What Are the Technical Requirements of Frost-proof Concrete?

The technical requirements of frost-proof concrete are: during winter construction, reliable technical measures are taken to make the concrete set and harden as early as possible after casting and no frost damage shall occur before reaching the critical strength by freezing.

When the temperature is 0 ~ 4 ℃, the activity of water is low, the hydration reaction of cement is extremely slow, and the strength development of concrete can not reach the requirements. When the temperature is below 0 ℃, most of the water inside the concrete freezes. Water into ice after the volume expansion of 9%, the concrete structure will suffer permanent damage. In addition, after the water freezes, there is not enough liquid water in the concrete to participate in the hydration reaction of cement, and the strength of concrete grows extremely slowly or even stops.

Therefore, the concrete poured during the winter construction period should be mixed with early strength agent or antifreeze. And should be in the early stage of concrete condensation and hardening, take appropriate insulation or temperature increase measures. Make full use of the concrete’s own heat or external heat (such as the electric heating method, warming shed method, etc.). Ensure that the starting curing temperature of concrete after casting: not less than 10℃ in cold areas; not less than 5℃ in cold areas. Make the concrete strength with normal growth conditions, as soon as possible to obtain the critical strength by freezing.

Technical measures taken for the winter construction of frost-proof concrete

Frost-proof Concrete

Winter construction is the season of concrete engineering quality accidents. This is due to the short preparation time, complex technical requirements, a certain link can not keep up or rushed construction. Therefore, according to the statistics of local meteorological data for many years, careful planning and preparation for winter construction should be done in advance to avoid engineering quality accidents. The technical measures taken in winter construction are mainly the following 4 methods.

Material preheating method

That is, the water, sand, and stone will be preheated first, and then used to mix concrete.

Heat storage and insulation method

That is the use of insulation materials to cover the concrete after pouring so that the concrete is poured in a certain period of time to maintain no cooling or slow cooling.

Warming shed method

That is, the construction site to build a thermal insulation heating shed, so that after the concrete is poured in the warming shed under positive temperature environment conditions, the best mixing, and pouring is also carried out in the warming shed when possible.

Mixing early strength agent or antifreeze agent

In addition, the winter construction of concrete is appropriate to increase the amount of cement, or the use of early-strength cement, as well as the use of high-performance water-reducing agents or high-efficiency water-reducing agents, to minimize the amount of water and other technical measures.

The purpose of taking the above technical measures is to control and improve the temperature of concrete out of the machine and the temperature after entering the mold, to ensure that the strength of concrete has the conditions for normal growth after casting and that no frost damage occurs before reaching the critical strength by freezing.

Early strength agent and antifreeze agent in frost-proof concrete

Concrete damage causes in order of importance are: corrosion of reinforcement, frost damage in a cold climate, and erosion of the physical and chemical effects of the environment. Therefore, the concrete mixed with the early-strengthening agent or antifreeze agent should be noted to limit the chloride salt content. Chlorine salt is a very important factor to induce the corrosion of reinforcement. Prestressed concrete and reinforced concrete should be strictly in accordance with the relevant standards and regulations to control the maximum chloride ion content in concrete, to avoid engineering quality accidents, resulting in huge waste. Many projects at home and abroad have paid a heavy price for this.

Concrete early strength agents and components

Can accelerate the development of early strength of concrete admixtures called early strength agents. In actual use, most of them are compounded early strength agents. Inorganic salt’s early strength agent is not good for the late strength of concrete. Chlorine salt early strength agent will cause corrosion of reinforcing steel. Sulfate early strength agents may produce volume expansion so that the concrete durability is reduced. Sodium salt early strength agent will increase the alkali content of the concrete, and active silica aggregates produce the alkali-aggregate reaction.

Excessive addition of early strength agent makes concrete early effect, but late strength loss is large. Increased salt precipitation affects the concrete finish. Increase the electrical conductivity of concrete and increase the risk of shrinkage and cracking of concrete. The main components of the concrete early strength agent are: sodium chloride, calcium chloride, sodium sulfate, calcium sulfate, aluminum sulfate, potassium dichromate, triethanolamine, triisopropanolamine, methanol, ethanol, calcium formate, lithium oxalate, sodium acetate, etc.

Concrete antifreeze agent and components

Concrete can be hardened at negative temperatures, and under the specified maintenance conditions to achieve the expected performance of the admixture called antifreeze. The vast majority of antifreeze is a compound admixture. Should control the early strength component and antifreeze component of inorganic salts into the amount. Otherwise, improper use will cause the late strength of concrete shrinkage, corrosion of reinforcing steel, and alkali-aggregate reaction occurs.

The main components of concrete antifreeze agents are: antifreeze components (such as sodium nitrite, sodium chloride, methanol, urea, calcium chloride, potassium carbonate, etc.), air-entraining components (such as rosin soap, rosin hot polymers, sodium alkyl sulfonate, etc.), early strength components (such as sodium sulfate, calcium chloride, calcium nitrate, triethanolamine, etc.), water-reducing components (such as naphthalene, melamine, sulfamic acid, etc.).

About the early strength agent and antifreeze agent selection

Concrete Admixture Application Technical Specification (GB50119), Article 7.3.1 provides as follows.

① In the minimum daily temperature of 0 ~ -5 ℃, the concrete using plastic film and insulation materials to cover the maintenance, can use an early strength agent or early strength water reducing agent.

② In the minimum daily temperature of -5 ~ -10 ℃, -10 ~ -15 ℃, -15 ~ -20 ℃, using the above paragraph insulation measures, it is appropriate to use the specified temperature of -5 ℃, -10 ℃, -15 ℃ antifreeze agent.

About winter construction of frost-proof concrete and critical strength by freezing

The critical strength is the minimum strength that the concrete poured in winter must reach before it is frozen.

Winter Construction Regulations for Building Construction” JGJ104 stipulates that: according to the local weather data for many years, when the average daily outdoor temperature is lower than 5℃ for 5 consecutive days, it enters winter construction. When the average outdoor daily temperature is higher than 5℃ for 5 consecutive days, the winter construction period is lifted. The “Regulations” stipulate that the critical strength of concrete poured during the winter period by freezing is as follows.

① Ordinary concrete using silicate cement or ordinary silicate cement preparation should be designed for 30% of the standard value of concrete strength. When using slag silicate cement, it should be 40% of the standard value of the designed concrete strength. But the concrete strength grade C10 and below shall not be less than 5.0MPa.

② For concrete mixed with antifreeze, when the minimum outdoor temperature is not less than -15 ℃ shall not be less than 4.0MPa. when the minimum outdoor temperature is not less than -30 ℃ shall not be less than 5.0MPa.

Concrete mixed with a qualified antifreeze agent can reduce the freezing point of water and change the structure of ice crystals. It makes concrete in negative temperature conditions will not occur frost damage, and there is still enough liquid water to enable the hydration of cement to continue. After the transition to positive temperature, concrete strength can further grow to reach or exceed the design strength requirements. Therefore, the “Regulations” stipulate that the critical strength of concrete mixed with antifreeze by freezing is significantly lower than that of non-doped.

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