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Concrete In Australia : December 2013
44 Concrete in Australia Vol 39 No 4 FEATURE: RHEOLOGY 2.2 .2 Stability (bleeding) – filtration press test Bleeding is a particular type of segregation, which has to be limited to ensure sufficient concrete stability. Common bleeding tests do not take into account additional hydraulic pressure with increasing depth. However, the filtration press tests (Figure 5) model concrete resistance in deep foundation elements against bleeding more realistically. This test method is used to measure the ability of a concrete mix with a maximum coarse aggregate size of 20 mm or less to retain water under pressure and determines the amount of water de-filtered from the sample when the concrete sample is subjected to a certain external pressure. The test results describe the filtration loss and the filter cake thickness. The filtration loss is to be determined by the following formula:1 3 000 . 1 3 / cm volume cylinder ml water filter ed m kg loss filtration (1) Fresh concrete is filled into the test cylinder in two layers and rodded to de-aerate the concrete. Then, the pressure unit is placed on top of the cylinder before the complete filter press unit is placed into the stand. The pressure valve is opened and 5 bar pressure should be applied within 10s. The applied pressure of 5 bar is maintained for a period of 5 m before it is released immediately. The volume of filtered water is measured and recorded. Then the concrete sample is de-moulded. The thickness of the filter cake (which is the de-filtered part of the concrete sample with usually stiff consistency) is measured and recorded. In general, with better concrete stability, the water retention ability of a mix is more and bigger, which indicates a lower filtration loss or a smaller filter cake. 3.0 CONTINUOUS FLIGHT AUGER (CFA) PILES CFA piles were introduced in the USA in the 1940s7 and the first trials in Australia were carried out about 40 years later. One of the main advantages of CFA piles is the high production rate, which can be achieved with the system. This can make CFA piles very economical and their installation is vibration free. The construction of CFA piles in difficult ground conditions is possible without extra measures like drilling fluid or casings. Contrary to conventional rotary bored piles (where an open excavation is created prior to pouring concrete into the pile) CFA augers are directly attached to the drill head of the piling rig. The most significant distinction from conventional bored piles is that the CFA pile does not create an open excavation. The auger and the soil inside the auger flights support the bore hole walls during penetration and extraction. CFA piles must be poured while the auger is extracted, concrete will fill the potential cavity created by the extracted auger. The CFA auger consists of a hollow stem with constant auger flight pitches and flights with a constant outer diameter (Figure 6). The tip of the auger is sealed with a temporary end cap to prevent soil or water ingress into the hollow stem of the auger during installation. The most common auger diameters range from 450 mm to 1200 mm and drilling depths in excess of 40 m can be achieved with modern piling rigs. The installation procedure for Continuous Flight Auger CFA piles is described in Figure 6 after Peiffer et al.8 The auger is rotated through the soil and auger rotation continues in the same direction throughout the whole auger penetration. Soil is cut and transported upwards out of the borehole and at the same time the auger (the flights of which are filled with soil) maintains the integrity of the borehole and prevents it from collapsing. When the proposed toe level is reached auger rotation is stopped and the first batch of pumped concrete is discharged through the hollow auger stem into the pile excavation. Concrete pressure of up to 5 bar (depending on ground conditions) blows off the end cap at the auger tip, which is then lifted at a constant rate without any rotation. Concrete is continuously pumped through the hollow auger stem and the auger tip must remain embedded in the fresh concrete at least half a metre to ensure shaft integrity and positive concrete pressure. After the concrete has reached working platform level a reinforcement cage can be installed in the fresh concrete. With modern concrete technology, concrete mixes should achieve sufficient workability that the use of a cage vibrator can be avoided (cage vibrators present a risk of causing segregation Figure 4: L-box test equipment and measurements of a standard L-box apparatus (1). 41-48 Larisch.indd 44 41-48 Larisch.indd 44 25/11/13 3:58 PM 25/11/13 3:58 PM