Combined effects of water film thickness and paste film thickness on rheology of mortar
A. K. H. Kwan
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L. G. Li
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A. K. H. Kwan (&) L. G. Li Department of Civil Engineering, The University of Hong Kong
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Hong Kong, China
In the mortar portion of a concrete mix, the water must be more than sufficient to fill the voids between the solid particles of cement and fine aggregate whereas the paste volume must be more than sufficient to fill the voids between the solid particles of fine aggregate so that there will be excess water to form water films coating all the solid particles and excess paste to form paste films coating the fine aggregate particles. Hence, it may be postulated that the water film thickness (WFT) and the paste film thickness (PFT) have major effects on the rheology of mortar. In this study, the combined effects of WFT and PFT on the rheology, cohesiveness and adhesiveness of mortar were investigated by testing mortar samples with varying water, cement and aggregate contents. It was found that whilst the WFT is the single most important factor governing the rheology of mortar, the PFT also has significant effects. Particularly, the PFT has certain interesting effects on the cohesiveness and adhesiveness of mortar, which should be duly considered in mortar design.
1 Introduction
With the advent of modern chemical and mineral
admixtures, it is now possible to produce many
different types of high-performance concrete (HPC)
with high performance in certain attributes at the fresh
or hardened state [1]. However, the mix design of
HPC, especially those containing many ingredients, is
not an easy task. In this regard, it is proposed herein to
develop a three-tier system for the mix design of HPC
and concrete in general. The first tier of materials
consists of the cementitious materials and microfillers
of size smaller than or similar to cement. These
materials will, after mixing with the water, form a
paste for filling the voids in the next tier of materials.
The second tier of materials consists of the fine
aggregate particles and fillers of size smaller than or
similar to fine aggregate. These materials will, after
mixing with the paste, form a mortar for filling the
voids in the next tier of materials. Finally, the third tier
of materials consists of the coarse aggregate particles.
With the concrete regarded as a three-tier system, the
mix design may then be carried out by considering
successively the paste and mortar portions of the
concrete.
This paper focuses on the mortar portion of
concrete. While considering the mortar portion, it
should be borne in mind that a layer of mortar, which
has to be sufficiently thick and flowable, should be
provided to coat every coarse aggregate particle [2]
and the fresh properties of a concrete are closely
related to the rheology of its mortar portion [3, 4].
For self-consolidating concrete (SCC), it has been
suggested that the mortar portion should be designed
first [5, 6]. Since a SCC has to flow a long distance
and fill into far-reaching corners without segregation,
the mortar portion has to have high flowability [36]
and high cohesiveness [57]. Furthermore, the
mortar portion needs to have high adhesiveness
(the ability to adhere to solid surfaces) so as to avoid
separation of the mortar from the coarse aggregate
particles or in the case of mortar works from the
substrate. However, the desired high flowability,
cohesiveness and adhesiveness are not easy to
achieve at the same time. One reason is that the
addition of a superplasticizer (SP) to increase the
flowability would substantially decrease the
cohesiveness [8] and probably also the adhesiveness.
Besides, although mortar with high adhesiveness has
been used in concrete repair and brick works [911],
a suitable test method for measuring the
adhesiveness of mortar is still lacking.
A number of studies have been carried out to
identify the main factors affecting the rheology of
mortar. Banfill [12] found that both the yield stress
and viscosity of mortar decrease exponentially with
the water content. In other studies, it has been found
that the characteristics of the fine aggregate also have
significant effects. For example, De Schutter and
Poppe [13] showed that the water demand of a mortar
is closely related to the packing density of the fine
aggregate. Reddy and Gupta [14] found that
generally a mortar made of a finer sand would need a
higher water content for a given workability and
explained that this is because of the larger solid
surface area of the finer sand used. From these
studies, it may be inferred that the main factors
affecting the rheology of mortar are the water
content, packing density and solid surface area of
the solidwater mixture. At a smaller particle size
scale, Kwan and Wong [15] demonstrated that
blending of cement with condensed silica fume could
increase the packing density, decrease the amount of
water needed to fill the voids and thus increase the
flowability of the paste formed. Hence, both the
packing density of the fine aggregate and the packing
densi (...truncated)