If concrete construction is not prepared and taken precautions before the arrival of winter, it will inevitably affect the quality and progress of the project. Improper measures will also bring hidden dangers to the quality of the project.
1. Requirements for ready-mixed concrete:
Issues such as the quality of the concrete raw materials used in the project, the mix ratio design, the measurement and control of materials during mixing, the temperature of the machine and the insulation during transportation, as well as the temperature of the concrete entering the mold, all need to be raised with the ready-mixed concrete batching station in advance. Requirements, and if necessary, send people to the mixing station to monitor. During the construction, the concrete manufacturer was notified in a timely manner to jointly master the various properties of the concrete, and both parties cooperated to do a good job in the winter construction of the concrete.
2. Requirements for concrete mix ratio
Due to the low temperature in winter, in order to increase the growth rate of concrete strength as soon as possible, the concrete mix proportion should be appropriately adjusted and a certain amount of early strength agent and antifreeze should be added. Changes in the mix ratio must be tested and qualified before use. The purpose of adding early strength and anti-freeze composite admixtures to concrete is (the anti-freeze performance reaches 15℃):
① In order to improve the early strength of concrete in the covering and insulation stage, the critical freezing strength of concrete should be reached as soon as possible (when ordinary concrete is prepared with Portland cement or ordinary Portland cement, it should be 30% of the designed concrete strength standard value) and demolition The required strength of the mold;
②Prevent concrete from freezing when the temperature suddenly drops;
③In order to enable the strength of concrete to continue to increase after the insulation is removed.
3. Concrete raw material requirements
(1) The cement strength grade in the concrete mix ratio shall not be lower than 42.5, and the cement dosage shall not be less than 300kg/m³. Ensure that the water-cement ratio is not greater than 0.6, and deduct the moisture brought in by the aggregate and antifreeze solution from the mixing water.
(2) The mixing station is required to heat the concrete raw materials: because the specific heat of water is about 5 times that of sand and stone aggregates, the method of heating water should be given priority when mixing concrete in winter, but the heating temperature of the water should not be higher than 80°. The use of warm water for mixing can prevent the concrete from losing too much heat and causing the surface to freeze. The aggregates used for mixing concrete should be clean and should not contain ice, snow, frozen blocks and other easily cracked objects. The storage site for aggregates in winter should be located on a higher ground. Places where water accumulates; cement must not be heated directly, and should be transported to a greenhouse for storage 1 to 2 days before use. The temperature of the greenhouse should be above 5°C.
(3) Ensure that the exit temperature of the concrete delivered to the construction site is above 15°C, thereby ensuring that the mold entry temperature of the concrete is controlled above 6°C, and an antifreeze agent with antifreeze performance reaching -15°C is added. It is not advisable to mix chlorine salt antifreeze in reinforced concrete.
4. Concrete mixing and transportation
(1) Concrete should not be mixed in the open air (controlled by commercial concrete stations). A warm shed should be set up as much as possible and a large-capacity mixer should be used to reduce the heat loss of concrete. Before blending, rinse the blender with hot water. The mixing time of concrete is 50% longer than the specified time at normal temperature.
(2) Because sudden solidification occurs when cement is mixed with water at about 80°C, when the materials are put in, the water and sand should be mixed first, and then the cement should be added.
(3) After the concrete leaves the station, it should be transported to the pouring site in time. During transportation, attention should be paid to preventing concrete heat loss, surface freezing, concrete segregation, cement mortar loss, slump changes and other phenomena. Tank trucks used for transportation should be wrapped with insulation materials to minimize heat loss during transportation and ensure that the temperature of the concrete coming out of the tank should not exceed 5 to 6°C per hour during transportation.
In addition, what measures are taken for the construction site, concrete pouring, test block retention, form removal and maintenance?
1. Insulation measures at construction sites
Plastic colored strips of plastic cloth are used as wind-blocking measures for the scaffolding around the construction floor. The fence range is generally two floors high, mainly on the working layer and the next concrete curing layer. Colored strips of cloth will be used to enclose the bottom of the floor, and the separation lines from the elevator entrance, stairwells, doors, windows, post-casting belts, etc. to the construction joint area will be blocked.
2. Concrete pouring
Before concrete is poured, ice, snow and dirt must be removed from the formwork and steel bars. In particular, snow and ice at the bottom of vertical components must be checked again to see if they have been removed before concrete is poured. A shed must be set up for the concrete delivery pump and the concrete must be sealed with colored strips of cloth for insulation. All delivery pump pipes must be insulated, first wrapped with a black quilt and then with a layer of plastic film. Before pouring the concrete construction joints, the cement film and loose stones should be removed, moistened and rinsed, and the temperature of the original concrete at the joints should be higher than 2°C. Only when the strength of the poured concrete reaches 1.2MPa can a layer of concrete be laid on it. Continue pouring concrete after layering a layer of mortar with the same composition as the mortar in the concrete. When pouring concrete during the winter construction period, the vibration of the concrete should be strengthened to increase the density of the concrete as much as possible. Mechanical vibration must be used to vibrate concrete in winter, and the vibration time should be longer than that at normal temperature.
3. Test block retention method
In addition to providing standard strength data for the acceptance of concrete projects, concrete test blocks during winter construction also provide a basis for removing insulation and removing formwork. There are three groups of test blocks, and they are maintained under the same conditions as the construction site. One group is used to test the strength of concrete before freezing and determine the insulation and curing period of concrete; one group is used to test the formwork strength of load-bearing floor slabs; the other group is a 28-day standard curing test block. The maintenance conditions of test blocks under the same conditions must be consistent with the structural maintenance conditions at the construction site.
4. Concrete form removal conditions
The formwork can be removed only when the concrete is cooled to 5℃, exceeds the critical strength and meets the requirements for normal temperature concrete formwork removal (concrete temperature is measured by a thermometer). For spans ≤8m, beam and plate formwork can be removed only after the concrete strength reaches 75% of the design strength. For spans >8m, the concrete strength can be removed only after the concrete strength reaches 100% of the design strength. For cantilever structures, the concrete strength reaches 100% of the design strength. Can be removed later.
5. Concrete maintenance measures
The insulation of the upper part of the roof and beam concrete is to first cover the surface of the newly poured concrete with plastic sheeting and then a layer of straw mat. Weak areas such as corners or windward surfaces should be covered with straw mats and overlapped. If the daily average outdoor temperature is 0℃ and below, cover the surface of the poured concrete with a layer of plastic film to prevent straw and other coverings from getting damp, then cover the concrete with straw, and finally cover the pit mouth with a layer of plastic to prevent it from getting wet. Enhance thermal insulation to ensure the quality of concrete. The molding temperature of concrete is controlled above 6°C; the initial curing temperature after concrete pouring is not lower than 5°C. If the above temperature is not reached, the curing water temperature must be increased to meet the requirements.
