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Effect of nano-particles on durability of fiber-reinforced concrete pavement
Construction and Building Materials 48 (2013) 934–941
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of nano-particles on durability of fiber-reinforced concrete pavement Niloofar Salemi ⇑, Kiachehr Behfarnia Department of Civil Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
h i g h l i g h t s Nano-silica improves the compressive strength of fiber reinforced concrete pavement considerably. The compressive strength of fiber reinforced concrete pavement can slightly be increased by using nano-alumina. PP fibers increase the frost resistance of concrete pavement slightly. Frost resistance of fiber reinforced concrete pavement can be considerably improved by the addition of nano-particles.
a r t i c l e
i n f o
Article history: Received 11 August 2012 Received in revised form 15 July 2013 Accepted 21 July 2013
Keywords: Concrete pavement Polypropylene fibers Nano-silica Nano-alumina Frost resistance
a b s t r a c t In this study frost resistance and mechanical properties of fiber-reinforced concrete pavement containing nano-particles are studied. Nano-particles are employed to be as substitute of a portion of cement. For comparison the frost resistance and mechanical properties of plain concrete (control concrete), concrete containing nano-particles (without fibers) and concrete containing polypropylene fibers are also experimentally studied separately in this work. The specimens were subjected to cycles of freezing and thawing in water according to ASTM C666A. Experimental results show that using 5% nano-silica (by the weight of cementitious materials) improves the compressive strength and frost resistance of concrete as much as 30% and 83% respectively. Substitution 3% of cement by nano-alumina particles can improve compressive strength and frost durability of concrete as much as 8% and 81% respectively. Experimental results also show that polypropylene fibers can slightly affect the frost durability and compressive strength of concrete. As a general result it can be concluded that using nano-particles incorporating polypropylene fibers enhances frost resistance and compressive strength of concrete pavement because of that the permeability and porosity of concrete are reduced due to the use of nano-particles and also the tensile strength of concrete is enhanced due to the use of polypropylene fibers. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years the durability and service life of cementitious materials have been playing an important role in their operation as construction and pavement materials[1]. The durability parameters are especially related to the concrete air void system and to the bond between the aggregates and matrix. Freeze and thaw resistance is one of the vital factors in durability of concrete [2]. Due to the severe climate condition of some regions characterized by very low temperatures below 0 °C for most part of a year, concretes of frost resistance to 300 cycles of freezing and thawing are required for construction of precast beam elements of railway bridges, slabs for concrete pavements and infrastructures [3].
⇑ Corresponding author. Tel.: +98 311 6620696; fax: +98 311 2682644. E-mail address: [email protected] (N. Salemi). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.037
Concrete pavement is mostly used for road surfaces, bridge decks, airfield runways and parking lots. It endures dynamical loads and subjects to rigorous environment. It has been a significant and technical problem to improve the durability and to prolong the service life of concrete pavement in the world; because it is directly exposed to the weather condition. It is now wellestablished that air entrainment can significantly enhance frost resistance, although it causes a reduction in strength of concrete [4]. It is also believed that mineral admixtures and pozzolans can improve permeation-related durability of concrete. Pozzolans could make microstructure of concrete more compact and improve frost resistance [5]. The effect of pozzolans is mainly to improve the interfacial transition zone, resulting in a reduction in porosity of this zone [6]. On the other hand, recently, nanotechnology has attracted considerable scientific interest due to the new potential uses of particles in nanometer scale. Nano-technology encompasses the
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techniques of manipulation of the structure at the nanometer scale to develop a new generation of tailored, multifunctional cementitious composites with superior mechanical performance and durability potentially having a range of novel properties such as: low electrical resistivity, self-sensing capabilities, self-cleaning, selfhealing, high ductility and self-control of cracks. Concrete can be nano-engineered by the incorporation of nano-sized building blocks or objects (e.g. nano-particles and nano-tubes) to control material behavior and add novel properties, or by the grafting of molecules onto cement particles, cement phases, aggregates and additives (including nano-sized additives) to provide surface functionality, which can be adjusted to promote specific interfacial interactions [7]. The nanoscale size of particles can result in dramatically improved properties from conventional grain-size materials of the same chemical composition. Thus industries may be able to reengineer many existing products and to design new and novel products that function at unprecedented levels. When ultra-fine particles are incorporated into Portland-cement paste, mortar or concrete, materials with different characteristics from conventional materials are obtained. The performance of these cementbased materials is strongly dependent on nano-sized solid particles of calcium–silicate–hydrates (C–S–H), or nano-sized porosity at the interfacial transition zone between cement and aggregate particles. Typical properties affected by nano-sized particles or voids are strength, durability, shrinkage and steel-bond [8]. Previous researches on mixing nano-particles in cement-based building materials indicate that the inclusion of nano-particles modifies fresh and hardened state properties, even when compared with conventional mineral additions. Li et al. investigated cement mortars with nano-SiO2 and nano-Fe2O3 to explore their super mechanical and smart (temperature and sensing) potentials [9]. Nano-SiO2 seems to be the most popular nano-particle in the researches. Nano-scale SiO2 can act as a nano-filler, filling the spaces between particles of gel of C–S–H. Moreover nano-silica is a pozzolan which has high rate of pozzolanic reaction because of its high surface area to volume ratio, providing the potential for tremendous chemical activity. Nano-silica acts as nuclei for cement phases, further promoting cement hydration due to their high reactivity, as nano reinforcement and as filler, densifying the micro structure; thereby leading to a reduced porosity. The pozzolanic reaction of nano-SiO2 with calcium hydroxide increases the amount of C–S–H which improves the strength and durability of the material [8]. Nano-silica decreases the setting time of mortar when compared with silica fume. It also reduces bleeding water and segregation of the concrete, while improving the cohesiveness of the mixtures in the fresh state [10]. Additionally nano-silica has been found to increase resistance to water penetration [11], to help control the leaching of calcium [12], which is closely associated with various types of concrete degradation and to accelerate the hydration reactions of both C3S and ash–cement mortar as a result of the large and highly reactive surface of the nano-particles [13,14]. There are few reports on using nano-Al2O3 in concrete. It has been stated that the use of nano-alumina as a partial replacement by cement leads to the C–A–S (calcium aluminum silicate) gel formation in concrete. Nano-alumina reacts with calcium hydroxide produced from the hydration of calcium aluminates. The rate of this reaction is proportional to the amount of surface area available for the reaction. Therefore it is possible to add nano-alumina of a high purity and a high blain fineness value in order to improve the characteristics of concrete [15]. Nano-alumina improves mechanical properties of concrete such as compressive and tensile strength. It also decreases the water absorption and chloride penetration; improving the durability of
concrete [16]. The investigation results show that nano-alumina particles blended concrete has higher compressive strength compared to that of the concrete without nano-alumina particles. It is found that the cement could be advantageously replaced with nano-alumina particles up to maximum limit of 2% with average particle size of 15 nm [17]. Nano-alumina has been shown to significantly increase the modules of elasticity (up to 143% at a dosage of 5%) but to have a limited effect on the compressive strength [18]. As it is well-established, one of the most important factors which affect the durability of concrete against freeze and thaw cycles is tensile strength. In recent years, efforts to modify the brittle nature of ordinary concrete have resulted in the production of fiber-reinforced concrete. Depending on the kind and properties of fiber, concrete tensile strength can be up to several hundred times that of normal concrete. The freeze and thaw durability of concrete with synthetic fibers was investigated by Richardson et al. They used synthetic fibers in the amounts of 0.9 and 1.8 kg/m3. In their research concrete containing 0.9 kg/m3 showed a superior performance using compressive strength and modulus [19]. Zhang and Li investigated the effect of polypropylene fibers on the durability of concrete composite containing fly ash and silica fume. Four different fiber volume fractions (up to 0.12%) were used in their study. As the result of the study, they indicated that the addition of polypropylene fibers has greatly improved the durability of concrete. Besides freeze–thaw resistance of polypropylene fiber reinforced concrete was found to slightly increase with increase of the fiber volume [20]. Lack of available test data in the performance of concrete containing polypropylene fibers incorporating nano-particles additions was a key factor justifying this research. Fibers have the ability to entrain air and to improve the tensile strength of cement blocks and these are believed to be part of the reason for the demonstrated improvement in freeze and thaw durability. In this study the effect of nano-particles on mechanical properties and frost resistance of concrete pavement are experimentally investigated. Nano-particles are employed to be as substitute of a portion of cement. For comparison the frost resistance and mechanical properties of plain concrete (control concrete), concrete containing polypropylene fibers and concrete containing both nano-particles and polypropylene fibers are also separately studied.
2. Materials and experimental program 2.1. Materials and mixture proportion An ordinary Portland cement, conforming to the ASTM C150 [21] with a specific area of 3000 (cm2/g) and a specific gravity of 3.15 (g/cm3) is used. Its chemical composition is shown in Table 1. Fine aggregate is natural river sand with a fineness module of 2.6. The course aggregate is crushed stone with diameter of 5–12 mm. The superplasticizer (SP) admixture [polycarboxylic acid based (Glenium 51P) with density of 1082 gr/m3], is employed as much as 0.5% by the weight of cementitious materials to aid the dispersion of nano-particles in concrete and achieve good workability of concrete. According to the Dransfield research [22] in 1987, superplasticizer agents may affect the frost resistance of concrete because of their property of making bubbles in cement paste; so in order to confine the effective parameters in frost resistance of concrete, the amount of superplasticizer is kept constant in all specimens; because in this work the effect of polypropylene fibers and nano-particles on the durability of concrete is investigated and the effect of superplasticizer is not concerned.
Table 1 Chemical composition of Portland cement. SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
K2O
Loss
Insol
21.86
5.90
3.20
63.50
1.80
1.70
0.20
0.70
1.24
0.50
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Table 2 The properties of nano-particles. Item
Diameter (nm)
Specific surface area (m2/g)
Density (gr/cm3)
Purity
SiO2 Al2O3
20 8.0
220 280
0.05 0.12
99.9 >99.8%
The nano-silica and nano-alumina particles are purchased from Nanopars company (which imports nano-particles from Korea). The properties of nano-particles are given in Table 2. The polypropylene fibers with 12 mm length, 90 lm diameter and 30% elongation are used. The water to binder (the sum of cement and nano-particles) ratio used for all mixtures is 0.48. Sand ratio is 0.51. The mixture proportions for cubic meter of concrete are given in Table 3. Herein C denotes control concrete. C1P and C2P denote concrete containing 0.1% and 0.2% polypropylene (by the volume of concrete) respectively. In naming other specimens, the number which is right before S or A denotes the amount of nano-silica or nano-alumina respectively. For example C2P3S denotes concrete containing 0.2% polypropylene fibers (by the volume of concrete) incorporating 3% nano-silica (by the weight of cementitious materials). Table 3 also shows the results of slump test. In this study the amount of superplasticizer in all specimens is the same in order to make other factors such as frost resistance comparable; So it can be seen that when the content of nano-particles are larger, the workability of concrete is worse.
2.2. Specimen preparation To cast the concrete containing nano-particles, superplasticizer is firstly mixed into water in a mixer, and then nano-particles are added and stirred at a high speed for 5 min. Course aggregate, sand and cement are mixed at a low speed for 2 min in a concrete centrifugal blender, then the mixture of water, superplasticizer and nano-particles are slowly poured in and stirred at a low speed for another 2 min to achieve proper workability [23]. To cast control concrete and concrete containing PP fibers, superplasticizer is firstly dissolved in water. Then nano-particles (if used) are added and stirred. After course aggregate, sand, cement and PP fibers (if used) are mixed uniformly in a concrete centrifugal blender, the mixture of water, superplasticizer and nano-particles (if used) is poured in and stirred for several minutes. Finally the fresh concrete is poured into molds to form cubes of size 100 100 100 mm. After pouring, an external vibrator is used to facilitate compaction and decrease the amount of air bubbles. The specimens are demolded at 24 h and then cured in a standard moist room at a temperature of 20 ± 3 °C.
Following tests are conducted in order to determine mechanical properties and frost resistance of concrete containing nano-particles and polypropylene fibers with respect to control concrete. Each test was conducted on six specimens. The results are the average of all tests. a. Compressive test is conducted after 7, 28 and 120 days of curing according to the standard test method of BS 12390-3 [24]. b. The percentage of water absorption in specimens is measured after 28 days of curing according to the ASTM C642 procedure [25]. The test procedure involves drying each specimen to a constant weight, weighing it, immersing it in water for specified time (24 h in this work), and weighing it again. The increase in weight as a percentage of the original weight is expressed as its water absorption (in percentage). c. The specimens are subjected to cycles of freezing and thawing in an automatic freeze–thaw machine which can apply freezing cycles at 18 °C and thawing cycles at 4 °C both in water according to ASTM C666A [26]. The loss of mass, change in length, increase in water absorption and reduction in compressive strength of specimens are measured in certain cycles. In this machine the specimens were subjected to freezing and thawing cycles for 6 and 2 h respectively. The freeze and thaw machine was designed by authors.
3. Experimental results and discussion 3.1. Compressive strength Table 4 shows the compressive strength of specimens after 7, 28 and 120 days. It can be found that the compressive strength is developed in concretes containing nano-particles in every case higher than that of control concretes. It can also be concluded that polypropylene fibers do not affect the compressive strength of concrete considerably. As Table 4 shows, the compressive strength of concrete is considerably improved by using nano-silica particles as a part of cementitious materials. This fact confirms the results of pervious researches [7,10,15]. The compressive strength of concrete is enhanced as much as 16.67% (in comparison to that of control concrete) by replacing 3% cement with nano-silica particles. The enhanced extent of compressive strength increases from 16.67% to 30.13% when the content of nano-silica increases from 3% to 5% by the weight of cementitious materials and then it decreases to 23.58% when the nano-silica content increases to 7% by the weight of cementitious materials. The enhancement of the compressive strength of concrete containing nano-silica in comparison
Table 3 Mix proportions of specimens (kg/m3). Mixture no.a
Water
Cement
Sand
Coarse aggregate
nSiO2
nAl2O3
UNF
Slump (cm)
C C1P C2P C3S C1P3S C2P3S C5S C1P5S C2P5S C7S C1P7S C2P7S C1A C1P1A C2P1A C2A C1P2A C2P2A C3A C1P3A C2P3A
168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168
350.0 350.0 350.0 339.5 339.5 339.5 332.5 332.5 332.5 325.5 325.5 325.5 346.5 346.5 346.5 343 343 343 339.5 339.5 339.5
960 958 956 953 951 949 950 948 946 947 945 943 956 954 952 955 953 951 954 952 950
920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920
– – – 10.5 10.5 10.5 17.5 17.5 17.5 24.5 24.5 24.5 – – – – –– – – – –
– – – – – – – – – – – – 3.5 3.5 3.5 7 7 7 10.5 10.5 10.5
1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75
12.0 10.5 9.0 9.0 7.5 6.0 6.0 5.0 3.5 2.0 0.5 0.0 10.5 9.0 7.5 8.5 7.0 5.5 6.5 5.0 3.5
a The number just before S, A or P defines the percentage of nano-silica, nano-alumina or PP fibers respectively. For example C2P1A defines concrete containing 0.2% polypropylene fibers incorporating 3% nano-alumina.
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N. Salemi, K. Behfarnia / Construction and Building Materials 48 (2013) 934–941 Table 4 Compressive strength of specimens (MPa). Mixture no.
Compressive Strength 7 days
C C1P C2P C3S C1P3S C2P3S C5S C1P5S C2P5S C7S C1P7S C2P7S C1A C1P1A C2P1A C2A C1P2A C2P2A C3A C1P3A C2P3A
28 days
120 days
Target
Enhanced extent (%)
Target
Enhanced extent (%)
Target
Enhanced extent (%)
27.10 27.69 28.11 29.27 29.85 30.01 31.16 31.48 31.92 30.19 30.61 30.95 27.65 28.34 29.22 27.84 28.31 29.47 27.97 29.10 30.03
0.00 2.17 3.73 8.00 10.15 10.74 14.98 16.16 17.78 11.40 12.95 14.21 2.03 4.57 7.82 2.73 4.46 8.75 3.21 7.38 10.81
42.11 43.04 43.52 49.13 49.87 50.12 54.80 54.89 55.07 52.04 52.37 52.06 43.81 44.92 46.21 44.64 45.97 47.15 45.48 46.52 45.91
0.00 2.21 3.35 16.67 18.43 19.02 30.13 30.35 30.78 23.58 24.36 23.62 4.03 6.67 9.74 6.01 9.17 11.97 8.00 10.47 9.02
47.15 49.75 50.92 60.35 61.28 61.42 68.36 68.83 69.15 64.12 64.76 65.15 50.45 51.91 52.48 51.39 52.29 53.61 52.34 53.30 54.78
0.00 5.51 8.00 28.00 29.97 30.27 44.98 45.98 46.66 35.99 37.35 38.18 7.00 10.10 11.30 8.99 10.90 13.70 11.01 13.04 16.18
to that of control concrete can be attributed to the pozzolanic reaction of nano-silica [7]. Nano-silica is thought to be very effective in pozzolanic reaction which leads to the formation of the C–S–H gel. Also, nano-silica would fill pores to increase the compressive strength. As a general result the compressive strength of the concrete was found to increase as the nano-silica content increases from 3% to 5% and then to decrease with the increase of nano-silica content from 5% to 7%, although the results of 7% replacement are still higher than those of control concrete and concrete containing 3% nano-silica. Two reasons may cause the reduction in enhanced extent of compressive strength (when 7% nano-silica is applied); It may be due to the fact that the quantity of nano-silica particles present in the mix is higher than the amount required to react with the liberated lime after the process of hydration, leading to leach out the excess silica and causing a deficiency in strength as it is substituted to a portion of cementitious materials but does not contribute in developing strength [23]. So in terms of compressive strength, 5% nano-silica is the optimum amount; because using nano-silica in high amounts (more than 5%) does not improve the compressive strength of concrete. Also it may be due to the agglomeration and defects generated in dispersion of nano-silica particles. It should be noted that using a high content of nano-particles must be accompanied by adjustments to the water superplasticizer dosage in the mix in order to ensure that no agglomeration would happen and specimens do not suffer from excessive self desiccation and cracking. In addition, because nano-particles are more difficult to uniformly disperse, when the content are large, the weak zone in concrete increases which results in the decrease of the strength of the concrete. But in this work the amount of superplasticizer is kept constant in order to confine freeze and thaw parameters. According to Table 4, the compressive strength of concrete containing nano-alumina particles is also improved as it is mentioned in previous studies [7,16–18]. The compressive strength of concrete is enhanced as much as 4.03% by replacing 1% of cement (by the weight) with nano-alumina particles. The enhanced extent of compressive strength increases from 4.03% to 8.00% when the content of nano-alumina increases from 1% to 3% by the weight of cementitious materials. The enhancement of compressive strength of concrete containing nano-alumina is due to the rapid consuming of Ca(OH)2 formed during hydration of Portland
cement, related to the high reactivity of nano-Al2O3 particles. As a consequence the hydration of cement is accelerated and larger volumes of reaction products are formed. Also nano-Al2O3 particles recover the particle packing density of the concrete and as a nanofiller improve the microstructure of it, directing to a reduced volume of larger pores in the cement paste. It also can be seen that the compressive strength of the concretes containing PP fibers enhanced only a little in comparison to that of control concrete. This conclusion had been reached in other studies too. Shuan-fa and Liao showed the same conclusions in separate researches [27,28]. They found that PP fibers have little effect on compressive strength of concrete, but their effect on tensile strength of concrete is considerable, as it is commonly believed. 3.2. Water absorption There are several methods to perform the water permeability tests such as percentage of water absorption, rate of water absorption and coefficient of water absorption. In this work, to evaluate the water permeability of the specimens, percentage of water absorption is a criterion of the pore volume or porosity of concrete after hardening, which is occupied by water in saturated state. Water absorption values of control concrete and concrete containing nano-particles were measured according to the ASTM C642 [25] after 28 days of moisture curing. Fig. 1 shows the result of water absorption test.
Fig. 1. Water absorption of specimens.
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Fig. 1 shows that the percentage of water absorption in concrete containing nano-particles is considerably lower than that of control concrete. Fig. 1 also shows that the percentage of water absorption of concrete decreases by increasing the nano-particles content. This may be due to the improved microstructure of concrete according to the use of nano-particles. As mentioned above, nano-Al2O3 and nano-SiO2 particles recover the particle packing density of the concrete and as nano-filler improve the microstructure of it; thus the water permeability of concrete decreases by the replacement of cement with nano-particles partially. In Fig. 1 it can
3.3.1. Compressive strength The compressive strength of specimens subjected to the freezing and thawing cycles, was determined after 50,150 and 300 cycles. Table 5 shows the results of compressive strength determination. Figs. 2 and 3 show the effect of nano-silica and nano-alumina on durability of concrete in terms of compressive strength. As Figs. 2 and 3 show, the strength loss of concrete containing nano-particles is much lower than that of control concrete. For example concrete containing 3% nano-silica (by the weight of
Table 5 The results of frost resistance determination in terms of strength loss for concrete using the rapid test method. Mixture no.
C C1P C2P C3S C1P3S C2P3S C5S C1P5S C2P5S C7S C1P7S C2P7S C1A C1P1A C2P1A C2A C1P2A C2P2A C3A C1P3A C2P3A
Residual/original compressive strength 50
150
300
0.77 0.81 0.83 0.93 0.94 0.95 0.95 0.95 0.96 0.94 0.95 0.96 0.94 0.95 0.96 0.94 0.95 0.95 0.94 0.95 0.96
0.35 0.48 0.55 0.83 0.85 0.87 0.89 0.90 0.91 0.85 0.86 0.88 0.86 0.88 0.90 0.87 0.89 0.90 0.88 0.90 0.91
0.00 0.00 0.00 0.72 0.76 0.79 0.84 0.86 0.87 0.77 0.80 0.82 0.76 0.79 0.80 0.79 0.81 0.84 0.82 0.85 0.87
be seen that the water absorption of concrete containing 7% nanosilica is lower than that of concrete which contains 5% nano-silica, while the compressive strength of concrete containing 5% nano-silica is higher than that of concrete containing 7% nano-silica. It can be explained as below: the specimen named C7S (concrete containing 7% nano-silica) has 2% excess nano-silica in comparison to C5S (concrete containing 5% nano-silica). This 2% nano-silica is substituted to a portion of cementitious materials but does not contribute in developing strength (C5S has 2% cement more than C7S that leads to a higher compressive strength). Fig. 1 shows that 2% excess nano-silica would fill pores leading to lower water absorption. In Fig. 1, it can be seen that polypropylene fibers increases the water permeability of concrete slightly. This may be due to the mixing procedure which may let micro bubbles entered the zone between fibers and cement paste. Using PP fibers in concrete leads to a lower workability compared to that of concrete without fibers; so it becomes harder for cement paste to flow and fill the pores. As a result there will be few more pores in fiber-reinforced concrete and the percentage of water absorption will slightly be more than that of control concrete.
Strength loss % (after 300 cycles)
Cracking
Frost resistance
100.00 100.00 100.00 28.00 23.74 21.41 16.28 14.09 12.52 23.20 19.82 17.93 23.84 21.36 19.72 21.31 19.01 16.25 18.19 14.82 12.76
Severe Severe Severe Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight
Poor Poor Poor Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory
Fig. 2. Strength loss of specimens containing nano-silica.
3.3. Frost resistance In order to determine the frost resistance of concrete, the decrease in compressive strength, change in length, loss of mass and increase in water absorption in specimens measured in different cycles. It should be noted that the specimens were subjected to cycles of freezing and thawing in water according to the procedure of ASTM C666A [26].
Fig. 3. Strength loss of specimens containing nano-alumina.
N. Salemi, K. Behfarnia / Construction and Building Materials 48 (2013) 934–941
Fig. 4. Strength loss of specimens containing polypropylene fibers.
cementitious materials) showed only 28% strength loss after 300 cycles of freezing and thawing, while the strength loss of control concrete after 300 cycles was 100%. The reduction of compressive strength of concrete containing nano-silica after freezing and thawing cycles (300 cycles) was found to decrease from 28% to 16.28% as nano-silica content increases from 3% to 5% and then to increase from 16.28% to 23.20% as nano-silica content increases from 5% to 7%. The percentage of strength loss in concrete containing nano-alumina is considerably lower than that of control concrete and it is shown to decrease from 23.84% to 18.19% as the nano-alumina content increases from 1% to 3%. The effect of polypropylene fibers on frost resistance of concrete can be seen in Fig. 4. In Fig. 4 it can be seen that PP fibers improve the durability of concrete slightly which confirms the results of pervious researches by Richrdson and Zhang [19,20]. For example, , Strength loss of concrete after 150 cycles of freezing and thawing decreases from 65% to 45.22% as the PP content increases from 0% to 0.2% (by the volume of concrete). The effect of PP fibers on improving the frost resistance of concrete in terms of compressive strength is much less than that of nano-particles. According to Figs. 2 and 3, it can be said that among the tested samples, concrete containing 5% nano-silica and concrete containing 3% nano-alumina showed the highest frost resistance among the tested samples in terms of compressive strength; because these specimens had the lowest strength loss after 300 cycles of freezing and thawing.
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3.3.2. Length change and mass loss After 50, 150 and 300 cycles of freezing and thawing, change in length and mass loss of specimens were measured in order to determine the frost resistance of concrete in terms of mass and length change. Fig. 5 shows the length change and mass loss of specimens under the freezing and thawing condition. In Fig. 5, it can be seen that the mass loss and length change of concrete containing nano-particles are much lower than that of control concrete. Control concrete showed 28.1% decrease in length after 300 cycles; while the decrease in length of concrete containing 1% nano-alumina and concrete containing 3% nano-silica after 300 cycles of freezing and thawing are 6.18% and 5.82% respectively. Fig. 5 also shows that the change in length decreases from 5.82% to 3.81% as the nano-alumina content of specimens increases from 1% to 3%. The length change of concrete containing nano-silica decreases from 6.18% to 1.41% by increasing the usage of nano-silica from 3% to 5% and then it increases from 1.41% to 4.05% when the applied content of nano-silica increases from 5% to 7%. Mass loss of specimens decreases from 17.28% to 11.47% by increasing the nano-alumina content from 1% to 3%. Mass loss of concrete containing nanosilica has the same order as strength loss and length change of them. According to Fig. 5, the mass loss and length change of concrete containing PP fibers decreases slightly as fiber content increases. In terms of length and mass change, among the tested concretes, concrete containing 5% nano-silica (by the weight of cementitious materials) incorporating 0.2% PP fibers (by the volume of concrete) showed to be the most frost resistant concrete.
3.3.3. Water absorption Water absorption of specimens measured after 50, 150 and 300 cycles of freezing and thawing. Fig. 6 shows the water absorption of specimens before and after freeze and thaw cycles. It can be seen that the nano-particles decrease the water absorption of specimens. Fig. 6 shows that using nano-particles in concrete, controls the rate of increase in water absorption during the freeze and thaw cycles. The water absorption of control concrete has been increased as much as 117% after 300 cycles of freezing and thawing; while the water absorption of concrete containing 1% nano-alumina and concrete containing 5% nano-silica have been increased as
Fig. 5. Length and mass loss of specimens after 300 cycles of freezing and thawing.
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Fig. 6. Percentage of increase in water absorption of specimens after 300 cycles of freezing and thawing.
much as 50.29% and 31.19% respectively. Water absorption of concrete was found to decrease by increasing nano-particles content. As Fig. 6 shows polypropylene fibers also control the rate of increase in water absorption percentage during the freeze and thaw cycles; although nano-particles are much more effective in improving the durability of concrete against freeze and thaw cycles in case of water absorption percentage.
and thawing cycles); however the index of frost resistance improvement of concrete containing nano-particles are much larger than that of concrete containing PP fibers. 4. Conclusion Based on the results of experiments in this study, the following primary conclusions can be obtained:
3.4. Discussion The mechanical properties and frost resistance of concretes containing nano-particles is improved. The mechanism of improving these properties of concrete by nano-particles can be interpreted as follows: supposed that nano-particles are uniformly dispersed and each particle is contained in a cubic pattern, the distance between nano-particles can be specified. After hydration begins, hydrate products diffuse and envelope nanoparticles as kernel. If the content of nano-particles and the distance between them are appropriate, the crystallization will be controlled to be a suitable state through restricting the growth of Ca(OH)2 crystal by nano-particles. This makes the cement matrix more homogeneous and compact. Moreover nano-alumina and nano-silica as pozzolanic materials react with calcium hydroxide formed from calcium silicate hydration. The rate of the pozzolanic reaction is proportional to the amount of surface area available for the reaction; so it is plausible to use these nano-particles in order to produce a concrete with higher strength and considerably improved microstructure. Nanoalumina and nano-silica not only behave as fillers to improve microstructure but also as activators to promote pozzolanic reaction; so the medium size of pores in concrete decreases with the use of nano-particles. As a result concrete containing nanoparticles absorbs less water in comparison to control concrete because of a denser microstructure; so the stress, which is produced due to the volume change of frozen water, decreases and the concrete will be more frost resistant. The frost resistance of concrete containing PP fibers is improved, which is mostly attributed to the tensile strength-enhancing, crack-arresting effect of PP fibers [27], the bridge effect of PP fibers on cracks and diversion effect of fibers on separated cement blocks [28]. In other words the frost resistance of concretes containing nano-particles and PP fibers is improved (in terms of strength, water absorption percentage, length and mass loss after freezing
(1) The compressive strength of concrete can slightly be increased by using nano-alumina as a substitute to cement materials partially. Compressive strength of concrete was found to increase as much as 8% by replacing cementitious materials by nano-alumina. (2) Nano-silica considerably improves the compressive strength of concrete. The optimum content of nano-silica in concrete in order to increase its compressive strength is 5% (by the weight of cementitious materials). The compressive strength of concrete improves as much as 30% by using 5% nano-silica (by the weight of cementitious materials). (3) Using nano-particles in concrete lower the workability of the concrete. (4) Frost resistance of concrete can be considerably improved by the addition of nano-particles. Nano-alumina and nano-silica behave not only as promoters of pozzolanic reaction (because of high surface area to volume ratio), but also as fillers, improving the pore structure of concrete and densifying the microstructure of cement paste. The strength loss of concrete containing 5% nano-silica was 16% while the strength loss of control concrete was 100% after 300 cycles. Using 5% nano-silica results in 84% reduction in deterioration of concrete after freezing and thawing cycles (in terms of compressive strength). Also 3% nano-alumina can improve the durability of concrete against freezing and thawing cycles as much as 82%. (5) PP fibers increase the frost resistance of concrete slightly. However the enhanced extent of frost resistance of concrete containing nano-particles is much larger than that of concrete containing PP fibers; so nano-particles are more favorable to improve the frost resistance of concrete than PP fibers. (6) The concrete containing both nano-particles and PP fibers showed to be the most frost resistant among the studied samples.
N. Salemi, K. Behfarnia / Construction and Building Materials 48 (2013) 934–941
(7) Among the tested concretes, concrete containing 5% nanosilica (by the weight of cementitious materials) incorporating 0.2% PP fibers (by the volume of concrete) showed the most frost resistance. The improvement of durability in concrete containing 5% nano-silica incorporating 0.2% polypropylene fibers was about 87% (in terms of strength).
References [1] Pospichal O, Kucharczykova B, Misak P, Vymazal T. Freeze–thaw resistance of concrete with porous aggregate. Proc Eng 2010;2(1):521–9. [2] Chandra S, Aavik J, Berntsson L. Influence of polymer microparticles on freezethaw resistance of structural lightweight aggregate concrete. Int J Cem Compos Lightweight Concr 1982;4(2):111–5. [3] Peng GF, Ma Q, Hu HM, Gao R, Yao QF, Liu YF. The effects of air entrainment and pozzolans on frost resistance of 50–60 MPa grade concrete. Constr Build Mater 2007;21(5):1034–9. [4] Pigeon M, Marchand J, Pleau R. Frost resistant concrete. Constr Build Mater 1996;10(5):339–48. [5] Pacewska B, Bukowska M, Wilinska I, Swat M. Modification of the properties of concrete by a new pozzolan – a waste catalyst from the catalytic process in a fluidized bed. Cem Concr Res 2002;32(1):142–52. [6] Gao XF, Lo YT, Tam CM. Investigation of micro-cracks and microstructure of high performance lightweight aggregate concrete. Build Environ 2002;37(5):485–9. [7] Sanchez F, Sobolev K. Nanotechnology in concrete – a review. Constr Build Mater 2010;24(11):2060–71. [8] Sneff L, Labrincha JA, Ferreira VM, Hotza D, Repette WL. Effect of nano-silica on rheology and fresh properties of cement pastes and mortars. Constr Build Mater 2009;23(7):2487–91. [9] Li H, Xiao H, Yuan J, Ou J. Microstructure of cement mortar with nano-particles. Composites Part B 2003;35(2):185–9. [10] Qing Y, Zenan Z, Deyu K, Rongshen C. Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume. Constr Build Mater 2007;21(3):539–45. [11] Ji T. Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2. Cem Concr Res 2005;35(10):1943–7.
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[12] Gaitero JJ, Campillo I, Guerrero A. Reduction of the calcium leaching rate of cement paste by addition of silica nano-particles. Cem Concr Res 2008;38(8– 9):1112–8. [13] Bjornstrom J, Martinelli A, Matic A, Borjesson L, Panas I. Accelerating effects of colloidal nano-silica for beneficial calcium–silicate–hydrate formation in cement. Chem Phys Lett 2004;392(1–3):242–8. [14] Lin KL, Chang WC, Lin DF, Luo HL, Tcai MC. Effects of nano-SiO2 and different ash particle sizes on sludge ash–cement mortar. J Environ Manage 2008;88(4):708–14. [15] Jo BW, Kim CH, Tae GH, Park JB. Characteristics of cement mortar with nanoSiO2 particles. Constr Build Mater 2007;21(6):1351–5. [16] Shekari AH, Razzaghi MS. Influence of nanoparticles on durability and mechanical properties of high performance concrete. Proc Eng 2011;14:3036–41. [17] Nazari A, Riahi S, Riahi S, Shamekhi SF, Khademno A. Influence of Al2O3 nanoparticles on the compressive strength and workability of blended concrete. J Am Sci 2010;6(5):6–9. [18] Li Z, Wang H, He S, Lu Y, Wang M. Investigations on the preparation and mechanical properties of the nano-alumina reinforced cement composite. Mater Lett 2006;60(3):356–9. [19] Richardson AE, Coventry KA, Wikinson S. Durability of concrete with synthetic fiber additions. Cold Reg Sci Technol 2012;83:49–56. [20] Zhang P, Li Q. Effect of polypropylene fiber on durability of concrete composite containing fly ash and silica fume. Compos B Eng 2013;45(1):1587–94. [21] ASTM C150/C150M. Standard specification for portland cement. Annual book of ASTM standards. ASTM: Philadelphia (PA); 2001. [22] Dransfield MJ. Durability of concrete with a superplasticizing admixture. Am Concr Inst 1987;100(1):741–64. [23] Li H, Zhang M, Ou J. Abrasion resistance of concrete containing nano-particles for pavement. Wear 2006;260(11–12):1262–6. [24] BS 12390-3. Compressive strength of hardened concrete. British-Adopted European Standard; 2009. [25] ASTM C642. Standard test method for density, absorption, and voids in hardened concrete. Annual book of ASTM standards. ASTM: Philadelphia (PA); 2001. [26] ASTM C666/C666M. Standard test method for resistance of concrete to rapid freezing and thawing. Annual book of ASTM standards. ASTM: Philadelphia (PA); 2001. [27] Shuan-fa C, Deng-liang Z, Jie Z, Feng L. The study of the road performance of the polypropylene fiber concrete. Northeast Highway 2001;24(2):23–5. [28] Liao X, He Y, Yang X. Interfacial behavior of PP fiber reinforced cement composite: study of abrasion resistance. Build Mater 1999;2:324–8.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of nano-particles on durability of fiber-reinforced concrete pavement Niloofar Salemi ⇑, Kiachehr Behfarnia Department of Civil Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
h i g h l i g h t s Nano-silica improves the compressive strength of fiber reinforced concrete pavement considerably. The compressive strength of fiber reinforced concrete pavement can slightly be increased by using nano-alumina. PP fibers increase the frost resistance of concrete pavement slightly. Frost resistance of fiber reinforced concrete pavement can be considerably improved by the addition of nano-particles.
a r t i c l e
i n f o
Article history: Received 11 August 2012 Received in revised form 15 July 2013 Accepted 21 July 2013
Keywords: Concrete pavement Polypropylene fibers Nano-silica Nano-alumina Frost resistance
a b s t r a c t In this study frost resistance and mechanical properties of fiber-reinforced concrete pavement containing nano-particles are studied. Nano-particles are employed to be as substitute of a portion of cement. For comparison the frost resistance and mechanical properties of plain concrete (control concrete), concrete containing nano-particles (without fibers) and concrete containing polypropylene fibers are also experimentally studied separately in this work. The specimens were subjected to cycles of freezing and thawing in water according to ASTM C666A. Experimental results show that using 5% nano-silica (by the weight of cementitious materials) improves the compressive strength and frost resistance of concrete as much as 30% and 83% respectively. Substitution 3% of cement by nano-alumina particles can improve compressive strength and frost durability of concrete as much as 8% and 81% respectively. Experimental results also show that polypropylene fibers can slightly affect the frost durability and compressive strength of concrete. As a general result it can be concluded that using nano-particles incorporating polypropylene fibers enhances frost resistance and compressive strength of concrete pavement because of that the permeability and porosity of concrete are reduced due to the use of nano-particles and also the tensile strength of concrete is enhanced due to the use of polypropylene fibers. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years the durability and service life of cementitious materials have been playing an important role in their operation as construction and pavement materials[1]. The durability parameters are especially related to the concrete air void system and to the bond between the aggregates and matrix. Freeze and thaw resistance is one of the vital factors in durability of concrete [2]. Due to the severe climate condition of some regions characterized by very low temperatures below 0 °C for most part of a year, concretes of frost resistance to 300 cycles of freezing and thawing are required for construction of precast beam elements of railway bridges, slabs for concrete pavements and infrastructures [3].
⇑ Corresponding author. Tel.: +98 311 6620696; fax: +98 311 2682644. E-mail address: [email protected] (N. Salemi). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.037
Concrete pavement is mostly used for road surfaces, bridge decks, airfield runways and parking lots. It endures dynamical loads and subjects to rigorous environment. It has been a significant and technical problem to improve the durability and to prolong the service life of concrete pavement in the world; because it is directly exposed to the weather condition. It is now wellestablished that air entrainment can significantly enhance frost resistance, although it causes a reduction in strength of concrete [4]. It is also believed that mineral admixtures and pozzolans can improve permeation-related durability of concrete. Pozzolans could make microstructure of concrete more compact and improve frost resistance [5]. The effect of pozzolans is mainly to improve the interfacial transition zone, resulting in a reduction in porosity of this zone [6]. On the other hand, recently, nanotechnology has attracted considerable scientific interest due to the new potential uses of particles in nanometer scale. Nano-technology encompasses the
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techniques of manipulation of the structure at the nanometer scale to develop a new generation of tailored, multifunctional cementitious composites with superior mechanical performance and durability potentially having a range of novel properties such as: low electrical resistivity, self-sensing capabilities, self-cleaning, selfhealing, high ductility and self-control of cracks. Concrete can be nano-engineered by the incorporation of nano-sized building blocks or objects (e.g. nano-particles and nano-tubes) to control material behavior and add novel properties, or by the grafting of molecules onto cement particles, cement phases, aggregates and additives (including nano-sized additives) to provide surface functionality, which can be adjusted to promote specific interfacial interactions [7]. The nanoscale size of particles can result in dramatically improved properties from conventional grain-size materials of the same chemical composition. Thus industries may be able to reengineer many existing products and to design new and novel products that function at unprecedented levels. When ultra-fine particles are incorporated into Portland-cement paste, mortar or concrete, materials with different characteristics from conventional materials are obtained. The performance of these cementbased materials is strongly dependent on nano-sized solid particles of calcium–silicate–hydrates (C–S–H), or nano-sized porosity at the interfacial transition zone between cement and aggregate particles. Typical properties affected by nano-sized particles or voids are strength, durability, shrinkage and steel-bond [8]. Previous researches on mixing nano-particles in cement-based building materials indicate that the inclusion of nano-particles modifies fresh and hardened state properties, even when compared with conventional mineral additions. Li et al. investigated cement mortars with nano-SiO2 and nano-Fe2O3 to explore their super mechanical and smart (temperature and sensing) potentials [9]. Nano-SiO2 seems to be the most popular nano-particle in the researches. Nano-scale SiO2 can act as a nano-filler, filling the spaces between particles of gel of C–S–H. Moreover nano-silica is a pozzolan which has high rate of pozzolanic reaction because of its high surface area to volume ratio, providing the potential for tremendous chemical activity. Nano-silica acts as nuclei for cement phases, further promoting cement hydration due to their high reactivity, as nano reinforcement and as filler, densifying the micro structure; thereby leading to a reduced porosity. The pozzolanic reaction of nano-SiO2 with calcium hydroxide increases the amount of C–S–H which improves the strength and durability of the material [8]. Nano-silica decreases the setting time of mortar when compared with silica fume. It also reduces bleeding water and segregation of the concrete, while improving the cohesiveness of the mixtures in the fresh state [10]. Additionally nano-silica has been found to increase resistance to water penetration [11], to help control the leaching of calcium [12], which is closely associated with various types of concrete degradation and to accelerate the hydration reactions of both C3S and ash–cement mortar as a result of the large and highly reactive surface of the nano-particles [13,14]. There are few reports on using nano-Al2O3 in concrete. It has been stated that the use of nano-alumina as a partial replacement by cement leads to the C–A–S (calcium aluminum silicate) gel formation in concrete. Nano-alumina reacts with calcium hydroxide produced from the hydration of calcium aluminates. The rate of this reaction is proportional to the amount of surface area available for the reaction. Therefore it is possible to add nano-alumina of a high purity and a high blain fineness value in order to improve the characteristics of concrete [15]. Nano-alumina improves mechanical properties of concrete such as compressive and tensile strength. It also decreases the water absorption and chloride penetration; improving the durability of
concrete [16]. The investigation results show that nano-alumina particles blended concrete has higher compressive strength compared to that of the concrete without nano-alumina particles. It is found that the cement could be advantageously replaced with nano-alumina particles up to maximum limit of 2% with average particle size of 15 nm [17]. Nano-alumina has been shown to significantly increase the modules of elasticity (up to 143% at a dosage of 5%) but to have a limited effect on the compressive strength [18]. As it is well-established, one of the most important factors which affect the durability of concrete against freeze and thaw cycles is tensile strength. In recent years, efforts to modify the brittle nature of ordinary concrete have resulted in the production of fiber-reinforced concrete. Depending on the kind and properties of fiber, concrete tensile strength can be up to several hundred times that of normal concrete. The freeze and thaw durability of concrete with synthetic fibers was investigated by Richardson et al. They used synthetic fibers in the amounts of 0.9 and 1.8 kg/m3. In their research concrete containing 0.9 kg/m3 showed a superior performance using compressive strength and modulus [19]. Zhang and Li investigated the effect of polypropylene fibers on the durability of concrete composite containing fly ash and silica fume. Four different fiber volume fractions (up to 0.12%) were used in their study. As the result of the study, they indicated that the addition of polypropylene fibers has greatly improved the durability of concrete. Besides freeze–thaw resistance of polypropylene fiber reinforced concrete was found to slightly increase with increase of the fiber volume [20]. Lack of available test data in the performance of concrete containing polypropylene fibers incorporating nano-particles additions was a key factor justifying this research. Fibers have the ability to entrain air and to improve the tensile strength of cement blocks and these are believed to be part of the reason for the demonstrated improvement in freeze and thaw durability. In this study the effect of nano-particles on mechanical properties and frost resistance of concrete pavement are experimentally investigated. Nano-particles are employed to be as substitute of a portion of cement. For comparison the frost resistance and mechanical properties of plain concrete (control concrete), concrete containing polypropylene fibers and concrete containing both nano-particles and polypropylene fibers are also separately studied.
2. Materials and experimental program 2.1. Materials and mixture proportion An ordinary Portland cement, conforming to the ASTM C150 [21] with a specific area of 3000 (cm2/g) and a specific gravity of 3.15 (g/cm3) is used. Its chemical composition is shown in Table 1. Fine aggregate is natural river sand with a fineness module of 2.6. The course aggregate is crushed stone with diameter of 5–12 mm. The superplasticizer (SP) admixture [polycarboxylic acid based (Glenium 51P) with density of 1082 gr/m3], is employed as much as 0.5% by the weight of cementitious materials to aid the dispersion of nano-particles in concrete and achieve good workability of concrete. According to the Dransfield research [22] in 1987, superplasticizer agents may affect the frost resistance of concrete because of their property of making bubbles in cement paste; so in order to confine the effective parameters in frost resistance of concrete, the amount of superplasticizer is kept constant in all specimens; because in this work the effect of polypropylene fibers and nano-particles on the durability of concrete is investigated and the effect of superplasticizer is not concerned.
Table 1 Chemical composition of Portland cement. SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
K2O
Loss
Insol
21.86
5.90
3.20
63.50
1.80
1.70
0.20
0.70
1.24
0.50
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N. Salemi, K. Behfarnia / Construction and Building Materials 48 (2013) 934–941 2.3. Experimental program and test methods
Table 2 The properties of nano-particles. Item
Diameter (nm)
Specific surface area (m2/g)
Density (gr/cm3)
Purity
SiO2 Al2O3
20 8.0
220 280
0.05 0.12
99.9 >99.8%
The nano-silica and nano-alumina particles are purchased from Nanopars company (which imports nano-particles from Korea). The properties of nano-particles are given in Table 2. The polypropylene fibers with 12 mm length, 90 lm diameter and 30% elongation are used. The water to binder (the sum of cement and nano-particles) ratio used for all mixtures is 0.48. Sand ratio is 0.51. The mixture proportions for cubic meter of concrete are given in Table 3. Herein C denotes control concrete. C1P and C2P denote concrete containing 0.1% and 0.2% polypropylene (by the volume of concrete) respectively. In naming other specimens, the number which is right before S or A denotes the amount of nano-silica or nano-alumina respectively. For example C2P3S denotes concrete containing 0.2% polypropylene fibers (by the volume of concrete) incorporating 3% nano-silica (by the weight of cementitious materials). Table 3 also shows the results of slump test. In this study the amount of superplasticizer in all specimens is the same in order to make other factors such as frost resistance comparable; So it can be seen that when the content of nano-particles are larger, the workability of concrete is worse.
2.2. Specimen preparation To cast the concrete containing nano-particles, superplasticizer is firstly mixed into water in a mixer, and then nano-particles are added and stirred at a high speed for 5 min. Course aggregate, sand and cement are mixed at a low speed for 2 min in a concrete centrifugal blender, then the mixture of water, superplasticizer and nano-particles are slowly poured in and stirred at a low speed for another 2 min to achieve proper workability [23]. To cast control concrete and concrete containing PP fibers, superplasticizer is firstly dissolved in water. Then nano-particles (if used) are added and stirred. After course aggregate, sand, cement and PP fibers (if used) are mixed uniformly in a concrete centrifugal blender, the mixture of water, superplasticizer and nano-particles (if used) is poured in and stirred for several minutes. Finally the fresh concrete is poured into molds to form cubes of size 100 100 100 mm. After pouring, an external vibrator is used to facilitate compaction and decrease the amount of air bubbles. The specimens are demolded at 24 h and then cured in a standard moist room at a temperature of 20 ± 3 °C.
Following tests are conducted in order to determine mechanical properties and frost resistance of concrete containing nano-particles and polypropylene fibers with respect to control concrete. Each test was conducted on six specimens. The results are the average of all tests. a. Compressive test is conducted after 7, 28 and 120 days of curing according to the standard test method of BS 12390-3 [24]. b. The percentage of water absorption in specimens is measured after 28 days of curing according to the ASTM C642 procedure [25]. The test procedure involves drying each specimen to a constant weight, weighing it, immersing it in water for specified time (24 h in this work), and weighing it again. The increase in weight as a percentage of the original weight is expressed as its water absorption (in percentage). c. The specimens are subjected to cycles of freezing and thawing in an automatic freeze–thaw machine which can apply freezing cycles at 18 °C and thawing cycles at 4 °C both in water according to ASTM C666A [26]. The loss of mass, change in length, increase in water absorption and reduction in compressive strength of specimens are measured in certain cycles. In this machine the specimens were subjected to freezing and thawing cycles for 6 and 2 h respectively. The freeze and thaw machine was designed by authors.
3. Experimental results and discussion 3.1. Compressive strength Table 4 shows the compressive strength of specimens after 7, 28 and 120 days. It can be found that the compressive strength is developed in concretes containing nano-particles in every case higher than that of control concretes. It can also be concluded that polypropylene fibers do not affect the compressive strength of concrete considerably. As Table 4 shows, the compressive strength of concrete is considerably improved by using nano-silica particles as a part of cementitious materials. This fact confirms the results of pervious researches [7,10,15]. The compressive strength of concrete is enhanced as much as 16.67% (in comparison to that of control concrete) by replacing 3% cement with nano-silica particles. The enhanced extent of compressive strength increases from 16.67% to 30.13% when the content of nano-silica increases from 3% to 5% by the weight of cementitious materials and then it decreases to 23.58% when the nano-silica content increases to 7% by the weight of cementitious materials. The enhancement of the compressive strength of concrete containing nano-silica in comparison
Table 3 Mix proportions of specimens (kg/m3). Mixture no.a
Water
Cement
Sand
Coarse aggregate
nSiO2
nAl2O3
UNF
Slump (cm)
C C1P C2P C3S C1P3S C2P3S C5S C1P5S C2P5S C7S C1P7S C2P7S C1A C1P1A C2P1A C2A C1P2A C2P2A C3A C1P3A C2P3A
168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168 168
350.0 350.0 350.0 339.5 339.5 339.5 332.5 332.5 332.5 325.5 325.5 325.5 346.5 346.5 346.5 343 343 343 339.5 339.5 339.5
960 958 956 953 951 949 950 948 946 947 945 943 956 954 952 955 953 951 954 952 950
920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920 920
– – – 10.5 10.5 10.5 17.5 17.5 17.5 24.5 24.5 24.5 – – – – –– – – – –
– – – – – – – – – – – – 3.5 3.5 3.5 7 7 7 10.5 10.5 10.5
1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75
12.0 10.5 9.0 9.0 7.5 6.0 6.0 5.0 3.5 2.0 0.5 0.0 10.5 9.0 7.5 8.5 7.0 5.5 6.5 5.0 3.5
a The number just before S, A or P defines the percentage of nano-silica, nano-alumina or PP fibers respectively. For example C2P1A defines concrete containing 0.2% polypropylene fibers incorporating 3% nano-alumina.
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N. Salemi, K. Behfarnia / Construction and Building Materials 48 (2013) 934–941 Table 4 Compressive strength of specimens (MPa). Mixture no.
Compressive Strength 7 days
C C1P C2P C3S C1P3S C2P3S C5S C1P5S C2P5S C7S C1P7S C2P7S C1A C1P1A C2P1A C2A C1P2A C2P2A C3A C1P3A C2P3A
28 days
120 days
Target
Enhanced extent (%)
Target
Enhanced extent (%)
Target
Enhanced extent (%)
27.10 27.69 28.11 29.27 29.85 30.01 31.16 31.48 31.92 30.19 30.61 30.95 27.65 28.34 29.22 27.84 28.31 29.47 27.97 29.10 30.03
0.00 2.17 3.73 8.00 10.15 10.74 14.98 16.16 17.78 11.40 12.95 14.21 2.03 4.57 7.82 2.73 4.46 8.75 3.21 7.38 10.81
42.11 43.04 43.52 49.13 49.87 50.12 54.80 54.89 55.07 52.04 52.37 52.06 43.81 44.92 46.21 44.64 45.97 47.15 45.48 46.52 45.91
0.00 2.21 3.35 16.67 18.43 19.02 30.13 30.35 30.78 23.58 24.36 23.62 4.03 6.67 9.74 6.01 9.17 11.97 8.00 10.47 9.02
47.15 49.75 50.92 60.35 61.28 61.42 68.36 68.83 69.15 64.12 64.76 65.15 50.45 51.91 52.48 51.39 52.29 53.61 52.34 53.30 54.78
0.00 5.51 8.00 28.00 29.97 30.27 44.98 45.98 46.66 35.99 37.35 38.18 7.00 10.10 11.30 8.99 10.90 13.70 11.01 13.04 16.18
to that of control concrete can be attributed to the pozzolanic reaction of nano-silica [7]. Nano-silica is thought to be very effective in pozzolanic reaction which leads to the formation of the C–S–H gel. Also, nano-silica would fill pores to increase the compressive strength. As a general result the compressive strength of the concrete was found to increase as the nano-silica content increases from 3% to 5% and then to decrease with the increase of nano-silica content from 5% to 7%, although the results of 7% replacement are still higher than those of control concrete and concrete containing 3% nano-silica. Two reasons may cause the reduction in enhanced extent of compressive strength (when 7% nano-silica is applied); It may be due to the fact that the quantity of nano-silica particles present in the mix is higher than the amount required to react with the liberated lime after the process of hydration, leading to leach out the excess silica and causing a deficiency in strength as it is substituted to a portion of cementitious materials but does not contribute in developing strength [23]. So in terms of compressive strength, 5% nano-silica is the optimum amount; because using nano-silica in high amounts (more than 5%) does not improve the compressive strength of concrete. Also it may be due to the agglomeration and defects generated in dispersion of nano-silica particles. It should be noted that using a high content of nano-particles must be accompanied by adjustments to the water superplasticizer dosage in the mix in order to ensure that no agglomeration would happen and specimens do not suffer from excessive self desiccation and cracking. In addition, because nano-particles are more difficult to uniformly disperse, when the content are large, the weak zone in concrete increases which results in the decrease of the strength of the concrete. But in this work the amount of superplasticizer is kept constant in order to confine freeze and thaw parameters. According to Table 4, the compressive strength of concrete containing nano-alumina particles is also improved as it is mentioned in previous studies [7,16–18]. The compressive strength of concrete is enhanced as much as 4.03% by replacing 1% of cement (by the weight) with nano-alumina particles. The enhanced extent of compressive strength increases from 4.03% to 8.00% when the content of nano-alumina increases from 1% to 3% by the weight of cementitious materials. The enhancement of compressive strength of concrete containing nano-alumina is due to the rapid consuming of Ca(OH)2 formed during hydration of Portland
cement, related to the high reactivity of nano-Al2O3 particles. As a consequence the hydration of cement is accelerated and larger volumes of reaction products are formed. Also nano-Al2O3 particles recover the particle packing density of the concrete and as a nanofiller improve the microstructure of it, directing to a reduced volume of larger pores in the cement paste. It also can be seen that the compressive strength of the concretes containing PP fibers enhanced only a little in comparison to that of control concrete. This conclusion had been reached in other studies too. Shuan-fa and Liao showed the same conclusions in separate researches [27,28]. They found that PP fibers have little effect on compressive strength of concrete, but their effect on tensile strength of concrete is considerable, as it is commonly believed. 3.2. Water absorption There are several methods to perform the water permeability tests such as percentage of water absorption, rate of water absorption and coefficient of water absorption. In this work, to evaluate the water permeability of the specimens, percentage of water absorption is a criterion of the pore volume or porosity of concrete after hardening, which is occupied by water in saturated state. Water absorption values of control concrete and concrete containing nano-particles were measured according to the ASTM C642 [25] after 28 days of moisture curing. Fig. 1 shows the result of water absorption test.
Fig. 1. Water absorption of specimens.
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Fig. 1 shows that the percentage of water absorption in concrete containing nano-particles is considerably lower than that of control concrete. Fig. 1 also shows that the percentage of water absorption of concrete decreases by increasing the nano-particles content. This may be due to the improved microstructure of concrete according to the use of nano-particles. As mentioned above, nano-Al2O3 and nano-SiO2 particles recover the particle packing density of the concrete and as nano-filler improve the microstructure of it; thus the water permeability of concrete decreases by the replacement of cement with nano-particles partially. In Fig. 1 it can
3.3.1. Compressive strength The compressive strength of specimens subjected to the freezing and thawing cycles, was determined after 50,150 and 300 cycles. Table 5 shows the results of compressive strength determination. Figs. 2 and 3 show the effect of nano-silica and nano-alumina on durability of concrete in terms of compressive strength. As Figs. 2 and 3 show, the strength loss of concrete containing nano-particles is much lower than that of control concrete. For example concrete containing 3% nano-silica (by the weight of
Table 5 The results of frost resistance determination in terms of strength loss for concrete using the rapid test method. Mixture no.
C C1P C2P C3S C1P3S C2P3S C5S C1P5S C2P5S C7S C1P7S C2P7S C1A C1P1A C2P1A C2A C1P2A C2P2A C3A C1P3A C2P3A
Residual/original compressive strength 50
150
300
0.77 0.81 0.83 0.93 0.94 0.95 0.95 0.95 0.96 0.94 0.95 0.96 0.94 0.95 0.96 0.94 0.95 0.95 0.94 0.95 0.96
0.35 0.48 0.55 0.83 0.85 0.87 0.89 0.90 0.91 0.85 0.86 0.88 0.86 0.88 0.90 0.87 0.89 0.90 0.88 0.90 0.91
0.00 0.00 0.00 0.72 0.76 0.79 0.84 0.86 0.87 0.77 0.80 0.82 0.76 0.79 0.80 0.79 0.81 0.84 0.82 0.85 0.87
be seen that the water absorption of concrete containing 7% nanosilica is lower than that of concrete which contains 5% nano-silica, while the compressive strength of concrete containing 5% nano-silica is higher than that of concrete containing 7% nano-silica. It can be explained as below: the specimen named C7S (concrete containing 7% nano-silica) has 2% excess nano-silica in comparison to C5S (concrete containing 5% nano-silica). This 2% nano-silica is substituted to a portion of cementitious materials but does not contribute in developing strength (C5S has 2% cement more than C7S that leads to a higher compressive strength). Fig. 1 shows that 2% excess nano-silica would fill pores leading to lower water absorption. In Fig. 1, it can be seen that polypropylene fibers increases the water permeability of concrete slightly. This may be due to the mixing procedure which may let micro bubbles entered the zone between fibers and cement paste. Using PP fibers in concrete leads to a lower workability compared to that of concrete without fibers; so it becomes harder for cement paste to flow and fill the pores. As a result there will be few more pores in fiber-reinforced concrete and the percentage of water absorption will slightly be more than that of control concrete.
Strength loss % (after 300 cycles)
Cracking
Frost resistance
100.00 100.00 100.00 28.00 23.74 21.41 16.28 14.09 12.52 23.20 19.82 17.93 23.84 21.36 19.72 21.31 19.01 16.25 18.19 14.82 12.76
Severe Severe Severe Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight Slight
Poor Poor Poor Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory Satisfactory
Fig. 2. Strength loss of specimens containing nano-silica.
3.3. Frost resistance In order to determine the frost resistance of concrete, the decrease in compressive strength, change in length, loss of mass and increase in water absorption in specimens measured in different cycles. It should be noted that the specimens were subjected to cycles of freezing and thawing in water according to the procedure of ASTM C666A [26].
Fig. 3. Strength loss of specimens containing nano-alumina.
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Fig. 4. Strength loss of specimens containing polypropylene fibers.
cementitious materials) showed only 28% strength loss after 300 cycles of freezing and thawing, while the strength loss of control concrete after 300 cycles was 100%. The reduction of compressive strength of concrete containing nano-silica after freezing and thawing cycles (300 cycles) was found to decrease from 28% to 16.28% as nano-silica content increases from 3% to 5% and then to increase from 16.28% to 23.20% as nano-silica content increases from 5% to 7%. The percentage of strength loss in concrete containing nano-alumina is considerably lower than that of control concrete and it is shown to decrease from 23.84% to 18.19% as the nano-alumina content increases from 1% to 3%. The effect of polypropylene fibers on frost resistance of concrete can be seen in Fig. 4. In Fig. 4 it can be seen that PP fibers improve the durability of concrete slightly which confirms the results of pervious researches by Richrdson and Zhang [19,20]. For example, , Strength loss of concrete after 150 cycles of freezing and thawing decreases from 65% to 45.22% as the PP content increases from 0% to 0.2% (by the volume of concrete). The effect of PP fibers on improving the frost resistance of concrete in terms of compressive strength is much less than that of nano-particles. According to Figs. 2 and 3, it can be said that among the tested samples, concrete containing 5% nano-silica and concrete containing 3% nano-alumina showed the highest frost resistance among the tested samples in terms of compressive strength; because these specimens had the lowest strength loss after 300 cycles of freezing and thawing.
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3.3.2. Length change and mass loss After 50, 150 and 300 cycles of freezing and thawing, change in length and mass loss of specimens were measured in order to determine the frost resistance of concrete in terms of mass and length change. Fig. 5 shows the length change and mass loss of specimens under the freezing and thawing condition. In Fig. 5, it can be seen that the mass loss and length change of concrete containing nano-particles are much lower than that of control concrete. Control concrete showed 28.1% decrease in length after 300 cycles; while the decrease in length of concrete containing 1% nano-alumina and concrete containing 3% nano-silica after 300 cycles of freezing and thawing are 6.18% and 5.82% respectively. Fig. 5 also shows that the change in length decreases from 5.82% to 3.81% as the nano-alumina content of specimens increases from 1% to 3%. The length change of concrete containing nano-silica decreases from 6.18% to 1.41% by increasing the usage of nano-silica from 3% to 5% and then it increases from 1.41% to 4.05% when the applied content of nano-silica increases from 5% to 7%. Mass loss of specimens decreases from 17.28% to 11.47% by increasing the nano-alumina content from 1% to 3%. Mass loss of concrete containing nanosilica has the same order as strength loss and length change of them. According to Fig. 5, the mass loss and length change of concrete containing PP fibers decreases slightly as fiber content increases. In terms of length and mass change, among the tested concretes, concrete containing 5% nano-silica (by the weight of cementitious materials) incorporating 0.2% PP fibers (by the volume of concrete) showed to be the most frost resistant concrete.
3.3.3. Water absorption Water absorption of specimens measured after 50, 150 and 300 cycles of freezing and thawing. Fig. 6 shows the water absorption of specimens before and after freeze and thaw cycles. It can be seen that the nano-particles decrease the water absorption of specimens. Fig. 6 shows that using nano-particles in concrete, controls the rate of increase in water absorption during the freeze and thaw cycles. The water absorption of control concrete has been increased as much as 117% after 300 cycles of freezing and thawing; while the water absorption of concrete containing 1% nano-alumina and concrete containing 5% nano-silica have been increased as
Fig. 5. Length and mass loss of specimens after 300 cycles of freezing and thawing.
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Fig. 6. Percentage of increase in water absorption of specimens after 300 cycles of freezing and thawing.
much as 50.29% and 31.19% respectively. Water absorption of concrete was found to decrease by increasing nano-particles content. As Fig. 6 shows polypropylene fibers also control the rate of increase in water absorption percentage during the freeze and thaw cycles; although nano-particles are much more effective in improving the durability of concrete against freeze and thaw cycles in case of water absorption percentage.
and thawing cycles); however the index of frost resistance improvement of concrete containing nano-particles are much larger than that of concrete containing PP fibers. 4. Conclusion Based on the results of experiments in this study, the following primary conclusions can be obtained:
3.4. Discussion The mechanical properties and frost resistance of concretes containing nano-particles is improved. The mechanism of improving these properties of concrete by nano-particles can be interpreted as follows: supposed that nano-particles are uniformly dispersed and each particle is contained in a cubic pattern, the distance between nano-particles can be specified. After hydration begins, hydrate products diffuse and envelope nanoparticles as kernel. If the content of nano-particles and the distance between them are appropriate, the crystallization will be controlled to be a suitable state through restricting the growth of Ca(OH)2 crystal by nano-particles. This makes the cement matrix more homogeneous and compact. Moreover nano-alumina and nano-silica as pozzolanic materials react with calcium hydroxide formed from calcium silicate hydration. The rate of the pozzolanic reaction is proportional to the amount of surface area available for the reaction; so it is plausible to use these nano-particles in order to produce a concrete with higher strength and considerably improved microstructure. Nanoalumina and nano-silica not only behave as fillers to improve microstructure but also as activators to promote pozzolanic reaction; so the medium size of pores in concrete decreases with the use of nano-particles. As a result concrete containing nanoparticles absorbs less water in comparison to control concrete because of a denser microstructure; so the stress, which is produced due to the volume change of frozen water, decreases and the concrete will be more frost resistant. The frost resistance of concrete containing PP fibers is improved, which is mostly attributed to the tensile strength-enhancing, crack-arresting effect of PP fibers [27], the bridge effect of PP fibers on cracks and diversion effect of fibers on separated cement blocks [28]. In other words the frost resistance of concretes containing nano-particles and PP fibers is improved (in terms of strength, water absorption percentage, length and mass loss after freezing
(1) The compressive strength of concrete can slightly be increased by using nano-alumina as a substitute to cement materials partially. Compressive strength of concrete was found to increase as much as 8% by replacing cementitious materials by nano-alumina. (2) Nano-silica considerably improves the compressive strength of concrete. The optimum content of nano-silica in concrete in order to increase its compressive strength is 5% (by the weight of cementitious materials). The compressive strength of concrete improves as much as 30% by using 5% nano-silica (by the weight of cementitious materials). (3) Using nano-particles in concrete lower the workability of the concrete. (4) Frost resistance of concrete can be considerably improved by the addition of nano-particles. Nano-alumina and nano-silica behave not only as promoters of pozzolanic reaction (because of high surface area to volume ratio), but also as fillers, improving the pore structure of concrete and densifying the microstructure of cement paste. The strength loss of concrete containing 5% nano-silica was 16% while the strength loss of control concrete was 100% after 300 cycles. Using 5% nano-silica results in 84% reduction in deterioration of concrete after freezing and thawing cycles (in terms of compressive strength). Also 3% nano-alumina can improve the durability of concrete against freezing and thawing cycles as much as 82%. (5) PP fibers increase the frost resistance of concrete slightly. However the enhanced extent of frost resistance of concrete containing nano-particles is much larger than that of concrete containing PP fibers; so nano-particles are more favorable to improve the frost resistance of concrete than PP fibers. (6) The concrete containing both nano-particles and PP fibers showed to be the most frost resistant among the studied samples.
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(7) Among the tested concretes, concrete containing 5% nanosilica (by the weight of cementitious materials) incorporating 0.2% PP fibers (by the volume of concrete) showed the most frost resistance. The improvement of durability in concrete containing 5% nano-silica incorporating 0.2% polypropylene fibers was about 87% (in terms of strength).
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