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1. Introduction
The transportation time of concrete from the first addition of water
until the beginning of its discharge at the construction site must be,
according to NBR 7212:
a) fixed, so that the end of compacting does not occur after the ini-
tial setting of the cast concrete and the layers or parts adjacent
to this batch;
b) under 90 min and fixed, so that by the end of the discharge no
more than 150 minutes have elapsed, in case of using a vehicle
equipped with an agitator;
c) under 40 min and fixed, so that by the end of the discharge no
more than 60 minutes have elapsed, in case of using a vehicle
without an agitator.
The transportation of pre-mixed concrete to the construction site
must be undertaken as fast as possible in order to minimize the
effects of stiffening and loss of workability, and to allow appropri-
ate setting and finishing after casting. Under normal conditions,
there is generally negligible slump loss during the first 30 minutes
after the initial hydration of Portland cement. When the concrete
is agitated at low speed or when it is periodically remixed, there
may occur some slump loss with time, which does not generally
cause any serious problems for the casting and setting of fresh
concrete in the first 90 minutes (Mehta and Monteiro, 2008).If that
time interval is exceeded, it is still possible to add plasticizers and/
or superplasticizer to recover the concrete’s slump for subsequent
casting and setting, as slump correction with water causes signifi-
cant damage to the mechanical properties.
The workability of concrete is the property that determines the ef-
fort required to manipulate a specific amount of fresh concrete, with
minimal loss of homogeneity. The term manipulate includeeesarly
stage operations, such as casting, setting and finishing (Mehta and
Monteiro, 2008).
The change from the fluid to the rigid state is, in general terms,
defined as setting (NEVILLE, 1995). Setting is the term used to
designate the stiffening of the cement paste, even though the defi-
nition of
stiffening
of the paste is arbitrary. According to MEHTA
and MONTEIRO (1994), what causes the stiffening of the paste
and, finally, its setting and hardening is the gradual loss of free wa-
ter due to hydration reactions, physical adsorption on the surface
of the hydration products with poor crystallinety (such as C-S-H
and ettringite) and evaporation. The beginning of the setting marks
the moment when the paste becomes unworkable. Consequently,
the casting, compacting and finishing of the concrete become dif-
ficult after this stage.
The paste does not solidify suddenly, it needs considerable time
to become totally rigid. The time necessary for the paste to totally
solidify marks the end of the setting, which should not be long, so
that other constructive activities can be resumed as soon as pos-
sible after the concrete casting (MEHTA and MONTEIRO, 2008).
The terms initial set and final set are used to arbitrarily describe
specific states of the setting. Although in the interval between the
initial and the final set, the paste acquires a certain resistance,
from a practical point of view, the development of the paste’s resis-
tance is considered after the end of the setting, when the harden-
ing starts (NEVILLE, 1995).
The standard test to determine setting times is the needle pen-
etration test. It is done almost universally by using the Vicat ap-
paratus. In Brazil, the testing method is prescribed by the NBR NM
65 (2002). The initial set time is designated as the time elapsed
between the moment water is added to the cement and the mo-
ment when the Vicat needle, with standard diameter and mass,
penetrates the standard consistency paste and stops 1 mm from
the bottom of a particular mold. Therefore, for each cement, it is
necessary to determine the water content of the paste that pro-
duces the standard slump (NBR NM 67, 1998). The final set time is
defined as the time interval between the pouring of water into the
cement and the first of three successive readings equal or higher
than 38 mm, done in the normal consistency paste, using a Vicat
needle (SILVA, 2007).
Specification norms establish the acceptable limits for each Port-
land cement designation. NBR 5733 – CPV, NBR 5735 – CPIII,
NBR 5736 – CPIV and NBR 11578 – CPII establish that the initial
set time should be equal to or over1 hour.
The reactions between cement and water are the basic
799
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
A. B. ROHDEN | D. C. C. DAL MOLIN | G. L. VIEIRA
Figure 1 – Schematic representation of the notions of the percolation theory
according to ACKER (1988) apud SILVA (2007)