CN103712855B - A kind of method of testing of concrete creep - Google Patents

Abstract

A concrete creep test method, using a small spring creep meter to load concrete specimens, the creep degree has a good correlation: the specimen is loaded by a universal pressure testing machine; the strain gauge pasted on the specimen and the static resistance The strain gauge is used to monitor the magnitude of the applied load; the spring-type compression creep mechanism is equipped with a half ball pad on the upper plate, and the pressure is preloaded and centered to 20% of the creep control load. After the specimen is centered, the load should be added to Xu After unloading after variable stress, the deformation difference of the four sides of the specimen should be less than 10% of its average value. After formal loading, the deformation difference of the four sides should be less than 10% of its average value, and the difference between the average strain and the previous loading value should be less than 1%. Load to the controlled load; the loading process should be controlled within 2 hours. After loading, the specimens are placed in a constant temperature and humidity chamber according to the number for continuous observation. The invention saves money, is convenient to load, effectively avoids partial loading of test pieces, has strong constant load capacity, and can move flexibly without supplementary loading.

Description

A test method for concrete creep

Technical field The present invention relates to a creep test method.

BACKGROUND OF THE INVENTION There are generally four types of loading devices for compression creep tests: lever type, spring type, spring lever type and hydraulic type. The spring-type compression creep mechanism has become the most widely used loading device in my country due to its small footprint, small size, convenient loading and reliable operation. It can be divided into single-rod, double-rod and three-rod according to the number of tension columns. Generally, the loading range is below 30t, and the relative displacement caused by concrete creep and shrinkage makes the load decrease, and the load needs to be adjusted frequently. Its shortcomings: the lack of loading tonnage and the cumbersome and complicated means of constant pressure limit the use of a large number of spring-type compression creep instruments for testing at the same time, which is a waste of money, inconvenient loading, complicated movement, and a large storage space.

SUMMARY OF THE INVENTION The object of the present invention is to provide a concrete creep test method that saves money, is convenient to load, effectively avoids unbalanced loading of specimens, has strong dead load capacity, and is flexible in movement without additional loading.

Research method of the present invention realizes by following steps:

(1) The creep degree of the 40mm×40mm×160mm concrete specimen made of coarse aggregate with a particle size of 5-16mm loaded with a small spring creep meter is the same as that of the 100mm×100mm×400mm specimen made with coarse aggregate with a particle size of 5-25mm There is a good correlation with the creep degree;

(2) Use a universal pressure testing machine to load a 40mm×40mm×160mm concrete specimen made of coarse aggregate with a particle size of 5 to 16mm;

(3) The strain gauge pasted on the test piece cooperates with the static resistance strain gauge to monitor the magnitude of the applied load;

⑷ Put ball half pads on the upper plate of the spring-type compression creep mechanism, first pressurize to 20% of the creep control load for preloading and centering, after the specimen is centered, the load should be added to the creep stress and then unloaded, at this time The deformation difference of the four sides of the test piece should be less than 10% of its average value, if it exceeds this range, it should be readjusted;

(5) Formal loading, after several loading cycles, the last loading, the difference between the deformation of the four sides should be less than 10% of its average value, and the difference between the average strain and the previous loading is less than 1%, and then loaded to the control load;

⑹The loading process should be controlled within 2 hours to prevent the influence of water evaporation and temperature changes on creep during the loading process of the specimen;

⑺ After the test pieces are loaded, put them into the constant temperature and humidity chamber according to the number, and conduct continuous observation according to the predetermined age.

The invention saves money, is convenient to load, effectively avoids partial loading of test pieces, has strong constant load capacity, and can move flexibly without supplementary loading.

Description of drawings

Fig. 1 Structural schematic diagram of spring type compression creep mechanism;

Fig. 2 Contrast curves of creep degree of specimens with different sizes of the same aggregate under different fly ash content;

Fig. 3 The creep degree comparison curves of the same size and different aggregate specimens under different fly ash content.

detailed description

In the schematic diagrams of the present invention shown in Fig. 1, Fig. 2 and Fig. 3, the spring type compression creep mechanism mainly includes: a nut, a spring washer, an upper support plate, a column, a middle support plate, a spring and a lower support plate. Wherein, the lower support plate 1 is fixedly connected with four uprights 2, and each upright is covered with a spring 3, and the lower ends of every two adjacent springs are sequentially connected to form a square, and the upper end of the spring is provided with a middle support plate 4. The middle support plate is provided with a through hole at the position corresponding to the column, and the column passes through the through hole of the middle support plate, and the middle support plate is arranged on the upper end of the above-mentioned spring. The test piece is placed on the top of the middle support plate, and the upper support plate 5 is arranged on the test piece. The position of the upper support plate corresponding to the column is provided with a through hole. The through hole is provided with a spring washer 6 with an inner diameter slightly larger than it at the top of the through hole, and a nut 7 corresponding to the thread of the above-mentioned column is provided on the washer.

Example 1

The raw material of the present invention adopts the portland cement of P.Ⅱ42.5 produced by Asano Cement Factory, the first grade fly ash produced by Suizhong Thermal Power Plant, its chemical composition, loss on ignition and specific surface area are as shown in Table 1; The fine aggregate is natural river sand with a fineness modulus of 2.8 and an apparent density of 2650kg/ ^m3 . In the creep test, the coarse aggregate particle size used in the standard prism specimen is different from that of the small prism specimen, and two different graded aggregates are used: the small specimen adopts continuous gradation crushed limestone with a particle size of 5-16mm; Continuously graded crushed limestone was used for large specimens.

Part of the particle size is 5-16 mm, and part of the particle size is 5-25 mm. The water reducer is a naphthalene-based water reducer with a water reducing rate of 21%.

Table 1 Chemical composition, loss on ignition and specific surface area of cement and fly ash

The invention adopts the method of fixing the concrete sand ratio, the total amount of cementitious material and the water-binder ratio, replacing the cement with the same amount of fly ash, and preparing concrete specimens of different sizes to study the effect of the size effect on the creep of the concrete. In the design of concrete mix ratio, the sand rate is fixed at 38%, the water-binder ratio is 0.45, the amount of concrete cement in the benchmark group is 390kg/m ³ , and fly ash replaces cement in equal amounts by mass fraction of 0%, 30%, and 60%. Apply water reducing agent to adjust the slump of concrete in the range of 90-110mm. Table 2 shows the specific mixing ratio and size of the fly ash specimens with different dosages.

Table 2 Concrete mix ratio

When loading the specimen, the small prism specimen with a size of 40mm×40mm×160mm is loaded with a small spring-type creep meter, and the standard prism specimen with a size of 100mm×100mm×400mm is loaded with a spring creep meter in the national standard. Stress level 33%. In an environment of (20±1)°C and relative humidity of (60±5)%, the shrinkage and creep deformation of concrete should be tested with a resistance strain gauge.

The following is the data processing of the present invention: it is the small concrete specimen of 40mm * 40mm * 160mm and the creep degree of each age of the large specimen of 100mm * 100mm * 400mm that the size is 100mm * 100mm * 400mm by comparison using the same graded aggregate size, different pulverized coal The creep degree comparison curves of specimens with the same aggregate and different sizes under the ash content are shown in Fig. 2.

It can be seen from Figure 2 that under different fly ash content, the creep curves of the large and small specimens are basically consistent, and the creep of the small specimen is slightly larger than that of the large specimen. The curve is flat and the variance is maintained at a certain level.

Comparing the creep degree of specimens with a size of 100mm×100mm×400mm made of coarse aggregate with a particle size of 5-16mm and the creep of specimens of the same size at different ages made with coarse aggregate with a particle size of 5-25mm under the same conditions Degree, each group of test pieces are classified according to the different amount of fly ash. The creep degree comparison curves of the same size and different aggregate specimens with different fly ash content are shown in Fig. 3.

It can be seen from Figure 3 that under different fly ash content, the creep curves of different aggregate specimens are basically consistent, and the creep degree of small aggregate specimens is slightly larger than that of large aggregate specimens. The creep degree curves of those are stable, and the difference is maintained at a certain level.

As shown in Figure 2 and Figure 3, the creep degree of each specimen has a good correlation. Therefore, taking the limestone with continuous gradation of 5-16mm as the benchmark, and taking the creep degree of different ages as the unit 1, the specimen with the size of 40mm×40mm×160mm and The size is 100mm×100mm×400mm, and the creep degree of specimens with continuous graded limestone of 5-16mm is used for comparison. After data processing, the statistical results of creep degree of each age are shown in Table 3.

It can be seen from Table 3 that the proportional relationship between the creep degrees of the three specimens in the later stage of loading gradually stabilized, and the average value of the last four values of the relative creep degrees of each group of specimens was calculated to obtain the creep degrees of different specimens correlation coefficient.

(1) Calculate the correlation coefficient between the creep degree of the prism with the size of 40mm×40mm×160mm and the creep of the prism with the size of 100mm×100mm×400mm in the concrete specimen with the same mix ratio:

When the amount of fly ash is 0%, the time-varying correlation coefficient k=(1.036+1.050+1.049+1.046)/4=1.045;

When the amount of fly ash is 30%, the correlation coefficient k=(1.087+1.049+1.062+1.064)/4=1.066;

When the amount of fly ash is 60%, the correlation coefficient k=(1.022+0.997+1.009+1.020)/4=1.012.

Taking the average value of the three correlation coefficients, the correlation coefficient of the creep degree of the concrete 40mm×40mm×160mm specimen with the same mix ratio as the creep degree of 100mm×100mm×400mm specimen is k=1.04.

Table 3-2 Comparison of creep degrees of three different sizes and different aggregate specimens

(2) Calculate the correlation coefficient between the creep degree of the 100mm×100mm×400mm specimen made of 5-16mm coarse aggregate and the creep degree of the specimen made of 5-25mm coarse aggregate under the same conditions:

When the fly ash content is 0%, the correlation coefficient of creep degree k=(0.933+0.926+0.927+0.900)/4=0.922;

When the amount of fly ash is 30%, the correlation coefficient k=(0.918+0.920+0.917+0.910)/4=0.916;

When the amount of fly ash is 60%, the correlation coefficient k=(0.989+0.926+0.922+0.933)/4=0.943.

Taking the average value of the creep degree correlation coefficients of the three fly ash additions, the creep degree of the 100mm×100mm×400mm specimen made of 5-16mm coarse aggregate is the same as that of the specimen made of 5-25mm coarse aggregate under the same conditions. The correlation coefficient between the variations is k=0.93.

(3) According to the correlation coefficient of creep degree obtained in (1) and (2), it can be concluded that the creep degree of a 40mm×40mm×160mm specimen made of coarse aggregate with a particle size of 5 to 16mm is the same as that of a specimen with a particle size of 5 to 16mm. The correlation coefficient between creep degrees of 100mm×100mm×400mm specimens made of coarse aggregate with particle size of 25mm is k=1.12.

Loading capacity calculation of the loading system, the present invention uses a spring-type compression creep mechanism, 4 steel springs with an elastic coefficient of 0.6-1kN/mm and an elastic limit of 10kN to form a spring group, and the size of the concrete specimen is 40mm×40mm×160mm , each mechanism loads a specimen. According to the previous experimental results, assuming that the elastic coefficient of the spring is 1kN/mm, the shrinkage strain of the concrete is 300×10 ^-6 , the creep degree is 30×10 ^-6 /MPa, and the calculation is based on the loading load of 30kN, which can be obtained as follows:

The initial deformation of the spring group is: Xs=F/K=30/4=7.5mm;

In the formula, Xs is the deformation of the spring group (mm); F is the load (kN); K is the elastic coefficient of the spring group (kN/mm)

The deformation of concrete due to shrinkage and creep after loading is:

X _c ＝X _sh +X _cr ＝L ₀ (ε _sh +ε _cr σ)＝160×[400×10 ^-6 +50×10 ^-6 ×25×10 ³ /(40×40)]＝0.138(mm )

In the formula, X _c is the deformation of concrete specimen (mm); X _sh is the deformation caused by shrinkage (mm); X _cr is the deformation caused by creep (mm); L ₀ is the length of concrete specimen (mm); ε _sh is Shrinkage strain; ε _cr is concrete creep degree (10 ^-6 /MPa); σ is stress of concrete specimen (MPa)

The load drop caused by shrinkage and creep is 1.84%, which is less than 2%, and there is no need for supplementary loading during the experiment. If the shrinkage and creep of the pre-recorded specimen are higher than the values set in this example, a spring set with a smaller elastic coefficient can be selected. Generally, it can meet the requirement that the specimen does not need to be reloaded within one year of loading.

Claims (1)

1. A test method for concrete creep, characterized in that: (1) The creep degree of the 40mm×40mm×160mm concrete specimen made of coarse aggregate with a particle size of 5-16mm loaded with a small spring creep meter is the same as that of the 100mm×100mm×400mm specimen made with coarse aggregate with a particle size of 5-25mm There is a good correlation with the creep degree; (2) Use a universal pressure testing machine to load a 40mm×40mm×160mm concrete specimen made of coarse aggregate with a particle size of 5 to 16mm; (3) The strain gauge pasted on the test piece cooperates with the static resistance strain gauge to monitor the magnitude of the applied load; ⑷ Put ball half pads on the upper plate of the spring-type compression creep mechanism, first pressurize to 20% of the creep control load for preloading and centering, after the specimen is centered, the load should be added to the creep stress and then unloaded, at this time The deformation difference of the four sides of the test piece should be less than 10% of its average value, if it exceeds this range, it should be readjusted; (5) Formal loading, after several loading cycles, the last loading, the difference between the deformation of the four sides should be less than 10% of its average value, and the difference between the average strain and the previous loading is less than 1%, and then loaded to the control load; ⑹The loading process should be controlled within 2 hours to prevent the influence of water evaporation and temperature changes on creep during the loading process of the specimen; ⑺ After the test pieces are loaded, put them into the constant temperature and humidity chamber according to the number, and conduct continuous observation according to the predetermined age.