Study of Near-surface properties of concrete with Metakaolin addition

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Study of Near-surface properties of concrete with Metakaolin addition Pavel Reiterman*1, Martin Keppert2, Monika Cachova3, Marcel Jogl4, Petr Konvalinka5 Faculty of Civil Engineering, Czech Technical University in Prague Thákurova 7, 166 29 Prague 6, Czech Republic pavel.reiterman@fsv.cvut.cz; 2martin.keppert@fsv.cvut.cz; 3monika.cachova@fsv.cvut.cz; 4marcel.jogl@fsv.cvut.cz; petr.konvalinka@fsv.cvut.cz

*1 5

Received 27 June 2013; Accepted 14 January 2014; Published 14 May 2014 © 2014 Science and Engineering Publishing Company

Abstract Surfacing, concrete mixture composition and curing are of great importance for the concrete surface resistance. That’s the reason, why many leading experts agree on the importance of monitoring the “concrete skin” as the most loaded area affected by the external environment.

Keywords Intrinsic Permeability; Metakaolin; Porous System

Introduction Many research works demonstrate that application of metakaolin improves properties of concrete as freezethaw resistance and resistance against de-icing salts with freeze-thaw cycles combination. A very important role in concrete performance in such severe environment is played by porous system and surface of hardened concrete. The paper presents an experimental program focused on the monitoring of water transport in surface layer of fair-face concrete aimed at monitoring the permeability of concrete, since concrete permeability is a property uniquely affecting durability of concrete. These methods are complemented by experimental results and other traditional tests. These findings will serve to further optimization of the structure being created, thus ensuring its better aesthetic and functional characteristics.

The origin and character of the concrete surface layer is given by the technology of production of concrete elements; during vibration compaction is going on initiation and rising of air bubbles upward. This eliminates air content in concrete, but also the concentration of fine particles takes place close the walls of the formwork. Coarse aggregate – due to its

geometry – cannot get its entire plane to formwork or to the corners (Reiterman). Hence the concrete surface exhibits completely different properties compared to bulk – it is called wall effect. The characteristic properties of the surface layer are low content of coarse aggregates and an increased amount of cement stone which consists of fine fractions of aggregate and cement hydration products. A very important feature is especially high content of pores in comparison with the inner layer. Such a surface layer having a thickness of 20-50 mm has highly very different, usually worse properties than the surrounding material. Therefore attention is drawn to quality of concrete surface layer.

Each void and crack decreases a steel covering concrete layer and thus accelerates the process of its corrosion which is associated with visual degradation of the building. Degradation processes are closely related to the quality of the surface layer and the pore system of concrete.

Environment induces several physico-chemical processes in concrete which can completely change its properties. Changes that are noticeable to human eye and reduce the aesthetic parameters of the material are important for the classification of visual elements (Reiterman). The material alterations – accompanied in some cases with visual changes – occur mostly due to interaction of environment components with concrete.

The intensity of these processes depends on the chemical and physical conditions, such as concentration of acting media, temperature, humidity, etc. The causes of degradation processes can be divided into the following groups: 

Physical action (temperature and moisture changes, freezing action, solar or UV radiation

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of another origin, leaching) 

Chemical influences (action of corrosion media, e.g. acid rains, carbonation, expansion processes)



Physico-chemical influences (impact of water, de-icing salts, corrosion under tension)

All the mentioned degradation processes are closely related to the quality of the surface layer and the pore system of concrete. Since the coating layer is the most exposed and thus the most polluted site of structure, there is an even distribution of tension across the section. Stresses are concentrated at the weakest point along the cracks and voids. There are mostly tensile stresses that exceed the material tensile strength and cause cracking which is becoming the reason of visual problems. Such a disruption creates a new access for aggressive media, which contribute to the gradual degradation of the material. It is an extensive and complex chemico-physical process and therefore it is very difficult to separate the individual simultaneous degradation processes from each other. Penetration of aggressive gaseous or liquid substances from the environment depends on the condition and character of pore system. Permeability of concrete surface layer is affected by many aspects - choice of used components, composition of fresh concrete mixture, formwork, the method of concrete compaction and its curing during setting and hardening (Kolář).

Influence of surface structure lies in the fact that the appearance of the facade elements - in particular its brightness - will vary depending on the surface roughness and degree of hydration. In other words, when one performs a color treatment to a perfectly smooth surface and to a rough surface, the rough surface will appear darker, especially when the color will be deeper. This is due to higher absorption of radiation on the structured surface. Contractor monitors the concrete shade preparation usually on the smooth surface; it may therefore happen that shade of structure is properly delivered and applied, but later the object appears darker.

Differences in surface color and other color defects are a usual problem in the case of requirement a uniform shade across the entire surface area. This inconvenience can not be removed or controlled by precast production, because the production of large amounts of elements is indeed possible to frontload and prevent complications with different production batches of individual components, but the elements are always produced under different climatic

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Construction Engineering (CE) Volume 2, 2014

conditions. It has been long generally known that when the concreting is performed at lower temperatures the surfaces reach a much richer and brighter tone due to slower hydration and subsequent finer crystalline structure of hydration products.

Susceptibility to defects in the visual elements can be relatively easy to suppress by using of a separation device or by structuring of concrete surface. Probably the oldest and simplest way to improve the surface quality is good curing of concrete but this method is in practice often associated with considerable problems (Konvalinka).

Experimental Program Permeability of porous materials is one of the principal parameter influencing their transport properties and durability. The intrinsic permeability of material K [m2] is defined in terms of Darcy’s law describing flow of a liquid through a porous material saturated by the liquid under the action of pressure gradient across the material. It is material property and thus does not depend on the kind of permeating liquid. One of possible definitions of K is given by equation (1)(Černý).

(1) There j is mass flux [kg m-2 s -1], ρl is density of the liquid [kg m-3], η is dynamic viscosity of the liquid [Pa s] and driving force is pressure gradient through the material. Intrinsic permeability is extremely dependent on properties of porous system of hardened concrete. That is one of the reasons of very different result of published experimental works.

Permeability was measured by means of high pressure permeameter made by CNE Technology. The design is based on work [6]. The method is based on definition equation (1). This method is not standardized in Europe, but there is an American standard (RILEM).

Altogether were measured three concrete mixtures (tab. 1), with dosage of metakaolin and different amount of plasticizer. Chemical properties of Portland cement and used metakaolin are shown in (tab.2).

Experiment based on Darcy’s law provides coefficient of permeability K valid for saturated state of material.

The dependence of K upon volumetric moisture content w can be derived by help of pore size distribution of the material. Mualem (Mualem) proposed equation (2) which uses experimentally determined pore size distribution function f(r); other

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input parameters are saturated moisture content wsat and parameter n. For computing was used software HydraPor developed at Department of Material Engineering and Chemistry Czech Technical University in Prague. Final properties of the studied mixtures are at (tab.3).

∙

(2)

experiment is dependence of pore volume (it corresponds to measurable volume of intruded mercury) upon pore diameter (corresponds to applied pressure). The experiment was carried out by means of Pascal 140 and 440 devices (Thermo).

The water vacuum saturation method was used for the measurements of bulk density, matrix density and open porosity (Vejmelková). Each sample was dried in a drier to remove majority of the physically bound water. After that the samples were placed into the desiccator with deaired water. During three hours air was evacuated with vacuum pump from the desiccator. The specimen was then kept under water not less than 24 hours. From the mass of the dry sample md, the mass of water saturated sample mw, and the mass of the immersed water saturated sample ma, the volume V of the sample was determined from the equation (3), (Vejmelková).

(3)

FIG. 1 HIGH PRESSURE PERMEAMETER SCHEME TABLE 1 COMPOSITION OF STUDIED CONCRETE MIXTURES

Mixture Components

A B C [kg/m3] [kg/m3] [kg/m3]

Cement CEM I 42,5 R Mokrá sand 0-4mm, Dobříň Aggregate: crushed 4-8mm, Zbraslav crushed 8-16mm, Zbraslav Plastisizer (Sika 1035)

400 930 315 600 3

400 930 315 600 2

370 930 315 600 3

water metakaolin

180 -

180 -

180 30

TABLE 2 CHEMICAL PROPERTIES OF PORTLAND CEMENT AND METAKAOLIN [% BY WEIGHT]

Chemical properties

CEM I 42,5 R

SiO2 Al2O3 Fe2O3 TiO2 MgO CaO K 2O K2O + Na2O Loss of ignition Specific surface

23.9 5.2 2.9 3.0 58.8 0.8 1.1 2.9 3.43 m2/g

Mefisto K 05 [% weight] 58.70 38.50 0.72 0.50 0.38 0.20 0.85 0.9 1.67 13.06 m2/g

Pore size distribution was determined by means of mercury intrusion porosimetry (MIP). The method is based on intrusion of mercury – non wetting liquid – to pores of studied sample. The penetrated pore’s diameter is inversely proportional to applied pressure (Washburn equation). The applied pressure is gradually increasing during the experiment and thus mercury penetrates narrower pores. The result of

Where ρl is the density of water. The open porosity ψ0, the bulk density ρ and the matrix density ρmat were calculated according to the equations (4), (5), (6).

(4) (5) (6) TABLE 3 PROPERTIES OF CONCRETE MIXTURES

Mixture Bulk density [kg/m3] ρmat [kg/m3]

A 2310 2598

B 2320 2600

C 2325 2579

ψ0 [-] Compressive strength [MPa]

10.18 67.8

11.48 60.5

12.65 68.5

Hydraulic conductivity [m/s]

1.2⋅10-11 1.2⋅10-11 1.0⋅10-11

Depth of pressure water penetration [mm]

38

48

25

Equilibrium volume moisture [-]

6.2

5.8

5.7

FIG. 2 PORE DISTRIBUTION

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FIG. 3 COEFFICIENT OF PERMEABILITY AS FUNCTION OF MOISTURE CONTENT

Sufficient concrete surface resistance depends not only on the quality of the surface layer, but also on the character of the pore system. It directly affects the rate of penetration of aggressive substances from the external environment to the internal structure of the concrete and thus the degradation rate of the material.

It is very important therefore to reduce transport of liquid water solutions and water vapor as a primary protection against corrosion of reinforcement.

Determination of depth of penetration was performed according standard ČSN EN 12390-8; water has a pressure of 0.5 MPa for 72 hours.

FIG. 4 WATER PENETRATION TEST SCHEME

FIG. 5 MOISTURE PROFILES AFTER WATER PENETRATION TEST

On the results is evident positive influence of metakaolin addition. This material reduces in concrete its ability to transport water to inner structure. It’s

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caused by continuing hydration of minerals contained in this pozzolanic additive. Durability of Portland concrete is one of the most fundamental parameter which defines the ability of material to resist acting of environment. Previous research (Kolář), (Doležel) confirms positive effect on fresh concrete and increasing resistance to thawing cycles by parallel effect of chemical de-icing salts on hardened concrete.

But decreasing of water/moisture transport properties is very significant in all degradation processes.

Conclusions Durability of concrete depends not only on the quality of the surface layer, but also on the nature of the pore system that is truly defining parameter because it affects the rate at which aggressive substances, gaseous or liquid, penetrate from the external environment into the internal structure of concrete, and thus the rate of degradation of the material and its durability. Very important is the limitation of water and moisture transport of having a role in reinforcement protecting against corrosion. Results of these tests will become the basis for a correlation dependence permeability of concrete and concrete methodologies for evaluation in terms of its durability.

At this point it should be noted that the in construction practice and research, the term of permeability is used quite freely (for different tests), it is always necessary to present the results along with the method and conditions of the experiment. It is obvious that a material will have a dramatically (ASTM C 1202-97) different permeability for air, for water or "permeability" of the chloride ions, reported in the Coulombs. In connection with the introduction of the new standard system came into issues of concrete durability on importance. Action of aggressive media concentrated in the external environment is to develop a wide range of corrosion mechanisms (CRD C 163-92:),(RILEM). Most buildings are protected from the effects of adverse conditions by other types of structures. In case of the fair-face concrete, however, this protective layer is formed by the material itself. Due to the fact that concrete degradation processes are going on a long-term, and aggressive substances act at low concentrations, it is very difficult to describe such effects and to quantify the resulting degradation phenomenon under test in the laboratory. In the case of porous building materials is intrinsic permeability one of the parameters that affect their behavior in structures; depend on it water resistance and indirectly resistance to action of frost,

Construction Engineering (CE) Volume 2, 2014

salts and other durability.

degradation

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processes,

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Reiterman, P. - Kostelecká, M. - Kolář, K. - Konvalinka, P. Influence of Covercrete Morphology on Durability

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Vejmelková, E. - Keppert, M. - Rovnaníková, P. - Keršner, Z. - Černý, R. Properties of lime composites containing a new type of pozzolana for the improvement of strength and durability In: Composites Part B: Engineering. 2012, vol. 43, no. 8, p. 3534-3540. ISSN 1359-8368.

Experimental Centre (EC) is accredited department of the Faculty of Civil Engineering at the Czech Technical University in Prague. EC provides experimental works in the civil engineering field regarding mainly static and dynamic testing of structural building elements and materials. Experimental works are performed in the specialized laboratories as well as in-situ. EC is fully open for the possibility of collaboration on research and development of new building materials and new structural elements.

Department of Materials Engineering and Chemistry is responsible for Material Engineering studies on the Faculty of Civil Engineering of the Czech Technical University in Prague. The research activities are related to the whole spectrum of work concerning the Materials Engineering subject, in particular to experimental and theoretical analyses of heat, moisture and chemical substances transport and storage in building materials, methodology of directed design of building materials and their multilayer systems, and testing.

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