Materials and Structures, 1994, 27, 153-156

TECHNICAL NOTES NOTES TECHNIQUES

On the behaviour of concrete under water jet impingement

A N D R E A S M O M B E R * , H A R T M U T L O U I S t

* WOMA Apparatebau GmbH, Werthauser Strasse 77-79, D-47228 Duisbur 9, Germany ? Universitdt Hannover, lnstitutffir WerkstofJ'kunde, Appelstrasse II A, D-30167 Hannover, Germany

It is important from the economics point of view to understand the behaviour of concrete under loading due to plain water jets. Investigations which are based on mass removal measurements, grain size analysis, and mercury intrusion show that the performance of failure can be described using fracture mechanics methods. The reactions of hardened cement paste and concrete differ widely from each other because of the influence of interfaces between the cement paste and the aggregate.

1. I N T R O D U C T I O N

The high-speed water jet has been used as a tool for concrete processing for some years. Its applications include cleaning, roughening, removing, cutting, and fragmentation, as shown on Fig. 1. Concrete being an inhomogeneous material and the structure of a water jet being complex, the mode of destruction is very complicated. To analyse this process, it is necessary to take into consideration the following special aspects of water jet loading:

extremely small geometrical sizes of loaded areas, short duration of loading, unusually high rates of loading, high-pressure environment, high-moisture environment, overlaying of static and dynamic portion of loading.

Macro-mechanical parameters cannot be used to describe

such variations in loading, because they are results of integral measurements on specimens which are very large compared with the loading area in the case of water jet impingement. Structural details like microcracks and interfaces will be considered only as summary effects. If concrete is assumed "to be a brittle material, fracture mechanics methods can be used to describe its behaviour, but with the limitations mentioned above.

2. KERFING OF H A R D E N E D C E M E N T PASTE

The results are based on tests which were performed with either hardened cement paste samples or with quartzite-concrete samples. The compositions and mechanical properties of all samples are described in detail elsewhere [1]. Investigations of the influence of aggregate on cement paste samples have been given priority. Fig. 2 shows the results of pump pressure

Fig. 1 Applications of plain water jets in concrete processing.

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E t ' -

E~ > O

E ~D

550= 5ooi 45o- 40o ,350 300 250. 2oo. 15o. lOO. 50. o

o 10 20 30 40

pump pressure in MPa Fig. 2 Mass removal in relation to water pressure for hardened cement paste.

o =

E

Ct.

50

macroscopic cracking. According to Schneider [3], the behaviour of hardened cement paste in the threshold pressure range is as shown in Fig. 4. The interval between 100 nm and 1000 nm, which is defined as the range of microcracking, does not change significantly at a pressure of 30MPa. This means that no accumulation of microcracks takes place, but there is a sudden development of one or a few large cracks, which leads to pieces breaking away from the material.

Fig. 3 Roughness profile of the fractured area of a hardened cement paste (pressure 40 MPa).

variation. At a certain pressure, in this case 30 MPa, initial mass removal and therefore the first macroscopic damage can be observed. A further increase of pressure led to the total destruction of the specimens. At pressures of approximately 30 MPa the induced stresses inside the material exceed the critical values. The rising of stresses allows the unrestrained growth of cracks up to total failure. The basis of crack propagation could be the existence of, e.g., shrinkage cracks. The surfaces of the fracture provide evidence of breakdown processes involved. From Fig. 3 it can be seen that the roughness of fractured surfaces increases with the length of cracking, as a result of the rise in supply of elastic energy. This results in crack acceleration, secondary cracking, and crack branching. Similar effects are observed with homogeneous brittle materials like glass [2]. This kind of failure generates only few relatively large broken pieces of material. Investigations using the mercury intrusion technique support the concept of

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�9 75 5 10 20 50 100 200 500 1000 2000

flaw radius in nm §

10 unlooded . . . . pressure: 30 MPo

Fig. 4 Pore distribution of unloaded and loaded hardened cement paste.

3. KERFING OF CONCRETE

Fig. 5 contains the pressure-mass removal relation of a concrete sample with the same water/cement ratio as that of the hardened cement paste, which was discussed before. Significant mass removal can be observed at a pressure of about 18 MPa. This value is lower than that for plain cement paste. Also, progress of the function is constant, which means that the dynamics of performance do not change significantly. Fig. 6 shows the general mode of failure. A lot of small particles break away from the structure. These particles are the result of a microcrack network. They illustrate that unrestrained cracks cannot be the source of failure, but that the crack growth has been interrupted due to events of energy dissipation. The behaviour seems to be a result of aggregate influence. The addition of the quartzite aggregate generates two new opportunities for crack movement in the material: the contact zone between aggregate and cement paste and the aggregate material. Fig. 7 shows that a significant change in material structure occurs due to aggregate addition. Dhir et aL [4] have reported a clear increase in the permeability of concrete compared with plain cement paste. Similar effects have been described by Hoshino [5]. Another aspect of aggregate addition is the modified stress-strain behaviour of concrete. It can be assumed that the critical stress intensity factor of the aggregate- paste interface falls below that of the cement paste. These facts explain the lower threshold pressure in concrete processing. The fracture mechanics basis of this behaviour are represented in Fig. 8. It can be seen that the interface between aggregate and cement matrix is a preferred area for crack propagation. Although the threshold pressure of

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154 Momber and Louis

Fig. 5 Mass removal in relation to water pressure for concrete.

Materials and Structures 155

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Klc -values [6] \ cement matrix: 55 kp/cr~ 2

, \ inferface 3/2 ~~ rfzife): 12 kp/cd

inferface

crack lengfh Fig. 8 Fracture mechanics basis for the influence of the interface on the threshold pressure of the materials.

Fig. 6 Concrete particles broken away by water jet attack.

the concrete falls below that of hardened cement paste, no total failure of the specimens at very high pressures (90 MPa) could be observed. In fact, the destruction can be described as controlled removal of material. The fracture mechanics basis of this phenomenon is shown in Fig. 9. A crack with critical length C propagates underjet stagnation pressure. The crack grows, and may be arrested by an aggregate grain, or branched by the matrix- aggregate interface. Both processes interrupt continuous crack propagation. Some mechanisms of crack branching and crack arresting are presented in Fig. 10. Another

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i i i i i

100 200 500 1000 2000

f law radius in nm

- - cement matrix . . . . . . concrete II ~ concrete I

Fig. 7 Pore distribution of hardened cement paste and of two concretes each containing a different aggregate.

K.c -values [6] cement matrix: 35 kp/c r~2

....~.~. aggregate �9 3/2 - marble: 200 kp/cm .

�9 3 2 ~ . - quartz,te: 330 kp/cr~ /

o, i aggregate

i cement matrix ~ t

crack C-"" aggregate grain

crack length Fig. 9 Fracture mechanics basis of crack arrest by an aggregate grain, based on Wit tmann [8].

Fig. 10 Mechanism of crack branching and crack arrest, according to [7].

156 Momber and Louis

0

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90-

80-

70-

60-

50- 40-

30-

20-

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/ capi((an/ pores ~'" ""~ i i i i l i I i

5 10 20 50 100 200 500 1000 2000

flaw radius in nm

wifh0u~ loading . . . . pressure: 20 MPa

Fig. 11 Pore distribution of unloaded and loaded concrete samples.

aspect is crack growth through the aggregates. The higher specific surface energy of aggregate materials dissipates more energy from the crack propagation process.

Mercury intrusion measurements at around the threshold pressure illustrate the damage process in a different manner. Fig. 11 shows a clear change in structure in the micro-crack range. Obviously this change is the result of micro-crack network generation. On increasing the pressure, these crack fronts will associate, and material particles will break away.

3. CONCLUSIONS

Concrete destruction by water jets can be described as a fracture mechanics process. The interfaces between aggregate and cement paste are of decisive importance for crack generation and propagation in this form of loading. This paper provides a basis for the evaluation of concrete resistance to water jet impingement in practical applications. One can check, e.g., this resistance

in relation to aggregate type, grain size, and grain distribution under site conditions.

ACKNOWLEDGEMENTS

These investigations were supported by grants from the Alexander-van-Humboldt Stiftung. The authors wish to thank the Institute of Material Sciences, the Institute of Building Material Sciences and Material Testing of Hannover University (Professor H. Wierig) and the Federal Association of Lime-Sandstone Industry, Hannover.

REFERENCES

1. Momber, A., 'Untersuchungen zum Verhalten van Beton unter der Belastung durch Druckwasserstrahlen', VDI-Fortschrittsberichte, Reihe 4, Nr. 109 (VDI-Verlag, Dfisseldorf, 1992), pp. 26-29.

2. Sch6nert, K., '/]ber die Eigenschaften van Bruchfl~.chen', Chemie-lng.-Teehnik. 49(17) (1974) 711-714.

3. Schneider, U. and Herbst, H. J., 'PorositS.tskennwerte van Beton', TIZ Int. 113(4) (1989) 311-321.

4. Dhir, R. K., Hewlett, P. C. and Chart, Y. N., 'Near surface characteristics of concrete - intrinsic permeability', Meg. Concr. Res. 41(147) (1989) 87-97.

5. Hoshino, M., 'Difference of the w/c ratio, porosity and microscopical aspects between the upper boundary paste and the lower boundary paste of the aggregate in concrete', Mater. Struct. 21 (1988) 337-340.

6. Hillemeier, B., ' Bruchmechanische Untersuchungen des Ril3fortschrittes in zementgebundenen Werkstoffen' (Dissertation, Universit~t Karlsruhe, 1976).

7. Zielinski, A. J., 'Model for tensile fracture of concrete at high rates of loading', Cem. Concr. Res. 14(2) (1984) 215-224.

8. Wittmann, F. J., 'Structure of concrete with respect to crack formation', in "Fracture Mechanics of Concrete' (Elsevier, Amsterdam, 1983), pp. 43-74.

R E S U M E

Comportement du b6ton soumis fi des jets d'eau haute pression

D'un point de rue &onomique, il est important de comprendre le comportement du b6ton soumis fi des jets d'eau haute pression. Des recherches qui s'appuient sur des

mesures de perte de masse, d'analyse granulom6trique et de p6n~tration de mercure montrent que l'on peut dOcrire le comportement m&anique en utilisant lee m6thodes de Ia m6eanique de la rupture. Lee r6actions de la pate de ciment et du bOton diffkrent beaucoup l'une de l'autre. Ceci est dfi fi l'influence des interfaces entre la p&e de ciment et le 9ranulat.