2011 gao,fan,lei_the failure mode and anti crack performance of the steel fiber reinforced high...

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The failure mode and anti-crack performance of the steel fiber reinforcedhigh strength concrete four-pile cap

Danying Gao1,a, Hua Fan1,b, * and Jie Lei 1,c 1Research Center of New Style Building Material & Structure, Zhengzhou University, Zhengzhou

450002, [email protected], [email protected], [email protected],

Keywords: Steel fiber, High strength concrete, Shear-span ratio, Ultimate load.

Abstract. Based on the experiments on 4 specimens with the dimension of 700mm×700mm and

different thickness, the failure mode and anti-crack performance of the steel fiber reinforced high

strength concrete four-pile cap was studied. The results show that the steel fiber mixed in high

strength concrete in four-pile cap enhances its cracking load, limits the propagation of crack

obviously and raises the ductility greatly. It also indicates that the ultimate load-carrying capacity of

four-pile cap can be improved significantly with the increase of effective thickness.

Introduction

In recent years, with rapid development of the city building and the construction of infrastructure,

the performance of the foundation has become more and more important. Pile foundation, which

can reduce the uneven settlement of foundation and has excellent seismic performance, has been

applied widely in high-rise building, roads and bridges, water conservancy project for its high

bearing capacity. As a connecting link, the cap of pile supports on the piles under column and it is

the so important specimen of foundation that the research of the mechanical properties of SFRC

four-pile cap becomes very meaningful.

With the development of concrete technology, the application of high strength concrete has

become more and more widespread. Though the specimen made of high strength concrete has

higher strength, larger stiffness and lighter weight [1], the thickness of pile cap must be large in

order to satisfy the bearing capacity in some practical projects. The mechanical behavior and

bearing capacity of pile caps with high strength concrete may be improved through the addition of

steel fiber; its thickness may also be reduced. Combining the advantages of high strength concrete

with steel fiber concrete, this paper is to study the performance of steel fiber reinforced

high-strength concrete pile cap [2].

Design of experimental

Specimens design. Four specimens of steel fiber reinforced high strength concrete (SFHSC)

four-pile caps were designed with the dimensions of 700mm×700mm and thickness of 150mm,

200mm, 300mm and 400mm respectively in the experiment. The concrete strength grade was C60

for each specimen, which was prepared with 42.5# ordinary cement, medium sand, rubble whose

diameter was from 10 to 20mm and steel fiber whose aspect ratio and volume fraction were 63.6

and 1.0% correspondingly. The reinforced bars with a diameter of 8mm were arranged double-sided

evenly in the bottom of the pile cap. The pile was simulated by steel pile with a diameter of 110mm

whose top extends 10mm into the pile caps. The concrete cover thickness of the pile caps on the bottom was 40mm and the others were 25mm [3]. The details of the specimen in the test are shown

in Fig.1, and the actually measured parameters and experimental are shown in Table 1.

Advanced Materials Research Vols. 306-307 (2011) pp 927-933Online available since 2011/Aug/16 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.306-307.927

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Test device and method. The whole experiment system consisted of loading device that was

YES-1000 type of press machine, strain measuring device that was CM-2B type of static

electric-resistance strain instrument and data gathering device that was strain measuring device and

computer. The deflection of pile cap was measured by displacement meter. The way of incrementalloading was adopted in the test. The increment of each load was 10％of predicted damage load, the

load was kept for 5 minutes after each loading, then observed and recorded the results. When the

load closed to the expected failure load, the incremental load was reduced to the 50%. According to

monitoring results of the strain gauge on the simulation pile, the load was similar for each pile. The

setup of the test is shown in Fig.2.

Analysis of test result

Table 1 The designed parameters and experimental results

Number cu f

[N.mm2] 0h

[mm]0

wh

s

ρ [%] f

ρ [%] cr

P [kN] u

P [kN]

CT41-1 77.40 110 1.13 0.52 1.0 150 350

CT41-2 84.52 160 0.78 0.36 1.0 300 570

CT41-3 78.14 260 0.48 0.22 1.0 400 950

CT41-4 85.18 360 0.35 0.16 1.0 1050 1850

Note: cu f is the cubic compressive strength of steel fiber reinforced high strength concrete;

0h is the effective height of cap; s ρ is the ratio of the longitudinal reinforcements on bottom； f ρ

is the volume fraction of steel fiber ； cr P is the crack load； u P

is the ultimate load; 0

wh

is the ratio

of shear span to effective thickness.

928 Emerging Focus on Advanced Materials



Cracks. The bottom cracks of the specimens are shown in Fig.3. The cracking load of pile cap

CT41-1 is 150KN. When the load arrives at 200KN, the bottom transfixion crack appears. When the

load reaches 300KN, the bottom concrete begins to abscise. When the load arrives at 350KN, the

cracks develop rapidly until the specimen destroys. The cracking load of pile cap CT41-2 is 300KN.

When the load reaches 550KN, the bottom crack in the mutual vertical direction intersects and the

sound of steel-fiber pull-out appears in internal of pile cap. Failure load arrived 570KN. The

cracking load of pile cap CT41-3 is 400KN. When the load reaches the failure load of 950KN, it is

kept unchanged about 2 minutes until the specimen destroyed. The cracking load of pile cap

CT41-4 is 1050KN, however, the cracks develops slowly after cracking. When the load arrived at

1750KN, the maximum crack width is 0.2mm. When the load reaches the failure load of 1850KN,

the cracks develops rapidly before the specimen destroys.

It was found from the experiments that the first crack appears on the position of mid-span when

the load reaches first cracking load. The first crack on each side surface appears at the same load

grade. With the increasing of load, the crack of each side surface develops upward until it arrives to

top surface. The bottom crack develops from edge to center until transfixion. When the load closesto failure load, the sound of steel-fiber pull-out appears continuously until specimen destroyed.

Load-deflection curve of pile cap. The load-deflection curves of pile cap with different ratio of

span to effective thickness are shown in Fig.4. It can be seen from the experimental results that

there is a significantly horizontal stage after the peak load for each pile cap, and the peak load

greatly increases with the decrease of the ratio of span to effective thickness, which indicates that

pile cap has a good ductility, deformability and flexural characteristics. The same conclusion was

got in ref. [4] and [5].

Advanced Materials Research Vols. 306-307 929



Effective thickness of pile cap. The four specimens were designed, which had the same

reinforcement ratio, concrete strength, the volume fraction of steel fiber and the geometry size

except the thickness varying from 150mm to 400mm to investigate the affect of the effectivethickness of pile cap on the load at cracking and ultimate. The relationship of load at cracking and

ultimate with effective thickness is shown in Fig.5, which shows the cracking load and the ultimate

load increase with the increasing of thickness. When the thickness increases from 300mm to

400mm, the cracking load and the ultimate load increases by 162.5% and 94.7% respectively, which

means that the effective thickness may be a main influence factor on the bearing capacity of pipe

cap.

Strain of reinforcing bar. The strains on reinforcing bar were arranged in the edge of pile and

inter-pile, which shows the differences of steel stress between different points. The arrangement of

measuring point is shown in Fig.6. The curve of the relationship between the load and the steel

strain is shown in Fig.7. From the figure, it can be seen that steel stress is very small before

cracking and increases significantly after cracking. With the increment of load, the steel stress of

odd points is larger than that in steel stress of even points and increases quickly, which reveals that

steel stress changes significantly along the length of the steel bar and stressed steel bar has the

feature of bending. When the load reaches the failure load, the steel stress at all odd points and part

of even points yields.




Strain of concrete. The concrete strain was measured by means of resistance strain gauges

pasted on side surface of pile cap and the arrangement of measuring point is shown in Fig.6. As

shown in Fig.8, Concrete strain distribution of CT41-1, whose shear span ratio is lower than 1,

shows that the concrete strain distribution obeys the plane section assumption , upper part concrete

of side surface is subjected to tension force and lower part concrete of side surface concrete is




subjected to compression, neutral axis is located at 2/5h. With the increase of the load, the neutral

axis is raising gradually, tensile stress increases quickly and compression stress increases slowly.

The compression area of concrete decreases gradually until specimen is destroyed.

Concrete strain distribution of CT41-2, CT41-3and CT41-4, whose shear span ratio are higher

than 1, as shown in Figure 1, shows that the distribution curves are straight line before cracking, and

change into curve after cracking. Generally, concrete strain distribution fits the characteristics of

flexural member. Through the overall analysis on concrete strain distribution and the development

of cracks, the failure mode of the steel fiber reinforced high strength concrete four-pile cap belongs

to flexural failure.

Conclusions

(1) The failure model of SFHSC four-pile cap belongs to flexural failure. The effective

thickness of cap has remarkable effects on the bearing capacity of four-pile cap and the increment

of effective thickness significantly enhances the cracking load and ultimate load.

(2) Steel fiber could limit the inclined cracks, restrain crack development, improve shearing,

anti-cracking and punching performance of pile cap, increase the ductility largely and reduce the

thickness.(3) Most of steel bar at the bottom of pile cap yield at the ultimate load.




References

[1] N.Q. Feng. High Performance Concrete Structures. Beijing: Mechanical Industry Press, 2004, 1.

[2] D.Y. Gao, H.T Zhu, J.Y. Tang. Experimental study on behavior of fiber reinforced high-strength

concrete under shear. J. Build. Struc. 25 (2004) 88-92.

[3] GB50007-2002, Code for design of building foundation．

[4] C.F. Sun, M.G. Wang, Q. Gian, S.M. Peng. Experimental research on punching and shearing

bearing capcity steel fiber reinforced concrete thick pile cap with two piles. J. Bulid, Struc. 25

(2004) 107-113.

[5] W. Jian, P.H. Long. Damage Control Modeing of a Single Column Cap Supported by 4 Piles

and Determination of the Anti-Cracking Characteristics. J. Beijing Inst. Civil Eng. Architecture,

2004, 20(2):15-19.




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The Failure Mode and Anti-Crack Performance of the Steel Fiber Reinforced High Strength Concrete

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