Page 169 - Riem-Vol6_nº2

340 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Is it possible to exceed the time limit specified by Brazilian Standard NBR 7212 for mixing and
transporting concrete?
1. Introduction
Today, considerations of cost and ease of use as well as market
requirements for improved proportioning, uniformity and homoge-
neity, much of the concrete used in Brazil is central-mixed. How-
ever, mixing plants pose problems of their own because they gen-
erate considerable amounts of waste and are therefore a cause
for environmental concern. Most concrete waste is the result of
leftover concrete that is rejected at site for failing to comply with the
time limits for placement after load into mixer trucks specified by
technical standards. Brazilian Standard NBR 7212 for ready-mixed
concrete sets a limit of 90 minutes for transport between the mixing
plant to the construction site and a limit of 150 minutes for concrete
discharge. Some of the factors that may affect the time for con-
crete use include cement hydration reactions, the onset of setting
and a reduction in workability observed in the first few hours, all of
which may make placement and consolidation more difficult.
1.1 Bibliographical review
Ready-mixed concrete should be transported to the construction
site in the shortest possible time to minimize hardening and loss
of workability and allow appropriate consolidation and finishing
operations after casting. Under normal circumstances, losses in
workability in the first 30 minutes after the hydration of Portland
cement are negligible. When concrete is kept at low mixing speeds
or remixed periodically, some slump loss may be observed after
some time, which usually poses no risk to the placement or com-
paction of fresh concrete in the first 90 minutes. Concrete work-
ability determines the amount of effort required to handle a given
amount of fresh concrete with a minimum loss of homogeneity.
Handling refers to initial operations such as placement, compac-
tion and finishing (MEHTA; MONTEIRO, 2008).
When subject to high temperatures, fresh concrete hardens faster
when compared to concrete exposed to normal conditions. This
faster setting time reduces workability during placement, com-
paction and finishing operations. In most situations, the interval
between initial and final set is reduced as curing temperatures in-
crease, a phenomenon related to the increase in cement hydration
rates, particularly in the first few instants (HEIKAL et al., 2005).
The major factors that influence concrete workability are evapo-
ration, hydration, absorption and agitation. Environmental condi-
tions may cause evaporation and hydration to accelerate over
time. (DEWAR; ANDERSON, 1992). Slump loss in fresh concrete
is a normal phenomenon and may be defined as a loss of flow
characteristics over time. This property of concrete in particularly
important for central-mixed concrete, because proportioning and
initial mixing take place in the mixing plant, while placement and/or
compaction will only take place minutes or even hours later, when
the mixer truck arrives at the construction site (WEIDMANN et al.,
2007). Prolonged mixing accelerates both hardening and the rate
of slump loss, which is a problem in most situations, particularly
when long transportation periods are involved, as is the case with
ready-mixed concrete (ERDOĞDU, 2005).
Kirca et al. (2002) investigated concrete with strength values of
25.0 and 35.0 MPa and found significant slump losses when mixing
periods increased, with greater losses reported for concrete com-
positions with higher cement consumption because of its hydration
process. An increase in compressive strength was observed when
mixing times were extended and no retempering was used. How-
ever, to make placement easier and allow suitable finishing opera-
tions, slump losses are often corrected by adding water. Teixeira
and Pelisser (2007) carried out a study in a mixing plant using
a concrete composition with a predefined strength of 20.0 MPa.
They measured strength losses after retempering and found a re-
duction of 34% in strength after 2.5 hours and of 44% after 4 hours.
Erdoğdu (2005) also observed this drop in compressive strength
after retempering was used in concrete compositions that had
been mixed for up to 150 minutes and in which water was added at
30-minute intervals to restore slump values. The drop is dramatic
in the first 90 minutes of mixing, and then slows down afterwards.
After 150 minutes of mixing, the loss of strength is greater than
40% when compared to initial values. The practice of retempering
should be abandoned as superplasticizers provide a useful alter-
native that will not affect the other properties of concrete.
Admixtures generate chemical interactions with the binders in the
concrete, thus affecting its performance both in the fresh and the
hardened state. They may be used to improve the workability of
the fresh mix or the strength and durability of hardened concrete.
Superplasticizers are a more useful alternative to other chemical
substances because of the range of improvements that can be
achieved from their use (COLLERPADI, 2005).
Kirca et al. (2002) attempted to simulate the reality of a construc-
tion site, where ready-mixed concrete is used and transported
in mixer trucks. Concrete mixes with two different compressive
strength levels were prepared in a laboratory and kept under pro-
longed mixing for up to 4 hours, with samples analyzed every hour.
The initial slump of 150 mm was restored at specific intervals by
means of four different processes: adding water only, and adding
water with three different superplasticizer concentrations (1.5%,
3.0% and 4.5% by weight of water). The addition of a superplas-
ticizer meant that less amount of water was required to restore
slump values. Therefore, the final w/c ratios of the mixes with the
superplasticizer admixture are lower than when only water was
used. Consequently, the reduction in compressive strength in
concrete where slump was restored with the aid of a superplasti-
cizer is smaller than the one observed when plain water is used. In
fact, in some cases minor increases in compressive strength were
observed in concrete adjusted with superplasticizer (particularly in
concentrations of 4.5%).
Erdoğdu (2005) found an increase in compressive strength in con-
crete that was mixed for 150 minutes and whose slump was main-
tained with the addition of a superplasticizer at 30-minute intervals.
An increase of 30% in compressive strength was observed after 90
minutes of mixing and of 10% when compared with the reference
sample, which was mixed for 150 minutes.
Superplasticizers offer a good choice to improve concrete proper-
ties, particularly when slump values need to be maintained over
time. They could be used alongside retempering, if necessary.
In real life, situations arise when it is necessary to restore slump
values. However, when water is used concrete properties are af-
fected, as several studies attest. Therefore, the use of a super-
plasticizer is a sound alternative.
1.2 Research significance
Brazilian standard NBR 7212 specifies a total time limit for mix-
ing, transporting and unloading concrete. In practice, it is often the