Horizontal Wedge Splitting Test Method (HWST) - a New Methodfor the Fracture Mechanics Testing of Large Samples

Herbert N. LinsbauerVienna University of Technology, Vienna, Austria

Aljoša ŠajnaIRMA - Institute for Research in Materials and Applications, Ljubljana, Slovenia

Karl FuchsVienna University of Technology, Vienna, Austria

1 Introduction

The principles and methods of fracture mechanics, besides their utmost importance in the fieldof metallic and ceramic materials, have also become powerful tools for the investigation ofcracking in structural elements and structures of concrete on the one hand and for materialoptimization to reduce cracking on the other. Especially non-reinforced (plain) concretestructures such as dams and lock walls are very sensitive to cracking due to the specialcharacter of mass concrete. Mass concrete in the juvenile state generates high temperaturesand thus increases thermal stress related cracking during the hydration process. Yet it also isquite vulnerable to tensile stresses after setting.

The theories of linear and nonlinear fracture mechanics in the case of concrete materialsseem to be fully developed in the sense that the physical process of cracking has been wellunderstood and mathematically simulated. The main problem in terms of a practicalapplication however is the formulation of an adequate fracture criterion which is directly tiedto the knowledge of a realistic materials parameter. This is of particular difficulty in thetesting of mass concrete [1] where the size of the aggregates can often be up to 120 mm andacross, requiring sample dimensions of 1 m and more to achieve homogeneous behavior as isgiven in the structure. Usual size effect models therefore must fail when using conventionaltesting sample dimensions.

There are two possibilities to overcome this problematic. One is to develop a procedureadapted for testing mass concrete with small samples as is given by the wet-screening method[2,3]. The other is to test adequate sized samples with a special testing equipment.

This paper presents a method for fracture mechanics testing of large samples of concretewhich can easily be used in the laboratory and on the construction site.

2 Problem outline

The main problem for the testing of large samples of (mass) concrete may be seen in thedetermination of realistic fracture mechanics parameters, which is a basic demand in LinearElastic Fracture Mechanics (LEFM). Test results of large samples also serve as calibration

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parameters for the testing methods using small samples and as a grade of the efficiency of sizeeffect models.

The most common methods of investigating fracture mechanics material parameters forconcrete are the three point bending test (3PB) recommended by RILEM [4] and the wedgesplitting test (WST) originally developed at the Vienna University of Technology [5,6]. Thesemethods require testing machines with stiff frames, which in the case of testing of largesamples, are available only in laboratories with a special equipment.

The objective of the project was to develop a new method for testing large samples withthe least possible effort which would be suitable both in the laboratory and in situ.

3 Horizontal wedge splitting test method (HWST)

The basic idea of the HWST is a horizontal arrangement of the cone-shaped loading device(Fig.1), thereby avoiding a very large, stiff and hardly portable loading frame and minimizingfriction effects usually resulting from the vertical to horizontal load conversion in theconventional wedge splitting test method.

The new loading device consists of two rigid steel blocks, two gabled sliding plates, twowedges, one force measuring beam equipped with strain gages and a drive axle. The forcemeasuring device is connected to both the sliding plate and the load transfer block, acting as afour-point bending beam. On each side of the beam (tension and compression side) two straingauges are mounted to form a Wheatstone’s bridge.

The splitting force is activated by two wedges facing horizontally within the groove of thetesting sample which are pulled together by a hand or a machine driven axle (with a screwthreaded rod) acting through holes in the wedges. Like the axle, one wedge hole has a screwthread. The second wedge can be moved freely on the axle. As the loading device isassembled and placed into the groove of the specimen, the wedges are in perfect contact withthe gabled gliding plates. When the axle is turned both wedges slide from the outside to theinside of the specimen, thereby inducing horizontal forces to the upper edge of the groove andsubsequently causing a starter notch opening. The acting force is measured through thedeformation of the previously calibrated four point bending beam. The calibration has to becarried out according to the usual standards in measuring technique under all aspects ofloading - unloading cycles, etc.. An advantage of this testing system compared to an alsohorizontal applied splitting loading device [7] may be seen in the practical use without specialload transferring adaptations and testing equipment.

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a)

b)Figure 1. Principle of the new Horizontal Wedge Splitting Test (WST) a) specified; b) assembled

4 Comparative tests

The HWST-method was developed for the testing of large samples of mass (dam) concrete(Fig. 2).

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Figure 2. HWS-Test: Sample 900⋅900⋅400 mm with loading and CMOD- measuring device.

In order to ascertain the efficiency of the new testing method, comparative studies werecarried out between the new method and the conventional wedge-splitting test on the basis ofthe same concrete and identical geometry. The sample dimensions were restricted to thetesting equipment available for the WST-test.

A series of six specimens was prepared, and three out of the six were tested by the standardWST – method. The remaining three were tested by the new device for large WST specimens.Within these test series, concrete containing a maximum aggregate size of 60 mm and anamount of 285 kg of normal Portland cement per m3 was fabricated. The w/c-factor was 0.6.For both tests, pre-notched (a=h/3=133 mm starter notch length) specimens of the same sizeand of almost the same geometry (length=400 mm, hight=400 mm, thickness=160 mm) wereprepared. The only difference was the size of the groove. For the WST specimen the groove

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width was 40 mm, while for the HWST-specimen the groove width was 170 mm. Thespecimens with the 40 mm groove were tested by a standard WST -method with a verticalwedge action .The specimens with the 170 mm groove were tested by the new device (Fig. 1).The crack mouth opening displacement CMOD was measured with a LVDT device. Theloading was performed via the crank drive by hand and because of this the process may beclassified as quasi-CMOD controlled.

From among of three samples prepared for the HWST-procedure one completely failed andone was usable only for the determination of the fracture toughness. This was caused by apartly break-off of the groove walls during the removal of the moulds. The test results in theform of the horizontal splitting force versus the crack mouth opening displacement diagramsfor both testing methods are summarized in Fig. 3 and in Table 1.

Figure 3. Standard (WST) versus new (HWST) loading device test results.

Table 1.: Test results

Fracture toughness KC

[MPa⋅m1/2]Work of fracture Wf

[Nm]*WST HWST WST HWST0.47 0.53 3.95 4.680.53 0.52 5.070.47 4.58

*analyzed for an intermediate phase (CMOD →1.0 mm) without normalization to the fracture area (Wf/Af → Gf)

As may be seen from the diagrams, the standard WST tests and the tests carried out with thehorizontal wedge splitting test method HWST show similar results with mean valuedeviations of 7.1% and 3.3% for the fracture toughness and the work of fracture respectively.

This proves that the HWST-method is of universal application.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00

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h60b - hor. loading dev. h60c -//- v60d - ver. loading dev. v60c -//- v60b -//-

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5 CONCLUSION

The demand for fracture mechanics materials testing of large samples, is especially given inthe case of mass concrete as mainly used in dam engineering. Maximum aggregate sizes ofmass concrete are within the range of 80 mm to 120 mm, therefore testing sample sizes of 1.0m and more are necessary in order to gain realistic material values. Since the possibilities oftesting such samples are limited by the usual laboratory equipment, a method has beendeveloped for the fracture mechanics material testing of large samples. The method can beused in the laboratory and on a construction site because, aside from the crane capacity, thereis no limitation regarding the sample size.

6 References

1. H.N. Linsbauer in Fracture Processes in Concrete, Rock and Ceramics (Eds.: J.G.M vanMier, J.G. Rots J.G. and A. Bakker ), RILEM, E.&.N.Spon, 1991, p.779.

2. Sajna, A. Dissertation Vienna University of Technology, 19983. Šajna, H.N. Linsbauer in Fracture Mechanics of Concrete Structures (Eds.:H. Mihashi and

K. Rokugo), Proc. FRAMCOS-3, AEDIFICATIO, 1998, Vol.1 p.1014. RILEM Draft recommendation (50-FMC) Mater.Struct. 1985, 287-2905. H.N. Linsbauer, E.K. Tschegg, Zement und Beton, 1986, 38-406. E.K. Tschegg, Materialprüfung 1991 338-3427. Vervuurt, J.G.M. Van Mier in Fracture Mechanics of Concrete Structures (Ed.: F.H.

Wittmann), Proc. FRAMCOS-2, AEDIFICATIO, 1995, p.295