COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1554
Measuring Thermal Mass of Sustainable Concrete Mixes
O. Damdelen1, C. Georgopoulos1 and M. C. Limbachiya1
1
School of Engineering, Faculty of Science, Engineering and Computing, Kingston
University, Penryn Road, Kingston-Upon Thames, KT1 2EE, UK; email:
k0838912@kingston.ac.uk.
ABSTRACT
One of the challenges in sustainable development is to optimize the energy efficiency
of buildings during their lifespan. Modern concretes offer both low embodied CO2
with the use of different types of cements and recycled aggregates and reduced
operational CO2 with the intrinsic property called “thermal mass” that reduces the
risk of overheating in the summer and provides passive heating in the winter.
Thermal mass is currently evaluated with “admittance” which is the ability of the
element to exchange heat with the environment and is based on specific heat
capacity, thermal conductivity and density. The aim of this study is to evaluate the
effect of thermal properties namely, density, specific heat capacity and thermal
conductivity on thermal mass. The objective of the study is to carry out laboratory
experiments by measuring such thermal properties of concrete mixes with various
percentages of GGBS (ground granulated blast furnace slag), PFA (pulverized fuel
ash), and SF (silica fume) and RCA (recycled coarse aggregates). The results
obtained from these tests would contribute to the evaluation of how such thermal
properties influence the thermal admittance and hence the thermal mass performance
of sustainable concrete elements in a building system.
INTRODUCTION
Sustainable construction is becoming more popular as this sector correspond the
world changing needs. The purpose of those variations is to increase the life of the
residence by lowering CO2 emissions and to increase the use of natural resources.
Examination of thermal mass can be used to prevent or minimize temperature swings
in the building and can also be used to eliminate the need for energy consuming air
conditioning systems. Thermal mass is related to the storage material. Storage
material is the mass of the building including walls, partitions, ceilings and floors
where all have high heat capacity. The most important factors associated to heat
storage (i.e. thermal mass) are thermal conductivity (λ), specific heat capacity (c) and
he density (ρ) of the concrete. Thermal mass can explain the ability of the concrete to
store the transferred heat/cool. Thermal mass can be determined by thermal
diffusivity (α) of the building material that can be expressed as;
∝= (1)
ƿ
As a conclusion, the usefulness of thermal storage depends on several parameters,
such as materials’ properties, the exposed surface area, the thickness of the storing
elements and its location and orientation within the building (as an external or an
internal partition). The storage capacity of the slab is determined by the thickness of
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1555
the penetration depth. If a building has a natural ventilation system with a concrete of
thin penetration depth (i.e. 50mm ˣ 75mm), it is operationally efficient for heat
transfer and storage. Daily temperature cycle which is called sinusoidal cycle has a
period of 24 hours. The slab reacts to variations occurred in this daily cycle.
According to CIBSE Guide (1999), exchange of heat and cool over the cycle is
measured by thermal admittance that can be defined as;
Y= (2)
From the above definition of thermal admittance, it can be resulted that for a given
temperature variation, heat/cool load that can be absorbed by the slab has a direct
relationship with the thermal admittance. Since heavy materials such as concrete,
brick or stone have a large, internal exposed capacity, those materials were storing
greater part of the daily energy cycle. High admittance of those materials results in
small temperature swing in the room. The unit for admittance (Y) is W/ m2 K.
Admittance values for several constructions components are given in CIBSE Guide
section A3. The aim of the admittance method described before is predicting indoor
temperature and by this way, evaluating peak environmental temperature for any
proposed building. Details about the technique of this method can be found from
CIBSE guide, section A8. As a conclusion, application of heat on different materials
will create different effect on each material. Efficient heat storage material should
have high density, thermal capacity and moderate thermal conductivity.
RESEARCH METHODOLOGY AND EXPERIMENTAL
Research Methodology. By using each cement replacement material separately,
such as SF (silica fume), PFA (pulverized fuel ash), and GGBS (Ground granulated
blasted slag) will be illuminating the factor that how these cement replacement
materials may affect the thermal properties of concrete.
Experimental Design
Preparation of Mixes. The mixes used in this study are described in Table 1.
There are 16 different specimens that can be classified in into three sections, namely,
natural aggregate (Group A), recycled coarse aggregate (Group B) and minimizing
water cement ratio concretes (Group C).Table 1 is shown the mixes for applying
thermal properties.
Table 1. The mixes prepared for the measurement of thermal properties
Mix No. Description Mix No. Description
A1 100% NA +100% PC C1 100% NA +100% PC
A2 100% NA+45% GGBS C2 100% NA+45% GGBS
A6 100% NA + 20% PFA C3 100% NA + 20% PFA
A10 100% NA+ 20%SF C4 100% NA+ 20%SF
B1 30% RCA + 100% PC C5 30% RCA + 100% PC
B2 30%RCA + 45% GGBS C6 30%RCA + 45% GGBS
B6 30% RCA + 20% PFA C7 30% RCA + 20% PFA
B10 30% RCA + 20% SF C8 30% RCA + 20% SF
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1556
Materials. • PC (Portland cement): A single source of Class 52.5N PC
confirming to BS EN 197-1 was applied;
• GGBS (ground granulated blast-furnace slag): A single source (Civil-Marine) of
GGBS confirming to BS 6699/BS EN 197 -1 was applied;
• SF (silica fume): A single batch of silica fume confirming to EN 13263-1 was
applied;
• PFA used in the UK is classified as CEM IV according to BS EN 197-1 (2011);
• Graded natural sand with a maximum particle size of 5 mm and complying with the
requirements of BS EN 12620-1 (2009)
• Natural aggregate used was Thames Valley gravel with a size fraction between 20
mm and 5 mm. The used RCA was obtained from processing concrete debris from
demolished concrete structures. The size of fraction of the RCA is between 20 mm
and 5 mm (BS EN 2620:2002 Classifying Aggregates)
Mix Proportions. Table 2 gives the mix proportions for the test concrete
mixes. All of the mixes applied in the study were designed to have a slump of 60-180
mm. which is the range of acceptable slumps according to EN 206-1. As well as this,
the range of the compacting factor of fresh concrete mixes was determined. All the
mixes applied in the study were designed to have a compacting factor of 0-3s.
Table 2. Mix proportions for concrete mixes (Group A, B and C).
Constituent Materials Kg/m3
Mix Coarse Aggregate Types of Cements FA/
No. W W/C
FA NA RCA SF PFA GGBS PC CA
A1 586 1240 - - - - 345 0.47 195 0.57
A2 586 1240 - - - 155 190 0.47 195 0.57
A6 580 1222 - - 60 - 295 0.47 185 0.49
A1
586 1240 - 70 - - 275 0.47 195 0.57
0
B1 597 850 365 - - - 345 0.49 204 0.59
B2 597 850 365 - - 155 190 0.49 204 0.59
B6 593 845 360 - 60 - 295 0.49 189 0.51
B10 597 850 365 70 - - 275 0.49 204 0.59
C1 460 1150 - - - - 557 0.40 195 0.35
C2 460 1150 - - - 306 251 0.40 195 0.35
C3 451 1132 - - 120 - 478 0.40 180 0.30
C4 460 1150 - 111 - - 501 0.40 195 0.35
C5 448 786 337 - - - 583 0.40 204 0.35
C6 448 786 337 - - 262 321 0.40 204 0.35
C7 440 772 331 126 - 502 0.40 190 0.30
C8 448 786 337 117 - - 466 0.40 204 0.35
SAMPLE OF TESTING CONCRETE MIXES
Thermal Conductivity. According to BS EN ISO 8990: 1996 and BS EN 1934:
1998, a method called “hot-box” is developed by Dundee University in order to
measure steady-state thermal transmission properties. This equipment contains two
sides: heating side and cooling side. By using the heating side of the equipment, total
power input (Фp) is calculated by using the measurement obtained from thermostat
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1557
of hot box and timer counting. Timer counting is used to determine the proportion of
the time needed to maintain constant temperature with heat source generated by 40
W light bulbs and a fan that is used to circulate the air.
( )
Ф = (3)
( )
The sample that will be used in this study is square shape slab with 300 mm length
and 75 mm thickness. Hence, thermal conductivity values are calculated by using the
equation below:
= (4)
∆
Where; Q1= Qp-Q3-Q4, therefore; Q4= (0.9763xQp)-6.2516
d is the thickness of the sample;
A= 0.004m2 (exposed area);
∆T is the temperature difference between the hot side of the equipment and cold side
of the equipment and
d= 0.075m which is the thickness of the samples;
Qp is the total heat input;
Q1 is the heat transferred from hot side of the equipment to cold side of the
equipment through the specimen
Q3 is the heat loss from hot side of the equipment to the environment;
Q4 is the flanking loss that is the heat lost through the gap between the specimen and
the equipment during the experiment.
3.2 Specific Heat Capacity
Thermal properties of building materials are examined in specific heat capacity, so
that it can be determined how much mass is needed per unit for one unit increase in
temperature of the sample. By this way, specific heat capacity can be used to explain
association between heat and temperature variation. Specific heat capacity is found
by performing an experimental procedure in an insulated box. This box consists one
stainless steel bucket which is approximately about half of the bucket. This preheated
sample of cube of concrete is placed in to the water which is inside the bucket, After
that, the insulated box is closed and the temperature of the sample, water and air are
recorded until the same temperature is observed on the sample, water and air. After
that, observed values from three elements are used to evaluate the value for specific
heat capacity by using the formula stated below;
Q=mc∆T (5)
Table 3. Known specific heat capacities.
Known Specific Heat Capacities
Material Specific heat capacity (J/KgK)
Stainless Steel 18Cr/8Ni 502
Water at 20,30,40 & 50oC 4181.6, 4178.2, 4178.3, 4180.4
Air at 20 to 100 oC (Dry) 1006
CC mC ∆TC = CS mS ∆TS + CW mW ∆TW + CA mA ∆TA + [Mw ] (6)
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1558
where, ι is specific latent heat of the water (226 ×104); Mw is mass of water in air
(evaporated water). The values needed to calculate the specific heat capacity using
the Eq. (6) can be found in Table 3
Density of Hardened Concrete. Hardened concrete density is determined either by
simple dimensional checks, followed by weighing and calculation, or by weight in
air/water buoyancy methods (BSEN 12390-7, 1097-6). The density of hardened
concrete specimens such as cubes and cylinders can be quickly and accurately
determined using a Buoyancy Balance.
EXPERIMENTAL RESULTS AND DISCUSSIONS
The thermal properties of concrete mixes were measured. The values of the measured
properties are summarized in Table 4.
Table 4. Thermal Properties of Concrete Mixes
Thermal Specific Heat Thermal Compressive
Mix Density
Conductivity Capacity Diffusivity Strength -28
No. (Kg/m3)
(W/mK) (J/KgK) (m2/sec) ×10-7 day(N/mm2)
A1 2270 0.921 785 5.2 45
A2 2255 0.880 836 4.7 42
A6 2220 0.820 847 4.4 41
A10 2250 0.84 850 4.4 43
B1 2150 0.720 882 3.8 39
B2 2135 0.670 940 3.3 42
B6 2120 0.610 950 3.0 41
B10 2135 0.65 958 3.2 38
C1 2345 0.990 686 6. 7 55
C2 2330 0.931 712 5.6 51
C3 2335 0.892 727 5.3 53
C4 2345 0.901 690 5.6 54
C5 2200 0.770 848 4.1 54
C6 2180 0.730 868 3.9 49
C7 2190 0.701 865 3.7 48
C8 2185 0.750 850 4.0 52
Group A and B concrete mixes were investigated. PC with natural coarse aggregate
exhibits the highest value on both thermal conductivity and density. On the other
hand, the lowest thermal conductivity and density of concrete occurred at 20% of
PFA replacement for ordinary Portland cement. Additions of different types of
cements material to the concrete affect the thermal conductivity and density.
Comparing PFA, GGBS and PC, PC concrete is slightly greater than GBBS and SF
concretes. On the other hand, recycled coarse aggregate decreased the thermal
conductivity and density of the concrete. RCA concretes are the light weight
aggregate concretes which have low density.. This means that SF and PFA are
probably related to the higher air content and partly to the amorphous structure of SF
and PFA. Types of cement materials increased with increasing specific heat capacity.
It is observed that PC concrete is the highest density and the lowest specific heat
capacity. On the other hand, the replacement of PFA for PC is the lowest density and
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1559
the highest specific heat capacity in natural and RCA concrete mixes. In Group C,
The water cement ratio is minimized with increasing the density of the concrete.
Clearly, if the cement content increases with increasing thermal conductivity and
density of the concrete mixes, this results in better dry density of concrete. The
results explained that W/C ratio is minimized with increasing the thermal
conductivity in all types of concrete mixes. The water cement ratio of concrete is
minimized, the density of concrete increases with increasing thermal conductivity.
As well as this, the investigation is carried out for the Recycled coarse aggregate
concrete. The results showed that when RCA is used with minimizing water cement
ratio; it is achieved to improve the thermal conductivity of concrete. When
minimizing water – cement ratio is used in the concrete, the thermal diffusivity of the
concrete is improved. For instance, when C4 concrete mix is used, nthe thermal
diffusivity is increased by 27.3 % when compared against A10 concrete mix.
THERMAL DYNAMIC CALCULATION
The excel file is set up to calculate the thermal dynamic properties of concrete mixes
by applying the thermal properties data (thermal conductivity, density and specific
heat capacity) of the concrete mixes. Factors which affect the thermal storage are
taken under examinations that include thermal admittance hence thermal mass,
decrement factor, and thermal transmittance (U-value) of the concrete mixes. The
main aim of this section is to understand the effects of different types of cements
materials, recycled coarse aggregate and minimizing water-cement ratio of the
concrete mixes on the thermal admittance, thermal transmittance and decrement
factor of the concrete mixes. Before setting up the excel calculator, the thermal
dynamic properties are calculated theoretically. BS EN ISO 13786:2007 standard is
used to calculate those parameters. The thickness of the samples is 0.075m
(constant). The results are provided in Table 5.
Table 5. Thermal Dynamic Results of concrete mixes.
U-Value Thermal admittance
Mix No. R-Value (m2·K/W) Decrement factor
(W/m·K) (W/m2·K)
A1 0.081 3.98 4.19 0.87
A2 0.085 3.92 4.20 0.86
A6 0.091 3.82 4.14 0.86
A10 0.089 3.86 4.17 0.86
B1 0.104 3.65 4.03 0.86
B2 0.112 3.55 4.03 0.85
B6 0.123 3.41 3.96 0.84
B10 0.115 3.50 4.03 0.84
C1 0.076 4.07 4.18 0.88
C2 0.081 3.99 4.14 0.88
C3 0.084 3.94 4.12 0.88
C4 0.083 3.95 4.10 0.88
C5 0.097 3.74 4.07 0.86
C6 0.103 3.67 4.04 0.86
C7 0.107 3.61 4.01 0.86
C8 0.100 3.70 4.05 0.86
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1560
Group A and B concrete mixes were investigated. Especially PFA content in
concrete mixes has the lowest U-value and However, GGBS content in concrete
mixes have greater R-value than silica fume content in concretes. When recycled
coarse aggregate content in concrete mixes is examined, it is concluded that RCA
content in concrete decreased the U-value more than different types of cement
materials of concrete mixes. When the results are examined that Natural aggregate
concrete mixes are increased slightly if comparing with 30% RCA content concrete
mixes at Group C. The laboratory results are provided that Water-cement ratio is
more important for RCA content concrete. The reasons for this, firstly because
minimizing Water-cement ratio is improved the density of RCA content concrete and
secondly, this is directly affected on the thermal conductivity and U-value of the
concrete. The laboratory results provided that when water-cement ratio, RCA content
and different types of cement materials are considered, water/cement ratio is act as a
main actor of affecting the thermal properties of the concrete. It can be concluded
that since minimizing the water-cement ratio increases the thermal conductivity of
the concrete. However, when the thermal conductivity increased, this decreases the
R-Value and hence increases the U-Value which will then increase the decrement
factor. Using GGBS in concrete mix, increase the thermal admittance of the concrete
and at the same time, R-Value increase with increasing thermal admittance in GGBS
concrete. When 20% silica fume is used in concrete mixes, a lower value of thermal
admittance is obtained compared to Portland cement mixes. However, PFA concrete
mixes, decrease thermal admittance with increasing R-Value of the PFA concrete
mixes. Beside of this, PFA concrete mixes have the highest R-Value compared
against all concrete mixes due to having less thermal conductivity in PFA concrete
mixes that means observing improvements in R-Value of the concrete. PFA concrete
mixes decrease the thermal admittance value more than others in group A. RCA
content concrete decrease the thermal diffusivity in all mixes in group B. This is due
to RCA material being lighter than normal concretes. PFA content concrete mixes
have lower thermal admittance and reasons for this depend on thermal properties of
concrete. The main reasons of group C having lowest thermal admittance and R-
Value are; minimizing water/cement ratio improve thermal conductivity and density
of the concrete. Therefore, in all groups, group C concrete mixes have the highest
value of thermal conductivity and density. As well as this, most of the concrete
mixes having lowest specific heat capacity values are present in group C. These
factors are directly affecting the thermal diffusivity values are increased. However,
these results are provided that higher thermal conductivity is not good for the thermal
admittance of the concrete mix. The important thing is using less dense concrete such
as lightweight concrete to achieve same or similar thermal admittance values in the
concrete mix which is more sustainable concrete. In other words, achieving moderate
thermal conductivity is more useful for the thermal admittance of the concrete. From
the observations, C1 is heaviest concrete mix than all the mixes. However, C5
contains RCA content and it is lighter than some of the concrete mixes in group C.
This results are lead to decide that the effect of materials and how these materials
have to be used properly in concrete to achieve lightweight concrete with functional
thermal properties and achieving optimum thermal mass in the concrete mix.
COMPUTING IN CIVIL AND BUILDING ENGINEERING ©ASCE 2014 1561
CONCLUSIONS
The laboratory tests results are shown that different types of cement materials are
affected thermal properties of concrete mix. The results obtained proved that type of
cements affected the density and thermal conductivity of the concrete. As well as
this, all types of cement materials are increased with increasing the specific heat
capacity of the concrete. The results also showed that different type of cements
content is important in considering the effect of thermal properties of the concrete.
On the other hand, RCA content increased with increasing of the specific heat
capacity of concrete. The laboratory tests showed that RCA content decreased the
density and thermal conductivity of concrete more than different type of cements
content. When the water cement ratio is minimized, the density of concrete is
increased with increasing the thermal conductivity of concrete. In the cases when
RCA and different type of cements content in concrete mixes are used, this improved
the density and thermal conductivity given that minimizing water cement ratio is
applied. Specific heat capacity of concrete is decreased with minimized water cement
ratio. This is due to water having greater specific heat capacity than cement and
aggregate. When the value of thermal admittance is concerned, it is found that this
value does not need to have high or low thermal conductivity of concrete mix. The
importance is to have a moderate thermal conductivity. The results are provided that
thermal admittance is increased with high specific heat capacity, high density and
moderate thermal conductivity of the concrete mixes. Therefore, those factors are
found to be vital for improving thermal admittance hence thermal mass of concrete
mix. As an application of this research, results obtained from laboratory section and
hence the calculations carried out afterwards can help in performing conceptual
design of framed buildings including options such as floor, slabs and walls.
Additionally, investigations of various materials used for concrete contribute in
specifying which materials are more suitable to use than others in sustainable
concretes.
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Arup/Bill Dunster architects, UK Housing and Climate Change Heavyweight versus
Lightweight Construction, Arup Research + Development, Bill Dunster
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B. Milovanovic, P.I. Banjad, I.Gabrijel, Measuring thermal properties of hydrating
cement pastes, in: 31st Cement and Concrete Science Conference, Novel
CIBSE Guide, Thermal Properties of Building Structures (A3), The Chartered
Institution of Buildings Services, London, 1980.
CIBSE Guide, Summertime Temperatures in Buildings (A8), The Chartered
Institution of Buildings Services, London, 1975.
EN ISO 13786:2007, Thermal Performance of Building Components—Dynamic
Thermal Characteristics— Calculation Methods International Organization
for Standardization, 2007.
Thermal Mass for Housing, TCC/04/05, The Concrete Centre, 2006.
