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Mestrado Integrado de Engenharia CivilMateriais de Construo 1

PALESTRA

DATA: 25 deOutubro de 2007 (quinta-feira)

SALA B004

HORA: 11,30H

Aggregate in concrete

Delft University of Technology

Professor Piet Stroeven

Born 18-08-37 in Amsterdam. Married, 3 sons. MSc at Delft University of Technology: project on conoid shells (1965)PhD at Delft University of Technology on Micromechanics of concrete. Engineering career at Delft University of

Technology as staff member of different groups. Activities: concrete structures, photo elasticity and moir techniques,

steel fibre concrete, technological research, cement blending by mineral admixtures, virtual concrete. Retirement in2001. Period 2001-2007: guest professor at DUT.Received gold medal of Wroclaw University of Technology, and is

honorary professor at Beijing Jiaotong University. Coordinator of research cooperation with Polish Academy of

Sciences, Wroclaw University of Technology, Hanoi University of Technology and 3 Universities in China.

Abstract

Presentation will focus on the conventional application of aggregate in

concrete , i.e. as inert filler that takes up about three-quarters of concretesvolume. It is a relatively cheap, strong and stiff component; it constitutes theload-bearing skeleton in compression. Compaction energy brings the aggregate

in the dense random packing state, whereby it is taken that mechanical

performance is promoted by highest density. Optimum mixture design is

based on this concept.The aggregate skeleton is filled up by the fresh particulate cement, which can

take up volume fractions approaching 60% in the ultra high performance range

of cementitious materials. These cement grains form a gradient structure at theaggregate surfaces; gradients that persist also in the hardened state. This size

segregation phenomenon is at the basis of the formation of Interfacial

Transition Zones (ITZs) around aggregate grains. It will be stipulated that most

of the hardened cement will be part of such ITZs, due to the close distances ofaggregate grains.

During hardening, pore space will gradually depercolate (i.e., will degenerateinto isolated branches). Final porosity depends on the size of the cement

pockets between aggregate grains; however most interesting is that percolated

porosity will be restricted to very narrow zones at the surfaces of aggregate

grains. As a consequence, percolated porosity in specimens is the result of

those spherical layers around aggregate grains that are connected due to near contact of the aggregate grains. Mostprobably, the denser the aggregate packing, the better external surfaces are connected by continuous pores. So, it seems

that durability performance may be negatively influenced by optimum density of aggregate packing.

This has been experimentally studied, asking very hard labor and is thus time-consuming and expensive. Modernapproach is to study virtual concrete or computercrete. The presentation will go into the fundamentally different

approaches in concrete technology, and how we should appreciate results obtained by these different approaches. This

is accomplished on the basis of materials science principles. It will be argued in this way that the reliability of

information generated by systems is inversely proportional to popularity of the system.