Flexural response of vertically cast steel fiber reinforced concrete members

doi:10.21203/rs.3.rs-4691894/v1

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Flexural response of vertically cast steel fiber reinforced concrete members

https://doi.org/10.21203/rs.3.rs-4691894/v1

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The presence of the short steel fibers dispersed uniformly in concrete improves its inherent flaws and makes it a versatile structural material. The improvement in material properties depends upon the alignment of the fibers in concrete members caused during their casting; those aligned along the principle tension trajectory of the beams are most efficient among all the fibers present therein and other fibers control their shear response. An attempt is made to quantify the effect of the mould position (vertical or horizontal) during the casting process on the flexural response of steel fiber reinforced concrete (SFRC) members. An analytical model is also presented to predict the response of the members in case some of the design constraints require their casting in the vertically placed moulds, such as for walls, columns, etc. instead of the routine horizontal position. A significant reduction in flexural strength and toughness of the SFRC members is observed if these were cast vertically instead of the horizontal position; additionally, they exhibit wider flexural cracks. Their shear capacity remained almost unchanged.

Analytical model

Beam

Concrete structures

Design methods & aids

Fibers

Moulds

Precast elements

Shear

Concrete in its modified form produced by adding short steel fibers at the time of mixing, known as steel fiber reinforced concrete (SFRC), has established itself as a versatile structural material in recent years. It has found numerous applications in the construction: initially in the construction of grade-supported elements, such as concrete pavements and now-a-days, in the elevated structural applications, such as slabs / plates, beams, etc. Mixing of short steel fibers in concrete during its production significantly improves its post-peak mechanical properties, such as toughness, residual tensile strength, etc. [Kang et al. (2011); Gettu et al. (2005); Dupont and Vandewalle (2005)]. The resultant fine crack widths in the SFRC members make them more corrosion resistant and durable in comparison to the conventionally reinforced concrete members. The orientation of the fibers in the hardened concrete plays an important role in improving the post-cracking response exhibited by SFRC [Žirgulis et al. (2016); Şanal and Ozyurt (2013)]. The size, shape and the placement of moulds with respect to the concrete pour direction are the major parameters that control the orientation of fibers in the concrete; the fibers aligned along the tension trajectory in the flexural members are more effective than the fibers aligned at any other angle [Singh (2016a, b), Soroushian and Lee (1990)]. Flexural members cast in the horizontally placed moulds results in the fiber alignment, mostly, along the member-axis; thereby, giving a better post-peak response than the members cast at some other angles [Singh (2017); Oldřich et al. (2014)]. However, most of the available test data in published literature pertain to the SFRC flexural members cast with moulds in horizontal position only. As such, all the subsequent design codes / guidelines for using SFRC in various structural applications have inherent limitation on its use for achieving the target performance levels. Use of SFRC members cast in position other than the horizontal needs further investigation to quantify the effect of orientation of the mould on response of the members. Some research work [Trabucchi et al. (2021), Mudadu et al. (2018)] in the recent past have established the influence of fiber orientation on the post-cracking characteristics of SFRC, but stops short of quantifying the same in some expression/ analytical form. Most of the publications have reported a reduction of around 25 to 30% in flexural load carrying capacity of SFRC members when the fiber orientation was changed from zero to 90^o at the time of their casting. As such, the only option for the design engineers and precast industry is to resort to the experimental testing of SFRC members if the field conditions warrant their casting in mould placement other than the horizontal.

Considering the potential benefits of using SFRC over the conventional concrete, its use in grade- supported and elevated structural applications will grow in the coming years. In all probability, its use as precast members will also witness growth as the members (both reinforced and pre-stressed) cast using steel fibers are in a better position to bear the unintentional stresses during their shifting from the casting yards/ factory and the subsequent installation on construction sites in comparison to the conventionally reinforced concrete members. This paper presents a set of the results from an experimental study conducted to see the feasibility of using SFRC members cast in the vertically placed moulds and quantify the effect of this mould orientation on the strength properties. An analytical equation is also presented to predict the response of members made by pouring concrete into vertically placed moulds from their top end. It will supplement the other material models available for SFRC cast traditionally in the horizontal position.

To validate the hypothesis that the response of the beam specimens cast using SFRC will change depending upon the orientation of the moulds used in their casting, a set of two different mould positions were considered in the experimental study: horizontal and vertical. Figure 1 depicts a typical set of moulds (of size: 150 x 150 x 750 mm) used in the investigations. Each set comprised of three specimens and three such sets were cast to investigate the role of mould position on the flexural capacity of SFRC beams.

In each set, the beam specimens were cast using the moulds placed horizontally as well as in a vertical position. First, the moulds were placed horizontally (Fig. 1a) and the concrete was poured in a direction normal to the mould-axis (representing the concreting operation during a routine beam casting) and then, the specially made moulds of identical dimensions were placed vertically (Fig. 1b) and the concrete was poured along the mould-axis, just similar to the routine column casting operations. The specimens at an age of 28 days were loaded to break them into two segments under the 3-point loading conditions in the laboratory. The main aim of this first phase of the investigations was to observe the orientation of the fibers in the fractured beam specimens casts in two different mould positions and, subsequently, to modify the analytical model proposed by the author [Singh (2016 a)] so as to help the analyst predict the failure load of SFRC beam specimens cast in the vertically placed moulds. The second phase of the study involved testing of another set of beam specimens designed using the proposed equation for a validation purpose and observe their load-displacement response and the subsequent failure pattern during the loading process.

Materials

43 grade Ordinary Portland cement conforming to IS 8112 (2003) and Fly ash (specific gravity: 2.2) conforming to IS 3812 (Part I) were used as binding materials in the concrete. The river sand (specific gravity: 2.61, zone-II) and coarse aggregates (specific gravity 2.63, FM-7.71) conforming to IS 383 (2016) was used as aggregates in the concrete mix. Maximum size of coarse aggregates was limited to 12.5 mm. Potable water was used to produce the concrete.

Two sets of hooked-end steel fibers, 30 mm and 60 mm long with an aspect ratio of 80 each, were used in the production of SFRC. The tensile strength of the steel fibers and their modulus of elasticity were found as 1035 MPa and 205 GPa, respectively.

Mix Proportions and Test Specimen Preparation

The cement concrete of grade M20 was designed as per the guidelines of IS 10262 (2009); Table 1 indicates typical mix proportions adopted in the concrete production. In the concrete mixture, no decrease in the weight of the aggregates was carried out because of very small presence of the steel fibers in the mix in comparison to the volume of aggregates. Standard cubes of size 150 mm and the prisms (750 mm long and 150 mm square cross section) were prepared to investigate the desired parameters of the study. The actual material strengths obtained from the standard tests were used in the calculations to determine the specimen load carrying capacity; 28-day average concrete compressive strength was found to be 24.87 MPa.

Table 1

Concrete mix proportions adopted in the study
Mix	Cement, kg/m³	Fly Ash, kg/m³	Fine Aggregate, kg/m³	Coarse Aggregate, kg/m³	Super plasticizer 'Master Glenium SKY 8630', kg/m³	Water, kg/m³
M20	340	185	1075	485	4.25	225

Revolving drum-type concrete mixer (0.06 m³ capacity) was used to prepare various batches of the concrete in the laboratory. The mixing procedure was carried out in several steps. Foremost, the sand and coarse aggregates were mixed together with half of the total water content to achieve a saturated surface-dry condition. Then, the cement and the remaining water along with admixture were added to the mixture and it was mixed thoroughly. The fibers were gradually sprinkled into the mix to get a homogeneous mix of good workability. Table 2 indicates the fresh properties of SFRC (produced using the mix proportions shown in Table 1) for two different values of the fiber volume fractions (i.e. 0.5% and 1%) adopted in the study; the fiber aspect ratio of 80 was kept constant.

Table 2

Properties of the concrete in the fresh state, for a fiber aspect ratio of 80
Volume fraction, V_f	Slump flow diameter, mm		V-funnel Flow time, Sec
Volume fraction, V_f	Fiber length: 30 mm	Fiber length: 60 mm	V-funnel Flow time, Sec
0.5%	654	634	8
1.0%	629	600	11

All test specimens, comprising three sets of beams by placing the moulds horizontally and another in the vertical position, were cast on a level ground with the designed concrete mix. The concrete was poured as described earlier in all the moulds and leveled to the uniform desired member thickness by a trowel and leveling bar. Figure 2 shows the mould positions with the freshly poured concrete. After casting, the test specimens were left for 28 days water curing.

Test Procedure

After 28 days, the first set of specimens was loaded under the standard 3-point loading conditions to break them into two parts. After the testing, the beam specimens were observed closely to count the number of the fibers protruding out of the fractured surface and measure their orientation with reference to the beam-axis. The test data so obtained was used to estimate the changes in the fiber orientation factor caused by the two mould positions considered in the study and see what further change is required in the analytical model [Singh, H. (2016 a)] to use it in predicting the load carrying capacity of SFRC beams cast in vertically placed moulds.

The accuracy of the proposed analytical model was examined by testing another set of similarly prepared test beams under the standard 3-point loading conditions; the beams were designed using the proposed equation. A center-to-center distance of 700 mm was used during the testing; thereby, giving a shear-span to depth ratio of 2.33. All the beam specimens were tested as per the design guidelines [RELIM (2003)].

A close examination of the fractured surfaces of the specimens revealed a significant change in the orientation angles of the fibers protruding out in the horizontally and the vertically cast beams. Figure 3 depicts an average percentage value of the fibers visible from the fractured surface against their observed angle of orientation measured from the beam-axis.

In case of the horizontally cast beams (H), most of the fibers are observed to be aligned along the beam-axis with an orientation angle ranging from 0^o to 30^o. This percentage goes on reducing with the higher angles of the fiber orientation; only 5% of the fibers are found to have an orientation angle of 80^o or higher. Whereas in case of the vertically cast beams (V), most of the fibers are found to be aligned at an orientation angle of 40^o to 60^o with a very small numbers orienting along the beam-axis. Only 13% of the fibers crossing the fractured beam surface were observed to be oriented along the beam-axis in the vertically cast beams against a value of 43% observed in case of the horizontally cast beams. This alteration in the fiber orientation caused by the mould position is expected to change the member post-peak strength parameters. The residual strength of SFRC depends only on the quantum of fibers that bridge the cracks being developed on the beam tension face. Their contribution towards the post-peak material response is expressed in the form of a parameter called as 'fiber-index' (β). This single parameter is reported to control the value of the crack-width that would develop on the tension face of the member and also, its residual tensile strength that it will exhibit in the post-peak range of load-displacement curves [Singh (2017)].

The residual tension/ flexural strength of SFRC specimen is a function of the three major constants, namely the constant γ_o that denotes the effect of fiber orientation with respect to the member-axis; the constant γ_d denoting their dispersion along the length and the width of flexural member caused during the concrete compaction and placement operations; and the length factor γ_l takes into account the non-linear distribution of the bond stress (τ_b) along the fiber length. The values of these three constants are reported as 0.8, 0.5 and 0.5, respectively, for the horizontally cast SFRC beams [Singh (2016 a)]. Accordingly, the final expression of the residual tensile strength (σ_th) of SFRC produced by using plain steel fibers in the pull-out failure mode is reproduced in Eq. 1. It must be multiplied by a suitable fiber-shape factor depending upon the fiber types, e.g. two for the hooked-end fibers and three for the wavy shaped fibers.

$$\:{\sigma\:}_{th}=2\:{\gamma\:}_{o\:}{\gamma\:}_{d}{\gamma\:}_{l}\left(0.75\sqrt{{f}_{c}}\:{V}_{f}\frac{l}{d}\right)=0.3\:{V}_{f}\frac{l}{d}\sqrt{{f}_{c}}$$

The value of the fiber orientation factor, γ_o in case of the vertically cast beams can be determined from the fiber distribution data shown in Fig. 3; it is obtained as 0.52. The values of other two factors (γ_d, and γ_l) will remain the same (= 0.5 each) as the fibers in the concrete will have equal probability to align themselves along the two dimensions of the cross-section during the pouring.

Equation 2 gives the modified value of the residual tensile strength (σ_tv) of SFRC for the vertically cast specimens by using the plain steel fibers. It should be multiplied by a suitable fiber-shape factor depending upon the fiber type used in the concrete production, i.e. two for hooked-end fibers and three for the wavy shaped fibers.

$$\:{\sigma\:}_{tv}=2\:{\gamma\:}_{o\:}{\gamma\:}_{d}{\gamma\:}_{l}\left(0.75\sqrt{{f}_{c}}\:{V}_{f}\frac{l}{d}\right)=0.2\:{V}_{f}\frac{l}{d}\sqrt{{f}_{c}}$$

Equations 1 and 2 show that the tensile strength exhibited by the vertically cast beams (V) reduces by a factor of 2/3 in comparison to the horizontally cast beams (H) having an identical size, fiber aspect ratio (l/d), fiber volume fraction (V_f) and concrete compression strength (f_c). The change in the fiber orientation caused by the wall effect in the moulds seems to be a major influential factor that leads to the reduction in the strength exhibited by the SFRC specimens in the two casting positions. The change in the material residual tension strength will also lead to modification of the section flexural capacity of the members; it can be calculated from Eq. 3 [reproduced from Singh (2016a)] for the rectangular beams reinforced using steel fibers only.

$$\:\frac{{M}_{u}}{{f}_{ck}\text{B}{\text{D}}^{2}}=0.24{\left(\frac{{h}_{1}}{\text{D}}\right)}^{2}+0.5\beta\:{\left(\frac{{h}_{2}}{\text{D}}\right)}^{2}$$

In Eq. 3, the parameters h₁/D and h₂/D denotes the position of the section neutral-axis, which can be determined from the following expressions:

$$\:\frac{{h}_{1}}{\text{D}}=\left(\frac{2.38{\beta\:}}{1+2.38{\beta\:}}\right)\:\text{A}\text{n}\text{d}\:\frac{{h}_{2}}{\text{D}}=\left(1-\frac{{h}_{1}}{D}\right)$$

Fiber index (β) in above two expressions denotes the ratio of the material residual tensile strength (σ_t) and its compressive strength (f_c) wherein; σ_t can be computed using equations 1 or 2 depending upon the mould position taken during the pouring.

The equations (1 to 3) show that the change in the fiber orientation caused by the mould position (H or V) changes the value of fiber-index (β), which directly influences the member flexural and shear capacity. The shear capacity of the beam section can be computed using the analytical model proposed by the author [Singh (2020)] to ensure that the designed beam specimens should not fail prematurely in shear. Table 3 illustrates the affect of the mould placement on different strength parameters of the test specimens, such as their flexural capacity (M_u), the value of the mid-span ultimate load (P_uth) and safe shear strength calculated using the proposed equations 1 to 3.

Table 3

Strength parameters for two different casting orientations of SFRC beams (fiber aspect ratio = 80)
Orientation of mould while casting	Volume fraction, V_f	Fiber index, β	M_u (kNm)	P_uth (kN)	Safe shear stress, MPa
Horizontal (H)	0.5	0.048	3.47	19.86	2.15
Horizontal (H)	1	0.096	6.07	34.67	2.37
Vertical (V)	0.5	0.032	2.43	13.90	2.09
Vertical (V)	1	0.064	4.42	25.26	2.23

Table 3 clearly established the changes brought in by different mould positions, for two values of the fiber dosage (i.e., 0.5% and 1%) used in the concrete specimens at the time of its production; the section flexural capacity reduces irrespective of the fiber dosage (V_f) used in the concrete; a similar trend is noticeable for their shear capacity though it is observed to be marginal. The flexural capacity and / or ultimate load of the beam sections reduce by ~ 30% when the position of the moulds during the casting process was changed from a horizontal to the vertical one. Interestingly, this change in the position of the moulds did not cause any significant change in the section shear capacity. The alignment and distribution of steel fibers along the span of the member and its depth are responsible for this reduction in the flexural capacity.

Figure 3 indicates the orientation of the fibers that become responsible for reduction in the flexural capacity as the fibers aligned along the beam-axis on its tensile face contribute maximum towards the flexural capacity; around 40% of all the fibers crossing the fractured surface are found to be aligned along the beam-axis, it reduces to 30% for an orientation angle of 20^o and it further reduces to 20% for an angle of 40^o for the beam specimens cast in the horizontal position; whereas, this percentage is only around 15% for the similarly cast beam specimens in the vertical moulds. On the other hand, the fibers aligned at angles of 40^o to 50^o contribute most towards the beam shear capacity and the average numbers of the fibers in this range are almost observed to be the same whether the beams were cast in the horizontally placed moulds or in the vertical moulds; thereby leading to an identical value of shear capacity being exhibited by them and as tabulated in Table 3.

In order to validate the efficacy of the proposed model (Eq. 2) and to examine the response of the beams cast in the horizontal and vertically placed moulds, another set of test specimens were cast using the concrete mix, given in Table 1. These were cast and cured using the same procedure described in the previous section. The test specimens were tested under a standard 3-point loading conditions on a strain-controlled loading machine. The shear-span to depth ratio of 2.33 adopted in the testing ensured that the failure of the beam specimens should occur in the opted bending mode only. The data presented in Table 3 also confirmed the design assumption that the beam specimens will not prematurely fail in shear. The actual shear stress at failure was found to be very small in comparison to the safe shear strength, tabulated in Table 3. The value of the load corresponding to the CMOD value of 0.5 mm and 3.5 mm were noted for all test specimens; it is tabulated in Tables 4 and 5 for the beam specimens cast in the horizontally placed moulds as well as those cast using the vertically placed moulds, respectively. The value of the theoretical load values (P_uth) computed for each case, using equations 1 to 3, is also shown in the Tables 4 and 5 for a comparison purpose.

Around 25 to 30% reduction in the experimental load carrying capacity (P_u) was observed in all the cases considered in the study, when casting position of the moulds was changed from a horizontal to the vertical. The analysis of the results shows that the average ratio of the predicted ultimate load (P_uth) to their experimental load value is found to be 0.91 with a standard deviation of 0.08 and a variance of 0.008 for a set of vertically cast beam specimens, whereas the same ratios are found to be 0.90 with a standard deviation of 0.04 and a variance of 0.002 for the beam specimens cast horizontally. The results predicted from the proposed analytical Eq. 2 is safe to use as the predicted values are found to be in good agreement with the values obtained from the independent experimental study with a very low value of the variance; thereby validating the hypothesis.

Irrespective of the fiber parameters (fiber aspect ratio, its dosage and length), the test data shown in Tables 4 and 5 also revealed that the SFRC beams cast in the vertically placed moulds would develop wider flexural cracks on their tension face in comparison to the beams cast horizontally for an identical value of the applied load. The reduction in the value of fiber index (β) due to the change in the orientation of the moulds during the casting process is primarily responsible for the wider cracks that developed on tensile face of various test specimens. This trend is found to be in conformity with the earlier reported findings [Singh (2017)]. An attempt should therefore be made to select a suitable value of the fiber parameters (concrete mix proportions; fiber shape, its dosage and aspect ratio) that gives the highest possible value of the fiber-index (β) as well as it meet the workability requirements of concrete mix.

Table 4

Load corresponding to different values of CMOD for SFRC beams cast horizontally
Fiber length	Volume fraction, V_f	P_uex (kN) at CMOD of		Shear stress, MPa	P_uth (theoretical)	$\:\frac{{P}_{uth}}{{P}_{uex\:\left(0.5mm\right)}}$
Fiber length	Volume fraction, V_f	0.5 mm	3.5 mm	Shear stress, MPa	P_uth (theoretical)	$\:\frac{{P}_{uth}}{{P}_{uex\:\left(0.5mm\right)}}$
30 mm	0.5	22.45	6.15	0.49	19.86	0.88
30 mm	1	36.30	9.03	0.80	34.67	0.96
60 mm	0.5	23.15	6.75	0.51	19.86	0.86
60 mm	1	38.60	9.45	0.85	34.67	0.90

Table 5

Load corresponding to different values of CMOD for SFRC beams cast vertically
Fiber length	Volume fraction, V_f	P_uex (kN) at CMOD of		Shear stress, MPa	P_uth (theoretical)	$\:\frac{{P}_{uth}}{{P}_{uex\:\left(0.5mm\right)}}$
Fiber length	Volume fraction, V_f	0.5 mm	3.5 mm	Shear stress, MPa	P_uth (theoretical)	$\:\frac{{P}_{uth}}{{P}_{uex\:\left(0.5mm\right)}}$
30 mm	0.5	13.53	2.55	0.30	13.90	1.02
30 mm	1	27.55	5.55	0.61	25.26	0.92
60 mm	0.5	17.30	2.85	0.38	13.90	0.80
60 mm	1	28.13	5.25	0.62	25.26	0.90

Figures 4 and 5 depict the average values of the test data in the form of a typical load-displacement response of the beam specimens cast using different mould positions, both for a horizontal position (Fig. 4a and Fig. 5a) and the vertical position (Fig. 4b and Fig. 5b). Almost similar trends were observed for the two fiber length and their dosages considered in the study. These figures also depict a significant reduction in the peak load carried by the test specimens as well as the material toughness when the casting position of the moulds was changed from a horizontal to the vertical.

It happened irrespective of the fiber length (30 mm & 60 mm) and its dosage (0.5% & 1%) used in the production of the concrete. However, the use of long fibers (60 mm) in the concrete is found to be more effective in maintaining the residual strength being exhibited by the SFRC test specimens in comparison to the concrete produced by using 30 mm long fibers. The reduced material toughness is a result of the less observed value of the peak load (less by a multiple of ~ 0.7) as well as the post-peak response of the material which undergoes a sudden drop in the load carrying capacity of the member post peak, followed by a descending response curve. This descending curve is observed to be steeped in the case of the test specimens cast using concrete produced by mixing shorter fibers (30 mm) in the mixture in comparison to the long fibers (60 mm). It seems that the 30 mm long fibers undergo an earlier pullout in comparison to the 60 mm long fibers. This happened due to the smaller anchorage of 30 mm fibers that led to a rapid loss of strength in the post-peak range, both for the horizontal as well as the vertical cast positions. The important takeaway of the study is to avoid casting of the SFRC flexural members in the vertically positioned moulds; else their design should be modified to take care of the reduction in the strength expected due to the change in the position of moulds during the casting process. The flexural members prepared using SFRC are observed to be more efficient if they are cast in the horizontal position, thereby allowing them to exhibit their maximum possible flexural and shear capacity along with the formation of relatively finer crack widths. Therefore, if the space permits the production of SFRC flexural members over the horizontal beds should be preferred in comparison to the casting in vertically positioned moulds.

The following salient conclusions may be drawn from the present study:

The orientation of SFRC members while they are being cast plays an important role in controlling their flexural strength and toughness.
The SFRC members cast in the vertically placed moulds undergo a reduction of around 30% in their flexural capacity in comparison to its counterparts cast horizontally.
Similarly, these specimens develop a set of relatively wider cracks on their tension face for an identical value of the applied load.
Shear Capacity of SFRC beams remain almost the same irrespective of their position during the casting process, whether vertical or horizontal.
Flexural as well as the shear capacity of the beam section cast using SFRC improves with the use of longer fibers in comparison to the shorter ones (having an identical fiber aspect ratio).
These two strength parameters also improve with the use of a higher fiber dosage (V_f) in the concrete mix.

Author Contribution

HVS, the author of this paper investigated the hypothesis, analysed the results and written the paper in all aspects as is being submiited for peer review and possible publication.

Acknowledgement

The author would like to express his heartiest appreciation for all those at Guru Nanak Dev Engineering College, Ludhiana who rendered help and support in this research project.

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No competing interests reported.

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Flexural response of vertically cast steel fiber reinforced concrete members

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Abstract

Figures

Introduction

Experimental validation of the hypothesis

Materials

Mix Proportions and Test Specimen Preparation

Test Procedure

Analysis of the results and analytical model

Validation of the proposed model

Conclusions

Declarations

Author Contribution

Acknowledgement

References

Additional Declarations

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