MECHANICAL PROPERTIES & DURABILITY OF CEMENT-STABILIZED ... MECHANICAL PROPERTIES &...

download MECHANICAL PROPERTIES & DURABILITY OF CEMENT-STABILIZED ... MECHANICAL PROPERTIES & DURABILITY OF

of 11

  • date post

    03-Jul-2018
  • Category

    Documents

  • view

    215
  • download

    0

Embed Size (px)

Transcript of MECHANICAL PROPERTIES & DURABILITY OF CEMENT-STABILIZED ... MECHANICAL PROPERTIES &...

  • MECHANICAL PROPERTIES & DURABILITY OF CEMENT-STABILIZED EARTH MORTARS FOR APPLICATION IN PREHISTORIC MONUMENTS

    Eleni-Eva Toumbakari*

    Directorate for Restoration of Ancient Monuments, Hellenic Ministry of Culture, 12, Karytsi Sq., 10561 Athens, Greece

    Maria Kaipanou, Aphroditi Ntziouni and Vassileia Kasselouri-Rigopoulou

    School of Chemical Engineering, National Technical University of Athens 9, Iroon Polytechniou str., 15773 Athens, Greece

    Keywords: Earth, Mortar, Conservation, Prehistory, Durability ABSTRACT The present work deals with the properties, mainly physical and mechanical, of repair mortars destined for the repair and strengthening of prehistoric masonry. The paper focuses on a specific prehistoric building type, the tholos tomb. Mixtures composed of Portland cement in limited quantities, clay and calcareous sand are examined. Aim of the mix design is to produce repair materials with Portland cement content as low as possible, to avoid mainly durability failures. Those materials should at the same time be able to ensure effective repair and strengthening, which is required in earthquake-prone areas such as Greece. The follow up of the evolution of some key parameters with time, such as compressive and tensile strength and apparent porosity, has shown that the cement-stabilized mortars can develop a great variety of properties, some of which can satisfy historic masonry requirements. Moreover, the study of the microstructure, using Scanning Electron Microscopy and X-Ray Diffraction, revealed the development of pozzolanic reactions, which is a key-factor for durability. However, a minimum percentage of cement is required, in order to improve the setting time and to ensure durability against sulphate attack as well. Cement-stabilized earth compositions permit the reduction of the Portland cement content and are able to produce a variety of mechanical and physical properties, sufficient for the repair and strengthening of prehistoric masonry structures as well as for use in archaeological sites. INTRODUCTION

    Earth-based binders were often used in the construction of prehistoric buildings in Greece. The function of those binders was either bedding mortars or plasters. The paper focuses on a specific prehistoric building type, the tholos tomb, where such binders were occasionally used. A tholos tomb consists of a circular burial chamber, called thalamos, initially roofed by a corbelled vault (Figure 1) and approached by an entrance passage, the dromos, which narrows abruptly at the doorway, the stomion. The thalamos is built of stone courses. The prehistoric way of assembling the stone blocks consists in the direct application of a stone on top of other, without the mediation of a mortar, as in Roman, Byzantine and later masonry assemblages. In the case, however, of irregular masonry, this technique results in the creation of, smaller or bigger, voids between the stone blocks which are often found filled with earth (Figure 2a). The fact that this earth fill was not systematically found during the excavations, led some researchers to the hypothesis that the tholos tombs were constructed solely of dry-stack masonries. The size, however, of many tholos tombs as well as stability requirements during construction call in favor of the use of this earth fill between the building stones as well. This assumption could be applicable to the majority of the tholos tombs, mainly those made of roughly carved stones, and it still remains an open subject.

  • Figure 1: Example of a Tholos Tomb: interior of Tholos IV in Thorikos, Attica

    Focusing on the structural pathology of the masonries envountered at the interior of many tholos tombs, it is systematically observed that the long-term exposure to environmental strains resulted in the creation of voids at the interstices between the stone blocks and layers (Figure 2b). Deterioration is then rapidly initiated, and sometimes leads to severe structural problems. A very common structural problem, in case of material loss, is the bending failure of the building stones, which were initially meant to lye one over the other (Figure 2b). If deformation proceeds, then local instabilities could take place, and eventually lead later to collapses. In this case, one of the techniques that could be used for the structural restoration of the tholos tomb masonries is the application of a mortar at the interstices between the stones, in replacement to the original earth fill (if it was) or simply in order to fill the voids created by material deterioration. This measure permits the increase of the frictional internal forces of the system and the overall Modulus of Elasticity.

    (a)

    (b)

    Figure 2: (a) Example of irregular masonry (tholos tomb of Acharnae, Attica), (b) severe stone deterioration and loss of material between the stones (Tholos IV in Thorikos, Attica)

  • Consequently, a more uniform stress distribution is expected to be restored, reducing the risk of stress concentrations, which lead to unwanted bending or even out-of-plane deformations, such as the ones shown in Figure 2b. On the basis of the aforementioned considerations, it is decided to study and develop suitable earth mortars for application in prehistoric masonries.

    EXPERIMENTAL

    Selection and characterization of the materials It is well-known that one of the requirements for the design of mortars for application in historic masonries is the study of the original materials in order to produce mortars, physicochemically and mechanically compatible to the original fabric [1]. Earth mortars are, however, vulnerable to environmental conditions and require protection to achieve durability. An alternative could be the design of stabilized earth mortars [2-4]. These are earth-based mortars, in which a part of the binder is typically replaced by cement or lime. To produce the mortar, an earth binder industrially produced in Attica was selected. Its grain size distribution is shown in Figure 3 [5]. In Figure 4 the morphology and the composition of the clay binder are also presented. The material mainly consists of albite and silica as it is also confirmed by the XRD pattern (Figure 5).

    Earth grain-size distribution

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0,001 0,01 0,1 1 10 100

    Diameter [mm]

    Passing [%]

    Figure 3: Grain-size distribution of earth binder

    Figure 4: SEM image (x3000) and EDX Spectrum of earth binder

  • 0

    100

    200

    300

    400

    500

    600

    700

    800

    900

    1000

    0 10 20 30 40 50 60

    2o

    cps

    Figure 5: XRD pattern of earth binder

    On the basis of the grain-size distribution, it is concluded that the earth is composed of fine-grain sand (2.9%), silt (79.8%) and clay (17.3%). The corresponding Atterberg limits are presented in Table 1.

    Table 1: Atterberg Limits Liquid Limit LL 38.2 % Plasticity Limit PL 22.2 % Plasticity Index PI 16.0%

    Selection of the mortar compositions In the present study, two stabilized earth mortar compositions were studied. The binder to aggregate ratio was kept constant and equal to 1:1 per weight. Calcareous sand of Attica was used as aggregate, whereas the binder consisted of silt and clay, to which cement type CEM II 32.5N in proportions of 12.5 % and 25%-wt was added. The water content was defined on the basis of flow-table tests. A requirement for a 16-17 cm mortar expansion was set. The compositions are presented in Table 2.

    Table 2: Composition of the earth-based mortars Composition Clay-silt Cement Sand

    A 25% 25% 50% B 37.5% 12.5% 50%

    Curing conditions Mortar preparation took place with a standard Hobart-type mixer. The specimens were prepared and cured as follows: first, the specimens were allowed to stay in the moulds for 7 days before

    Quartz Albite * Muscovite + Montmorillonite

    * *

    + +

  • demoulding, because of their slow strength increase. They were stored inside the moulds in the wet chamber for 7 days. Then, they were carefully removed from the moulds, and were stored in a laboratory closet under environmental conditions (22C at approximately 60-65% R.H.) until the day of testing. Specimen destined for mechanical tests had dimensions 40x40x160mm whereas those used for durability tests were measuring 25x25x285mm following Norm ASTM C 490. Fragments of the mortars, obtained after the mechanical tests, were immediately immersed once in acetone and twice in diethyl ether and then let to dry under vacuum overnight, in order to stop hydration. The fragments were subsequently stored in air-tight vessels to avoid carbonation until the day of testing.

    Experimental procedures A very important part of the study was the assessment of the mortars hygric properties and durability. The mortars hygric behaviour was studied through the determination of the water absorption and drying capacity of the mixtures at the age of 28 days. Durability was studied through constant immersion of the mortar specimen in deionized water and in sodium sulphate solution. At specific intervals, the specimens were removed from the water or the solution and their length and weight were measured. Then, the specimens were put back to water or the solution until the next measurement. The mortars were also subjected to cycles of wetting and drying. Moreover, the mortar shrinkage under exposure in laboratory conditions (22C and 60%R.H.) was equally monitored. To measure open porosity at a certain age, the specimens were first oven-dried at 60C until constant weight. After having cooled off, they were immersed in water until weight stabilisation. The saturated sample was then hung in a weight balance and immersed in water for v