# Hybrid Fibres as Shear Reinforcement in High-Performance Concrete Beams with and without Openings
## 2 Experimental Work  
### Materials  
Portland cement CEM I 52.5R is characterised by high initial strength and high heat release in the early period of curing. The aggregate consists of 0.05/2 mm fraction quartz sand and 2/8 mm fraction granodiorite. A superplasticiser based on polycarbonate ethers causes significantly higher cement particle dispersion and has the unique ability to create the appropriate consistency. The steel fibres are hook-ended with a length of 50 mm and diameter of 1 mm. The polypropylene fibres are gathered in bundles and cut to 12 mm. To ensure the workability of the HPC, a maximum of 0.1% polypropylene fibres were added to the mixtures. The use of such components gives the ability to produce homogeneous concrete mixtures. This is particularly important for concrete with fibres and a low W/B ratio. The mixture compositions used to produce the specimens are shown in Table 1.  
At the beginning of mixing, the granodiorite and quartz sand were homogenized with half of the water. Subsequently, cement, silica fume, the remaining water and superplasticiser were added. After the components had been thoroughly mixed, fibres were added by hand to four mixtures. The beams were cast in plywood moulds. After compacting, the specimens were protected with foil to reduce the loss of moisture. The total curing period of the control samples and test beams was 28 \(\pm\) 2 days.  
The concrete and fibre concrete properties such as slump, compressive strength, tensile splitting strength, flexural strength and modulus of elasticity were measured for each type of concrete mixture. Cubes, cylinders and prisms were cast. The mechanical properties were determined according to standards PN-EN 12390-3:2002 [55], PN-EN 12390-6:2001 [56], PN-EN 12390-5:2009 [57] and ASTM C469/C469M-14 [58], respectively. The measurement results of the properties of the concrete cast for six beams are given in Table 2.  
The water to binder ratio (W/B) in beams with openings was reduced to 0.25 in order to improve the concrete properties. This effect has been achieved only for HPC without fibres. As a result of the reduction of the W/B ratio by 0.05 in BO mixtures with fibres as compared to the B series, lower concrete strengths were obtained. The inhibition of concrete flow as the w/b ratio decreases resulted in the occurrence of poor adhesion between fibres and the matrix as well as had a decisive influence on the inferior concrete properties. The reduction in the workability of concrete was also ascribed to the presence of fibres in the mix tending to lump in balls, and absorb some of the free water required for lubrication and paste formation.  
The longitudinal and diagonal tension reinforcements were fabricated from Class A-III nominally 410 MPa grade, hot rolled, deformed bars. Plain steel bars 6 mm in diameter were used for stirrups. They were fabricated from Class A-I nominally 240 MPa grade drawn wire reinforcement according to standard PN-B-03264:2002 [59].  
The mean results of testing three samples from each size of bars 6, 12, 22 mm and the test results for the two types of fibres are shown in Table 3.