Study of durability of sprayed concrete

Transcription

1 Study of durability of sprayed concrete ADAM HUBÁČEK, RUDOLF HELA Department of Technology of Building Materials and Components Brno University of Technology, Faculty of Civil Engineering Veveří 95, Brno CZECH REPUBLIC Abstract: - Durability of sprayed concrete has been discussed very much recently. It is the property of a material, which characterizes its resistance to actions from surrounding environment (physical, chemical and biological inlvuences). The paper focuses on possibilities of preparation and study of basic physical parameters of sprayed concrete prepared in common laboratory mixer and compacted by vibration combined with defined pressure. The aim is assessment of basic characteristics of sprayed concrete, like strength of young sprayed concrete and strength of drilled test cores. At the same time, durability of sprayed concrete will be studied, in particular resistance to pressure water and surface resistance to water and chemical de-icers. Key-Words: - Sprayed concrete, durability, compressive strength, chemical aggressivity, chemical de-icers, exposure class 1 Introduction Sprayed concrete is the type of concrete, which is sprayed onto base material with special apparatus powered by compressed air. Sprayed concrete creates homogeneous layer thanks to its own kinetic energy, which develops through pressure in the spraying nozzle. Other technological requirements have to be fulfilled just like with concrete placed in standard way (water cement ratio, amount of cement and aggregate or appropriate consistency.) Sprayed concrete (so-called shotcrete) is nowadays an essential part of modern building industry. It is mostly applied in underground building industry for temporal and final tunnel lining. Primary lining ensures stability of the stope by co-action of concrete (which can also be complemented with arc bracing, reinforcement or anchor iron) with rock mass (a new tunneling method NÖTM a new Austrian method of tunneling with tunneling machines TMB). This lining transmits the load of rock mass until building of final tunnel lining. Sprayed concrete is often used as construction material of finale lining; therefore, it has to have sufficient durability. Durability is the property of material, which characterizes its resistance to action of surrounding environment (physical, chemical and biological influences). These undesired influences cause degradation of concrete stains, efflorescence, flaking, cracks or deformation. Degree of degradation depends on aggressivity of environment (intensity, type and concentration of the chemical, temperature, relative humidity), surface and properties of concrete (in particular water-tightness and frost resistance connected with porosity of concrete). Further factor with influence on durability is cracks in cement matrix. Cracks are formed as a consequence of inner effects (shrinking, thermal expansion) or outer forces (load, temperature). Mentioned phenomena cause local stress in cement matrix, which is higher, than strength of the matrix. Cracks decrease durability of concrete by faster action of aggressive environment in the concrete structure; therefore, it is necessary to prevent their formation by technological and static design [1]. 2 Sprayed concrete and aggressive environment 2.1 Permeability Sprayed concrete placed in aggressive environment XA can be exposed to several corrosive chemicals. These chemicals degrade concrete structure in various ways. An important measure increasing resistance to these influences is mainly considerable reduction of permeability increasing of homogeneity of concrete. Permeability is one of the main factors with influence on final durability of sprayed concrete. It is defined by penetration of a medium (liquid or gas) or flow of energy through porous structure of ISBN:

2 cement matrix. It is closely related to density or porosity of cement matrix and type of aggregate used. If aggregate is dense (hence non-permeable), permeability depends on porosity of cement matrix. Liquids and heat penetrate more easily through porous structure. The decisive factor is size, distribution and shape of pores. If capillaries are not interconnected and pores are small, permeability of concrete is lower. If open pores are larger than 10-7 m, water can pass through. Properties defining permeability are following: Hygroscopic property (capacity of absorbing atmospheric moisture), water absorbing capacity (capacity of saturation through open pores and hydrostatic pressure in capillaries) and capillarity (progression of water in capillaries thanks to capillary forces and surface tension of water). Cracks in concrete are another important factor. Permeability then depends on width, length, shape of a crack and hydraulic gradient of water. Shrinking and creep caused by temperature cause formation of cracks [2]. 2.2 Water-tightness Requirements for water-tight concrete are apparent from location of concrete in the structure with respect to wash water (its level), static function and dimensions of the structure. [1]. The important factors are dimensions, static function of the structure from sprayed concrete, position and chemical properties of wash water. Depth of infiltration of water into sprayed concrete exposed to exposure classes XF1, XF2 and XA1 must be at maximum 50 mm. Maximal depth of infiltration for exposure classes XF3, XF4 and XF2 is specified as 30mm, for XA3 20 mm at maximum. Watertightness of cement stone is influenced by quality and quantity of cement, admixtures (air-entrainers), additions (microsilica, blast furnace slag, fly ash), porosity (water cement ratio), voids content (granulometric curve) and homogeneity of placing sprayed concrete [1], [3]. 2.2 Frost resistance Effect of freeze-thaw cycles is more dangerous for horizontal structures than for vertical structures. Water, which is in contact with the surface of vertical structure, can flow down thanks to gravitation force, which causes less saturation than that of horizontal structures. Therefore, horizontal structures are more loaded after water freezes. For sprayed concrete, which is exposed to frost but not to chemical de-icers, it is not necessary to determine other frost resistance requirements than the frost resistance test exposure class XF1. For exposure classes XF2, XF3 and XF4, it is necessary to prove fulfillment of requirements for frost resistance and resistance of concrete surface to action of water and chemical de-icers [1], [3]. 3 Properties of sprayed concrete For definition of sprayed concrete, it is important to realize that shotcrete differs from commonly compacted concrete in by the technology of application on base material. Even though composition corresponds to common compacted concrete, properties begin to differ after application by spraying. Shotcrete differs not only by strength, which is influenced mainly by addition of accelerators but also water-tightness, frost resistance and durability of shotcrete. After spraying concrete onto base material, porous structure of concrete changes, a part of the material falls off which increases content of fine parts, which can cause higher values of shrinking. All this has to be taken into account when designing structures from shotcrete. One of the main observed properties is development of compressive strength within the first 24 hours after application. This value is determined in accordance with CSN EN [4] with penetration needle and consequently by the method of shooting nails. However, both methods provide only informative values of strength on the basis of calculation according to calibration curves. The advantage of these methods is that they can be used in situ. The method of penetrating needle determines strengths from 0.2 N/mm 2 to 1.2 N/mm 2. For strengths from 1.0 N/mm 2 to 16 N/mm 2 the method of shooting nails is used. Real values of strength are then determined on cores drilled from the construction or from testing slabs [4]. Compressive strength of commonly compacted concrete is measured on testing specimens (usually cubes with side 150 mm) made from tested concrete. Test cubes are loaded up to the failure limit in a test press. However, it is not so simple to determine strength of sprayed concrete. Shotcrete cannot be sprayed into cube forms because counter pressure of the form causes segregation of concrete mix. Coarse fractions then gather in corners of the form, fine fractions and cement are removed from the form. ISBN:

3 In practice, spraying concrete into test boxes with dimensions 500 x 500 x 200 mm is used. Size of such box is large enough to prevent abovementioned problems of 150 mm cube form. Test cubes can be cut from these concrete blocks or a test core can be drilled. Test cores can also be drilled directly from the construction [5]. 4 Testing of sprayed concrete Designing concrete mix, its preparation and testing is quite complicated nowadays. Since machinery has to be used for spraying, it is also costly. For this reason, there are efforts to formulate correlations, which could predict strength of shotcrete on the basis of development of strength of shotcrete prepared in laboratory. Concrete prepared in laboratory has the same composition as concrete applied on the construction; application by spraying is simulated by compacting concrete placed into a form by pressure board with defined pressure. There are several factors with influence on strength of sprayed concrete made in laboratory conditions compared to concrete sprayed with spraying machine. It is particularly process of dosing concrete components, level of compaction, temperature of concrete and influence of surrounding environment. If exact composition of concrete is known, it is easy to make in laboratory the same mix-design as the one used for spraying onto the construction. Then the most difficult part if defining of level of pressure applied on concrete to achieve similar density as concrete sprayed with the machine has. Based on test results it was proved that pressure 250 kg/m 2 brings the best results of concrete density. 5 Experimental part The aim of the article is assessment of individual properties of sprayed concrete, in particular its durability. Tested concrete was made from Portland cement CEM I 42.5 R, made by the Company Ceskomoravsky cement, plant Mokrá combined with plasticizer SIKA Viscocrete 2700 and accelerator of setting for sprayed concrete SIKA Sigunit L 53 AF. Dosage of accelerator was 5%, 7% and 9% by the amount of cement used. Aggregate used was of size fractions 0-4 washed from Zabcice, coarse crushed 4-8 from Olbramovice. In the first stage, tests of concrete sprayed by a machine were carried out. To observe properties, testing blocks with dimensions 500 x 500 x 150 mm were manufactured. For laboratory mixing, these forms were also used with compacting on vibration table with pressure board and pressure 250 kg/m 2. After adding accelerator into the laboratory mix, concrete was further mixed for another 60 seconds, then it was immediately placed in the forms. In practice, concrete is applied on the structure within seconds from adding accelerator in mixing nozzle. Unfortunately, this cannot be achieved during laboratory tests (in the lab, concrete was placed and compacted within 2-3 minutes from adding accelerator). It can be presumed that shotcrete made in laboratory will show lower density than shotcrete applied by spraying machine. Different density can of course influence resulting compressive strengths, which are compared. After compaction, strength of concrete was tested by the method of penetration needle, later by the method of shooting nails. Test cores were drilled from test slabs and tested on compressive strength at the age of 3, 7, 14 and 28 day; at the same time volumetric weight oc concrete was determined. Resistance of concrete to pressure water and water with chemical de-icers was also studied. Achieved results were evaluated and compared with reference concrete (concrete without accelerator). Following tables and diagrams show test results of studied concrete. Table 1. Properties of reference concrete Testing method Value Slump [mm] 210 Volume weight of fresh concrete [kg/m 3 ] 2270 Air content in fresh concrete [%] 2.1 ISBN:

4 Table 2. Comparison of compressive strength values Penetration needle average values [N/mm 2 ] HILTI-Tester average values [N/mm 2 ] Time of Shotcrete testing 5% 7% 9% 3 min min min min min min min min hrs hrs hrs Compressive strength of hardened concrete was determined on reference zero concrete (labeling Z) on test specimens in the form of cube with side 150 mm. Compressive strength of individual versions of sprayed concrete was tested on test cores of diameter and length 100 mm. Paragraph 7.2 of ČSN EN stated that if relation of length and diameter is 1.0, then determined strength of concrete is comparable to cube strength [7]. Table 3. Strengths on test cores Shotcrete Testing method Z 5% 7% 9% 3 days Compressive 7 days strength [N/mm 2 ] 14 days days Table 4. Depth of penetration with pressure water Mix design Unit Value Z 10 5% 18 mm 7% 15 9% 12 Table 5. Test results of resistance to chemical deicers Testing Shotcrete method Z 5% 7% 9% Resistance to chemical de-icers [g/m 2 ] 25 cycles cycles cycles cycles Conclusion The aim of the article was assessment of durability of sprayed concrete. Sprayed concrete with 5% and 7% of accelerator showed strength development of young sprayed concrete between curves J1 and J2. Concrete with 9% of accelerator by the amount of cement fulfilled requirements of development curve of young sprayed concrete J2 throughout the whole curve. According to the values of strength, all concrete samples can be classed into C30/37, which is the most often used strength class for concrete resistant to frost and corrosive chemicals. As for depth of ingression, all assessed mix-designs fulfilled requirements for concrete applied in environment with exposure classes XF4 or XA3 in accordance with CSN EN [6]. It means that these concretes can be used in environment with cyclical freezing and chemical de-icers as well as in strongly aggressive chemical environment (with the exception of high concentration of sulphates). Based on tests of resistance to chemical de-icers only zeroconcrete fulfilled requirements of required durability. All mix-designs of sprayed concrete showed low resistance. This was also due to the fact, that no air-entrainers were added into mixdesigns. Based on achieved results it can be stated that studied mix-designs of sprayed concrete fulfill only requirements of resistance to exposure class XA1 - XA3 but they are not resistant to exposure classes XF1 XF4. Currently, tests of durability of sprayed concrete with effective aeration are in process. These mix-designs are expected to be resistant to exposure classes XF1 XF4. ISBN:

5 7 Acknowledgements The paper was financially supported by the project GACR P104/11/P411 Problems of determination of calibration relations for strength characteristics of shotcrete and financial support of EU, OP Research and Development for innovations, project reg. No. CZ.1.05/2.1.00/ , as a part of actions of regional Centre AdMaS Advanced Building Materials, Constructions and Technologies.The paper was financially supported by the project GACR P104/11/P411 Problems of determination of calibration relations for strength characteristics of shotcrete and financial support of EU, OP Research and Development for innovations, project reg. No. CZ.1.05/2.1.00/ , as a part of actions of regional Centre AdMaS Advanced Building Materials, Constructions and Technologies. References: [1] Pytlik, P., Concrete technology, Brno University of technology, Vol.I, 2000, 280 pages (In Czech) [2] Czech tunnel committee ITA-AITES: Sprayed concrete in underground constructions, Prague 2008 [3] Czech tunnel committee ITA-AITES: Principles of application of sprayed concrete, Prague 2003 [4] CSN EN Testing Sprayed Concrete - Part 2: Compressive strength of young sprayed concrete, Czech standards institute, Prague, 2007 (in czech) [5] Hela, R., Hubacek, A. (2009) RVC - Technical quallity conditions of Building Directorate waterways Czech Republic. Chapter 2 - Sprayed concrete. 2009, Prague, Czech Republic (in czech) [6] ČSN EN Concrete Part 1: Specification, properties, manufacture and compliance. [p.l.] : [s.n.], September pp. [7] ČSN EN Testing concrete in structures - Part 1: Cored specimens - Taking, examining and testing in compression, Czech standards institute, Prague, 2009 (in czech) ISBN: