• Journal of the Korea Concrete Institute
• /
• v.21 no.4
• /
• pp.465-471
• /
• 2009

To prevent the explosive spalling of the high strength concrete and control the rise of temperature in the steel rebar during fire, a fiber cocktail method has been proposed simultaneously with the use of polypropylene and steel fiber. After applying the fiber cocktail (polypropylene and steel fibers) into the mixture of high strength concrete with a compressive strength of between 40 and 100 MPa and evaluating the thermal properties at elevated temperatures, the fire test was carried out on structural members in order to evaluate the fire resistance performance. Two column specimens were exposed to the fire without loading for 180 minutes based on the standard curve of ISO-834. No explosive spalling has been observed and the original color of specimen surface was changed to light pinkish grey. The inner temperature of concrete dropped rapidly starting from 60mm deep. After 60 minutes of exposure to the fire, the temperature gradient of fiber cocktail reinforced high strength concrete was measured as 2.2oC/mm, which is approximately 5 times less than that of normal concrete. The final temperatures of steel rebar after 180 minutes of fire test resulted in 488.0oC for corner rebar, 350.9oC for center rebar, and 419.5oC for total mean of steel rebar. The difference of mean temperature between corner and center rebar was 137.1oC The tendency of temperature rise in concrete and steel rebar changed between 100oC and 150oC The cause of decrease in temperature rise was due to the water vaporization in concrete, the lower temperature gradient of the concrete with steel and polypropylene fiber cocktails, the moisture movement toward steel rebars and the moisture clogging.

• Journal of the Korea Concrete Institute
• /
• v.21 no.4
• /
• pp.473-480
• /
• 2009

The 180 minutes fire test based on the standard curve of ISO-834 were conducted on three RC column specimens with different constant axial loading ratios to evaluate the fire performance of fiber cocktail (polypropylene+steel fiber) reinforced high strength concrete column. The columns were tested under three loading levels as 40%, 50%, and 61% of the design load. No explosive spalling has been observed and the original color of specimen surface has been changed to light pinkish grey. The maximum axial displacements of three specimens were 1.5~2.2 mm. There was no reduction in load bearing capacity of each specimen exposed to fire and no effect were observed on the fire performance within 61% of the design load. The tendencies of the results with loading, such as the temperature distribution of in concrete and the changes in temperature rise due to the water vaporization in concrete, are very similar to those without loading. The final temperatures of steel rebar after 180 minutes of fire test resulted in 491.4${^{\circ}C}$ for corner rebar, 329.0${^{\circ}C}$ for center rebar, and 409.8${^{\circ}C}$ for total mean of steel rebar. The difference of mean temperature between corner and center rebar was 153.7${^{\circ}C}$ㅍ. The tendency of temperature rise in concrete and steel rebar changed after 30~50 minutes from the starting time of the fire test because the heat energy influx into corner rebar is larger than that into center rebar. The cause of decrease in temperature rise was due to the water vaporization in concrete, the lower temperature gradient of the concrete with steel and polypropylene fiber cocktails, the moisture movement toward steel rebars and the moisture clogging.

• International Journal of Concrete Structures and Materials
• /
• v.2 no.1
• /
• pp.41-48
• /
• 2008

This study investigates the spalling properties of high strength concrete designed with various types of mineral admixture and diverse content ratios of polypropylene (PP) fiber. Experimental factors considered in series I are four pozzolan types of mineral admixture and series II consists of three shrinkage reducing types of mineral admixture. PP fiber was added 0.05, 0.10 and 0.15vol. % in each mixture of series I and series II, so that totally 27 specimens including control concretes in each series were prepared. Test results showed that the increase of fiber content decreased the slump flow of fresh concrete and increased or decreased the air content depending on the declining ratio of slump flow. For the properties of compressive strength, all specimens were indicated at around 50 MPa, which is high strength range; especially all specimens in series II were 60 MPa. Fire test was conducted in standard heating curve of ISO 834 with ${\phi}100{\times}200\;mm$ size of cylinder moulds for 1 hour. The specimens incorporating silica fume exhibited severe spalling and most specimens without the silica fume could be protected from the spalling occurrence in only 0.05vol % of PP fiber content. This fire test results demonstrated that the spalling occurrence in high strength concrete was not only affected by concrete strength related to the porosity of microstructure but also, even more influenced by micro pore structure induced by the mineral admixtures.

• Coupled systems mechanics
• /
• v.9 no.2
• /
• pp.163-182
• /
• 2020

Coupled thermo-mechanical analysis of reinforced concrete slab at elevated temperatures from a fire accounting for nonlinear thermal parameters is carried out. The main focus of the paper is put on a one-way continuous reinforced concrete slab exposed to fire from the single (bottom) side as the most typical working condition under fire loading. Although contemporary techniques alongside the fire protection measures are in constant development, in most cases it is not possible to avoid the material deterioration particularly nearby the exposed surface from a fire. Thereby the structural fire resistance of reinforced concrete slabs is mostly influenced by a relative distance between reinforcement and the exposed surface. A parametric study with variable concrete cover ranging from 15 mm to 35 mm is performed. As the first part of a one-way coupled thermo-mechanical analysis, transient nonlinear heat transfer analysis is performed by applying the net heat flux on the exposed surface. The solution of proposed heat analysis is obtained at certain time steps of interest by α-method using the explicit Euler time-integration scheme. Spatial discretization is done by the finite element method using a 1D 2-noded truss element with the temperature nodal values as unknowns. The obtained results in terms of temperature field inside the element are compared with available numerical and experimental results. A high level of agreement can be observed, implying the proposed model capable of describing the temperature field during a fire. Accompanying thermal analysis, mechanical analysis is performed in two ways. Firstly, using the guidelines given in Eurocode 2 - Part 1-2 resulting in the fire resistance rating for the aforementioned concrete cover values. The second way is a fully numerical coupled analysis carried out in general-purpose finite element software DIANA FEA. Both approaches indicate structural fire behavior similar to those observed in large-scale fire tests.

• Journal of the Korea Concrete Institute
• /
• v.26 no.3
• /
• pp.247-254
• /
• 2014

Strain properties of concrete member which acts as an important factor in the stability of the concrete structure in the event of fire, significantly affected the characteristics of the coarse aggregate, which accounts for most of the volume. For this reason, there are many studies on concrete using artificial lightweight aggregate which has smaller thermal expansion deformation than granite coarse aggregate. But the research is mostly limited on concrete using clay-based lightweight aggregate. Therefore, in this study, the high temperature compressive strength and elastic modulus, thermal strain and total strain, high temperature creep strain of concrete was evaluated. As a result, remaining rate of high-temperature strength of concrete using lightweight aggregate is higher than concrete with general aggregate and it is determined to be advantageous in terms of structural safety and ensuring high-temperature strength from the result of the total strain by loading and strain of thermal expansion. In addition, in the case of high-temperature creep, concrete shrinkage is increased by rising loading and temperature regardless of the type of aggregate, and concrete using lightweight aggregate shows bigger shrinkage than concrete with a granite-based aggregate. From this result, it is determined to require additional consideration on a high temperature creep strain in case of maintaining high temperature like as duration of a fire although concrete using light weight aggregate is an advantage in reducing the thermal expansion strain of the fire.

• Proceedings of the Korean Institute of Building Construction Conference
• /
• 2008.11a
• /
• pp.199-203
• /
• 2008

With recent trend in domestic and global market requiring architectures' conversion into skyscrapers seasoned with the features of landmarks, structural problems in relation with explosive spatting during fire emergencies are arising as controversial issues. Accordingly, many productive researches have been made in relation to the reinforcement techniques for improving fire resistance and the number of applications in the field is gradually increasing. In this study, a ductile outline form using ECC (Engineered Cementations Composites) was made with improvements on the structure and fire resistance to examine its applicability. Also, currently in Japan, the number of studies and applications is increasing focusing on reduction of construction time and improvement of workability with application of Half-PCa method. However, using such method of construction, large structural members decrease the utilization of space and architecture-wise, there is a disadvantage of the weight increase. Therefore, in such context, it would be worth reducing the weight of the structural members by reducing the size using ECC. In addition, its excellent pseudo strain-hardening due to fiber may have great effects on seismic designs. In the mean time, this study planned 3 equal conditions for mix water, PVA fiber and additives excluding binder and refractory to evaluate the mechanical properties of resistance against pressure and internal force. Finally, an evaluation was executed on the fire resistance of the newly made ductile outline form. As a result, from ECC-I to ECC-III, all showed excellent mechanical properties due to pseudo strain-hardening and in the fire resistance test conducted with ISO 834 heating curve, most of them tended to be in the range of the reference temperature (538℃-180min), so there was no occurrence of any explosive spatting.

• Fire Science and Engineering
• /
• v.25 no.1
• /
• pp.42-49
• /
• 2011

In the high temperature situation like in a fire, the high strength of concrete (HSC) has extreme danger named explosive spalling. It is assumed that the major cause of explosive spalling is water vapour pressure inside concrete. This paper examines the effect of the compressive strength and the moisture content on the initial occurrence of explosive spalling. For the effective experiment of the initial explosive spalling, the curve of ISO834 temperature profile is applied on the basis of 15 minute and 30 minute. As a result, the more increase the compressive strength and the moisture content, the more increase the occurrence and phenomenon of explosive spalling. This paper analyzes the territory of explosive spalling depending the compressive strength and the moisture content. The explosive spalling is not examined in the case of the compressive strength 50~100 MPa and the moisture content below 3% and the compressive strength over 100 MPa and the moisture content below 1%. Also, due to the HSC, which makes it more difficult to transport vapour and moisture, very high vapour-pressure may occur close to the surface, there is a greater risk that HSC spalls compared with normal strength concrete (NSC).

• Journal of the Korea Concrete Institute
• /
• v.24 no.2
• /
• pp.173-183
• /
• 2012

Recently, experimental studies to prevent explosive spalling based on spalling mechanism and addition of Polypropylene fiber in high strength concrete (HSC) are performed actively. However, with respect to ultra high strength concrete (UHSC), its compact internal structure is more difficult release vapor pressure at rapid rising temperature compared to HSC. Therefore, in this study, an experiment was conducted to evaluate spalling properties of UHSC using ${\Box}$ $100mm{\times}100{\times}H200mm$ rectangular specimen according to ISO-834 standard fire curve. With respect melting point of fiber, three fiber types of Polyethylene, Polypropylene, and Nylon fibers with melting temperature of $110^{\circ}C$, $165^{\circ}C$, and $225^{\circ}C$, respectively, were considered. Mixed fiber of 0.15% and 0.25% of concrete volume was used to consider spalling properties based on water vapor pressure release. Then, TGDTA test on fiber and FEM analysis were performed. The results showed that it is difficult to prevent initial spalling without loss of fiber mass even if fiber melting temperature is low. Also, in preventing thermal spalling, fiber that melts to rapidly create porosity within 10 minutes of fire is more effective than that of low melting temperature property of fiber.