Page 170 - Riem-Vol6_nº2

341 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
E. POLESELLO | A. B. ROHDEN | D. C. C. DAL MOLIN | A. B. MASUERO
2.1 Materials
2.1.1 Cement
This study used blended Portland cement (CP II Z 32). Its proper-
ties are shown in Table 1.
2.1.2 Fine aggregate
Quartz sand was used a fine aggregate, quarried from a local river
with specific mass of 2.62 kg/dm³ according to Brazilian Standard
NBR NM 45 (2006), maximum nominal size of 4.75 mm and fine-
ness modulus of 2.54 according to Brazilian Standard NBR NM
248 (2003).
2.1.3 Coarse aggregate
Two types of coarse aggregates of basaltic origin were used, iden-
tified as crushed aggregate #1 and crushed aggregate #0, with
maximum nominal size of 19mm and 9.5mm and fineness modu-
lus of 6.82 and 5.70, respectively, according to Brazilian Standard
NBR NM 248 (2003). Specific masses were determined accord-
ing to Brazilian Standard NBR NM 45 (2006), with 2.80 kg/dm³ for
crushed aggregate #1 and 2.82 kg/dm³ for crushed aggregate #0.
2.1.4 Water and admixture
Mains water from the city of Porto Alegre was used. The admix-
tures used in this research were a standard plasticizer, designed
for concrete production, with mean density of 1.05 g/cm³ (manu-
facturer’s data) and a polycarboxylate-based superplasticizer used
to correct slump, with mean density of 1.08 g/cm³, both with normal
setting times.
2.2 Test Program
Three w/c ratios and five mixing periods were used as control vari-
ables in this study. Concrete mixes were prepared in a laboratory
case that mixer trucks may have to hold their concrete load for 4
or 5 hours because of delays in transportation or unloading, which
means that the time limits specified by the standard are exceeded.
In such cases, two situations arise:
a) The concrete is accepted by the site engineer for the simple
reason that no changes in concrete temperature are noticed, in
which case it is likely that slump values will be corrected by add-
ing water. This, in turn, will affect w/c ratios and the mechanical
properties and durability of concrete, rendering this approach
unacceptable;
b) The concrete is returned to the mixing plant, which must find a
destination for this rejected concrete. This creates other prob-
lems given that this concrete reject poses an environmental
hazard. The problem is compounded by the large volumes in-
volved and the financial losses incurred.
Faced with these choices, users need to decide whether to use
concrete in these conditions. This is a serious issue as there is no
conventional wisdom regarding the final properties of concrete that
is placed after the time limit specified by the standard. For this rea-
son, and also because of the lack of data and studies that analyze
the time limit for mixing and transporting concrete, it is imperative
to advance and further expand scientific knowledge on the impact
of mixing times on the properties of concrete mixes that are used
when the time specified by the standard is exceeded.
This aim of this study was to assess the compressive strength
properties of concrete that is used when the time limit of 150 min-
utes for mixing and transporting after the first contact of cement
with water, as specified in Brazilian Standard NBR 7212, is ex-
ceeded and concrete slump is maintained by adding a polycarbox-
ylate-based superplasticizer.
2. Materials and test program
The selection and choice of the materials used in the research took
into account the reality of concrete mixing plants in the city of Porto
Alegre and surrounding areas area. All materials were assayed in
the laboratories of the Science and Technology Foundation of Rio
Grande do Sul (CIENTEC).
Table 1 – Characteristics of Portland cement compost used
Characteristics and properties Manufacturer
information
Values
obtained
Specifications by NBR 11578
(ABNT, 1991)
CaO
MgO
SO3
Loss on ignition
2 Specific surface - blaine (cm /g)
3 Specific mass (kg/dm )
Fineness on sieve nº 200
Initial setting time (min)
Final setting time (min)
Compressive strength (MPa) 3 days
Compressive strength (MPa) 7 days
Compressive strength (MPa) 28 days
1,44
6,01
2,90
4,98
4763
3,02
0,9%
184
262
27,6
33,2
41,1
–
–
–
–
4310
2,96
0,4%
185
235
27,2
31,0
43,6
–
≤ 6,5
≤ 4,0
≤ 6,5
≥ 2600
–
≤ 12,0
≥ 60
≤ 600
≥ 10,0
≥ 20,0
≥ 32,0