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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
Self-compacting concretes (SCC) – comparison of methods of dosage
amount of cement has been determined, the water/cement ratio
must be established according to local requirements and aggres-
sive environmental exposure.
In order to establish the amount of water to be added to the mix-
ture, the relevant ABNT NBR 6118:2007 [8] requirements were
fulfilled, and the aggressive environmental exposure was defined
as III, with a water/cement ratio ≤ 0.50, which was the limit used.
From Equation 4 the amount of water in the mixture is then known.
(4)
W
aC
= (
a
/
c
)*
C
Hence WaC = 196 kg/m³.
The fine material selected for the dosage was fine sand. Since the
amounts for all the other SCC components are known, the remain-
ing volume missing to complete 1 m³ belongs to the to the fine
material (Vf) which should be found through Equation 5.
(5)
V
f
= 1 -
W
g
-
Ws
-
C
-
W
aC
- V
ar
1000*G
g
1000*G
s
1000*G
c
1000*G
a
Where:
Gg - coarse aggregate specific mass;
Gs - fine aggregate specific mass;
Gc - cement specific mass;
Ga - water specific mass;
Var - amount of air incorporated into the SCC (in %).
The result obtained, 0.13, is applied in Equation 6 and thus the
final result for the amount of fine sand in the mixture is known.
(6)
W
f
=Vf * 1000 * G
af
Where:
Wf - amount of fines in kg/m³;
Gaf - specific mass for the fine material used in kg/m³;
Vf - volume of fines in the mixture.
Wf = 303 kg/m³ of fine sand in the SCC mixture.
After the amounts of all aggregates are defined and the cement and wa-
ter consumption is obtained, the percent additive is yet to be determined.
The admixture/binder ratio adopted was 0.70% (amount of solids
and liquids in the admixture), which was the value selected as an
initial parameter, although such value might be changed depend-
ing on the workability tests results.
The amount of water in the superplasticizer was considered as part
of the water from the mixture. The quantity of admixture (in kg/m³)
used in the mixture was calculated using Equation 7.
(7)
W
sp
= n%*(C)
aggregate and the aggregate total (S/t) should be verified. According
to this method, this ratio should range between 50% and 57%. Given
these figures, a 53% S/t ratio was established, and the efficacy of the
mixture for self-compactibility on the fresh state tests was analyzed.
The coarse aggregate composition was made up of 60% of 25
mm gravel and 40% of 19 mm gravel to be then employed in
Equation 1. This was the arrangement which represented the
largest number of voids.
(1)
W
g
= PF*W
gL
*(1-S/t)
Where:
Wg - amount of coarse aggregate in kg/m³;
WgL - density of coarse aggregate, in loose state in kg/m³ (1528
kg/m³ for the materials studied);
PF - loose state aggregate and compacted state aggregate ratio, in
mass (0.941 for the materials studied);
S/t - volume ratio between fine aggregate and aggregate total,
which ranges from 50% to 57% (53% adopted).
The result was Wg = 677 kg/m³ of coarse aggregate, being 406.20
kg/m³ for the 25 mm gravel and 270,80 kg/m³ for the 19 mm gravel.
The fine aggregate, i.e., regular sand, was determined through
Equation 2.
(2)
W
s
= PF*W
sL
*S/t
Where:
Ws - amount of fine aggregate in kg/m³;
WsL - density of fine aggregate, loose state, in kg/m³ (1470 kg/m³
for the materials studied);
PF - loose state aggregate and compacted state aggregate ratio,
in mass (0.902 for the materials studied);
S/t - volume ratio between fine aggregate and aggregate total,
which ranges from 50% to 57% (53% adopted).
The result was Ws = 703 kg/m³ of fine aggregate.
After having established the amounts of gravel and regular sand,
the amount of cement is determined. So as to obtain the amount of
cement per m³, it is first necessary to adopt the required compres-
sive strength in MPa (f’c) at 28 days.
The f’c selected for the calculation was 55 MPa. Therefore the
compressive strength of the SCC at 28 days is supposed to be
55 MPa. From Equation 3 the cement consumption for the mixture
can then be known.
(3)
C = f c
0,14
Where:
C - cement consumption in kg/m³;
f’c - required compressive strength in MPa.
Hence the cement consumption was set at 393 kg/m³. After the