Mike Lawrence

Hygrothermal Performance of an Experimental Hemp-Lime Building

The use of hemp-lime as a construction technique is a novel approach which combines renewable low carbon materials with exceptional hygrothermal performance. The hemp plant can grow up to 4m over a four month period, with a low fertilizer and irrigation demand, making it very efficient in the use of time and material resources. All parts of the plant can be used the seed for food stuffs, the fibre surrounding the stem for paper, clothing and resin reinforcement, and the woody core of the stem as animal bedding and aggregate in hemp-lime construction. The unique pore structure of the woody core (shiv) confers low thermal conductivity and thermal and hygric buffering to hemp-lime. The construction technique promotes good air tightness and minimal thermal bridging within the building envelope. All these factors combine to produce low carbon, hygrothermally efficient buildings which are low energy both in construction and in use, and offer opportunities for recycling at end of life. This paper reports on the hygrothermal performance of an experimental hemp-lime building, and on the development of a computerized environmental model which takes account of the phase change effects seen in hemp-lime.

Porosity, Pore Size Distribution, Micro-structure

The high porosity and microstructure of bio-aggregates are fundamental to their physical properties. Typically they have a low density and a complex pore structure. This has two principal effects. In the first instance, low density is associated with low strength, but also with low thermal conductivity. For this reason most bio-aggregates are not suitable for use as structural materials, but are eminently suited to act as a low density filler in composite materials conferring low thermal conductivity on the resulting bio-composite. The complex nature of their porosity results in a material that is able to readily adsorb moisture and humidity. This results in a material that has an exceptionally high moisture buffering capacity, a characteristic that is of great interest in building materials, because it tends to stabilise the internal environment of a building, thereby resulting in a much more healthy indoor environment. This chapter considers the range of methods that can be used to measure porosity and to characterise the microstructure of materials in general, and discusses how some of these techniques have been used on bio-aggregates. It also identifies opportunities to use novel techniques on bio-aggregates in order to improve our understanding of their porosity, pore size distribution, pore connectivity and microstructure, all of which are characteristics that are essential to the optimisation of the performance of bio-aggregates within the construction industry.

Reducing the Environmental Impact of Construction by Using Renewable Materials

The relative importance of embodied energy and operational energy on the environmental impact of construction are examined in this article. It highlights the fact that the targets set by the Kyoto Protocol are primarily being met by the reduction of in-use energy, and that the implications of that are that the energy embodied in buildings will increase in signifi cance from its current 17% level to 50% by 2050. The article describes how the use of bio-based renewable materials can make a signifi cant contribution to reducing not only the embodied energy of buildings by using the sequestration of CO2 through photosynthesis, but also in-use energy demand through passive environmental control. Case studies are presented showing ways in which this has been achieved.

Non-homogeneous Thermal Properties of Bamboo

A Phyllostachys edulis (Moso Bamboo)sample’s density, heat capacity and thermal effusivity were obtained by a series of experiments. The porosity, thermal conductivity and thermal diffusivity were calculated. Based on these experimental values, this study discusses the Phyllostachys edulis sample’s microstructure characteristics and the causes of the variation of thermal properties along the radial direction.

Non-Hydraulic Lime Mortars: The influence of binder and filler type on early strength development

Abstract Lime-based mortars are now widely acknowledged as generally superior to cement-based mortars in the repair of appropriate historic infrastructure. Increasingly the benefits of hydraulic lime mortars are also being realized in new masonry construction. In order to standardize the expected performance of mortars, designers will specify the type of lime, the type of filler (aggregate), the proportions of each and quantity of water or the required workability. Limes can be non-hydraulic (calcium or dolomitic) or hydraulic (natural or artificial). This paper reports on results of tests conducted on non-hydraulic lime conservation repair mortars at early stages of curing. Results to date show that the type of both non-hydraulic lime and filler used have a significant effect on the early (up to 28 days) mechanical performance of a lime mortar. The mortars in the study were made using five non-hydraulic lime binders: dry hydrate; 4 month-old lime putty; 20 year-old lime putty; ‘hot’ lime; and dispersed hydrated lime. The fillers were silicate sand, crushed bioclastic limestone, and crushed oolitic limestone. No pozzolanic material was added to the mortars. Compressive strengths at 14 days ranged from 0.3 MPa to 2.5 MPa. Comparisons are made between the structural performance and rates of carbonation up to 28 days of each binder:aggregate combination. In conclusion, observations are made on factors to be considered when specifying non-hydraulic lime mortar mixes for repair work.

Influence of Interfacial Material Pore Structure on the Strength of the Brick/Lime Mortar Bond

This paper builds on previous work investigating the flexural bond strength, initial shear strength and compressive strength of fired clay brickwork built using hydraulic lime mortars. It has been shown that whilst flexural bond strength and initial shear strength of the brickwork generally increased with mortar strength, flexural bond strength was significantly impaired by both low and high brick water absorption. This paper describes a study of the pore size distribution of the surfaces of brick and mortar at the brick/mortar interface using Mercury Intrusion Porosimetry. The paper identifies critical pore sizes at the brick surface which would appear to govern resultant bond strength.

Monitoring of the Moisture Content of Straw Bale Walls

This paper describes an investigation into moisture levels in straw bale walls used to clad a newly constructed building. The moisture content was monitored up to 10 months after the building was handed over. The sensors used for this purpose were readily available, low cost and easily installed. The moisture levels fluctuate during the first 4 months following installation of the instrumentation, followed by a period of greater stability where it is believed that the straw acts as a moisture buffer, managing the humidity levels within the building and contributing to a healthier internal environment. This ongoing study makes a contribution towards raising confidence levels in the use of straw bales as low carbon building material in mainstream construction.

Modification of Hemp Shiv Properties using Water-repellent Sol-gel Coatings

AbstractFor the first time, the hydrophilicity of hemp shiv was modified without the compromise of its hygroscopic properties. This research focused on the use of sol–gel method in preparation of coatings on the natural plant material, hemp shiv, that has growing potential in the construction industry as a thermal insulator. The sol–gel coatings were produced by cohydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) using an acidic catalyst. Methyltriethoxysilane (MTES) was added as the hydrophobic precursor to provide water resistance to the bio-based material. Scanning electron microscopy (SEM) and focused ion beam (FIB) have been used to determine the morphological changes on the surface as well as within the hemp shiv. It was found that the sol–gel coatings caused a reduction in water uptake but did not strongly influence the moisture sorption behaviour of hemp shiv. Fourier transformed infrared (FTIR) spectroscopy shows that the coating layer on hemp shiv acts a shield, thereby lowering peak intensity in the wavelength range 1200–1800 cm−1. The sol–gel coating affected pore size distribution and cumulative pore volume of the shiv resulting in tailored porosity. The overall porosity of shiv decreased with a refinement in diameter of the larger pores. Thermal analysis was performed using TGA and stability of coated and uncoated hemp shiv have been evaluated. Hemp shiv modified with sol–gel coating can potentially develop sustainable heat insulating composites with better hygrothermal properties.

The influence of the casting process on the internal structure and physical properties of hemp-lime

Bio-aggregate composites such as hemp-lime offer a more sustainable alternative to traditional walling infill material. Hemp-lime, whether in situ or prefabricated, is generally either cast or sprayed, which results in a directionally dependent, typically layered, physical structure. This paper considers the impact of compaction and layering on the directional thermal conductivity, compressive strength and internal structure of the material through use of a novel image analysis method. The results presented indicate that production variables have a significant, and crucially, directionally dependent impact on the thermal and mechanical properties of cast hemp-lime.

Green composites for the built environment

Abstract Green composites used in construction are unlike natural fiber composites developed for automotive and other structural composites where particles or fibers are combined with a polymer matrix to form a composite material, often in the form of relatively thin sheets. Green composites for construction are designed to satisfy the requirements of low-energy, zero-carbon green buildings where walls and other structural building components are highly thermally insulating and breathable, ensuring effective climatic control. Coatings have also been developed for these materials which improve indoor air quality, impacting positively on the health of occupants.