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Potential of structural pozzolanic matrix–hemp fiber grid composites
Construction and Building Materials 25 (2011) 2867–2874
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Potential of structural pozzolanic matrix–hemp fiber grid composites Domenico Asprone a,⇑, Massimo Durante b, Andrea Prota a, Gaetano Manfredi a a b
Department of Structural Engineering, University of Naples ‘‘Federico II’’, Naples, Italy Department of Materials and Production Engineering, University of Naples ‘‘Federico II’’, Naples, Italy
a r t i c l e
i n f o
Article history: Received 1 September 2010 Received in revised form 13 December 2010 Accepted 24 December 2010 Available online 17 January 2011 Keywords: Hemp fiber Pozzolanic mortar Sustainability Inorganic composite Structural retrofit
a b s t r a c t Currently, sustainability represents a primary issue for construction industry. New material and technological solutions are widely proposed and investigated to meet sustainability requirements and natural fibers represent one of the most studied materials. The work presented here investigated the mechanical behavior of a sustainable composite system made by pozzolanic mortar reinforced with hemp fiber grids. To improve the durability of the system and in particular of the fibers in the pozzolanic mortar environment a latex coating was used. The objective of the study was to investigate the mechanical behavior of the proposed composite system and assess the feasibility of using the system for structural retrofit applications on existing structures. A mechanical characterization of the fibers was conducted and the effectiveness of the latex coating in improving the durability of the fibers was investigated. The mechanical behavior of the composite system was studied, through a three-point bending test program. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Low environmental impact of operations on built environment is often a primary condition to be respected by construction designers and operators. Thus, recently, new technological solutions are proposed and new materials are investigated and used [1,2]. For this reason, in recent years, natural fibers have been widely investigated, to be used as an alternative to carbon, glass or plastic fibers, in several composite applications for construction industry. In fact, given their low environmental impact both in production and in disposal phase, natural fibers represent a highly ‘‘sustainable’’ material. Furthermore, natural fibers can be locally supplied, ensuring a sustainable production chain. The recent increasing scientific interest in natural fibers as a component of construction applications is also due to the good mechanical properties exhibited by natural fibers. Available literature provides the mechanical characterization of natural fibers, in terms of elastic properties and tensile strength [3–5]; in particular, attention has been focused on different fibers, e.g. flax [6], jute [7,8], hemp [9,10], sisal [11]. The available reviews [3,5] reports the main mechanical properties of various natural fibers. It can be observed that the tensile strength can reach more than 1000 MPa, in case of flax fiber and vary from about 400 MPa to 800 MPa in case of jute fiber, whereas hemp fiber exhibits a tensile strength of 690 MPa. Furthermore, the ultimate tensile strain varies from 1.5% for the jute fiber to 3.2% for the flax fiber, whereas ⇑ Corresponding author. Address: via Claudio, 21 80125 Naples, Italy. Tel.: +39 081 7683672; fax: +39 081 7683491. E-mail address: [email protected] (D. Asprone). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.12.046
hemp fiber presents an ultimate tensile strain of 1.6%. Young’s modulus is equal to 26.5 GPa and 27.6 GPa for jute and flax fiber, respectively, whereas, according to Dhakal et al. [10], Young’s modulus for hemp fiber varies from 30 GPa to 60 GPa. Thus, the values of the main mechanical properties of natural fibers are not so far from those exhibited by the most used synthetic fiber, i.e. glass or carbon. Hence, given these properties, natural fibers can be feasibly used as a component of composite materials, in different applications. In fact, whereas in industrial applications natural fibers are already used in fiber-reinforced plastic composites, structural applications in construction industry represent an interesting development for natural fiber use. In scientific literature, several works from structural and material engineering communities have investigated these applications. In particular, fiber reinforced mortars composed of short natural fibers reinforcing inorganic matrices have been studied [12–14]. Also textile reinforced laminates, composed of inorganic matrices reinforced by long natural fibers have been investigated [15–17]. A number of works have been also conducted on the development of natural fiber reinforced concrete [18–21]. The objective of these works is to study and develop new materials and technological solutions for structural applications. On the contrary, the objective of the current paper is to develop a composite material, made by a pozzolanic mortar reinforced by a hemp fiber grid, to be potentially used in retrofitting application of civil structures and in particular in seismic retrofitting operations of existing masonry structures. In fact, Italian guidelines from National Research Council [22] permit to use, for seismic retrofitting of masonry structures, externally bonded fiber reinforced composite systems, using inorganic
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matrices. From the structural point of view, the use of natural fibers and in particular hemp fibers guarantees a good mechanical compatibility of the composite system with the masonry elements, given the low Young’s modulus of the fibers. Here, the results of the preliminary experimental characterization of the composite system are presented. In particular, the single hemp fibers have been characterized in terms of tensile strength; furthermore, the durability of the fibers in the matrix environment was also addressed and a latex coating was investigated. Finally, the flexural behavior of the composite was investigated using different configurations of the fiber reinforcement. It is emphasized that the choice of the specific components for the composite system is led by sustainability criteria. In fact, both hemp fibers and pozzolanic mortar are very common in Italy and the production of the composite system can be provided by a local supply chain. In particular, the used mortar is a commercial product, PLANITOP HDM from MAPEI, composed by natural hydraulic lime, pozzolan and natural fine aggregate. It is a GP-CS IV masonry mortar, according to EN 998-1 [23]. 2. Mechanical characterization and durability assessment of the hemp fibers The hemp fibers employed in the investigated composite systems were produced in Italy, from a local growing. Fig. 1 reports a bundle of the investigated fibers. In order to characterize the mechanical properties of the fibers 10 tensile failure tests were carried out on fiber specimens, using a displacement control testing machine. The tests were conducted and elaborated according to ASTM C1557 [24]. The free length of the fiber specimens was 15 mm and the tests were conducted at 0.1 mm/s of elongation velocity. The specimens used for the tests came from five fibers; each of them were divided into two specimens. During the tests, the load and the specimen elongation were acquired and elaborated into stress–strain relationships. To do this, the cross area A of the fibers was evaluated as
A¼
P Lc
ð1Þ
being P the weight of the fiber, L the length of the fiber and g the specific weight of the fiber, which can be considered equal to
1.4 104 N/m3 for cellulose based materials [25]. The specimens exhibited an elastic behavior up to failure. Table 1 reports the average failure stress, ultimate strain and Young’s modulus from each couple of specimens. As it was expected, the values are highly variable; the obtained average tensile failure stress and average Young’s modulus were equal to 898 MPa and 21.3 GPa, respectively, whereas the average ultimate strain was equal to 6.1%. The durability of natural fibers represents a critical issue for the use of such fiber in composite systems. Physical and mechanical properties of natural fibers can be affected by significant modifications due to environmental factors [26–28]. In particular, inorganic matrix environment can induce a significant degradation of fiber properties [27,15]. In case of cementitious environment, durability of natural fibers is affected by different factors: alkali attacks, chemical reactions with products of cement hydration and fibers volume variation due to water absorption [15,29–31]. In order to increase natural fiber durability in inorganic matrices, different approaches have been proposed in literature. Gram [26] proposed different fiber treatments with blocking agents, such as sodium silicate, magnesium sulphate, iron or copper compounds and others, but fiber durability did not increase significantly. Toledo Filho et al. [15] proposed to use silica fume products in order to reduce the degradation due to alkali attacks. Bilba and Arsene [32] studied a silane coating of natural fibers to reduce external agents attacks and then the degradation of the mechanical properties. In order to assess the sensitivity of the used hemp fibers to such degradation phenomena and in particular to alkali attacks and water adsorption, the tensile failure tests described above were repeated for conditioned fibers. Fiber conditioning consists of 15 days of immersion in water with a value of pH of 13. In particular, as for tensile tests on unconditioned fibers, five couples of specimens were tested. Results are reported in Table 2, where a strong decrease of both tensile failure stress and Young’s modulus can be observed. This confirmed that environmental alkalinity and water adsorption represent critical issues for the mechanical properties of the used hemp fibers. Actually, in the present study, the use of a pozzolan-based mortar, characterized by a lower alkalinity than those presented by purely cementitious mortars, can reduce the effects of the alkali attacks on the hemp fibers. However, in order to improve the dura-
Table 1 Tensile properties of the hemp fibers.
Average Standard deviation
Fiber cross area (mm2)
Tensile failure stress (MPa)
Ultimate strain (%)
Young’s modulus (GPa)
0.0036 0.0025 0.0050 0.0020 0.0040 0.0034 0.0012
676 738 533 1488 1055 898 381
3.9 5.7 6.0 6.0 8.7 6.1 1.7
16.3 19.4 12.6 43.4 14.9 21.3 12.6
Table 2 Tensile properties of conditioned hemp fibers.
Fig. 1. Hemp fibers.
Fiber cross area (mm2)
Tensile failure stress (MPa)
Ultimate strain (%)
Young’s modulus (GPa)
0.0036 0.0025 0.0050 0.0020 0.0040
80.0 77.6 288 60 124
3.0 2.1 9.0 2.5 10.7
3.1 2.0 13.8 1.0 3.8
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bility of the hemp fibers in the proposed composite system, a latex coating was investigated, as described in the following sections. 3. Latex coating of the hemp fibers To overcome the degradation of both physical and mechanical properties that could occur to hemp fibers in the inorganic matrix environment, a latex coating was proposed and used. The objective is to provide an external protection, in order to insulate fibers from alkali agents and moisture of the inorganic mortar. In order to assess the effectiveness of this solution, tensile failure tests were conducted on hemp fiber strings, before and after moisture and alkaline conditioning, in presence of the latex coating and in the unprotected configuration. The employed hemp fiber strings are made by discontinuous fibers twisted together to form a bundle. Due to its configuration, the string presents a lower ultimate stress, if compared with that exhibited by the single fiber. In particular, tensile failure tests were conducted on string specimens, of 100 mm of free length, using a displacement control testing machine, with a maximum load capacity of 50 kN. Fig. 2 depicts a string specimen during the test. Different typologies of specimens were tested; in particular, for both the specimens with and without the latex coating, the tests were conducted (i) in case of no conditioning, (ii) in case of immersion in basic water, to test the contemporary effect of alkali and moisture induced degradation, and (iii) in case of immersion in neutral water, to test the effect of the water adsorption. In details, four tests were carried out, for each of the following typology (labels in parentheses are used hereafter to indicate the corresponding specimen): strings without coating and without conditioning (NL–NC); strings with latex coating and without conditioning (L–NC); strings without coating, subjected to 25 days of immersion in pH 13 water (NL–AC); strings with latex coating, subjected to 25 days of immersion in pH 13 water (L–AC); strings without coating, subjected to 60 days of immersion in neutral water (NL–MC); strings with latex coating, subjected to 60 days of immersion in neutral water (L–MC).
Fig. 2. Tensile test on string specimen.
Table 3 reports the results of the tests, in terms of average tensile failure stress. It can be observed that in case of the NL–NC specimens, corresponding to the unprotected and unconditioned string, the ultimate stress is much lower than that exhibited by the single fibers, since, in case of the string, the failure is due to the fraying of the specimens and not to the failure of the fibers. This can be observed in Fig. 3, which depicts a string specimen close to failure. Furthermore, it can be observed that, the contemporary effect of alkali attacks and water adsorption produces a more significant degradation than that induced only by the water adsorption, as it was expected, since the ultimate stress for the specimens NL–MC is higher than that for the specimens NL–AC. However, in both cases, latex coating is able to reduce such degradation, as it results from ultimate stress values from L–MC and L–AC, which are higher than ultimate stress value of NL–MC and NL–AC, respectively. In particular, for the water adsorption and alkali attack conditioning the ultimate stress, if compared with that of the reference specimens, presents a reduction of the 49% and of the 36%, in case of the unprotected and the protected specimens, respectively. On the contrary, in case of the water adsorption conditioning, the ultimate stress presents a reduction of the 40% and of the 26%, in case of the unprotected and the protected specimen, respectively. Hence, it is observed that, even if the specimens present a latex coating the conditioning phenomena induce a reduction of the ultimate stress. However, it should be mentioned that the adopted conditioning procedures are much more severe than the actual degrading processes, which the hemp fibers would be subjected to, when submerged into the inorganic matrix environment. In order to assess the mechanical behavior of the bond between the hemp fibers and the pozzolanic mortar, a pull-out test program was carried out. The tests were conducted on hemp strings partially embedded in pozzolanic mortar specimens.
Table 3 Tensile properties of conditioned fiber strings. Specimen type
Bundle cross area (mm2)
Tensile average failure stress (MPa)
NL–NC L–NC NL–AC L–AC NL–MC L–MC
1.00 1.78 2.00 1.60 1.70 0.90
108.4 112.4 55.5 69.0 64.9 80.4
Fig. 3. A string specimen close to failure.
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The tests were carried out also for specimens with an epoxy resin coating, in order to have a reference for the pull-out behavior of coated hemp fibers, since this type of resin presents good mechanical properties in terms of bond with the cementitious mortar. Hence, it could represent an optimal solution for the durability of the fibers, but the use of epoxy resin, if compared with latex, can be more expensive and can lead to a higher environmental impact, according to Life Cycle criteria [33,34]. Furthermore, latex is less dangerous than epoxy resin both for workers health and for disposal phase, as confirmed by USA National Institute for Occupational Safety and Health (NIOSH) database [35]. The specimens were prepared by placing the hemp strings in prismatic mortar specimens, 10 mm 30 mm 50 mm in dimen-
sions, at different embedment length, from 10 mm to 40 mm. The tests were conducted after 28 days of curing of the mortar, using a displacement control testing machine, with a maximum load capacity of 50 kN. During the tests, a pull out velocity of 2 mm/ min was used. Fig. 4 depicts the pull-out test setup. Table 4 reports the main results of the tests in terms of ultimate pull-out force F, corresponding to the bond failure and the slip of the string from the mortar. The equivalent ultimate bond stress t is reported and defined as the ratio of F over the string lateral area Al, which is equal to the string circumference multiplied by the embedment length; the corresponding average values for each type of specimen are also presented. As it could be expected, it can be observed that, as the embedment length increases, t decreases, since a stress
Fig. 4. Pull-out test setup.
Table 4 Results from pull-out tests on strings.
a
String type
Embedment length (mm)
Diameter (mm)
Ultimate pull-out force F (N)
Equivalent ultimate bond stress t (N/mm2)
t, average values (N/mm2)
Without coating
10
1.30
20
Without coating
40
With latex coating
10
1.48
With latex coating
20
1.48
With latex coating
30
1.48
With latex coating
40
1.48
With resin coating
30
1.60
3.02 1.81 1.77 1.19 1.45 0.98 1.07 4.01 2.62 3.14 1.77 2.24 2.17 2.17 1.72 2.50 >1.45a 1.60 1.83 1.85 >3.63a >2.82a >1.51a >1.63a >0.58a
2.20
Without coating
40 24 23 32 39 52 57 69 45 54 61 77 75 112 89 129 100 110 126 127 219 170 91 98 35
String tensile failure occurred.
1.32 1.02 3.26
2.06
2.13
1.68
>2.03
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concentration occurs at the top of the embedded portion of the string. It is also observed that both the latex and the epoxy resin coating improve the mechanical bond of the string to the mortar; indeed, given the embedment length, higher values of t have been obtained for the specimens with latex and resin coating, compared with those without latex protection. In particular, resin coating determines the highest values of bonding stress, causing even the tensile failure of the string. Finally, the conducted test program justified the use of the latex coating, since a significant protection from fiber degradation is provided and even an improvement in bond between fibers and mortar is obtained.
Table 5 Results from pull-out tests on bundles. Coating type
Diameter (mm)
Embedment length (mm)
Ultimate pull-out force F (N)
Equivalent ultimate bond stress t (N/mm2)
Latex
0.3
5 5 20 20 30 30 40 10 10 20 20 30 5 5 10 20 20 30 40 5 5
14 24 51 59 12 30 29 192 173 416 161 387 76 65 51 75 52 105 99 83 210
>39.63a >67.94a >36.09a >41.76a >5.66a >14.15a >10.26a 24.46 22.04 26.50 >10.25a >16.43a >215.15a >184.01a >72.19a >53.08a >36.80a >49.54a >35.03a 21.15 53.50
1.0
4. Mechanical characterization of the composite system The investigated composite system consists of a thin pozzolanic mortar slab reinforced with different layers of hemp fiber grids. To set-up the grids, long fiber bundles were preferred to the fiber strings used in pull-out test, since in tensile failure tests the latter exhibited a worse behavior in terms of ultimate stress, due to the fraying of the strings, occurring at failure. To clarify the difference between strings and bundles a sketch is presented in Fig. 5. To assess the bond between the fiber bundles and the mortar, a pull-out test program was also conducted on coated bundles, embedded into mortar specimens. Again, as for pull-out tests on strings, both latex and resin coating were tested. The dimensions of the specimens and the set-up characteristics were the same as those used for pull-out tests conducted on fiber strings. Two values of the bundle diameter were tested, 0.3 mm and 1.0 mm, whereas the embedment length varied from 5 mm to 40 mm. The main results are reported in Table 5. In case of 0.3 mm diameter bundle, for each embedment length and for both coating types, fiber tensile failure occurred outside the mortar specimens. Hence, for 0.3 mm diameter bundle, the critical embedment length, representing the threshold between tensile failure and bond failure, is even shorter than 5 mm, for both latex and resin coatings. On the contrary, for 1.0 mm diameter bundle, in case of resin coating, bond failure occurred only for 5 mm embedment length whereas, for latex coating, bond failure occurred for 10 mm and 20 mm embedment length. Hence, resin coating exhibits a better bond behavior than latex coating, as it was already observed in pull-out tests on fiber strings. Fig. 6 depicts a latex coated bundle after bond failure. It can be observed that failure occurred between fibers and latex coating. This revealed that the bond between the latex and the mortar is even stronger than the bond between the latex and the hemp fibers. To assess the mechanical properties of the hemp fiber reinforced composite system a bending test program was conducted on different composite specimens, with different grid reinforcement configurations. Fig. 7 depicts a composite specimen during preparation, where a hemp fiber grid has just been placed on the fresh mortar. In particular, three-point bending tests were carried out. The samples were almost 300 mm long, 15 mm thick and 100 mm wide. The span length L used for the test was equal to 240 mm. Up to six reinforcement layers were placed in the composite samples. The position through the thickness of each layer is such as indicated in Fig. 8; in particular, two different configurations were tested, using four and six layers, adding the inner layers in the latter case.
single fiber
Resin
0.3
1.0 a
String tensile failure occurred.
fiber bundle Fig. 5. Fiber configurations.
Fig. 6. Bond failure for a latex coated bundle.
Fig. 7. A composite specimen during preparation.
fiber string
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top
reinforcement layer 2
reinforcement layer 4
reinforcement layer 6
reinforcement layer 5
reinforcement layer 3
reinforcement layer 1
bottom Fig. 8. Reinforcement layers position.
r¼ e¼
3PL 2bd
The average flexural stress–flexural strain curves are reported from Figs. 9–12. The average maximum flexural stress is also reported in Table 6. It can be observed that in the unreinforced spec25
Flexural stress [MPa]
Two different values of grid spacing through the width of the sample, Sw, were used, equal to 10 mm and 20 mm, whereas the grid spacing through the length of the sample, Sl, was equal to 40 mm in each specimen. Two different values for the diameter of the single fiber bundle, Db, were used, equal to 1 mm and 0.3 mm, corresponding to a weight per length of the bundles of 1.1 g/m and 0.1 g/m, respectively. Both latex and resin coatings were used. Table 6 reports the main characteristics of the tested samples. For each type, three tests were conducted. The tests were conducted after 28 days of curing the mortar, using a displacement control testing machine, with a maximum load capacity of 50 kN. During the tests, the applied load P, and the midspan deflection D, were acquired. According to ASTM C1018 approach [36], the load and deflection data were elaborated to obtain flexural stress–strain curves, through the following relationships:
15
10 A
5
ð2Þ
2
20
B P
6Dd
0
ð3Þ
L2
0
5
10
15
20
25
30
35
40
Flexural strain [‰]
where s is the flexural stress, e is flexural strain, P is applied load, D is midspan deflection, L is span length, b is width of the specimen, and d is the thickness of the specimen.
Fig. 9. Flexural stress–flexural strain curves (latex coating – four reinforcing layers).
Table 6 Three-point bending tests. Sample
Coating type
Number of reinforcing layers (#)
Bundle diameter, Db (mm)
Bundle spacing through the width, Sw, (mm)
Reinforcement weight ratio (%)
Average maximum flexural stress (MPa)
P B A D H L C F E G I
– Latex
0 4
– 0.3 1.0 0.3 0.3 1.0 0.3 1.0 0.3 1.0 1.0
– 20 20 20 10 10 20 20 20 20 10
– 1.2 4.0 1.8 3.0 12.0 1.2 4.0 1.8 7.0 12.0
7.03 10.20 9.35 9.27 13.70 13.20 9.90 10.27 11.10 15.02 21.64
6
Resin
4 6
D. Asprone et al. / Construction and Building Materials 25 (2011) 2867–2874
25
Flexural stress [MPa]
20
15
10 L H
5 D P
0
0
5
10
15
20
25
30
35
40
Flexural strain [‰] Fig. 10. Flexural stress–flexural strain curves (latex coating – six reinforcing layers).
25
Flexural stress [MPa]
20
15
values of strain are obtained. In particular, higher strain values are reached for the specimens with latex coated reinforcing fibers, whereas higher values of stress are obtained in case of the specimens with resin coated reinforcing fibers. This behavior reveals that at the first peak stress, tensile failure of the mortar occurs and cracks open at the bottom of the specimen; in the unreinforced configuration, this cause the complete failure of the specimen, whereas in the reinforced configuration a further tensile capacity is provided by the fibers. Hence, in case of latex coating, specimens exhibited a more ductile behavior, whereas, in case of resin coating, higher stress values were obtained, but a more brittle behavior was experienced. This behavior is probably due to different failure mechanisms of the fibers, occurring in case of latex and resin coating. In fact, in case of resin coating, a tensile failure was experienced, as it can be observed in Fig. 13, reporting a resin coating sample after failure, where the fibers present a sharp cut. On the contrary, fibers slipped from the mortar in case of latex coating, as it can be observed in Fig. 14, where a failed latex coating sample is depicted. In particular, the maximum flexural stress increased up to the 1.95 and 3.08 times the value obtained in the unreinforced configuration, for the latex and the resin coating, respectively. With regard to the diameter of the fiber bundles, it can be observed that, in case of the latex coating, unlike the resin coating, the increase of the diameter does not correspond to an increase of the maximum flexural stress. This reveals that latex coating is less effective than resin coating in transferring bond stress inside the bundles. Finally, it can be observed that, both for latex and resin
10 F C
5 P
0
0
5
10
15
20
25
30
35
40
Flexural strain [‰] Fig. 11. Flexural stress–flexural strain curves (resin coating – four reinforcing layers).
Flexural stress [MPa]
25
20 Fig. 13. Specimen reinforced with resin coated fibers after failure.
15 I
10
E
G
5 P
0
0
5
10
15
20
25
30
35
40
Flexural strain [‰] Fig. 12. Flexural stress–flexural strain curves (resin coating – six reinforcing layers).
imen a maximum stress of 7.03 MPa is reached; then a rapid failure occurs with a drop of the flexural stress. On the contrary, for the reinforced specimen, after a peak stress occurring almost at the same flexural strain value, the stress increases again and higher
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Fig. 14. Specimen reinforced with latex coated fibers after failure.
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coating, an increase of the reinforcement ratio produces a growth of the maximum flexural stress, both in case of reduction of the bundle spacing and in case of increase of the reinforcing layers. However, such variation is much less significant in the latter case, as it could be expected, since the added reinforcing layers are less effective than the others, since they are closer to the neutral axis in the bended cross section of the specimen. 5. Conclusions The activities here presented addressed a preliminary mechanical characterization of an inorganic composite system that could become a potential solution for the retrofit of existing structures. The system is made by pozzolanic mortar reinforced with hemp fiber grids, with a latex coating. Based on the results of the conducted experimental activities it can be concluded that: latex coating can improve the durability of the hemp fiber in the pozzolanic mortar environment; the bond behavior between the hemp fibers and the pozzolanic mortar is even improved by the latex coating; hemp fiber grid reinforcement can provide a significant improvement of the flexural behavior of the pozzolanic mortar, increasing the flexural strength and providing a considerable durability enhancement. At this step, further tests will be conducted to assess the durability of the composite system within different environmental conditions and to investigate the feasibility of applications of the reinforced mortar on structural elements. However, the preliminary outcomes here illustrated reveal that the composite system presents considerable mechanical properties and can be further developed to be used as an effective solution for structural retrofit of existing structures. Furthermore, once the mechanical behavior of the composite per se has been optimized, it will be necessary to assess issues related to its bond to existing structural members. As a final word, it is underlined that, as it was expected for natural fibers, results from both fiber and composite tests presented a high dispersion. This represents a critical issue for structural applications, where a design value for each mechanical property of the used materials is needed. Acknowledgement Authors gratefully acknowledge italian association Assocanapa and Mr. Michele Castaldo for the support and the assistance to the conducted activities. References [1] Berge B. Ecology of building materials. 2nd ed. Elsevier; 2007. p. 453. [2] Khatib J. Sustainability of building materials. Woodhead Publishing in Materials; 2009. p. 368. [3] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Progress Polym Sci 1999;24(2):221–74. [4] Gassan J. Natural fibre-reinforced plastics – correlation between structure and properties of the fibres and the resultant composites. Dissertation at the Institute of Materials Engineering. Kassel: University of Kassel; 1997. [5] Torgal FP, Jalali S. Cementitious building materials reinforced with vegetable fibres: A review. Constr Build Mater 2011;25(2):575–81. [6] Stamboulis A, Baillie CA, Peijs T. Effects of environmental conditions on mechanical and physical properties of flax fibers. Compos Part A: Appl Sci Manuf 2001;32(8):1105–15.
[7] Mannan KhM, Talukder MAI. Characterization of raw, delignified and bleached jute fibres by study of absorption of moisture and some mechanical properties. Polymer 1997;38(10):2493–500. [8] Gassan J, Bledzki AK. Possibilities for improving the mechanical properties of jute/epoxy composites by alkali treatment of fibres. Compos Sci Technol 1999;59(9):1303–9. [9] Kostic M, Pejic B, Skundric P. Quality of chemically modified hemp fibers. Bioresour Technol 2008;99(1):94–9. [10] Dhakal HN, Zhang ZY, Richardson MOW. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos Sci Technol 2007;67(7–8):1674–83. [11] Li Y, Mai YW, Ye L. Sisal fibre and its composites: a review of recent developments. Compos Sci Technol 2000;60(11):2037–55. [12] Ramakrishna G, Sundararajan. Impact strength of a few natural fiber reinforced cement mortar slabs: a comparative study. Cem Concr Compos 2005;27:547–63. [13] Savastano Jr H, John VM, Agopyan V, Pellegrino Ferreira O. Weathering of vegetable fibre-clinker free cement composites. Mater Struct 2002;35:64–8. [14] Aggarwal LK. Bagasse reinforced cement composites. Cem Concr Compos 1995;17:107–12. [15] Toledo Filho RD, Scrivener K, Ghavami K, England GL. Development of vegetable fibre-mortar composites of improved durability. Cem Concr Compos 2003;25:185–96. [16] Ramarishka G, Sundararajan T. Studies on the durability of natural fibers and the effect of corroded fibers on the strength of mortar. Cem Concr Compos 2005;27:575–82. [17] Toledo Filho RD, Silva FA, Fairbairn EMR, Melo Filho JA. Durability of compression molded sisal fiber reinforced mortar laminates. Constr Build Mater 2009;23:2409–20. [18] Boghossian E, Wegner LD. Use of flax fibres to reduce plastic shrinkage cracking in concrete. Cem Concr Compos 2008;30(10):929–37. [19] Elfordy S, Lucas F, Tancret F, Scudeller Y, Goudet L. Mechanical and thermal properties of lime and hemp concrete (‘hempcrete’) manufactured by a projection process. Constr Build Mater 2008;22(10):2116–23. [20] Savastano Jr H, Warden PG, Coutts RSP. Potential of alternative fibre cements as building materials for developing areas. Cem Concr Compos 2003;25(60):585–92 [Infrastructure Development]. [21] Li Z, Wang L, Wang X. Compressive and flexural properties of hemp fiber reinforced concrete. Fibers Polym 2004;5(3):187–97. [22] CNR. Guide for the design and construction of externally bonded FRP systems for strengthening existing structures – materials, RC and PC structures, masonry structures. Italy; 2004. [23] EN 998-1. Specification for mortar for masonry – Part 1: rendering and plastering mortar; 2003. p. 26. [24] ASTM. Standard test method for tensile strength and Young’s modulus of fibers. ASTM C1557; 2008. p. 10. [25] Fakirov S, Bhattacharyya D. Handbook of engineering biopolymers: homopolymers, blends, and composites. Hanser Gardner; 2007. p. 932 [26] Gram HE. Durability of natural fibres in concrete. CBI Research No. 183. Stockholm: Swedish Cement and Concrete Research Institute; 1983. p. 255. [27] Jayamol G, Sreekala MS, Sabu T. A review on interface modification and characterization of natural fiber reinforced plastic composites. Polym Eng Sci 2004;41(9):1471–85. [28] Bledzki AK, Reihmane S, Gassan J. Properties and modification methods for vegetable fibers for natural fiber composites. J Appl Polym Sci 1998;59(8):1329–36. [29] Toledo Filho RD, Scrivener K, England GL, Ghavami K. Durability of alkali sensitive sisal and coconut fibres in cement based composites. Cem Concr Compos 2000;6(22):127–43. [30] Canovas SK. New economical solutions for improvement of durability of portland cement mortars reinforced with sisal fibres. Mater Struct 1992;25:417–22. [31] Tolêdo Filho RD. Natural fibre reinforced mortar composites: experimental characterisation. PhD thesis, DEC-PUC-Rio, Brazil; 1997. p. 472. [32] Bilba K, Arsene M-A. Silane treatment of bagasse fiber for reinforcement of cementitious composites. Compos Part A: Appl Sci Manuf 2008;39(9):1488–95. [33] Althaus HJ, Chudacoff M, Hischier R, Jungbluth N, Osses M, Primas A. Life cycle inventories of chemicals. Final report ecoinvent data v2.0 No. 8. Dübendorf (CH): Swiss Centre for Life Cycle Inventories; 2007. [34] Hischier R. Life cycle inventories of packaging and graphical paper. Final report ecoinvent data v2.0 No. 11. Dübendorf (CH): Swiss Centre for Life Cycle Inventories; 2007. [35] NIOSH. National Institute for Occupational Safety and Health. NIOSHTIC-2 database, USA; 2010. [36] ASTM. Standard test method for flexural toughness and first-crack strength of fiber-reinforced concrete (using beam with third-point loading). ASTM C101897; 1997. p. 7.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Potential of structural pozzolanic matrix–hemp fiber grid composites Domenico Asprone a,⇑, Massimo Durante b, Andrea Prota a, Gaetano Manfredi a a b
Department of Structural Engineering, University of Naples ‘‘Federico II’’, Naples, Italy Department of Materials and Production Engineering, University of Naples ‘‘Federico II’’, Naples, Italy
a r t i c l e
i n f o
Article history: Received 1 September 2010 Received in revised form 13 December 2010 Accepted 24 December 2010 Available online 17 January 2011 Keywords: Hemp fiber Pozzolanic mortar Sustainability Inorganic composite Structural retrofit
a b s t r a c t Currently, sustainability represents a primary issue for construction industry. New material and technological solutions are widely proposed and investigated to meet sustainability requirements and natural fibers represent one of the most studied materials. The work presented here investigated the mechanical behavior of a sustainable composite system made by pozzolanic mortar reinforced with hemp fiber grids. To improve the durability of the system and in particular of the fibers in the pozzolanic mortar environment a latex coating was used. The objective of the study was to investigate the mechanical behavior of the proposed composite system and assess the feasibility of using the system for structural retrofit applications on existing structures. A mechanical characterization of the fibers was conducted and the effectiveness of the latex coating in improving the durability of the fibers was investigated. The mechanical behavior of the composite system was studied, through a three-point bending test program. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Low environmental impact of operations on built environment is often a primary condition to be respected by construction designers and operators. Thus, recently, new technological solutions are proposed and new materials are investigated and used [1,2]. For this reason, in recent years, natural fibers have been widely investigated, to be used as an alternative to carbon, glass or plastic fibers, in several composite applications for construction industry. In fact, given their low environmental impact both in production and in disposal phase, natural fibers represent a highly ‘‘sustainable’’ material. Furthermore, natural fibers can be locally supplied, ensuring a sustainable production chain. The recent increasing scientific interest in natural fibers as a component of construction applications is also due to the good mechanical properties exhibited by natural fibers. Available literature provides the mechanical characterization of natural fibers, in terms of elastic properties and tensile strength [3–5]; in particular, attention has been focused on different fibers, e.g. flax [6], jute [7,8], hemp [9,10], sisal [11]. The available reviews [3,5] reports the main mechanical properties of various natural fibers. It can be observed that the tensile strength can reach more than 1000 MPa, in case of flax fiber and vary from about 400 MPa to 800 MPa in case of jute fiber, whereas hemp fiber exhibits a tensile strength of 690 MPa. Furthermore, the ultimate tensile strain varies from 1.5% for the jute fiber to 3.2% for the flax fiber, whereas ⇑ Corresponding author. Address: via Claudio, 21 80125 Naples, Italy. Tel.: +39 081 7683672; fax: +39 081 7683491. E-mail address: [email protected] (D. Asprone). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.12.046
hemp fiber presents an ultimate tensile strain of 1.6%. Young’s modulus is equal to 26.5 GPa and 27.6 GPa for jute and flax fiber, respectively, whereas, according to Dhakal et al. [10], Young’s modulus for hemp fiber varies from 30 GPa to 60 GPa. Thus, the values of the main mechanical properties of natural fibers are not so far from those exhibited by the most used synthetic fiber, i.e. glass or carbon. Hence, given these properties, natural fibers can be feasibly used as a component of composite materials, in different applications. In fact, whereas in industrial applications natural fibers are already used in fiber-reinforced plastic composites, structural applications in construction industry represent an interesting development for natural fiber use. In scientific literature, several works from structural and material engineering communities have investigated these applications. In particular, fiber reinforced mortars composed of short natural fibers reinforcing inorganic matrices have been studied [12–14]. Also textile reinforced laminates, composed of inorganic matrices reinforced by long natural fibers have been investigated [15–17]. A number of works have been also conducted on the development of natural fiber reinforced concrete [18–21]. The objective of these works is to study and develop new materials and technological solutions for structural applications. On the contrary, the objective of the current paper is to develop a composite material, made by a pozzolanic mortar reinforced by a hemp fiber grid, to be potentially used in retrofitting application of civil structures and in particular in seismic retrofitting operations of existing masonry structures. In fact, Italian guidelines from National Research Council [22] permit to use, for seismic retrofitting of masonry structures, externally bonded fiber reinforced composite systems, using inorganic
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matrices. From the structural point of view, the use of natural fibers and in particular hemp fibers guarantees a good mechanical compatibility of the composite system with the masonry elements, given the low Young’s modulus of the fibers. Here, the results of the preliminary experimental characterization of the composite system are presented. In particular, the single hemp fibers have been characterized in terms of tensile strength; furthermore, the durability of the fibers in the matrix environment was also addressed and a latex coating was investigated. Finally, the flexural behavior of the composite was investigated using different configurations of the fiber reinforcement. It is emphasized that the choice of the specific components for the composite system is led by sustainability criteria. In fact, both hemp fibers and pozzolanic mortar are very common in Italy and the production of the composite system can be provided by a local supply chain. In particular, the used mortar is a commercial product, PLANITOP HDM from MAPEI, composed by natural hydraulic lime, pozzolan and natural fine aggregate. It is a GP-CS IV masonry mortar, according to EN 998-1 [23]. 2. Mechanical characterization and durability assessment of the hemp fibers The hemp fibers employed in the investigated composite systems were produced in Italy, from a local growing. Fig. 1 reports a bundle of the investigated fibers. In order to characterize the mechanical properties of the fibers 10 tensile failure tests were carried out on fiber specimens, using a displacement control testing machine. The tests were conducted and elaborated according to ASTM C1557 [24]. The free length of the fiber specimens was 15 mm and the tests were conducted at 0.1 mm/s of elongation velocity. The specimens used for the tests came from five fibers; each of them were divided into two specimens. During the tests, the load and the specimen elongation were acquired and elaborated into stress–strain relationships. To do this, the cross area A of the fibers was evaluated as
A¼
P Lc
ð1Þ
being P the weight of the fiber, L the length of the fiber and g the specific weight of the fiber, which can be considered equal to
1.4 104 N/m3 for cellulose based materials [25]. The specimens exhibited an elastic behavior up to failure. Table 1 reports the average failure stress, ultimate strain and Young’s modulus from each couple of specimens. As it was expected, the values are highly variable; the obtained average tensile failure stress and average Young’s modulus were equal to 898 MPa and 21.3 GPa, respectively, whereas the average ultimate strain was equal to 6.1%. The durability of natural fibers represents a critical issue for the use of such fiber in composite systems. Physical and mechanical properties of natural fibers can be affected by significant modifications due to environmental factors [26–28]. In particular, inorganic matrix environment can induce a significant degradation of fiber properties [27,15]. In case of cementitious environment, durability of natural fibers is affected by different factors: alkali attacks, chemical reactions with products of cement hydration and fibers volume variation due to water absorption [15,29–31]. In order to increase natural fiber durability in inorganic matrices, different approaches have been proposed in literature. Gram [26] proposed different fiber treatments with blocking agents, such as sodium silicate, magnesium sulphate, iron or copper compounds and others, but fiber durability did not increase significantly. Toledo Filho et al. [15] proposed to use silica fume products in order to reduce the degradation due to alkali attacks. Bilba and Arsene [32] studied a silane coating of natural fibers to reduce external agents attacks and then the degradation of the mechanical properties. In order to assess the sensitivity of the used hemp fibers to such degradation phenomena and in particular to alkali attacks and water adsorption, the tensile failure tests described above were repeated for conditioned fibers. Fiber conditioning consists of 15 days of immersion in water with a value of pH of 13. In particular, as for tensile tests on unconditioned fibers, five couples of specimens were tested. Results are reported in Table 2, where a strong decrease of both tensile failure stress and Young’s modulus can be observed. This confirmed that environmental alkalinity and water adsorption represent critical issues for the mechanical properties of the used hemp fibers. Actually, in the present study, the use of a pozzolan-based mortar, characterized by a lower alkalinity than those presented by purely cementitious mortars, can reduce the effects of the alkali attacks on the hemp fibers. However, in order to improve the dura-
Table 1 Tensile properties of the hemp fibers.
Average Standard deviation
Fiber cross area (mm2)
Tensile failure stress (MPa)
Ultimate strain (%)
Young’s modulus (GPa)
0.0036 0.0025 0.0050 0.0020 0.0040 0.0034 0.0012
676 738 533 1488 1055 898 381
3.9 5.7 6.0 6.0 8.7 6.1 1.7
16.3 19.4 12.6 43.4 14.9 21.3 12.6
Table 2 Tensile properties of conditioned hemp fibers.
Fig. 1. Hemp fibers.
Fiber cross area (mm2)
Tensile failure stress (MPa)
Ultimate strain (%)
Young’s modulus (GPa)
0.0036 0.0025 0.0050 0.0020 0.0040
80.0 77.6 288 60 124
3.0 2.1 9.0 2.5 10.7
3.1 2.0 13.8 1.0 3.8
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bility of the hemp fibers in the proposed composite system, a latex coating was investigated, as described in the following sections. 3. Latex coating of the hemp fibers To overcome the degradation of both physical and mechanical properties that could occur to hemp fibers in the inorganic matrix environment, a latex coating was proposed and used. The objective is to provide an external protection, in order to insulate fibers from alkali agents and moisture of the inorganic mortar. In order to assess the effectiveness of this solution, tensile failure tests were conducted on hemp fiber strings, before and after moisture and alkaline conditioning, in presence of the latex coating and in the unprotected configuration. The employed hemp fiber strings are made by discontinuous fibers twisted together to form a bundle. Due to its configuration, the string presents a lower ultimate stress, if compared with that exhibited by the single fiber. In particular, tensile failure tests were conducted on string specimens, of 100 mm of free length, using a displacement control testing machine, with a maximum load capacity of 50 kN. Fig. 2 depicts a string specimen during the test. Different typologies of specimens were tested; in particular, for both the specimens with and without the latex coating, the tests were conducted (i) in case of no conditioning, (ii) in case of immersion in basic water, to test the contemporary effect of alkali and moisture induced degradation, and (iii) in case of immersion in neutral water, to test the effect of the water adsorption. In details, four tests were carried out, for each of the following typology (labels in parentheses are used hereafter to indicate the corresponding specimen): strings without coating and without conditioning (NL–NC); strings with latex coating and without conditioning (L–NC); strings without coating, subjected to 25 days of immersion in pH 13 water (NL–AC); strings with latex coating, subjected to 25 days of immersion in pH 13 water (L–AC); strings without coating, subjected to 60 days of immersion in neutral water (NL–MC); strings with latex coating, subjected to 60 days of immersion in neutral water (L–MC).
Fig. 2. Tensile test on string specimen.
Table 3 reports the results of the tests, in terms of average tensile failure stress. It can be observed that in case of the NL–NC specimens, corresponding to the unprotected and unconditioned string, the ultimate stress is much lower than that exhibited by the single fibers, since, in case of the string, the failure is due to the fraying of the specimens and not to the failure of the fibers. This can be observed in Fig. 3, which depicts a string specimen close to failure. Furthermore, it can be observed that, the contemporary effect of alkali attacks and water adsorption produces a more significant degradation than that induced only by the water adsorption, as it was expected, since the ultimate stress for the specimens NL–MC is higher than that for the specimens NL–AC. However, in both cases, latex coating is able to reduce such degradation, as it results from ultimate stress values from L–MC and L–AC, which are higher than ultimate stress value of NL–MC and NL–AC, respectively. In particular, for the water adsorption and alkali attack conditioning the ultimate stress, if compared with that of the reference specimens, presents a reduction of the 49% and of the 36%, in case of the unprotected and the protected specimens, respectively. On the contrary, in case of the water adsorption conditioning, the ultimate stress presents a reduction of the 40% and of the 26%, in case of the unprotected and the protected specimen, respectively. Hence, it is observed that, even if the specimens present a latex coating the conditioning phenomena induce a reduction of the ultimate stress. However, it should be mentioned that the adopted conditioning procedures are much more severe than the actual degrading processes, which the hemp fibers would be subjected to, when submerged into the inorganic matrix environment. In order to assess the mechanical behavior of the bond between the hemp fibers and the pozzolanic mortar, a pull-out test program was carried out. The tests were conducted on hemp strings partially embedded in pozzolanic mortar specimens.
Table 3 Tensile properties of conditioned fiber strings. Specimen type
Bundle cross area (mm2)
Tensile average failure stress (MPa)
NL–NC L–NC NL–AC L–AC NL–MC L–MC
1.00 1.78 2.00 1.60 1.70 0.90
108.4 112.4 55.5 69.0 64.9 80.4
Fig. 3. A string specimen close to failure.
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The tests were carried out also for specimens with an epoxy resin coating, in order to have a reference for the pull-out behavior of coated hemp fibers, since this type of resin presents good mechanical properties in terms of bond with the cementitious mortar. Hence, it could represent an optimal solution for the durability of the fibers, but the use of epoxy resin, if compared with latex, can be more expensive and can lead to a higher environmental impact, according to Life Cycle criteria [33,34]. Furthermore, latex is less dangerous than epoxy resin both for workers health and for disposal phase, as confirmed by USA National Institute for Occupational Safety and Health (NIOSH) database [35]. The specimens were prepared by placing the hemp strings in prismatic mortar specimens, 10 mm 30 mm 50 mm in dimen-
sions, at different embedment length, from 10 mm to 40 mm. The tests were conducted after 28 days of curing of the mortar, using a displacement control testing machine, with a maximum load capacity of 50 kN. During the tests, a pull out velocity of 2 mm/ min was used. Fig. 4 depicts the pull-out test setup. Table 4 reports the main results of the tests in terms of ultimate pull-out force F, corresponding to the bond failure and the slip of the string from the mortar. The equivalent ultimate bond stress t is reported and defined as the ratio of F over the string lateral area Al, which is equal to the string circumference multiplied by the embedment length; the corresponding average values for each type of specimen are also presented. As it could be expected, it can be observed that, as the embedment length increases, t decreases, since a stress
Fig. 4. Pull-out test setup.
Table 4 Results from pull-out tests on strings.
a
String type
Embedment length (mm)
Diameter (mm)
Ultimate pull-out force F (N)
Equivalent ultimate bond stress t (N/mm2)
t, average values (N/mm2)
Without coating
10
1.30
20
Without coating
40
With latex coating
10
1.48
With latex coating
20
1.48
With latex coating
30
1.48
With latex coating
40
1.48
With resin coating
30
1.60
3.02 1.81 1.77 1.19 1.45 0.98 1.07 4.01 2.62 3.14 1.77 2.24 2.17 2.17 1.72 2.50 >1.45a 1.60 1.83 1.85 >3.63a >2.82a >1.51a >1.63a >0.58a
2.20
Without coating
40 24 23 32 39 52 57 69 45 54 61 77 75 112 89 129 100 110 126 127 219 170 91 98 35
String tensile failure occurred.
1.32 1.02 3.26
2.06
2.13
1.68
>2.03
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concentration occurs at the top of the embedded portion of the string. It is also observed that both the latex and the epoxy resin coating improve the mechanical bond of the string to the mortar; indeed, given the embedment length, higher values of t have been obtained for the specimens with latex and resin coating, compared with those without latex protection. In particular, resin coating determines the highest values of bonding stress, causing even the tensile failure of the string. Finally, the conducted test program justified the use of the latex coating, since a significant protection from fiber degradation is provided and even an improvement in bond between fibers and mortar is obtained.
Table 5 Results from pull-out tests on bundles. Coating type
Diameter (mm)
Embedment length (mm)
Ultimate pull-out force F (N)
Equivalent ultimate bond stress t (N/mm2)
Latex
0.3
5 5 20 20 30 30 40 10 10 20 20 30 5 5 10 20 20 30 40 5 5
14 24 51 59 12 30 29 192 173 416 161 387 76 65 51 75 52 105 99 83 210
>39.63a >67.94a >36.09a >41.76a >5.66a >14.15a >10.26a 24.46 22.04 26.50 >10.25a >16.43a >215.15a >184.01a >72.19a >53.08a >36.80a >49.54a >35.03a 21.15 53.50
1.0
4. Mechanical characterization of the composite system The investigated composite system consists of a thin pozzolanic mortar slab reinforced with different layers of hemp fiber grids. To set-up the grids, long fiber bundles were preferred to the fiber strings used in pull-out test, since in tensile failure tests the latter exhibited a worse behavior in terms of ultimate stress, due to the fraying of the strings, occurring at failure. To clarify the difference between strings and bundles a sketch is presented in Fig. 5. To assess the bond between the fiber bundles and the mortar, a pull-out test program was also conducted on coated bundles, embedded into mortar specimens. Again, as for pull-out tests on strings, both latex and resin coating were tested. The dimensions of the specimens and the set-up characteristics were the same as those used for pull-out tests conducted on fiber strings. Two values of the bundle diameter were tested, 0.3 mm and 1.0 mm, whereas the embedment length varied from 5 mm to 40 mm. The main results are reported in Table 5. In case of 0.3 mm diameter bundle, for each embedment length and for both coating types, fiber tensile failure occurred outside the mortar specimens. Hence, for 0.3 mm diameter bundle, the critical embedment length, representing the threshold between tensile failure and bond failure, is even shorter than 5 mm, for both latex and resin coatings. On the contrary, for 1.0 mm diameter bundle, in case of resin coating, bond failure occurred only for 5 mm embedment length whereas, for latex coating, bond failure occurred for 10 mm and 20 mm embedment length. Hence, resin coating exhibits a better bond behavior than latex coating, as it was already observed in pull-out tests on fiber strings. Fig. 6 depicts a latex coated bundle after bond failure. It can be observed that failure occurred between fibers and latex coating. This revealed that the bond between the latex and the mortar is even stronger than the bond between the latex and the hemp fibers. To assess the mechanical properties of the hemp fiber reinforced composite system a bending test program was conducted on different composite specimens, with different grid reinforcement configurations. Fig. 7 depicts a composite specimen during preparation, where a hemp fiber grid has just been placed on the fresh mortar. In particular, three-point bending tests were carried out. The samples were almost 300 mm long, 15 mm thick and 100 mm wide. The span length L used for the test was equal to 240 mm. Up to six reinforcement layers were placed in the composite samples. The position through the thickness of each layer is such as indicated in Fig. 8; in particular, two different configurations were tested, using four and six layers, adding the inner layers in the latter case.
single fiber
Resin
0.3
1.0 a
String tensile failure occurred.
fiber bundle Fig. 5. Fiber configurations.
Fig. 6. Bond failure for a latex coated bundle.
Fig. 7. A composite specimen during preparation.
fiber string
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top
reinforcement layer 2
reinforcement layer 4
reinforcement layer 6
reinforcement layer 5
reinforcement layer 3
reinforcement layer 1
bottom Fig. 8. Reinforcement layers position.
r¼ e¼
3PL 2bd
The average flexural stress–flexural strain curves are reported from Figs. 9–12. The average maximum flexural stress is also reported in Table 6. It can be observed that in the unreinforced spec25
Flexural stress [MPa]
Two different values of grid spacing through the width of the sample, Sw, were used, equal to 10 mm and 20 mm, whereas the grid spacing through the length of the sample, Sl, was equal to 40 mm in each specimen. Two different values for the diameter of the single fiber bundle, Db, were used, equal to 1 mm and 0.3 mm, corresponding to a weight per length of the bundles of 1.1 g/m and 0.1 g/m, respectively. Both latex and resin coatings were used. Table 6 reports the main characteristics of the tested samples. For each type, three tests were conducted. The tests were conducted after 28 days of curing the mortar, using a displacement control testing machine, with a maximum load capacity of 50 kN. During the tests, the applied load P, and the midspan deflection D, were acquired. According to ASTM C1018 approach [36], the load and deflection data were elaborated to obtain flexural stress–strain curves, through the following relationships:
15
10 A
5
ð2Þ
2
20
B P
6Dd
0
ð3Þ
L2
0
5
10
15
20
25
30
35
40
Flexural strain [‰]
where s is the flexural stress, e is flexural strain, P is applied load, D is midspan deflection, L is span length, b is width of the specimen, and d is the thickness of the specimen.
Fig. 9. Flexural stress–flexural strain curves (latex coating – four reinforcing layers).
Table 6 Three-point bending tests. Sample
Coating type
Number of reinforcing layers (#)
Bundle diameter, Db (mm)
Bundle spacing through the width, Sw, (mm)
Reinforcement weight ratio (%)
Average maximum flexural stress (MPa)
P B A D H L C F E G I
– Latex
0 4
– 0.3 1.0 0.3 0.3 1.0 0.3 1.0 0.3 1.0 1.0
– 20 20 20 10 10 20 20 20 20 10
– 1.2 4.0 1.8 3.0 12.0 1.2 4.0 1.8 7.0 12.0
7.03 10.20 9.35 9.27 13.70 13.20 9.90 10.27 11.10 15.02 21.64
6
Resin
4 6
D. Asprone et al. / Construction and Building Materials 25 (2011) 2867–2874
25
Flexural stress [MPa]
20
15
10 L H
5 D P
0
0
5
10
15
20
25
30
35
40
Flexural strain [‰] Fig. 10. Flexural stress–flexural strain curves (latex coating – six reinforcing layers).
25
Flexural stress [MPa]
20
15
values of strain are obtained. In particular, higher strain values are reached for the specimens with latex coated reinforcing fibers, whereas higher values of stress are obtained in case of the specimens with resin coated reinforcing fibers. This behavior reveals that at the first peak stress, tensile failure of the mortar occurs and cracks open at the bottom of the specimen; in the unreinforced configuration, this cause the complete failure of the specimen, whereas in the reinforced configuration a further tensile capacity is provided by the fibers. Hence, in case of latex coating, specimens exhibited a more ductile behavior, whereas, in case of resin coating, higher stress values were obtained, but a more brittle behavior was experienced. This behavior is probably due to different failure mechanisms of the fibers, occurring in case of latex and resin coating. In fact, in case of resin coating, a tensile failure was experienced, as it can be observed in Fig. 13, reporting a resin coating sample after failure, where the fibers present a sharp cut. On the contrary, fibers slipped from the mortar in case of latex coating, as it can be observed in Fig. 14, where a failed latex coating sample is depicted. In particular, the maximum flexural stress increased up to the 1.95 and 3.08 times the value obtained in the unreinforced configuration, for the latex and the resin coating, respectively. With regard to the diameter of the fiber bundles, it can be observed that, in case of the latex coating, unlike the resin coating, the increase of the diameter does not correspond to an increase of the maximum flexural stress. This reveals that latex coating is less effective than resin coating in transferring bond stress inside the bundles. Finally, it can be observed that, both for latex and resin
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Flexural strain [‰] Fig. 11. Flexural stress–flexural strain curves (resin coating – four reinforcing layers).
Flexural stress [MPa]
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Flexural strain [‰] Fig. 12. Flexural stress–flexural strain curves (resin coating – six reinforcing layers).
imen a maximum stress of 7.03 MPa is reached; then a rapid failure occurs with a drop of the flexural stress. On the contrary, for the reinforced specimen, after a peak stress occurring almost at the same flexural strain value, the stress increases again and higher
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Fig. 14. Specimen reinforced with latex coated fibers after failure.
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coating, an increase of the reinforcement ratio produces a growth of the maximum flexural stress, both in case of reduction of the bundle spacing and in case of increase of the reinforcing layers. However, such variation is much less significant in the latter case, as it could be expected, since the added reinforcing layers are less effective than the others, since they are closer to the neutral axis in the bended cross section of the specimen. 5. Conclusions The activities here presented addressed a preliminary mechanical characterization of an inorganic composite system that could become a potential solution for the retrofit of existing structures. The system is made by pozzolanic mortar reinforced with hemp fiber grids, with a latex coating. Based on the results of the conducted experimental activities it can be concluded that: latex coating can improve the durability of the hemp fiber in the pozzolanic mortar environment; the bond behavior between the hemp fibers and the pozzolanic mortar is even improved by the latex coating; hemp fiber grid reinforcement can provide a significant improvement of the flexural behavior of the pozzolanic mortar, increasing the flexural strength and providing a considerable durability enhancement. At this step, further tests will be conducted to assess the durability of the composite system within different environmental conditions and to investigate the feasibility of applications of the reinforced mortar on structural elements. However, the preliminary outcomes here illustrated reveal that the composite system presents considerable mechanical properties and can be further developed to be used as an effective solution for structural retrofit of existing structures. Furthermore, once the mechanical behavior of the composite per se has been optimized, it will be necessary to assess issues related to its bond to existing structural members. As a final word, it is underlined that, as it was expected for natural fibers, results from both fiber and composite tests presented a high dispersion. This represents a critical issue for structural applications, where a design value for each mechanical property of the used materials is needed. Acknowledgement Authors gratefully acknowledge italian association Assocanapa and Mr. Michele Castaldo for the support and the assistance to the conducted activities. References [1] Berge B. Ecology of building materials. 2nd ed. Elsevier; 2007. p. 453. [2] Khatib J. Sustainability of building materials. Woodhead Publishing in Materials; 2009. p. 368. [3] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Progress Polym Sci 1999;24(2):221–74. [4] Gassan J. Natural fibre-reinforced plastics – correlation between structure and properties of the fibres and the resultant composites. Dissertation at the Institute of Materials Engineering. Kassel: University of Kassel; 1997. [5] Torgal FP, Jalali S. Cementitious building materials reinforced with vegetable fibres: A review. Constr Build Mater 2011;25(2):575–81. [6] Stamboulis A, Baillie CA, Peijs T. Effects of environmental conditions on mechanical and physical properties of flax fibers. Compos Part A: Appl Sci Manuf 2001;32(8):1105–15.
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