Influence of reinforcement arrangement in flexural fire behavior of hollow core slabs
Abstract This paper describes standard scale fire tests conducted on four precast prestressed hollow core slabs configurations under flexure. The influence of the reinforcement arrangement was investigated using two test parameters: the average axis distance (from 30.1 to 46.7 mm), and the type of reinforcement (5 mm diameter wires and 9.5 mm diameter strands). An ISO 834 standard fire was applied maintaining the load level in a four-point test arrangement constant. The results presented different failure mechanisms, types of cracks, spalling patterns, and fire resistance ratings and it was shown that wires displayed optimal behavior as opposed to strands. A comparison was carried out with the current verification methods provided in Eurocode 2 part 1–2, and showed that verification methods tend to overestimate the calculation of fire resistance when both wire and strands are moved away from the exposed surface.
New regulations for hollow core slabs after premature partial collapse
In October 2007, a floor made up of pre-cast prestressed hollow core floor elements suffered early partial collapse during a fire in a carpark of a moderately high new building in Rotterdam, predominantly in use for housing and for limited enterprise. The damage pattern revealed horizontal cracks through the webs of the hollow core slabs, causing collapse of the underflanges which contain the prestressing strands. As a consequence, the structural integrity of the floor and the entire building was jeopardized. In 2007, the municipal authorities of Rotterdam commissioned Efectis Nederland to investigate the damage that occurred during the fire. The results of the study strongly recommended further investigation and addressing the observed damage pattern in the assessment and design rules for hollow core slabs. Based on this conclusion, TNO was commissioned in 2008 by BFBN (Dutch precast industry) to further investigate the incident and whether or not this early partial collapse concerns an omission in the current regulations. TNO concluded that the structure did meet the requirements stated by the authorities, the fire development did not significantly deviate from the standard fire curve, the damage pattern has been observed more often, and the fire resistance of hollow core slabs in fire tests is often lower than the expected fire resistance based on current regulations. Therefore, TNO concluded that the current regulations contain an omission. Therefore, additional detailing and design rules for hollow core slabs were advised by BFBN, which are partially based on new rules for hollow core slabs developed by CEN. This paper addresses the research commissioned by the municipal authorities of Rotterdam, the research commissioned by BFBN and the additional rules advised by BFBN. This includes the fire development and damage to the hollow core slabs in the carpark in Rotterdam, the relevant collapse mechanisms for hollow core slabs in case of fire, their relative importance and respective uncertainties, and the measures taken in the new rules.
Phase formation during mechanical alloying in the Ti–Si system
Abstract Mechanical alloying of Ti–Si powder mixture or TiH 1.924 –Si powder mixture was performed by high-energy ball milling, in which the Ti to Si mole ratio was 1:2. The phase formations in both mixtures during milling were investigated by X-ray diffraction. Mechanical alloying a mixture of elemental powders led to the formation of TiSi 2 equilibrium compound, while when starting from Si + TiH 1.924 , a metastable phase was formed. The metastable phase completely transforms to TiSi 2 when the mechanically alloyed powder is annealed at temperatures above 800 °C. Two different pathways of phase transitions during mechanical alloying are attributed to the existence of hydrogen atoms, i.e., when starting from Si + TiH 1.924 compared to Si + Ti, the kinetics for the formation of the TiSi 2 intermetallic compound slows down. The kinetics related to the different phase transitions depending on the starting powder mixture are discussed.