Concrete strength test during winter construction

First of all, let me tell you that the two standards you mentioned have now been updated, "Concrete Strength Inspection and Evaluation Standards" (GBJ107-1987) has been updated to, "Concrete Strength Inspection and Evaluation Standards" (GB T50107-2010), "Concrete Structure Engineering Construction Quality Acceptance Code" (GB50204-2002) Although there is no new standard, on August 1, 2011, a partial revision has been made, and now the revised standards have begun to be implemented. For the validity query of the new standard, I suggest that you can go to the blog of the search tube Zhengbiao, and find a software that can check the validity of the standard after entering, which can greatly facilitate your future work.

In addition, the understanding of the effective age of the same condition, the cumulative 600 degree days refers to the age corresponding to the average temperature of each day when you maintain the same condition (i.e., the average of the highest and lowest temperatures) and when the cumulative temperature reaches 600 degrees.

Temperatures below the supplementary degree are not counted; 2. When assessing, the representative value of the strength of the test block under the same conditions should be multiplied by the conversion factor, and the conversion coefficient is. If there's anything you don't understand, feel free to talk to me on my blog.

The strength standard of concrete cured under the same conditions is the strength of the test block after reaching 600 degrees. This sentence is too general, and the explanation is as follows: we usually say that the compressive strength can be tested after 30 days of standard breeding, which is calculated by the average daily temperature of 20 degrees, 30 days * 20 degrees = 600 degrees of days, which is the condition under the standard curing room, and the same condition of maintenance, that is, the average temperature value of the day * 1 day, and then add up these values by day, and when this value reaches 600 degrees of days, this group of test blocks will also reach the standard of testing its strength.

The critical intensity value is 28 days.

In order to explore how strong the freshly poured concrete can be before it freezes in order to basically avoid frost damage, a large number of experiments have been done in various countries. According to the current Chinese regulations, this critical strength value is 40% of the maintenance intensity of the normal beam banquet for 28 days, and is not less than 5 megapascals. This is the basis for the winter construction of concrete.

When the minimum temperature is not lower than -15 °C, and the surface coefficient (ratio of surface area to volume) of the structure is not greater than 5 on the surface or underground project, it is advisable to use the heat storage method; When the surface coefficient of the structure is large, such as columns, beams, plates, etc., and there is sufficient steam to be used, the steam method can be used.

.What are the methods for determining the strength of concrete in winter construction?

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The methods for measuring the strength of concrete in the pro-winter period are: the heat storage method and the comprehensive heat storage method of concrete construction are the construction methods that use the heat of raw material heating and hydration heat under the winter conditions, and delay the cooling of concrete through appropriate thermal insulation to reduce the concrete temperature to 0 or the construction method that achieves the expected strength before the design specified temperature: the comprehensive heat storage method is the construction method of adding chemical admixtures on the basis of the heat storage method.

1。Although the determination of the standard strength of the heat storage method is under the conditions of winter, the concrete strength growth under the condition of the artificial microclimate environment is not much different from the normal temperature condition, therefore, the winter construction of the concrete of the heat storage method and the comprehensive heat storage method, the determination of the standard strength is the same as the construction under the condition of room temperature, and it is still inspected and evaluated in batches according to the provisions of the national standard "Concrete Strength Inspection and Evaluation Standard" (GBJ107-198), which is used for the inspection and batch acceptance of concrete sub-projects. 2。

Determination of curing intensity under the same conditionsThe concrete under the winter conditions adopts the heat storage method or the comprehensive heat storage method for construction, and the concrete is not hardened in the atmospheric environment at room temperature, but is carried out in the cover in the artificial small environment, therefore, the concrete construction of the heat storage method or the comprehensive heat storage method is determined under the same conditions, and the difference between it and the maintenance temperature under the condition of room temperature is only the difference in the value of the curing temperature, the former is the measured temperature in the covering, and the latter is the atmospheric environment temperature. At the same time, it should be noted that the atmospheric ambient temperature should be calculated by the average of the daily maximum temperature and the minimum temperature announced by the meteorological station at the time of the weather forecast, and the measured temperature in the cover should be measured according to the "Winter Construction Regulations for Construction Projects" (GJ104-1997), from the beginning of the concrete manhood to the concrete to reach the critical strength of freezing, or the concrete temperature drops to 0 or the design temperature before measuring every 6, The concrete mixed with antifreeze is measured every 2 hours before the strength reaches its freezing critical strength, and every 6 hours after reaching the freezing critical strength, and the daily average temperature is calculated from this. Under the same conditions, it is advisable to keep more curing specimens, so as to determine whether the concrete strength of the structural entity has reached the critical strength of freezing and the construction strength of the follow-up project at an appropriate age.

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Winter construction makes the concrete frozen, cement and other raw materials cannot play a role, and the strength cannot grow. Therefore, winter construction has a great impact on the strength of concrete, and if the concrete is seriously frozen, it is very likely that the concrete strength will not meet the requirements or be much lower than the requirements, which will cause "tofu slag engineering".

In areas with long low temperature time, finer soil particles and abundant recharge water, during the freezing period of the soil, due to the action of film water and capillary water around the soil particles, the moisture in the soil continues to accumulate to the freezing line, forming an ice layer, and the volume increases, so that the gap between the soil particles can not be accommodated and rises upward, resulting in frost heave, and its swelling force can be as large as megapascals.

If left untreated, the foundation can be lifted, causing the building to crack, tilt, raise or even collapse. Therefore, when building on frost heave soil, it is necessary to consider the foundation form, minimize the bottom and side area, reasonably determine the depth of its embedding, strengthen soil insulation, and do a good job of surface drainage.

1. In addition to the quality inspection according to the requirements of normal temperature construction, the following items should be inspected for the winter construction of concrete engineering:

1) the quality and dosage of admixtures;

2) heating temperature of water and aggregate;

3) the temperature of the concrete at the time of discharge, after pouring and during the hardening process;

4) The strength of the concrete when the temperature drops to 0 (the strength of the negative temperature concrete when the temperature is lower than the specified temperature of the admixture).

2. The temperature of water, aggregate and concrete should be measured at least 4 times per shift.

3 Measurement of concrete temperature:

1) When the heat storage method is used to cure concrete, the curing period is measured 4 times per day and night;

2) Negative temperature Xiangxiang concrete, before the strength reaches the critical strength of frost resistance, measure once every 2 hours; Measured 2 times every day and night thereafter;

3) When the heating method is used to cure concrete, it is measured once every 1h during the lifting and cooling period, and once every 2h during the constant temperature period;

4) Adopt comprehensive curing concrete, measured 4 times per day and night;

5) Outdoor air temperature and ambient temperature should be measured 4 times a day and night.

4. The temperature measurement of concrete shall be carried out in accordance with the following provisions:

1) All temperature measuring holes and points should be numbered, the layout drawing should be drawn, and the measurement results should be written into the official record;

2) The temperature measuring hole and point should be located in the representative structural part and the temperature change is large, easy to cool the part, and the depth of the temperature measuring hole is generally 10 15cm, or 1 2 of the thickness of the plate and wall.

3) When measuring the temperature, the thermometer should be properly isolated from the outside air temperature, which can be plugged with thermal insulation material around the orifice, and the thermometer should be left in the temperature measurement hole for more than 3min before it can be read.

5 When measuring readings, keep the line of sight and the apex of the mercury column of the thermometer at the same level to avoid parallax. When reading, be quick and accurate, and do not bring the head, hands, or lamp head close to the lower end of the thermometer. After finding the apex of the mercury column of the thermometer, read the decimal first, then the large number, and then recheck it again after recording to avoid misreading.

6. The temperature measurement personnel should check the covering and insulation at the same time, and should understand the pouring date, required temperature, and maintenance period of the structure. If it is found that the concrete temperature is too high or too low, the relevant personnel should be notified immediately and effective measures should be taken in time.

7. In the process of concrete construction, random sampling should be made at the pouring site, and 3 groups of specimens should be made at the same time for each sampling. 1 group was cured to 28 days under 20 standard conditions, and the strength f28 was obtained. 1 group is cured with the component under the same conditions, and when the concrete temperature drops to 0 (when the temperature of negative temperature concrete drops below the specified temperature of the antifreeze), the pressure is tested to check whether the concrete reaches the critical strength of frost resistance; 1 group and the component under the same conditions to 14 days, and then transferred to 20 standard conditions to continue maintenance for 21 days, in the total age of 35 days when the pressure test, the strength of F14'+21。If f14'+21 f28, which proves that the concrete has not been frost damaged, and f28 can be used as the basis for strength assessment.

5 degrees. Below 5 degrees, corresponding measures can be taken for concrete pouring.

When the outdoor daily average temperature is stable below 5 °C for 5 consecutive days, it enters the winter construction, the winter temperature drops, the temperature in many areas is below 0 °C (i.e., negative temperature), the moisture contained in the soil, concrete, mortar, etc. is frozen, and the building materials are easy to be brittle, which brings many difficulties to the building construction.

When the average temperature is lower than 5 °C or the lowest daily temperature is lower than 3 °C for 5 consecutive days, winter construction measures should be taken to ensure the quality of the project. Due to the fact that the construction needs to be insulated and covered and consumes more heat energy in winter, which increases the cost of the project, such as site leveling, foundation treatment, outdoor decoration, roof waterproofing and high-altitude pouring concrete, etc., the construction should be avoided in winter as much as possible.

Mainly moisture, strength, mixture, etc.

It should be selected according to the temperature situation, structural characteristics and on-site energy conditions. The minimum temperature is not lower than -15 °C, and the surface coefficient (ratio of surface area to volume) of the structure is not more than 5 for surface or underground works, and the heat storage method should be used. In order to prevent early frost damage, it is required that the temperature of the concrete at the beginning of curing should not be lower than 5°C.

When preparing concrete at negative temperature, water and sand can be properly heated, but the temperature of the water and sand mixture should not exceed 40 °C, otherwise false coagulation is easy to occur when putting cement in. In addition to heating and curing, concrete can also be mixed with early strength agent and antifreeze in winter to reduce its freezing point and harden the concrete at negative temperature.