Initial tensile test setup

Taking the literature review presented in Section 2.2 and Section 4.1 into account and utilising the materials and apparatus available, an initial tensile testing setup was designed, assembled and used to conduct tensile experiments on early age concrete. This section firstly gives a description of the initial test setup and the method used to conduct experiments. Next, the results obtained from the tensile tests are presented. Finally, an evaluation of the setup and results is presented.

4.2.1 Description of initial test setup

The initial test setup is shown in Figure 4.6. The tensile testing mould, described in Appendix A, had its moving halve supported on four adjustable roller bearings while its fixed halve was bolted to a wooden box at the same height as the roller bearings. The wooden boxes on which both mould halves were supported could be adjusted individually in terms of height so that both mould halves were supported evenly and were level to one another. A pulley system in the form of a bicycle chain and gear was used to convert a vertical force provided by the loading machine into a horisontal force acting on the early age concrete specimen. Since the strength of early age concrete is extremely low and an extremely slow constant displacement rate is required to obtain accurate results, any slack in the pulley system or build-up of elastic strain energy in the entire system would result in a sudden jerk causing the specimen to fail abruptly or the results to be unreliable. The pulley system was connected to a load cell which measured the force exerted on the concrete specimen, as shown in the top right hand corner of Figure 4.6. A PC260 x 90 steel channel section was used as a base to ensure sufficient stiffness. All the components of the test setup were firmly mounted to this rigid base and the rigid base in turn was firmly mounted to a Zwick Z250 uniaxial testing machine which was used to apply the load through the pulley system.

LVDT’s were used to measure the average displacement over the gauge length of the specimen as well as the mould halves moving apart. A LVDT was connected to each side of the mould, as shown in Figure 4.6, while four LVDT’s were connected to the surface of the concrete, as shown in Figure 4.7. Since the material of the specimen at both the moving and fixed sides of the gauge length deforms during the application of tension, it was necessary to measure the displacement of both ends of the gauge length as the relative displacement between these points are needed in order to calculate the strain over the specimen gauge length. The two pairs of LVDT’s were connected to freely rotating lightweight PVC pipes, to account for possible settlement of the specimen and to measure the movement of lightweight plastic placers, which had 25 mm long needles that were inserted into the specimen surface at the edges of the gauge length to track the displacement at these points.

Figure 4.6: Supporting structure

Figure 4.7: LVDT displacement measurement setup over gauge length

4.2.2 Test procedure

All the constituents of the concrete mix were placed in a room with a controlled temperature of 23°C and a relative humidity of 60% at least 24 hours prior to testing. After the constituents were mixed sufficiently, the fresh concrete was placed in the prepared mould and compacted on a vibration table until air bubbles were no longer visible on the specimen surface. The mould was then carefully placed on the wooden box and mechanical bearings, as to disturb the specimen as little as possible, after which the LVDT’s and load cell were connected to the specimen. A displacement rate of either 0.05 or 0.5 mm/min was used to perform the tests. The slower testing rate did however lead to undesirably long testing periods. The concrete mix design used in this initial study is shown in Table 4.1. This mix had a water to cement ratio of 0.4, a slump of 80 mm and was designed for minimum segregation and bleeding, and rapid setting.

Mechanical bearings Fixed mould halve Moving mould halve

Wooden boxes PC260 x 90 channel section Load cell Pulley system Drection of force Fixed mould halve Mould LVDT

Table 4.1: Mix proportions and constituents of the concrete mix design used for initial testing.

Constituent [kg/m3]

Water 205

CEM I 52.5N cement 513

13 mm Greywacke stone 631

Natural quarry sand 1105

4.2.3 Test results

Tensile tests were conducted on specimens from 1 to 4 hours after mixing had occurred. For this mix, the 1 hour test was a fresh concrete, still in a plastic state, while the 4 hour specimen was tested at the time at which final set had occurred. Before initial set had occurred, the hydrostatic force that the plastic concrete exerted on the inside of the mould combined with the weight of the pulley system, caused the slightest movement of the mould halves before the test was initiated. The tests before initial set could therefore not be successfully completed as the specimens were disturbed before the time of testing. This showed the need for a more rigid method of applying a tensile load that would be able to withstand the hydrostatic effect of plastic concrete. The stress-displacement curves of the tests conducted at 3 and 4 hours using displacement rates of 0.05 and 0.5 mm/min are shown in Figure 4.8 and Figure 4.9. From Figure 4.9 it can be seen that failure occurred before adequate data could be captured in the linear elastic region of the material’s behaviour, despite the use of a data sampling rate of 50 Hz and an extremely slow displacement rate on some tests. The Young’s modulus of the concrete specimen could thus not be determined. It is believed that the method of displacement measurement, i.e. the LVDT’s which were weakly connected to the concrete surface, did not account for any significant deformation or cracks that may have occurred far below the concrete surface. Such deformation and cracks may have occurred before the LVDT’s measured any deformation on the surface. This was more apparent on plastic fresh concrete specimens that displayed some shearing during testing, leading to the conclusion that a horisontal displacement gradient existed over the depth of the specimens. The cause of the abrupt failure of the specimens at final set can be accredited to the build-up of strain energy in the system, either at the fixed end of the wooden box or in the pulley mechanism. This can be seen in Figure 4.10, which shows that the 4 hour specimen which was tested at a rate of 0.5 mm/min reached its maximum tensile strength before any displacement was measured on the tensile testing mould. A sharp jump in the displacement shortly after the peak stress was reached, indicating failure, was measured. However, despite this the 4 hour specimens showed a maximum tensile strength of 9.6 and 8.1 kPa while the 3 hour specimens had a

maximum strength of 3.5 and 3.7 kPa, which are comparable with the results discussed in the Section 2.2.

Figure 4.8 Complete sress-displacement curve of 3h and 4h specimens

Figure 4.9: Initial section of the stress-displacement curve of 3h and 4h specimens

0 2 4 6 8 10 12 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Str e ss [kPa] Strain [microstrain] 3h_0.05 3h_0.5 4h_0.05 4h_0.5 0 2 4 6 8 10 12 0 200 400 600 800 1000 1200 1400 1600 Str e ss [kPa] Strain [microstrain] 3h_0.05 3h_0.5 4h_0.05 4h_0.5

Figure 4.10: Stress and displacement measured on mould vs. time for the 4h specimens tested at a rate of 0.5 mm/min

4.2.4 Evaluation of initial tests setup and results

After critically evaluating the initial test setup and the results obtained, the following shortcomings and areas of improvements were identified:

 The method of load application through the loading machine and pulley system could not account for the influence of hydrostatic forces in a concrete specimen which has not reached the initial setting time.

 The method of load application and the fixing of the tensile testing mould were not rigid enough to capture the ascending part of the stress-strain curve and allowed for unwanted movement and strain build-up as illustrated by Figure 4.10.

 A rigid base is needed for the mounting of the tensile testing apparatus. The wooden boxes provided insufficient stiffness and caused slippage during testing that resulted in inaccurate results.

 The method of displacement measurement only accounted for deformation and cracking near the surface of the concrete. Assuming a displacement gradient exists in the depth of the plastic specimens, no reliable displacement can be captured using this method.

 It was cumbersome to align and level the fixed halve of the mould on the wooden box with the moving halve of the mould on the four mechanical bearings. A system where

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 0 200 400 600 800 1000 1200 1400 1600 1800 2000 D isp lac e m e n t [m m ] Str e ss [kPa] Time[s] Stress Displacement

both mould halves are on the same level and therefore simultaneously levelled would be ideal.