Time-dependent response of ECC: Characterisation and modelling of creep and creep fracture.

The phenomenon creep fracture is well-known for concrete. Since 1960 it is known that delayed fracture may occur for concrete structures at sustained loads beyond roughly 80% of the short term peak resistance in compression and flexure. More recently, evidence has been produced of an even lower creep fracture limit exists for concrete, potentially as low as 60% of the peak short term resistance in flexure. Such characterisation and consideration of creep and creep fracture are essential for design guidelines for HPFRC and form the focus of this paper. A comprehensive experimental program has been launched to characterise the time dependent behaviour of a particular HPFRC, including creep and creep fracture, as well as shrinkage. In this paper, results are presented of direct tensile tests, as well as three point bending tests towards establishing the creep and creep fracture behaviour of a particular HPFRC. 1. TIME DEPENDENCE OF CEMENT-BASED COMPOSITES The mechanical behaviour of cement-based material is highly time-dependent and rate-dependent not only for dynamic cases, but also in the quasi-static loading range, where inertia and wave effects are negligible[1]-[5]. Creep is a well-known phenomenon in terms of continued deformation under sustained load. Less well-known is the reduced strength, of which an example is given in Figure 1(a), which shows the dependence of peak resistance of notched concrete beams on the deflection rate in three point bending[3]. An approximate 45% increase in peak resistance was the result of an increase in the deflection rate from 0.05 μm/s (peak load after about 80 minutes) to 50 μm/s (peak load after about 5 s). By the same mechanisms causing such reduction in resistance with loading rate, namely time dependent crack propagation, delayed failure may occur in structures loaded at levels below the “static” peak resistance, as is evident from Figure 1(b) for uniaxial tensile creep experiments[6]. The time to failure varies from a few seconds at a high sustained load level of 90% of the peak load, to several hours at 65-75% of the peak load. Figure 1. (a)Concrete rate-dependent strength increase in quasi-static loading range [3]. (b)Time to failure of concrete elements under creep load in uniaxial tension [6]. The main micromechanical mechanism of the time dependence is moisture migration. For example, it is generally agreed that creep is caused by the de-bonding and re-bonding of micro particles, in which process the moisture flux between microand macro-pores plays an important role [7], [8]. Furthermore, heterogeneities in the microstructure cause stress peaks, which may exceed inter-particular bond and cause micro-cracking. Through processes of softening and viscous stress relaxation, stressredistribution occurs for global equilibrium with the creep load, leading to crack propagation and eventual delayed fracture. It is emphasised that the illustrations of time-dependence in Figure 1 were performed on sealed specimens, or specimens at equilibrium with the surrounding environment. Thereby additional important sources of time-dependence, namely drying (hygral) and thermal shrinkage, as well as the so-called Picket-effect [9] of stress-dependent shrinkage, or drying creep, have been excluded. In fibre-reinforced cement-based composites (FRCC) the same mechanisms act, while more are introduced in terms of fibres and the fibre-matrix interaction. Experimental studies on FRC report conflicting findings on the effect of the fibres on creep and shrinkage. It has been reported [10], [11] that steel fibres are effective in reducing creep in concrete. However, in contrast [12], [13] reported that the creep strains of concrete specimens reinforced with small amounts of steel and polypropylene fibres are consistently higher than that of specimens without fibres. This trend was recently confirmed in pilot studies of shrinkage and compressive creep [14] of Engineered Cement-based Composites (ECC), a class of high performance fibre-reinforced cementbased composites (HPFRC). ECC specimens exhibited higher drying shrinkage and creep strains than corresponding specimens without fibres. In order to appropriately apply FRCC in the construction industry, it is essential to understand and characterise its long-term behaviour. This inspired the research project to both characterise and model the time-dependent behaviour of HPFRC. A constitutive model for macroscopic finite element analysis of HPFRC has been developed and implemented [15]. To characterise the parameters for this model, an experimental programme has been launched. The program envisages to test the time-dependent response of a particular HPFRC in direct tension as well as bending. By testing sealed and unsealed specimens, the time-dependent response caused by hygral shrinkage, as well as the Picket-effect may be distinguished. Preliminary results of this experimental program are reported here. The results of varying rate direct tensile, as well as flexural tests are reported, whereby the time dependent reduction in peak resistance in both these modes can be characterised. 2. EXPERIMENTAL PROGRAM For this study of the time-dependant behaviour of HPFRC, experimental tests were done to determine the nature and try to identify the source of the time-dependant behaviour. All tests were done with the same mix proportions, namely a water:binder ratio of 0.4, aggregate to binder ratio of 0.5, fly ash:binder ratio of 0.5, all by mass, and 2% by volume PVA REC15-12 fibres. Displacement and force controlled tests were executed in both direct tension, as well as three point bending. The test program is summarized in Table 1. Table 1: Experimental program All the test specimens were stripped from their moulds after three days, water cured for 11 days, subsequently sealed with Sikagard 63N and then tested at the age of 14 days from casting. Casting and curing for the full 14 days were at a controlled environmental temperature of 23 ̊C. The ultimate loads referred to in Table 1 are the average peak resistances at normal rates. For tension this deformation rate is 0.5 mm/min and for bending the normal deflection rate is 2.5 mm/min. 2.1 Displacement controlled tensile test setup The direct tensile tests were done using a Zwick Z250 Universal Materials Testing Machine. The dog bone shaped specimens are shown in Figure 2. The displacement of the Zwick crosshead was prescribed for each set of tests according to the rates shown in Table 1. The strain readings were taken with an HBM DD1 extensometer with a gauge length of 80 mm. The extensometer was placed as shown in Figure 2. The range of this extensometer is 2.7% strain when used with the chosen gauge length. No strain results beyond this range are analysed or discussed in this paper. The force readings are however accurate throughout the entire test. 2.2 Force controlled tensile tests For the force controlled (creep) tensile tests, the load was applied indirectly by hanging weights on a lever arm, as shown in Figure 3. The load is applied to two specimens in series. The strain is measured using an instrument made of a thin (0.2 mm) spring steel plate. Two strain gauges are fixed on the centre of the plate on opposite sides. Each of these instruments was calibrated using a micro-meter to relate the displacement between the two points on the specimen to the readings of the strain gauges. An example of this instrument is shown in Figure 3(b). These tests are still in a preliminary stage, thus the strain readings are normalised. 2.3 Displacement and force controlled bending tests All three point bending tests were done in the same test setup. A 500 kN Instron servo controlled hydraulic actuator was used to apply the load. A force or displacement controlled test can be prescribed by the control unit of the actuator at the required loading rate. The force was measured using a 2 ton HBM U1 load cell and the deflection with a HBM W50 LVDT. The beam dimensions and the test setup are shown in Figure 3.