Normal Strength Concrete Columns Research Articles

A comparative study between ultra-high-performance concrete structures and normal strength concrete structures exposed to fire

The use of ultra-high-performance concrete (UHPC) in civil infrastructure is increasing due to its numerous advantages compared to normal strength concrete (NSC). In this research, thermal analysis of NSC and UHPC beams and columns was developed to understand and compare the fire performance of these structures. A numerical analysis was performed using finite element (FE) modeling with ABAQUS software. Compared to NSC, UHPC structures in some cases have lower fire resistance due to their slender cross-sections and unfavorable thermal properties and mechanical capacity degradation due to temperature effect. The results show that UHPC structures experience significantly higher cross-sectional temperatures when exposed to fire compared to NSC. The results are particularly relevant for high-rise buildings, where fire safety requirements are more stringent and the use of UHPC is attractive, leading to an unprecedented contradiction. Alternative strategies for the application of UHPC, such as the contradictory increases in cross-section or the use of fire protection coatings, must be considered. In some cases, NSC may be more attractive as a structural material than UHPC if structural fire design requirements are taken into account.

Fire performance of aged and corroded reinforced concrete columns

The existing guidelines for determining the fire ratings of reinforced concrete (RC) columns only take into account undamaged components, without considering any deterioration caused by age or corrosion. An experimental study was conducted to examine the effect of corrosion on the fire performance of RC columns. In this study, six full-scale RC column specimens were cast, corroded, and tested in fire conditions, consisting of two made of normal strength concrete (NSC) and four made of high-strength concrete (HSC). Out of these six, three RC columns (1-NSC and 2-HSC) were corroded using a specifically designed accelerated corrosion setup for a target mass loss of 30 %. After the completion of the accelerated corrosion exposure time, these three corroded column specimens, along with three other non-corroded control column specimens, were tested in a fire furnace, simulating the ISO-834 fire curve. It was found that due to corrosion exposure, the steel reinforcement underwent significant mass loss, and severe corrosion cracks were observed. These cracks were discovered to serve as convenient channels for heat transfer. This facilitated the earlier and more severe spalling of the concrete cover, ultimately leading to premature deterioration in the strength of the corroded RC columns during the fire. Consequently, this contributed to the catastrophic failure of the corroded RC columns, with a significant reduction in fire ratings- 44 % for NSC columns and 40.49 % for HSC columns. This paper also addresses the spalling characteristics of the corroded columns and their structural responses, including axial deformations, lateral deformations, and load variations.

Testing the effect of UHPFRC jacket thickness and steel fiber volume fraction on NSC‐UHPFRC column compressive behavior

AbstractSeveral efficient applications of ultrahigh‐performance fiber‐reinforced concrete (UHPFRC). Among them are UHPFRC jackets utilized for strengthening or permanent formworks of the steel reinforcing normal strength concrete (NSC) columns since UHPFRC jackets could enhance the compressive strength and other properties of the NSC column. However, the efficiency of a UHPFRC jacket simultaneously depends on its thickness and the added steel fiber volume fraction. The paper aims to explore the interaction effect of UHPFRC jacket thickness and steel fiber volume fraction on the behavior of UHPFRC jackets confining the NSC core under compressive load. The circular ultrahigh performance concrete jacket filled inside with the normal strength concrete, named NSC‐UHPFRC column specimen, is tested under loading on the NSC core. UHPFRC jacket thickness of 18‐49 mm with three adding steel fiber volume fractions was conducted for the experimental study. The investigation and analysis of the effect of the UHPFRC jacket and the steel fiber volume fraction on compression behaviors of NSC‐UHPFRC columns proceeded after the compressive strength test results were validated using Richart's model. This comprised a review of various parameters such as specimen failure pattern, stress–strain relationship, and ductile indices. The Weibull distribution was employed to explain the interaction effect on the variation in the experimental results. Finally, an interaction model was proposed and used to discuss the interaction effect of UHPFRC jacket thickness and steel fiber volume fraction and the optimum condition of the compressive strength of the NSC‐UHPFRC columns.

Performances of Concrete Columns with Modular UHPC Permanent Formworks Under Axial Load

This research proposed the modular prefabricated permanent formwork system made of ultra-high-performance concrete (UHPC). Two kinds of modular formwork shapes were designed: the flat formwork and the ribbed. The experimental investigation on the axial compression performance of the composite columns that consist of the normal strength concrete (NSC) core and the modular UHPC permanent formwork was demonstrated. Compared with the flat formwork, the ribbed formwork exhibited better bonding with the NSC core. As observed from the test results, the composite column with the ribbed formwork presented a similar axial behavior as the NSC column with a slight improvement in ultimate loads. Therefore, the modular UHPC ribbed permanent formwork could be regarded as the additional cover to the conventional NSC column. In addition, the finite element analysis (FEA) model was also developed to simulate the composite columns numerically. The predicted capacities agreed with the experimental results, which validated the numerical models. The crack pattern estimated by the FEA model revealed that the interaction between the permanent formwork and the inner concrete introduced many tiny cracks to the concrete core. However, as protected by the UHPC permanent formwork, the overall durability of the composite columns can still be enhanced.

Advanced Study of Columns Confined by Ultra-High-Performance Concrete and Ultra-High-Performance Fiber-Reinforced Concrete Confinements

The need for concrete with ‘super’ strength and ‘super’ ductility for greater sustainability has been answered by the existence of ultra-high-performance concrete (UHPC) and ultra-high-performance fiber-reinforced concrete (UHPFRC). Over the last decades, UHPFRC has been implemented in actual concrete structures, as well as used to retrofit structural elements, including columns. However, the use of UHPC and UHPFRC confinement to strengthen normal concrete columns is still limited. Therefore, this research aims to investigate the advanced performance of columns using UHPC and UHPFRC confinement in the context of the strength and ductility of such columns, such as load capacity, stress–strain behavior, and the crack pattern in the failure mode. This research is an advanced study of several investigations previously carried out by other authors on the characteristics of UHPC and UHPFRC, as well as columns confined by UHPC and UHPFRC. The methods used in this research are experimental and analytical. The experimental results were compared to analytical calculations for validation. This research produced 12 short-column specimens confined by UHPC (CF0 series) and UHPFRC (CF1 and CF2 series) that contained 0%, 1%, and 2% fiber and were also tested for axial loading and various eccentricities as follows: e = 0, 35, and 70 mm. The results found that the normal strength concrete (NSC) columns confined by UHPC and UHPFRC could sustain a higher maximum load and stress, and also sustain greater vertical deformation and strain compared to the control specimens. It was noted that specimen CF2-35 had the highest load capacity, vertical deformation, maximum stress, and maximum vertical strain compared to specimen C-0 (control column with no confinement). The specimen CF2-35 (column confined by UHPC with a 2% fiber volume with an eccentricity of 35 mm) also exhibited a ductile failure mode and very minor cracks. It was also found that 75% of the specimens had 0–39% errors and 25% had 0–13% errors. The research proved that the addition of a volume of 2% fiber to the UHPFRC minimizes the crack of the failure mode and prevents confinement spalling of the column. This research has led to the conclusion that UHPC and UHPFRC confinements will increase the strength and ductility of columns.

Experiments and Stress-Strain Model of Concrete Confined by UHPC Materials

Compressive behaviors of 12 stub normal-strength concrete (NC) columns were experimentally investigated, which were confined by ultra-high performance concrete (UHPC) jackets. The governing influencing factors consisted of the thickness of the UHPC jacket, shape of specimens, and compressive strength of the core concrete. Generally, the UHPC jacket greatly improved the ductility of the stub columns. The final failure pattern transferred from crushing for concrete stub columns to the expansive crack of the UHPC jacket for UHPC-confined stub concrete columns. Moreover, the thickness of the UHPC jacket was found to greatly affect the compressive strength of specimens. In particular, the circular UHPC-confined stub columns, of lower-strength core concrete, were prone to gain more increments in compressive strengths. Debonding of the UHPC-NC interface was observed in the rectangular and square UHPC-confined stub columns. Enlightened from experimental observations, an analytical model is proposed to predict the axial stress-strain behavior of UHPC-confined concrete columns. It is noteworthy that the UHPC-NC interfacial behavior is identified and accounted for. The accuracy of the analytical model is validated by the reported experimental results. The model provides a useful tool to predict the axial strain-stress relation of UHPC-confined concrete columns in closed-form expressions for design-oriented purposes.

Experimental Study on the Behavior of Axially Loaded Reinforced Concrete Square Columns Strengthened with SIFCON Shell

Slurry infiltrated fiber concrete (SIFCON) is an advanced generation of fiber-reinforced concrete (FRC) with higher fiber content. SIFCON offers excellent potential for use in areas that require high ductility and impact resistance, particularly when designing seismic retrofits and repairing or strengthening structural reinforced concrete members. This study investigated the behavior of square normal strength concrete (NSC) columns of grade M35 strengthened with SIFCON shell and compared them with unstrengthen NSC. The effect of different SIFCON shell thicknesses (2 and 3) cm, fiber type and volume fraction (4 and 6%), and tie spacing (8 and 16) cm were studied. The fiber types employed were polypropylene and hooked-end steel fiber. Eleven columns were cast and tested for the current investigation in two groups, where the first group (control specimens) consists of two unstrengthen NSC columns and one square NSC column strengthened with a 2 cm SIFCON shell with a 6% steel fiber ratio. The second group comprises eight NSC columns strengthened with a hybrid fiber SIFCON shell. The NSC square columns had dimensions of (8x8x80) cm. 3 cm SIFCON shell thickness was observed to evolve the strengthen columns' load-carrying capacity and energy absorption. The maximum load achieved is about 223% as compared with unstrengthened NSC columns. The energy absorption was about 16 times that of the control. At the same time, the stiffness of strengthened columns is less than that of NSC columns.

Performance evaluation of normal- and high-strength concrete column specimens reinforced longitudinally with different ratios of GFRP bars

Glass fibre-reinforced polymer (GFRP) bars, as reinforcement in concrete structures, have several desirable properties over conventional steel bars, such as high tensile strength, low density and resistance to corrosion. However, there are still some concerns about using GFRP bars as longitudinal reinforcement in concrete columns primarily due to their low compressive strength and low modulus of elasticity. It is evident that the compressive behaviour of GFRP bars as longitudinal reinforcement in normal strength concrete (NSC) and high strength concrete (HSC) columns has not been extensively investigated to formulate adequate design guidelines. In this study, 12 reinforced concrete column specimens were tested under concentric compression loading. All the tested column specimens were 600 mm in height and 154 mm in diameter. The column specimens were divided into two groups as per the compressive strength of the concrete used in casting the column specimens, which were 45 MPa and 90 MPa. Each group consisted of six column specimens, one plain concrete column specimen and five column specimens reinforced transversely with 6 mm steel helices spaced at 40 mm pitch (transverse reinforcement ratio ρt = 2.2 %). Out of those five column specimens, one had no longitudinal reinforcement (ρ = 0), one was reinforced longitudinally with steel bars (ρ = 2.4) and the rest were reinforced longitudinally with GFRP bars (ρ = 2.7, 3.2, and 4.3 %). From testing results, it was observed that steel and GFRP bar reinforced NSC column specimens exhibited two peak axial loads, with a higher second peak axial load than the first peak axial load, whereas steel and GFRP bar reinforced HSC column specimens exhibited one peak axial load only. It was also observed that the contribution of the longitudinal steel bars was about two times the contribution of GFRP bars in both NSC and HSC column specimens. Finally, the ductility of GFRP bar reinforced NSC column specimens was greater than the steel bar reinforced NSC counterpart, whereas this trend was the opposite in the column specimens cast with HSC.

Compressive behavior of concrete-filled ultra-high performance concrete tube with FRP stirrups

Ultra-high performance concrete (UHPC) has excellent durability, compressive strength and toughness which can be combined with normal strength concrete (NSC) columns to improve the performance. In order to investigate the compressive behavior of concrete-filled UHPC tube, twelve circular composite columns with carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) stirrups were tested. The effects of stirrup spacing and type on the axial behavior and failure mechanism of the composite columns were further analyzed. The research proved that the large-area spalling of the UHPC tube did not occur and the UHPC tube worked well with the internal concrete. Reducing the stirrup spacing and using the CFRP stirrups could significantly improve the ductility of specimens, but it had little influence on the initial stiffness. Then, a three-dimensional meso-scale model considering UHPC as two-phase material which composed of steel fibers and UHPC matrix was established. The simulated errors of the key mechanical performance indexes were almost within 15%, indicating that this finite element model could effectively reflect the compressive behavior of the concrete-filled UHPC tube columns. The influence rules of other variables including internal concrete strength, UHPC tube thickness and stirrup elastic modulus on the initial stiffness and peak load of specimens were also studied based on the validated model. Furthermore, the axial bearing capacity of the concrete-filled UHPC tube was predicted.

Role of cross-diagonal reinforcements in lieu of seismic confining stirrups in the performance enhancement of square RC columns carrying axial load subjected to close-range explosive loading

Exposure of building infrastructures to accidental or intentional blasts is an extreme load condition that may cause irreparable damage leading to the collapse of buildings. Columns being principal elements are the most important for the stability and safety of the buildings under accidental explosions and subversive blast events and therefore attract the attention of structural engineers and researchers. Some recent examples are the Beirut seaport explosion (August 2020), the explosion at an ammunition warehouse in Ryazan City of Russia (October 2020), the gas explosion in China’s Hubei Province (June 2021), a blast at a chemical factory on the outskirts of Bangkok (July 2021), and the explosion on a container ship docked at Dubai’s Jebel Ali Port (July 2021). In the crises like ongoing conflict between Russia and Ukraine, the enhanced response of the principal components of a structure may save the life of the building users by limiting severe damage to the structure. In this study, three experimentally tested 3000-mm-long normal strength concrete columns, 300mm x 300mm, provided with (i) conventional reinforcement, (ii) seismic reinforcements over top and bottom confining regions (600 mm), and (iii) seismic reinforcement over confining and mid-height regions, carrying an axial working load of 950 kN available in the literature, are modeled in the ABAQUS 2020 code and are subjected to 82 kg TNT close-range explosive load at a scaled distance 1.0 m/kg1/3using the software’s explicit module. In addition to this, one column with seismic reinforcement over its entire length has been considered and modeled. The concrete damage plasticity model is explored for nonlinear elastic and inelastic behaviors, degradation of stiffness, and loading rate effect on concrete. Following the validation of the numerical models, the seismic reinforcements of the columns have been replaced by the cross-diagonal reinforcements between the conventional stirrups with the same axial load. Blast performance of the columns with the seismic reinforcements and with replaced diagonal reinforcements is critically examined and discussed. The results show that the application of cross-diagonal reinforcements as a replacement for the seismic reinforcements enhances the blast resistance of the reinforced concrete column significantly by reducing the damage and displacement.

Artificial neural networks approach for prediction of axial loading capacity of circular normal strength concrete columns confined by both transverse steel reinforcement and carbon fiber reinforced polymer wraps

A few empirical models for the axial loading capacity (ALC) of circular normal strength concrete (NSC) columns wrapped by carbon fiber reinforced polymer (CFRP) sheets and interior transverse steel reinforcement (TSR) (CSC columns) are available in the literature. The deficiency of those models is that they were proposed based on a small number of tests by considering limited parameters of CSC columns. Therefore, the main aim of the current investigation is to propose the improved empirical models for the ALC of CSC columns by including the interaction mechanism between TSR and FRP confining behavior. To secure this aim, a general regression analysis technique and artificial neural networks (NNs) on the experimental outcomes of 76 CSC columns collected from the previous investigations were employed. The proposed NN model was adjusted for the different number of hidden layers and neurons to achieve an optimized model. The suggested NN and empirical models portrayed a close agreement with the testing database with R2 = 0.998 and R2 = 0.892, respectively. The NN model reported a higher accuracy than the theoretical model. The comparative investigation solidly authenticated the superiority and accuracy of the anticipated strength models for CSC columns.

Experimental and Numerical Investigation of the Seismic Performance of Bridge Columns with High-Strength Reinforcement and Concrete

Despite the numerous advantages high-strength reinforcement (HSR) and high-strength concrete (HSC) offer over conventional materials, the practical use of these materials for bridge column design in seismic zones has somewhat been limited. This is due to the insufficient research data and guidelines for the seismic design of bridges using HSC and HSR and the lack of a reliable analytical model. Therefore, to address this issue and promote the application of HSR and HSC, this paper investigates high-strength bridge columns’ seismic performance experimentally and numerically. Six large-scale reinforced concrete (RC) bridge columns and one multi-column bent frame were tested under a quasi-static cyclic loading with constant axial compression. The primary design parameters were axial load ratio, longitudinal and transverse reinforcement yield strength, and transverse reinforcement spacing. The failure pattern of high-strength columns was similar to conventional RC columns and satisfied the requirements for seismic design in terms of failure mode, hysteresis behavior, ductility, and energy dissipation capacity. The experimental ductility values of the high-strength columns were satisfactory and capable of meeting the ductility demand of most codes. Furthermore, a numerical model was built using the OpenSees program to predict the seismic performance of the specimens and then verified by comparing them with the test results of 12 columns. The numerical model’s results were in good agreement with the experimental results. The results suggested that numerical modeling techniques commonly used for normal strength concrete (NSC) columns can be used for HSC bridge columns by incorporating a proper material model.

Data-driven analysis on axial strength of GFRP-NSC columns based on practical artificial neural network tool

Previous research is deficient in the theoretical estimation of the axial load-carrying strength (ALCS) of glass fiber reinforced polymer (GFRP) composite columns. This study aims to calculate the ALCS of GFRP normal strength concrete (NSC) columns (GFRP-NSC) using two different approaches; empirical modeling and artificial neural networks (ANNs). The geometric, material, and testing details of 250 GFRP-NSC columns were collected from the previous studies. An initial assessment of the previous strength models for the ALCS of GFRP-NSC columns was accomplished over the constructed database. General regression and curve-fitting were used for suggesting an improved empirical model. The various hidden layers and neurons were calibrated to find an improved ANN model. Both new empirical and ANN models showed higher accuracy than the previous model for capturing the ALCS with R2 = 0.740, and R2 = 0.882, individually. A comparison of the predictions through one-way analysis of variance (ANOVA) at a 5% significance level depicts that the proposed models presented a higher accuracy than the previous models for capturing the ALCS of GFRP-NSC columns. Additionally, a detailed parametric investigation of GFRP-NSC members was carried out using the suggested empirical model to explore the influence of different variables of studied members.

Experimental and analytical investigation of UHPC confined concrete behavior

Ultra-High Performance Concrete (UHPC) is a self-levelling cementitious material with enhanced mechanical and durability properties when compared to normal strength concrete. Accelerated construction of concrete columns can be realized using a precast hollow UHPC shell as permanent formwork. Also, as a retrofit solution, UHPC can be placed around existing concrete columns with deficient confinement details. The continuous confinement provided by UHPC shell to both the concrete core and longitudinal reinforcement could be used to improve the seismic behavior of columns or reduce the amount of confining hoop reinforcement required in traditional column designs. An experimental and analytical study is performed with a main objective of quantifying the confinement effect provided by plain UHPC shell. The experimental investigation compressed of axial testing of 228 mm (9 in.) and 280 mm (11 in.) dia circular columns, 203 mm (8 in.) and 305 mm (12 in.) square columns confined with UHPC shell. Experimental investigation included a total of two normal strength concrete column specimens as control specimens and 11 column specimens confined with 1 in. and 2 in. thick UHPC shell. All the specimens were tested to failure under axial compression and were extensively instrumented. The confinement provided by the unreinforced UHPC shell improved the peak compressive stress and the crushing strain values of the core concrete by 15%–30% and 26%–46% respectively. The effectiveness of plain UHPC shell to provide confinement was found to considerable in circular columns compared square columns. A detailed 3D FEA models were developed in ATENA software and were validated using the experimental data. The FEA models captured observed responses accurately and provided insight into the observed experimental behavior. Based on the experimental and analytical study, it is recommended to include some transverse reinforcement in UHPC shells, especially for non-circular cross-section for improved confinement effect.

Numerical investigation of hybrid UHPC columns subject to lateral impact

This paper presents extensive numerical and parametric studies on the dynamic performance of axially-loaded hybrid ultra-high-performance concrete (UHPC) columns subject to low-velocity impact loading. While existing studies have been performed on columns comprised fully of UHPC, the case of hybrid UHPC columns has not been fully explored. Hybrid columns involve the localised use of UHPC in combination with normal strength concrete (NSC) and can result in both performance and economic efficiencies. The present study uses Abaqus to develop a validated finite element (FE) model which allows exploration of key parameters such as impact location and energy, slenderness ratio and the length of the UHPC zone and their effect on hybrid column behaviour. It was found that localised adoption of UHPC at the predicted impact location significantly enhanced the performance and resulted in a change from brittle shear failure to flexural failure with minimal damage. The length of the UHPC segment was also found to have greater effect on columns with low reinforcement ratio. Meanwhile, the impact location was shown to have significant influence on the column response especially under a low pre-loading level. Hybrid columns experienced less displacement and higher plateau impact load under low impact energy. In comparison to the normal strength concrete column, the hybrid columns exhibited reduced mid-span displacement and increased impact load capacity which confirmed the ability of the UHPC segment to reduce the local damage resulting from the impact. In addition, in many cases, it was found that the application of higher pre-loading levels can provide an improvement in total impact performance of the hybrid columns. This is mainly attributed to the shifting of the pre-loading axis as a result of cracking at the top of the column ends. Finally, the presence of a UHPC segment with various lengths can change the location of the main (central) plastic hinge to the end of the UHPC segment, this reflects the ability of UHPC in improving the entire response of hybrid columns.

Cyclic behavior of high strength lightweight concrete slab-edge column connections with and without openings

In this study, the seismic behavior of nine full-scale slab-edge column connections with and without openings constructed from Normal-Strength Concrete (NSC with fc` = 28 MPa) and High-Strength Lightweight Concrete (HSLWC with fc` = 49 MPa) is investigated experimentally and analytically. The nine specimens included five specimens from NSC and four specimens from HSLWC. The punching strength provisions used by international codes, such as the International Building Code (International Code Council 2021), the American Concrete Institute (ACI Committee 318 2019), the New Zealand Code (New Zealand Standard NZS 3101: Part 1 2006), the Japan Society of Civil Engineering (JSCE 2007), and the Eurocode 2 (Eurocode 2 2008), are investigated when applied to HSLWC slab-edge column connections with openings. The most significant factors to consider are the effect of using HSLWC, the location and size of the openings. The investigation was widened by performing a nonlinear finite element analysis with the computer application ABAQUS. According to the test results, the punching shear strength estimated using the equations of the (International Code Council 2021), (ACI Committee 318 2019), (New Zealand Standard NZS 3101: Part 1 2006), and (Eurocode 2 2008) is safe for HSLWC. For both NSC and HSLWC, the punching shear strength estimated using the JSCE-2007 is either on the borderline or unsafe. The ultimate punching load of HSLWC with fc` = 49 MPa is increased by about 72% than that of NSC with fc` = 28 MPa for slab-edge column connections without openings and by approximately 81% for connections with opening size equals to one-eighth of the column strip width and positioned next to the face of the column. The finite element program ABAQUS can be used successfully to forecast the behavior of normal strength concrete and high strength lightweight concrete slab-edge column connections under seismic loadings, both with and without opening.

Is it reasonable to use high-strength concrete columns under fire conditions?

PurposeThis paper aims to answer two questions. First, are there any differences in the fire performance of columns made of normal and of high-strength concrete? Second, under which circumstances does the fire design govern the cross-sectional dimensions of concrete columns? Is it feasible to replace columns out of normal strength concrete by more slender high-strength concrete columns?Design/methodology/approachThe author conducted numerical studies using the finite element code “Infocad” of the German company “Infograph”. The studies included the effect of different parameters on the fire performance of columns out of normal and high-strength concrete, i.e. the load ratio and eccentricity, boundary conditions and times of fire exposure.FindingsResults from the numerical investigations showed that high-strength concrete columns suffer much more from heating than normal strength concrete columns. This is the outcome of the unfavourable mechanical properties of high-strength concrete at elevated temperatures. Although the relative fire performance of columns out of high-strength concrete is worse than that of columns out of normal strength concrete, initial load reserves are beneficial to achieve even high fire ratings.Originality/valueMany researchers addressed in experimental and numerical studies the fire performance of columns out of normal and high-strength concrete. A special emphasis was often laid on the spalling of fire-exposed high-strength concrete. However, there are no systematic investigations when the fire design governs the cross-sectional dimensions of high-strength concrete columns. Based on a previous comparison of the relative fire performance of columns out of normal and high-strength concrete, this paper, hence, addresses the question whether there is a reasonable lower limit for the use of these columns. This is an important aspect for designers since there is a tendency to replace columns out of normal strength concrete by columns out of high-strength concrete. Higher concrete strengths allow for smaller cross sections of the columns, and designers may, hence, increase the usable space of buildings.

Determination of Axial Capacity for Enhanced Normal and Ultra - High Performance Concrete Columns

Abstract Concrete Technology has been developing for more than a century. One of the most exceptional achievements in concrete technology is the evolving of Ultra-High Performance Concrete (UHPC) which has been a research focus in a wide applications diversity. In this paper, an experimental work has been carried out for investigating the transverse and longitudinal reinforcements’ variation influence on the axial capacity of UHPC columns. Eight columns (five UHPC columns and three Normal Strength Concrete (NSC) columns) have been poured and tested under a concentric axial compression load till a failure is reached. Then, the results are reported herein. The experimental results show that UHPC columns failed in a controlled manner and no concrete chips or a concrete cover spalling are observed. Also, the longitudinal reinforcements have not buckled away beyond the peak load because of the presence of the reinforcing steel fibers in UHPC. Correspondingly, the steel ties spacing proportionally affects the load carrying capacity of columns as presented hereinafter.

Behavior of Eccentrically Loaded Slender High-Strength Concrete Columns.(Dept.C)

The buckling behavior of slender High-Strength Concrete (HSC) columns under axial load with different end eccentricities has been studied experimentally. The results of these tests have been used to examine the applicability of the design requirements of Normal Strength Concrete (NSC) columns of the Eurocode 2 (EC-2), the Egyptian code (ECC-9S) and ACI 318-99 building code, when applied to slender HSC columns with a characteristic compressive strength of 85 MPa. The program included tests of22 columns with slenderness ratios λ1 ranged between 17.3 and 69.3. The main parameters examined in this study were the values of end eccentricity of the applied axial load (e/t=O.3, 0045 and 0.90), the transverse reinforcement ratio (p v) and the longitudinal reinforcement ratio (Pt ). The results of this experimental work showed that, slender columns constructed with HSC increased the ultimate load of the tested columns in comparison with the same columns constructed with NSC. The Model Column method. used by EC·2 and also the Moment Magnifier method used by ACl 318-99 building code for the design of HSC slender columns showed to be conservative for r design of HSC slender columns. The slenderness ratio limit between short and long columns required by the ECC-95 need to be modified when applied for HSC columns. Based on the results or these tests. the second order effect should be taken into account for HSC columns when λ1> 40 (λb> 12 ).

Experimental study on the spalling behaviour of ultra-high strength concrete in fire

High strength construction materials are now attractive owing to their economic and architectural advantages. The higher the material strength, the smaller member size is required. Ultra-high strength concrete (UHSC) encased columns are being developed for the erection of high-rise buildings due to their higher load bearing capacity and smaller cross section size compared to normal strength concrete encased columns. When the UHSC is subject to elevated temperature, explosive fire-induced spalling is more often observed than in normal strength concrete. The consequence of spalling could cause serious life loss and damage to the close key infrastructure. Spalling is mostly due to the UHSC increased density, lower permeability and brittleness. Most of the previous studies show that polypropylene fibres have been found effective in preventing fire spalling. The aim of this experimental study is to discover the minimum polypropylene fibre dosage to control the fire spalling of steel fibre reinforced concrete of 115–135 MPa strength. The experimental study was carried out on 15 concrete specimens with different parameters and two fibre-reinforced concrete encased columns exposed to ISO 834 fire. The study indicates that a polypropylene fibre dosage of 1.365 kg/m3 can prevent the 115–135 MPa ultra-high strength concrete from explosive fire spalling. This polypropylene fibre dosage is lower than that proposed in Eurocode 2, which is 2 kg/m3. The proposed lower polypropylene fibre dosage can potentially bring sustainability (use less polypropylene fibres that are made of crude oil) and economy, as well as improve constructability by improving the workability of fresh concrete. It is also found steel fibres may relieve the fire spalling but not adequate to prevent spalling. Moreover, there is no significant effect of the size and inner temperature of the centre of the concrete specimen on spalling.