The effects of silica fume and polypropylene fibers on the impact resistance and mechanical properties of concrete

Impact resistance and strength performance of concrete mixtures with 0.36 and 0.46 water–cement ratios made with polypropylene and silica fume are examined. Polypropylene fiber with 12-mm length and four volume fractions of 0%, 0.2%, 0.3% and 0.5% are used. In pre-determined mixtures, silica fume is used as cement replacement material at 8% weight of cement. The results show that incorporating polypropylene fibers improves mechanical properties. The addition of silica fume facilitates the dispersion of fibers and improves the strength properties, particularly the impact resistance of concretes. It is shown that using 0.5% polypropylene fiber in the silica fume mixture increases compressive split tensile, and flexural strength, and especially the performance of concrete under impact loading.

Construction and Building Materials 24 (2010) 927–933 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat The effects of silica fume and polypropylene fibers on the impact resistance and mechanical properties of concrete Mahmoud Nili *, V Afroughsabet Civil Eng Dept., Bu-Ali Sina University, Hamedan, I.R, Iran a r t i c l e i n f o Article history: Received 21 August 2009 Received in revised form 21 November 2009 Accepted 21 November 2009 Available online 24 December 2009 Keywords: Polypropylene fibers Silica fume Mechanical properties Impact resistance a b s t r a c t Impact resistance and strength performance of concrete mixtures with 0.36 and 0.46 water–cement ratios made with polypropylene and silica fume are examined Polypropylene fiber with 12-mm length and four volume fractions of 0%, 0.2%, 0.3% and 0.5% are used In pre-determined mixtures, silica fume is used as cement replacement material at 8% weight of cement The results show that incorporating polypropylene fibers improves mechanical properties The addition of silica fume facilitates the dispersion of fibers and improves the strength properties, particularly the impact resistance of concretes It is shown that using 0.5% polypropylene fiber in the silica fume mixture increases compressive split tensile, and flexural strength, and especially the performance of concrete under impact loading Ó 2009 Elsevier Ltd All rights reserved Introduction It is well known that concrete is a quasi brittle material Brittleness increases with increasing strength This may be due to low tensile strength and lack of bonding in the transition zone of the cement matrix which obviously restricts utilization of high strength concrete under static and, in particular dynamic loading [1–3] However, despite the defects in high strength concrete, demand for this material continues to grow It is well understood that silica fume, due to high pozzolanic activity, is inevitable material when producing high strength concrete; however, it causes the concrete to have a more brittle structure [4–5] Therefore, ductility improvement is a vital matter in concrete science that must be taken into account by researchers One possible solution to improve the ductility and resistance of concrete structures [6–10] to dynamic loading, such as impact, fatigue and earthquakes, is incorporating fibers in the concrete Adding fibers to concrete increases the energy absorption capacity of concrete and provides a more ductile structure The fibers are mainly made of steel, carbon or polymer [11] Among the polymer fibers, polypropylene (PP) has attracted the most attention among researchers because of its low cost, outstanding toughness and enhanced shrinkage cracking resistance in concrete reinforced with this type of fiber [11–16] Many studies have evaluated the ductility of fibrous specimens; the impact test is a well known method for assessment of concrete ductility [17] Hibbert and Hannant [18] designed an instrument to control the * Corresponding author Tel.: +98 9181112615; fax: +98 8118224205 E-mail address: nili36@yahoo.co.uk (M Nili) 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd All rights reserved doi:10.1016/j.conbuildmat.2009.11.025 impact resistance of fiber-reinforced concrete A 100  100  400-mm specimen is supported in a Charpy test apparatus and completely fractured by one blow; the fracture energy is measured from the amplitude of the pendulum swing The drop weight test was also used to perform impact tests on plain and steel fiber-reinforced concrete beams by Mohammadi et al [19] The Committee 544 [20] ACI proposed a drop weight impact test to evaluate the impact resistance of fiber concrete Disc specimens that were of 150 mm in diameter and 64 mm in thickness were cut from 150  300-mm cylinders The number of blows required to cause the first visible crack and to cause failure were recorded Because of the nature of the impact test, and especially because of the in homogeneity of concrete, the data obtained from the impact test can be scattered noticeably, as reported by Schrader [21] This test is widely used because of its simplicity and economy The variation in the impact resistance determined from this test is reported in the literature for some types of FRC, but less data can be found for polypropylene fiber-reinforced concrete [22] Thus, several impact test methods have been used to demonstrate the relative brittleness and impact resistance of concrete However, none of these test methods have been standardized yet In the present research, the impact resistance and strength performance of fibrous and non-fibrous specimens with and without silica fume are experimentally examined Test program and procedures In this research, two series of concrete mixtures with 0.46 and 0.36 water–cement ratios, were prepared and labeled A1 and B1, respectively Some specimens were reinforced with 0.2%, 0.3% and 0.5% (by volume) polypropylene fibers Silica 928 M Nili, V Afroughsabet / Construction and Building Materials 24 (2010) 927–933 the specimens was determined as well in accordance with the ACI committee 544 proposal [20] For this purpose, six 150  64-mm discs, which were cut from 150  300-mm cylindrical specimens using a diamond cutter were prepared and placed on a base plate with four positioning lugs; they were then struck with repeated blows The blows were introduced through a 4.45 kg hammer dropping frequently from a 45.7-cm height onto a 6.35-cm steel ball, which was located at the center of the top surface of the disc Figs and show the specimens and impact base plate and the test procedure The numbers of blows producing the first visible crack and cause ultimate failure were recorded In each test, the number of blows to produce the initial visible crack was recorded as the first crack strength, and the number of blows to cause complete failure of the disc was recorded as the failure strength 2.1 Materials and mixing procedure Fig Flexural test machine Ordinary Portland cement (ASTM Type 1) produced by Hekmatan Factory and silica fume, a by-product of the silicon and ferrosilicon Semnan factory, were used in this work The cement and silica fume properties are given in Table Coarse aggregate with a maximum size of 19 mm and fine aggregate with a 3.4 fineness modulus were used in this experiment The specific gravity and water absorption of the coarse and fine aggregates were 2.69 and 0.56% and 2.61 and 1.92%, respectively A high range water reducer agent with a commercial name of Carboxylic 110 M (BASF) was used to adjust the workability of the concrete mixtures The mixing procedure for fresh concrete mixtures was as follows: the cement (or cement and silica fume) and fine aggregate were mixed initially for min; and superplasticizer with half mixing water were mixed for Coarse aggregate and the rest of water were added and mixed for Finally fiber was added to the mixtures and mixed for The polypropylene fiber properties, as well as the mix proportions of the mixtures, are provided in Tables and 3, respectively Table Properties of cement and silica fume Composition (%) Fig Disc type specimens for the impact test fume as a cement replacement was also added (8% by weight) to some specimens Compressive strength tests were performed at the ages of 7, 28 and 91 days on 100  100  100-mm cubic specimens and the flexural strength test was also performed (see Fig 1) on 80  100  400-mm specimens The tensile strength test was also performed on 100  200-mm cylindrical specimens The Impact resistance of Chemical compositions Sio2 Al2O3 Fe2O3 MgO Na2O K2O CaO C3S C2S C3A C4AF Physical properties Specific gravity Specific surface (cm2/gr) Cement 21.20 5.35 3.40 1.44 – – 63.95 51.46 22.00 6.42 10.35 3.1 3000 Fig (a) Base plate within four positioning lugs and subjected to repeated blows and (b) procedure for the impact test Silica fume 85–95 0.5–1.7 0.4–2 0.1–0.9 0.15–0.2 0.15–1.02 – – – – – 2.21 14,000 929 M Nili, V Afroughsabet / Construction and Building Materials 24 (2010) 927–933 Table Properties of polypropylene fiber Length (mm) Effective diameter (lm) Density (kg/m3) Shape 12 22 0.91 Straight 2.2 Specimen molding Each type of freshly mixed concrete was cast into cubic (100 mm), cylindrical (100 mm  200 mm specimens), prismatic and cylindrical cutting specimens for compressive, splitting tensile, flexural and impact tests, respectively All specimens, before de-molding, were stored at 23 °C and 100% relative humidity for about 24 h The concrete specimens were then cured in lime-saturated water until the day of testing Results and discussion The compressive, tensile and flexural strength results are summarized in Table and graphically illustrated in Figs 4–6 3.1 Compressive strength The variations of the compressive strength versus fiber volume fractions, at the ages of 7, 28 and 91 days, are illustrated in Fig It can be generally seen that, for all specimens, as the fiber volume increases the compressive strength increases As shown, for 0.46 water–cement ratio specimens, the increase in compressive strength are 3% at 0.2% fiber volume and 14% at 0.5% fiber volume Adding silica fume to non-fibrous specimens also improves compressive strength Increase in compressive strength up to 13%, 21% and 23% are observed in No at the ages of 7, 28 and 91 days compared to No 1, respectively Whereas in fibrous specimens, for instance No 8, when silica fume was added to a 0.5% fiber specimen, an increase of 20% at days, 27% at 28 days and 30% at the ages of 91 days are obtained In series B1, the specimens with a water–cement ratio of 0.36 demonstrated a similar trend in the, results In the case of specimen Nos 10 and 12, increase of 1–6% in compressive strength is observed as the fiber volume varies between 0.2% and 0.5%, respectively On the other hand, introducing silica fume to the specimens (No 13) improves the compressive strength by about 7–14% at the ages of and 91 days, respectively When silica fume and polypropylene fiber are simultaneously incorporated into the specimens (Nos 14 and 16) an improvement in compressive strength between 9–18% and 11–20% at the ages of 7–91 days, compare to reference specimen, No 9, are observed This indicates that the pozzolanic properties of silica fume and also the crack restriction effect of fiber can promote the compressive strength of concrete Table Mix proportions of the concrete mixtures Mix no W/ (C + Sf) Water (kg/ m3) Cement (kg/ m3) Silica fume (kg/ m3) Fine agg (kg/ m3) Coarse agg (kg/ m3) Vf (%) Weight (kg/ m3) Sp (%) Slump (Cm) A1 0.46 0.46 0.46 0.46 0.46 0.46 0.46 0.46 177 177 177 177 177 177 177 177 385 385 385 385 354.2 354.2 354.2 354.2 – – – – 30.8 30.8 30.8 30.8 920 918 916 914 915 912 911 908 884 882 880 878 879 876 875 873 – 0.2 0.3 0.5 – 0.2 0.3 0.5 – 1.82 2.73 4.55 – 1.82 2.73 4.55 0.60 0.90 1.25 1.70 0.70 1.10 1.35 1.75 5.0 6.0 6.0 4.0 7.0 7.0 7.0 5.0 B1 10 11 12 13 14 15 16 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 162 162 162 162 162 162 162 162 450 450 450 450 414 414 414 414 – – – – 36 36 36 36 912 910 908 906 906 903 902 899 877 874 873 870 871 868 867 864 – 0.2 0.3 0.5 – 0.2 0.3 0.5 – 1.82 2.73 4.55 – 1.82 2.73 4.55 1.10 1.50 1.60 1.90 1.20 1.55 1.65 1.95 6.5 6.5 8.0 6.0 10.0 10.0 8.5 5.0 Table Compressive, tensile and flexural strength of the specimens Mix no 10 11 12 13 14 15 16 Compressive strength (MPa) Tensile strength (MPa) Flexural strength (MPa) 28 days days 28 days 91 days days 28 days 91 days 32.95 33.88 36.15 37.56 37.28 37.51 38.12 39.41 47.58 48.12 49.01 49.64 51.19 51.88 52.30 52.68 41.30 42.32 44.05 46.09 49.88 50.29 50.88 52.61 55.58 55.97 56.46 58.24 63.34 65.93 66.16 66.33 46.65 48.96 50.21 53.56 57.44 58.71 59.43 60.49 61.01 62.27 63.43 64.54 69.48 72.28 72.41 73.26 2.67 2.81 2.85 3.01 2.95 2.97 3.13 3.24 3.56 3.63 3.73 3.90 3.98 4.04 4.15 4.27 3.22 3.49 3.66 3.68 3.52 3.69 3.87 4.09 4.39 4.41 4.49 4.68 4.71 5.05 5.09 5.43 3.89 3.97 4.03 4.16 3.97 4.01 4.06 4.39 4.74 4.93 5.04 5.22 5.52 5.36 5.71 5.86 4.45 4.48 5.17 5.58 5.09 5.46 5.68 6.14 6.30 6.58 6.63 6.36 6.97 7.06 7.56 7.83 930 M Nili, V Afroughsabet / Construction and Building Materials 24 (2010) 927–933 3.2 Splitting tensile strength Split tensile strength results versus fiber volume fractions are shown in Fig The results show that for both water–cement ratios, tensile strength rises as the fiber volume fractions increases For example, the tensile strength of A1 mixture, at the age of 28 day increases 8%, 14% and 14% when the fiber volume fractions in the mixes are 0.2%, 0.3% and 0.5%, respectively Adding silica fume to the specimen (No 5) the tensile strength increases by 9% compared to reference ones However, when silica fume is introduced to fibrous specimens, the rate of tensile strength increases by 15%, 20% and 27% in specimens Nos 6–8, respectively Although splitting tensile strength is greatly affected due to a reduction in water–cement ratio, in B1 mixtures, the same tendency as A1 specimens is observed In other words, introducing the fiber and silica fume to the mixtures improves tensile strength Furthermore, the combined effect of fiber and silica fume leads to increases of 15%, 16% and 23% in tensile strength in specimens’ Nos 14–16, respectively 3.3 Flexural strength The flexural strength results versus fiber volume fractions, at the age of 28 days, carried out on sixteen different mixtures are presented in Fig As explained in the tensile strength results, 3.4 Impact test The impact resistance performance of the A1 and B1 series of concrete are given in Table and are also shown in Fig As it is shown, the number of blows at the first crack (N1) and the number of blows for failure (N2) are provided in the results The percentage increase in the number of post first crack blows to failure (N2–N1/ N1) is labeled the termed as PINPB and is also given in Table As the results suggest, by incorporating PP fibers into the A1 mixtures, N1 is increased by 31%, 100% and 360% by adding 0.2%, 0.3% and 0.5%, fiber, respectively When silica fume is introduced to the mixture (No 5) N1 increases six times However, in silica fume fibrous specimens (Nos 6–8), N1 increases about 6.6, 7.6 and 8.5 times, (b) 80 70 Compressive Strength [MPa] W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf 60 50 40 30 20 0.1 0.2 0.3 0.4 0.5 80 70 60 50 40 W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf 30 20 0.6 0.1 Fiber Volume Fraction [%] (c) Compressive Strength [MPa] Compressive Strength [MPa] (a) the flexural strength of fibrous specimens increases compared to the reference specimen However, the rate of increase is higher in A1 specimens Silica fume, as in the tensile strength results, improves flexural performance The combined effect of fiber and silica fume is considerable, and typically, an improvement in flexural strength of 22% in No 6, 27% in No and 38% in No are observed In B1, the increase in specimens flexural strength is the same as A1, but at a lower rate However, the highest flexural strength value of 7.83 MPa is belongs to specimen No 16 in series B1, which contain silica fume and 0.5% polypropylene fiber 0.2 0.3 0.4 0.5 Fiber Volume Fraction [%] 80 70 60 50 40 W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf 30 20 0.1 0.2 0.3 0.4 0.5 0.6 Fiber Volume Fraction [%] Fig Compressive strength versus fiber volume fractions at the ages of: (a) days, (b) 28 days and (c) 91 days 0.6 931 M Nili, V Afroughsabet / Construction and Building Materials 24 (2010) 927–933 (b) W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf Tensile Strength [MPa] W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf Tensile Strength [MPa] (a) 6 2 0.1 0.2 0.3 0.4 0.5 0.6 0.1 Fiber Volume Fraction [%] (c) 0.3 0.4 0.5 0.6 W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf Tensile Strength [MPa] 0.2 Fiber Volume Fraction [%] 0.1 0.2 0.3 0.4 0.5 0.6 Fiber Volume Fraction [%] Fig Splitting tensile strength versus fiber volume fractions at the ages of: (a) days, (b) 28 days and (c) 91 days (a) (b) 11 11 W/C=0.36 W/C=0.46 10 W/C=0.46-Sf Felxural Strength [MPa] Felxural Strength [MPa] 10 W/C=0.36-Sf 0.2 0.3 0.5 Fiber Volume Fraction [%] 0.2 0.3 0.5 Fiber Volume Fraction [%] Fig Flexural strength and fiber volume fractions at the age of 28 days: (a) w/c = 0.46 and (b) w/c = 0.36 respectively This may be attributed to the fact that adding silica fume improves dispersion of the fibers in the specimens [7,22] A similar trend to that specified for N1 is observed for N2 values On the other hand, PINPB values that indicate the ability to absorb kinetic energy suggest that adding fiber delays failure strength On the other hand, the results also reveal that adding silica fume (No 5) despite increment the strength, leads to higher brittleness However, initiation and cracks propagation under 932 M Nili, V Afroughsabet / Construction and Building Materials 24 (2010) 927–933 Table Test results for impact resistance of polypropylene fiber-reinforced concrete Mix no Impact resistance (blows) 10 11 12 13 14 15 16 First crack (N1) Failure (N2) First crack Failure 35 46 70 161 243 268 300 331 132 139 192 239 281 299 325 365 38 54 79 181 246 277 315 371 134 152 211 307 284 322 343 399 712.1 935.9 1424.2 3275.5 4943.8 5452.5 6103.5 6734.2 2685.5 2827.9 3906.2 4862.5 5716.9 6083.2 6612.1 7425.9 773.1 1098.6 1607.3 3682.4 5004.9 5635.6 6408.7 7547.9 2726.2 3092.4 4292.8 6245.9 5777.9 6551.1 6978.3 8117.7 8.6 17.4 12.9 12.4 1.2 3.4 5.0 12.1 1.5 9.4 9.9 28.5 1.1 7.7 5.5 9.3 Percentage increase in number of post-first-crack blows to failure Number of Blows at First Crack (a) (b) 450 W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf 400 350 450 W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf 400 Number of Blows at Failure a PINPB (blows)a Impact energy (kN mm) 300 250 200 150 100 50 350 300 250 200 150 100 50 0 0.1 0.2 0.3 0.4 0.5 0.6 Fiber Volume Fraction [%] 0.1 0.2 0.3 0.4 0.5 0.6 Fiber Volume Fraction [%] Fig Impact strength versus percentage of polypropylene fiber volume fractions at: (a) first crack and (b) failure strength impact loading are reduced in fibrous and nearly silica fume fibrous specimens As the water–cement ratio decrease in the B1 mixtures, lower ductility and increased strength of the paste can be attained Adding of silica fume, despite increasing the strength, led to higher brittleness Although N1 increases compare to A1 mixtures the rate of increase resulting from fiber or silica fume, in N1 and N2 decreases considerably Adding fibers also increases the PINPB value seven times over the specimens made by silica fume and without fiber specimens This means that fibers effectively reduced the brittleness of the specimens In Fig 8, a comparison of the failure pattern in the disc specimens with and without fiber is shown It can be concluded that, by adding fiber, the failure crack pattern changed from a single large crack to a group narrow cracks, which demonstrates the beneficial effects of fiber-reinforced concrete subjected to impact loading Conclusions The increase of polypropylene fiber in the mixtures from 0.2% to 0.5%, generally increased the compressive strength The compressive strength of fibrous specimens at the age of 91 days, with 0.5% fiber, increased by 15% compared with those of the reference When silica fume is added into the non-fibrous and fibrous mixtures, the compressive strength, at the age of 91 days, was enhanced by 23% and 30%, respectively On the other hand, adding of silica fume into the fibrous specimens led to an increased in compressive strength up to 30% at the age of 91 days This may be due to pozzolanic effect of silica fume and crack restriction effect if fiber Splitting tensile and flexural strength of 0.5% fibrous silica fume concretes was enhanced considerably The number of blows at first cracks and failure, as impact indices, increased considerably in fibrous specimens Incorporating 0.2%, 0.3% and 0.5% polypropylene fiber into the 0.46 watercement ratio specimens led to an increase in the number of blows by 31%, 100% and 360%, respectively at first crack and 42%, 107% and 376%, respectively, at failure compared to those of the reference Likewise, a similar trend was observed, but at a lower rate, in 0.36 water-cement ratio specimens The results revealed that silica fume improved the fiber dispersion in the mixtures Adding silica fume to fibrous specimens improved the specimens strength more than adding silica fume by itself These results show that silica fume can strengthens the transition zone and reduces crack initiation, and therefore, improves the failure strength of polypropylene fiber concretes M Nili, V Afroughsabet / Construction and Building Materials 24 (2010) 927–933 933 Fig Fracture pattern of concrete with different fiber volume fractions under the drop weight test: (a) plain concrete, (b) 0.2% fiber, (c) 0.3% fiber and (d) 0.5% fiber A ductile failure, under impact loading, was observed in fibrous 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construction Press; 1978 p 107–20 [19] Mohammadi Y, Carkon-Azad R, Singh SP, Kaushik SK Impact resistance of steel fibrous concrete containing fibers of mixed aspect ratio J Construct Build Mater 2009;23:183–9 [20] ACI Committee 544 Measurement of properties of fiber-reinforced concrete ACI Mater J 1988;85(6):583–93 [21] Schrader EK Impact resistance and test procedure for concrete ACI J 1981;78:141–6 [22] Löfgren I Fiber-reinforced concrete for industrial construction—a fracture mechanics approach to material testing and structural analysis Thesis for the degree of doctor of philosophy, Göteborg, Sweden; 2005  ... that the pozzolanic properties of silica fume and also the crack restriction effect of fiber can promote the compressive strength of concrete Table Mix proportions of the concrete mixtures Mix... by itself These results show that silica fume can strengthens the transition zone and reduces crack initiation, and therefore, improves the failure strength of polypropylene fiber concretes M... incorporating silica fume and polypropylene considerably improved the ability of concrete to absorb kinetic energy References [1] Yan H, Sun W, Chen H The effect of silica fume and steel fiber on the dynamic