07-12-2012, 01:34 PM
THE USE OF GLASS FIBER–REINFORCED CONCRETE AS A STRUCTURAL MATERIAL
A STRUCTURAL MATERIAL.pdf (Size: 517.29 KB / Downloads: 228)
INTRODUCTION
Glass fiber–reinforced concrete (GRC) consists basically
of a cementitious matrix composed of cement,
sand, water, and admixtures, in which shortlength
glass fibers are dispersed. The effect of the
fibers in this composite leads to an increase in the tension and
impact strength of the material (Bentur and Mindess1). GRC
has been used for over 30 years in several construction elements,
mainly nonstructural ones, like facade panels (about
80% of the GRC production), piping for sanitation network
systems, decorative nonrecoverable formwork, and other
products (Bentur and Mindess1).
In the beginning of the GRC development, one of the most
concerning problems was the durability of the glass fibers,
which became fragile with time, due to the alkalinity of the
cement mortar. Since then, significant progresses have been
made, and presently, the problem is practically solved with
the new types of alkali-resistant glass fibers and with mortar
additives that prevent the processes that lead to the embrittlement
of GRC (Bentur and Mindess1, Majumdar and
Ryder2, Cem-FIL3, Liang et al.4).
The light-weight characteristics and improved tensile
strength of GRC as compared with concrete led to a recent
research program to study the viability of its use as a structural
material (Ferreira,5 Branco et al.,6–8 Branco,9 Viegas,10
Cian and Della Bella11). The research was developed in association
with concrete precast companies for which the referred
improved characteristics are especially appealing as the
reduced weight of the precast elements is important for transportation
and installation. To obtain a GRC with high durability,
reinforcement systems were also analyzed, considering
carbon or glass strands and stainless steel bars, leading to
corrosion-free solutions (Ferreira5).
PRODUCTION OF GRC
There are two main production techniques of GRC, usually
referred as spray-up and premix (Bentur and Mindess,1 and
Cem-FIL3). In the spray-up process, the mortar is produced
separately from the fibers, which are mixed only at the jet of
the spray gun. The glass fiber strands are cut within the spray
gun to the required size, typically between 25 mm (0.98 inch)
and 40 mm (1.57 inch), and are about 5% of the GRC total
weight. The subsequent compaction with a cylindrical roll
guarantees the adaptation of GRC to the form, the impregnation
of the fibers within the mortar, the removal of the air
retained within the mix, and an adequate density.
In the GRC production method by premixture, mortar and
precut fibers are previously mixed. The quantity of fibers
added to the mortar is usually up to 3.5%, in terms of weight,
and the length of the fibers is around 12 mm (0.47 inch).
Longer fibers lead to an excessive reduction of the mix’s workability.
Production with premix GRC may involve several procedures
such as injection and vibration, pressing, or
shotcreting (Fig. 1).
Compression Strength
Compression strength was obtained with spray-up and premix
specimens. The compositions of each production technique
were optimized, based on former experience and on
workability tests. The specimens were tested according to
the national standard LNEC E226.18
Four series of specimens were tested (Table 2). Spray-up GRC
mortar was identical to the one used for Young’s modulus
determination, while premix GRC mortar had the following:
white cement type BR I 42,5R, 100 kg; sand, 67 kg
(148 pounds); polymer Primal MC 76 S, 1.8 L (110 inch3);
fluidizer type Sikament: 163, 1.0 L (61 inch3); and water,
29 L (1770 inch3).
The plain mortar specimens (without fiber reinforcement) had
an explosive rupture, while the GRC ones, despite the crack
pattern, almost maintained the initial shape at rupture,
denoting a much more ductile behavior (Fig. 3). This distinct
type of GRC behavior, when compared to that of the plain
mortar, is relevant for structural use and will be highlighted
in the analysis of the stress–strain diagrams.
TENSILE STRENGTH TESTS
The tensile behavior of GRC is one of the most important
parameters when considering its structural use. It has been
recognized (Chanvillard19 and Banthia et al.20) that the standard
flexural tests do not provide reliable values of tension
strength and should be used mainly for quality control. On the
other hand, pure tensile tests are not usually performed
because of their operative difficulty. The tensile tests presented
in this paper were performed using a tension-testing
machine (Fig. 5) with controlled pressure hydraulic grabs.
The specimens were 30 cm (11.8 inch) long, generally with
cross-section of 1 3 5 cm (0.39 3 1.97 inch). A large number
of tests were carried out to analyze different aspects, namely,
production techniques, compositions, aging, and continuous
reinforcement.
Plain Spray-Up GRC
The composition used in the spray-up GRC specimens was
equal to that referred in Compression Behavior Tests. The
different series are distinguished by the type, quantity (percentage
of total weight) and length of dispersed fibers, and
type of sand (regular or sieved).
The two different types of fibers correspond to Cem-FIL fiber
roving designated, respectively, as Cem-FIL 53/76 and Cem-
FIL 250/5. The first type is the most commonly used in GRC
hand-spray process, while Cem-FIL 250/5 presents improved
long-term strength.
Within each series, all the specimens have identical characteristics.
Table 3 presents the results (average [fm] and characteristic
[fk] tension strength values) for each series within
each group.
CREEP BEHAVIOR TESTS
Because some of the structural uses of GRC include prestressed
elements, evaluation of creep behavior was required
to evaluate long-term losses. For this purpose, two specimens
were tested during 5 months under a constant load. To ensure
the stability of applied compressive stress, the specimens
were subjected to a gravity load of 85 kN (19,109 pounds),
as shown in Fig. 8. The strain variation in a control specimen
was also measured to evaluate the strain component due to
shrinkage and temperature variation.
To evaluate the creep coefficient for loads applied in different
ages, two specimens were loaded, one 8 days after production
(S1) and the other 28 days afterward (S3). With this procedure,
the creep coefficient for loads applied on the 8th
and 28th days was obtained. The test procedures were based
in the national standard LNEC E399,22 for creep evaluation
in concrete, and LNEC E398,23 for shrinkage evaluation of
concrete.
CONCLUSIONS
An experimental test program carried out on small specimens
allowed for the assessment of the main mechanical characteristics
of GRC concerning its structural use. The GRC compositions
of the final tested specimens were achieved based on
an optimization of the fabrication procedures and on previous
test results.