1,721,017 research outputs found
Pulse-Echo Monitoring of Concrete Damage and Spalling during Fire
Full text linkMonitoring concrete damage and spalling progression in structural members during fire tests (hot conditions) is a central but challenging task, since the high temperatures involved make difficult the implementation of most of the common Non-
Destructive evaluation methods. Hence, an advanced ultrasonic technique – Ultrasonic Pulse-Echo (UPE) – was recently adapted for real time survey in fire test, in order to evaluate the material damage during heating. The UPE technique was implemented at the cold (upper) face of concrete slabs (800x800x100 mm) heated at the bottom face according to the Standard Fire and subjected to biaxial compressive membrane loading. Different concretes were tested, with grades ranging from 40 to 60 MPa, with and without different kinds of fibre (monofilament or fibrillated polypropylene, or steel fibres). Furthermore, different load levels were applied, from 0 to 25% of the original compressive strength. During tests, spalling was generally observed in loaded plain concrete (up to 50-60 mm depth), while only slight scaling was experienced on
unloaded samples or if polypropylene fibre was added. The method proved to be very effective in recognizing the decay of the Ultrasonic Pulse Velocity (UPV) with temperature and the role played by external loading and fibre type
Experimental Methods for Spalling Monitoring During and After a Fire
Full text linkMonitoring the progress of spalling and moisture front in concrete elements subjected to heating is a challenging task, since most of the available techniques can be hardly implemented in the test furnace. This is a critical issue, because the measurement of temperature and pressure can be not sufficient to define the conditions for spalling if depth and area involved, as well as occurrence time are not known. Generally speaking, monitoring can be performed by means of Real-Time or Post-Event survey. As concerns the former approach, promising results are expected to come from ultrasonic Pulse-Echo and Ground-Penetrating Radar methods, both based on the measurement of the time delay of (ultrasonic and electromagnetic, respectively) echoes reflected by the specimen side exposed to fire. Other methods for Real-Time monitoring can be Digital Image Processing of pictures taken during heating, and Acoustic Emission. As regards spalling survey after fire, laser profilometry, optical size measurement and weighing of the collected splinters can be co-ordinately used to depict some statistical trends of the fracture process due to fire. These methods (except Acoustic Emission) have been – or will be – implemented in the case of fire tests performed on concrete slabs subjected to heating at the bottom face, and the results are discussed in the present paper
Calcestruzzi Leggeri e Pesanti ad Alta Temperatura
No full textRecenti prove condotte al Politecnico di Milano sulla caratterizzazione termo-meccanica di calcestruzzi speciali hanno riguardato due mescole, l’una con aggregato a base di polistirene espanso sinterizzato – EPS e l’altra con aggregato baritico, maturata quest’ultima in due condizioni ambientali differenti. L’obiettivo è stato quello di rendere disponibili dati nuovi o aggiornati su due famiglie estreme di calcestruzzo strutturale (fc = 26-35 MPa). Una quarta mescola con aggregato ordinario è stata poi studiata per confronto. Circa 100 cilindri riscaldati a diverse temperature di riferimento fino a 700-750°C hanno permesso di valutare le resistenze in compressione ed in trazione indiretta per spacco, il modulo elastico, e la diffusività termica. L’introduzione di due indici di danno, basati rispettivamente sui decadimenti del modulo elastico e della velocità delle onde ultrasoniche, ha poi permesso di mettere in luce l’effetto che la temperatura ha sulla deviazione dalla linearità nei conglomerati studiati, il cui decadimento meccanico è risultato in linea con quello dei calcestruzzi ordinari. Tuttavia, il calcestruzzo baritico conserva più a lungo il proprio comportamento lineare, mentre il calcestruzzo con EPS richiede più cemento per garantire proprietà meccaniche accettabili
Bond shear modulus in reinforced concrete at high temperature: General trends from test results
No full textThe scanty attention devoted so far to bond shear modulus (secant or tangent slope of the loading branch of the bond-slip curve) limits the ability to numerically model such phenomena as tension stiffening, that controls the stiffness of cracked RC structures even past a fire. To improve the knowledge of bond shear modulus at high temperature, the bond stress-bar slip curves resulting from the tests of eleven selected experimental campaigns spanning a forty-year period are re-examined in this paper. Bond shear modulus is derived as the initial slope of the bond stress-bar slip curves of stressed or unstressed specimens in either hot or residual conditions. The criteria to compare the test results coming from very different experimental campaigns are discussed at length. Bond shear modulus is shown to be a decreasing function of the residual compressive strength of the concrete (at high temperature or past cooling). The envelopes of the test data allow to assess the roles of bar diameter and concrete grade. Trend curves are derived for normal-strength and high-strength/high-performance concretes, as a first necessary step for the formulation of parametric design-oriented laws, that may be useful to model tension stiffening and to describe the distribution of the bond stresses in long anchored bars. A numerical example is also developed and the consistency with EuroCode EC2 is checked
Spalling sensitivity test on concrete
No full textConcrete sensitivity to spalling in fire is still a critical issue, as no reliable predictive model is currently available. Hence, so far, experimental testing is the most effective means of investigation. This is the reason why an experimental setup has been designed (and discussed in the Rilem TC 256 SPF) by the authors, based on 800x800 mm concrete slabs installed in a steel frame, aimed at applying a biaxial membrane compression. Load and slab thickness can be adjusted in order to simulate the actual service conditions of concrete elements such as tunnel lining segments. The loading system is placed on a horizontal furnace pow-ered by a propane burner fitted with an automatic control system, allowing to follow the prescribed heating curve. This setup allows comparing different concrete mixes as regards their sensitivity to spalling in realistic service conditions and can be of considerable help in initial material testing for strategic infrastructures such as tunnels
Spalling Test on Concrete Slabs Under Biaxial Membrane Loading
Full text linkConcrete spalling is a rather complex phenomenon ensuing from the interaction of different aspects – often hard to be monitored – namely, temperature, pressure in the pores and stress. It is, in fact, commonly agreed that spalling is triggered by the mutual influence of hygro-thermal and thermo-mechanical processes. Aimed at investigating these two critical issues, an ad hoc test setup was developed at the Politecnico di Milano, based on in plane-loaded slabs. Concrete specimens of dimensions 800x800x100 mm were subjected to the Standard Fire at the intrados, while a constant biaxial compressive load was applied. Pore pressure and temperature at 6 different depths, as well as the flexural behaviour, were continuously monitored during the test. Taking advantage of this facility, an experimental campaign was carried out on one High-Performance Concrete (fc ≥ 60 MPa with silico-calcareous aggregate), without or with one among 3 different fibre types (steel fibre, monofilament or fibrillated polypropylene fibre). So far, tests on concrete without and with polypropylene fibre were carried out. Explosive spalling was observed in plain concrete only, with a remarkably homogeneous spalled layer. In all cases, the mechanical response was characterized by sagging deflection due to thermal strain followed by hogging deformation due to creep and plastic strain
Bending and shear behavior in one-way dapped-end reinforced concrete slabs
No full textSix one-way reinforced concrete slabs, simply supported along the short sides by means of corbels (dapped ends), were recently tested for bending and shear behavior in Milan under two different crosswise load distributions and with three partially different reinforcement layouts in the supporting corbels and in the main body (size 2200 x 1300 mm [7.22 x 4.27 ft]; thickness/length ratio close to 1/14; corbel depth and overhang/length ratio close to 1/23). The tests in bending under the service loads and in shear up to and beyond the peak load show that load crosswise-distribution plays a minor role. In shear, the quite complex crack patterns in the D-regions close to the dapped ends clearly indicate the formation of very effective strut-and-tie systems if the bottom bars of the main body are bent up, and of shallow arch-and-tie systems if the same bars are straight. In the former case, a proper introduction of the bond along the tension bars of the corbels is a must to define the position of bond-related joints and to make strut-and-tie models more reliable in predicting the bearing capacity, while in the latter case, the design equations provided by the codes for constant-section shear-unreinforced beams (ACI 318, EC2, and fib Model Code 2010) prove to be adequate also in the case of corbels
Effect of Biaxial Mechanical Loading and Cement Type on the Fire Spalling Behaviour of Concrete
No full textFire spalling of concrete is a complex phenomenon, which might occur due to pressure build-up in the pores, thermal- and load-induced stresses. In this context, eight mid-size ordinary concrete slabs (4of B40-II and 4 of B40-III concrete: fc28 ≈
40 MPa) were heated at the bottom face according to Standard Fire curve (ISO 834-1), while a constant biaxial compressive load was applied. Four different levels of biaxial mechanical loading have been investigated on both concretes. The test results showed that the loaded specimens are more prone to spalling than unloaded specimens, with increasing amount of spalling for higher values of applied load. Concrete made with CEM III cement (B40-III: 43 % of slag) exhibited less spalling than CEM II cement concrete (B40-II: 3 % of slag)
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