Effect of silica fume, steel fiber and ITZ on the strength and fracture behavior of mortar
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- Wang, X.H., Jacobsen, S., Lee, S.F. et al. Mater Struct (2010) 43: 125. doi:10.1617/s11527-009-9475-1
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Two sets of parameters known to affect the quality and thickness of the interfacial transition zone (ITZ), i.e. water/binder ratio and content of silica fume were varied in a series of mortars without and with steel fiber. Compressive and three-point bending tests were performed and the dissipated energies were calculated. Nanoindentation characteristics of the steel fiber–matrix and fiber–matrix-aggregate interfacial zones in the steel fiber reinforced mortars were studied. Influence of water/binder ratio, steel fiber, silica fume and ITZ on the strength and toughness of the mortar was analyzed, respectively. It is found that mortar compressive strength can be increased with low volume addition of steel fiber if the air content is well controlled; the interfacial characteristic and microstructural morphology near the fiber surface play a critical role on the three-point bending strength and the toughness of the steel fiber reinforced mortar.
KeywordsSteel fiberSilica fumeStrength and toughnessNanoindentationInterfacial transition zone (ITZ)
Concrete is a highly complex and heterogeneous composite material. The properties of the concrete depend on the aggregate, matrix, steel reinforcement and/or fiber, as well as the interfacial transition zone (ITZ) between the aggregate and matrix, and the steel and/or fiber and matrix. Those transition zones determine many of the important properties of concrete, such as strength, cracking and fracture behavior. In studies [1–3] carried out to determine the influence of aggregate, matrix and ITZ properties on the tensile and compressive strengths of concrete, it was found that the interfacial bond was the deciding factor for the tensile strength and played a little role on the compressive strength. It was concluded that the properties of the ITZ may have a moderate influence on mechanical properties of concrete and have a drastic effect on the mechanical properties of fiber reinforced cement composites .
In addition, the ITZ properties have a particular importance on the cracking mechanism and fracture behavior of concrete [3, 5, 6]. Fracture of concrete was shown to be influenced by the presence of the paste–aggregate interface [3, 6]. The strength of the interface affected the fracture energy in different ways depending on the shape of the particles; the critical crack opening decreased when the interface was strong, approaching the value corresponding to the matrix, and it increased when the crack was rough . As the w/c ratio increased, the porosity in ITZ was also increased, resulting in the initiation and development of cracks in this zone .
The above mentioned properties and behaviors of cementitious materials at the macroscale are all significantly affected, if not dominated, by their structural features and properties at the micro/nanoscale where the deterioration and failure process starts—properties at ITZ. The morphological characteristic of the microstructure of ITZ was primarily characterized via electron microscopy. Throughout those studies, Scanning Electron Microscopy (SEM) [3, 8], Environmental Scanning Electron Microscopy (ESEM)  and Transmission Electron Microscopy (TEM)  have been extensively used. In order to investigate and quantify microstructural gradients across the ITZ, backscattered electron imaging [10, 11] has been also used. However, for the above mentioned microscopical techniques, only two dimensional sections of a three-dimensional microstructure can be observed .
It is not until recent years that progress and improved availability of advanced instruments and techniques, such as depth-sensing micro/nanoindentation, have made it possible to study mechanical properties of various micro/nanoscale features in cement-based materials. Microindentation testing was firstly used to characterize gradients in mechanical properties to determine the mechanical characteristics of microsize zones in various cement based composites [12–14]. Due to the limitations of microindentation, such as larger indent comparing with the thickness of ITZ , nanoindentation, which has already been used for many other materials , began to be used in the cementitious composites. By using the nanoindentation technique, better representation of heterogeneous features exhibited by cementitious materials can be obtained . Till now, nanoindentation was widely used to measure elastic modulus and hardness of cement paste cured at different conditions [18–20]. Few research works were focused on the studying of the ITZ between a rigid inclusion and matrix [12, 21, 22].
In the present paper, in order to link the nano-mechanical properties of the ITZ with the macroscopic mechanical properties, mortars without and with steel fiber were prepared; where water/binder ratios of 0.3 and 0.5, and 0% and 10% contents of silica fume were considered. Uniaxial compressive strength testing and three-point bending strength testing were performed. Dissipated energies Welastic and Wpost-elastic before and after maximum elastic load were calculated to analyze the effect of steel fiber on the toughness of the mortar. Samples prepared from the same steel fiber reinforced mortars were used for nanoindentation test and the steel fiber–matrix and fiber–matrix-aggregate interfaces were studied. The whole sizes of each indented areas were determined by Scanning Electron Microscopy (SEM) and bond conditions across the interfaces were observed. Characteristics of the profiles of the interfacial transition zones were studied. It is found that the compressive strength of the mortar can be increased due to the low volume (0.3 vol%) addition of steel fiber on condition that the air content of the hardened mortar is well controlled; while the three-point bending strength and the toughness of the steel fiber reinforced mortar are mainly dependent on the interfacial characteristics and microstructural morphology near the fiber surface, which are mainly related to the water/binder ratio, the presence of silica fume, ITZ thickness as well as voids and microcracks within those zones.
2 Experimental programme
The detailed information of the specimens
Silica fume (percentage of the weight of cement)
Steel fiber (volume percentage)
The mortar mixtures were prepared with cement, sands 0–4 and 0–2 mm, water, limestone powder, and/or silica fume, and/or steel fiber. Norcem Anlegg cement which is an ordinary Portland cement was used. Silica fume (Elkem Microsilica grade 940-U) in powder form with the content of SiO2 larger than 90% was used. The mass retained on 45 micron sieve is small than 1.5%. The limestone powder, used as a filler, has a fineness of 57.2% <24 μm, with a specific weight of 2,730 kg/m3. Sands 0–4 and 0–2 mm have fineness modulus of 2.37 and 1.51, respectively. The percentages of particle <0.125 mm of sands 0–4 and 0–2 mm are 6.1% and 23.6%, respectively. Glenium 151 was incorporated in all mixtures as a superplasticizer, having 15% solids content and a density of 1,030 kg/m3. Straight high carbon steel fibre OL13/.16 has a length of 13 mm and a diameter of 0.16 mm, and a tensile strength more than 2000 MPa.
2.2 Proportions of mixes used
Characteristics of the mortar mixes
Mix proportion (kg/m3)
Sand 0–4 mm
Sand 0–2 mm
2.3 Mixing procedures
The mortars were mixed in a flat-bottomed mixer with maximum volume of 12 l. For the mortars without steel fiber, the mixing procedure was as follows: (1) cementitious materials (including silica fume) and sands were blended for 1 min at lower speed; (2) following the next 4 min, half of the mixing water was firstly added during the mixing, then all the superplasticizer and the remaining water were added; (3) stop the mixing, substances sticking in the sides of the mixer were cleaned and mixed into the mixes within 1 min; (4) the mixture was allowed to rest for 5 min, then mixed for an additional 1 min at lower speed. For the steel fiber mortar, after finishing the adding of the water and superplasticizer in Step 2, the steel fibers were added. It is suggested that the fibers should be added in small batches to get good dispersion of fibers in the mixture.
2.4 Rheology and curing
For each type of fresh mortar, slump test with a small cone and rheological measurements with the ConTec Viscometer were performed. Then, 50 × 50 × 50 mm cubes were cast for the compressive test and 40 × 40 × 160 mm prisms were cast for three-point bending strength test and nanoindentation test. During the casting, steel molds were vibrated in order to evacuate parts of the entrapped air. Then, surfaces of the specimens were carefully smoothed and covered with plastic sheets. Specimens were demolded after 24 h, and then each specimen was weighed in air and water, respectively. After that, the specimens were cured at 20°C under water for 28 days.
2.5 Sample preparation for studying interfacial properties
After 28 days curing in a water bath, the central part of the 40 × 40 × 160 mm prism was taken out using a diamond saw. This part was cut into small samples of the size about 15 × 15 × 15 mm, which were embedded in epoxy resin in air and prepared for grinding and polishing.
For fiber-reinforced composites, due to the lack of published grinding and polishing procedures in the literature, the best possible grinding and polishing procedures were finally determined after several trial tests. The TegraForce-5 grinding and polishing machine was used. The MD-Piano plates (220, 600 and 1200) were selected for the coarse grinding to obtain flat surface. The Largo (9 μm), Dur (3 μm) and Dac (1 μm) were selected for the fine polishing. The Nap was used for the final polishing. At every step, a microscope was used to check the effectiveness of the grinding and polishing and ultrasonic bath cleaning was performed to remove all dust and diamond particles.
2.6 Nano-mechanical properties of ITZ
3 Results and discussion
3.1 Properties of fresh and hardened mortar
Properties of the fresh and hardened mortar
Flow diameter (mm)
Yield value (Pa)
Plastic viscosity (Pa·s)
Air content (%)
The air content of fresh mortars with w/b = 0.3 increases slightly with the addition of silica fume. Along with this effect is a decrease in density after demoulding. However, due to the use of superplasticizer in the mortars with high w/c ratios (e.g., w/b = 0.5), segregation of the mixture may occur  and the air content of fresh mortars with silica fume is smaller than that of mortars without silica fume.
3.2 Compressive and bending strengths
Compressive and bending strengths of the mortar
Strength at 28 days (MPa)
w/b = 0.3
w/b = 0.5
It can be seen from Table 4 that the compressive and bending strengths of the mortars are influenced by the addition of silica fume and steel fiber. For the mortars without steel fiber, both the compressive strength and bending strength are increased by the addition of silica fume, where the increase of compressive strength is very obvious for mortars with low and high water/binder ratio, as pointed out by Kjellsen et al. . For the steel fiber reinforced mortar, the compressive strength is also increased by the addition of silica fume. However, the bending strength is little influenced or even reduced. For mortars without and with silica fume, although it was reported that the fibers are increasingly being used to enhance the strength and toughness of the cementitious composites , this enhancing effect of strength is not obvious for the 0.3 vol% addition of steel fiber. Similar test result was also obtained, where the density and compressive strength of fiber-reinforced concretes are about the same as those of the plain counterparts due to 1.0 vol% addition of steel fiber .
Due to the expected positive effect of the interfacial bond on the compressive strength  for the same batch of aggregate, different matrix compressive strength should result in different mortar compressive strength. For fiber reinforced mortars with low fiber volume content, if the contribution strength of steel fiber outweighs the decrease of the matrix compressive strength resulting from the increase of air content, the compressive strength of the fiber reinforced mortar may increase due to the addition of the low content of steel fiber; otherwise, the addition of the low content of steel fiber may have little or adverse effect on the compressive strength of the fiber reinforced mortar. With the increased fiber volume content, the compressive strength of the fiber reinforced mortars can greatly increase. For instance, the 4.67% higher air content of the hardened mortar and 0.3 vol% steel fiber results in a lower compressive strength of mortar 030003, compared with mortar 030000 with 4.03% air content; on the other hand, the 3.47% lower air content of the hardened mortar and 0.3 vol% steel fiber results in a higher compressive strength of mortar 051003, compared with mortar 051000 with 4.84% air content. When the fiber volume content increases from 0.3 to 1.0 vol%, such as mortar 030010, although the air content of hardened mortar increases, the compressive strength of mortar 030010 increases correspondingly.
3.3 Toughness of the mortar
Dissipated energy of the mortar
Dissipated energy (kN·mm)
w/b = 0.3
w/b = 0.5
Then, the influence of factors on the toughness of the mortar was analyzed, where the toughness is generally defined as the energy absorption capacity . It can be seen from Table 5 that the Welastic, Wpost-elastic and Wtotal of mortars with low water/binder ratio are larger than those of the counterparts with larger water/binder ratio, indicating high toughness of mortars with low water/binder ratio. In addition, the toughness of the mortar is influenced by addition of steel fiber. For the mortars with w/b = 0.3, both the Welastic and Wpost-elastic increase with the addition of steel fiber. As a result, the Wtotal of mortar 030010 is more than twice of that of mortar 030000 due to the addition of 1.0 vol% steel fiber. For the mortars with w/b = 0.5, although Welastic is nearly identical, Wpost-elastic is still increased due to the addition of steel fiber, resulting in an increase of the Wtotal. For steel fiber reinforced concrete, the toughness is also remarkably improved by the addition of 1.0 vol% steel fiber . Silica fume, however, has little or even negative effect on the mortar without steel fiber. For the steel fiber reinforced mortars, the Wtotal is increased due to the 10% addition of silica fume.
3.4 Properties of the interfacial transition zone in steel fiber reinforced mortars
Nanoindentation tests were carried out to study the steel fiber–matrix and fiber–matrix-aggregate interfaces in mortars 030003, 031003 and 050003. Firstly, the P–h curves of each indented area were checked to determine the validity of each indentation. Irregular P–h curves and curves exhibiting large displacement jump at any portion of the loading portion were discarded. The irregular nature of those curves may be due to the presence of a large void  while the discontinuous load–displacement curves may lie in the surface cracking during the force-driven indentation tests . Since the contact stiffness is measured only at peak load, and no restrictions are placed on the unloading data being linear during any portion of the unloading , curves showing nonlinear characteristic in the unloading portion were adopted.
The hardness profile of the ITZ between steel fiber and matrix in Fig. 5a shows the trend of type Ι in Fig. 2. In the distance 5–35 μm approaching the steel fiber surface, there is an obvious increase in hardness. Away the steel fiber surface, i.e. in the 35–65 μm zones, the hardness becomes relatively constant; while in Fig. 5b, the bonds across the interfaces are also efficient, as seen by the obvious rise in the hardness profile as the steel fiber and aggregate are approached. Low hardness is observed in the distance of 20–40 μm away the steel fiber–matrix interface. Within the steel fiber–matrix-aggregate zone, there are also some partially hydrated cement clinkers near the aggregate and fiber surfaces. As a whole, in mortar 030003, the bonding across the interfaces is quite efficient.
In Fig. 6a, the whole hardness profile of the ITZ between steel fiber and matrix likes type III in Fig. 2, but it is more uniform. Due to the addition of the silica fume, the hardness profile of the steel fiber–matrix interfacial zone is quite different from that of the mortar 030003. It is interesting to note that there is no obvious trough in the interfacial zone in Fig. 6a; comparatively low hardness is observed just in the 10–30 μm zones. Compared with 0.1636 GPa at the weakest point in Fig. 5a, this value reaches 0.2391 GPa in Fig. 6a, indicating an increase of the hardness value at the weakest point owing to the 10% addition of the silica fume. In Fig. 6b, small rise in the hardness profile as the steel fiber is approached indicates good bonding across the steel fiber-matrix interface. However, this bond is not as efficient as that of the steel fiber-matrix interface shown in Fig. 5b; the hardness value at the weakest point in Fig. 6b is very low, just 0.034 GPa. The influence of the silica fume is not as obvious as that shown in Fig. 6a. With the approaching of the aggregate, rise in the profiles is observed, indicating at least partial bonding at the aggregate–matrix interface, as suggested by Igarashi et al. . As a whole, the bond behavior in 031003 is good, but not as efficient as that in 030003. The addition of silica fume results in no distinct presence of weak ITZ between steel fiber and matrix in some parts of the mortar; however, in the steel fiber–matrix-aggregate interfacial zone, the effect of the silica fume is not obvious.
The hardness profile of the ITZ in Fig. 7a shows typical characteristic of type IV in Fig. 2. Compared with the comparatively high matrix hardness in mortars 030003 and 031003, comparatively low matrix hardness in mortar 050003 is observed due to the larger water/binder ratio. The lowest hardness is observed near the steel fiber surface, just 25.6 μm from the fiber surface. This value is 0.1452 GPa, indicating a decrease of the hardness at the weakest point owing to the increase of the water/binder ratio. A trough is shown in the 0–35 μm zones. This indicates that the interfacial bonding at the actual interface in specimen 050003 is poor and not as effective as that obtained in specimens 030003 and 031003 with low water/binder ratio. In Fig. 7b, small rise in the hardness profile as the steel fiber surface is approached can be indicative of at least partial bonding at the interface. Unlike the cases in mortars 030003 and 031003, the smallest hardness value occurred near the fiber surface. It is very surprising to note that in the distance 5–20 μm from the aggregate surface, the hardness are quite similar to those of the aggregate although it is part of matrix. It is doubted that there maybe a piece of aggregate underneath this area. As a whole, partial bond exhibits at the interfaces in mortar 050003.
3.5 Relationship between the ITZ and macroscopic mechanical properties
As pointed out above, the addition of the 1.0 vol% ductile steel fiber can greatly improve the three-point bending strength and fracture toughness of cementitious materials even the compressive strength is little improved . The magnitude of the bending strength and fracture toughness increase of fiber-reinforced cementitious composites is mainly controlled by their interfacial characteristics and microstructural morphology near the fiber surface. Thus, the effect of interfacial characteristics and microstructural morphology near the fiber surface on the bending strength and toughness of the fiber reinforced mortar are mainly discussed.
Properties of the fresh mortar are greatly influence by water/binder ratio, the addition of silica fume and steel fiber. With the increase of the water/binder ratio, the yield stress and plastic viscosity decrease corresponding. Due to the addition of the silica fume, the yield stress increases more for mortars with low water/binder ratio. For mortars with low water/binder ratio, yield stress and plastic viscosity increase greatly resulting from the addition of steel fiber; while for mortars with large water/binder ratio, the workability and consistency is less influenced by the low volume addition of steel fiber.
At both water/binder ratios, compressive strength increased with silica fume. If the air content of the hardened mortar is well controlled, compressive strength can be increased due to the low volume (0.3 vol%) addition of steel fiber; otherwise, the addition of the low steel fiber content may have little or adverse effect on the increase of the compressive strength of the mortar. With the increased volume addition of steel fiber from 0.3 to 1.0 vol%, the compressive strength of the mortar is remarkably increased.
Failure modes of the mortar are influence by the addition of steel fiber. Good ductile behavior is shown by steel fiber reinforced mortars. The toughness of the mortar is denoted by the dissipated energy. Mortars with lower water/binder ratio show higher toughness. For mortars with low water/binder ratio, the toughness is increased with the increased volume addition of steel fiber; for mortars with large water/binder ratio, the elastic energy is nearly identical while the post elastic energy is still increased due to the addition of steel fiber.
The three-point bending strength and the toughness of the steel fiber reinforced mortar are related to the interfacial characteristics and microstructural morphology near the fiber surface. For mortar with the same water/binder ratio, comparatively low bending strength and Welastic in mortar with 10% silica fume result from the wider thickness of the ITZ in the steel fiber–matrix-aggregate interfacial zone and voids and microcracks within this zone. Those discontinuous pores and the increase in microcracks may result from the reaction with silica fume and autogenous shrinkage. For mortar with different water/binder ratio, due to the poor bonds across the steel fiber–matrix and fiber–matrix-aggregate interfaces in mortar with higher water/binder ratio, the bending strength and fracture toughness are obviously lower than those of mortar with lower water/binder ratio.
The present work aims at linking the nano-mechanical properties of the ITZ with the macroscopic mechanical properties. It will help to proceed in the development of more ductile cement based materials and obtain fiber reinforced cementitious composites with high tensile strength, flexural strength and fracture toughness by tailoring the microstructure of the fiber–matrix interface and fiber–matrix-aggregate interface.
This research was done at the Norwegian University of Science and Technology (NTNU), when the first author is working at the Department of Structural Engineering as a visiting Professor for one year. The first author gratefully thank the support by the Norwegian Research Councils a part of the Cultural Agreement between Norway and China—Government scholarships 2007/2008 (No. 26X35003) and the National Natural Science Foundation of China (No. 50508020). The invitation provided by Professor Stefan Jacobsen in NTNU to cooperate with this research work is gratefully appreciated. In addition, the financial support from the COIN (Concrete Innovation Centre) Project 3-3.5—Nanotechnology applied to Cement-based Materials is greatly appreciated. Help given by Dr. Hilde Lea Lein, engineer Ove Edvard Loraas, engineer Arild Monsøy and Wilhelm Dall in NTNU is also acknowledged.