Page 59 - Riem-Vol6_nº2

230 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Influence of the environment and loading age on SCC drying creep
According to NBR8224 [13], the creep strain is given by subtract-
ing the elastic deformation and the unloaded specimen strain from
the total strain. The elastic deformation is the one obtained after
the 30s after the third load.
2.2 Creep predicting models
The purpose of the prediction models is to provide, to designers,
conditions to estimate the creep and shrinkage strains with some
accuracy, using known parameters [15].
There are several factors that affect creep, and it is hard to ac-
curately predict it since different physical mechanisms interacts
among each other and are influenced by these parameters [16].
Because it is a long-term test, these properties are not often exper-
imentally determined and, in the absence of experimental results
for creep or shrinkage, it can be estimated by prediction models
that are available in standard codes and the literature. This paper
uses the creep prediction models from NBR6118 [17], Eurocode
2 [18], ACI 209R [19], B3 [16], and GL [20]. It is also discussed
some factors that might interfere its use for SCC and the input data
adopted for each model.
The comparison between experimental results and the ones ob-
tained by the prediction models will be gauged by the compliance
function (J).
All the analyzed prediction models can be used for an applied load
of up to 40% of the compressive strength of the specimen at the
loading age, with the exception of EC2, which allows stress up
to 45% of the characteristic compressive strength of the concrete
and B3 which can be used for a stress up to 45% of the average
compressive strength of the concrete.
Other input data is the slump, which is used only by NBR6118 [17]
and ACI 209R [19] prediction models. According to the NBR6118
[17], the concrete slump must be used without additives. As for
ACI 209R [19], it is considered the slump of the mixture. This paper
The applied load both in the first group (14 days specimens) and
the second group (49 days specimens) were set at 30MPa, which
corresponds to 30% of the compressive strength of the specimen
at 14 days. This value is fixed due to the limitation of the load appli-
cation system which maximum stress allowed is 30MPa. The load
systems used inside and outside the climated-controlled chamber
can be seen at Figures 3 and 4.
As previously said, the specimens undergo two cycles of loading and
unloading before the final load is applied. Therefore, the initial strain is
the one obtained after the removal of the second load. After that, the
specimens are loaded for the third time and measured the strain after
30s (as prescribed in NBR8224 [13]). The data acquisition equipment
was set to register the measurement data every 30s on the first day
and every hour after the second day until the end of the test.
Figure 2 – Split creep mold
Orifice for the
strain gage cable
Figure 3 – Manual pump and hydraulic
accumulators coupled to the manifold
in order to decrease the pressure loss
hydraulic
accumulators
Figure 4 – Manual pump used to load the
specimens outside the climated-controlled chamber