World Journal of Engineering and Technology, 2014, 2, 48-54
Published Online September 2014 in SciRes.
How to cite this paper: Löber, P. and Holschemacher, K. (2014) Structural Glass Fiber Reinforced Concrete for Slabs on
Ground. World Journal of Engineering and Technology, 2, 48-54.
Structural Glass Fiber Reinforced Concrete
for Slabs on Ground
Philipp Löber, Klaus Holschemacher
University of Applied Sciences, Institute of Concrete Construction, Leipzig, Germany
Received Ju ly 2014
This paper aims to contribute to the classification and specification of glass fiber reinforced con-
crete (GFRC) and to deal with the question if structural glass fiber reinforced concrete as a special
kind of glass fiber reinforced concrete is suited for use in load-bearing members. Despite excellent
material properties, the use of glass fibers in a concrete matrix is carried out so far only in non-
structural elements or as a modification for the prevention of shrinkage cracks. The aim of re-
search at the University of Applied Sciences in Leipzig is the use of alkali-resistant macro glass fi-
bers as concrete reinforcement in structural elements as an alternative to steel fiber reinforce-
ment. Slabs on ground, as an example for structural members, provide a sensible application for
the new material because they can be casted as load bearing and non-load bearing and are mostly
made of steel fiber reinforced concrete. In the future, structural glass fiber reinforced concrete
shall provide a simple and visually appealing alternative to conventional steel bar or steel fiber
reinforced concrete. The glass fibers can also be used in combination with conventional reinforc-
ing bars or mat reinforcements. Initial investigations have announced some potential.
Glass Fiber Reinforced Concrete, GFRC, Slab on Ground, Structural Concrete, Bending Test
1. Introduction
Approximately 25% of all industrial floors were made of steel fiber reinforced concrete [3 ]. Hence, steel fibers
have been successfully used for foundations such as slabs on ground for years. They made their way into several
other fields of civil engineering such as tunnel or bridge construction. So far, the use of glass fibers as rein-
forcement material for load-bearing concrete structures has failed because there are no suitable fibers for normal
concretes, which are able to transmit forces reliable, durable and can be mixed economically in large quantities.
An ongoing research project at the University of Applied Sciences is intended to examine whether special ma-
croglass fibers can be introduced reliably in vibrated concrete and are as composite material suite d for the u se in
structural members especially slabs on ground.
2. State of the Art
Due to the use of glass fibers in a concrete matrix a lot of properties such as crack distribution and crack devel-
P. Löber, K. Holschemacher
opment in the concrete can be improved. Glass fibers are ab le to impro ve the fle xural st rengt h due to t heir hi gh
tensile strength of 1.700 - 3.700 N/mm 2 and their good bond with the cement matrix. However, this effect ap-
pears only at very high fiber contents from 3% - 5% of concrete volume [5]. In these ratios the fibers cannot
anymore mixed in the concrete because of the loss of consistency and bad fiber distribution. Due to their rela-
tively low stiffness co mpared to steel fibers, glass fibers are able to bridge very small cracks and to support the
conc re te al re ad y dur in g se tti n g ( flow of hyd r at io n hea t a nd shrinka ge ) and contrib ute to its i mpermeabilit y. Con-
crete covers as well as minimal cement contents must not be ensured. Therefore GFRC is used for thin elements
or for repair of existing components. Examples for usage are prefabricated elements in façade construction,
noise barriers, place formwork, fire-resista nt panels, design elements in the i nterior or for the renovatio n of old
floors as glass fiber modified concrete [1]. In Germany there are a few standards dealing with test methods for
glass fiber reinforced concrete [9]. These standards are used for testing thin panels of traditional glass fiber
reinforced concrete with high fiber concrete. For struc tural use of GFR C no rul es exis t. At the moment slabs on
ground are constructed with reinforcement bars, steel fiber reinforced concrete, with combined reinforcement
(bars and steel fibers) or are made pre-stressed. If sla bs on ground may also be cons t ructed with struc tural GFRC,
has still to be inve stigated.
Structural Glass Fiber Reinforced Concrete-Conceptual Alignment
The term glass fiber reinforced concrete is often used in civil engineering representative of a whole group of
composite building materials and has to be differentiated with respect to material be havior [5]. A distinction is
made between [3]:
textile reinforced concrete (use of glass fiber rovings (continuous fibers) as reinforcement, analogous to bar
or mat reinforcement);
glass fiber modified concrete (using microfibers of 12 - 30 mm length, produced in mixing, good for preven-
tion of shrinkage cracks in concrete applications, fiber content up to 1 vol%) and;
(traditional) glass fiber reinforced (fine) concrete or mortar (using macro fibers up to 50 mm length in spray-
ing metho d for very thin GFRC applications, fiber content up to 5 vol%).
Structural glass fiber reinforced concrete shall be the fourth kind of GFRC wherein the fibers, similar to glass
fiber modified concrete in the form of short macrofibers are mixed with a three-dimensio nal arr ange ment i n the
concrete matrix. Due to excellent mechanical properties, glass fibers (see Table 1) can be used as reinforcing
material in the concrete. However, as usual they have to have a high durability in typical concrete alkaline envi-
ronment. Therefore o nly al kali re sistan t fibe rs are suited .
In structural GF RC integral AR-macr o glass fibers with a le ngth of 36 mm and a slenderness of 67 are used.
As matrix, normal vibrated concrete with a maximum grain size of 16 mm was used in former tests. Structural
GFRC should be economical and easy to prepare, that is why it has to be produced in mixing process and shall
be transported via pump to the respective point of use. That is the reason why the fibers can be interfered only in
s mal l a moun t s in t he mixin g pr oce ss due to the ir le n gth. App ro ximatel y fi ber co ntent s up to 1 5 kg/ m3 are po ssi-
ble with this kind o f pro duction. Glass fiber s reac t sensitive and br ittle at tra nsverse press ur e. This is due to their
low stiffness and the composition of individual filaments. Therefore, it is important to restrict the mechanical
impacts duri ng mixing to a minimum. In structural glass fiber reinforced concrete, the fibers are not used to in-
crease the te nsile strength of the composite buildi ng material, but to make the concrete mo re ductile and give it a
post cracking tensile strength as it is known from steel fiber reinforced concrete with a strain-softening material
behavior after concrete cracking. This shall be su fficient for use in statica lly highl y indet er mina te str uct ures l ike
slabs on ground. In these members structural GFRC can represent a visually appealing alternative to steel fiber
reinforced concrete. It is resistant to corrosion and can normally be used without surface protection system also
because fibers coming out of the surface are harmless.
3. Investigations
At the University of Applied Sciences, Leipzig, four test series with 14 specimens each were carried out. The
mix design was done on experiences with steel fiber reinforced concrete (see Table 2). The four series contain
two different fiber contents (5 and 7 kg/m3) e ac h wit h t wo d if fer e nt mi xi ng ti mes ( 4 5 a nd 1 80 sec) after addition
of fibers. Seven specimens of each series were tested in three- and fo ur-point bending tests according to G e rma n
[2] and European [6] regulations, respectively. The number of specimens has to be at least six [2] on the basis of
P. Löber, K. Holschemacher
Table 1. Mechanical properties of dif ferent fib er materials [8] .
Property Materials
AR-Gl ass St eel PVA (plastic) Carbon fiber
Density, g/cm3 2.68 7.85 1.30 1.80
Tensile strength, N/mm2a 1500 - 3700 1000 - 2600 880 - 1600 3000 - 5000
Modulus of elast ic ity, N/mm2 72,000 200,000 30,000 230,000
Ultimate strain, % 2.0 2.5 - 10 8 1.50
a. Tensile strengths vary depending on the fiber material composition and the type of sample the test was carried out. Tensile strengths of fiber
filaments are much higher compared with fib erbundles such as strands or rovin gs.
Table 2. Used GFRC mixtures.
Ingredients Series
I (5 - 180) II (7 - 180) III (5 - 45) IV (7 - 45)
San d 0/ 2, kg /m3 732 731 7 27 7 26
Gr a vel 2/8, kg /m3 465 465 4 62 4 62
Gr a vel 8/16 , kg / m 3 670 670 6 66 6 65
CEM I 42.5 R, kg/ m3 300 300 3 00 3 00
Water, l/m3 180 180 1 85 1 85
Superplast icizer , % 0.75 1 1 1
Water/ C em en t 0 .6 0.6 0.62 0.62
Fibe r c o nte nt kg /m 3 5 7 5 7
Mixing time, sec 180 180 45 45
statistical rea so ns. Table 2 shows the investigated versions.
3.1. Materials
The matrix used for the glass fibers was a normal weight, vibrated concrete (C25/30) with a maximum grain size
of 16 mm and a grading curve A/B 16. As additives only superplasticiser “Muraplast FK 43” was used to
achieve a flowable consistency (F3 or higher, [7]). The fibers used were integral AR-macro glass fibers with a
length of 36 mm and a slenderness of 67. Cement and aggregates were first placed in a pan forced mixer and
mixed for 30 s. Water was then added, followed by flow agent, and finally the fibers. The mixing times of 180 s
and 45 s are related to the time after the addition of glass fibers. The mixtures used are listed in Table 2. The
slump in each case was around F3 and the air content at around 3.5%. For hardened concretes properti es the
compressive strength, splitting tensile strength and the residual flexural strength according to German and
European regulations were measured. The aim was to achieve a C25/30 which was given with an average
compressive strength of 36 N/mm2.
3.2. Bending Tests
Normally, the flexura l bearing capacity of glass fiber reinforced concrete according to [4] is determined on thi n
plate stripes in four-point bending tests. However, the tests described are based on glass fiber reinforced fine
concrete or mortar. As part of the research program at the University of Applied Sciences, Leipzig, it was de-
cided to perform these tests in accordance with the guidelines for steel fiber reinforced concrete [2] and [6]. A
total of 56 fiber concrete beams have been tested on their structural behavior, especially after the first peak.
With a defined speed the deformation fields 1 and 2 were covered and recorded the load-deflection or load-
CMOD curves. These are used to determine the residual tensile strengths and the performance classes according
to [2]. The tests differ in the definition of the deformation fields for the SLS and ULS, specimen geometry,
loading r ate and evaluation of results.
P. Löber, K. Holschemacher
3.2.1. German Regulatio n “DAfStb Guideline for Steel Fiber Reinforced Concrete”
The investigation in accordance to the German guideline from the German Committee for Reinforced Concrete
Struc t ure s ( DAfStb) is done with fiber concrete beams with dimensions of 70 × 15 × 15 cm. The clear span is 60
cm and the displacement-controlled loading is done in the third points (see Figure 1).
Deformation field 1 stands for the SLS and describes a deflection of 0.5 mm at midspan. Up to 0.75 mm th e
loading speed is 0.1 mm/min and afterwards up to 0.3 mm/min. Defor mation field 2 (UL S) ends at 3 .5 mm de f -
lection. The average load-deflection curves of all specimens are shown in Figure 2.
Steel fiber reinforced concrete in Germany is divided in the so-called performance classes. These classes d e-
scribe the residual strengths for defined deflections of the composite material and are needed for the design
process. Therefore the characteristic value of the residual flexural stren gth for the two defor mation fields is de -
termined. The average value of the residual flexural strength of at least n = 6 beams is give n by:
,1 2
cflm Liii
For deformation field 2 F3,5 has to be used instead of F0,5. F0, 5 a nd F3,5 are the forces corresponding to deflec-
tions of 0.5 and 3.5 mm.
The chars l, b und h sta nd for the length, wide a nd he ight o f the specimen. The characteri stic value of t he re -
sidual flexural strength is then calculated wi th:
cfml Li
Lfk Ls
cflk Licflm Li
fe f
= ≤⋅
Lffcfml,Li i s the average value of the logarithmized single test values ffcfl,Li,I with
( )
, ,,
1/ ln
cfmlLicflLi i
Lfn f= ⋅
, (3)
Ls is the standart deviation of the logarithmized si ngle test values of the series with
( )
( )
, ,,
cflm LicflLi i
Lf f
Ls n
and ks the fractile factor for unknown standart deviat ion for the 5% -quanti le with 75% pr opability. If six speci-
mens are taken, ks is 2336. The corr esponding strengths are given in Figure 3.
3.2.2. European R egula tion “EN 14651 or RILEM TC 162-T DF”
The tests in accordance with EN 14651 are done with fiber concrete beams with dimensions of 55 × 15 × 15 c m
and up to 5 mm wide notch at the bottom in the middle. The clear span is 50 cm and the displacement-controlled
loading is applied at midspan (see Figure 4). In comparison to the four-poi nt b end i ng t e st no t t he d e flectio n, but
the CMOD (crack mouth opening displacement) is taken into evaluation. Alter natively, if no t measured, CMOD
can be calculated linearly out of the deflec tion.
The deformation field 1 is up to a CMOD of 0.5 mm. The loading rate is 0.05 mm/min until 0.1 mm d eflec-
tion. From this point, the loading rate can be increased to 0.2 mm/min. The deformation field 2 ends at 2.5 mm.
In addition, the test should be continued up to a crack opening of 4 mm. Figure 5 shows the load-crack opening
curve of the mean value curves of all test series.
The residual flexural strength fR,j according to [6] is determined as follows:
For the calculation of the limit of proportionality (LOP) ffct,L insert FL instead of Fj. The corresponding
stre ngth s are s hown in Fi gure 6.
3.3. Evaluation
The evaluation showed, that the concrete in region 2 (ultimate limit sta te) has insufficie nt load be aring capacity
to substitute normal reinforcement. In region 1 (limit state of serviceability), however, the Series II (7 - 180)
P. Löber, K. Holschemacher
Figure 1. Experimental setup: four-point bending test according to German guide-
line [2].
Figure 2. Average load-displacement curves of the four-point
bending tests.
Figure 3. Characteristic flexural strength values according to
Figure 4. Experimental setup: three-point bending test according to RILEM
guideline [6].
P. Löber, K. Holschemacher
Figure 5 . Average lo ad -di spl acement curves o f the th ree-point bending
Figure 6 . Charact er istic flexural str ength values according to [6].
with performance class 0.4/0 and IV (7 - 45) with performance class 0.6/0 announced some potential. The Ger-
man guideline allows the use of this fiber concrete for plane members (width > 5 times height).
Similar conclusions occurred after evaluation according to EN 14651. The influence of mixing time after ad-
dition of fibers on the load bearing capacity was quite significant. Using wash o ut test s it was found, that the fi -
ber damage is obviously increased with increasing mixing time. This statement was confirmed in the evaluation
of the load-displacement curves. In both diagrams, an increase of loading capacity after first crack can be seen
whe n using higher fiber contents. The influence o f mixing t ime on t he resid ual strengths is e ven mo re signi fica nt.
Whereas a 40% increase of fiber content results in a 59% increase of flexural strengths, an increase of 144%
could be obtained by reduction of the mixing time to a quarter with the same fiber content. The influence of
mixin g time o n the res idual te nsile str ength d ecrease s with i ncreasi ng fiber content , but pl ays an i mport ant role
in the production of load-bearing parts consisting of glass fiber reinforced concrete. A minimum mixing time,
howe ver, with rega rd to a uni form fi ber d istrib ution has to be ensured. Therefore balance between a uniform fi-
ber distribution and a minimal fiber damage degree by the mixing process has to be found.
4. Conclusion
The focus of research at the University of Applied Sciences, Leipzig is the design of a glass fiber reinforced
conc rete for str uct ural c o mpone nts a nd the stud y o f the sui ta bili ty o f this co ncre te i n slab s o n gr ou nd. T he men-
tioned tests were a ll carried out with normal concrete as the matrix. Further tests should show whether a perfor-
mance gain can be achieved with a different concrete mixture as well as other fiber contents and mixing times.
At the moment it i s still d ifficult to detec t the fail ur e mec ha ni s m o f t he ind i vid ua l fi b er s. T he goa l i s a fai l ur e by
fiber pull-out, to have ductile failure of the member. However , it turns out that it is difficult to detect and dete r-
mine whether a glass fiber is broken or pulled out of the matrix. The reason is the very slight color difference
from the matrix and the fact that the fiber surface after testing appears very worn.
P. Löber, K. Holschemacher
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suring Bendi ng Strength. Complete Bending Test Meth od , German Vers ion .
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(Limit or Proportionality (LOP), Residual), German Version.
[7] (2009) EN 12350-5:2009: Testing Fresh ConcretePart 5: Flow Table Test, German Version.
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