BEHAVIOUR OF STRUCTURAL LIGHTWEIGHT CONCRETE SUBJECTED TO

HIGH TEMPERATURES

Elisabete Moreira de Oliveira Pino

EXTENDED ABSTRACT

JURY

President: Professor Doutor Pedro Manuel Gameiro Henriques

Supervisor: Professor Doutor José Alexandre de Brito Aleixo Bogas

Supervisor: Professor Doutor João Pedro Ramôa Ribeiro Correia

Examiner: Professor Doutor Jorge Manuel Caliço Lopes de Brito

Examiner: Professor Doutor João Paulo Janeiro Gomes Ferreira

October, 2013

1

1. INTRODUTION

The reduction of density in lightweight aggregate concrete (LWAC) compared to the normal

weight concrete (NWC), is the most attractive feature of this concrete, for solutions where the

influence of the permanent load and thermal insulation are relevant. For the use of LWAC, it’s

fundamental to understand theirs main physical and mechanical properties, and in particular

its behaviour when exposed to high temperatures. It is therefore important to quantify the

strength loss of these concretes at high temperatures and the expected damage and the

correspondent security risk for uses during fire. To this end, this work aims to contribute for

the improved knowledge on this domain.

The LWAC strongly affected by the properties of the aggregates (FIP 1983, Chen, Yen e Lai

1995, Faust 2000, Zhang e Gjørv 1989). Replacement of normalweight aggregates (NWA) by

lightweight aggregates (LWA) allows a reduction in the density of concrete, but also influences

the behaviour of the fresh concrete (workability, compaction and concrete curing) as well as

the behaviour of the hardened concrete (strength, modulus of elasticity, thermal properties,

shrinkage, creep and durability), affecting their mechanical properties and long-term

performance (EuroLightConR2 1998).

The compressive strength of LWAC depends, as in NWC, on various parameters related to the

type and volume of its constituents, including the amount and type of aggregate, the water /

cement ratio, the type and content of cement and the introduction of air. However, in LWAC,

the effect of aggregates is more relevant, affecting how the other constituents influence the

strength of the concrete.

Figure 1 – Distribution of forces: NDC (left) and LWAC (right) (adapted FIP 1983 and Bogas 2011)

2

As the stress characteristics in concrete depends on the stiffness of each of their components

as well as bonding between them, the failure mode is controled by the elastic characteristics of

each phase (aggregate and cement). In LWAC, the aggregates are more deformable than the

surrounding paste, and there the failure path goes throw the LWA (Figure 1) (Bogas 2011, FIP

1983, Holm e Bremner 2000, Gerritse 1981, FIP 1983, Chandra e Berntsson 2003, Cembureau

1974, Newman 1993, Virlogeux 1986, Faust 2000).

The modulus of elasticity of concrete depends on the stiffness of their constituents, including

the cement paste and aggregates (EN1992-1-1 2004, Newman 1993, FIP 1983). Due to the low

stiffness of the LWA, the modulus of elasticity in LWAC is lower than that of NDC (Zhang e

Gjørv 1991b, FIP 1983, Newman 1993, Holm e Bremner 2000, Chandra e Berntsson 2003,

ACI213R 2003).

The characterization of the fire resistance of concrete is complex. In fact, on the one hand

concrete is a composite material with components of different thermal characteristics. On the

other hand, its behaviour is strongly affected by their level of moisture and porosity.

According to several authors, as the cement paste is exposed to increasing temperatures the

following process takes place: the expulsion of evaporable water at a temperature of 100 °C;

the beginning of the dehydration of the hydrates of calcium silicate at 180 °C; the

decomposition of calcium hydroxide at a temperature of 500 °C; the initial decomposition of

hydrate calcium silicate around 700 °C. The concrete damage at high temperatures are more

evident when the temperature exceeds 500 °C. Around this temperature, most changes in

concrete are considered irreversible. Since aggregates materials are normally 65 to 75% of the

concrete volume, the behaviour of concrete at high temperature is strongly influenced by the

type of aggregate. The loss of strength is considerably lower when the aggregates are not

siliceous: siliceous aggregate concretes tend to spall due to their high thermal conductivity. As

long as no spalling occurs, lightweight concrete, characterized by lower thermal conductivity,

behaves better than normal weight concrete when exposed at high temperatures.

The increase of temperature induces several of physical and chemical reactions in concrete,

which combined with the thermal incompatibility of their constituents, leads to a global

strength reduction of concrete. Therefore, further investigations on the behaviour of LWAC at

high temperature are needed.

3

This dissertation aims characterizing the behaviour of LWAC after exposed at high

temperatures and their relative performance when compared to normal weight concrete of

equal composition.

2. EXPERIMENTAL PROGRAM

Two types of concrete were produced: lightweight aggregate concrete (L450) and normal

weight concrete (REF450). Moreover, this study also involved the characterization of several

specimens produced by Bogas (2011), which were subjected to the temperature and relative

humidity conditions of the laboratory, during 6 years.

Three types of lightweight concretes with different types of aggregates, designated by L450,

A450 and AL450 were analysed and compared with two different types of normal weight

concretes, REF450 and REF350. Their compositions are listed in table 1.

Table 1 - Mixture proportion of concretes

Designation of

mixture

Materials

Coarse aggregate

(l/m3)

Coarse sand

(kg/m3)

Fine sand

(kg/m3)

Cement

(kg/m3)

Effect.

w/c

L450 350 593 254 450 0.35

A450 350 583 250 450 0.35

AL450 350 211(LWF) 280 450 0.35

Designation of

mixture

Coarse gravel

(70%)

Fine gravel

(20%)

Coarse sand

(kg/m3)

Fine sand

(kg/m3)

Cement

(kg/m3)

Effect

w/c

REF450 736 185 592 254 450 0.35

REF350 0.45 735 184 652 280 350 0.45

The residual mechanic properties in LWAC was measured in different temperature stages,

T200, T400 and T600, corresponding to a maximum exposure temperature of 200, 400 and

600 °C, over 60 minutes. The temperature is increased at a rate of 2 °C/min.

The concretes were characterized in terms of theirs compressive strength, tensile strength (by

diametral compression) and modulus of elasticity. The weight and the capillarity water

absorption were measured before and after exposure to high temperatures, in order to

evaluate the residual resistance capacity of the produced concrete.

2.1. Curing and heating regimes

Despite the specimens produced by experimental campaign of Bogas (2011), further, 150 mm

cubic specimens (150 mm) were prepared to determine the effect of different temperatures

4

on compressive strength. Cylinders specimens (ø 150 x 300 mm) were also produced to

determine the effect of different temperatures on splitting tensile strength, modulus of

elasticity and capillarity water absorption. For each temperature, three specimens were

produced.

For each mix, the cubic and cylindrical specimens were demolded after 24 h ± 4 h and then

water cured for 28 days (temperature = 20 °C; relative humidity = 95%). Subsequently,

specimens were moved to a dry chamber (temperature = 20 ± 2 °C; relative humidity = 50 ±

5%), where they were kept for 772 days. It should be noted that the fire exposure of

specimens in a saturated state or with high moisture content would most likely lead to

excessive spalling.

In order to monitor the evolution of the temperature inside the specimen when exposed to

high temperatures, type K thermocouples were introduced inside the specimens.

After curing, the specimens were heated in an electric furnace (Figure 2) up to 200, 400 and

600 °C. Each temperature was maintained for 1h to achieve the thermal steady state. The

heating rate was about 2 °C/min. The specimens were allowed to cool naturally at the room

temperature for 1 day. Then they were moved to a dry chamber.

Figure 2 – Provision of the samples in the electric furnace

2.2. Tests on fresh concrete

The workability of fresh concrete was determined by means of the Abrams cone test (EN

12350-2, 2002). The fresh density was determined according to EN 12350-6 (2002). Table 2

shows the main results.

5

Table 2 - Slump and density

Designation of

mixture

Slump

(cm)

Density

(kg/m3)

L450 19,5 1841

REF450 21,5 2351

L450 – B 19,7 1830

AL450 – B 18 1519

A450 – B 18,4 1940

REF450 - B 17,7 2411

REF350 - B 2356

To simplify the reading of the results, the compositions produced in Bogas (2011) are identified

by their names followed by the letter "B".

2.3. Tests on hardened concrete

Tests were carried characterization, strength and modulus of elasticity at 28 days. Table 3

shows the main results.

Table 3 - Compressive strength and modulus of elasticity at 28 days

Designation of

mixture

Compressive strength

at 28 days (MPa)

Modulus of elasticity

at 28 days (GPa)

L450 45,8 ± 1,88 23,7 ± 0,57

REF450 72,2 ± 5,84 43,2 ± 0,74

As expected, the compressive strength of normal density concrete is higher than that of

lightweight concrete of the same composition, because of their lower strength aggregates. The

modulus of elasticity of normal density concrete is higher than that of lightweight concrete,

due to the lower stiffness of the LWA than that of NWA.

2.3. Tests on hardened concrete after thermal exposure

In order to evaluate the post-fire residual mechanical properties of concrete, the following

tests were carried out: compressive strength (EN 12390-3, 2003), splitting tensile strength

(EN 12390-6, 2003), elasticity modulus (LNEC E-397, 1993) and capillarity water absorption

(LNEC E 393, 1993). Test results of residual mechanical properties are presented in chapter 3.

These values were compared with those values obtained at the reference temperature (T20).

6

3. RESULTS AND DISCUSSION

Table 4 shows the residual mechanical properties of each type of concrete, for the different

levels thermal expositions.

Table 4 - Residual mechanical properties

Residual mechanical

properties Concrete

Thermal exposure

T20 T200 T400 T600

Compressive

strength (MPa)

L450 –B 46,7 ± 6,27 49,3 ± 0,94 - -

AL450 – B 37,6 ± 0,62 37,8 ± 0,35 - -

A450 – B 69,2 ± 2,21 68,8 ±2,44 66,2 ± 1,41 -

REF450 – B 75,9 ± 6,67 73,4 ± 7,40 71,9 ± 3,74 42,0 ± 19,06

REF350 - B 67,5 ± 2,77 59,2 ± 2,22 57,0 ± 1,71 40,0 ± 12,18

Splitting tensile

strength (MPa)

L450 1,2 ± 0,12 1,2 ± 0,16 0,8 -

REF450 2,1 ± 0,16 1,7 ± 0,25 1,1 ± 0,09 -

Elasticity modulus

(GPa)

L450 22,4 ± 0,39 20,4 ± 0,11 10,5 -

REF450 43,8 ± 2,14 40,5 ± 0,48 19,6 ± 0,15 -

The results of the residual compressive strength after thermal exposure for the different

concrete compositions are shown in Figure 3.

Figure 3 – Residual compressive strength

The results indicate that the increase of temperature caused a general decrease in the residual

compressive strength. However, after exposure to 200 °C, the residual compressive strength of

concrete L450 increased by about 6% compared to the reference ambient temperature. After

exposure to 400 °C, the compressive strength of concretes REF450, REF350 and A450

decreased about 10%, 15% and 5%, respectively. After exposure to 600 °C the reference

concretes presented a rather high level of degradation. The worst results were obtained for

concrete REF450 in which the compressive strength reduction was around 45%.

0,4

0,5

0,6

0,7

0,8

0,9

1

1,1

20 200 400 600

Re

sid

ua

l co

mp

ress

ive

str

en

gth

(-)

Temperature (°C)

REF450

REF350

L450

AL450

A450

7

It is also worth mentioning the occurrence of spalling in concrete compositions AL450 and

L450 for temperatures of about 350 °C.

The results obtained for the residual tensile strength are depicted in Figure 4, for compositions

REF450 and L450, only for exposure temperatures of 200ºC and 400ºC (for higher

temperatures, spalling occurred).

Figure 4 – Residual tensile strength

Figure 4 shows a reduction of tensile strength with increasing temperature, as expected. When

exposed to 200 °C, the tensile strength reduction for lightweight concrete L450 was about

24,5%, while for concrete REF450 it was 17%.When exposed to 400 °C there was a reduction of

approximately 50% in both types of concrete.

The results obtained for the modulus of elasticity after the different thermal exposures are

presented in Figure 5. As for the splitting tensile strength, and for the same reason, results are

presented only up to 400ºC and for concrete L450 and REF450.

Figure 5 shows that as expected there is a reduction in the modulus of elasticity with the

temperature increase. The reduction was very similar for both compositions, being about 10%

after exposure to 200 °C. After exposure to 400 °C the reduction was higher, being more than

50% compared to the reference temperature. As expected, the reduction experienced by the

modulus of elasticity is higher than that suffered by the compressive strength. This can be

explained by the higher sensitivity of the modulus of elasticity to micro cracking during thermal

exposure.

0,4

0,5

0,6

0,7

0,8

0,9

1

20 200 400

Re

sid

ua

l te

nsi

le s

tre

ng

th (

-)

Temperature (°C)

REF450

L450

8

Figure 5 – Residual modulus of elasticity

The results concerning the weight of cubic specimens before and after thermal exposure are

shown in Figure 6. As expected, the weight loss increased with higher temperatures of

exposure.

Figure 6 – Relative weight loss

The weight loss was higher in concrete REF350, which suffered a mass reduction of 4,3% after

exposure to 600ºC. However, for the exposure to 200 °C, there was a similar weight loss

between concretes L450, REF450 and REF350, all presenting reductions of around 1,5%. For

that exposure concrete AL450 presented a weight reduction of about 3%. Due to the

occurrence of spalling in lightweight concretes AL450 and L450, for higher temperatures it was

not possible to measure their weight.

Figure 7 presents the increase of water absorption by capillarity after exposure to elevated

temperatures. It can be seen that exposure to increasing temperatures results in an increase of

0

0,2

0,4

0,6

0,8

1

1,2

20 200 400Re

sid

ua

l m

od

ulu

s o

f e

last

icit

y

Temperature (ºC)

L450

REF450

0,95

0,96

0,97

0,98

0,99

1

20 200 400 600

Re

lati

ve

we

igh

t lo

ss (

-)

Temperature (°C)

L450

REF450

REF350

A450

AL450

9

the water absorption. This can be justified by the porosity increase and cracking due to

internal stresses, which are both caused by the thermal exposure.

Figure 7 – Water absorption by capillarity

The similar results obtained for the different types of concrete allows concluding that, when

spalling does not occur, the deterioration is similar in conventional concrete and lightweight

concrete, i.e. there are no significant differences in their microstructure.

4. CONCLUSIONS

The results obtained in this study show that after exposure to elevated temperatures, the

mechanical properties of the different types of concrete are reduced, and the magnitude of

such reduction increases with the exposure temperature. The replacement of traditional

aggregates by lightweight aggregate generally resulted in an increase in the occurrence of

spalling for temperatures above 350 °C. Results obtained for lightweight concrete L450

together with visual observations after thermal exposure, show that the higher susceptibility

to spalling of this type of concrete does not involve a higher degradation of its mechanical

properties. The occurrence of spalling in lightweight concrete is due to its lower tensile

strength, higher moisture content and the development of higher thermal gradient during heat

exposure (confirmed in the tests).

0,0000

0,0005

0,0010

0,0015

0,0020

0,0025

0,0030

0,0035

0,0040

0,0045W

ate

r a

bso

rpti

on

by

ap

illa

rity

(g

/mm

2)

L450-T20

L450-T200

L450-T400

L450-T600

REF450 - T20

REF450 - T200

REF450-T400

10

When spalling does not occur, the deterioration of conventional concrete and lightweight

concrete is similar. In fact, the residual strength is generally higher in lightweight concrete than

in conventional concrete due to the higher thermal compatibility of the constituents of the

former type of concrete. In fact, the difference between the coefficients of thermal expansion

of the aggregate and the cement paste in the conventional concrete is higher than in

lightweight concrete, making the former type more prone to cracking.

As expected, the lightweight concrete with lightweight fine aggregates AL450 presented the

worst performance due to its lower tensile strength and higher moisture content.

5. REFERENCES

ACI213R (2003) - Guide for Structural Lightweight-Aggregate Concrete”. Amer. Concrete

Institute.

Anderberg, Y., 1997. “Spalling phenomena of HPC and OC”. International Workshop on Fire

Performance of High-Strength Concrete, Gaithersburg, MD.

Bogas, J. A., 2011. “Characterization of Structural Concrete made of Lighhtweight Expanded

Clay Aggregate”. PhD Thesis. Instituto Superior Técnico, Universidade Técnica de Lisboa,

Lisboa. (in Portuguese)

Chandra, S., e Berntsson, L., 2003. “Lightweight aggregate concrete. Science, Technology and

Applications”. Noyes publications-Wiliam Andrew Publishing, USA, 2003.

Chen, H-J, Yen, T., Lai, e T-P., 1995. “A new proportion method of light-weight aggregate

concrete based on dividing strenght”. International Symposium on structural lightweight

aggregate concrete,20-24 June . Sandefjord, Norway: Editors: I. Holand et al.

EN12390-6, 2000. “Testing hardened concrete-Part 6: Tensille Splitting strength of test

specimens”European Committee for standardization CEN, English version.

EN 13055-1, 2002. - Lightweight aggregates - Part1: Lightweight aggregates for concrete,

mortar and grout." European Committee for standardization CEN.

EN 1992-1-1, 2004 - Eurocode 2: Design of concrete concrete structures - Part 1-1: General

rules and rules for buildings”. European Committee for standardization CEN.

EuroLightConR2, 1998. “LWAC Material Properties, State-of-the-Art.” “European Union – Brite

EuRam III, BE96-3942/R2, December.

Faust, T, 2000. “Properties of diferent matrixes and LWAs and their influences on the

behaviour of structural LWAC.” Second International Symposium on structural lightweight

aggregate concrete, 18-22 June. Kristiansand, Norway: Editors: Helland et al.

11

FIP, 1983. "FIP Manual of Lightweight Aggregate Concrete”. Fédération Internationale de la

Précontrainte (FIP), second edition, Surrey University Press. 259 p.

Holm, T. A. e Bremner, T. W., 2000. “State-of-the-art report on high-strength, high-durability

structural low-density concrete for applications in severe marine environments.” Us Army

corps of engineers. Structural Laboratory, ERDC/SL TR-00-3.

LNEC E-393, 1993 - Concrete: Determination of water absorption by capillarity. LNEC, Lisboa.

(in Portuguese)

LNEC E-397, 1993 – Concrete: Determination of elasticity modulus in compression. LNEC,

Lisboa.

Neville, A.M., 1995. “Properties of Concrete”. Fourth edition, Longman.

Newman, J.B., 1993. "Properties of structural lightweight aggregate concrete." In Structural

Lightweight Aggregate Concrete, by J.L.Clarke, pp. 19-44. Chapman & Hall, 1993.

NP EN 12350-2, 2002 - Testing fresh concrete: Slump-test. IPQ, Lisboa. (in Portuguese)

NP EN 12350-6, 2002 - Testing fresh concrete: Density. IPQ, Lisboa.

NP EN 12390-3, 2003 - Testing hardened concrete: Compressive strength. IPQ, Lisboa. (in

Portuguese)

RILEM TC HTC, 2004 – Behavior of concrete at high temperatures - Part 1 - Ordinary concrete.

A Report of the State-of-the-Art.

Schneider, U., 1988. “Concrete at high temperatures: A general review”. Fire Safety Journal, v.

13.

Zhang, M-H, e Gjørv, Odd E., 1989. “Characteristics of lightweight aggregates for high strength

LWA concrete.” Materialutvikling Hoyfast Betong. Report Nº2.2. STF70 A92022. 41 p.