Objetivos específicos

This section provides a comprehensive overview of the adopted methodology used for SFPO experiments.

3.3.1 Specimen preparation, mixing procedure, moulds and curing

Single fibres were embedded either in a virgin or premixed fibre state. Virgin fibre state refers to the original fibre appearance as received by the suppliers, while premixed fibre state refers to the fibre appearance achieved as a result of the mixing process, causing roughening of the fibre surface. It was found that fibres looked somewhat different from their virgin state when added to the concrete and mixed for several minutes. The difference is illustrated in Figure 3.3 for the EPC BC MQ58 macro synthetic fibre as an example.

Figure 3.3: Virgin (a) and premixed (b) macro synthetic fibres

Eight specimens containing virgin fibres and twelve specimens containing premixed fibres were prepared in order to account for potential variability in test results caused by the additional surface roughening as a consequence from premixing.

Typical 100 x 100 x 100 mm concrete cube moulds were used and divided using a wooden cross-like separator into four equal compartments measuring 39 x 39 x 100 mm, as shown in Figure 3.4. The wooden separator was coated using PLASCON ROOFSEAL waterproofing

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paint in order to eliminate water absorption of the wood. Additionally, the moulds along with the separator were oiled using mould release oil.

Figure 3.4: Wooden separator

The FRC materials described in Section 3.1 were batched by weight and added to a 50 litre concrete pan mixer in the order of sand, cement, stones. Prior to mixing, the mixer was rinsed with water and dried using industrial tissue paper in order to ensure the same conditions for each mixing batch and eliminate additional water absorption of the pan. The dry materials were mixed for one minute before the mixing water was added, followed by an additional mixing time of five minutes.

Premixed fibres were prepared using a 10 litre concrete pan mixer. The aforementioned mixing procedure was adopted with the addition of fibres after the five minutes of wet mixing. This was followed by an additional five minutes of mixing in order to cause the anticipated fibre roughening as a result of the mixing process. Afterwards, the premixed fibres for the SFPO experiments were rinsed using water with the aim to only retain the roughened (premixed) fibres.

After mixing, a slump test according to SANS 5862-1 (2006) was performed as a measure of consistency as well as to ensure sufficient workability for different concrete batches.

Specimens cast for SFPO tests did not contain any fibres as only the behaviour of a single- fibre was of interest. Single fibres were embedded with great care into the concrete at specific pre-marked embedment lengths. The embedment lengths were taken as 12.5, 25.0 and 37.5 mm as well as half the fibre length for fibres not being 50 mm in length. Fibres were embedded as straight as possible by hand with the aid of visual assessment for straightness from all directions.

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The main obstacle was however the insertion of flat and bundled fibres as a result of the low axial stiffness. Non-stiff fibres tend to bend within the paste matrix, resulting in skew embedment. In order to simplify the insertion process, the concrete was sieved using a 2.36 mm sieve to eliminate the stone content. This furthermore had the effect of the fibres being in continuous contact with surrounding paste without stones causing weak spots creating additional possible variability in the test results. The individual fibres were inserted by hand up to a pre-marked embedment length. Depending on the fibre colour, either correction fluid or black ink was used to pre-mark the desired embedment length. In order to embed the fibre exactly in the middle of the 39 x 39 x 100 mm specimen, a piece of flat roof sheeting was slightly stencilled across opposite corners into the fresh paste leaving behind a mark on the paste surface. Moulds were gently vibrated after fibre insertion, in order to ensure a good paste compaction around the fibre. Subsequently, fibres were visually assessed from all sides to ensure orthogonality with the exposed fresh paste surface. The SFPO test specimen as well as a schematisation of the specimen preparation is shown in Figure 3.5. The freshly cast SFPO specimens were left to set for 24 hours before de-moulding.

a) b)

Figure 3.5: SFPO specimen (a) and specimen casting scheme (b)

After SFPO specimens were de-moulded, these were transferred to temperature controlled curing tanks with a water temperature of 25°C. All specimens were cured by complete immersion in water and were left to cure for additional 27 days before the preparation for testing. SFPO specimens were prepared for testing shortly after their removal from the curing tanks.

3.3.2 Test setup

The SFPO test was performed using a ZWICK/ROELL Z250 universal materials testing machine with a capacity of 250 kN. The SFPO specimen as well as the gripping system was held in position by means of hydraulic clamps. The fibre-portion protruding from the hardened paste was gripped with the aid of a fibre clamp, consisting of two 40 x 8 mm flat

39 22 39 100 39 22 39 39 22 39 10 FIBRE FIBRE FIBRE FIBRE A ---- A

---- FIBRE NOT TO SCALE

le WOODEN SEPARATOR FIBRE FIBRE WOODEN SEPARATOR

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mild steel bars as shown in Figure 3.6a. The fibre was gripped as close as possible to the hardened paste surface in order to ensure consistency as well as to simulate the in-service behaviour of the fibre i.e. the formation of a crack. Two linear variable displacement transducers (LVDTs) were used on either side of the fibre to measure the pull-out displacement. The average LVDT reading linearly interpolates the true pull-out displacement of the fibre. These had a differential range of 50 mm, allowing sufficient measurement for the maximum embedment length of 37.5 mm. In addition, a load-cell with a capacity of 500 kg was used to record the pull-out force. The SFPO test method was displacement controlled using a crosshead displacement rate of 0.2 mm/s. The SFPO test setup is shown in Figure 3.6b.

a) b)

Figure 3.6: SFPO experiment fibre grip (a) and setup (b)

The edges of the fibre grip used were rounded in order to avoid sharp edges which potentially could cause premature fibre rupture. Furthermore, it was important to ensure that no fibre slip is permitted by sufficient tightening of the two 6 mm bolts, clamping the two steel bars.