KR100644082B1 - The reinforced concrete column improved fire resistance - Google Patents

Abstract

A fireproof steel concrete column is provided to increase improve fire resistance by increasing lateral reinforcing efficiency of a tie band and restricting longitudinal crack in a fire and temperature increase of a corner longitudinal steel reinforcing bar. A main steel reinforcing bar is arranged longitudinally, and a tie band for surrounding the main steel reinforcing bar is arranged horizontally in a steel concrete column. The tie bands are arranged at the interval of less than 150 mm, and organic fiber is mixed with high strength concrete in mixing high strength concrete having the design compression strength of more than 40 MPa for the steel concrete column. The steel concrete column is formed of a rectangular post, and the tie band is anchored with a hook of 90‹. The extension length of the tie band is more than the distance from the corner of the tie band to the longitudinal crack portion after heating the steel concrete column.

Description

Reinforced concrete column with improved fire resistance

1 is a cross-sectional view (1a) and a transverse direction detailed view (1b) of a column test body used in the heating test

2 is a view showing the installation position (2a) and the installation method (2b) of the temperature sensor installed in the reinforced concrete column.

3 is an SEM photograph of polypropylene fibers incorporated into high strength concrete.

4 is a comparison graph between the standard fire curve and the actual heating curve applied to the test body.

5A to 5C are graphs of temperature history according to strip reinforcement intervals and are graphs of temperature history for intervals of 150 mm (5a), 210 mm (5b), and 300 mm (5c), respectively.

6A and 6B are photographs of longitudinal cracking after three hours of heating of the cross section size 500x500 mm pillar 6a and the cross section size 305x305 mm pillar 6b, respectively.

FIG. 7 is a cross-sectional view illustrating the development of longitudinal cracks generated as shown in FIGS. 6A and 6B.

8 is a cross-sectional view of the reinforced concrete column showing the length of the reinforcing band reinforcing bars in accordance with the present invention.

The present invention relates to a reinforced concrete column with improved fire resistance, and more particularly, it is confirmed that the decrease in the structural strength of the reinforced concrete column at the time of fire is due to the rapid temperature rise of the edge longitudinal bars according to the longitudinal cracking. The present invention relates to a reinforced concrete column that has a structural stability by effectively improving fire resistance by improving lateral reinforcement ability.

When the reinforced concrete member is exposed to a high temperature, its strength decreases. At this time, the time at which the strength remains above a certain standard is called fire resistance. In general, fire resistance is divided into 1 hour, 2 hours, and 3 hours. In particular, reinforced concrete columns are designed to have fire resistance of 3 hours, although they are different depending on their location because they are the main members of a building structure. There are two ways to evaluate the fire resistance of reinforced concrete columns.

The first method is to evaluate the contraction degree of the column by applying a constant load to the column and conducting a heating test as shown in KS F 2257-1. Fire resistance is defined as the time measured within the reference value.

The second method is briefly mentioned in KS F 2257-7, but it is a non-heating heating test that is widely used in Japan. It is a method of defining the fire resistance performance until the temperature of the longitudinal rebar in the reinforced concrete column does not exceed 500 ℃. .

In the event of fire, the concrete changes temperature greatly up to 125 mm from the surface. In the case of reinforced concrete column members, the concrete cover is about 40 mm, so the center of the main reinforcing bar is usually located at 60 to 70 mm from the surface. As a result, the reinforcing bar in the column member shows a high temperature in case of fire. In the three-hour heating test according to the ISO-KS heating curve, the temperature of the edge reinforcing bar was increased to 500 ℃ after 2 hours and 20 minutes and to 600 ℃ after 3 hours. This is a result showing that a conventional reinforced concrete column having a coating thickness of 40 mm lacks fire resistance.

When the rebar is exposed to high temperature, the elastic modulus and yield strength of the rebar are lower than at room temperature, and the longitudinal concrete reinforcement is prone to buckling failure because the outer concrete cover of the reinforcing bar is in very low strength. The local buckling failure of the cast iron will ultimately lead to the destruction of the entire column, so it should be avoided as much as possible.Therefore, additional measures to improve the fire resistance of reinforced concrete columns will be required.

Conventionally, the method of improving the fire resistance of a reinforced concrete column is to improve the first concrete coating, the method of adding a fireproof plate with resistance to high temperature to the surface of the column, and the third, a fireproof material with excellent fire resistance. The method of apply | coating to the whole surface, etc. have been applied.

The first method has the disadvantage that the cross section of the pillar is too large and the available space is reduced. For example, the concrete covering thickness of the reinforced concrete column developed by Kajima Construction in Japan is 72 mm, which is 32 mm higher than the general coating thickness of 40 mm. Increased. In this case, the width of the column is 64mm (32 + 32), which increases, so that not only concrete use is increased but also the distance between the pillars is shortened, thus reducing the available area and increasing the weight of the building. The second and third methods have a disadvantage in that economic efficiency is reduced because the front surface of the pillar is coated with a refractory material, and workability is inferior because a fireproof coating process is required separately from the construction of the structure.

The present invention has been made to improve the above-mentioned conventional problems, and the test confirmed that the decrease in structural strength of the reinforced concrete column during the fire is due to the rapid temperature rise of the edge longitudinal reinforcement due to the longitudinal cracking. The purpose of the present invention is to provide reinforced concrete columns that effectively improve fire resistance by improving lateral reinforcement ability.

It is another object of the present invention to provide a reinforced concrete column with improved fire resistance that can eliminate the fireproof coating process by solving the improvement of the fire resistance through the reinforcement process of the rebar.

In order to achieve the above object, the present invention is a reinforcement in which the reinforcing bar is reinforced in a longitudinal direction and the reinforcing steel bars surrounding the main reinforcing bars in the transverse direction are reinforced at a predetermined interval, the reinforcing bar is reinforced at intervals of 150 mm or less Provides reinforced concrete columns with improved performance. When the reinforced concrete column is designed to be mixed with high-strength concrete having a compressive strength of 40 MPa or more, it is preferable to design the organic fiber into the high-strength concrete.

In particular, the present invention is the reinforcement concrete pillar is a square pillar, the band reinforcement is fixed while having a 90 ° hook, the fixing length has an extension length that extends further from the design fixing length, but the extension length is the band reinforcement We propose a reinforced concrete column that extends beyond the length from the edge of to the point where longitudinal cracking occurs after the heating test of the reinforced concrete column. Specifically, when the cover thickness of the reinforcing bar is designed to be 40 mm in the reinforced concrete column, it is proposed to extend the strip reinforcing bar to 60 mm or more.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and preferred embodiments.

The present invention confirms that the decrease in the structural strength of the reinforced concrete column at the time of fire is due to the rapid temperature rise of the longitudinal longitudinal reinforcing bars (cast bars) due to the longitudinal cracking, and the fire resistance performance of the reinforced concrete column is improved by improving the lateral reinforcement ability of the band reinforcing bars. There are technical features to enable effective promotion. In other words, by reducing the reinforcement spacing of transverse reinforcement bars (reinforcing bars) reinforced around the reinforcing bar in the reinforced concrete column, it improves the lateral restraint force, thereby suppressing the longitudinal cracking of the reinforced concrete column and consequently reducing the buckling fracture due to the temperature rise of the reinforcement concrete column. To induce stable behavior.

On the other hand, concrete becomes water vapor when there is a sudden temperature rise inside the concrete and escapes into the gaps in the concrete. When the steam generation rate is faster than the exiting speed, steam pressure is generated. Spalling is caused by the premature dropping of concrete cladding, which causes the longitudinal rebar to be exposed to high temperatures, causing the reinforced concrete member to collapse quickly. Therefore, when the reinforced concrete member is designed to be mixed with high strength concrete having a concrete compressive strength of 40 MPa or more, it is preferable to mix the organic fiber in the high strength concrete and mix the designed design. This is because the thermal explosion usually does not occur in normal strength concrete, because the thermal phenomenon easily occurs in high strength concrete.

In other words, high-strength concrete is manufactured by adding water, fine aggregates, coarse aggregates, and high performance water reducing agents to improve fluidity to powder materials composed of cement, fly ash, slag, or silica fume. It is defined as high-strength concrete when the compressive strength level is 40MPa or more.The higher the concrete strength, the less the voids inside, so that the void pressure (mainly water vapor pressure) is not effectively discharged. This happens easily. Therefore, in high-strength concrete, organic fibers are incorporated into concrete to prevent such thermal expansion. Since the organic fibers are melted before the thermal explosion occurs, the passage in which the organic fibers are melted becomes a passage through which the void pressure (mainly water vapor pressure) is effectively discharged, thereby preventing the thermal explosion phenomenon.

The present invention was completed as a result of selecting the appropriate reinforcement spacing and anchoring length of the reinforcing bar reinforcement after grasping the fire resistance performance of the reinforcement concrete column according to the spacing of transverse reinforcement (strip reinforcing bar).

Example 1

1. Test plan

 In order to investigate the effect of lateral reinforcing bars on the fire resistance of reinforced concrete columns, a rectangular high-strength concrete column with reinforced bars was selected as a model.

(1) reinforcement plan

Cross section and reinforcement details of the concrete pillar member are as shown in FIG. Experimental variables are the band reinforcement intervals, the cross section of the rectangular reinforced concrete column with the reinforcing bars is shown in Fig. 1 (a), and the band reinforcement interval is shown in Fig. 1 (b).

The column section size was determined to be 12in (305mm) for 3 hours fire resistance, based on ACI / TMS Committee 216 'Standard Method for Determining Fire resistance of Concrete and Masonry Construction Assemblies'. As shown in Fig. 1b, the interval of the band reinforcing bars was placed in the center at 300 mm, 210 mm, and 150 mm, and as shown in Fig. 1a, the band reinforcing bars were set at 135 degree standard hooks. In order to measure the temperature at the center of the column cross section, a temperature sensor was installed as shown in FIG. 2.

(2) concrete combination plan

Concrete mixing ratio is as shown in Table 1, and in order to prevent the thermal expansion of PP fiber (length 6mm, diameter 45.2㎛) 0.9kg / ㎥ was mixed. 3 is a SEM photograph of the PP (polypropylene) fibers used.

[Table 1] Concrete mixing ratio

Water Binder Ratio W / B (%) Fine Aggregate S / a (%) unit weight (kg / m ³ ) Water (W) Cement (C) Fly Ash (FA) Fine Aggregate (S) Coarse aggregate (G) Polypropylene (PP) High Performance Supervisor (SP) 30 41.2 168 476 99 629 878 0.9 5.04

2. Heating test

The concrete members were subjected to air curing after the concrete was placed, and the compressive heating test was carried out at 60 days of age, and the compressive strength was 68.7 MPa.

Before heating, the pillar member was subjected to a central axial load of 142 tons. The heating temperature in the furnace was fitted to the standard time-heating temperature curve of the ISO-KS standard as shown in FIG. 4. The ISO-KS standard time-heat temperature curve is a reference value used to evaluate the fire resistance performance of various materials and members.

3. Temperature history for each location after heating test

5A, 5B and 5C show temperature hysteresis curves at each column position. As can be seen in each figure, the temperature of the longitudinal reinforcing bars located at the corners was greatly affected by the spacing of the band reinforcing bars.

In other words, the temperature of one of the longitudinal bars in the corner increases sharply when about 132 minutes have elapsed in the case of the 300 mm strip reinforcement and about 157 minutes has elapsed when the strip reinforcement spacing is 210 mm. . In particular, in the case where the band reinforcement is 150 mm, the temperature of the longitudinal reinforcing bars located at the corners does not increase rapidly even after 180 minutes.

4. Longitudinal crack of column after heating test

6A and 6B show the test specimens after the heating test, and it can be seen that distinct longitudinal cracks were generated near the edges. These longitudinal cracks begin to appear fine about 40 minutes after the start of the heating test. After that, the width of the longitudinal cracks will be further enlarged and eventually develop into cracks which partially penetrate the edges as shown in FIG. 7.

The longitudinal cracking at the corners of the column acts as a cause of the rapid temperature increase of the longitudinal reinforcing bars located at the corners. This is because the hot heat from heating will penetrate into the concrete column along the longitudinal crack and directly affect the temperature increase of the longitudinal reinforcing bars located at the corners.

However, the width of these longitudinal cracks will be narrower as the strip rebar spacing is smaller. This is because the narrower the rebar spacing, the smaller the width of longitudinal cracks generated at the edges by suppressing the expansion of the concrete inside the reinforcing bar. The test results also show that it is possible to control the rapid temperature rise of the longitudinal reinforcing bars when the rebar spacing is 150 mm in the square reinforced concrete column.

On the other hand, Figure 6a and Figure 6b shows the appearance of the column after 3 hours heating according to the thickness of the column, showing that the longitudinal crack occurs at the point about 100mm from the corner in relation to the column thickness. That is, FIG. 6A corresponds to a pillar having a thickness of 500 mm (concrete coating 40 mm), and FIG. 6B corresponds to a pillar having a thickness of 305 mm (concrete coating 40 mm), in which the longitudinal crack is 1 In the case of the / 5 point and the 305mm thickness column, it occurred at one third of the thickness.

Longitudinal cracks are caused by temperature differences because the temperature near the edge of the column is relatively higher than elsewhere, and the location of the crack corresponds to the boundary between the very high and low temperatures. In other words, the area where the temperature is high from the edge of the column to the crack is generated, and the remaining sections (near the center) are classified into relatively low temperatures. As described above, the temperature of the longitudinal reinforcing bars located at the relatively low central temperature did not exceed 500 ° C. even when heated for 3 hours.

When longitudinal cracking occurs in this way, ultimately, a problem occurs in the fixing performance of the band reinforcing bar 90 ° standard hook. Because longitudinal cracking occurs in concrete cladding, the concrete thickness is not sufficient for 90 ° standard hooks (with 135 ° hooks, the tip is towards the center of the column, so there is no problem with its anchoring performance). As a result, the length that can resist the tensile force based on the outer line (edge) of the strip reinforcement is reduced to the point where the longitudinal crack occurs. In other words, the tensile strength of the strip reinforcement is exerted after the point where the longitudinal crack occurs, and the fixing performance decreases accordingly.

The problem of deterioration of the fixing performance of the band reinforcing bar can be solved by extending the fixing length of the band reinforcing bar longer than the design fixing length, as shown in FIG. It should be at least as far as the point where the direction crack occurs. For example, when the cover thickness of the reinforcing bar is designed to be 40 mm in the reinforced concrete column as in the present embodiment, the extension length of the band reinforcing bar is 60 mm (cracking point (100 mm from the edge of the reinforced concrete column)- It is expected that the tensile strength generated in the strip reinforcing bar can be sufficiently withdrawn when the thickness of the reinforcing bar is 40 mm or more.

According to the present invention as described above, the lateral reinforcement capacity of the band reinforcement is improved to suppress cracks in the longitudinal direction during the fire and as a result the temperature rise of the longitudinal longitudinal bars is suppressed, thereby effectively improving the fire resistance performance of the reinforced concrete column It becomes possible.

Claims (4)

In the reinforced concrete column in which the main reinforcing bars in the longitudinal direction and the band reinforcing bars surrounding the main reinforcing bars in the transverse direction are reinforced at a predetermined interval, Reinforced concrete pillars with improved fire resistance, characterized in that the reinforcing strip is reinforced at intervals of 150mm or less. In claim 1, When the reinforced concrete column is designed to be mixed with high-strength concrete having a compressive strength of 40 MPa or more, the high-strength concrete is characterized by the fact that the organic fiber is mixed in the reinforced concrete column. The method of claim 1 or 2, The reinforced concrete column is a square pillar, The band reinforcing bar is settled with a hook of 90 ° and the fixing length has an extension length that extends further from the design fixing length, the extension length is longitudinal crack after heating test of the reinforced concrete column from the edge of the band reinforcing bar Reinforced concrete column with improved fire resistance, characterized in that more than the distance to the point of occurrence. In claim 3, The cover thickness of the reinforcing bar in the reinforced concrete column is designed 40mm, Reinforced concrete pillars with improved fire resistance, characterized in that the extension length of the strip reinforcing bar 60 mm or more.

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