Precautions for winter concrete production and selection of antifreeze admixtures

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Precautions for winter concrete production and selection of antifreeze admixtures

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Winter has arrived in the Northern Hemisphere again!

I. Precautions for Concrete Production in Winter

1. Raw material control (core foundation, avoiding the source of frost damage)

(1) Water: Warm water (water temperature ≤ 60℃) should be used first. Water containing ice or snow is strictly prohibited. The water tank should be insulated and equipped with an electric heating device. The water temperature should be monitored in real time to avoid local overheating.

(2) Aggregates: Store in an insulated shed in advance. Sand and gravel containing ice, snow, frozen blocks or frost on the surface are prohibited. Coarse aggregates must be screened to remove frozen blocks with a particle size > 40mm. The moisture content of fine aggregates should be controlled within 3% to prevent freezing and clumping.

(3) Cement: Use ordinary Portland cement with a strength grade of ≥42.5 (high heat of hydration), store it in a dry and insulated warehouse, raise the bottom of the warehouse by more than 30cm to prevent moisture, and keep the temperature at the warehouse not lower than 5℃ to avoid the cement from getting damp and clumping.

(4) Admixtures: Select standard antifreeze admixtures (such as air-entraining antifreeze admixtures), and strictly prohibit the use of expired or frozen admixtures; adjust the dosage according to the temperature (3%-5% at -5℃, 5%-8% at -10℃), and conduct tests on compatibility with cement in advance.

2. Mix ratio optimization (adapting to low-temperature environments and ensuring strength development)

(1) Increase the amount of cement (5%-10% more than at room temperature) to use the heat of hydration to raise the internal temperature of the concrete and avoid early freezing.

(2) Reduce the water-cement ratio to below 0.45 to reduce the free water content and enhance the freeze resistance; appropriately increase the amount of mineral admixtures such as fly ash (admixture amount ≤20%), but the strength development rate at low temperature needs to be verified in advance.

(3) Adjust the aggregate gradation and use continuous gradation aggregate to reduce the void ratio, reduce the amount of cementitious materials, and improve the density.

(4) The proportion of each batch must be adjusted in real time according to the temperature and moisture content of the raw materials on the day. An adjustment form shall be issued by the laboratory. On-site changes are prohibited.

3. Control of the stirring process (uniformity and temperature compliance to avoid quality defects)

(1) Mixing equipment: The outer layer of the mixer is covered with a heat insulation cover. Before mixing, the mixing drum is preheated with hot water (water temperature is about 50℃) and the ice water in the drum is discharged to avoid the cold drum absorbing heat and lowering the concrete temperature.

(2) Feeding sequence: First, add aggregate and hot water and stir for 30 seconds, then add cement and admixture and stir for 60-90 seconds to ensure that the admixture is evenly dispersed. Cement should not come into direct contact with cold water (to prevent false setting).

(3) Outlet temperature: The outlet temperature of the forced mixer shall be ≥10℃ (the air temperature below -5℃ shall be ≥15℃). It shall be tested once every hour and recorded. If the temperature does not meet the standard, the water temperature shall be increased first (not exceeding 60℃). It is strictly forbidden to simply increase the amount of cement.

(4) Mixing time: 50% longer than normal temperature (e.g., 90s at normal temperature, 135s in winter) to ensure that the concrete mixture is uniform and free from segregation and bleeding . The slump should be controlled at 120-160mm (adjusted according to construction needs, but should not be too large).

4. Transportation safety measures ( temperature control and damage prevention to avoid freezing during transit)

(1)Transport vehicle: The tanker is wrapped with double-layer insulation cotton quilt + waterproof outer layer, and insulation cover is installed at the inlet and outlet. The tank is preheated with hot water before transportation, and the tank is kept rotating slowly (2-4 r/min) during transportation to avoid the mixture from clumping.

(2) Transportation route: Plan the shortest transportation route, avoid congested sections, and the transportation time should be ≤45min ( ≤30min below -10℃ ). It is strictly forbidden to stay in the open for a long time.

(3) Unloading management: Before unloading, check the temperature of the mixture (the temperature of entering the mold is ≥5℃). If the temperature is too low or freezing or segregation occurs, it is strictly forbidden to use it. During the unloading process, avoid the mixture being directly exposed to the cold wind. A temporary heat preservation shed can be built at the discharge port.

5. Production environment and equipment maintenance (stable production+safety assurance)

(1) Mixing plant site: Clear the ice and snow on the ground, lay anti-slip mats, check the equipment foundation and scaffolding for frost heave and cracks; install insulation sheds on the silos and batching machines , and do a good job of frost protection and insulation for the batching scale sensors (to avoid freezing and affecting accuracy).

(2) Water and electricity supply: Water pipes are wrapped with insulation layer, and exposed pipes are equipped with electric heat tracing devices. The remaining water in the equipment is drained before the end of each workday (to prevent freezing and cracking); the backup generator is filled with fuel to deal with sudden power outages (to prevent the concrete in the mixing drum from solidifying).

(3) Equipment maintenance: Low-temperature anti-wear hydraulic oil (grade ≤ -20℃) is used for hydraulic systems such as mixers and pump trucks, and low-temperature grease is selected; check the equipment operation status every day and clean the residual concrete in the mixing drum in time (to prevent freezing and hardening).

6. Quality Inspection and Emergency Response (Full Traceability + Risk Management)

(1) Temperature monitoring: Assign a dedicated person to record the raw material temperature, outlet temperature, and mold temperature every 2 hours. In case of any abnormality (such as the outlet temperature being lower than 8℃), immediately stop the machine for adjustment.

(2) Test block preparation: Prepare standard curing test blocks and test blocks cured under the same conditions according to specifications (the quantity is increased by 50% compared to room temperature). The temperature of the test block curing box should be controlled at 20±2℃. Test blocks cured under the same conditions should be placed in an insulated curing shed (temperature ≥5℃).

(3) Emergency frost protection: If a sudden and severe temperature drop occurs (temperature drops by ≥10℃), production should be stopped immediately. The already produced concrete should be transported and poured more quickly. If the temperature of the unpoured concrete is below 5℃, an appropriate amount of antifreeze (confirmed by the laboratory) should be added and the mixture should be stirred again until homogeneous.

(4) Quality traceability: For each batch of concrete, production records (raw material batch, mix proportion, mixing time, temperature, transport vehicle, etc.) should be kept for at least 6 months to facilitate subsequent quality traceability.

II. Different types of antifreeze additives

Based on different sub-zero temperature environments and project types in winter, we recommend antifreeze admixtures, covering various categories and highlighting their core advantages and applicable scenarios to facilitate matching production needs:

1. Salt-free compound antifreeze

This is a composite product that combines early strength, air entrainment, and water reduction. It is chlorine-free and salt-free, will not corrode steel bars, and can inhibit alkali-aggregate reaction. It is suitable for environments from -5℃ to -25℃ and is suitable for pumped concrete and commercial concrete batching plants. Its fully water-soluble properties ensure the workability of concrete, with minimal slump loss, making it suitable for most winter-use civil and industrial building concrete production.

2. Antifreeze type polycarboxylate superplasticizer

As a high-performance new admixture, it has a water reduction rate of 25%-45%, outstanding antifreeze properties, and a strength loss rate of ≤90% after 50 freeze-thaw cycles. It is chloride-free, has extremely low alkali content, is suitable for environments from -5℃ to -15℃, and is well- compatible with admixtures such as fly ash. It is suitable for the production of high-strength, self-compacting commercial concrete, and can also reduce pumping resistance, ensuring the performance of concrete transportation and pouring in winter.

3. Early-strength antifreeze

Available in various specifications suitable for temperatures ranging from -10℃ to -20℃ and -15℃ to -20℃, conforming to building materials industry standards. In addition to frost protection, it can rapidly improve the early strength of concrete and shorten the curing cycle, making it suitable for construction projects with tight schedules in winter, such as the production of concrete for parts that need to reach load-bearing strength as soon as possible, such as bridge bearings and building beams and columns.

4. Chloride-based antifreeze (calcium chloride)

The core components are calcium chloride and magnesium chloride, which provide freeze protection and significantly improve the early strength of concrete. It is suitable for plain concrete projects at temperatures ranging from -5℃ to -10℃, such as concrete subbases and non-load-bearing masonry mortar used in winter construction. Note that it is strictly prohibited for use in reinforced concrete, and the dosage must be strictly controlled to avoid affecting the later strength of the concrete.

5. Chloride salt rust-inhibiting composite antifreeze (sodium nitrite composite antifreeze)

It is composed of chlorine salt and sodium nitrite and other rust-inhibiting components, which not only retains the advantages of chlorine salt fast frost protection and early strength, but also avoids steel bar rust. Reinforced concrete projects suitable for -10°C to -15°C, such as ordinary residential floor slabs and road side stones constructed in winter, need to be adapted before use to ensure that the anti-rust and anti-freezing effects are balanced.

6. Organic antifreeze agents (calcium formate)

With calcium formate as its core component, it offers excellent compatibility while providing freeze protection, and also helps improve the workability of concrete, making it suitable for medium and low temperature environments around -5℃. It is often used in combination with water-reducing agents and air-entraining agents, making it suitable for concrete projects with high requirements for environmental protection and salt-free properties, such as fine aggregate concrete for interior decoration and precast component production, without negatively impacting the later-stage durability of the concrete.

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