Microstructure investigation of the interface between lightweight concrete and normal-weight concrete

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Abstract

Lightweight concrete (LWC) and normal weight concrete (NWC) are used in combination in structural engineering practice. Introduction of LWC brings advantages in terms of energy efficiency and reduction of costs. The mechanical properties of the interfacial transition zone (ITZ) between LWC and NWC has a crucial effect on the durability of structures. In this study, the bond properties of normal weight concrete-to-lightweight concrete (NWC-to-LWC) and lightweight aggregates-to-cement paste (LWA-to-CP) have been investigated at the microscale conducting micro-hardness tests and scanning electron microscope observations, whereas phase composition at the ITZ was measured with micro-Raman spectroscopy. The results indicated that the micro-hardness profile across the NWC-to-LWC ITZ is “U-shaped” and is surrounded by many fine cracks. The minimum in micro-hardness corresponds to a higher concentration of hydration products and ettringite. The LWA-to-CP ITZ is narrower; it appears compact with an asymmetric micro-hardness profile, thicker on the side of the cement paste (about 50 μm). On this side, a weak zone, with hardness lower than the one of the cement paste and enriched in ettringite and carbonated Ca(OH)2, is observed.

Graphical abstract

Introduction

Lightweight aggregate concrete (LWC) is widely used in structures in combination with normal weight concrete (NWC) because of its excellent performance (evaluated as high strength per unit weight) while offering thermal insulation, remarkable energy-saving effect, high durability, high fire resistance, low comprehensive engineering cost. [[1], [2], [3], [4], [5]]. At the surface of contact between LWC and NWC, joints frequently occur, and an interfacial transition zone (ITZ), in which mechanical properties differ from those of LWC and NWC, develops. An ITZ also forms between aggregates and the matrix of cement paste (CP). It has been reported to host a higher concentration of Ca(OH)2 crystals and calcium silicate gel (C-S-H), oriented towards the aggregate side and CP side, respectively, as a so-called ‘duplex film’ about 1 μm thick [6,7]. Compared to conventional aggregates, the whole ITZ (with thickness of about 40–50 μm) in presence of LWAs is interested in a higher amount of hydration products and C-S-H filling the aggregate porosity and realizing an effective mechanical interlock [6]. However, the grain size distribution of the cement, the water-cement ratio, the aggregate size, and the aggregate type have significant effects on the ITZ properties [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. In fact, in LWC absorption and water release processes in the lightweight aggregates have been considered to play a crucial role in the development of the ITZ [7,8].

Artificial ceramsite, made from rock, clay or sintered fly ash following thermal treatment, is frequently employed as the material for lightweight aggregates. Being largely amorphous and rich in SiO2 and Al2O3, in principle, it may exhibit pozzolanic activity, with impact on the characteristics of the ITZ and the mechanical performance of LWC. However, in this respect, experimental evidence is contrasting [21].

In the case of NWC-to-LWC ITZ, it has been observed that it develops thanks to the formation of a water film on the surface of the NWC (more hydrophilic) after the LWC is poured. The local increase in the water-to-cement ratio of the new concrete close to the interface, favors the development of ettringite and calcium hydroxide, impairing the interfacial strength [22,23]. Furthermore, the confinement induced by NWC (old concrete), known as ‘wall effect’, promotes bleeding and accumulation of bubbles in the new concrete close to the surface of contact, increasing the large porosity and the number of micro-cracks in the area, with a significant decrease in the interfacial strength. These alterations of the microstructure and chemistry strongly affect the nature of the bonding between the new and the old concrete [[2], [3], [4], [5],[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]].

Considering the applications of LWC and NWC in structures and the remarkable differences in material properties between LWC and NWC, the interfacial performance needs further study. Noteworthy, concrete being a multi-scale material, a link exists between the macroscopic properties and the composition and microstructure. At present, there is no study reporting on the bond properties NWC-to-LWC in relationship with the identification of the phase composition with the spatial resolution compatible with the thickness of the ITZ. Therefore, several aspects relevant to structure durability and structural integrity, are still unclear. In this paper, the bond properties NWC-to-LWC and lightweight aggregates-to-cement paste (LWA-to-CP) have been investigated making use of micro-hardness tests and scanning electron microscope (SEM). Moreover, because the ITZ may be as narrow as 100 μm, making its mineralogical composition hard to tackle with most conventional techniques, for the first time, its nature has been characterized with micro-Raman spectroscopy, which enabled to build up maps of phase composition with spatial resolution comparable to micro-hardness tests, by focusing the laser beam down to less than 1 μm.

Section snippets

Materials and preparation of specimens

P.O 42.5 N Portland cement, natural sand, gravel, shale ceramsite (as lightweight aggregate), tap water, fly ash, silica fume, and polycarboxylate superplasticizer were used to produce the concrete specimens to investigate the bond properties in NWC-to-LWC and LWA-to-CP ITZ. The cement satisfied the definition of “Common Portland cement”(GB175-2007) [35]. The grading of the used sand and gravel aggregates satisfied the definition of “Pebble and crushed stone for construction”(GB/T 14685-2011) [

Micro-hardness tests

In Fig. 2, the micro-hardness data are plotted in function of the position with respect to the interface between LWA and CP. The right side, inside CP, was taken as a positive direction, and the left side, inside the LWA, was taken as a negative direction. The values decrease in the LWA-to-CP ITZ approximately from -30 to 30 μm. Beyond the ITZ, on the LWA side, the micro-hardness is constant, with values of about 3500 MPa. The ITZ, on the CP side, is slightly larger, because, after a minimum of

Conclusions

Based on the macroscopic experimental program and micro-mechanism herein described, the following conclusions can be drawn:

  • (a)

    The micro-hardness profile across the LWA-to-CP ITZ is asymmetric, being thicker on the side of the cement paste (about 50 μm). From 20 and 50 μm the hardness values are even lower than for the cement paste. Indicating a weak zone tends to bond failure in the cement volume.

  • (b)

    The micro-hardness curves of the NWC-to-LWC ITZ exhibit a “U-shaped” distribution across the ITZ, with

Acknowledgments

The authors would like to acknowledge the financial support provided by the National Science Foundation of China (No. 51778060), and the Fundamental Research Funds for the Central Universities, CHD (No. 300102289401).

References (46)

  • A. Zhou et al.

    Structural performance of FRP confined seawater concrete columns under chloride environment

    Compos. Struct.

    (2019)
  • D.A. Silva et al.

    Evidence of chemical interaction between EVA and hydrating Portland cement

    Cem. Concr. Res.

    (2002)
  • S. Chandra et al.

    Freezing as a method of study of early cement paste hydration

    Cem. Concr. Res.

    (1980)
  • L. Kong et al.

    Chemical reactivity of lightweight aggregate in cement paste

    Constr. Build. Mater.

    (2014)
  • S. Diamond et al.

    The ITZ in concrete - a different view based on image analysis and SEM observations

    Cem. Concr. Compos.

    (2001)
  • P.R. Rangaraju et al.

    An investigation into the influence of inter-aggregate spacing and the extent of the ITZ on properties of Portland cement concrete

    Cem. Concr. Res.

    (2010)
  • L. Basheer et al.

    Influence of coarse aggregate on the permeation, durability and the microstructure characteristics of ordinary Portland cement concrete

    Constr. Build. Mater.

    (2005)
  • P. Vargas et al.

    Microstructural analysis of interfacial transition zone (ITZ) and its impact on the compressive strength of lightweight concretes

    Constr. Build. Mater.

    (2017)
  • T. Wu

    Mechanical properties and microstructure of lightweight aggregate concrete with and without fibers

    Constr. Build. Mater.

    (2019)
  • T. Li et al.

    Hydration process modeling of ITZ between new and old cement paste

    Constr. Build. Mater.

    (2016)
  • B. Wang et al.

    Evaluation of tensile bonding strength between UHTCC repair materials and concrete substrate

    Constr. Build. Mater.

    (2016)
  • H. Costa et al.

    Influence of lightweight aggregates concrete on the bond strength of concrete-to-concrete interfaces

    Constr. Build. Mater.

    (2018)
  • J.P. Ollivier et al.

    Interfacial transition zone in concrete

    Adv. Cement-Based Mater.

    (1995)
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