Flexural creep

For a constitutive model to be verified, the analysis of flexural test using parameter obtained from uniaxial tensile test should accurately simulate the experimental results. Hence, the essence of the flexural creep is to study the time-dependent behaviour of cracked macro synthetic FRC under bending stresses as well as for the verification of the finite element analysis (FEA) of the time- dependent behaviour of macro synthetic FRC using material model parameters obtained from the uniaxial tensile creep tests.

Experimental test setup

The test setup for the flexural creep followed the design concept reported in Arango et al. (2012) and Zerbino & Barragan (2012) with some minor modifications to the components of the creep frame

67 and the measuring device. The flexural creep frame developed and used in this research consisted of the following arrangement (see Figure 3.18):

Figure 3.18: Flexural creep frame

• A support base made of two longitudinal 100 mm × 50 mm channel sections with transverse steel channel sections welded to it at defined interval to ensure a stiff base. The longitudinal sections are 1.6 mm long and spaced 200 mm apart.

• Vertical supports for the column of specimens made of square hollow steel sections (100 mm ×100 mm × 6 mm) which were filled with concrete were welded to be base. The supports were located such that a test specimen span of 450 mm could be achieved.

• The load was applied through a lever arm mechanism. The lever arm was also made of 100 mm × 50 mm channel sections similar to the base with two transverse channel sections placed only at the free end for supporting the counterweight. Two vertical flat plates (10 mm thick) bolted to the internal surface of the supporting base and the lever arms on either side were used to support the lever arm at one end. The point of connection between the plates and lever arm has a hole drilled to a diameter of 27 mm to allow for the placement of

Supporting base Support for specimens

Lever arm Specimen

68 roller bearings. A Bright mild steel round bar (diameter = 25 mm) was then passed through both roller bearings on both sides to act as the fulcrum for the lever arm.

• Holes were drilled on the flanges of the lever arm on either side at the points where the threaded bars will lie exactly at mid-point of the column of specimens. A couple of M20 threaded bars were then connected vertically to the lever arm between the support points for the application of load. M20 galvanised hex nuts and washers were used to fasten the threaded bars to the level arm.

• Each test specimens was supported on a circular hollow steel section; one of the circular sections was prevented from rotating by using wedges while the other was not wedged. Specimens were stacked in column of three specimens referred to in this dissertation as top, middle and bottom specimens. After the positioning of the bottom and middle specimens, the top specimen then had a load transmission element (made of square hollow section with two hollow circular sections attached) placed on it.

• A 20 mm thick flat load plate connecting both threaded bars as a couple was finally placed on the load transmission element. It was ensured in the design of the frame that the setup was stiff enough for the level of load applied such that no additional deformation from the threaded bars, lever arm, specimen supports and load transmission element was expected. The specimens were positioned in a four point bending test setup with a span of 450 mm and specimen supports at 150 mm. Due to the shortage of LVDT’s, dial gauges with a resolution of 0.01 mm were used to measure the crack tip opening displacement (CTOD) of specimens for a period of eight months. The dial gauges measuring the CTOD of specimens were borne by angle plates glued to the specimens across the notched area. They were only placed on one surface of each specimen as shown in Figure 3.19c. Though the measurements reported were not average values, it will nevertheless give an indication of the deferred deformation of Macro synthetic FRC under sustained flexural loadings. Measurements were taken at regular intervals throughout the test period.

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Experimental test programme

The flexural creep test was commenced after the specimens had been cracked in the Zwick Z250 Universal Testing Machine to a crack width of 0.2 mm and then unloaded under a 3-point bending test following the same procedure already discussed in Section 3.3.3. Specimens were tested under three stress levels as presented in Table 3.1. The equivalent sustained loads for each stress level was obtained as a percentage of the average residual strength at CMOD = 0.2 mm.

Prior to loading the cracked specimens on the creep frame, the frames were calibrated with a 50 kN HBM load cell placed on the setup (Figure 3.19a).

a) b)

c)

Figure 3.18: Flexural creep setup (a) Calibration of frame showing load cell (b) Constant load and Enerpac cylinder (c) Stacked beams already loaded

70 I-steel section was put in the position where the actual test specimens occupied and the transverse load plate connecting both threaded bars together was in turn placed on top of the load cell. It was ensured that the nuts did not exert any force on the specimens before the gravity loads were applied. It should be remarked that the load cell was not left in the frame during the actual tests as the constant gravity load needed to be sustained at the stress levels had been determined (Figure 3.19b). Three beams were stacked in each frame and the gravity loads already placed during the calibration were gradually applied by releasing an extended Enerpac hydraulic cylinder that was connected to a hand- operated hydraulic pump beneath the weights as shown in Figure 3.19c.

3.7 Summary

This chapter presented all the experimental test materials used, preparation of moulds for casting, concrete mixing, workability investigation, preparation of specimens for test, test procedures performed to investigate the short term and the time-dependent behaviour of macro synthetic FRC at the macro, single fibre and structural levels. A well defined experimental methodology is necessary for the successful achievement of all the stated objectives in this study.

At the macro level, the short and long term behaviour of macro synthetic FRC were investigated. These included the performance of specimens under compression, uniaxial tension, uniaxial tensile creep and drying shrinkage tests. Methodology for the investigation of the long term behaviour of cracked specimens subjected to sustained loadings was also presented in detail. These tests included the uniaxial tensile and flexural creep, time-dependent single fibre pull-out and fibre creep tests to study the mechanisms causing the time-dependent crack opening of cracked macro synthetic FRC.

Other tests conducted at the single fibre level were the single fibre pull-out rate test and tensile rate test of single fibres. Results of all tests and the discussion of findings are presented in the subsequent chapters.