Spalling In High Strength Concrete Research Articles

Early residual splitting tensile strength of concrete at elevated temperature

Assessment of the fire-induced spalling of high-strength concrete in concrete structures requires knowledge of the tensile strength of concrete at an elevated temperature. However, previous research has mainly focused on determining residual tensile strength, measured at the post-fire stage after cooling the specimens to ambient temperature. However, such residual tensile strength is only applicable to concrete after fire exposure and not during the fire event, which is usually more critical. In this study, the early residual splitting tensile strength of concrete at high temperature is determined experimentally. The test results indicate that early residual splitting tensile strength decreases with temperature. To understand this phenomenon, temperature distribution is examined. Compared to residual tensile strength, loss of early residual splitting tensile strength is found to be faster due to the elevated temperature effect. Lastly, in order to reproduce early residual splitting tensile strength, a numerical model is developed and empirical expressions are proposed for engineering application.

Spalling of high strength concrete in fire

This paper reviews a series of experimental tests programmes undertaken at BRE in the UK on the performance in fire of high strength concrete with a particular focus on mix design to assess the effects of spalling. The experiments include a series of fire tests on reinforced and unreinforced concrete columns and tunnel linings. The paper will provide information on the parameters that lead to explosive spalling of tunnel linings in a fire situation and on the measures available to mitigate this hazard.

Compressive behaviour of ECC confined concrete partially encased steel composite columns using high strength steel

When using high strength materials in concrete partially encased steel (CES) composite columns, the major issues encountered are strain incompatibility of high strength steel and high strength concrete, premature concrete spalling and explosive brittle failure with poor ductility. In order to improve the compressive behaviour and ductility of CES columns, Engineered Cementitious Composites (ECC) encasement is proposed as a confinement and protection layer for concrete and steel, respectively. In order to evaluate the effectiveness of ECC cover on performance of CES columns, compressive behaviours of ECC-CES short columns were experimentally investigated to study the effects of material strengths, ECC encasement thickness, steel section shape and column’s aspect ratio. Findings were discussed and reported in terms of failure behaviours, load-deformation responses, post-peak ductility and toughness. Test results suggested that while only an insignificant increase of axial compressive resistance was observed, ECC encasement improved failure behaviour of ECC-CES columns by effectively controlling brittleness and explosive spalling of high strength concrete. ECC-CES generally showed improved ductility and energy absorption capacity but the strength enhancements were minimal. Reliability of current design codes in predicting the column ultimate strength was investigated and a new approach based on effective stress method was proposed. Finally, a 3D non-linear numerical model was also developed to predict the compressive behaviours of the proposed form of columns and a small scale parametric study was conducted.

High-Strength Concrete Containing Recycled Coarse Aggregate Subjected to Elevated Temperatures

High-strength concrete (HSC) containing recycled coarse aggregate (RCA), as a partial replacement of natural coarse aggregate, is tested in this paper to have information on RCA concrete behavior at high temperature. The high-temperature mechanical properties were investigated with reference to a number of parameters, such as initial moisture content, ductility, heating rate, and preloading. Various factors including heating rates, moisture content, and the effects of preloading were investigated. Displacement measurements were obtained with the non-contact optical technique of digital image correlation. A scanning electron microscope (SEM) was used to view the fractured surfaces of the RCA concrete. Concrete specimens containing 100% of natural limestone aggregate or 15 and 30% (by volume) replaced with recycled aggregate were cast and heated up to 550–600°C with different heating rates (1.5, 2.0 and 10.0°C/min). A slight decrease in the compressive strength was observed (at room temperature, i.e. after cooling down to room temperature) as an effect of partially replacing natural aggregate with recycled aggregate. Nearly all HSC containing coarse RCA explosively spalled. The SEM images showed that most of the RCA concrete fractures were located along the interfacial transition zones. It appears, therefore, that increasing the replacement ratio may contribute to reducing the risk of explosive spalling in high-strength concrete.

Performance of high-strength concrete walls exposed to fire

High-strength concrete is becoming very popular around the world due to its many advantages over normal-strength concrete. There are significant behavioural differences between high-strength concrete and normal-strength concrete, most notably the brittleness and sudden spalling under elevated temperatures, whereby pieces of hardened concrete explosively dislodge. Although all high-rise and even many medium-rise buildings have high-strength concrete walls, the spalling of high-strength concrete walls in fire has generally been ignored by the designers and the fire resistance of walls has been calculated using the rules specified for normal-strength concrete. Catastrophic failures could occur due to this ignorance of an important issue. Major design codes including the American and Australian Codes do not cover spalling adequately. Even the Eurocode rules are based on limited research. After a brief discussion on the present design practice, this article presents a summary of spalling research. The relevant results from a comprehensive study conducted at the University of Melbourne are briefly discussed. The authors are not aware of any other comprehensive research projects covering the fire behaviour of normal-strength concrete and high-strength concrete walls exposed not only to standard fires but also hydrocarbon fires. The results showed that spalling in high-strength concrete is more significant when subjected to hydrocarbon fire compared to normal-strength concrete. The level of compressive load on the panels was also found to have a significant effect on the fire performance of the high-strength concrete panels. The finite analysis element program, ANSYS, was used to model the concrete walls subjected to load and fire (both ISO834 Standard fire and hydrocarbon fire). The test results were used to validate the computer model.

High Strength Polypropylene Fibre Reinforcement Concrete at High Temperature

Concrete is an inherently brittle material with a relatively low tensile strength compared to compressive strength. Reinforcement with randomly distributed short fibres presents an effective approach to the stabilization of the crack and improving the ductility and tensile strength of concrete. A variety of fibre types, including steel, synthetics, and natural fibres, have been applied to concrete. Polypropylene (PP) fibre reinforcement is considered to be an effective method for improving the shrinkage cracking characteristics, toughness, and impact resistance of concrete materials. Also, the use of PP fibre has been recommended by all of the researchers to reduce and eliminate the risk of the explosive spalling in high strength concrete at elevated temperatures. In this study, constitutive relationships are developed for normal and high-strength PP fibre reinforcement concrete (PPFRC) subjected to high temperatures to provide efficient modelling and specify the fire-performance criteria for concrete structures. They are developed for unconfined PPFRC specimens that include compressive and tensile strengths, elastic modulus, modulus of rupture, strain at peak stress as well as compressive stress–strain relationships at elevated temperatures. The proposed relationships at elevated temperature are compared with experimental results. These results are used to establish more accurate and general compressive stress–strain relationships prediction. Further experimental results for tension and the other main parameters at elevated temperature are needed in order to establish well-founded models and to improve the proposed constitutive relationships, which are general, rational, and fit well with the experimental results.

고온에 노출된 고강도콘크리트의 열적특성에 관한 실험적 연구

화재 시 고온에 고강도콘크리트가 노출되었을 경우에는 심각한 성능저하 및 손상이 발생할 가능성이 매우 높다. 화재 시 고강도 콘크리트는 수밀성으로 인해 폭렬 현상이 더욱 심하게 발생할 것으로 판단되며, 열전도율은 물질의 종류, 온도구배 및 형상에 크게 의존한다. 또한 콘크리트 내부에서 물리 화학적 반응이 중요한 역할을 할 것으로 판단된다. 본 연구에서는 고온에 노출된 고강도콘크리트의 열적특성을 검토하기 위하여 <TEX>$200{\sim}800^{\circ}C$</TEX>의 측정온도에서 열선법에 의해 열전도율(W/mK), 열팽창율(<TEX>$m^2/s$</TEX>), 비열(J/gK)을 측정하였고, 포로시메타를 이용 누적세공량을 측정하여 고강도콘크리트의 내화성능확보를 위한 기초 자료를 제시하였다. There is a strong possibility that high strength concrete(HSC) has a serious chance of decreasing efficiency or loss of dfficiency in case of exposure to high temperature. It is thought that in fire, the Spalling of High Strength Concrete will more seriously occur because of water tightness. Generally, heat conductivity depend on transition between a object and heat, so a physical and chemical reaction inside concrete has an important role. In this experiment, we measure heat conductivity(W/mK), thermal diffusivity(<TEX>$m^2/s$</TEX>), and specific heat(J/gK) in points at a range of from <TEX>$200^{\circ}C$</TEX> to <TEX>$800^{\circ}C$</TEX> to review the thermal characteristics of high strength concrete exposed to high temperature. In results, fundamental data, which has experimentally examined and is to achieve fire resistive performance of HSC, is provided.

The spalling mechanism of high-strength concrete under fire

The spalling properties of concrete were investigated in relation to surface moisture content. Changes in the microstructure of concrete, moisture transfer and internal pressure were also investigated from a microscopic aspect. Based on the results of the investigation, experimental research was conducted on the effects of heat and moisture transfer on spalling as well as the causes of vapour pressure (internal stress) surges and spalling. Spalling of high-strength concrete was found to be significantly affected by moisture transfer in the surface region of concrete and moisture transfer was found to be closely related to the temperature profiles resulting from the characteristics of pore structures, which are the main passages for moisture. It was also found that, in the high-strength (80 MPa) concrete used in this study, the probability of spalling and scale of damage could be reduced according to dryness in the surface region (0–30 mm depth).

Effect of Types and Contents of Polymer Resin on Spalling Prevention of High-Strength Concrete Subjected to Fire

In this study, the fundamental and spalling properties of high-strength concrete were examined, especially when various types and varying content of polymer resin were added. Two types of polymers were used in this study: ethylene vinyl acetate copolymer (EVA-P) and polyvinyl acetate copolymer (PVA-P) as powders and polyvinyl acetate copolymer (PVA-F) and polypropylene copolymer (PP-F) as fibers. Test results showed that the addition of EVA-P and PVA-F to concrete slightly decreased flowability, whereas the addition of PP-F and PVA-P enhanced the viscosity, leading to a remarkable reduction in flowability. The air content of concrete containing EVA-P, PVA-F, and PP-F showed no significant variation. The addition of PVA-P to concrete also caused a slight reduction in compressive strength, whereas the other additives had insignificant effects. After a fire test, the control concrete and concretes with EVA-P, PVA-P, and PVA-F exhibited severe explosive spalling regardless of the dosages. This was because the polymer does not provide sufficient void networks, which is important for vapor evacuation, which enables the release of steam pressure inside the concrete. However, when more than 0.10% of PP-F was added, spalling was effectively prevented. For the residual compressive strength, higher polymer dosage in the concrete produced better results regardless of the polymer type. The powder-type polymers did not contribute to preventing spalling in concrete subjected to fire. This is due to their geometric shape and high melting point. It is concluded that a high aspect ratio and low melting point is critical during polymer selection to prevent spalling in high-strength concrete.

Fire Induced Spalling in High Strength Concrete Beams

A macroscopic finite element model is extended to account for fire induced spalling in high strength concrete (HSC) beams. The model is based on the principles of mechanics and thermodynamics and utilizes pore pressure calculations to predict fire induced spalling in concrete. For validating the model, spalling measurements were made by conducting fire resistance experiments on four normal strength and high strength concrete beams. Spalling predictions from the model are compared with the measured values of spalling at various stages of fire exposure. The validated model is applied to investigate the influence of fire scenario, concrete strength (permeability) and axial restraint on the fire induced spalling and fire response of RC beams. Results from the analysis show that fire scenario, and concrete permeability largely influence the extent of fire induced spalling in concrete beams. Further, it is also shown that the extent of spalling has significant influence on the fire resistance of RC beams.

Study on the Optimum Mixing Condition for Control of Spalling in High Strength Concrete by a Orthogonal Array

This study proceeded to find the optimum mixing rate of a high strength concrete with 80MPa of compressive strength using meta-kaolin, waste tire chips, polypropylene, and steel fiber together for improvement of fire resistance performance. To estimate of optimum mixing rate, the orthogonal array method was used to plan the factors (slump flow, air content, compressive and flexural strength, steel bar temperature and concrete spalling volume) of high strength concretes and experiment was executed. Then the functional relationship of each factor was valuated by general linear model of ANOVA and functional relation between the factors and the data was estimated using response surface analysis. The optimum mixing rate was computed as 80% replacement of silica fume volume by meta-kaolin, 3% replacement of fine aggregates volume by waste tire chips and 0.2% additive rate of polypropylene fiber without steel fiber to the total mixed volume with this modeling of the functional relationship.

Pore pressure and explosive spalling in concrete

Results of an experimental study, which measured thermally-induced pore pressure and corresponding concrete temperatures in high-strength concrete (HSC) and normal strength concrete (NSC), to quantify the effects of factors influencing pore pressure buildup and potential for explosive spalling in HSC and NSC are presented. The specimens are 100 × 200 × 200 mm concrete blocks, heated to a maximum temperature of 600°C (1,112°F) at 5°C/min (41°F/min) and 25°C/min (77°F/min). The complex heat-induced moisture transport process, which varied with specific levels of concrete temperature and significantly influenced the developments of pore pressure and concrete temperature, is explained. Pore pressure developments are shown to be directly related to the moisture transport process and have a significant influence on occurrence of explosive spalling. Effects of water-to-cementitious materials ratios (w/cm), curing conditions, heating rates, and polypropylene (PP) fibers on pore pressure buildup and explosive spalling are quantified and described.

Spalling of High Strength Concrete at Elevated Temperatures

Spalling of High Strength Concrete at Elevated Temperatures