Thermal performance of building envelopes with structural layers of the same density: Lightweight aggregate concrete versus foamed concrete [original]

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Strzałkowski, J., Sikora, P., Chung, S.-Y., & Abd Elrahman, M. (2021). Thermal performance of building

envelopes with structural layers of the same density: Lightweight aggregate concrete versus foamed

concrete. Building and Environment, 196, 107799. https://doi.org/10.1016/j.buildenv.2021.107799

Jarosław Strzałkowski, Pawel Sikora, Sang-Yeop Chung, Mohamed Abd

Elrahman

Thermal performance of building

envelopes with structural layers of the

same density: Lightweight aggregate

concrete versus foamed concrete

Accepted manuscript (Postprint)Journal article |

Accepted manuscript of: Strzałkowski, J., Sikora, P., Chung, S.-Y., Abd Elrahman, M. (2021). Thermal

performance of building envelopes with structural layers of the same density: Lightweight aggregate concrete

versus foamed concrete. Building and Environment, 196, 107799.

https://doi.org/10.1016/j.buildenv.2021.107799

This manuscript version is made available under the CC-BY-NC-ND 4.0 license

http://creativecommons.org/licenses/by-nc-nd/4.0/

Thermal performance of building envelopes with structural layers

of the same density: Lightweight aggregate concrete versus

foamed concrete

Jarosław Strzałkowski1, Pawel Sikora1,2,*, Sang-Yeop Chung3, Mohamed Abd Elrahman 4

1 Faculty of Civil and Environmental Engineering, West Pomeranian University of Technology Szczecin, Poland

2 Building Materials and Construction Chemistry, Technische Universität Berlin, Germany

3 Department of Civil and Environmental Engineering, Sejong University, South Korea

4 Structural Engineering Department, Mansoura University, Mansoura, Egypt

Abstract

This study presents a comparative analysis of the effects of lightweight aggregate concrete (LWAC) and

foamed concrete (FC), with dry densities of 500, 750 and 1000 kg/m3, on the thermal performance of a

typical multi-family (residential) building. Typical two-layer walls consisting of an essential layer

(LWAC or FC), with an insulating layer of foamed polystyrene were evaluated. To ensure fixed U values

for all variants tested, the thicknesses of the support layers were adjusted accordingly, in such a way

that in each variant the load-bearing layer had the same value of the thermal resistance, thus ensuring

the same thermal transmittance value for the entire wall. Calculations were made for four different

climate zones, making it possible to determine the impact of each variant used, in different climatic

conditions. For a hot climate, the data for Cairo (Egypt) was used. A moderate, warm climate was

represented by Vienna (Austria), a moderate cold climate by Kołobrzeg (Poland) and a cold climate by

Tromsoe (Norway). Significant correlations between the type/density of concrete and climate zones

were established. The study shows that, despite comparable densities and thermal conductivity values

between LWAC and FC, their specific heat and thus dynamic thermal properties are different. Study

provides valuable guidelines and knowledge on choice between proper lightweight concrete type

depending on the climate zone. Meaningful conclusions were drawn, showing that the pursue for

developing the material with “the lowest” thermal conductivity itself is not the key factor to develop a

residential building with satisfactory thermal comfort.

Keywords: lightweight concrete; ultra-lightweight concrete; lightweight aggregate concrete; foamed

concrete; thermal performance; building envelopes

Corresponding author:

Pawel Sikora - Building Materials and Construction Chemistry, Technische Universität Berlin,

Germany; Faculty of Civil and Environmental Engineering, West Pomeranian University of Technology

Szczecin, Poland; pawel.sikora@zut.edu.pl

1. Introduction

Concrete is the most popular building material used in engineering structures. Compared to other

building materials, normal-weight concrete has a wide range of superior properties, such as good

mechanical properties, low sorptivity and high durability. In terms of material properties, the thermal

performance of concrete is not good enough to use it solely as a building envelope. To reduce its thermal

conductivity, other insulation materials needs to be taken into consideration when designing concrete

elements. To satisfy the European Energy Efficiency Directive for climate neutrality of building stock

by the year 2050, work is actively being carried out to improve the thermal insulation of building

envelopes, along with attempts to decrease the total energy used for heating [1]. Such challenges are of

high importance, as in a traditional building heat loss system, 70% of the loss is due to the enveloping

structure [2]. This problem is not only significant in countries with cold climates, but also in those

located in warm climate zones, where a substantial amount of energy is used for cooling [3-6]. There is,

therefore, a strong interest in developing modern building materials to tackle these environmental

challenges.

One type of construction material known for its favorable thermal properties is lightweight concrete

(LWC), which has a dry density in the range of 800–2000 kg/m3, according to EN 206-1. The concrete

below this range is considered to be a nonstructural material and is called ultra-lightweight (ULWC) or

infra-lightweight concrete [7]. Lightweight concrete can be classified into two main categories: namely,

lightweight aggregate concrete (LWAC) and foamed concrete (FC). The former is produced by

incorporating lightweight aggregates in a mixture while the latter is a cellular concrete containing large

numbers of pores produced by foaming agents. Although, the production technology of modern LWC

is well-known for more than a century, this technology has experienced a significant boom in the last

two decades. This is attributed to the development of inter alia: high-strength artificial aggregates, new

admixtures and additives, advanced measuring techniques as well as substantial improvement of the

production methods (e.g. high-shear mixers). These enable to produce advanced lightweight

cementitious composites with remarkably improved durability and mechanical properties which allow,

despite extremely low density, to use them for structural applications.

The use of such materials in the construction of engineering structures has manifold benefits: the use of

LWC in multi-story buildings can decrease the dead weight of a building, as well as providing significant

cost saving related to work time. It has beneficial effects on improving seismic structural response,

provides longer spans, improves fire and acoustic resistance, as well as lowering reinforcement ratios

and foundation materials [8]. When used in prefabricated elements, it decreases transportation costs.

Since it exhibits significantly lower thermal conductivity than normal-weight concrete, it can play a

substantial role in decreasing the energy consumption required for heating and cooling [8]. Pan et al. [9]

have reported that the incorporation of lightweight concrete blocks as partition walls, leads to a decrease

in the direct cost of an apartment by about 8.2 % - 13.9 %, as compared to one built with conventional

partitions. Furthermore, additional factors, such as transportation costs, enlarged net room area or project

duration, are of significance. Kurpińska et al. [10] have performed a comparative study on the production

of the prefabricated elements of ordinary and lightweight concrete walls in residential construction,

showing that the incorporation of LWAC during the production process can increase the initial costs of

a construction process, but on the other hand its use leads to a decrease in construction time and transport

costs. Muda et al. [11] have reported that the use of fiber-reinforced structural LWAC in high-rise

residential buildings, as compared to the use of conventional concrete and sand-cement brick and clay

brick walls, results in energy savings between 3.2% and 14.8%, across various climatic regions.

Moreover, such material can be used without additional insulation in hot-humid tropical and temperate

Mediterranean climates. Liu et al. [12] have reported that a properly designed foamed concrete mixture

composition noticeably increases the energy savings in a building. The evaluation of a typical, large

office building model has shown that in both cold and warm climates in parts of the USA, the use of FC

can reduce cooling energy consumption, as all as saving cooling water usage [12].

It is widely agreed that concrete density has a major influence on the thermal conductivity of LWC. In

contrast, a higher volume of pores within it results in deterioration of mechanical performance and the

durability of related properties. Therefore, there is significant interest in research related to the

production of LWC with satisfactory density, mechanical and thermal properties [2], [6], [8], [13], [14].

Among the thermal properties, thermal conductivity is not the only representative property that can

classify a material as being suitable for insulation. Thermal capacity and thermal mass should also be

taken into account when designing a building envelope.

The potential of using ULWC in building envelopes and its performance in relation to LWC has been

extensively evaluated by Roberz et al. [15]. The authors have reported that, despite the relatively low

density of ULWC (ca. 800 kg/m3), an extremely high thickness would be necessary to apply solely this

material in envelope construction in colder climates (Amsterdam, the Netherlands). Robati et al. [16]

have evaluated the effects of ULWC used in floor designs, on the energy performance of a typical office

building in Australian conditions. The research has revealed that structures with a higher thermal mass,

could moderate fluctuations between inside and outside air temperatures. In other words, buildings with

a higher concrete mass (thermal mass) stored more heat, which then reduced the peak indoor air

temperatures. Moreover, it was found that slab structures produced with ULWC resulted in indoor

temperatures that were more sensitive to fluctuations in external air temperatures. Thus, such buildings

required more energy to achieve the desired indoor temperature range, as compared to elements built

from conventional concrete. Similarly, Kumar et al. [10] have reported that lightweight thermal mass

materials have poor dynamic characters (time lag and decrement factor), which should be considered

when designing residential buildings. Besides, lightweight construction materials have higher embodied

energy than conventional materials.

As such, there is still no agreement between researchers as to whether LWC and ULWC are suitable

materials for use in the production of building envelopes. Moreover, more study is required to

understand which type of LWC, i.e. lightweight aggregate concrete or foamed concrete, is more

favorable for this purpose. For instance, while LWAC is more expensive than FC due to the cost of

lightweight aggregates, its compressive strength-to-density ratio is much higher than that of FC. In

addition, LWAC is more durable than FC, though its production requires much more energy than the

production of FC. Finally, both materials can be recycled easily, thus making them much more

sustainable than conventional concrete.

Based on the above-mentioned studies, it can be concluded that whilst there is a certain amount of

knowledge regarding the role of foamed concrete and LWCs, the literature is limited to only one type

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