Accelerated engineering properties of high and low volume fly ash concretes reinforced with glued steel fibers

Abstract

The present study focuses on the improvement of pozzolanic reaction of fly ash particles with the cement hydration products. Low and high volume fly ash concrete mixtures were studied systematically with the addition of accelerating admixtures and accelerated curing of the concrete specimens in a steam chamber for 18 h at 75°C. Also, the reinforcing effects of glued steel fibers addition on the compressive and flexural performance of fly ash concrete were investigated. The test results indicated that the addition of accelerator improved the rate of hardening and the inclusion of steel fibers provided higher flexural performance. Also, it can be noted that the high volume fly ash (50%) addition in concrete showed a reduction in strength; however, the addition of accelerator has compensated the deceleration in strength gain. The proper selection of concrete ingredients, addition of accelerator and initial steam curing for 18 h showed better improvement on the engineering properties in fly ash concrete. A maximum increase (41.7%) in compressive strength of fly ash concrete around 52.90 MPa was noticed for 25% fly ash substitution and 1.5% steel fibers addition. Dynamic elastic modulus was also calculated in loaded concrete specimen using ultrasonic pulse velocity test and showed a good agreement with the experimental value.

Keywords

fly ash / pozzolanic index / steam curing / superplasticizer / accelerator / steel fibres / elastic modulus

Introduction

The rapid increase in the fly ash substitution in concrete is attributed to its positive effects on the mechanical properties of cementitious composites. It is well documented from earlier studies that the use of fly ash as a partial replacement for cement in combination with super plasticizers provides a significant increase in the fresh and hardened properties of concrete [1,2]. However, the large scale addition of fly ash is restricted owing to its poor pozzolanic reaction at early ages of concrete setting [3]. This is attributed due to the delay in the reaction with the cement hydration products (CaOH₂)_. The negative effects on the setting properties and deceleration in the rate of strength gain restricts the maximum replacement levels of fly ash upto 30% in cement concrete. Many Studies in fly ash concrete showed that the beneficial properties of fly ash can be realized in terms of the improved mechanical properties in concrete after longer curing period [4-7]. The use of finer fly ash is one technique that can improve the pozzolanic efficiency and can completely offset the negative aspects of fly ash addition in concrete. Addition of finer fly ash particles are known to improve the micro structural properties and thereby improve the permeation resistance of concrete. It is also noted that the cementing efficiency of fly ash depends primarily on the particle size as well as its silica content. It is also reported that the pozzolanic efficiency of fly ash with cement depends on the careful mix design procedures leading to improved strength gain properties [8,9]. However, the test results were found to vary widely due to presence of large number of variables which affect the pozzolanic reaction between cement hydration products. The reaction efficiency of high volume fly ash addition in concrete is affected with the increased water cement ratio and poor binder to aggregate proportioning [10]. Earlier studies shown that the proper selection of chemical admixtures and tailoring concrete ingredients to meet the strength attainment have significantly improved the large volume fly ash addition in concrete [11-14]. The strength gain in fly ash based concrete systems can be monitored using ultrasonic pulse measurements and can be used to check the reaction efficiency of various replacement levels of fly ash incorporated in the fresh cementitious system [15]. A wide range of research studies indicated that the addition of 30% fly ash substitution in cement showed better pozzolanic reactivity. However, the inclusion of chemical admixtures at low water to binder ratio showed better performance levels of high volume fly ash addition in concrete with careful mixture proportioning of mix ingredients [16]. It can be concluded from the earlier studies that the incorporation of low and high volume additions of fly ash depends on the early age reaction and faster strength gain properties which can substantially increase the performance levels of fly ash in concrete [17]. Also the effect of curing regime on the strength properties of cementitious system greatly influence the hydration charactersitics [18]. It is well documented from the previous studies that much focus was not shown to improve the pozzolanic reactivity of fly ash with the addition of accelerating admixtures and the effect of steam curing on the rate of hardening. Also, the influence of selecting appropriate binder to aggregate ratio and fine to coarse aggregate ratio has to be explored systematically in different fly ash based cementitious system. Further a detailed study needs to be conducted systematically to evaluate the various mechanical properties of accelerated fly ash concrete incorporating 25% and 50% fly ash addition in concrete.

Research significance

The present study explores the improvement on the rate of hardening characteristics of fly ash concretes with the addition of accelerator and steam curing. A special attention was given for designing concrete mixture proportions for obtaining high strength concrete using conceptual proportioning methodology by considering binder to total aggregate ratio and fine to coarse aggregate ratio. Further, the improvements on the hardened concrete properties were studied with the inclusion of special type of glued steel fibers in fly ash concrete systems. Comprehensive experimental tests comprising compressive, split tensile, flexural, residual load, elastic modulus, dynamic modulus under loading and ultrasonic pulse velocity tests have been conducted to assess the performance of high and low volume fly ash concretes.

Material used and experimental methods

The details of materials used in the present experimental investigation are as follows.

Cement

Ordinary Portland cement of 53 grade conforming to IS: 12269-1987 [19] was used in the investigation. The specific gravity of cement was 3.17 with a consistency limit of 31.5% and the chemical composition of cement is given in Table 1.

Fine aggregates

River sand obtained from locally available source passing through 4.75 mm IS sieve, conforming to grading zone-II of IS: 383-1970 [20] was used with a fineness modulus of 2.57, specific gravity of 2.71 and water absorption of 0.67% tested under standard conditions.

Coarse aggregates

Machine crushed well graded angular blue granite stone with 12.5 mm maximum size, conforming to IS: 383-1970 was used. The specific gravity was found to be 2.75, fineness modulus of 7.2 and water absorption is 0.62% at 24 h.

Fly ash

A low calcium fly ash (class F) obtained from the Ennore thermal power station, Chennai was used in the present study and the detailed chemical properties are given in Table 1.

Chemical admixture

A commercially available calcium nitrate based accelerator (MEYCO MP 355 1K) was used to accelerate the pozzolanic reaction in fly ash concrete; which had a specific gravity value of 1.82 and solid content of 25%. To improve the workability properties of fresh concrete, polycarboxylate ether (PCE) based super-plasticizer (SP) was added at 1% for reference concrete. In the case of 25% and 50% fly ash addition, the SP dosage was added at 1% and 1.5% (by weight of binder) respectively to obtain the desired workability range of 75 to 100mm slump.

Glued steel fibers

Glued steel fibers (as shown in Fig. 1) imported from Korea was used in the present study and the various properties of the material are given in Table 2. The glued steel fibers were bundled together using a water soluble glue and upon addition in concrete the glue dissolves and dispersed in the concrete. The steel fibers were consisting of hooked ends to provide adequate end anchorages and bond strength in the concrete matrix.

Conceptual concrete mix design

Concrete mixtures adopted in this study were designed using a conceptual mix proportioning method with target strength of M40 grade concrete. The water content for the reference mix and various mix proportions was found to be 142 kg/m³ and were established by keeping w/c ratio 0.3 with a fine to coarse aggregate (F/c) ratio of 0.6 and 0.8. In this research study, a total of 24 different concrete mixture proportions (presented in Table 3) were proportioned with a reference mix without fly ash (MC1 and MC1A). The remaining 22 concrete mixes contain 25% and 50% of fly ash by weight of cement with varying percentage of glued steel fibers at 0.5%, 1.0% and 1.5% volume fraction of concrete. Also, the addition of accelerator dosage was fixed at 1% by weight of binder (from trial studies) and it was noted that the further increase in accelerator dosage beyond 1% resulted in the rapid stiffening and loss in consistency. This restricted the maximum dosage of accelerator at 1% for both low and high volume fly ash concretes. The workability of concrete mixes at low water cement ratio was improved by the addition of polycarboxylic ether based superplasticizer and the maximum dosage is restricted upto 1.5% by weight of cement since, the desired slump range of 75 to 100 mm were obtained in all the concrete mixes.

Pozzolanic activity index [PAI] test

The reaction efficiency of fly ash with cement was assessed using pozzolanic activity index. Mortar specimens of size 70.6 mm × 70.6 mm × 70.6 mm were casted for testing the pozzolanic activity index of various fly ash cement mortar specimens. A reference cement mortar and fly ash replaced cement mortar with and without accelerator were casted as per the standard codal provisions IS 1727-1981[21]. The samples were tested after required curing and tested in a compression testing machine.

Mixing and casting details of the concrete mixtures

The concrete ingredients were mixed in a pan type concrete mixer of capacity 40 kg for a period of 5 min. Initially, the required amount superplasticizer and accelerator was mixed thoroughly with the calculated mix water and added to the dry ingredients in the mixer machine followed by the addition of steel fibers. Concrete is then casted in steel molds and finally all specimens were compacted on a table vibrator. The surface finishing was done very carefully to obtain a uniform smooth surface. Except the reference concrete, all the fly ash concrete mixes were essentially cured in a steam chamber at 75°C (shown in Fig. 2) for 18 h for accelerated curing and thereafter it was kept in water curing for the remaining days till testing. The concrete specimens were cured in the potable water with respect to different curing days and also to maintain the uniformity of curing in the concrete specimens. Tests were performed at 7, 28 and 56 days of curing period for various mixture proportions of the concrete.

Compressive strength and split tensile strength

Concrete specimens were casted in a steel cube mold of size 100 mm × 100 mm × 100 mm for measuring compressive strength and cylindrical specimens were casted using steel molds of 100 mm × 200 mm size for determining split tensile strength tests as per IS 5816 [22]. The hardened concrete specimens after sufficient water curing were tested as per the respective curing age with the help of a digital compressive testing machine of capacity 2000 kN.

Flexural strength

A flexural testing machine of capacity 100 kN was used to study the flexural tensile properties of hardened concrete using a third point loading arrangement shown in Fig. 3. The concrete prism of size 100 mm × 100 mm × 500 mm was tested, to study the effect of fiber reinforcements in fly ash concrete beam to resist the crack after bending with the help of displacement controlled testing machine of capacity 100 kN operating at the rate of 1.5 mm/min.

Residual flexural strength ratio

The influence of steel fibers in bridging the cracks of concrete matrix can be best determined using the residual flexure strength ratio which is a measure of the post crack strength index of the fiber reinforced concrete. It is obtained from the ratio of the residual load to the maximum load. The residual load is defined as the maximum load achieved on reloading the specimen after unloading at 90% of the original maximum load in the descending region (as shown in Fig. 4).

The residual flexural strength ratio is given by the following expression:

(1){\alpha }_b＝P_o/P,

where α_b is the residual flexural strength ratio, P_o is the residual maximum load at reloading after unloading at the point of 90% of maximum load on the descending region and P is the original maximum load taken by the specimen.

Ultrasonic pulse velocity

Quality of the concrete and the rate of strength improvement were measured indirectly using an ultrasonic pulse velocity method, which involves measurement of the time of travel of an ultrasonic pulse passing through the concrete. The pulse generator circuit (shown in Fig. 5) consists of an electronic circuit for generating pulses and a transducer for transforming these electronic pulses into mechanical energy having a vibration frequency of 50 KHz. The path length between transducer divided by the time of travel gives the average velocity of wave propagation and the test results were verified as per IS 13311 [23].

Modulus of elasticity

Elastic modulus of concrete represent the resistance of concrete against compressive strain and this was evaluated using a concrete specimen of size 150 mm diameter and 300 mm height After capping, the cylindrical concrete specimens was attached with a compressometer at the gauge points about the center of the specimen. The test specimen was placed in a compressive testing machine loads were applied up to 40% of the failure load and the deformation was recorded for every 10 kN load. A stress-strain graph is drawn from the observed compression test results and the slope of the line is calculated to evaluate the elastic modulus of the concrete test set up as shown in Fig. 6.

Dynamic modulus of elasticity

The elastic modulus value calculated from the ultrasonic pulse velocity test is referred to as dynamic modulus of elasticity. This was determined in the present study by the measuring the pulse velocity through the loaded concrete specimen in compression. The concrete specimens were given a compressive load equal to 40% of the ultimate load and the transducers were fixed to the unloaded sides of the concrete specimens (as shown in Fig. 7) to measure the ultrasonic pulse velocity. The dynamic elastic modulus (E) value is then calculated as per IS 13311 part 1 and is given by the following equation:

(2)E=\rho V^{\mathrm{2}}(\mathrm{1}+\mu )(\mathrm{1}-\mathrm{2}\mu )/(\mathrm{1}+\mu ),

where V is pulse velocity; ρ is density of concrete (kg·m^-3); μ is poisson’s ratio (0.2).

Experimental test results and discussions

Test results obtained from the entire study are presented systematically and have been discussed under various categories.

Pozzolanic activity index

The effects of fly ash addition in cementitious system can be better assesed in terms of pozzolanic activity index. It can be justified from the results given in Table 4 that the fly ash addition at 25% showed a marginal reduction in 7 days strength compared to reference cement mortar. However, with the addition of 1% accelerator the pozzolanic reaction was much effective in 25% fly ash cement mortar and exhibited a faster setting at 7 and 28 days. A similar trend was observed in the case of 50% fly ash cement mortar wherein, the addition of 1% accelerator has increased the strength gain at 7 and 28 days but the strength was lower than reference cement mortar. This can be concluded that the addition of accelerator has improved the reaction efficiency of silica present in fly ash with the cement hydration product (CaOH₂).

Fresh concrete properties

The workability of various fresh concrete mixtures is reported in Fig. 8. It was observed during the workability test conducted that the desired slump range between 75 to 100 mm in the case of reference concrete mixtures was achieved with the superplasticizer addition at 1%. However, with the 25% fly ash addition the workability was slightly affected and the slump loss was more at higher substitution of fly ash (50%). The slump loss was reinstated with the addition of high range water reducers (1.5% by weight of cement). This loss was reported as a result of the increase in the binder volume (due to fineness of fly ash) and the addition of superplasticizer has compensated the loss in consistency of fly ash concrete mixes. Also it can be noted that the workability of all fly ash concrete mixes still decreased with the addition of steel fibers with increasing fiber volume fraction. It was ensured that all the fly ash concrete mixes were well compactable in the steel molds without any difficulty when the slump range was maintained between 75 to 100 mm. Among all the steel fiber reinforced fly ash concrete mixes, a lowest slump (71 mm) was reported for 50% fly ash addition with 1.5% steel fibers which required additional time for compaction.

Compressive strength of concrete

The compressive properties of various concrete specimens are summarized in Table 5 and shown in Figs. 9. The test results indicated that the addition of 25% fly ash in concrete reported a reasonable increase in strength compared to reference concrete. However, the effect of accelerator was better realized in terms of reaction efficiency of fly ash at early stages of hydration. It can be noted from Fig. 9(a) that for F/c ratio of 0.6 with 25% fly ash substitution showed an increase in strength up to 10.93% compared to reference concrete. Also in the case of 50% fly ash substituted concrete the strength enhancement for F/c ratio of 0.6 was better than F/c ratio of 0.8. However, the accelerator substituted fly ash concrete mixes showed better improvement in strength gain up to 13.81% compared to reference concrete as seen in Fig. 9(b). Whereas the 50% fly ash substituted concrete with 0.6 F/c ratio showed a marginal improvement in strength compared to 50% fly ash concrete without an accelerator. The experimental trend showed that for high volume fly ash (50%) substituted concretes the strength loss reported was not high compared to reference concrete. The effect of fine to coarse aggregate ratio had shown direct implication on the strength gain in different types of fly ash concretes investigated. A reasonable increase in compressive strength up to 28.39% was reported with the addition of steel fibers at higher volume fraction of 1.5% (V_f). A maximum compressive strength value of 52.90 MPa was noticed in the case of fly ash concrete mixes at 25% replacement level with 1% accelerator compared to reference concrete as seen in Fig. 9(c). The maximum and early strength gain was observed for lower fly ash substitution at 25% with glued steel fibers of 1.5% and 1% accelerator dosage, which exhibited the attainment of high early strength concrete. It is clearly evident from the experimental test results that the early strength gain properties of fly ash based concrete mixes can be improved with the addition of accelerator. The results demonstrated that the effects of accelerator in fly ash concrete systems provided adequate initiation of the early pozzolanic reaction at the age of 7 days itself compared to reference fly ash based concretes without accelerator and showed delayed pozzolanic reaction only after 28 days. It can be justified that the higher replacement level of fly ash up to 50% with the addition of steel fibers and 1% of accelerator showed marginal increase (4.61%) in the strength upto 43.40 MPa at 28 days which was higher compared to reference concrete as seen in Fig. 9(d). It can be concluded from the strength results that the influence of accelerator on the early strength gain in fly ash concretes was appreciable as well as exhibited higher strength. Also, a reasonable strength increase was noted with the addition of 1.5% V_f of glued steel fibers in fly ash concretes. The addition of fibers were found to be effective due to random distribution and uniform fiber spacing (without balling effects) during concrete mixing which were identified to be an important factor for obtaining the quality and strength of steel fiber concrete mixes.

Split tensile strength properties

The split tensile strength values of various concrete specimens are provided in Table 5. It can be noted that the split tensile strength of concrete was found to increase marginally up to 4.57% and 1.88% at 28 days for concrete mixes containing F/c ratio of 0.6 with 25% and 50% fly ash (without accelerator and steel fibers) respectively compared to reference concrete. Also a slight decrease in the trend was observed for F/c ratio of 0.8 with different percentage of fly ash as seen in Fig. 10(a). Further, the addition of 1% accelerator in 25% fly ash concretes showed a reasonable improvement in strength up to 2.59% (F/c ratio-0.6); whereas, compared to F/c ratio of 0.8, the strength was increased up to 6.68% at 28 days. In the case of high volume fly ash (50%) concrete mixes the addition of accelerator showed a marginal increase in split tensile value due to early start of pozzolanic reaction as shown in Fig. 10(b). However, the strength increase was found to be higher for the concrete mix containing low fine to coarse aggregate ratio (F/c = 0.6) with fly ash addition 25%, steel fiber addition of 1.5% (V_f) and a accelerator dosage of 1% . This showed better improvement in the split tensile strength upto 4.94 Mpa at 28 days with an increase up to 32.80% due to the inclusion of steel fibers as shown in Fig. 10(c). Compared to reference concrete, the higher replacement of fly ash at 50% with the inclusion of 1.5% steel fibers also exhibited improved pozzolanic reaction due to the addition of 1% accelerator. A similar trend was also noted for concretes with higher F/c ratio of 0.8 which showed a maximum split tensile strength value of 4.76 MPa at 28 days as shown in Fig. 10(d). It can be concluded from the results that, the effect of steel fibers on the improvement of split tensile value was noted for higher volume fraction of steel fibers (1.5%) and lower fly ash substitution. fly ash (25%). The addition of accelerator plays a major role in the initiation of pozzolanic reaction and thereby improves the rate of hardening. Further, the effect of fine to coarse aggregate ratio on the strength attainment was noticed at lower 0.6 F/c compared to 0.8 F/c. Also, the strength gain properties of high volume fly ash concrete had shown a convincing improvement at later ages due to the addition of accelerator and this strength gain was on par that of reference concrete at 28 days curing. The results indicate that the addition of accelerator in low volume flaysh concrete systems has provided early strength gain in 7 days itself ; whereas, the strength gain in high volume fly ash concrete was noticed at 28 days curing.

Flexural strength

The flexural strength for various mixture proportions of concrete is given in Table 6. Among the various concrete mixes tested the steel fibers substituted concretes showed higher flexural strength value. In the case of 25% of fly ash substituted concretes with an accelerator dosage of 1.0%, the strength enhancement was found to be 7.23 MPa (as shown in Fig. 11(a). Whereas, for concretes with low volume fraction (0.5% and 1%) the strain hardening properties was found to be decreased, due to less fiber density available in the crack front. It can be noted that, compared to reference concrete the flexural strength of 25% fly ash substituted concrete with 1% accelerator exhibited a maximum flexural strength value of at 28 days (as seen in Fig. 11(b). This is due to early strength gain properties achieved with the addition of accelerator and showed a strength increase up to 20.50% compared to reference concrete. It can be also noted that, the concrete containing 50% fly ash with 1.5% steel fibers showed a maximum strength attainment up to 6.10 MPa at 28 days. Further, experimental trends showed that for higher F/c ratio of 0.8, the fly ash addition at 25% with 1.5% steel fibers showed a maximum flexural strength of 7.10 MPa at 28 days (as seen in Fig. 11 (c)). The increase in strength was up to 24.34% as compared to reference concrete. It is well demonstrated that higher replacement of fly ash at 50% with addition of 1.5% GSF and 1% accelerator the strength gain was increased up to 8.33% on par as compared to reference concrete the strength was 5.40 MPa at 28 days as shown in Fig. 11(d). The effective fiber bridging by steel fibers (shown in Fig. 11(e) occurred even after the failure had resulted in the concrete beam and showed a significant enhancement on the post peak characteristics. It is understood from the above test results that the pre peak strain hardening characteristics is dependent on the matrix densification and hardening properties of cementitious systems. However, the post peak strain softening characteristics of concrete is dependent on the crack bridging properties offered by the steel fibers. In general the steel fiber addition had a synergistic effect with the pozzolanic reaction of fly ash with cement and showed improved strength compared to controlled concrete mixtures.

Residual load/residual flexural strength ratio

The experimental results on the residual stress carrying capacity of various fiber concrete specimens were calculated from the loading, unloading and reloading cycles during the flexural testing. It can be observed from the results given in Table 6 that, in the case of reference concrete the specimens were broken immediately after peak load and hence the residual load was zero. Whereas in the case of fly ash concrete mixes containing steel fibers a sudden failure was not anticipated even after reaching the peak load. However, a sudden drop in load was observed due to crack bridging of steel fibers. In the case of increased fiber dosage up to 1.5%, it was observed that steel fibers were acting as stress transfer mechanism and as a result the load sustainability in the matrix was still realized Also, in the case of concretes with low volume fraction of steel fibers (at 0.5%) there resulted a sudden drop in load after reaching the maximum load and hence the residual flexural strength ratio was found to be lower due to reduced flexural load carrying capacity after the initiation of first visible crack on the concrete. Whereas, the residual load value was found to be higher (0.68) for low volume fly ash (25%) substituted concretes at higher steel fiber content of 1.5% V_f. The increased fiber dosage has resulted in significant stress carrying capacity even after the initiation of visible first crack without a sudden drop in the load carrying capacity. The effective crack bridging of fibers after failure shows significant residual load carrying capacity of fiber reinforced concrete specimens. It can be inferred from the test results after conducting the loading and unloading cycles of different concrete specimens that, the presence of steel fibers dominates the post crack response of concrete subjected to monotonic loading. In the case of reference concrete the residual load is completely absent since the initiation of first crack propagates unsteadily leading to complete failure. Whereas it is noted for fiber reinforced fly ash concrete specimens that the fibers transfer stress across the crack and exhibit the post cracking stress carrying capacity. Also, with the increase in steel fiber dosage the residual load value was found to be higher. This essentially implies the performance characteristics of steel fibers in the matrix with effective crack bridging phenomena and effective bond strength in the high strength matrix. The matrix strength in the case of fly ash substituted concrete was originally contributed due to the early pozzolanic reaction with the cement hydration products. This can be also seen in the case of low volume fly ash substituted concretes exhibited a good pozzolanic reaction leading to early strength gain compared to reference concretes at 7 days. Whereas, in the case of high volume fly ash concretes the strength gain was appreciable at 28 days without any significant reduction in the ultimate strength compared to reference concrete. It can be concluded that, for fiber reinforced fly ash concretes the pre peak behavior is controlled by the matrix densification as a result of pozzolanic reaction of fly ash and the post peak behavior is dominated by the presence of steel fibers with high fiber density as a result of high fiber volume fraction. However, the high strength matrix contributed for the effective fiber bonding with the matrix and resulted in synergistic interaction leading to improved flexural properties and residual load characteristics.

Rate of hardening measurements using ultrasonic pulse velocity test

The ultrasonic pulse velocity values for all fly ash based concrete with respect to different curing ages are given in the Table 7. The investigation test results showed that good hardening properties of the concrete were noticed for optimized fly ash substitutions at 25% replacement level at 28 days. The pulse velocity values in different concrete specimens tested were starting in the range of 4.10 km/sec to a maximum of 4.51 km/sec and the plotted values are shown in Fig. 12. The ultrasonic pulse velocity values were showing satisfactory performance levels of concrete tested at one day (after demolding) which exhibited a good rating as per Indian standard specifications and were useful to assess the quality of concretes prepared for different mixture proportions.

Young’s modulus of concrete (E)

The Young’s modulus of elasticity of various concrete mixes is presented in Table 8. Test results showed that a higher value of elastic modulus around 43.50 GPa was obtained for low volume flyash concrete containing 1.5% V_f of steel fibers and 1% accelerator which was higher than the reference concrete of which the elastic modulus value was around 37.26 GPa. It was also noted that in the case of 50% replacement of fly ash, with 1.5% steel fiber addition and 1% accelerator, the modulus of elasticity was 39.45 GPa at 28 days (as seen in Fig. 13(a) which was higher than reference concrete. A similar increase in the trend was also observed from Fig. 13(b) that, in the case of concrete containing F/c ratio of 0.8 exhibited an elastic modulus value of 42.97 GPa and 34.08 GPa for 25% and 50% fly ash substitution respectively. It can be inferred from the results that elastic modulus of concrete is a function of matrix densification and improved microstructural properties resulting in high strength and high stress-strain capacity. Matrix properties were improved with the flyash pozzolanic reaction leading to refinement of pore structure. Whereas, the stress transfer and redistribution in the matrix was increased with the steel fiber addition and this leads to higher elastic modulus of the composite material.

Dynamic elastic modulus of concrete (E_d)

The ultrasonic pulse velocity values calculated upon loading the concrete specimens were used to calculate the dynamic elastic modulus values (calculated using Eq. (1)) and were plotted graphically. It can be seen from the results that a good correlation (linear trend line) was existing between the measured ultrasonic pulse velocity value and calculated elastic modulus value. The linear increasing trend line represented in the Figs. 14 shows that the dynamic elastic modulus value is an indicator of the concrete strength and dictated by the pulse velocity values. However the initiation of micro cracks in concrete is controlled by steel fibers and results in higher stress carrying capacity. In addition to the presence of steel fibers a good positive correlation exists between the concrete mix constituents and rate of strength gain. The test results also indicates that when the concrete gains adequate strength earlier the improvement in the microstructural properties due to the adequate particle packing and addition of accelerators can lead to the delay in micro cracking upon loading. This eventually leads to faster travel of ultrasonic pulse in stressed concrete specimens. This demonstrates that the dynamic modulus value of concrete specimens under stressed condition can exhibit higher pulse velocity values in fiber concretes as well as carefully designed concretes mix ingredients for high early strength gain. It can be inferred that the delay in crack initiation and subsequent crack propagation showed pulse velocity value equivalent to that of uncracked concrete which was seen in all fiber concretes tested in this study. Hence, this test method was found to be useful for deriving the quantitative and qualitative assessment in fiber reinforced concrete specimens.

Conclusions

Based on the experimental investigation, the following conclusions are drawn within the limitations of the test results.

1) Compressive strength enhancement was reported for both low and high volume fly ash concrete which was found to be higher than reference concrete. The increase in strength for flyash concretes was appreciable with the proper selection of mix constituents, addition of accelerator and accelerated initial steam curing.

2) In the case of both low and high volume fly ash concretes, the addition of accelerator were found to be instrumental to improve the pozzolanic reaction. This resulted in earlier strength gain at 7 days for low volume flyash concretes and 28 days in the case of high volume fly ash concretes.

3) The improvement on the strength properties was higher in the case of concretes containing higher binder to total aggregate ratio (0.24) and lower fine to coarse aggregate ratio (0.8); whereas in the case of concretes with lower binder to total aggregate ratio (0.24) and higher fine to coarse aggregate ratio (0.8), a marginal reduction in strength properties was noticed possibly due to poor cementitious content and granular packing.

4) The rate of strength gain was appreciable in the case of accelerated fly ash concrete mixes at various curing ages of 7, 28 and 56 days due to increased pozzolanic reactivity of fly ash particles and subsequent rate of hardening .

5) The addition of glued steel fibers in fly ash concrete showed a reduction in workability; however the loss was compensated with the addition of superplastcizers.

6) The real benefits of steel fiber addition in fly ash concretes was noted at higher substitution of steel fibers at 1.5% V_f and demonstrated a satisfactory improvements on the compressive and flexural properties of concrete.

7) An increase in compressive strength up to 41.07% for maximum fiber dosage up to 1.5% V_f was noted for low volume fly ash (25%) addition and the strength enhancement was around 14.93% noted in the case of high volume fly ash (50%) addition. However the increase in strength was very conspicuous due to careful proportioning of ingredients, mixing of accelerators and fibers.

8) The effect of steam curing for 18 h in the case of all fly ash concretes were essential to improve the hydration properties and subsequently leading to dense microstructural formation.

9) Residual load capacity of fiber concretes were found to exhibit the flexural strength capacity of all fiber concrete mixes after the initiation of first crack and subsequent reloading till failure.

10) It was observed from the test results that the higher residual flexural strength ratio (0.68) was noticed in the case of low volume fly ash concrete with 1.5% V_f of steel fibers; however, the further increase in steel fiber dosage beyond 1.5% has to be restricted due to poor workability of concrete mixes leading to low compactability during placing.

11) Steel fiber additions in fly ash concrete showed good attainment of elastic modulus due to matrix strengthening effects. A maximum value of elastic modulus around 43.50 GPa (MSF3) was noticed for steel fiber reinforced fly ash concrete containing 25% fly ash and 1.5% V_f of steel fibers.

12) The dynamic elastic modulus test was found to be an important measure for assessing the concrete strength subjected to loading and a maximum value of around 26.75 GPa was noticed for high volume fly ash based concrete (MSF7). However, the dynamic elastic modulus values of various fly ash concrete mixes tested were in the range of 21.34 to 27.75 GPa.

13) It can be concluded that a maximum design grade of M40 can also be achieved in the case of high volume fly ash concrete which requires the presence of accelerating admixtures for improving the strength gain at 28 days. Also, the comprehensive test results conducted in this study showed that, a significant faster rate of hardening for early attainment of ultimate strength in fly ash based cementititous systems depends on the early initiation of pozzolanic reaction with the cement hydration products. This could be synergised when the fly ash concrete systems are designed with due caregiven to proper mix design, accelerated steam curing, optimal steel fiber addition and the presence of accelerator; all of which dictates the beneficial addition of high volume fly ash addition in concrete.

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