CHAPTER 6 POLYPROPYLENE FIBRE REINFORCED GEOPOLYMER CONCRETE COMPOSITES

Transcription

1 113 CHAPTER 6 POLYPROPYLENE FIBRE REINFORCED GEOPOLYMER CONCRETE COMPOSITES 6.1 GENERAL This chapter describes the effect of addition of polypropylene fibres on the strength characteristics of geopolymer concrete composites. The fresh and hardened properties such as workability, density, compressive strength, split tensile strength, flexural strength, impact strength, modulus of elasticity, water absorption and sorptivity of Polypropylene Fibre Reinforced Geopolymer Concrete Composites (PFRGPCC) is presented in this chapter. A comparison on the strength and durability aspects between GPCC and PFRGPCC is also discussed. 6.2 EXPERIMENTAL PROGRAMME Parameters of Study investigation: The following parameters were considered in this experimental (a) Volume fraction of polypropylene fibres: 0%, 0.1%, 0.2% and 0.3% (b) Age of concrete at time of testing: 1day, 3days, 7 days and 28 days

2 Materials Used Fly ash: Class F dry fly ash conforming to IS obtained from Mettur thermal power station of Tamilnadu from southern part of India was made use of in the casting of the specimens. Cement: Ordinary Portland Cement (OPC) conforming to IS: , having a specific gravity of 3.15 was made use of, in the casting of the specimens. Fine Aggregate: Locally available river sand having a bulk density of 1693 kg/m 3,fineness modulus of 2.75, specific gravity of 2.81 and conforming to grading zone-iii as per IS: was used. Coarse Aggregate: Crushed granite coarse aggregates of 19 mm maximum size having a fineness modulus of 6.64 and a specific gravity of 2.73 were used. Bulk Density of the coarse aggregate used is 1527 kg/m 3. Sodium Hydroxide: Sodium hydroxide solids in the form of flakes with 97% purity manufactured by Merck Specialties Private Limited, Mumbai was used in the preparation of alkaline activator. Sodium Silicate: Sodium silicate in the form of solution supplied by Salfa Industries, Madurai was used in the preparation of alkaline activator. The chemical composition of Sodium silicate solution supplied by the manufacturers is as follows: 14.7%, of Na 2 O, 29.4% of SiO 2 and 55.9% of water by mass. Super plasticiser: To achieve workability of fresh Geopolymer Concrete, Sulphonated napthalene polymer based super plasticizer Conplast SP 430 in the form of a brown liquid instantly dispersible in water,

3 115 manufactured by Fosroc Chemicals (India) private limited, Bangalore, was used in all the mixtures. Water: Distilled water was used for the preparation of sodium hydroxide solution and for extra water added to achieve workability. Polypropylene fibre: Polypropylene fibres having a length of 6 mm and a diameter of 0.02 mm were used and they are shown in Figure 6.1. These fibres have a density of 910 kg/m 3, modulus of elasticity of 3500 MPa and yield strength of 550 MPa. (Source: Manufacturer s data) Figure 6.1 Polypropylene fibres Preparation of Alkaline Activator Solution A combination of sodium hydroxide solution of 12 molarity and sodium silicate solution was used as alkaline activator solution for geopolymerisation. To prepare sodium hydroxide solution of 12 molarity (12 M), 480 g (12 x 40 i.e, molarity x molecular weight) of sodium hydroxide

4 116 flakes was dissolved in distilled water and madeup to one litre. The mass of solid NaOH was measured as g/ kg in the 12 M NaOH solution Mix Proportion of PFRGPCC In case of PFRGPCC mixes polypropylene fibres were added to the GPCC mix in three volume fractions such as 0.1%, 0.2% and 0.3% by volume of the concrete. The mix proportions of GPCC and PFRGPCC are given in Table 6.1. Table 6.1 Details of mix proportions of PFRGPCC Mix ID Fly Ash kg/m 3 OPC kg/m 3 FA kg/m 3 CA kg/m 3 NaOH Solution kg/m 3 Na 2 SiO 3 Solution kg/m 3 Extra Water kg/m 3 SP kg/m 3 PP fibres GPCC kg/m 3 P P P Preparation of PFRGPCC Specimens The prepared solution of sodium hydroxide of 12 M concentration was mixed with sodium silicate solution one day before mixing the concrete to get the desired alkalinity in the alkaline activator solution. Initially Fine aggregates, fly ash, OPC, coarse aggregates and polypropylene fibres were dry mixed for three minutes in a horizontal pan mixer. After dry mixing, alkaline activator solution was added to the dry mix and wet mixing was done for 4 minutes. Finally extra water along with super plasticizer was added.

5 Curing of PFRGPCC Specimens PFRGPCC specimens were removed from the moulds immediately after 24 hours since they set in a similar fashion as that of conventional concrete. All the specimens were left at room temperature in ambient curing till the date of testing. 6.3 WORKABILITY All the freshly prepared PFRGPCC mixes were tested for workability by using the conventional slump cone apparatus. The slump cone was filled with freshly mixed polypropylene fibre reinforced geopolymer concrete composite mix and was compacted with a tamping bar in four layers. The top of the slump cone was leveled off, then the cone was lifted vertically up and the slump of the sample was immediately measured. All the mixes were generally cohesive and shiny in appearance due to the presence of sodium silicate solution. Inclusion of polypropylene fibres reduces the slump values. Increase in fibre content dosage additionally reduces the workability of PFRGPCC specimens as shown in Figure GPCC P0.1 P0.2 P GPCC P0.1 P0.2 P0.3 Figure 6.2 Effect of polypropylene fibres on workability

6 DENSITY Density was calculated by measuring the weight of cube specimens before subjecting them to compression test. Density of all the mixes is presented in Table 6.2. Specimens have been given descriptive names, composed of two terms. Each of these terms gives information about some aspect of the specimens which is described as follows: The first term describes the volume fraction of polyproylene fibres in the geopolymer concrete composite mix. P 0 refers to GPCC specimens without polypropylene fibres. P 0.1 refers to PFRGPCC specimens containing 0.1% volume fraction of polypropylene fibres. Similarly P 0.2 and P 0.3 refers to PFRGPCC specimens containing 0.2% and 0.3% volume fraction of polypropylene fibres respectively. The second term refers to the age of concrete at the time of testing A 1, A 3, A 7 and A 28 refer to tests conducted at respective age of concrete in days. The density of GPCC without polypropylene fibres ranges from 2347 kg/m 3 to 2458 kg/m 3, density of GPCC containing 0.1% of polypropylene fibres ranges from 2376 kg/m 3 to 2415 kg/m 3, density of GPCC containing 0.2% of polypropylene fibres ranges from 2336 kg/m 3 to 2406 kg/m 3 and density of GPCC containing 0.3% of polypropylene fibres ranges from 2299 kg/m 3 to 2421 kg/m 3 as shown in Figure 6.3. The density of GPCC and PFRGPCC is found close to that of ordinary Portland cement concrete. It was found from the test results that for most of the cases, inclusion of polypropylene fibres in concrete resulted in a marginal decrease in unit weight.

7 119 Table 6.2 Density of PFRGPCC specimens Spec. Avg. Weight in kg Avg. Density kg/m 3 P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A GPCC P 0.1 P 0.2 P Specimen number Figure 6.3 Density ranges of PFRGPCC specimens

8 COMPRESSIVE STRENGTH Test Specimens Totally thirty six cubes of size 150 mm x 150 mm x 150 mm were cast to study the compressive strength of PFRGPCC. Standard cast iron moulds were used for casting the test specimens. Before casting, machine oil was smeared on the inner surfaces of moulds. Geopolymer concrete with polypropylene fibres was mixed using a horizontal pan mixer machine and was poured into the moulds in layers. Each layer of concrete was compacted using a table vibrator Instrumentation and Testing Procedure For the evaluation of compressive strength, all the PFRGPCC cube specimens were subjected to a compressive load in a digital Compression Testing Machine with a loading capacity of 2000 kn. Specimens were tested as per the procedure given in Indian Standards I.S.516. The maximum load applied to the specimen was recorded. The compressive strength of the specimen was calculated by dividing the maximum load applied to the specimen by the cross-sectional area Results and Discussion The effect of addition of polypropylene fibres in different volume fractions and age of concrete at the time of testing on the compressive strength of geopolymer concrete composite has been investigated and presented. Test results of compressive strength are presented in Table 6.3. At the age of 1 day, PFRGPCC specimens gained 9 % to 15 % of its 28 days compressive strength. Similarly at the age of 3 days PFRGPCC specimens gained 37% to 40% of its 28 days strength and at the age of 7 days PFRGPCC specimens gained 49% to 53% of its 28 days strength as shown in Figure 6.4.

9 121 Table 6.3 Compressive strength of PFRGPCC specimens Spec. Avg. Ultimate load in kn Avg. Compressive Strength MPa P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A % 51% 50% 47% 20% 26% 25% 24% 12% 14% 12% 14% 20% 9% 13% 15% Volume fraction of polypropylene fibres in % 28 days 7 days 3 days 1 day Figure 6.4 Gain in compressive strength with age

10 122 As the age of concrete increases from 1 day to 28 days, compressive strength also increases for all the mixes. From the test results it can be seen that, the 28 days compressive strength of geopolymer concrete composites containing polypropylene fibres was slightly higher than those of GPCC without polypropylene fibres. The increase in compressive strength due to addition of polypropylene fibres is not much significant and it was only about 3%, 1.82% and 1.66% for 0.1%, 0.2% and 0.3% volume fraction respectively with reference to GPCC mix without polypropylene fibres as shown in Figure Volume fraction of Polypropylene fibres in % Figure 6.5 Gain in compressive strength due to PP fibres 6.6 SPLIT TENSILE STRENGTH Test Specimens Totally eighteen cylinders with a diameter of 150 mm and 300 mm length were cast to evaluate the split tensile strength of PFRGPCC. Standard cast iron moulds were used for casting the test specimens. Before casting, machine oil was smeared on the inner surfaces of moulds. Geopolymer concrete with polypropylene fibres was mixed using a horizontal pan mixer machine and was poured into the moulds in layers. Each layer of concrete was compacted using a table vibrator.

11 Instrumentation and Testing Procedure In order to evaluate the splitting tensile strength of polypropylene fibre reinforced geopolymer concrete composites, all the cylinder specimens were subjected to split tensile test in a 2000 kn digital Compression Testing Machine. Specimens were tested as per the procedure given in Indian Standards IS The maximum load applied to the specimen was recorded and the split tensile strength of the specimen was calculated Results and Discussion The effect of various factors such as addition of polypropylene fibres in different volume fractions and age of concrete at the time of testing on the split tensile strength of geopolymer concrete composite has been investigated and presented. Test results of split tensile strength are presented in Table 6.4. Table 6.4 Split tensile strength of PFRGPCC specimens Spec. Avg. Ultimate load in kn Avg. Split tensile Strength MPa P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A

12 124 Within 7 days, PFRGPCC specimens gained 51%, 49% and 48% of its 28 days split tensile strength for volume fraction of 0.1%, 0.2% and 0.3% respectively as shown in Figure 6.6. As the volume fraction of polypropylene fibres increases from 0% to 0.3%, the split tensile strength also increases as shown in Figure % 49% 51% 52% 46% 51% 49% 48% Volume fraction of Polypropylene fibres in % 28 days 7 days Figure 6.6 Gain in split tensile strength with age days 28 days Volume fraction of polypropylene fibre in % Figure 6.7 Gain in split tensile strength due to PP fibres The improvement in the split tensile strength at 28 days was found to be 1%, 9% and 12% for volume fractions of 0.1%, 0.2% and 0.3%

13 125 respectively. Once the splitting occurred and continued, the polypropylene fibres that bridges across the split portions of the geopolymer matrix acted through the stress transfer from the matrix to the fibres and, thus, gradually supported the total load. The stress transfer improved the tensile strain capacity of the PFRGPCC specimens thereby increasing the split tensile strength over the unreinforced control GPCC specimens. The increase in split tensile strength may be also due to the role of polypropylene fibres to resist cracking and spalling across the failure planes. Based on the test results, using least square regression analysis, an equation for predicting the 28 days split tensile strength of polypropylene fibre reinforced geopolymer concrete composites in terms of the split tensile strength of plain GPCC and percentage volume fraction of fibres (V f ) is obtained and given in Equation (6.1). f ts = f t V f (6.1) where, f ts = 28 days split tensile strength of PFRGPCC f t = 28 days split tensile strength of GPCC without fibres V f = Percentage volume fraction of polypropylene fibres. The split tensile strength of PFRGPCC predicted from the proposed analytical equation was compared with the experimental results as shown in Table 6.5. It was found that a good correlation was obtained between the experimental results and those got from the equation. It can be seen that the proposed equation predicts the split tensile strength of PFRGPCC well with good accuracy.

14 126 Table 6.5 Comparison of experimental and analytical results Volume fraction of fibres in % Split tensile strength MPa Analytical / Experimental Analytical Experimental FLEXURAL STRENGTH Test Specimens Totally eighteen prisms of 500 mm x 100 mm x100 mm were cast to study the flexural strength of PFRGPCC. Standard cast iron moulds were used for casting the test specimens. Before casting, machine oil was smeared on the inner surfaces of moulds. Geopolymer concrete with polypropylene fibres was mixed using a horizontal pan mixer machine and was poured into the moulds in layers. Each layer of concrete was compacted using a table vibrator Instrumentation and testing procedure Flexural strength of polypropylene fibre reinforced geopolymer concrete composites was determined using prism specimens by subjecting

15 127 them to two point loading in Universal Testing Machine having a capacity of 1000 kn. Specimens were tested as per the procedure given in Indian Standards IS.516. The maximum load applied to the specimen was recorded and the flexural strength of the specimen was calculated Results and Discussion The effect of addition of polypropylene fibres with different volume fractions and age of concrete at the time of testing on the flexural strength of geopolymer concrete composite has been investigated and presented. Test results of flexural strength are presented in Table 6.6. Table 6.6 Flexural strength of PFRGPCC specimens Spec. Avg. Ultimate load in kn Avg. Flexural Strength MPa P 0 A P 0.1 A P 0.2 A P 0.3 A P 0 A P 0.1 A P 0.2 A P 0.3 A Geopolymer concrete composite specimens harden immediately and start gaining flexural strength without any need of heat curing. In ambient curing at room temperature, within 7 days, PFRGPCC specimens gained 67% to 70% of its 28 days flexural strength as shown in Figure 6.8. As in the case of split tensile strength, PFRGPCC specimens resulted in significant increase of flexural strength when compared to control GPCC specimens as shown in

16 128 Figure 6.9. The flexural strength improves by about 1%, 6% and 12% for volume fractions of 0.1%, 0.2% and 0.3% of polypropylene fibres respectively at the age of 28 days. This increase in flexural strength might have resulted primarily from the polypropylene fibres intersecting the cracks in the tension zone of the flexure beam. These fibres accommodated the crack face separation by stretching themselves, thus providing an additional energy absorbing mechanism % 30% 30% 32% 28 days 7 days % 70% 70% 68% Volume fraction of Polypropylene fibres in % Figure 6.8 Gain in flexural strength with age Volume fraction of Polypropylene fibre in % 7 days 28 days Figure 6.9 Gain in flexural strength due to PP fibres

17 IMPACT RESISTANCE Test Specimens The impact resistance of the specimens was determined in accordance with ACI committee 544 recommendations. The test specimen consists of concrete discs of 150 mm diameter and 64 mm thickness. Specimens were cast using cast iron moulds as shown in Figure In this investigation, totally twelve geopolymer concrete composite discs were cast with and without fibres. Three specimens were cast for each volume fraction of fibre as shown in Figure 6.11 and remaining three discs were cast as control specimens without any fibres. Figure 6.10 Specimens with moulds Figure 6.11 PFRGPCC specimens for impact test

18 Instrumentation and Testing Procedure Drop weight impact test, also known as repeated impact test, is conducted for evaluating the impact resistance. The impact test equipment was fabricated according to standards for testing as per ACI Committee 544. Specimens were tested as per the recommendations given by ACI Committee Results and Discussion The effect of addition of polypropylene fibres in different volume fractions in improving the impact strength has been investigated and presented. Test results of impact strength are presented in Table 6.7. Table 6.7 Test Results of impact strength Spec. First Crack strength (blows) Spec. 1 Spec. 2 Spec. 3 Avg. Failure strength (blows) Spec. 1 Spec. 2 Spec. 3 Avg. GPCC P P P It is observed from the test results, that the specimens without fibres failed in a brittle manner. Plain GPCC specimens do not have considerable post crack resistance as it resisted only a few additional blows after the crack. The increase in number of blows for the first crack and the ultimate failure is significantly higher in the case of polypropylene fibre reinforced GPCC specimens. Even for a small addition of fibres the enhancement in first crack resistance as well as ultimate resistance is quite

19 131 considerable when compared to that of plain GPCC specimens as shown in Figure First crack Ultimate failure Volume fraction of fibres in % Figure 6.12 Effect of polypropylene fibres on Impact strength Due to the addition of polypropylene fibres, the first crack resistance increases by about 7.2 times, 11.4 times and 12.1 times for volume fractions of 0.1%, 0.2% and 0.3% respectively. A similar trend to that specified for first crack resistance is observed for ultimate resistance also. For volume fractions of 0.1%, 0.2% and 0.3% of polypropylene fibres, the ultimate resistance increases by about 8 times, 13 times and 14 times respectively. The percentage increase in number of post crack blows (PINPCB) is about 31%, 39% and 38% for the fibre volume fractions of 0.1%, 0.2% and 0.3% respectively and thus inclusion of polypropylene fibres considerably improved the ability of concrete to absorb kinetic energy leading to delayed failure strength. In Figure 6.13, a comparison of failure pattern in the disc specimens with and without polypropylene fibres is shown. It can be seen that the addition of fibres changes the crack pattern from a single large crack to a

20 132 group of narrow cracks, which demonstrates the beneficial effects of fibre reinforced GPCC subjected to impact loading. Figure 6.13 Failure pattern of PFRGPCC impact discs 6.9 MODULUS OF ELASTICITY Specimens and Test Procedure The modulus of elasticity was determined in accordance with IS.516. The test specimen consists of concrete cylinders 150 mm diameter by 300 mm height. In this investigation, totally twelve geopolymer concrete composite cylinders were cast with and without fibres. Three specimens were cast for each volume fraction of fibre. Three cylinders were used as control specimens without any fibres added to them. All specimens were loaded in axial compression, using a digital CTM of capacity 2000 kn. Specimens were tested as per the procedure given in Indian Standards IS Evaluation of Modulus of Elasticity The strains at ten equal load intervals upto an average stress of (C+1.5) kg/sq.cm were measured and the stress-strain values are listed in

21 133 Appendix 3. For each volume fraction of polypropylene fibres, a graph was drawn by plotting the average strains against their corresponding stresses as shown in Figures 6.14 to Then best fit straight line was drawn through the plotted points. From the best fit straight line equation, the slope of the line is expressed as modulus of elasticity y = 27246x R² = Axial strain Figure 6.14 Axial stress Vs axial strain for GPCC specimens y = 28177x R² = Axial strain Figure 6.15 Axial stress Vs Axial strain for P 0.1 Specimens

22 y = 27425x R² = Axial strain Figure 6.16 Axial stress Vs Axial strain for P 0.2 Specimens y = 27301x R² = Axial strain Figure 6.17 Axial stress Vs Axial strain for P 0.3 Specimens Results and Discussion The effect of addition of polypropylene fibres on the modulus of elasticity has been investigated and presented. Test results of modulus of elasticity are presented in Figure Incorporation of polypropylene fibres in geopolymer concrete does not affect positively the modulus of elasticity. Elastic modulus of concrete containing polypropylene fibres is slightly higher than the elastic modulus of concrete without fibres. The elastic modulus improves by only about 3%, 1% and 0.2% for volume fractions of 0.1%, 0.2% and 0.3% respectively.

23 GPCC P0.1 P0.2 P0.3 Figure 6.18 Modulus of elasticity of PFRGPCC 6.10 WATER ABSORPTION Test Procedure The water absorption test has been carried out according to ASTM C , to study the relative porosity or permeability characteristics of PFRGPCC specimens at 28 days. The specimens used for this test were 100 mm cubes as shown in Figure The difference between the saturated mass and oven dried mass expressed as a fractional percentage of oven dried mass gives the water absorption. Figure 6.19 PFRGPCC specimens for water absorption test

24 Results and Discussion The values of saturated water absorption of the specimen at 28 days were found out and tabulated in the Table 6.8. The initial absorption values (at 30 min) for all the concretes were compared with recommendations given by Concrete Society (CEB). From the test results, it can be seen that the water absorption values at 30 minutes for the PFRGPCC specimens for all the volume fractions of fibres were lower than the limit of 3% specified for good concretes. The water absorption capacity of PFRGPCC specimens having 0.1% and 0.2% volume fraction of fibres were less when compared with control GPCC specimens, whereas the specimens having 0.3% of fibres have higher water absorption capacity as compared to control GPCC specimens. Within the fibrous specimens, specimens containing 0.2% of polypropylene fibres performs better by showing lower value for water absorption as shown in Figure minutes immersion 24 hours immersion GPCC P 0.1 P 0.2 P 0.3 Figure 6.20 Water absorption at 30 minutes and 24 hrs

25 137 Table 6.8 Test Results of water absorption Spec. GPCC P0.1 Initial weight g At 30 minutes immersion Weight g At 24 hours immersion Water absorption % At 30 minutes At 24 hours P P Average water absorption % At 30 minutes At 24 hours SORPTIVITY Test Procedure Oven dried cube specimens of 100 mm size were exposed to the water by placing it in a pan as shown in Figure At certain times, the mass of the specimens was measured using a balance, then the amount of water adsorbed was calculated and normalized with respect to the cross section area of the specimens exposed to the water at various times such as 1, 4, 9, 16, 25, 36, 49, 81and 100 minutes. To determine the sorptivity value,

26 138 Q was plotted against the square root of time A t.the sorptivity value was calculated from the slope of the linear relation between A Q and t. Figure 6.21 PFRGPCC Specimens during sorptivity test Results and Discussion The sorptivity test results of GPCC and PFRGPCC specimens are presented in Appendix 4. From the test results, cumulative absorbed volume after time t per unit area of inflow surface is calculated and given in Table 6.9. When polypropylene fibres were added into the GPCC mix the sorptivity coefficient decreases as shown in Figure This is due to the decreased porosity in the geopolymer paste and this lower sorptivity value emphasizes the beneficial effect of adding polypropylene fibres to increase the durability of concrete. Sorptivity values of specimens containing 0.1% and 0.2% of fibres were too low which indicates that the porosity of concrete is lesser with lesser number of interconnected pores. These specimens have a denser structure as compared to specimens containing 0.3% of fibres because

27 139 P 0.3 specimens showed a maximum sorptivity value of This higher value of sorptivity coefficient is due to larger number of capillary pores. Table 6.9 Cumulative water absorption t Q / A in mm min 1/2 GPCC P 0.1 P 0.2 P GPCC P 0.1 P 0.2 P 0.3 GPCC P 0.1 P 0.2 P 0.3 Figure 6.22 Sorptivity values for PFRGPCC specimens

28 CONCLUSIONS Based on the results obtained in this investigation, the following conclusions are drawn: Inclusion of polypropylene fibres reduces the slump values. Increase in fibre content dosage additionally reduces the workability of PFRGPCC specimens. The density of GPCC without polypropylene fibres ranges from 2347 kg/m 3 to 2458 kg/m 3.Density of GPCC containing polypropylene fibres ranges from 2376 kg/m 3 to 2415 kg/m 3, 2336 kg/m 3 to 2406 kg/m 3 and 2299 kg/m 3 to 2421kg/m 3 for volume fractions of 0.1%, 0.2% and 0.3% respectively. It was found from the test results that for most of the cases, inclusion of polypropylene fibres in concrete resulted in marginal decrease in unit weight. The increase in compressive strength due to addition of polypropylene fibres is not much significant and it was only about 3%, 1.82% and 1.66% for 0.1%, 0.2% and 0.3% of polypropylene fibres respectively with reference to GPCC mix without polypropylene fibres. As the volume fraction of polypropylene fibres increases from 0% to 0.3%, the split tensile strength also increases. The improvement in the split tensile strength at 28 days was found to be 1%, 9% and 12% for volume fractions of 0.1%, 0.2% and 0.3% respectively. The increase in split tensile strength is due to the role of polypropylene fibres to resist cracking and spalling across the failure planes.

29 141 Based on the test results, using least square regression analysis, an equation for predicting the 28 days split tensile strength of polypropylene fibre reinforced geopolymer concrete composites in terms of the split tensile strength of plain GPCC and percentage volume fraction of fibres is obtained. The split tensile strength of PFRGPCC predicted from the proposed analytical equation was compared with the experimental results and it is found that a good correlation is obtained. Addition of polypropylene fibres to GPCC resulted in enhancement of flexural strength. At the age of 28 days, the flexural strength increases by about 1%, 6% and 12% for volume fractions of 0.1%, 0.2% and 0.3% of polypropylene fibres respectively. This increase in flexural strength might have resulted primarily from the polypropylene fibres intersecting the cracks in the tension zone of the flexure beam. These fibres accommodated the crack face separation by stretching themselves, thus providing an additional energy-absorbing mechanism. In case of impact testing, the number of blows at first cracks and failure, increased considerably in fibrous specimens. Incorporating 0.1%, 0.2% and 0.3% polypropylene fibres into the GPCC specimens led to an increase in the number of blows by 620%, 1040% and 1110%, respectively at first crack and 683%, 1225% and 1292%, respectively, at failure compared to those of control GPCC specimens. The percentage increase in number of post crack blows is about 31%, 39% and 38% for the fibre volume fractions of 0.1%, 0.2% and 0.3% respectively and

30 142 thus inclusion of fibres considerably improved the ability of concrete to absorb kinetic energy. Incorporation of polypropylene fibres in geopolymer concrete does not affect positively the modulus of elasticity. Elastic modulus of concrete containing polypropylene fibres is marginally higher than the elastic modulus of concrete without fibres. The elastic modulus improves by only about 3%, 1% and 0.2% for volume fractions of 0.1%, 0.2% and 0.3% respectively. Water absorption values at 30 minutes for the PFRGPCC specimens for all the volume fractions of fibres were lower than the limit of 3% specified for good concretes. The water absorption capacity of PFRGPCC specimens having 0.1% and 0.2% volume fraction of fibres were less when compared with control GPCC specimens, whereas the specimens having 0.3% of fibres have higher water absorption capacity as compared to control GPCC specimens. Within the fibrous specimens, specimens containing 0.2% of polypropylene fibres perform better by showing lower value for water absorption. The addition of polypropylene fibres into the GPCC mix decreases the sorptivity coefficient. Sorptivity values of specimens containing 0.1% and 0.2% of fibres were too low which indicates that the porosity of concrete is lesser with lesser number of interconnected pores. These specimens have a denser structure as compared to specimens containing 0.3% of fibres because P 0.3 specimens showed a maximum sorptivity value of This higher value of sorptivity coefficient is due to larger number of capillary pores.