Causes and preventive measures of appearance quality defects of bridge prefabricated components
As bridges are increasingly used in transportation facilities, precast concrete members of bridges are also widely used, but the appearance quality of some bridge prefabricated components is not satisfactory. The reasons for the formation are not only due to the outdated process equipment, but also the extensive operation. People pay attention to the lack of appearance quality. Some people think that the prefabricated components are only semi-finished products of bridges. Therefore, as long as the structural performance is qualified, it can be The appearance quality is insignificant, and this kind of understanding leads to 68.8% of the unqualified appearance quality during the inspection of prefabricated components throughout the country. The appearance quality of prefabricated components is a direct reflection of the quality of structural concrete, which is of great significance to concrete engineering. Therefore, in-depth analysis and exploration of the appearance quality defects of concrete prefabricated components, gradually improve its preventive measures and continuously study effective repair methods, control and Improving the appearance quality of concrete prefabricated components has certain practical effects.
1 Prefabricated component appearance quality defect manifestation. It refers to the irregular surface of the concrete surface which is not formed by the cement mortar to form the corresponding outer surface or the outer surface is damaged by the template, and the diameter is generally less than 5 mm; Due to the lack of cement mortar, the surface of the concrete is exposed to the overhead of the stone or there is a gap between the aggregates. The concrete forms a honeycomb shape with irregular or more holes; holes. The depth and length of the concrete that exceeds the thickness of the protective layer but does not exceed 1/3 of the cross-sectional dimension of the void or the void existing in the structure, partially or partially without concrete, if there is a cavity that can be seen through the structural member is called a hole; . Deep into the gap inside the concrete member, it can be divided into dry shrinkage and self-shrinkage. The shrinkage is the volume reduction caused by the evaporation of excess water in the concrete and the decrease of humidity. The self-shrinkage is the volume reduction caused by cement hydration. Small; shape defects. Refers to the phenomenon that the end of the member is inclined, the bending is not straight, the edge is missing, and the flash, rib cancer and the like formed by the leakage of the slurry;
2 Causes of appearance quality defects and prevention measures
The reasons for the formation of the prefabricated parts are mostly due to the poor venting of the upper slant of the horseshoe. The anchors under the anchor section are dense and restricted by the position of the drawing tube and the bellows, which makes the part of the concrete difficult to vibrate or the vibration is insufficient. It stays between the concrete and the formwork to form a pockmark; the continuity in the pouring process is poor, and the residence time of the poured concrete before the fresh concrete is put into the mold is too long, and even solidification occurs, and the pockmark is prone to occur between the new and old concrete joint layers; The cement mortar flows along the stencil gap or the surface layer to the underlying concrete surface, resulting in intermittent tear-like phenomenon after the mold is removed. If the template is not cleaned before the pouring or the mold release agent and the wet process are improper, the moisture in the concrete is sucked away by the template. Or the template joint seam leakage causes the surface of the upper part to be damaged by the mortar and the surface is produced. The prevention measures are mainly to clean the surface of the formwork before pouring the concrete, and the template is tightly assembled to ensure that the gap meets the requirements of the specification. The concrete is poured and vibrated in strict accordance with the construction process. Applying a dilute oxalic acid solution to the formed psoriasis will cause oil on the psoriasis caused by oil stains The soil is washed with a brush and moistened with water before repairing. Then it is repaired with 1:2 or 1:2.5 cement mortar. The amount of mortar to be repaired is small. To avoid solidification of the cement mortar, mix it in a small bucket. When used with the mixture, if necessary, it can be mixed with white cement. The mortar is pressed into the pockmark with a scraper. Then it is smoothed to meet the appearance quality requirements. After repairing, cover the moisturizing and curing.
2.2 The causes and prevention measures of honeycombs are mainly caused by template leakage, insufficient vibrating or excessive vibration, which leads to serious leakage phenomenon; the concrete itself has small slump or insufficient vibration or vibration of vibrating equipment; concrete pouring Inappropriate pouring method in the process, using the slurry method or the slurry method, the vibration is not applied properly or the concrete itself is segregated; the concrete itself is insufficiently stirred, resulting in unevenness and inconsistency, causing serious local mortar defects; the prevention measures are strictly in accordance with the construction. Before the process is carried out, the formed honeycomb should first be cleaned and cleaned with soft concrete around the honeycomb and cleaned with high-pressure water or wire brush. Then, the cement is repaired with cement mortar with the same strength as the original concrete. With a ratio of 1:2 or 1:3, if the component has waterproof requirements, 1-3% of the iron oxide waterproofing agent should be added to the cement mortar, and the cement mortar should be densely pressed into the honeycomb with a trowel. After careful scraping, the corners of the components should be straightened by the ruler to ensure the appearance is consistent. After the repair is completed, When covering wet curing.
2.3 The reason for the formation of the hole hole is generally due to the difficulty of vibrating the concrete when the internal and external formwork is too small, or the particle size of the aggregate is too large and the steel bar is too dense, which causes the aggregate to be stuck by the steel bar during the concrete blanking process. The concrete in the lower part of the aggregate forms a hole; the poor fluidity of the concrete or the phenomenon of segregation leads to the poor pouring of the concrete; the vibration is not vibrated in the order of vibrating, or there is no layering or thick layering. The radius of the vibrating effect of the lower concrete is too small to form a loose part around the concrete. The cement in the concrete mixing material has agglomeration or the inclusion of ice and mud in the aggregate; the repair method of the hole is generally to first fix the concrete to be repaired. And the protruding aggregate particles are carefully cut out and ensure that the upper part of the hole is inclined upwards and the lower part is horizontal. After that, the base layer of the hole is cleaned with high-pressure water or wire brush, and then filled and wetted with wet cotton yarn and other materials for 72 hours. In order to achieve full wetting of the concrete around the hole, the repair generally uses fine stone concrete or a supplement higher than the original concrete strength. Fill the shrinkage concrete, fill the hole around the hole with cement mortar before filling, then fill the concrete and carefully vibrate it, smooth the surface after smoothing, and then cover the surface with plastic film to maintain. For hollows with small diameters but large depths, if the cutting is cleaned, the size of the defects will be increased to weaken the structure. Therefore, the hydraulic slurry with a water-cement ratio of 0.7-1.1 can be used to reinforce the cement slurry. If necessary, it can be used in the slurry. When adding a certain amount of water glass as a coagulant, the pores should generally be no less than two, one used as a grout for exhaust and drainage, and two times of repeated grouting, in the first grouting After the initial setting, the second grouting is carried out, and the grouting pipe can be cut off after 2-3 d of grouting, and the remaining grouting pipe gap is treated according to the small shallow hole or the honeycomb processing method to meet the quality and appearance requirements.
2.4 The cause of cracks is mainly due to improper curing after concrete pouring, resulting in excessive surface moisture loss, surface cracking caused by uneven shrinkage inside and outside the concrete; segregation due to excessive vibration during concrete pouring, or excessive cement content on the surface The increase of the shrinkage amount leads to the occurrence of cracks; for the fine cracks, the cement paste can be poured into the cracks, and after the embedded cracks are covered, the cracks are cleaned, and after the cracks are dried, the epoxy paste is applied twice or affixed. Epoxy glass cloth is used for surface sealing, and for deep or penetrating cracks, the surface of epoxy resin grouting should be closed with reinforced epoxy cement.
2.5 Surface bubble member bubble generation mechanism is due to the fact that the vibration is not dense during construction, the internal gas can not be completely discharged, and the free water in the concrete forms blisters on the surface of the template. As the concrete condenses the blister, the component is removed after the mold is removed. The surface forms pits of different sizes; the oil used as the release agent is too much and unevenly applied on the surface of the template, resulting in the formation of oil droplets for the discharge of bubbles on the surface of the template; the concrete slump control is not good. The vibration of the small vibrating device can be attenuated too fast and the effective vibrating range becomes small. The concrete should not be dense between the two effective vibrating ranges, or the slump is too large, the water-cement ratio is too large, and the blisters are increased. The probability of formation; the excessive coarse aggregate in the concrete or the improper aggregate size causes the aggregate to be less dense and has more free voids to form bubbles; the influence of the admixture causes more air inside the concrete to increase the bubble. The probability of formation; the control measures are to control the layer thickness during the pouring process, the insertion vibrator should not exceed 0.8 times the length of the rod, and the surface vibrator Not more than 20cm, the attached vibrator should not exceed 40cm, the vibration time should be flat, slurry, stop sinking, and it is best to use the vibrating method combined with the plug-in type. During the insertion process, the principle of quick insertion and slow pull should be adhered to, and the residence time and insertion distance of each rod should be controlled; and the brushing condition of the template should be strictly controlled to ensure that the attached oil is uniform and cannot be dripped. If the concentration is too large, soap liquid should be added or soap liquid should be used directly as a release agent. 3 Conclusion The appearance quality of bridge concrete prefabricated components is not an isolated appearance, which reflects the intrinsic quality of the whole project, the proportion of raw materials in the construction process and the construction process, and also reflects the enterprise from a certain angle. The management level, so in-depth analysis of the factors affecting the appearance quality, and the targeted development of prevention and control measures to improve the quality of precast concrete components has a very practical significance.