Quality control of fair-faced concrete construction?

Fair-faced concrete is also called decorative concrete, that is, the surface of concrete is directly decorated with the natural color of cast-in-place concrete without any external decoration. Therefore, it is required that the concrete surface should be smooth, the color should be uniform, there should be no collision and pollution, and the setting of bolts and construction joints should be neat and beautiful, and the common quality defects of ordinary concrete are not allowed.

Quality standard: ① straight axis and accurate size; ② The edges and corners are square and the lines are straight; (3) the surface is smooth, clean and consistent in color; ④ There are no obvious bubbles, sand belts and black spots on the surface; ⑤ There are no beehives, pits, cracks and exposed tendons on the surface; ⑥ Formwork joints, opposite bolt holes and construction joints have regularity; All landowners template joints and construction joints without hanging slurry, slurry leakage phenomenon.

Its construction technology originated in the 1920s, which is characterized by high strength grade of concrete, fine workmanship and excellent skills of operators.

process flow

Template (steel template, bamboo glue template) customization, mobilization → elastic line positioning → steel bar binding → template installation → substructure concrete pouring → template removal and displacement → scaffold erection → column steel bar binding → template support → pumping concrete pouring → template removal → maintenance → finished product protection → surface repair → transparent protective film coating.

Technical requirements for concrete appearance

At present, there is no specification and standard for fair-faced concrete in China, and there is a lack of judgment basis and control requirements for color and color difference. According to the construction quality supervision and management experience accumulated by various construction units for many years, fair-faced concrete should not only meet the technical requirements of ordinary concrete, but also carry out construction quality control according to the following requirements:

(1) Tone: Tone plays an important role in the quality control of fair-faced concrete. Different proportion, dosage and origin of raw materials will affect the color of concrete. Therefore, the color of concrete must be blended before construction to determine the color of concrete in engineering construction. The color requirements of fair-faced concrete are basically the same (it is advisable to face the wall 5 meters without obvious color difference).

(2) Cracks: There are no obvious cracks on the surface of fair-faced concrete, and cracks with a width greater than 0. 12mm or a length greater than 50mm are not allowed.

(3) Bubbles: The bubbles on the surface of fair-faced concrete should be kept even and small to prevent honeycomb pits and basically eliminate the bubbles on the surface. The pore diameter of fair-faced concrete surface is 0.8mm ~ 2mm ~ 2mm, there should be no pore above 2.5mm, the pore depth is not more than 2mm, and the porosity is not more than 0.09% (not more than 10 per square meter).

(4) Smoothness: it is required to meet the advanced plastering quality acceptance standard, that is, the surface flatness error is not more than 2mm, the interlayer verticality error is not more than 3mm, and the allowable deviation is not more than 2mm.

(5) Smoothness: After demoulding, the surface is flat and smooth, with uniform color and no traces of oil stains, rust spots, powder, flowing and scouring.

(6) The axis is straight and the geometric dimension is accurate: the bolts are aligned in rows, and the embedded and reserved rules are orderly.

(7) Appearance defects: no defects caused by slurry leakage, slurry hanging, mold running and expansion, no dislocation, no cold joint or inclusion, no honeycomb, pits, holes and exposed tendons, and no traces of planing or brushing.

Selection of raw materials for fair-faced concrete

The selection of raw materials should follow two principles besides the relevant specifications. One is to improve the workability of concrete mixture, reduce segregation and bleeding, and the other is to improve the visual effect of hardened concrete.

0 1, cement

When selecting cement, the color, water retention, adaptability with additives and alkali content of cement should be considered. The color of cement plays a decisive role in the color of concrete. Even the products of the same cement factory will have different cement colors due to different production processes or additives, so the color of cement must be uniform and stable. If the water retention of cement is good, it will be beneficial to improve the water retention of concrete mixture and reduce segregation and bleeding. The adaptability of cement and admixture is good, which is beneficial to control the slump of concrete mixture and reduce the deviation of concrete slump when pouring. After drying and wetting cycles, hardened concrete will pan-alkali on the surface of concrete, which will affect the appearance color of concrete. Moreover, fair-faced concrete lacks the protection of decorative layer and is greatly influenced by natural environment. Choosing low-alkali cement can reduce the harm of alkali aggregate, so choosing low-alkali cement is preferred.

02, total

The gradation and particle shape of sand and gravel have great influence on the workability of concrete mixture, and sand and gravel with good gradation and particle shape are beneficial to improve the workability of concrete mixture. Therefore, natural medium sand should be preferred in Zone II, coarse sand and artificial sand should not be selected, and pebbles with continuous gradation should be preferred, and the pebble particle size should be small rather than large. In addition, the content of mud in sand and gravel is strict, and too high mud content will affect the color of concrete.

03, admixture

When choosing additives, we should focus on the adaptability, water retention and alkali content of additives and cement. Admixtures have good adaptability to cement, which can effectively control the slump loss of concrete mixture and reduce the slump deviation during concrete pouring. Admixtures have good water retention, which is beneficial to improve the water retention of concrete mixture and reduce the segregation and bleeding of concrete mixture. The alkali in admixture has the same effect on the appearance of hardened concrete as cement. The lower the alkali content in the admixture, the more beneficial it is to control the appearance color of hardened concrete and improve the durability of concrete. If the admixture is mixed with air-entraining components, high-quality air-entraining components should be selected, and calcium lignosulfonate and dodecyl air-entraining components should not be selected, because these air-entraining components introduce bubbles with large diameter and poor stability. In addition, defoamer can be selected to reduce the generation of bubbles in concrete. In addition, the retarding time of admixture should not be too long, and the setting time of concrete after admixture should be controlled within 12h.

04, mineral admixture

At present, the commonly used mineral admixtures are slag powder and fly ash. The purpose of choosing ground slag powder is to reduce the dosage of cement, thus reducing the shrinkage of cement, increasing the volume stability of concrete and reducing the drying shrinkage cracks of concrete. The improvement of the surface compactness of concrete is beneficial to the durability of concrete, and the workability of concrete can be improved by adding high-quality fly ash, which is convenient for casting and molding. When selecting mineral admixture, besides activity, fineness and color should also be considered emphatically. The color of mineral admixture should be uniform and stable. Grade S95 slag powder with surface area ratio above 4000cm2 /g should be selected for slag powder, and grade I fly ash should be selected for fly ash. The content of fly ash should be controlled within the range of 13%, because the content will affect the color of concrete.

Concrete mix design requirements

In the design of concrete mixture ratio, besides meeting the design requirements of strength and durability, the workability of concrete mixture and slump during pouring should also be considered emphatically. The optimized concrete mixture ratio should meet the requirements that the mixed concrete has good workability and water retention, is not easy to separate and bleed, and has small slump loss. The content of concrete admixture should be determined by experiments, and it is not suitable to mix too much, which is easy to separate and secrete water. In order to ensure the workability of concrete mixture, the total amount of cementitious materials per square meter should not be less than 350kg, because there are few cementitious materials, the total amount of concrete mixture is easy to be poor, and it is easy to isolate and secrete water. In addition, for ordinary concrete pouring, the slump should be small but not large, the slump of pumping concrete should not be greater than 180mm, and the slump of skip concrete should be less than150 mm. If the raw materials change during construction, it should be re-proportioned.

Construction control technology of fair-faced concrete

0 1, template and release agent

Commonly used templates are steel template and bamboo veneer, and the effect of steel template is better than bamboo veneer. When using steel formwork, the steel formwork should be derusted and polished first, and then coated with release agent. At the same time, the size of the steel formwork is strictly required, and the joints should be sealed, so that no slurry can be leaked. New templates with good quality should be selected for bamboo veneer to prevent the color of bamboo veneer from polluting the color of concrete. The joints of bamboo veneer should also be dense, so as not to leak slurry.

02, steel engineering

(1) After the reinforcement enters the site, the certificate of approval and material report are complete, the sampling inspection (re-inspection) is qualified, and the inspection is completed; The varieties and specifications of steel bars meet the requirements of design drawings.

(2) The variety, specification, quantity, position, anchorage length, joint position and connection mode (socket joint and welding) of steel bars meet the design and specification requirements.

(3) There is no rust on the surface of the steel bar, and the negative error must be guaranteed in the preparation size, the shape meets the design requirements, and there is no obvious deformation in appearance.

(4) Reinforced cushion blocks must use plastic cushion blocks with high strength and low elasticity; Don't use stones or other objects as cushions.

(5) When the steel bar is bound, the wire head is required to face the inside of the wall; It is forbidden to collide with embedded parts; Ensure that the position of the embedded pipeline is correct.

03, concrete pouring and vibrating

Before pouring, it is necessary to plan and coordinate preparations, strictly control the quality of ready-mixed concrete, and ensure the uniformity, stability and good workability of concrete performance. Concrete must be poured continuously to reduce the unloading time of concrete and the interval between layers during pouring. Construction joints should be left at open joints to avoid the influence of cold joints on the visual quality of concrete during construction. When pouring, the drop of concrete into the mold should be reduced, because the greater the drop, the easier it is for concrete to separate; The pouring thickness of each layer should be uniform, and should not exceed 300mm, otherwise it is not conducive to the discharge of gas in concrete.

Before vibrating, vibrating equipment should be selected according to different pouring parts and different concrete, and vibrating parameters should be determined. The frequency and amplitude of the vibrator should be low but not high, because the vibrator with high frequency and high amplitude is more likely to cause concrete bleeding. For steel formwork, both vibrating rods and attached vibrators can be used at the same time, because attached vibrators are more likely to discharge air bubbles on the concrete surface.

When vibrating, grasp the vibrating time of concrete until the concrete surface presents uniform cement slurry, and there is no obvious sinking and a large number of bubbles pop up upwards. In order to reduce the air bubbles on the concrete surface, the secondary vibrating process should be adopted. The first time after the concrete is put into the mold, the second time before the second layer of concrete is poured, the top layer of concrete is generally 0. After 5 hours, the second vibration was carried out. The vibrating process should be uniform, and it is forbidden to leak and over-vibrate, so as to ensure the concrete is solid.

04, template removal and maintenance

When the strength of the same concrete specimen reaches 3MPa (not less than 4MPa in winter), the formwork shall be removed. For different concrete structures and different concrete, the relevant specifications stipulate the time of formwork removal. In addition to the relevant specifications, the formwork removal time of fair-faced concrete should be sooner rather than later. When the formwork is removed too early, the cement on the surface of concrete structure is less hydrated and the strength is low, which not only affects the brightness of concrete surface, but also easily leads to surface cracks. Under normal circumstances, when there is a requirement in the specification, the form removal time should not be earlier than the specification, and when there is no requirement in the specification, the form removal time should not be earlier than 24h.

After form removal, it should be cured in time to reduce the color difference and shrinkage cracks on the concrete surface. The curing of concrete should start from the initial setting of concrete, and the curing water should be clean, and the covering used for curing should not fade to prevent the surface color of concrete from being polluted during curing.

Maintenance is divided into two parts: formwork maintenance and demoulding maintenance. Formwork curing is only to water the upper surface of concrete, and the curing after demoulding should cover the exposed surface of the whole concrete structure. When spraying water for maintenance, the side and bottom of the concrete structure should be fully wetted first, and then sprayed from the top. Because the top surface of concrete will precipitate a certain amount of alkali and admixture due to water evaporation, this part of alkali and admixture will be washed down when sprinkling water. If the concrete on the side and bottom surface is dry, the water flowing down from the top surface will infiltrate and pollute the appearance of the concrete on the side and bottom surface.

For the parts that can't be watered and cured, curing agent should be used, or protective film should be sprayed directly on the concrete surface after formwork removal, which can avoid curing and improve the visual effect and durability of concrete. The curing time of fair-faced concrete shall not be less than 7 days, and the curing time shall not be less than 14 days during winter construction.

05, finished product protection of concrete

Strengthen the education of construction personnel to avoid man-made pollution or destruction.

It is forbidden to throw the scaffold when it is dismantled, and it must be lowered slowly with a lifting rope.

Pay attention to the protection of fair-faced concrete in the construction of subsequent processes, and do not collide and pollute the concrete surface; Before the completion of concrete, protect the external wall with plastic film to prevent pollution; For the parts that are easy to be touched, stairs, reserved holes, columns, doors, external corners, etc. , after ripping, you can nail thin strips of wood or paste hard plastic strips for protection.

06, fair-faced concrete surface repair

Although every link from formwork processing, mix design to construction control ensures that the poured concrete is naturally bright and clean in color and smooth in lines, there are many unforeseen factors in construction, and errors in any link directly affect the appearance quality of fair-faced concrete, resulting in color difference, capillary cracks, sand holes, bubbles, spots, honeycombs, pits and other phenomena. Therefore, it is necessary to restore the original natural beauty of fair-faced concrete by carefully repairing the appearance defects.

When repairing, the repairing slurry with the same color as the concrete slurry should be prepared and evenly coated on the concrete surface (if necessary, appropriate building glue should be mixed into the slurry, or the surface of the structure to be repaired should be coated with building glue to increase the cohesive force), and the curing should be carried out by using the hydration heat process of the early strength development of concrete, so that the repairing slurry can be combined with the main part of concrete to achieve the effect of consistent color.

Attention should be paid to when repairing:

A. Generally, the casting body only has some appearance quality problems, such as color difference, capillary crack, sand mark, mottle or coarse aggregate transparent layer, etc. However, when filling slurry, the whole body should be repaired in coordination to avoid color difference between the repaired part and the unrepaired part, which will affect the appearance.

B. Before repairing, attention should be paid to the place where there are capillary cracks, sand marks, color spots or coarse aggregate transparent layers. Sand particles and loose mortar should be brushed with a steel wire brush, and then washed with clear water to avoid the repaired mortar from being separated from the whole to form cracks or even falling off. After repair, the whole body is covered with plastic film and sealed with transparent adhesive. The hydration heat process generated during the development of concrete strength is used for maintenance to ensure that the repaired cement slurry does not delaminate from the body itself.

Quality problems and solutions

0 1, template deformation

(1) The reason is that the pouring speed is too fast; The template processing is not standardized, and the residual stress and stiffness of the outer purlin of the column template are insufficient.

(2) According to the technical specification for construction and the load during template design, and in strict accordance with the requirements of safety technical disclosure, measures shall be taken to control the concrete pouring speed, and the pouring speed shall be ≤ 3m/h ... According to the design requirements of pier formwork, the weak parts of the template entering the site, such as the corner of outer purlin, shall be reinforced by welding steel plates; In the part with the maximum lateral pressure of concrete, it is reinforced by welding channel steel and dense purlin.

02, color difference problem

The reason of (1) is the change of raw materials and the instability of concrete mixture ratio; Insufficient stirring time; Uneven release agent or unstable curing.

(2) The countermeasures adopted adopt the same raw materials; Please ask the measuring department with corresponding qualifications to recalibrate the measuring equipment of the concrete mixing station to stabilize the concrete mixture ratio; According to the change of climate and raw materials, the mixing time is prolonged, the water content is sampled, and the water cement ratio is adjusted; Colorless release agent is adopted; Timely maintenance.

03, sand problem

(1) The reason is that the joint is not tight and the template mouth is not tight enough; Too much water in concrete and too high fluidity; The vibration is too strong.

(2) Countermeasures: the external control of construction joints is straight, and there is a groove in the middle; Stick a strip-shaped double-sided adhesive tape at the flange interface of the template. Strictly control the scope of concrete slump; Vibration properly to avoid direct vibration of joints.

04, oil, surface pollution, too many bubbles.

(1) The reason is that rust spots remain on the steel mold, and the surface dust is brushed with release agent without cleaning; Too much oiling; Pollution to pier shaft when washing concrete pump; Excessive sand ratio of concrete; The surface wettability of the template is poor; Insufficient vibration.

(2) Countermeasures: Sprinkle water on the hoisting site before formwork removal to avoid dust; Before template installation, derusting and sundries should be carried out. After the release agent is painted, the formwork shall be covered with striped cloth and removed before hoisting to prevent dust from adhering to the inner wall of the formwork. Control the sand content, slump and workability of concrete; Control the thickness of concrete layer; Control the depth and vibrating time of vibrating insertion into the next layer; Wet the template.

05, the watermark problem

The reason of (1) is that the concrete oozes along the slab joint after shrinkage; Premature maintenance.

(2) Set a tongue-and-groove joint on the countermeasure template and add a sealing strip; Fixing the lower relative steel bars; Exudate is discharged in time during perfusion; Cured after 24 hours.

Key points of concrete quality control

0 1, formwork project

(1) The formwork must be tight and firm, with correct geometric size and position, and it will not leak slurry or expand the formwork when pouring.

(2) In order to achieve the goal of fair-faced concrete, different types of templates are designed for different structural parts, and large templates are designed as much as possible to better reflect the fair-faced effect.

(3) Carefully check the formwork support to ensure that the support has sufficient strength, rigidity and stability.

(4) Do a good job in the selection, painting and daily cleaning of release agent to prevent its deformation and mold sticking. The turnover times of bamboo veneer should be controlled to avoid affecting the quality of concrete forming.

(5) In order to obtain the quality effect of fair-faced concrete, it is very important to strengthen the construction management of formwork, especially steel frame plywood formwork.

02, steel engineering

(1) The hook and bending of steel bars shall meet the requirements of specifications.

(2) The material and reinspection results of reinforcement conform to the relevant provisions of current national standards and mandatory provisions.

(3) The variety, specification, quantity, position, anchorage length, joint position and welding of reinforcement shall meet the design and specification requirements.

(4) The positions of embedded parts and reserved holes are accurate and fixed firmly.

(5) Ensure the accurate rooting position of reinforcement and prevent the displacement of column reinforcement when pouring concrete.

03, concrete engineering

(1) Fair-faced concrete requires the same color, and it is stipulated that cement of the same manufacturer, the same label and the same variety should be selected, and ordinary portland cement with pure color, good stability and strength should be used;

(2) The slump of concrete should be strictly controlled to prevent it from being too thick or too thin, and it is forbidden to add water on site;

(3) Concrete vibrating shall be carried out by experienced workers or professional teams, and the vibrating interval, time and speed shall be controlled to prevent vibration leakage and excessive vibration;

(4) The formwork removal and maintenance shall be carried out according to the specification requirements. When the pier concrete is poured, it should be covered with plastic film in time to prevent surface cracks caused by water loss, and the concrete curing time should be no less than 7 days. The pier should be covered with cotton wool in time (sacks are not allowed) for watering maintenance or brushing maintenance liquid for maintenance.

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