Hygric performance of a massive masonry wall: How do the mortar joints influence the moisture flux?

https://doi.org/10.1016/j.conbuildmat.2012.12.024Get rights and content

Abstract

Although masonry walls are a composite of mortar and brick, in numerical simulations the actual composition is often simplified to a homogenous brick layer. This article studies the impact of the mortar joints and the interface resistance for a massive masonry wall exposed to climate conditions. Also, a comparison to the moisture behaviour during an imbibition experiment is made. Whereas during an imbibition experiment the joints and brick–mortar interface resistances influence the moisture transport significantly, for real climate conditions a negligible impact is found. Consequently, for the investigated case a simplification to a homogenous brick layer is allowed.

Highlights

► The hygric impact of mortar joints and interface resistances in masonry is studied. ► Mortar joints and interface resistances significantly retard free water uptake. ► For a real climate, the impact of mortar joints/interface resistances is limited. ► For a real climate, the bed joints serve as the preferential pathway for moisture. ► For the investigated case a simplification to a homogenous brick layer is allowed.

Introduction

In many European countries, masonry walls are the traditional construction technique for exterior building walls. Although masonry walls are a composite of brick and mortar, in most HAM (Heat, Air and Moisture)-simulations the actual composition of the masonry wall is simplified to a homogenous brick layer (e.g. [1], [2], [3], [4]). By doing so, a one-dimensional approach is feasible which reduces the computational time substantially. However, e.g. when assessing the durability of masonry (frost damage, salt efflorescence, algae growth, liquid moisture reaching the interior surface, etc.), the reliability of a simplified approach should be questioned.

In general, most mortars have a lower capillary absorption coefficient than ceramic brick. In addition, between brick and mortar an imperfect contact is found. Hence, at first sight when simplifying the masonry wall to a homogenous brick layer an overestimation of the moisture flux trough the wall is expected. However, since real climate conditions consist of alternating wetting and drying periods also the moisture storage capacity and the moisture dependent liquid permeability of the brick and the mortar have an influence on the hygric behaviour of the composite masonry wall. To investigate the hygric behaviour of a massive masonry wall exposed to alternating wetting and drying periods, the current article studies the influence of the mortar and brick and the interface resistance between both. To do so, in Section 2 the current state-of-the-art related to brick–mortar composites and the contact phenomenon can be found. In Section 3 an overview of the investigated wall structure and the material properties used in the analysis is given. Section 4 shows the simulated water uptake in a brick–mortar composite during an imbibition experiment. The response of the composite structure (with and without interface resistance) is compared with the simplified homogenous brick layer. In Section 5 the hygric behaviour of a masonry wall exposed to real climate conditions is investigated. Also, in this analysis attention is paid to the impact of the mortar as well as the interface resistance between brick and mortar. To end, the main conclusions are drawn.

Section snippets

State-of-the-art

Most building elements are a composite of different material layers. Moisture transport in multi-layered building elements can deviate from the moisture transport found for the combination of the single material elements. This deviation is projected in a retardation of the liquid transport across the material interface. In cases of a brick–mortar composite the reason for this deviation is multiple. First, during the curing process water will be extracted from the fresh mortar [5], [6], [7], [8]

Energy and mass balance

To study the hygro(thermal) performance of masonry walls, the coupled heat and moisture transfer in the building components is simulated with HAMFEM [22], a finite element method verified in [1]. The conservation of energy in the walls and the transport and mass storage in the walls is described by respectively:(c0ρ0+clw)Tt+clTwpcpct=(λT-clTgl-(cvT+Lv)gv)wt=(Klpc)+δvpvρlRvTpc+δvpvρlRvT2(ρlLv+pc(Tγ-1))Twhere c0, cl and cv the specific heat capacity of respectively the material,

Imbibition experiment

To study the liquid transport in a (simplified) masonry wall, an imbibition experiment on the different configurations (1A, 1B, 1C, 1D and 2, see inset in Fig. 3, Fig. 5) is simulated.

Case study: wall exposed to real conditions

Apart from water uptake, an exterior masonry wall will be exposed to alternating wetting and drying periods. Hence, also the drying potential of the composite materials will be of interest in the moisture response of a masonry wall. Therefore, in what follows the hygric behaviour of a massive masonry wall exposed to real climate conditions will be investigated. Note that whereas for the imbibition experiment no interior finishing layer was taken into account, now an interior plaster layer is

Conclusion

In many European countries, masonry walls are the common construction method for the building envelope. Although massive masonry walls are a composite of brick and mortar, in HAM-simulation the masonry wall is mostly simplified to a homogenous brick layer. By doing so, the less absorptive mortar joint as well as an interface resistance between mortar and brick are neglected. In the current analysis the potential impact of both simplifications was investigated based on the recent

Acknowledgements

The results in this paper have been partially obtained within IWT 3E90050 ‘Global performance approach and economic analysis of interior insulation with regard to renovation projects’ funded by the Flemish Government and KUL OT/09/23 ‘Towards a reliable application of interior insulation for the retrofit of existing buildings′ funded by the KU Leuven. These financial supports are gratefully acknowledged. In addition, the authors would like to acknowledge Prof.dr.ir. H. Janssen for the initial

References (28)

  • C. Hagentoft et al.

    Assessment method for numerical prediction models for combined heat, air and moisture transfer in building components: benchmarks for one-dimensional cases

    J Therm Envelope Build Sci

    (2004)
  • P. Häupl et al.

    Moisture atlas for building envelopes

  • H. Stopp et al.

    Hygrothermics of wooden beam end in envelope parts of buildings

  • J.I. Davison

    Loss of moisture from fresh mortars to bricks

    Mater Struct

    (1961)
  • Cited by (0)

    View full text