1. Purpose of the experimental investigation
In series 1 basic mechanical material properties of cement mortars with 3 types of basalt fibres were investigated and compared to the properties of cement mortar without fibre addition and with addition of glass fibres made of alkali resistant glass (AR-glass). The mechanical tests performed were:
- bending test,
- uni-axial compression test
All tests were performed in a sample age of only 14 days. The length of all 3 types of chopped basalt fibre was 6.4 mm. The types of basalt fibres selected for investigation as well as its fibre volume fraction results from the investigation in series 0. For comparison, 2 types of AR-glass fibres with chop length of 6 mm were used as well for matrix modification. The fibre volume fraction of AR-glass fibres was adapted to the maximum volume fraction which was reached in case of basalt fibres.
2. Fibre Material
In Table 1 geometrical and further properties of the basalt fibres and AR-glass fibres used in the tests were reported. The basalt fibres were produced by Kammeny Vek company, the AR-glass fibres are made by Owens Corning Company.
3. Matrix composition
The cementitious matrix used in the mechanical tests of series 1 was the same as used in series 0 – tests. The matrix is characterised by high binder content and maximum aggregate diameter of 1 mm. The binder is composed of CEM III/B, fly ash and microsilica. The amount of chopped basalt fibres added to the matrices resulted from the tests in series 0 and were 0.8 Vol.-% for KV05/1-6.4, 1.2 Vol.-% for B1.5-6.4 and 0.8 Vol.-% for KV12-6.4. The bulk density of fibres is assumed to be 3 g/cm³ for all 3 fibre types. The amount of chopped AR-glass fibres CF70-6 and CF 62-6 was chosen according to the maximum fibre volume fraction of basalt fibres by 1.2 Vol.-%. The bulk density of AR-glass fibres is 2.7 g/cm³ for both fibre types. These chosen fibre volume fractions of AR-Glass fibres are not yet the maximum amount of fibres which can be added to the matrix. In case of CF70-6 the fibre fraction can be increased up to approx. 1.5 Vol.-%. When using CF62-6 the maximum fibre volume fraction is approx. 1.8 Vol.-%. For all matrices the plasticizer Glenium ACE 30 (BASF company) was used to control the rheological properties of fresh mortar. The amounts of fibres as well as of plasticizer were not considered in the volume calculation of mortars. In Table 2 the matrix compositions are indicated.4. Sample Production, Sample Curing, Sample Batches
The matrix volume produced for each mixture was 4.5 litres. The mixer used for
mortar mixing was a laboratory mixer as fabricated from Hobart Company (cf.
Figure 1, Figure 2). The mixer provides 3 different mixing speeds referred as I, II
and III. Speed I correspond to 120 rotation per minute (rpm), speed II to 200
rpm and speed III to 380 rpm. The mixing procedure was as described following:
· Mixing of cement and fly ash at speed I for approx. 10 seconds,
- · Addition of water, mixing at speed I for 30 seconds,
- · Addition of microsilica, mixing at speed I for 60 seconds,
- · Addition of sand, mixing at speed I for 30 seconds,
- · Addition of superplasticizer, mixing at speed II for 60 seconds,
- · Addition of fibres, mixing at speed I for 10 seconds.
- · After homogenization of fibres mixing at speed III for 15 seconds.
Immediately after mixing the slump of fresh fibre-mortar-mix was determined according DIN EN 1015-3. For a short description of the test device as well as the test procedure refer to the series 0 - report. After that the fresh mortar was filled into the moulds. At one time up to 3 bending test specimens (prisms 160x40x40 mm; cf. figure 3) were produced. The bending test specimens were as well used for compression tests. After filling the moulds were vibrated for duration of 30 seconds by a vibration frequency of 50 1/s. Additional, the air volume content of fresh mortar was measured according DIN EN 12350 - T7 for the basalt fibre mortars and the reference mortar. The air content measuring box was vibrated the same time and with the same intensity as the mortar filled moulds. The specimens were un-moulded one day after casting and stored in water at 20°C. At a specimen age of 7 days the samples were removed from the water and stored in air at 20 °C and at 65 % rel. humidity until the testing in sample age of 14 days. 5. Mechanical Test Setup
5.1. Bending Test
The bending tests were performed as well as 3-point bending tests. The span of specimens was 100 mm and the loading point was at the mid span of specimens. The tests were performed with a displacement rate of 0.5 mm/min, controlled at the bottom surface of specimen. The measuring pin was pressed by a constant load at the bottom surface of specimen, counter-acting to the dead weight of specimen half parts after crack initiation. During the test the load, the cross head displacement and the mid span displacement of bottom surface of the specimen
were recorded. The recorded mid span displacement of specimen’s bottom surface contains only the elastic deformation of support beam (made of steel) and can be nearly considered as specimen deformation. A linearization-shift of measuring values was done to exclude non-linear deformations of mechanical parts of bending test setup.
5.2. Uni-axial Compression Test
For the compression tests were used the half parts of bending specimens after bending test. The tests were performed according DIN 18555 – T3. During compression tests no continuously recording of load or deformation were done. Only the maximum compression load was recorded.
6. Results of Basalt Fibre Mortars and Reference Mortar
6.1 Fresh matrix properties, density of hardened mortar
The slump test values of two subsequent performed tests, the air content of fresh mortars and the density of hardened mortars are indicated in Table 3. In Figures 4 – 7 one of both “slump cakes” is pictured for each mortar. In case of M-KV05/1 and M-KV12 the slump was close to the maximum slump. A further addition of plasticizer leaded to pronounced “bleeding” of mortars (“bleeding”: separation of fine binder components from aggregates and coarser
binder particles due to “over-plasticizing”). In case of M-B1.5 the addition of a higher amount of plasticizer would be possible to reach a softer consistency – or the addition of higher fibre volume content and higher amount of plasticizer at the same slump flow of approx. 150 to 160 mm. The air content in all fresh mortars with fibre addition is very high compared to ordinary concretes (approx. 2 to 3 Vol.-%) as well as to the reference mortar MRef. Therefore the density of hardened mortar with basalt fibres was relatively low with approx. 1.9 g/cm³.
6.2 Bending Test
The results measured during bending tests are pictured in Figures 8 – 11. For each fibre type 6 prisms were tested. In Table 4 characteristic values (bending strength and the mid-span deformation at bending strength) are indicated for each specimen as well as the average values and standard deviations. The mortars with fibre addition M-KV05/1, M-B1.5 and M-KV12 show a higher bending strength compared to the reference mortar M-Ref without fibres. The
highest bending strength was measured at M-B1.5 specimens, followed by mortar M-KV12 and M-KV05/1. For all basalt fibre mortars (M-KV05/1, M-KV12 and M-B1.5) ductile post crack behaviour could be shown. The shapes of the beginning descending branches are different: KV05/1 and M-KV12 posses a more “sharp” tip of the curve than MB1.5 where the tip is less curved. In case of M-B1.5 this indicates a favourable partial fibre de-bonding or fibre pull-out during crack widening and crack propagation. This is one reason for the high energy absorption of M-B1.5 after crack initiation compared to KV05/1 and M-KV12. The other reason is the higher maximum bending load of M-B1.5. At all specimens only one crack was formed underneath the load transmitting facility in specimen’s middle.6.3 Compression Test
The results of compression test on the half parts of bending specimens areindicated in Table 5. The scattering of the results is low. The fibre reinforced
mortars M-KV05/1 and M-KV12 posses a higher compression strength than the
reference mortar M-Ref. The strength of mortar M-B1.5 and M-Ref are similar.
7. Results of AR-Glass Fibre Mortars
7.1 Fresh matrix properties, density of hardened mortar
The slump test values of fresh mortars and the density of hardened AR-glass
fibre mortars are indicated in Table 6. The slump of AR-glass fibre mortars was not yet close to the maximum slump. A higher amount of plasticizer would be possible to reach a softer consistency – or the addition of higher fibre volume content and higher amount of plasticizer to get the same slump flow value. The amount of chopped AR-glass fibres CF70-6 and CF 62-6 was chosen according to the maximum fibre volume fraction of basalt fibres by 1.2 Vol.-% (cf. section 3).7.2 Bending Test
The results measured during bending tests are pictured in Figures 12 and 13. For each fibre type 6 prisms were tested. In Table 7 characteristic values (bending strength and the mid-span deformation at bending strength) are indicated for each specimen as well as the average values and standard deviations. The mortars with addition of glass fibres show a relatively high scattering of curves instead of good workability of the fresh mortars (slump flow 170 and 180
mm, cf. Table 6). Both mortars show clearly a post cracking behaviour after single crack initiation underneath the loading point in specimen’s middle. In case of mortar M-CF70 the crack bridging behaviour of disperse AR-Glass fibres only allow a softening post crack branch of load-deflection curve (cf. Figure 12). In contrast after crack initiation the integral AR-glass fibres in mortar M-CF62 enables at low deformations (deflections between approx. 0.05 and 0.3 mm) a rising curve branch (cf. Figure 13). Therefore the bending strength of M-CF62 is higher compared to M-CF70. Regarding the crack initiation stress mortar M-CF70 shows higher average values (approx. 6.3 MPa) compared to mortar M-CF62 (approx. 5.7 MPa). Caused by the specific crack bridging behaviour of integral AR-Glass fibres mortar M-CF62 show the highest energy absorption of all tested mortars.
7.3 Compression Test
The results of compression test on the half parts of bending test specimens equipped with AR-glass fibres are indicated in Table 8. The scattering of results is low. Mortar M-CF70 shows lower compression strengths than mortar M-CF62. The average compression strength of both glass fibre mortars is lower than the average compression strength of reference mortar M-Ref (cf. Table 5).8 Results Discussion
Generally, both Cem-FIL and Basfiber products have shown the expected behaviours. There are two main applications of fibre reinforcement of concrete –
1. Improvement of mechanical properties (strength, toughness)
2. reduction of susceptibility to concrete cracking
For the first application integral fibres are used, such as CemFIL CF62 or Basfiber
B1.5. Testing results of mortars with these fibres showed ductile, strain softening
character of failure during bending tests and high energy absorption compared to
mortars without fibre addition. The second application requires highly dispersible fibres, such as CemFIL CF70 or Basfiber KV05 and KV12 respectively. Mortars with these fibres also show a change of failure type from brittle to ductile and strain softening, but in less degree. The comparison of the results obtained for similar types of fibres is presented in Table 9 and Table 10. This is obvious from the comparison, that the results of basalt fibres and AR-glass fibres are more or less identical. The only high difference observed is the increase in compression strength in case of mortars with disperse basalt fibres (KV05 and KV12).9 General Comment
All mechanical tests of mortars were performed in sample age of only 14 days. The used binder composition shows a relatively slow hardening. This is caused by
the use of CEM III cement, where the use of grounded blast furnace slag leads to
a reduced hydration speed. The continued hydration will influence the morphology of the interphase between fibres and binder. Thus an increased
crack initiation stress and more brittle post crack behaviour are to expect in
higher specimen age. In addition the chemical stability of fibres bulk material, its
sizing as well as further durability issues can be not traced back from these tests
in age of 14 days.
10 Conclusion
Based on performed test it is possible to conclude that in relatively young mortar
or concrete age (some weeks after mortar or concrete mixing) basalt fibres
Basfiber® are suitable for concrete reinforcement – as integral fibres in the same
applications as Cem-FIL 62 and as disperse fibres in the same applications as
Cem-FIL 70.
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