1 Control of concrete raw materials：
The control of raw materials mainly starts from the entry and storage of materials and whether they meet the requirements of relevant standards and specifications. The raw materials are carefully and comprehensively inspected and tested to ensure that the raw materials have no hidden quality risks.
1.1 The same batch of raw materials should be used as far as possible for the same unit project.
Choose the same origin, the same brand, the same color of cement, sand, gravel and admixtures and other raw materials.
1.2 Strengthen the on-site inspection of cement
Test inspection: For each batch of cement entering the site, its factory certificate should be checked, and samples should be sent to the laboratory for testing to determine its strength, initial and final setting time, stability and other indicators. Only all the cement can be used.
Check the storage conditions. Make sure cement is stored in a dry, ventilated, weatherproof and moisture-proof place. The floor is paved with sleepers, at least 30cm above the ground, and the cement should be used as soon as possible after entering the site. Special attention should be paid to prevent the cement that has expired or been agglomerated by rain from flowing into the construction site. This kind of cement will not only affect the strength of the concrete, but also make the poured concrete have dark spots, affecting the appearance quality of the concrete.
1.3 Strengthen the on-site inspection of sand and gravel materials
Test inspection: test at any time, and conduct tests on the sand and gravel materials in the sand field, such as screening, impurity content, crushing value and other items. Control the gradation of sand and gravel. The selection of sand should be selected and roughly uniform, and the maximum limit percentage of the specified gradation cannot be used, which is conducive to the compactness and smoothness of the concrete.
Coarse aggregate inspection: The coarse aggregate used for concrete should have a good gradation, and its maximum particle size should not exceed 1/2 of the slab thickness or 1/4 of the minimum size of the structural section, nor larger than the minimum size between the steel bars. 3/4 of the clear distance, and shall not exceed 100mm. When the structural section is small and the steel bars are denser, bean stone concrete can be poured.
2 Control of key process of concrete mixture：
Whether the process control of the concrete mixing process is tight or not directly affects the quality of the concrete. Therefore, it is the key to ensure the quality of concrete mixture to be familiar with the key processes of concrete mixture and to carry out careful and strict control of each process. The main procedures and precautions of concrete mixture are described.
2.1 Mix ratio design and selection of cement labels
Strictly control the concrete mix design. Calculations and tests are carried out in accordance with relevant technical specifications, and inspections and verifications are carried out frequently during the construction process. Select a reasonable cement grade, so that the ratio of the cement grade to the concrete design strength grade is controlled between 13-20. When the objective situation cannot be achieved, measures such as adding admixtures (such as finely ground fly ash) or water reducer to the concrete mixture can be taken to improve the workability of the concrete mixture and increase the density of the concrete. and finish.
Strictly control the water-cement ratio. Note that when mixing concrete on site, the water-cement ratio should be adjusted according to the moisture content of each sand in order to maintain the good workability of the concrete and reduce the formation of blisters and pores.
2.2 Measurement and slump control
Strictly control the slump of concrete, and check the slump of concrete at the mixing site and pouring site according to regulations. The slump during concrete pouring can be taken as 1-9mm according to the nature of the components and the specific reinforcement of the components. Values can be found in relevant specifications or information. During the construction process, the parking time of the concrete mixture should be shortened as much as possible to reduce the loss of slump.
Establish a post responsibility system for raw material measurement, and strive to be simple, easy and reliable in measurement methods, especially for water measurement, standard measurement buckets should be made; admixtures should be measured with a small platform scale.
How to control the appearance defects of precast concrete?
1 Control of concrete raw materials
The control of raw materials mainly starts from the entry and storage of materials and whether they meet the requirements of relevant standards and specifications. The raw materials are carefully and comprehensively inspected and tested to ensure that the raw materials have no hidden quality risks.
1.1 The same batch of raw materials should be used as far as possible for the same unit project.
Choose the same origin, the same brand, the same color of cement, sand, gravel and admixtures and other raw materials.
1.2 Strengthen the on-site inspection of cement
(1) Test inspection: For each batch of cement entering the site, its factory certificate should be checked, and samples should be sent to the laboratory for testing to determine its strength, initial and final setting time, stability and other indicators. Only all the cement can be used.
(2) Check the storage conditions. Make sure cement is stored in a dry, ventilated, weatherproof and moisture-proof place. The floor is paved with sleepers, at least 30cm above the ground, and the cement should be used as soon as possible after entering the site. Special attention should be paid to prevent the cement that has expired or been agglomerated by rain from flowing into the construction site. This kind of cement will not only affect the strength of the concrete, but also make the poured concrete have dark spots, affecting the appearance quality of the concrete.
1.3 Strengthen the on-site inspection of sand and gravel materials
(1) Test inspection: test at any time, and conduct tests on the sand and gravel materials in the sand field, such as screening, impurity content, crushing value and other items. Control the gradation of sand and gravel. The selection of sand should be selected and roughly uniform, and the maximum limit percentage of the specified gradation cannot be used, which is conducive to the compactness and smoothness of the concrete.
(2) Coarse aggregate inspection: The coarse aggregate used for concrete should have a good gradation, and its maximum particle size should not exceed 1/2 of the slab thickness or 1/4 of the minimum size of the structural section, nor larger than the minimum size between the steel bars. 3/4 of the clear distance, and shall not exceed 100mm. When the structural section is small and the steel bars are denser, bean stone concrete can be poured.
2 Control of key process of concrete mixture
Whether the process control of the concrete mixing process is tight or not directly affects the quality of the concrete. Therefore, it is the key to ensure the quality of concrete mixture to be familiar with the key processes of concrete mixture and to carry out careful and strict control of each process. The main procedures and precautions of concrete mixture are described.
2.1 Mix ratio design and selection of cement labels
(1) Strictly control the concrete mix design. Calculations and tests are carried out in accordance with relevant technical specifications, and inspections and verifications are carried out frequently during the construction process. Select a reasonable cement grade, so that the ratio of the cement grade to the concrete design strength grade is controlled between 13-20. When the objective situation cannot be achieved, measures such as adding admixtures (such as finely ground fly ash) or water reducer to the concrete mixture can be taken to improve the workability of the concrete mixture and increase the density of the concrete. and finish.
(2) Strictly control the water-cement ratio. Note that when mixing concrete on site, the water-cement ratio should be adjusted according to the moisture content of each sand in order to maintain the good workability of the concrete and reduce the formation of blisters and pores.
2.2 Measurement and slump control
(1) Strictly control the slump of concrete, and check the slump of concrete at the mixing site and pouring site according to regulations. The slump during concrete pouring can be taken as 1-9mm according to the nature of the components and the specific reinforcement of the components. Values can be found in relevant specifications or information. During the construction process, the parking time of the concrete mixture should be shortened as much as possible to reduce the loss of slump.
(2) Establish a post responsibility system for raw material measurement, and strive to be simple, easy and reliable in measurement methods, especially for water measurement, standard measurement buckets should be made; admixtures should be measured with a small platform scale.
2.3 Stirring time and admixture control
(1) Check the concrete mixing time at any time. The shortest time for continuous mixing of concrete (all materials are put into the mixing drum until the discharge is stopped) can be determined according to the slump requirements of the concrete pair, the model and capacity of the mixer. For specific time, please refer to relevant specifications or design materials.
(2) Strictly control the admixtures. The admixtures used in the concrete should be qualified according to the relevant standards and tested to meet the construction requirements before use. Powder admixtures should be kept dry to prevent moisture agglomeration. The powdered admixture that has been agglomerated should be dried, crushed, and passed through a 0.6mm sieve before use.
3 Control of construction process：
The main objects of construction process control are the installation and cleaning of formwork, the distributing and tamping of concrete, and the treatment of construction joints during intermittent construction. The descriptions are as follows: 3.1 Template installation and cleaning
(1) Try to use steel molds instead of wooden molds. Although the cost is high, it can greatly improve the work efficiency, and can significantly improve the finish of the concrete and improve the appearance quality of the concrete.
(2) Strictly control the cleaning of the steel mold. Before each mold installation, use a small grinding wheel to remove rust on the steel mold. After the rust removal, wipe it with a rag and apply light oil in time to keep the inner surface of the steel mold free of any sundries and dye spots.
(3) The release agent of the steel formwork should be brushed evenly and should not be missed. Practice has proved that it is better to choose light engine oil as the release agent, which is easy to volatilize under the action of sunlight after the mold is removed, and will not leave any traces, so that the appearance of the concrete is bright and clean, and it can prevent the steel mold from rusting.
(4) Make sure that the formwork is reinforced firmly. The key is to check that the template seams are tightly assembled. The joint of the template is controlled at about 2mm, and the glass glue is used to coat it densely and flat to prevent the leakage of the slurry, and the appearance of honeycomb pitted surface or unclear lines.
(5) Before pouring concrete, the wooden formwork should be fully moistened with clean water and cleaned without leaving water.
(6) Always pay attention to keep the inner surface of the formwork clean during the construction process. When the construction workers step on the dirty formwork or the concrete slurry splashes on the formwork or the formwork is stained for other reasons, after pouring a layer of concrete, the stains on the formwork on the upper section must be wiped clean with cotton gauze in time to avoid the appearance of deep concrete. Spots of color appear.
(7) When pouring concrete, the formwork, brackets, caulking, etc. should be observed frequently. If the formwork is found to be moving, the pouring should be stopped immediately, and the concrete should be repaired well before setting.
(8) Before each use, check the deformation of the template. It is forbidden to use the deformed template such as bending, unevenness or lack of edges and corners.
3.2 Concrete placing and vibrating
(1) Before pouring concrete, check whether the position of the steel bars and the thickness of the protective layer are accurate, and whether the pads are fastened as required; do not step on the steel bars during operation. Good, so as not to show the tendons.
(2) When the free fall height of concrete exceeds 2m, use a stringer or chute to avoid concrete segregation.
(3) To control the vibration spacing, the plug-in vibrator should not be larger than 1.5 times its action radius; for light aggregate concrete, it should not be larger than 1 time its action radius. The distance from the vibrator to the template should not be greater than 0.5 times its radius of action. Control the thickness of the pouring layer of concrete to be about 1.25 times the length of the vibrator's action part. The new layer of vibrating should be inserted into the first poured concrete 5~10cm, and strive to closely combine the upper and lower layers.
(4) Control the vibrating time, so as not to under-vibrate, but not to vibrate. Set up a full-time vibrating team to facilitate the accumulation of experience. The appropriate vibrating time can be judged by the following phenomena: the concrete does not sink significantly, no air bubbles appear, the surface of the concrete is in a horizontal state, and the corners of the formwork are filled.
(5) Pay attention to the vibrating method. When vibrating vertically, vibrate the vertical concrete surface; when vibrating obliquely, the vibrating rod and the concrete surface are at an angle of 40°~45°; the depth of the rod inserted into the concrete should not exceed 2/2 of the length of the rod. 3~3/4; the vibrating rod should be twitched up and down in time, and the layers should be vibrated and compacted evenly. After the vibrating is done, the vibrating rod should be slowly pulled out to fill the cavity caused by the vibrating rod with concrete.
(6) Control the vibrating procedure, first around the middle and then in the middle, and pay attention to the height of the concrete paving around the middle, so as to drive out the air bubbles to the middle and avoid gathering at the formwork.
(7) When vibrating, the vibrating rod should not collide with the steel bar, formwork, embedded parts, etc., and the vibrating rod with a blade can be used for vibrating in the place where the steel bar is dense.
(8) Pay attention to the vibrating of the concrete at the mortar pad of the protective layer, and make sure that the cement mortar is fully wrapped; or take a small section of vibrating and remove a small section of the pad first. In this way, bright spots or dark spots on the surface of the pad can be effectively avoided.
3.3 Treatment of concrete construction joints
(1) When pouring columns, beams, walls and similar structures, if the interruption time exceeds the time shown in the table below, it should be treated as a construction joint, and the pouring should be allowed when the concrete compressive strength is not less than 12MPa.
(2) In order to improve the strength and compactness of the joints, the concrete can be vibrated twice. In the construction of mass concrete, if the joint time exceeds the time specified in the table above, it is also possible to vibrate the concrete to be poured first before and after the final setting (4~6h), and then pour the next layer of concrete. The method of secondary vibrating to deal with construction joints should be tested first to find out the law before it can be applied.
(3) Before pouring concrete, the surface cement film and loose stones or weak concrete layer should be removed from the construction joint, the surface should be chiseled, rinsed with water, fully moistened, and the water remaining on the concrete surface should be removed.
(4) In order to facilitate cleaning and flushing, the formwork should be opened along the construction joint. During construction in winter, if there is ice and snow, it should be melted with hot air and cleaned up; debris such as sawdust can be purged with high-pressure air. After all is cleaned up, seal the opening of the through bar, and apply cement slurry or stone-reducing concrete mortar to pour concrete.
(5) Be sure to handle the construction seam edges so that the construction seams are on the same plane and overlap with the formwork joints as much as possible.
