Large-size precast concrete slabs for urban road pavements with tramway tracks

In 1967 precast concrete slabs were experimentally used in lieu of granite paving sets as surfacings fur carriageways used by trams in Vienna's public transport system. After some tests, increasingly large areas were paved by this method from 1970 onwards. The evident advanta~es of precast slabs were the deciding factor: - rapid paving construction progress, - little noise due to the wheels of vehicles travellin~ over them, - no maintenance expenses, - sound-insulat~d pavement structure is possible (little structure-borne sound transmission), - visual separation from adjacent roaù surfacin~. Form of construction: - foundation structure: in-situ concrete sub-base (concrete grade B 225) is placed on sealed and compacted subgrade; sub-base is 20 - 25 em thick, depending on soil conditions, and is finished to level of 225 mm below rail top level; - resilient rail support on IS mm thick rubber slabs or styropor (expanded polysterene) slabs on in-situ concrete sub-base; - rail section encased with appropriately shaped precast concrete units; - IS cm thick precast concrete slabs laid on 7 cm bed of chirpings (5 - 8mm); - joints filled with chippin~s (2 - 5 mm). Features of the precast slabs: - length: 272 em on straight sections of track (corresponding to spacing of rail tie bars), appropriately shorter on curved sections and transition curves; - width: running trough 130 mm; middle trough: 140, 120, 160 em, depending on centreto- centre distance between rails; - edge slabs 57,37,47,68,98 em, depending on clearance gauge; - longitudinal edge members curved in shape on curves with radius <60 m; slab thickness 15 em. For handling and placing, each slab is provided with two lifting sockets (30mm x 60mm) on the centroidal axis. At the rail tie bars the transverse edges of the slabs are appropriately rebated. To facilitate lifting the slabs for maintenance operations, all the lateral faces have a 1:10 batter. Quality characteristics: the quality of the precast concrete slabs complies with the "General technical cond i ri ons for the construction of concrete pavements for motorways, expressways and Federal highways" published by the Road Research Society (affiliated to the Austrian Association of Engineers and Architects), working ~roup "Concrete Roads". Top-course concrete: 3 cm thick surfacin~ layer, resistant to frost and de-icing salts, water-cement ratio 0.45 , made with air-entraining agent complying with RVS 8.171, brushed finish to obtain non-skid surface. Abrasion loss in wet testin~ in Böhme apparatus <21 cm3/S0 cm2. The longitudinal edges are provided wi t h 10 mm chamfers, the transverse edp,es with max. 3 mm chamfers. Bottom course concrete: grade B 500; flexural strength 5.5 N/mm2; two-layer reinforcement (fabric mesh). Installing the slabs: with the aid of mobile cranes or fork-lift trucks the precast slabs are deposited on a bed of chippin~s (5 - 8 mm particle size) finished with screeds and are so adjusted with the aid of crowbars that the top of the slab is 5 mm below the top of the rail. After the joints (10 mm - max. 25 mm width) have been filled with chippin?,s (2 - 5 mm), traffic can immediately be allowed on the slabs. Constructing tbe connections to the adjacent pavement: the edge slabs should not be installed until just before the adjacent road navement is to be constructed, 50 as to avoid damage due to crushing or chipping of arrises. In the case of asnhalt pavements the top surface thereof should be finished to about 5 mm above the top of the slabs, to prevent the edges of the slabs being dama~ed when the asphalt is compacted by road rollers. On completion of all the construction operations a durable pavement surfacinp, is obtained with this construction method. Over a service oeriod of 15 - 18 years the amount of ~brasive wear found to have occurred on the surface of the slabs is about Smm. Maintenance work (correction of the position of the slabs) is necessary onlv to a very limited extent. After various experiments, the sound-insulated pavement is increasingly beinp.installed since 1984. In this form of construction rubber ~ranule slabs or roofing slabs (2 - 2.5 cm thick) are installed between the outer edge of the rail and the precast concrete slabs and also around the in-situ concrete sub-base. In this way the structure-borne sound level is reduced by about 10 dB.