KR20180116668A - An Estimation Method of Concrete Properties at Early Ages by Using Thermal Stress Device - Google Patents

Abstract

The present invention relates to an estimation method of a concrete property value at early ages, which comprises the following steps of: measuring a deformation rate of a temperature stress measurement device to deposit a concrete sample; calculating concrete stress based on the measured deformation rate; and estimating the concrete property value at early ages by comparing the measured stress and interpretation stress. Moreover, a simple indoor experiment is carried out to estimate various concrete property values.

Description

[0001] The present invention relates to a method for evaluating the properties of early aged concrete,

The present invention relates to a method for evaluating early age concrete properties, and more particularly, to a method for evaluating elastic modulus, creep, autogenous shrinkage, and thermal expansion coefficient after a concrete sample is poured.

The present invention relates to a method for evaluating early age concrete properties, comprising the steps of measuring a strain of a temperature stress measuring apparatus with a concrete sample placed thereon, calculating a concrete stress based on the measured strain, comparing measured stresses and analytical stresses And evaluating the properties of early age concrete, and it is possible to evaluate various concrete property values by carrying out a simple indoor test.

When cement reacts with water, about 120 cal of heat is generated per gram by the exothermic chemical reaction, which is called hydration heat. Heat of hydration raises the temperature inside the concrete and causes the accompanying volume change. When this volume change is restrained, thermal stress occurs in the concrete structure. This phenomenon is evident in a mass concrete structure with a thick concrete member, and it causes tensile stresses exceeding the tensile strength of the concrete, thereby becoming a main cause of the early cracks of the concrete. Most of these temperature stresses are predicted by numerical analysis. Accuracy of numerical analysis is strongly influenced by concrete property value, so accurate evaluation of concrete property value is required.

U.S. Patent No. 6,591,691 (Apparatus for measuring thermal stress of concrete structure) German Patent Registration No. 10122112 (Apparatus for measuring the thermal stresses in the construction form unit)

Currently, in most construction sites, concrete properties such as ACI, KCI, etc., or some experiments (eg, compressive strength) are evaluated. However, since the characteristic value of concrete changes not only with time but also with curing conditions (temperature and humidity) and materials used, experiments are indispensable. In other words, all concrete property values affecting the temperature stress of the mass concrete structure concrete should be tested. In addition, it is necessary to consider the curing condition of the mass concrete structure where the temperature changes with time at the time of the experiment, and since the characteristic value changes with time, Day, etc.), it is difficult to carry out the experiment considering all these conditions. In order to overcome these limitations, the present invention relates to a technique for evaluating various concrete property values using simple laboratory test results.

The method includes measuring a strain of a device using a concrete sample; Calculating a concrete stress using the measured strain; And evaluating the concrete property value using the developed program.

The method of evaluating early age concrete properties according to the present invention has the effect of improving the accuracy of stress prediction of a mass concrete structure by evaluating various concrete property values by using a simple laboratory test and using it in the analysis of a mass concrete structure, It can be used in construction sites.

FIG. 1 is a diagram illustrating an analysis procedure of mass concrete structures. FIG.
FIG. 2 is a diagram illustrating an experimental procedure of a thermal stress device. FIG.
3 is a diagram illustrating the shape and dimensions of the modified thermal stress device (in mm).
FIG. 4 is a diagram illustrating an estimation procedure. FIG.
FIG. 5 shows a set up of thermal stress measurement with a thermal stress device.
6 is a graph showing temperature history for experiment.
7 is a graph showing thermal stresses measured from thermal stress devices.
8 is a graph showing concrete properties by the estimation method.
9 is a set up of compressive and elastic modulus test.
10 is a graph showing the compressive strength and elastic modulus.
FIG. 11 is a view showing set up of the specimen for autogenous shrinkage.
12 is a graph showing non-stress strain obtained from the restraint-free specimen.
13 is a graph showing a comparison of elastic modulus obtained from estimation method and experiment.
14 is a graph showing a comparison of autogenous shrinkage obtained from estimation method and experiment.
17 is a graph showing a comparison of non-stress strain obtained from estimation method and experiment.

Before describing the embodiments according to the present invention in detail, the present invention is not limited to the configurations shown in the following description or the accompanying drawings, but may be used or performed in various ways.

The present invention relates to a method for evaluating an early age concrete property value,

Measuring strain of the apparatus using a concrete sample;

Calculating a concrete stress using the measured strain; And

And evaluating the concrete property value using the developed program.

The strength, elastic modulus, shrinkage, and creep characteristics of concrete materials are influenced by time, temperature, humidity and working load, and are particularly influenced in early ages. In the case of mass concrete, the influence of time, temperature, humidity and working load must be considered since cracks due to hydration heat occur early in the ages. Until now, most of the construction sites have been evaluating the concrete characteristics through predictive modeling or some experiments. In the present invention, a method of evaluating various concrete properties through a simple laboratory test is presented. In this method, an experiment is performed under the same conditions as the curing conditions of the actual structure, and the concrete characteristics are determined by comparing the experimental results with the analytical results. The thermal stress measurement device developed in the previous study was used for the experiment and the thermal expansion coefficient, elastic modulus, autogenous shrinkage and creep were evaluated by optimizing the existing prediction model. Also, the evaluated results were verified by comparing with the experimental results, and the two results were similar. In conclusion, the proposed concrete characterization method can be used to evaluate the characteristics of early age concrete affected by various factors.

One. Introduction

The characteristics of concrete, such as strength, elastic modulus, shrinkage, creep, etc., are affected by time, temperature, humidity and working load. For example, the strength of high-temperature cured concrete is higher than that of low-temperature cured concrete in the early age, but the strength of low-temperature cured concrete is higher than that of high-temperature cured concrete. This effect is very large at the beginning of the age when the hydration reaction occurs rapidly. In the case of mass concrete, the cracking occurs in the early age due to the heat of hydration, so concrete properties that vary greatly depending on time, temperature, humidity and operating load must be considered

In general, the concrete characteristics are evaluated through a characteristic prediction model or an experiment according to each characteristic. However, when the prediction model is used, the concrete characteristics are greatly affected by the combination and the aggregate, and are greatly influenced by the curing conditions such as the temperature and the humidity, the working load and the time described above. There can be a large difference in characteristics. On the other hand, when evaluating concrete properties through experiments, it is necessary to carry out experiments on each characteristic to be evaluated and with time-dependent experiments. In addition, since it is necessary to carry out an experiment reflecting the temperature and humidity history of actual structures, a laboratory capable of simulating temperature and humidity changes is required to carry out the experiment. Therefore, it is not economically feasible to carry out an experiment considering all these conditions in a construction site, and most of the cases are impossible due to actual construction site conditions.

In order to solve the above-mentioned problem, we have developed a device that can measure the direct temperature stress experimentally without evaluating the concrete characteristics in the previous research. However, due to the difference in constraint between the developed device and the actual structure, it is difficult to apply the measured stress to the structure. The present invention proposes a concrete property prediction technique using this apparatus and an experimental verification.

2. Estimation procedure of 물질 properties

The analysis of mass concrete structures can be divided into heat transfer analysis and stress analysis including strain analysis. There are differences in the characteristics used for each analysis. As shown in FIG. 1, the characteristics used in the heat transfer analysis are heat generation (hydration heat), heat conductivity, specific heat, unit weight and convection coefficient, and the thermal conductivity coefficient, specific heat, unit weight and convection coefficient except for calorific value , It can be used assuming a constant value. In addition, since the accuracy of the heat transfer analysis depends heavily on the accuracy of the calorific value evaluation, studies on the properties used in the concrete heat transfer analysis have focused on calorific value evaluation and various methods have been developed. In particular, the adiabatic temperature rise test for measuring the change in concrete temperature in the adiabatic state is a typical test method for accurately evaluating the amount of concrete heat. Also, since the adiabatic temperature rise tester is commercialized and sold, and since many tester have the tester, it can be tested without any difficulty. Therefore, the heat transfer analysis of the mass concrete structure can be predicted relatively accurately through the adiabatic temperature rise test.

On the other hand, the thermal expansion coefficient, shrinkage, elastic modulus, and creep, which are characteristics used in concrete stress prediction, change not only with time but also with curing conditions such as temperature and humidity, It is very important to consider precisely the changes depending on conditions and loads.

Therefore, this study proposed a method to evaluate the properties used in concrete stress prediction. The proposed method compares the measured stresses with the predicted stresses through simple laboratory tests and evaluates all the properties required for stress prediction. In other words, since only the temperature stress measurement test is required rather than various experiments, the proposed concrete property evaluation method can be easily used in a construction site where various testing equipments are not provided. The temperature stress measurement is improved by using the temperature stress measuring device proposed by Amin. The analysis program can utilize various programs developed in the past.

In the proposed concrete property evaluation method, it is very important to ensure the accuracy of the measured temperature stress because the concrete characteristics are evaluated by comparing the temperature stress measured through the experiment with the predicted temperature stress through the analysis. To this end, the following must be considered. First, since the characteristics of early age concrete are largely affected by the curing conditions, experiments should be conducted under the same curing conditions as the structure. Second, the stress generated in the concrete is proportional to the constraint, and the generated stress affects the concrete characteristics. Therefore, it is almost impossible to simulate the constraint conditions such as the structure, .

In the present invention, temperature stress is measured using a temperature stress measuring apparatus proposed by Amin. Since the temperature stress measuring device performs the experiment under the same curing conditions as the mass concrete structure, there is an advantage that the error according to the curing condition can be minimized. In addition, it is possible to simulate the restraint conditions of the surface and center of the concrete structure, as well as to simulate restraint of various sizes, so that the difference according to the restraint size can be considered.

The specific experimental procedure is shown in Fig. 2, and the temperature stress experiment is carried out with at least two constraints in order to reflect the influence of the constraint. Through the heat transfer analysis, the temperature distribution for the target structure is predicted. After selecting the location of the structure to predict the temperature stress, the predicted temperature is programmed into the thermostat and the heat pump through the heat transfer analysis. Humidity is maintained above 95% to negligible effect on drying shrinkage. After that, concrete is placed in a temperature stress measuring device set on a thermo-hygrostat, and temperature and strain are measured.

Despite the various advantages described above, existing temperature stress measuring devices have some disadvantages. First, the temperature stress measuring device consists of a structure in which the main frame and the restraining bar are connected by bolts. That is, the constraint of the temperature stress measuring device is largely influenced by the relatively low-rigidity bolt connection portion, and the measured stress becomes small. In addition, the frictional force of the bolt connection changes due to the different thermal expansion coefficients of the bolt and the device, which changes the restraint of the device. Second, when calculating the temperature deformation of the apparatus, the thermal expansion coefficient varying with the temperature is used as a constant value, resulting in an error due to this. Third, when the gauge temperature changes, the strain measured by the temperature stress measuring device should be temperature compensated to the measured value as shown in Equation (1).

[Formula 1]

는 strain of thermal stress device이고,here,

Is a strain of thermal stress device,

는 strain of measurement이며,

Is the strain of measurement,

는 strain of temperature compensation이다.

Is the strain of temperature compensation.

Generally, gauge temperature compensation is provided by the gauge manufacturer. However, since the temperature compensation value changes depending on the gauge installation conditions, the adhesive used, and the material on which the gauge is installed, an error may occur when using the compensation value provided by the gauge maker.

In order to reduce the error caused by the above-mentioned problems, the existing devices have been improved. As shown in FIG. 3, the improved apparatus is composed of a temperature stress measuring apparatus and a dummy specimen for measuring only strain due to temperature change, and the bolt connection is removed. In other words, by removing the temperature strain of the device directly by using the measured value from the dummy specimen, the error due to temperature deformation and temperature compensation is eliminated.

[Formula 2]

는 concrete stress at time t 이고,here,

Is the concrete stress at time t,

는 stain of constraint bar at time t 이며,

Is the stain of constraint bar at time t,

은 stain of dummy specimen at time t 이고,

Is the stain of dummy specimen at time t,

는 elastic modulus of constraint bar이며,

Is the elastic modulus of the constraint bar,

은 cross sectional area of constraint bar이고,

Is the cross sectional area of the constraint bar,

은 cross sectional area of concrete이다.

Is a cross sectional area of concrete.

Since the concrete properties used in the stress prediction vary with time, they should be evaluated taking into account changes over time. That is, the concrete properties should be evaluated according to age. However, since the thermal expansion coefficient, shrinkage, elastic modulus and creep, which are characteristic values to be evaluated, influence each other, when there is an optimal value for each age, there are many optimum values. More specifically, the strain generated in concrete by temperature deformation and shrinkage is equal to the sum of temperature deformation and shrinkage, so that in any case where the sum of temperature deformation and shrinkage is the same, the strain generated in concrete is calculated equally. This feature also occurs when evaluating creep and modulus of elasticity. Therefore, when the optimum value for each age is searched, only the strain due to thermal expansion and shrinkage, the elastic modulus and the strain due to creep can be evaluated. These problems were solved by using existing prediction models. The characteristic prediction model to be evaluated is selected and the characteristics are evaluated in such a manner that the characteristic size and the value used for the characteristic expression are optimized in the selected model so that a large number of optimal values are not present. The evaluation procedure is shown in Fig. First, select the property values to be evaluated, and select the model to use and the variables to optimize. The stress analysis is performed using the initial values of the variables to be optimized and compared with the experimental stress. If the stop criteria is not satisfied, the value of the variable is changed and the analysis result is compared with the experimental result. This procedure is repeated until the stop criteria is satisfied.

Table 1 shows the predictive model expressions, variables, and variable ranges used in the present invention.

[Table 1]

Since the thermal expansion coefficient does not exist according to age, a random function that decreases with time is used to reflect the results of previous studies. The modulus of elasticity was calculated using the ACI model, which was modified to take into account the rapid increase in modulus of elasticity due to high temperature curing. The autogenous shrinkage was modeled by B3. For creep, the change in the final creep coefficient with respect to the load age in the ACI creep model was evaluated. The range of the variable used in the evaluation was used by the proposed value for each model. When there is no proposed value, the variable range was determined considering the general concrete characteristic range. Equation (3) is used to represent the difference between experimental and analytical values.

[Formula 3]

및

는 measurement and analysis stress respectively이고,here,

And

Are the measurement and analysis stress respectively,

는 degree of restraints,

은 number of degree of restraints이다.

The degree of restraints,

Is the number of degree of restraints.

3. Experiments and verification of concrete properties

In order to verify the proposed concrete characterization method, the temperature stress test was conducted and the concrete characteristics were evaluated by comparing the experimental results with the analytical results. In the temperature stress test, both the center and the surface of the structure were tested, and the experiment was carried out for four constrained sizes. The property values were evaluated by considering the effects on the four constraints equally, and the concrete properties with respect to the average constraint size. Also, it is compared with actual concrete material property test results. Material properties tests were conducted on non-stress strain, which is the strain due to compressive strength, elastic modulus, temperature and autogenous shrinkage. The compressive strength and elastic modulus were measured at the same temperature as that of the temperature stress test, so that no difference was caused by the experimental temperature. Concrete temperature deformation and autogenous shrinkage have a great influence on each other at early ages. Therefore, the non-stress strain, which is the strain due to temperature and autogenous shrinkage, was measured and compared with the evaluation results without conducting each experiment. The creep test was not carried out due to the difficulty of the experiment. The test specimens were cured in the same environment as the temperature stress test, so that no difference was caused by curing. The temperature stress test and the material characteristic test specimens were made using the same batch of concrete. The formulation used is shown in Table 2. Water - cement ratio was 0.44 which is frequently used in mass concrete and crushed aggregate was used. The maximum aggregate size is 25 mm.

[Table 2]

In order to evaluate the concrete characteristics by the proposed property evaluation method, the temperature stress test was performed as shown in FIG. 5, and the temperature history of the central part and the surface part of the mass concrete structure used in the experiment is shown in FIG. In the case of the center of the structure, the temperature stress measuring device manufactured by Invar was used, and the surface of the structure was tested using a temperature stress measuring device made of aluminum. The variables and mechanical properties used in the experiment are shown in Table 3.

[Table 3]

The temperature stress of the concrete measured through the experiment is shown in FIG. The temperature stress measured by the invariant temperature stress measuring device was initially compressive stress as the thermal expansion was constrained by the device. As the temperature decreased, the compressive stress decreased and then the tensile stress was changed. On the other hand, the temperature stress measured by the temperature stress measuring device made of aluminum showed a tensile stress as the temperature increased, and then changed to the compressive stress as the temperature decreased. The temperature stress was the smallest when using a 5 mm restraining bar and the largest when using a 40 mm restraining bar.

The concrete properties were evaluated using the above experimental results. The hydration heat analysis program was used to supplement the CONSA / HS developed in KAIST concrete laboratory. The optimization program was based on the firefly algorithm proposed by Yang. The evaluation result is shown in Fig. As shown in FIG. 8 (a), the coefficient of thermal expansion sharply decreases before 1 day of age, and then converges to a substantially constant value. The difference between the center portion of the structure and the surface portion hardly occurs. The reason is that the hydration heat is generated rapidly at the beginning of the ages and the curing conditions of the surface part and the surface part are similar before 1 day. As shown in Fig. 8 (b), the modulus of elasticity was estimated to increase slightly more than the surface portion of the structure due to the influence of temperature. However, the elastic modulus of 20 days was reversed at about 4 days, It was evaluated as large. The autogenous shrinkage was also significantly affected by temperature at the beginning of the ages, but the difference between the center and the surface of the structure was not large due to the same curing history at the beginning of the age. Finally, the final creep coefficient was also estimated to be similar to that of the center of the structure.

mm의 원주형 실험체이며, 모든 실험체는 polyester film으로 밀봉 후 습도 95%에서 양생되었다. 양생온도는 구조물 표면온도, 중심온도, 20^oC 세가지 경우이며, 모든 실험은 도 10과 같이 간이항온실 내부에서 실시하여, 각 재령별 양생온도와 동일한 온도에서 실험을 수행하였다. 양생조건 및 재령에 따른 실험체 개수는 표 4와 같다. Compressive strength and elastic modulus were measured at 1, 3, 7, 14 and 28 days of age and tested in accordance with ASTM C39 and C469. The specimen was pressurized using an Instron 2500 kN UTM, and the strain was measured using three LVDT as shown in FIG. The size of the specimen

mm, and all specimens were cured at 95% humidity after sealing with polyester film. The curing temperature was three times at the surface temperature of the structure, the center temperature, and 20 ^o C, and all the experiments were carried out in the simple thermostatic chamber as shown in FIG. 10, and the experiment was performed at the same temperature as the curing temperature for each age. Table 4 shows the number of specimens according to curing conditions and age.

The results of the experiment are shown in Fig. Compressive strength and modulus of elasticity increased in the order of center, surface, and 20 ^o C of the structure with high curing temperature in the early ages. However, the compressive strength and elastic modulus of the specimens cured at 20 ^° C were the largest at 28 days, reversed at 7 days, and the specimens cured at the same curing temperature as the center of the structure were the smallest. In addition, the elastic modulus of the specimens cured at the center and surface temperature of the structure was almost constant after 7 days, but the elastic modulus of the specimens cured at 20 ^° C continuously increased until 28 days.

[Table 4]

mm 각주형 실험체를 이용하였으며, 구조물 중심온도와 표면온도에 대하여 실험을 실시하였다. 변형률은 실험체 중앙에 매립게이지를 매립하여 계측하였으며, 열전대를 이용하여 실험체의 온도를 계측하였다. 실험체는 타설 직후 polyester film으로 밀봉하였으며, 재령 1일에 탈형하여, aluminum tape로 밀봉한 후, 실험체 아래 롤러를 두어 마찰을 제거하였다.The unstressed strain test was conducted to measure the strain due to autogenous shrinkage and thermal expansion. 11

mm square test specimens were used. Experiments were conducted on the center temperature and surface temperature of the structure. The strain rate was measured by embedding a buried gage in the center of the specimen, and the temperature of the specimen was measured using a thermocouple. The specimens were sealed with a polyester film immediately after casting. The specimens were demolded on the first day of age, sealed with aluminum tape, and then friction was removed by placing rollers under the test specimen.

10^-6, 구조물 표면부와 동일한 온도이력을 가진 실험체가 약 250

10^-6이었다.The results of the experiment are shown in Fig. The unstressive strain was much higher than the autogenous shrinkage and showed a similar temperature change. The maximum strain of the specimens with the same temperature history as the center of the structure is about 420

10 ^-6 , and the specimen with the same temperature history as the surface part of the structure is about 250

10 ^-6 .

 The concrete properties evaluated by the proposed concrete property evaluation method and the concrete property values evaluated by the experiment were compared. Fig. 13 shows the results of elastic modulus comparison. The modulus of elasticity was slightly higher at early age when cured with the temperature history of the structure, but the results were consistent with the experimental results in both the center and surface of the structure. The unstressed strain was calculated by using the proposed method and the autogenous shrinkage and the thermal expansion coefficient. Fig. 14 shows the evaluation results and the experimental results. In both the center of the structure and the surface, the results are almost the same as the experimental results.

In the present invention, a method of evaluating various concrete property values by performing an experiment using a temperature stress measuring apparatus and comparing with the analysis is proposed. The conclusion is as follows.

(1) In order to increase the accuracy of the measured temperature stress through the experiment, the bolt connections were removed from the temperature stress measuring device and dummy specimens were manufactured to minimize errors due to temperature compensation.

(2) Since the concrete characteristics are influenced by each other, the concrete characteristics were evaluated by optimizing the existing model formula without evaluating the optimum value for each age.

(3) The thermal expansion coefficient and creep were evaluated to be the same at the surface and center of the structure, and the autogenous shrinkage and elastic modulus were evaluated to be faster at the center of the structure with higher curing temperature at the early age. In addition, the elastic modulus, thermal expansion coefficient, and autogenous shrinkage except for creep were compared with experimental results, and the results were similar to experimental results.

(4) The proposed concrete property evaluation method predicts the elastic modulus considering the hydration reaction difference according to the temperature history, and is similar to the experimental result.

(5) To verify the evaluated creep, additional testing is required and additional verification of various concrete and temperature is required.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Do. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Accordingly, the actual scope of the invention is defined by the appended claims and their equivalents.

Claims (1)

Measuring strain of the apparatus using a concrete sample;
Calculating a concrete stress using the measured strain; And
And evaluating a concrete property value by using the developed program.

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