Steam curing technology of subway segment and its improvement?

The following is the related content of steam curing technology of subway segment brought by Zhong Da Consulting for your reference.

According to the current steam curing technology of subway segment, the influence of steam curing on the strength of subway segment concrete is analyzed. Aiming at the phenomenon that the steam curing of subway segments consumes a lot of energy and takes a long time, this paper puts forward some improvement ways.

Preface of 0

Segment is a kind of precast concrete lining (usually assembled into a ring by several pieces in a certain way), which is mainly used to build subways or large sewage pipes. In the process of tunnel excavation, the segment mainly plays a supporting and waterproof role.

Steam curing is a method of heating concrete with external heat source to accelerate the hydration reaction of cement and the formation of internal structure, thus accelerating the hardening of concrete, with the aim of shortening the turnover period of formwork and increasing the output. The maintenance method of subway segments is generally atmospheric steam maintenance, and the maintenance system is basically divided into four stages: pre-maintenance (Y), heating (S), constant temperature (H) and cooling (J).

At present, the main problems existing in the steam curing process of subway segments are as follows: during the curing period, the heat exchange intensity of air in the cover is not enough, which is easy to cause uneven heating of different parts of the same segment for some large segments; Low steam energy utilization rate and high energy consumption; At present, the steam curing of subway segments often adopts linear heating method (that is, it rises to the highest temperature at a certain heating rate), which has the disadvantage of long curing time.

Through four stages of curing, this paper analyzes the influencing factors of steam curing on the strength of segment concrete, and puts forward some suggestions on the improvement of curing system.

1 subway segment maintenance technology and the influence of each stage on subway segment.

1. 1 incubation period

Generally speaking, it refers to the time from the casting of subway segments to the static parking time before heating. The function of pre-curing stage is to improve the hydration degree of cement before the start of thermal curing, so that concrete has the necessary initial structural strength, thus enhancing the resistance of concrete to structural damage during heating period. The longer the pre-curing time of concrete products, the greater the initial structural strength of concrete, and the smaller the internal damage of concrete products after steam curing. However, the pre-curing period should not be too long, otherwise it will affect the production cycle. There is an optimal pre-curing period here. Professor Wu Zhongwei, a famous concrete expert, pointed out that the best pre-curing period is the time required for concrete strength to reach 0.39 ~ 0.49 MPa [1].

1.2 heating period

The time when the temperature of the medium in the curing equipment rises from the initial temperature to the constant temperature is called the heating period. The structural failure of concrete mainly occurs in the heating stage, which is mainly manifested by the increase of coarse pore volume and the increase of gas-liquid phase quantity. The faster the heating rate, the greater the damage to concrete. The temperature rising speed of subway segment maintenance shall not exceed 25℃/h, and the maximum temperature shall not exceed 55℃.

1.3 Constant temperature period

Constant temperature period is the main growth period of concrete strength. The hardening rate of concrete at constant temperature depends on the type of cement, water-cement ratio and constant temperature conditions. The factors affecting the constant temperature time are cement variety, cement strength grade, pre-curing time, heating rate and constant temperature [1]. The smaller the water-cement ratio, the faster the concrete hardens and the shorter the constant temperature time. Constant temperature time is not necessarily good, and intensity fluctuation may occur.

1.4 cooling-off period

The time when the temperature of the medium drops from constant temperature to the allowable temperature of the product is called cooling period. During the cooling period, the water inside the subway segment evaporates, and at the same time, it produces shrinkage and tensile stress. If the cooling speed is too fast, the subway segment will produce excessive shrinkage stress, which will lead to structural damage such as cracks and looseness on the surface of the subway segment, and even lead to quality accidents. Structural damage during cooling is related to cooling speed, concrete strength, surface modulus of products (surface area to volume ratio) and steel bars [1]. Products with low strength, small surface modulus and few reinforcements should be cooled slowly. Refer to table 1 for the maximum cooling rate.

2 Factors affecting the strength of steam-cured subway segments

2. 1 cement mineral composition

The hydration speed of C3S in cement is faster at the initial stage of hydration, and the C3S content is between 45% and 60%, which is beneficial to the formation of the initial structure of the segment and can shorten the pre-curing time of the segment. In the process of steam curing, the strength of C2S in cement increases rapidly, and the structural holes formed are also low, but the absolute value of strength is not high, so the content should not be too high. If the durability of the block is high, cement with high C2S content can be selected. The content of C3A in cement is generally 4%~5%, too high will affect the later strength of segment. The content of C4AF should be controlled between10% and15%, and the strength will be obviously reduced when the content reaches 20%.

2.2 admixture

Steam curing concrete and natural curing concrete have different requirements for additives. Air entraining agent should not be added to concrete, otherwise it will cause the expansion and looseness of the outer surface of the segment. The general principle of choosing steam curing concrete admixture is to have early strength admixture which can promote cement hydration, low gas content and high water reduction rate.

2.3 mineral admixture

Slag and fly ash have good adaptability to steam curing, which can reduce the hydration heat and absolute temperature of concrete and reduce the number of cracks produced by steam curing [3]. The strength of subway segment concrete is generally C50, mineral admixture is generally controlled at about 20%, and low calcium secondary ash is generally suitable for fly ash.

2.4 Water consumption

A certain amount of water is an important condition to ensure the normal hydration of cement and the workability of concrete mixture, but water will bring damage to concrete structure when it is cured in damp heat. Due to the thermal expansion of water and the migration of thermal medium, the porosity of concrete will increase. Therefore, water consumption has a great influence on the formation and strength of concrete structures. Low water-cement ratio not only contributes to the rapid growth of strength, but also improves the compactness and forms high-quality concrete.

3. Improvement of maintenance methods

There are many ways to improve the curing process, such as reasonable pre-curing, variable speed heating and improving curing conditions.

3. 1 variable speed heating and phased heating curing method

Linear heating is a common heating system, which is used for moist heat curing at atmospheric pressure. The disadvantage of this system is that the steaming time is too long, and it is a passive method to inhibit the heating rate. The author thinks that the variable speed heating system is reasonable, which can gradually improve the strength of concrete at a lower temperature, and then quickly raise the temperature to the highest temperature after being able to withstand the damage of damp heat. This method will be more effective when the temperature is low. For example, manual steam supply, 1 ~ 2 hours can raise the temperature to 20 ~ 35℃, 1 ~ 2 hours can keep the temperature, and then quickly raise the temperature to the highest temperature. This method can greatly weaken the damage of concrete structure and reduce the pre-curing time, thus reducing the steam curing time. The solid line is a variable speed heating wire, and the dotted line is a linear heating wire. Variable-speed heating (shown by solid line) pre-incubating for 2h, then heating to 35℃ for 2h, keeping the temperature at 65438±0h, then rapidly heating to 50℃, keeping the temperature at 2h, and finally cooling to 65438 00℃. Compared with linear heating, variable-speed heating reduces the pre-culture time by 65438 0 hours, and the total time is also reduced by 65438 0 hours.

3.2 Directional cyclic curing method of thermal medium

Directional circulation moist heat curing of heat medium is an improvement of heating mode. Because the heat exchange is accelerated during heating, the heating speed must be controlled when it is used [2].

The essence of this method is to make the mixed gas in the steam curing hood flow directionally and forcibly, so as to change the static state and heat exchange intensity of the steam in the hood and achieve the purposes of uniform curing, shortening the cycle and saving energy.

One of the characteristics of this process is to obtain the driving force of forced flow by increasing the speed of steam passing through the valve nozzle, that is, the nozzle is not only a channel for steam injection, but also a key facility to realize the directional circulation flow of mixed gas. The upper gas collector should be located at 2/3 of the upper part of the curing hood, and the nozzle on it should face down; The lower gas collector can be above the cover bottom 1/3, with the nozzle facing upwards. Laval pipe is a kind of gradually shrinking and expanding pipe, which can be determined according to the amount of steam evaporated by the boiler per hour and the number of steam curing hoods in the workshop. Generally, the valve nozzles can be arranged above and below.

3.3 Natural curing method of subway segment

In the high temperature season, the exothermic reaction of cement hydration can be used for natural curing without steam curing. In order to make full use of the hydration heat of cement, a steam curing cover must be placed on the subway segment after water accumulation. The better the seal, the less heat loss and the shorter the curing time. For example, we assume the following maintenance parameters: air temperature 5℃, heating time 3h, constant temperature 2h, cooling time 2h, heating speed 15℃/ hour, maximum constant temperature 50℃ and minimum temperature 25℃. The product ε (degree-time product) of temperature and time in subway section can be approximately calculated by the following formula:

ε= 0.5(t 1 T2)×τ 1 T2×τ20.5(t2t3)×τ3

=0.5(550)×350×20.5(5025)×2

= 257.5℃ hours

Where t 1, t2 and t3 are the temperatures at the beginning of heating, constant temperature and the end of cooling respectively; τ 1, τ2 and τ3 are the time of heating, constant temperature and cooling respectively.

Similarly, if the temperature-time product is 257.5℃h, and we assume that the average weather temperature is 25℃, t 1=t2=t3=30℃ (the hydration heat of cement can make the average temperature of subway segments higher than the average temperature, if it is higher than 5℃), then we can find τ1τ 2 τ 38.6h by substituting the above formula. This example.

4 conclusion

(1) It is necessary to fully realize the influence of each stage of the subway segment curing system on steam curing segments. The best pre-feeding period is often overlooked by everyone, so this stage should be paid enough attention. The most important stage of steam curing of segment is heating and cooling, which is an important stage to reduce turtle cracks on segment surface and ensure segment internal quality.

(2) Besides reasonably determining the curing system, we should also pay attention to the influence of raw material composition, admixture, admixture and water-cement ratio in concrete on the strength and sectional quality of steam-cured concrete. The components in raw materials should be in a reasonable proportion, otherwise it is not conducive to the development of concrete strength. Adding a certain amount of slag powder or fly ash into concrete is beneficial to the quality of steam-cured segments, and concrete with low water-cement ratio is more suitable for steam-cured segments.

(3) There are many ways to improve the staged maintenance mode, such as heating up in stages according to your actual needs, improving steam maintenance equipment, or adopting natural maintenance mode in high temperature season to reduce steam maintenance time and energy consumption.

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