Page 57 - Riem-Vol6_nº2

228 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Influence of the environment and loading age on SCC drying creep
1. Introduction
Creep basically consists in the deformation increasing on a loaded
element when stress is kept constant [1]. This constant external
load becomes the driving force for the movement of the adsorbed
and capillary water. Thus, the creep deformation can occur even
with a 100% relative humidity [2].
It is usually classified into basic and drying creep. The basic creep
occurs without moisture exchange with the environment. Accord-
ing to Mehta and Monteiro [2], basic creep usually occurs in large
structures where drying shrinkage can be neglected. As for the
drying creep is observed when concrete is submitted to a less than
100% relative humidity environment.
Considering that the gradual increase of the creep deformation
can be several times greater than the one observed at the time of
loading, it proves to be very important for structures. It affects the
deformation and vertical displacements and often also the stress
distribution. However depending on the structure, these effects
may vary. For example, to the concrete mass it is an important
parameter, because its effect results in relaxation of the thermal
stresses from the concrete cooling. As for the prestressed con-
crete, it contributes to the loss of prestressing tension [3].
The moisture movement in the hardened cement paste, which es-
sentially controls the drying shrinkage and creep strains in con-
crete, is influenced by many factors that interact simultaneously
[2]. Among the factors that influence this property are: material
type and concrete dosage, additives and admixtures, chemical
composition and cement fineness, the relative humidity and ambi-
ent temperature, curing conditions and conservation, the geometry
of the concrete element, age at loading and concrete strength.
Neville [4] states that one of the most important factors that influ-
ence the creep of concrete is the relative humidity of the air that
surrounds it. For creep, the relative humidity affects the concrete
drying, and it is important to distinguish between the drying that
occurs before and after loading. The drying while the member is
loaded, increases creep, which is called drying creep. However,
in concrete members which have reached the hygroscopic equi-
librium with the environment before loaded, the influence of the
relative humidity is lower.
Regarding the temperature, it may have two opposing effects on
creep. Mehta and Monteiro [2] reported that if a concrete member
is subjected to a temperature greater than the environment as a
curing before loaded, the strength will increase and creep strain is
smaller than that from a concrete stored at a lower temperature.
This is because the temperature accelerates hydration, reducing
creep. On the other hand, the exposure to high temperatures dur-
ing the load can increase the creep.
In mortar tests, made by Neville [5], considering the variation of
humidity over time, the results show that creep is increased when
the specimens are exposed to moisture variation only if the load
is applied before the first drying. Exposure of mortar specimens
to an alternating relative humidity between 50 and 70% show that
creep is almost as big as the measured on specimens kept in a
constant relative humidity of 50%, and higher than the average
relative humidity of 60%. This suggests that by alternating the rela-
tive humidity, creep will be increased beyond that obtained with
the lower limit of moisture. The same author also show the study of
the behavior of concrete exposed to relative humidity variations in
an environment protected from weather (humidity ranging from 60
to 90%), comparing them with results from laboratory specimens
kept at 50% moisture. From the results it seems to be no substan-
tial difference in the total strain or creep between the specimens
kept in an environment with relative humidity ranging from 60 to
90%, when compared to specimens kept in constant humidity of
50%. Therefore, caution should be exercised in applying results
with constant moisture for field applications.
As for the age at loading, the creep of concrete loaded at early
ages is greater in the first weeks of loading compared to concrete
loaded at higher ages. This behavior is due to the higher degree of
hydration of the older concrete, which have internal structure more
compact and less water available [3]. For loading ages superior to
28 days, the influence of age is very small [6].
In this paper comparisons are made between results obtained
from drying creep specimens loaded at different ages and kept
in a climate-controlled chamber and specimens left in an uncon-
trolled environment of relative humidity and temperature. In addi-
tion, comparisons are made between the experimental results and
prediction models available in the literature.
2. Materials and methodology
The self compacting concrete of this paper is tested both in fresh
and hardened state. The tests performed in the fresh SCC were:
slump-flow, flow-rate, V-funnel, and L-box. The tests performed in
the hardened SCC were: compressive strength, splitting tensile
strength, modulus of elasticity and drying creep.
The concrete was made with cement-type CP-II-E-40 from Vo-
torantim Cimentos Brasil S.A., granitic coarse aggregate (maxi-
mum size 12,5mm), coarse aggregate from limestone, quartz sand
and artificial sand (from crushed stone). The superplasticizers used
were polyfunctional Mira 94 from Grace Construction Products and
viscocrete 3535 from Sika SA. The mixture used is shown in Table
1 and the physical characterization of coarse and fine aggregates
are in Table 2. In Figure 1 are shown the gradation curves of all
aggregates used.
The materials of the mixture were weighted and the aggregates
Table 1 – Mixture proportion of the concrete
Components SCC
3 Cement (kg/m ) 370.0
3 Quartz sand (kg/m ) 512.0
3 Crushed stone sand (kg/m ) 420.0
3 Crushed stone (19mm) (kg/m ) 520.8
3 Crushed stone (12,5mm) (kg/m ) 347.2
Water (l) 180.0
3 Superplasticizer viscocrete 3535 (kg/m ) 2.59
3 Superplasticizer Mira 94 (kg/m ) 2.40
a/c (kg/kg) 0.49
f (MPa)ck 50