The aim of this work is to describe the strength of Ultra High Performance Concrete (UHPC) under triaxial compression. The main goal is to find a trend in the triaxial compressive strength development under various values of confinement pressure. The importance of triaxial tests lies in the spatial loading of the sample, which simulates the real loading of the material in the structure better than conventional uniaxial strength tests. In addition, the authors describe a formulation process for UHPC that has been developed without using heat treatment, pressure or a special mixer. Only ordinary materials available commercially in the Czech Republic were utilized throughout the material design process

Beckmann, Birgit

Sovják, Radoslav

Vogel, Filip

English

Radoslav Sovják

Filip Vogel

Birgit Beckmann

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Acta Polytechnica

TRIAXIAL COMPRESSIVE STRENGTH OF ULTRA HIGH PERFORMANCE CONCRETE

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/AP.2013.53.0901Acta Polytechnica 53(6):901–905, 2013 © Czech Technical University in Prague, 2013available online at http://ojs.cvut.cz/ojs/index.php/apTRIAXIAL COMPRESSIVE STRENGTH OF ULTRA HIGHPERFORMANCE CONCRETERadoslav Sovjáka,∗, Filip Vogela, Birgit Beckmannba Czech Technical University in Prague, Faculty of Civil Engineering, Experimental Centre, Thákurova 7,16629 Prague, Czech Republicb Dresden University of Technology, Faculty of Civil Engineering, Institute of Concrete Structures,George-Bähr-Straße 1, D-01062 Dresden, Germany∗ corresponding author: sovjak@fsv.cvut.czAbstract. The aim of this work is to describe the strength of Ultra High Performance Concrete(UHPC) under triaxial compression. The main goal is to find a trend in the triaxial compressivestrength development under various values of confinement pressure. The importance of triaxial tests liesin the spatial loading of the sample, which simulates the real loading of the material in the structurebetter than conventional uniaxial strength tests. In addition, the authors describe a formulation processfor UHPC that has been developed without using heat treatment, pressure or a special mixer. Onlyordinary materials available commercially in the Czech Republic were utilized throughout the materialdesign process.Keywords: UHPC; triaxial strength; material design.1. IntroductionUltra High Performance Concrete (UHPC) can becharacterized as a composite material with a highcement and silica fume content, a low water-binderratio and absence of coarse aggregate, i.e. aggregatelarger than 4mm [1, 2]. It has outstanding materialcharacteristics such as self-consolidating workability,very high mechanical properties and low permeability,which results in excellent environmental resistance[3–5]. Typical strengths are 100 to 200 MPa in uniax-ial compression and 6 to 15MPa in uniaxial tension[6]. Moreover, these materials exhibit strain harden-ing under tension [7, 8] and high energy absorptioncapacity [6, 9–12].Structural engineers have long recognized the im-portance of concrete behaviour under multiaxial stressstates. Several researchers have studied the behaviourof Normal Strength Concrete (NSC) [13–15] or HighPerformance Concrete (HPC) [16–20] under triaxialcompression. It has been widely established that thecompressive strength increases when confinement pres-sure is applied to the sample. In addition, it has beenspecified for NSC that the strength increments arelinearly adequate to the lateral stress increments [13].For HPC, the gradient of the strength increments isnot constant but tends to decrease as the confinementpressure increases. Using the least square method tocover this trend, polynomial regression is applied bysome researchers [21]. However, power law regressionsare mostly applied for HPC [13, 18].This study focuses on an evaluation of the triaxialcompressive strength of UHPC developed in the CzechRepublic from local materials in order to verify itsbehaviour with respect to available literature sources.The results provided in this study can serve as valuableinformation for verifying material models, and alsofor design purposes.2. Material2.1. UHPC DesignIn the first phase of the research, several concretemixtures were produced to find the best combina-tion of constituents with respect to maximal com-pressive strength and workability. The first mixturewas designed following the proportions of cement :silica fume : glass powder recommended by Wille etal. [1] as 1 : 0.25 : 0.25, with a water-to-binder ratio of0.2. Subsequent changes in the most important param-eters, e.g. water content, silica fume and high-rangewater reducers, led to an optimized cementitious ma-trix in terms of compressive strength and workability.From the 24 tested mixtures [2], the best performingcementitious matrix composition denoted as UHPCis shown in Figure 1. Figure 1 also shows the ba-sic material properties of the selected mixtures. Inthe “average spread” row, a diameter is shown of thepaste spread measured after filling and removing thestandard cone and impacting the table 15 times. Theflexural strength was evaluated on 40× 40× 160mmprisms, and the compressive strength was evaluated onthe halves of these prisms, following CSN EN 1015-11.It is well established that the addition of short steelfibres increases the mechanical properties of plainUHPC mixtures [2, 6]. The shear action of the fibreshelps to destroy any remaining agglomerates in themixture, thus improving the workability and conse-quently the mechanical properties. However, UHPCwithout fibres was used for the triaxial compressive901R. Sovják, F. Vogel, B. Beckmann Acta PolytechnicaComponent [kg/m3]Cement CEM I 52,5R 800Silica fume 200Glass powder 200Water 176HRWR: Sika SVC 20 Gold 24.2HRWR: Sika ViscoCrete 20He 14.8Fine sand 0.1/0.6mm 336Fine sand 0.3/0.8mm 800Water/binder ratio 0.176Average spread 150mmAverage compressive strength 141.9MPaAverage flexural strength 22.1MPaTable 1. UHPC composition.strength tests, as it has been found that fibres haveno effect on triaxial compressive strength [19].2.2. Mixing ProcedureDuring the mixing of UHPC, it is very important toachieve good workability, particle distribution andpacking density. In comparison with NSC, UHPCcontains more constituents and finer particles. Sev-eral researchers have recommended mixing all finedry particles first before adding water and high-rangewater reducers. This is because the small particlestend to agglomerate, and it is easier to break thesechunks when the particles are dry. The specific mix-ing procedure was as follows: In the first step, bothtypes of aggregate and silica fume were mixed for fiveminutes. In the second step, cement and glass powderwere mixed for another five minutes. At the end ofthe procedure, water and high-range water reducerswere added. The water and the high-range water re-ducers were added gradually. The mixture becamefully workable after another 5 more minutes.3. Testing3.1. Basic Mechanical PropertiesThe compressive strength and the secant modulus ofelasticity were measured on cylinders 100mm in diam-eter and 200mm in height. Because the strength of thebest available capping material (100MPa) was signifi-cantly lower than the expected measured strengths,the tops of the cylinders were cut off and ground.The compressive strength was measured on the cylin-ders, and additionally on the cubes, by monotonicincrements of the load with an average speed of36MPa/min up to a level of 70% of the expectedcompressive strength. At this point, the loadingwas switched to deformation control with a speedUniaxial compressive strength 123/148MPaSecant modulus of elasticity 41.1GPaPoisson ratio 0.17Modulus of rupture 9.9MPaDirect tensile strength 6.6MPaTable 2. Average mechanical properties of the UHPC.of 0.48mm/min in order to keep the test stable whenfailure occurs.The modulus of elasticity was measured using twoextensometers with a 100mm base, attached to thesides of the cylinder specimen. A DSM2500-100 hy-draulic loading machine was used, and the loadingprocedure was stress controlled. In the first step, thespecimens were loaded to 1/3 of the expected maximalcompressive strength — in this case 40MPa — for60 seconds. Afterwards the specimens were unloadedto 4MPa. This procedure was repeated three times.The secant modulus of elasticity was calculated fromthe third loading and unloading branch.The Poisson ratio, representing the ratio of trans-verse to axial strain, was determined on the cylinders,using a pair of strain gauges glued to the perimeter ofthe cylinder in the middle of its height. The Poissonratio was determined on the same samples using thesame loading procedure as for the secant modulus ofelasticity. Thus, the Poisson ratio was determined asan average value up to the stress level correspondingto 1/3 of the expected uniaxial compressive strength.The modulus of rupture was determined on prisms100 × 100 × 400mm. The clear span was 300mm.A three-point bending configuration was applied andthe maximal force was measured. The loading wascontrolled by the deformation at all times. The loadingspeed was 0.2mm/min.Direct tensile tests were carried out on dog-boneshaped specimens without a notch. The specimenswere 330mm in length, and the cross-section of thenarrowed part was 30 × 30mm. The direct tensiletests were performed on an MTS loading machine.The specimens were mounted into specially developedgrips. The loading speed was 0.1mm/min.3.2. Strength in Triaxial CompressionThe triaxial compressive strength was determined si-multaneously on the cylinder and cubes. The cylinderswere 200mm in height and 100mm in diameter; thesides of the cube were 100mm long. The cylinderswere tested in a triaxial chamber, where the confine-ment pressure was provided by mineral oil. A water-proof coating was provided for all cylinders in order toavoid the ingress of mineral oil into the UHPC struc-ture. It was verified experimentally that this kind ofcoating has no influence on the uniaxial compressivestrength. At first, the cylinders were prestressed by902vol. 53 no. 6/2013 Triaxial Compressive Strength of Ultra High Performance ConcreteFigure 1. Testing procedure for UHPC cylinders and cubes.Figure 2. Triaxial hydraulic loading machine.the crossbeam of the hydraulic loading machine. Af-terwards, the chamber with the UHPC cylinder wasflooded by the mineral oil, which was subsequentlypressurised to the prescribed confinement pressure(Figure 1).The cubes were tested in a triaxial hydraulic loadingmachine, in which each side of the cube was pushedby the loading plate with dimensions 95×95mm (Fig-ure 2). The cubes were placed in the hydraulic loadingmachine, where all the loading plates developed thecompressive stress simultaneously until the prescribedconfinement pressure. At this point, two directionswere fixed to the prescribed stress, while the stress inthe third direction continued on until failure of theUHPC cube (Figure 1). A further detailed descriptionof the testing device and the testing procedure can befound in the work of Hampel et al. [22]. In both cases,i.e. cylinders and cubes, the loading was controlled bythe increments of the deformation. The loading speedwas 0.48mm/min.SampleConfinementpressure Cylinder Cube[MPa] [MPa] [MPa]0 123 14810 178 –15 – 23120 209 –30 231 28060 – 36290 – 432Table 3. Triaxial compressive strength of the UHPC.4. Results and Discussion4.1. Basic Mechanical PropertiesTable 2 shows the compressive strength, the secantmodulus of elasticity, the Poisson ratio, the modu-lus of rupture, and the direct tensile strength of theUHPC mixture. The values presented in the table areaverages from three samples.4.2. Strength in Triaxial CompressionThe triaxial compressive strength was described usingvarious samples and loading procedures. The triaxialcompressive strength was determined under confine-ment pressure of 10, 20, 30MPa for the cylinders and15, 30, 60, 90MPa for the cubes (Figure 3). The tri-axial compressive strength determined under elevatedconfinement pressure was standardised by the uniax-ial compressive strength in order to obtain the firstevaluating parameter. The ratio of the confinementpressure and the uniaxial compressive strength wasused as a second parameter in order to describe the903R. Sovják, F. Vogel, B. Beckmann Acta PolytechnicaFigure 3. Development of triaxial compressive strength.strength of the UHPC in triaxial compression. As theconfinement pressure increased, the UHPC compres-sive strength also increased. NSC tends to follow alinear trend up to the level where the confinement pres-sure is equal to the uniaxial compressive strength. Atthis level, the triaxial compressive strength is roughlyfive times the uniaxial compressive strength. The bestreliability, using the least square method, was achievedby fitting the development of UHPC triaxial strengthby the power law function. The triaxial compressivestrength derived from the power law regressions, estab-lished for the UHPC used in this study, were uniaxialcompressive strength 3.1 for the cylinders and 3.7 forthe cubes (Figure 3).5. ConclusionsA UHPC mixture was developed in this study to findthe best-performing combination of constituents withrespect to workability and strength. As fibres play norole in triaxial compressive strength, a UHPC mixturewithout fibres was subjected to triaxial compressionusing both cylinders and cubes made of plain UHPC.It was verified experimentally that the development ofthe UHPC triaxial strength on confinement pressurecan be fitted by the power form regression. Using es-tablished regression and confinement pressure equal tothe uniaxial compressive strengths, it was determinedthat the triaxial compressive strength is 381MPa forcylinders and 548MPa for cubes. Therefore, the fail-ure surface of UHPC falls below the failure surface ofNSC, especially at higher confining pressures.AcknowledgementsThe authors gratefully acknowledge the support providedby the Czech Science Foundation under project num-ber GAP 105/12/G059. The authors would also liketo acknowledge the support provided by the Otto-Mohr-Laboratory at TU Dresden for the triaxial compressiontest, and also the assistance given by the technical staff ofthe Experimental Centre, CTU in Prague and by studentswho participated in the project.References[1] Wille, K., Naaman, A. E., Parra-Montesinos, G. J.:Ultra-high performance concrete with compressivestrength exceeding 150MPa (22 ksi): a simpler way, ACIMater. 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TRIAXIAL COMPRESSIVE STRENGTH OF ULTRA HIGH PERFORMANCE CONCRETE

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