Effect of embedment length of untreated natural fibres on the bond behaviour in cement mortar

Abstract

The present investigation is focused on studying the effects of various matrices with 1:3, 1:4 and 1:5 mortars and fibre types of sisal and coir on the bond behavior at various ages of curing, i.e., 24 h, 3 d, 7 d and 28 d. The other parameters included in the investigation are water/cement (w/c) ratio, sand gradation and embedment length of fibres. In addition, the type of failure of sisal and coir fibres for different mixes of mortars at various curing ages is also reported. From the results, it is seen that the bond strength is improving with respect to age of curing in case of sisal fibres, but decreases in case of coir fibres. The failure of fibres due to fibre fracture is observed in sisal fibres and fibre pullout is observed in coir fibres. The other varying parameters such as mortar mixes, sand gradation, w/c ratio and embedded length also showed significant effect on bond behaviour of sisal and coir fibre with the cement mortar mixes.

Keywords

bond strength / cement mortar / natural fibre / pullout test / type of failure

Introduction

Over the past decades, fibre reinforced cement and concrete have progressed from laboratory trails to full scale commercial production. Simultaneously, considerable effort has been devoted to the development of theoretical models which simulate and predict the behaviour of FRC observed in practice. A fibre reinforced cement mortar is a composite with three distinct phases namely the fibre, the cement matrix and the fibre-matrix interface [¹,²]. The nature of bond in fibre reinforced composites is very complex and this includes physical and chemical adhesion between fibre and matrix, fibre to fibre interlock and friction generally influenced by confinement [³]. These fibre-matrix bond characteristics are required to analyze the transition zone or predict the composite mechanical performance [⁴], in the energy absorption during and after matrix cracking and in the resistance to deflection it imparts after cracking. The resistance of crack propagation by fibre and achievement of bond is mainly affected by varying matrix and fibre parameters [⁵]. After cracking of the matrix, the strength of the composite is mostly influenced by the pull-out behaviour of fibres. The bond strength and bond-slip relationship between the fibres and the matrix will influence the process of multiple cracking, deformation, strength and toughness of the composites [⁶]. To understand better, the mechanism of fibre reinforcing and to develop a model to predict the stress-strain relation of the composite, reliable pull-out load-slip relationship is needed. Many pull-out tests [⁷–¹¹] have been conducted to determine the bond strength in which the matrix is restrained in compression on one face and the protruding fibre is subjected to direct tension. But all these pull-out tests and the predicting models cannot be applied constantly to all types of fibre and matrix used. Fundamental information on the nature of bond between fibre and the matrix in a fibre reinforced composite material can be obtained using a single fibre pull-out test. Resistance to debonding and pullout process is principally a function of the fibre matrix interfacial bond shear strength and interfacial bond area, i.e., embedded fibre length times the fibre perimeter. Some authors worked on natural fibres for predicting pullout behaviour and critical research findings are presented here. The natural fibres namely sisal, coir, jute etc have been used so far in pullout studies. In case of sisal fibres, the fibre embedded length is shorter, fibre pullout out from the matrix and other hand longer fibre tends to break during pullout. Also the critical embedded length of sisal fibre has been considered around 30 mm based on the probability of local anchorage becomes high [¹²]. The bond behaviour of coir fibres are studied by considering various parameters Viz. embedded length, diameter, pre-treatment condition and mix design ratio. Single fibre pullout tests reveal that the bond strength and pullout energy increases with increase in embedded length but the highest value achieved with embedded lengths of 30mm. Also chemical pretreatment reduces the bond strength significantly [¹³]. The jute fibre was also investigated with asphalt matrix and the interfacial bond effects were reported. This suggests that, for understanding interface bond, the tensile rupture behaviour of composite was assessed for asphalt matrix with and without jute fibres. The jute fibre asphalt matrix shows higher interface bond than the plain asphalt [¹⁴]. In this paper, a fibre is cast into cement matrix and loaded under tension until fibre debonds or fractures. The pull-out test is used to investigate the effect of varying fibre and matrix properties.

Experimental program

Materials

The materials used for making mortar are cement, fine aggregate and water. The OPC-53 grade of cement confirming to IS 12269-1987 and fine aggregate of locally available river sand with free of impurities and passing through 4.75 mm sieve were used. Potable water available in PEC campus was used for mixing and curing purposes. The physical properties of materials are provided in Tables 1 and 2. The natural fibres such as sisal fibre (SF) and coir fibre (CF) were used in untreated and dry condition for performing pull out test in mortar specimens.

Mix proportion and test specimen

Sisal and coir fibres used in this investigation are locally available in untreated form. These fibres are used to prepare the test specimens for tensile strength test for gauge lengths of 30 mm, 40 mm, 50 mm, 60 mm, 70 mm and 80 mm respectively. The mortar mixes for pullout specimens were prepared by considering various parameters namely mortar mixes (1:3, 1:4, and 1:5), w/c ratio (0.4, 0.5, and 0.6), sand gradation (zone I, zone II, zone III, zone IV), age of curing (24 h, 3 d, 7 d and 28 d), fibre type (sisal and coir) and fibre embedded length (EL) (25, 40 and 50 mm). The cube specimen of size 50 ´ 50 ´ 50 mm with 80 mm length of fibre extends out of specimen was prepared for pullout test and is shown in Fig. 1. Firstly, the mixes were prepared for investigating the effect of various mixes of 1:3, 1:4 and 1:5 mortars on bond strength at various curing ages by varying embedded length and fibre type at constant w/c ratio of 0.5. Secondly, the mix prepared only for 1:3 mortars with fibre embedded length of 50 mm for studying the effect of various zones of sand gradation and various water-cement ratios.

Test program

Tensile test

The tensile test was conducted by using tensile tester of 5 kN capacity for sisal and coir fibre with 20 numbers of prepared specimens for each gauge lengths of 30 mm, 40 mm, 50 mm, 60 mm, 70 mm and 80 mm respectively. The specimen is locked between rollers at both top and bottom positions and load is applied till the fibre fails. The fibre diameter and failure load is noted for calculation of tensile strength of the specimen. Tensile strength of the fibre is calculated using Eq. (1).

(1){\sigma }_{\mathrm{f}}=P_{\max }/A

where, σ_f is the tensile strength in MPa; P_max is the failure load or maximum load taken by the fibre before failure in N; A is the cross sectional area of fibre in mm².

Pullout test

The pullout test was conducted by using tensile tester of 5 kN capacity and the graphical representation of test setup is shown in Fig. 1. To perform the pullout test, the specimen is placed on the mould for holding it. The mould has a gap of 1 cm to allow the fibres to extend out. The fibre is then tightened on to the rollers at the bottom of the tester before any application of load on it. The tensile tester is attached to computer for acquisition of test data. Finally set zero and start applying the load, which to be continued till the fibre failure either by fracture or pullout. Bond strength is calculated using Eq. (2).

(2)t_{\mathrm{sb}}=P_{\max }/\pi d{l}_{\mathrm{e}}

where, τ_sb is the bond strength in MPa; P_max is the failure load or maximum load taken by the fibre before failure in N; d is the fibre diameter in mm; l_e is the fibre embedded length.

Results and discussions

Direct tensile strength of natural fibres

Sisal fibre

The fibre diameter, tensile strength and percentage of elongation of fibre for various gauge lengths are provided in the Table 3. From this, it is seen that the tensile strength and percentage of elongation decreases as gauge length increase from 30 mm to 80 mm. This may be due the fact that the axial stiffness of fibre reduces as the length of fibre increases and this is evident from the relation that the Stiffness is directly proportional to cross-sectional area and modulus of elasticity as well as indirectly proportional to gauge length. The percentage decrease in tensile strength and percentage of elongation with respect to gauge length from 30 mm to 80 mm are noticed to be about 70.34% and 69.3% respectively.

Coir fibre

The fibre diameter, tensile strength and percentage of elongation of fibre for various gauge lengths are provided in the Table 4. From this, it is seen that the tensile strength and percentage of elongation increases as gauge length increase from 30 mm to 80 mm, showing opposite trend with that of sisal fibre. The increase of tensile strength and percentage of elongation of fibre with gauge length of 80 mm are noticed to be about 6 times and 3 times with respect to the gauge length of 30 mm.

Also, it is observed that the tensile strength of sisal fibre is higher than coir fibre but percentage of elongation is higher for coir fibre.

Effect of various mortar mixes and embedded length on bond strength and type of fibre failure

Sisal fibre

From the Fig. 2, it is noticed that the effect of various mortar mixes such as 1:3, 1:4 and 1:5 on the bond strength is significant in case of sisal fibre. This shows that the decreases of bond strength are observed as mix becomes leaner. The age of curing shows increase in bond strength for 1:3 and 1:5 mortar mixes, whereas 1:4 mortar mixes shows small and uneven variations with respect to various EL. On the other hand, the EL has very little effect on the bond behaviour of various mortar mixes. It is seen in general, at constant mortar mix, the lower EL of 25 mm shows higher bond strength than that of higher EL of 50 mm. In all mortar mixes, the EL of 25 mm and 40 mm doesn’t show much difference in bond strength. Therefore, the percentage improvement in bond strength for EL of 25 mm ranges between 2% to 23% when compared with EL of 50 mm. The maximum decrease in bond strength is noticed to be about 52%, when mix changes from richer (1:3 mortar) to leaner (1:5 mortar). The various failure type of sisal fibre with respect of various mixes, EL and various ages of curing is given in Table 5. From the table it is seen that the fibre pullout and fibre fracture are the two failures occurred commonly. In 1:3 mortar mixes, the fibre fracture is more predominant in almost all embedded lengths of sisal fibre. The same aspect of failure type is noticed with 1:4 mortar mix but the both fibre pullout and fibre fracture are noticed in 1:5 mixes. In all the three mixes, the fibre fracture is observed in case of EL of 50 mm. In addition, the fibre fracture is noticed in all mixes cured at 28 d with all EL.

Coir fibre

The effect of various mortar mixes and embedded lengths of coir fibre on the bond strength for different curing ages are shown in Fig. 3. From this, it is seen that as the age of mortar increases bond strength is getting decreased in all mortar mixes, showing opposite trend as that of sisal fibre embedded mortar specimens. This can be attributed to the fact that the unprocessed coir used may have coir pith particles on the fibre surface, leading to the fibre degradation as the age of curing increases. Also, the increase of bond strength with respect to mortar mixes is noticed for richer mixes at constant EL. However, the bond strength for various EL is not showing the linear trend as mixes become leaner or richer. The effect of EL on bond behavior shows that as decrease in EL increases the strength. The maximum improvement of bond strength at 25 mm of EL is 52% with that bond strength at EL of 50 mm. The maximum percentage reduction of bond strength for mixes become leaner from 1:3 to 1:5 with EL of 25 mm, 40 mm and 50 mm are 30%, 39% and 48% respectively. Moreover, the decrease in bond strength with respect of age of curing from 24 h to 28 d is noticed to be about 50%. In the Table 6, the type of failure of coir fibre for various mortar mixes and various EL at different curing ages are presented. It shows that the fibre fracture is predominant in 1:3 mortar but the mix become leaner the fibre pullout appears to be more predominant. In 1:4 mortar, fibre pullout is observed for 25 mm and 40 mm of EL, while in 50 mm of EL, fibre fracture is noticed. The fibre pullout is mostly observed for 1:5 mortar even at the EL of 50 mm.

Effect of sand gradation on the bond strength

The evaluation of effect of sand gradation on the bonds characteristics of sisal and coir fibre has been made by using four zones of sand (zone I, zone II, zone III and zone IV), which were prepared as per IS 383-1970. These four grading zones become progressively finer from Grading Zone I to Grading Zone IV. The obtained gradation curve for four different zones is shown in Fig. 4. From Fig. 5, it is seen that the increase of zone increases the bond strength in both sisal and coir fibre, but the increase is not felt much significant from zone II to zone IV. The maximum increase in bond strength is 13% and 45% for sisal and coir fibres respectively, when changing from zone I to zone IV. Also it is noticed that the increase in age of curing increases the bond strength in sisal fibres and decreased trend is observed in coir fibres irrespective of zones of sand. The increase in bond strength ranging between 40% to 50% for sisal fibres and decreases by about 50% for coir fibres when the age of curing varies from 24 h to 28 d. Moreover, the sisal fibre embedded mortar shows higher bond strength than that of coir fibre embedded mortar.

Effect of w/c ratio on the bond strength

Figure 6 shows the effect of w/c ratio on bond strength of sisal and coir fibre of 50 mm embedded length in 1:3 mix. From this, it is seen that the increase in w/c ratio from 0.3 to 0.4 increases the bond strength both for sisal and coir fibres, but decrease of bond strength is noticed beyond 0.4 water/ratio. The bond strength below and above 0.4 w/c ratio is reduced and this proves that the normal consistency of mortar attained only at w/c ratio of 0.4. The increase in bond strength from 0.3 to 0.4 w/c ratio ranges from 58% to 68% for sisal fibres and 61% to 81% for coir fibres at various ages of curing. Also the percentage decrease of bond strength is computed for increase of w/c ratio from 0.4 to 0.6 and this shows that the decrease of bond strength ranges between 28% to 54% for sisal fibres and 31% to 36% for coir fibres respectively.

Conclusions

For sisal fibres, as the gauge length varies from 30 mm to 80 mm, both tensile and percentage elongation decreases by 70.34% and 69.3% respectively.

For coir fibres, as the gauge length varies from 30 mm to 80 mm, both tensile and percentage elongation increases by 6 times and 3 times respectively.

The bond strength increases with increase of age of curing for sisal fibres with 1:3 and 1:5 mortar mixes and decreases as age increases for coir fibres for all mortar mixes of 1:3, 1:4 and 1:5. The EL of 25 mm shows higher bond strength than other embedded lengths of 40 mm and 50 mm irrespective of age of curing for both sisal and coir fibres. Moreover, the bond strength appears to decrease when mixes becoming richer to leaner, i.e., from 1:3 mix to 1:5 mix.

The bond strengths obtained at 28 d for sisal and coir fibres is maximum at the embedded length of 25 mm and it is given below.

In all the mortar mixes, the fibre fracture is observed to be more common at 28 d cured cement mortar in both sisal and coir fibres. This may be due to the fact that the interfacial bond strength is stronger than the tensile strength of the embedded fibre. For the 1:5 cement mortar mix with coir fibre showed the pullout type of failure even at 28 days curing.

The bond strength with respect of gradation of sand shows that increase of zones from zone I to zone IV improves the bond strength, but there is no significant improvement is noticed beyond zone II for both sisal and coir fibres. Moreover, in all pullout tests the sisal fibre has higher bond strength than that of coir fibre.

The w/c ratio also produced noteworthy effects on the bond behavior of sisal and coir fibre embedded mortar for all curing ages. The improved and highest bond strength is achieved at the w/c ratio of 0.4, which matches with the w/c ratio of the normal consistency.

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