Civil Engineering Reference
In-Depth Information
increases and gradients that will occur in a concrete place-
ment can be predicted by procedures that provide data for
this purpose. With this technique, measures to heat, cool,
or insulate a concrete placement can be determined and
applied to significantly reduce both micro- and macro-
cracking of the structure and assure durability. The
increasing use of these techniques will be required in most
structures using HPC to assure that the cover concrete
provides long term protection to the steel, and results in
the intended service life of the structure.
Confederation Bridge across the Northumberland Strait
between Prince Edward Island and New Brunswick has a
100-year design life (see Mix No. 2 in Table 17-3). This
bridge contains HPC designed to efficiently protect the
embedded reinforcement. The concrete had a diffusion
coefficient of 4.8 x 10
-13
at six months (a value 10 to 30
times lower than that of conventional concrete). The elec-
trical resistivity was measured at 470 to 530 ohm-m,
compared to 50 for conventional concrete. The design
required that the concrete be rated at less than 1000
coulombs. The high concrete resistivity in itself will result
in a rate of corrosion that is potentially less than 10 percent
of the corrosion rate for conventional concrete (
Dunaszegi
1999
). The following sections review durability issues that
high-performance concrete can address.
Quality Control
A comprehensive quality-control program is required at
both the concrete plant and onsite to guarantee consistent
production and placement of high-strength concrete.
Inspection of concreting operations from stockpiling of
aggregates through completion of curing is important.
Closer production control than is normally obtained on
most projects is necessary. Also, routine sampling and
testing of all materials is particularly necessary to control
uniformity of the concrete.
While tests on concrete should always be made in
strict accordance with standard procedures, some addi-
tional requirements are recommended, especially where
specified strengths are 70 MPa (10,000 psi) or higher.
In testing high-strength concrete, some changes and
more attention to detail are required. For example, card-
board cylinder molds, which can cause lower strength-
test results, should be replaced with reusable steel or
plastic molds. Capping of cylinders must be done with
great care using appropriate capping compounds.
Lapping (grinding) the cylinder ends is an alternative
to capping. For specified strengths of 70 MPa (10,000 psi)
or greater, end grinding to a flatness tolerance of
0.04 mm is recommended.
The physical characteristics of a testing machine can
have a major impact on the result of a compression test. It
is recommended that testing machines be extremely stiff,
both longitudinally and laterally.
The quality control necessary for the production of
high compressive strength concrete will, in most cases,
lead to low variance in test results. Strict vigilance in all
aspects of quality control on the part of the producer and
quality testing on the part of the laboratory are necessary
on high-strength concrete projects. For concretes with
specified strengths of 70 MPa (10,000 psi), or greater, the
coefficient of variation is the preferred measure of
quality control.
Abrasion Resistance
Abrasion resistance is directly related to the strength of
concrete. This makes high strength HPC ideal for abrasive
environments. The abrasion resistance of HPC incorpo-
rating silica fume is especially high. This makes silica-
fume concrete particularly useful for spillways and
stilling basins, and concrete pavements or concrete pave-
ment overlays subjected to heavy or abrasive traffic.
Holland and others (1986)
describe how severe abra-
sion-erosion had occurred in the stilling basin of a dam;
repairs using fiber-reinforced concrete had not proven to be
durable. The new HPC mix used to repair the structure the
second time contained 386 kg/m
3
(650 lb/yd
3
) of cement,
70 kg/m
3
(118 lb/yd
3
) of silica fume, admixtures, and had a
water to cementing materials ratio of 0.28, and a 90-day
compressive strength exceeding 103 MPa (15,000 psi).
Berra, Ferrara, and Tavano (1989)
studied the addition
of fibers to silica fume mortars to optimize abrasion resist-
ance. The best results were obtained with a mix using slag
cement, steel fibers, and silica fume. Mortar strengths
ranged from 75 MPa to 100 MPa (11,000 psi to 14,500 psi).
In addition to better erosion resistance, less drying
shrinkage, high freeze-thaw resistance, and good bond to
the substrate were achieved.
In Norway steel studs are allowed in tires; this causes
severe abrasion wear on pavement surfaces, with resur-
facing required within one to two years. Tests using an
accelerated road-wear simulator showed that in the range
of 100 MPa to 120 MPa (14,500 psi to 17,000 psi), concrete
had the same abrasion resistance as granite (
Helland
1990
). Abrasion-resistant highway mixes usually contain
between 320 and 450 kg/m
3
(539 and 758 lb/yd
3
) of
cement, plus silica fume or fly ash. They have water to
cementing materials ratios of 0.22 to 0.36 and compressive
strengths in the range of 85 to 130 MPa (12,000 to 19,000
psi). Applications have included new pavements and
overlays to existing pavements.
HIGH-DURABILITY CONCRETE
Most of the attention in the 1970s and 1980s was directed
toward high strength HPC; today the focus is more on
concretes with high durability in severe environments
resulting in structures with long life. For example, the
