T Sedran

Optimization of ultra-high-performance concrete by the use of a packing model

Abstract The concept of high packing density has been recently rediscovered, as a key for obtaining ultra-high-performance cementitious materials. First, this paper presents two models allowing to predict the packing density of a particle mix. These models are derived from the Mooneys suspension viscosity model. At a second step, considerations on the parameter to be maximized during the mix-design process are presented. Reference is made to the Maximum Paste Thickness concept, which leads to choose a fine sand for optimizing the compressive strength of cementitious materials. Then, an optimal material is sought, based on the following requirements: fluid consistency, classical components (i.e. ordinary aggregate, sand, Portland cement, silica fume, superplasticizer, water), and moderate thermal curing. A selection of mixes is made with the help of the Solid Suspension Model, and tests are performed in order to verify that the mix obtained is definitely optimal. The final result is the production of a fluid mortar having a water/binder ratio of 0.14 and a compressive strength of 236 MPa.

A NEW RHEOMETER FOR SOFT-TO-FLUID FRESH CONCRETE

This article describes the development of apparatus designed to characterize the rheological behaviour of fresh concrete , both in the lab and on site, in the range of consistency from soft to fluid. The principles that governed the design of the rheometer are first recalled. It is a torsional rheometer, in which a sample shaped like a hollow cylinder is sheared between two horizontal planes. The conduct of the tests is then described, followed by their processing, which yields, in particular, two physical quantities to characterize a concrete, the shear yield stress and the plastic viscosity. These parameters may be measured with or without vibration, giving different values in each case. Later, the results of experiments are described. They deal with the detection, with the detection, with only one sample, of the phenomenon of loss of workability, and with the pumpability of the concrete on site, respectively. The article concludes with a look at the prospects for industrial use of the BT RHEOM rheometer.

Removable urban pavements: an innovative, sustainable technology

By definition, a removable urban pavement (RUP) can be quickly opened and closed, using lightweight equipment, for easy access to underground networks. The premises of the concept can be found in certain military paths or industrial soils. According to a survey conducted among French municipal authorities, the RUP concept could decrease the public nuisance caused by pavement and road network maintenance work. Two French cities agreed to play host to an RUP experiment. Functional specifications were provided for streets to be built in new residential areas. Then original pavement designs were proposed based on a configuration of precast, hexagonal concrete slabs over an excavatable hydraulic base course. After successful trials using accelerated loading facilities, two RUPs were built and subjected to real traffic. Should the satisfactory behaviour observed so far last into the future, RUPs would appear to be a highly sustainable response to the needs of modern cities.

Prediction of the volume of concrete backfill materials excavated using a pick

Pick excavatability of controlled low-strength materials was experimentally studied to circumvent the fact that current classification systems or empirical excavatability criteria lack relevance (they do not refer to digging facilities and depend on the mix design and the granular skeleton of the material). Manual excavation is separated into two steps: the impact of the pick and decompaction. This was analysed by a mechanical approach. Energy is mainly consumed during the first step. A laboratory punching test was therefore proposed to evaluate excavatability (a confined cylindrical specimen was loaded by a cylindrical punch). Results were compared between the mechanised and the manual pick excavation tests on field trenches backfilled with air-entrained mortars or very lean concretes. These comparisons confirmed that a limited compressive strength was an indicator of excavatability. A model was proposed to evaluate the excavated volume by punching penetration for a given level of energy and a maximum size of aggregates in the material.

Suivi du comportement thermo-mécanique d’une voie ferroviaire sur dalles en béton

At the beginning of 2013, a new ballastless track structure (designed and tested in laboratory) based on concrete slabs was built in France. Since December 2013, several trains circulate every day and a monitoring of some sections of the structure i s performed continuously. Data collected are compared to the results of finite element simulati ons used to design the structure in order to verify that the experimental structure can be operated saf ely. In this paper, a focus is made on the mechanical behavior of the track slabs with ther mal gradients. MOTS-CLÉS : instrumentation, voie ferroviaire sur dalles béton, gradient thermi que.