Effects of Spalling Area and Interfacial Shear Stress on Fiber Pullout Behavior in Ultra-High Performance Concrete

doi:10.16552/j.cnki.issn1001-1625.2025.1236

Abstract

Abstract:

To investigate the influence of the spalling zone and interfacial shear stress on the pullout behavior of steel fibers in ultra-high performance concrete （UHPC）， the pullout behavior of steel fibers at different curing ages （7， 14， 28 d） and embedment angles （0°， 30°， 45°， 60°） was studied. The degree of matrix spalling was evaluated， and the pullout behavior was analyzed using a micromechanical model. The results show that the pullout load and average bond strength are maximized at 45° for different embedment angles， while the maximum pullout energy occurrs at 60°. The spalling area of the matrix is less affected by the curing age but significantly influenced by the embedment angle， with the maximum spalling area occurring at 60°. An equation is established to describe the relationship between the spalling area and the embedment angle. The degradation of interfacial shear stress determines the trend of the descending branch （debonding stage）， with the shape changing from concave to convex and the curvature of the convex shape gradually increasing with different curing ages and embedment angles. Extending the value of shape coefficient η from 0.10 to -0.05 and introducing an interfacial enhancement factor b with a range of values from -8 to -2 can effectively evaluate the change of the interfacial shear stress， the results of the numerical analysis are in good agreement with experimental results.

Key words: ultra-high performance concrete （UHPC）, embedment angle, pullout behavior, matrix spalling, interfacial shear stress, micromechanical model

CLC Number:

TU528

XIANG Gucong, NIU Yanfei, LIN Xihua, YUAN Zong, GUO Yanlong, YUAN Lijun. Effects of Spalling Area and Interfacial Shear Stress on Fiber Pullout Behavior in Ultra-High Performance Concrete[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(7): 2275-2289.

Figures/Tables 25

Fig.1 Particle size distribution of raw materials

Table 1 Chemical composition of raw materials

Material	Mass fraction/%
Material	SiO₂	CaO	Fe₂O₃	MgO	SO₃	Al₂O₃	Na₂O	Other
OPC	18.62	64.88	3.74	2.29	4.23	4.89	0.12	1.23
SF	97.04	0.48	—	0.45	0.24	0.17	0.26	1.36
QP	98.50	0.02	0.15	—	—	0.89	—	0.59
QSⅠ/QSⅡ	99.16	0.01	0.04	—	—	0.47	—	0.32

Table 2 Main parameters of steel fibers

Diameter/mm	Length/mm	Density/（kg·m^-3）	Tensile strength/MPa	Elastic modulus/GPa
0.2	13.0/20.0	7 850	2 850	206

Fig.2 Actual partical size distribution curve and target curve

Fig.3 Marsh time at different water reducer dosage

Table 3 Mix proportion of UHPC

Material	OPC	SF	QP	QSⅠ	QSⅡ	Water	Steel fiber	SP
Mix proportion/（kg·m^-3）	1 025	100	75	850	105	190	157	92

Fig.4 Schematic diagram of steel fiber embedment in UHPC matrix

Fig.5 Compressive and flexural strength of UHPC at 3， 7， and 28 d

Fig.6 Load-slip curves of steel fibers at different embedment angles

Fig.7 Load-slip curves of steel fibers at different curing ages

Table 4 Maximum pullout load of steel fibers with different curing ages

Series	P_max/N
Series	7 d-45°	14 d-45°	28 d-45°
S1	65.04	70.83	82.73
S2	63.73	71.74	79.90
S3	61.94	72.00	79.81
Average	63.57	71.52	80.81
SD	1.55	0.61	1.66

Table 5 Maximum pullout load of steel fibers at different embedment angles

Series	P_max/N
Series	28 d-0°	28 d-30°	28 d-45°	28 d-60°
S1	47.74	85.14	82.73	70.31
S2	44.87	80.44	79.90	72.06
S3	47.09	84.11	79.81	72.76
Average	46.56	83.23	80.81	71.71
SD	1.50	2.46	1.66	1.26

Fig.8 Pullout energy and average bond strength of

Fig.9 Pullout energy and average bond strength of steel steel fibers with different embedment angles fibers with different curing ages

Fig.10 Schematic diagram of spalling effect and spalling specimen

Fig.11 Comparison of experimental and predicted spalling area values

Fig.12 Typical load-slip curves of smooth straight fiber pullout

Table 6 Basic parameters of matrix and fibers for ascending stage

Parameter	Value	Reference
A_f/A_m	0.01	［38］
E_m/GPa	45	［37］
E_f/GPa	200	—
γ	2	［22］
n	100	［22］

Fig.13 Comparison of experimental and predicted results for ascending segment of load-slip curves

Fig.14 Comparison of experimental and predicted result for descending segment of load-slip curves

Fig.15 Schematic diagram of snubbing effect and contact pressure during slip

Fig.16 Load-slip curves of steel fibers with different curing ages

Table 7 Basic parameters of matrix and fibers for descending stage

Parameter	Value	Reference
$v f$	0.3	—
$α$	1.0	［38］
$μ$	0.3	［38］
$v m$	0.2	［38］
$η$	0.05~0.10	［38］

Table 7 Basic parameters of matrix and fibers for descending stage

Parameter	Value	Reference
$v f$	0.3	—
$α$	1.0	［38］
$μ$	0.3	［38］
$v m$	0.2	［38］
$η$	0.05~0.10	［38］

Fig.17 Effect of η on descending shape curves （b=1）

Fig.18 Effect of b on descending shape curves

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