; ;

(3) The distance between the frame and the joint is large, some of which are more than 40cm, which makes the joint stiffness insufficient;

(4) The welding is not in place when the template is made, and the repair welding due to thermal deformation is beyond the error range;

(5) The bottom plate is not set on the same horizontal plane, resulting in warping.

2. 1.2 template installation reasons

(1) The installation template is not controlled according to the specified method, and the geometric dimension deviation is large;

(2) The butt wedge for adjusting the elevation is not made of square wood, but made of scrap, or the butt wedge is not connected with nails, or a single wedge is used, and the wedge "retreats" due to the working vibration of the attached vibrator during construction;

(3) The adjacent surfaces at both ends of the formwork joint should be in the same plane but not in the same plane, and the adjacent straight lines should be in the same plane but actually in different planes, resulting in bending;

(4) The two screws of the (upper and lower) counter-pulling screw are not tightened.

2. 1.3 concrete construction reasons

During the construction, the working vibration of the vibrator (especially the attached vibrator) causes the screw of the opposite screw to loosen or the wedge of the butt wedge fails to be reinforced in time. Excess concrete mixture is not removed, resulting in beam height error beyond the allowable range.

2.2 Preventive measures

In order to prevent the common quality defects of prestressed concrete beams with geometric size errors beyond the allowable range, the basic requirements for formwork fabrication and installation should be clarified first. These basic requirements are: ① the geometric dimensions should be accurate, that is, all geometric dimensions and sections should be completely consistent with the design (within the allowable range); (2) The rigidity requires that the concrete will not deform during construction; (3) Stability requires effective connections and firm support during construction. This is the basic requirement that must be followed in the process of template production and installation, and it must be fully realized after the above process is completed. In addition, if the template should be flat or smooth (non-planar template), the solder joints should be polished and smooth, the angular lines should be distinct, and the template assembly should be sealed tightly. These are also the conditions that a template should have.

(1) The bottom formwork of prestressed concrete beam is an important part of formwork and must be placed on the same horizontal plane. The maximum relative error of each point should be controlled within 3 mm, and the floor supporting the frame must be hardened with cement concrete. The reserved hole distance of the opposite screw under the bottom die should not be greater than 100cm. The thickness of the steel plate used to make the template should be 5 mm ~ 6 mm ... The frame spacing varies with the thickness of the steel plate, and it should be 70 cm ~ 90 cm, and the distance from the center of the template joint should not be greater than 25cm. The geometric dimensions of the template must be accurate, angular, sharp lines and smooth solder joints. Screw adjustment is most suitable for vertical lifting of formwork. The "chicken leg" should be made of channel steel, and its height should be coordinated with the height of the bottom die, so that it can be adjusted with the butt wedge. The wood wedge (butt wedge) for fine-tuning and reinforcing the "chicken leg" should be square wood (6cm×8cm) to ensure that any two butt wedges are parallel to each other at any joint distance. Tenon joint should be adopted between side plate and bottom plate and side plate. The female tenon and the male tenon at the tenon joint should be matched (tenon joint). If butt joint is not used, the joint width shall be ≮2mm during installation.

(2) the template installation, should strictly control the geometric size. The principles of geometric lofting and detection during template installation are as follows: ① First, find out the geometric center or central axis as a benchmark to prevent deviation; (2) Measure from the geometric center or symmetry axis to both sides to prevent eccentricity error; (3) After accurately determining the plane position and relative elevation of each corner (point) on the top surface of the structure, determine the plane projection or straight line position of each side with a straight line (tension rope); (4) The above (1) to (3) should be repeated several times until they are qualified.

(3) During construction, a special person shall be appointed to check the formwork, especially after the attached vibrator vibrates, to check whether the screws are loose and whether the wedge retreats, and reinforce it at any time. After pouring wing plate concrete, take the top of both sides of the plate as the benchmark, scrape off the excess mixture with a ruler to ensure that the beam height error is within the allowable range (box girder is +0mm and -5mm).

3 poor appearance quality of concrete

3. 1 reason

The main reasons for the quality defects of concrete appearance such as pits, bubbles, bubbles, honeycombs and fish scales are:

4. 1. 1 Improper material selection and mix design.

Contractors pay more attention to the procurement, storage and use of cement, but often ignore the selection of aggregate. Coarse aggregate needs the cooperation of two or more materials, which is easily overlooked. Improper gradation of concrete mixture and large void ratio lead to unconsolidated concrete and easy formation of honeycomb pits. Improper mixing ratio of cement concrete is also one of the reasons for poor appearance quality of concrete. Concrete mixing must be carried out in accordance with the approved mixture ratio.

3. 1.2 concrete construction reasons

The main reasons for the poor appearance quality of concrete are that the mixing time of concrete mixture is not strictly controlled, the mold is not carefully inserted, and the vibrating time and method of concrete mixture are improper.

3.2 Preventive measures

3.2. 1 Selection of qualified materials and optimization of mix design

The mixture ratio of modern cement concrete is already a five-component theory, in which cement, water, coarse aggregate and fine aggregate are four basic materials for pouring concrete, and admixture is an auxiliary additive to improve the technical performance of concrete. Our goal is to mix all kinds of raw materials evenly according to proper gradation and proportion, and discharge air and free water from concrete by appropriate methods, so that cement slurry can fill the gap between coarse and fine aggregates and become a cemented entity with solid inside and light outside.

Controlling the gradation of sand, selecting medium sand and making it roughly uniform, and not using the maximum limit percentage of specified gradation are beneficial to the compactness and smoothness of concrete. Coarse aggregate for concrete should have good gradation, and its maximum particle size should not exceed 1/4 of the minimum size of the structural section, nor should it be greater than 3/4 of the minimum clear distance between steel bars. Generally speaking, the maximum particle size of coarse aggregate of precast beam and slab should not exceed 3 1.5mm, and the maximum particle size of coarse aggregate of other concrete projects should not exceed 37.5 mm In the design of concrete mixture ratio, the appropriate water cement ratio and slump should be selected according to the structural size and steel bar density.

3.2.2 Improve concrete construction technology

(1) Check the mixing time of concrete construction at any time;

(2) Before shuttering, 8cm mortar screed-coat shall be applied to the lower mouth of the side formwork, and the screed-coat shall be embedded in the column and slab wall no more than 1cm, so as to ensure that the lower mouth is tight and prevent slurry leakage;

(3) When the free pouring height of concrete exceeds 2m, it is necessary to use a string tube or chute for blanking to avoid concrete segregation. The feeding speed should be moderate to prevent too many bubbles from entering;

(4) Control the vibrating distance, and the internal vibrator should not be greater than 0.5 times of its action radius. The distance between the vibrator and the template should not be greater than 0.5 times of its action radius. Control the thickness of concrete pouring layer to be about 1.25 times of the length of vibrating part. When the new layer is vibrated, it should be inserted with concrete pouring 5 cm ~ 10 cm to make the upper and lower layers closely combined. The insertion time of the plug-in vibrator is 20s~30s, and the action radius is 40cm~50cm. The stroke of the vibrator can be moved or staggered in a straight line, and the spacing between insertion points is 75cm. The plug-in vibrator adopts fast insertion and slow pumping, and extends into the lower layer by 5cm ~ 10cm. Control the time of the vibrator, not under-vibrating, but not over-vibrating. The suitable vibrating time can be judged by the following phenomena: when vibrating concrete, there will be no obvious settlement and a large number of bubbles, and the concrete surface is even, flat and submerged. Vibrating time should not be too long or too short. If it is too long, the concrete may be separated, the floating slurry on the upper layer will cause pits and cracks, and the coarse particles on the lower layer will cause fish scale pits. If it is too short, the concrete will have virtual vibration, and it is easy to have honeycomb holes.

Pay attention to the vibrating method. When vibrating vertically, the vibrator is perpendicular to the concrete surface; When vibrating obliquely, the included angle between the vibrator and the concrete surface is 40 ~ 45; The depth of steel bar inserted into concrete should not exceed 2/3 ~ 3/4 of the length of steel bar; Vibrating rods should be twitched up and down in time, and vibrated evenly and densely in layers. After vibrating, the vibrating rod should be slowly pulled out to make the concrete fill the holes caused by the vibrating rod. Control the vibrating procedure, first four weeks later, pay attention to the concrete paving around the high school, try to drive the bubbles out of the middle, and avoid gathering at the template.

When vibrating, vibrating rods shall not collide with steel bars, templates, embedded parts, etc. In places with dense steel bars, vibrating rods with blades can be used for vibrating.

Attached vibrators shall vibrate symmetrically at the same time. Vibrating shall be subject to no obvious subsidence on the concrete surface.

Pay attention to the use order of vibrators. Whether the plug-in vibrator is used in conjunction with the flat vibrator or the attached vibrator, the initial vibration should be carried out by the plug-in vibrator and the final vibration should be carried out by the latter.

(5) Concrete shall be collected at least twice. For the first time, it is advisable to wipe the pulp with wood, and press the surface coarse aggregate under the laitance. The last pulp collection should be carried out with an iron brush after bleeding and sizing. It is forbidden to sprinkle water when it is difficult to collect slurry, but some concrete can be eradicated and fresh concrete can be mixed to absorb slurry. When repairing plastering, excess surface laitance shall be daubed on other parts, and the surface shall not be dusted for repairing. Pay attention to the concrete vibrating at the protective layer mortar pad to ensure that the cement mortar is fully wrapped; Or adopt the method of vibrating for a short time and removing the small vibrating pad first. In this way, bright spots or dark spots on the surface of the polishing pad can be effectively avoided.

4 conclusion

Municipal bridge construction is affected by many factors, such as limited construction site, great interference and short construction period. , so it is difficult to guarantee the construction quality. It is necessary to think about different quality defects repeatedly, constantly sum up experience, and avoid recurring common problems.

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