Resumen de Construction of precast high performance concrete segmental bridges.
Lidia Ruiz Ripoll
The construction of both medium and long span precast concrete segmental bridges is widely spread throughout Spain. Usually, the segments have multiple-keyed epoxy joints, and are assembled by internal prestressing. Yet, there is a more recent type of bridge with dry joints and external prestressing. In these last ones, shear is transferred through physical support between keys and friction between faces of the compressed joint. This shear force is evaluated using friction coefficients from tests, where joints were subjected to service compressive stresses. However, normal stresses at ultimate limit state can reach much higher values. Hence, the friction coefficient used currently to design segmental concrete bridges is an extrapolation of the experimental values. It is assumed that the value advocated is the same for any normal stress level and any concrete type, although the roughness of their flat surfaces is different in each case. Nowadays, the absence of friction data for both segments made out of ultra-high strength and self-consolidating concrete, and the very few tests performed on high strength concrete, complicate the innovation of precast segmental bridges. The optimization of their design and construction is also affected by the lack of standards for mix designing ultra-high strength concrete with local materials, and for quantifying formwork pressure due to self-consolidating concrete. This doctoral thesis presents a simple experimental method for measuring friction coefficient between precast segments based on push-off tests. Flat dry joints of six concrete types (conventional, self-consolidating, high strength, high strength self-consolidating, high strength fibre reinforced, and ultra-high strength concrete) were tested under two confining pressure levels. The combination of both parameters provided friction values for conventional and high performance concrete subjected to normal stresses of around 50 and 100% of the ones registered at ultimate limit state. All concrete tested were made with local materials. For this reason, this work also provides a mix design procedure to achieve ultra-high performance concrete with compressive strength above 150 MPa at industrial production. It consisted of the modification of the mix proportions according to the results obtained after applying the trial-and-error techniques at distinct scales. The possibility of using self-consolidating segments in this bridge type led to analyse the influence of the mix design on the evolution of formwork pressure. This doctoral thesis contributes to understand better which materials can achieve a balance between superior fresh properties of self-consolidating concrete and low lateral pressure build-up. Five typical precast self-consolidating mixes were subjected to slump flow, rheological and thixotropic tests. As a result, slump flow diameter, spreading rate, yield stress, plastic viscosity, and rate of structural rebuilding (stress relaxation) were measured. The correlations made between these different parameters allowed estimating the implication of water content, nanoclays and viscosity-modifying admixtures on the development of lateral pressure and its rate of pressure drop. The results of each test were compared with standards and recommendations and other research for corroborating the data and information used so far, for determining their structural safety, and for improving and completing current design codes. From the points of view of cost, time and safety, all this research contributes to promote the use of high performance concrete, and to optimize the design and the construction of segmental bridges, especially precast viaducts built span-by-span.
