The International Conference of Composites on Construction (CCC 2003)
Seismic Resistant Properties of
Lightweight Aggregate Concrete
Chung-Ho Huang, and How-Ji Chen*
Department of Civil Engineering
National Chung-Hsing University
Taichung, Taiwan, R.O.C.
ABSTRACT
Lightweight aggregate concrete possesses lower unit weight and better damping
characteristic than normal weight concrete. Using it as a building material can enhance the
seismic resistance of structures. This research investigates the seismic resistant properties of
lightweight aggregate concrete and compares with those of normal weight concrete.
The test results of lightweight aggregate concrete and normal weight concrete show that
the unit weight of reinforced lightweight concrete is 20% approximately lower than that of
reinforced normal weight concrete at same strength level. For low strength concrete near 20
MPa, lightweight aggregate concrete appears larger damping ratio. The stiffness of
lightweight aggregate concrete is similar to that of normal weight concrete. As the concrete
strength is higher near 40 MPa, lightweight aggregate concrete has similar damping ratio to
normal weight concrete but the stiffness is lower. Moreover, the natural frequency and
damping ratio of reinforced concrete beam are lower than those of plain concrete beam
because of the reinforcing bars effect.
Keywords：lightweight aggregate concrete、stiffness、natural frequency、damping ratio
*correspondent
How-Ji Chen
Associate Professor of Department of Civil Engineering, National Chung-Hsing University
NO.250, Kuo-Kwang Road, Taichung 40227, Taiwan
Tel: 886-4-22859390
Fax: 886-4-22855610
E-mail: hjchen@mail.ce.nchu.edu.tw
1
Introduction
The seismic force is a response of inertia of ground acceleration. In general,
the magnitude of internal force of member due to seismic force is related to the
seismic resistance properties of structure such as the mass, stiffness, nature
frequency and damping ratio.[1-3] For the seismic force design, to know the
seismic resistance properties of structure building is very important.
Using of lightweight aggregate concrete (LWAC) in building structure has
many advantages, especially reducing the mass of structure and possesses better
damping characteristics.[4-6] the lightweight building structure has smaller
seismic force when subjected to earthquake. For the seismic resistance of
concrete per unit weight, lightweight aggregate concrete is better than normal
weight concrete (NWC). This research investigates the seismic resistant
properties of lightweight aggregate concrete such as unit weight of concrete,
stiffness, nature frequency and damping ratio. The results are compared with
those of normal weight concrete.
Experimental program
A total of 32 flexural reinforced LWAC beams as well as NWC beams were
made. In the meantime, 18 non-reinforced concrete beams were also made. The
size of all specimens was 150 mm×200mm×1500mm. The test items included
both damping and frequency tests. Besides, the reinforced concrete beams were
also tested for stiffness. The summary of the specimen quantity are shown in
Table 1.
Materials
The mixture proportions for LWAC and NWC are shown in Tables 2 and 3.
2
Type 1 Portland cement was used with natural sand having a fineness modulus
of 2.67. The coarse lightweight aggregate used was expanded clay with
maximum size of 12.5 mm. The aggregate properties are given in Table 4.
Two sizes of the longitudinal reinforcement were used in this investigation
(no.4 and no.6 deformed steel bars). The stirrups used were no.6 deformed steel
bars of Grade 40. The different yield strength in tension was 283 MPa and 445
MPa for the longitudinal reinforcements and 281 MPa for the stirrup
reinforcement.
Casting and curing
The concrete was placed in two layers in the beam and was appropriately
vibrated. Besides, six 100 mm*200 mm cylindrical specimens were also cast.
Immediately after casting, all specimens covered with polyethylene sheets to
avoid escape of moisture. Twenty-Four hours after casting, the beams and
cylindrical specimens were stripped and moved to curing room with 100% RH,
23℃ for 28 days curing.
Specimen details and testing
The beams were divided into two groups (reinforced beam and plain
concrete beam without reinforcement). The specimen details are shown in
Figure 1. The tested variables included concrete type, concrete strength, space
of web reinforcement, amount of tensile reinforcement. The summary of the
specimen quantity are shown in Table 1.
The test method of compressive strength and modulus of elasticity of
concrete is in accordance with ASTM C469. Reinforced concrete beam was
3
loaded with a central point load in the stiffness test. At each end of a reinforced
beam, a hinge was used to allow the flexure beam to rotate freely. A linear
voltage differential transducer (LVDT) was used to measure the vertical
deflection at mid-span section. The span was 142 mm in length. Details of the
test setup are shown in Figure 2. The total load on the test facility was increased
linearly with a loading speed of 200 N/sec. When the total load reached 8000 N,
stop test and release load. The outputs from the load cell and LVDT were
continuously recorded by use of a computer and a 7V14M data acquisition
system.
In the frequency and damping tests, the bounded condition of the specimens
was similar to the stiff test. The specimens were support by two hinges, as
shown in Figure 3. An accelerometer was fixed to the central point of the span
for measure the variation of acceleration. Using a hammer beat the specimen;
let it induce a slight free vibration. The frequency and damping of the
specimens were acquired by the analysis of the decayed acceleration. The
analytic method was in accordance with the theory of structural dynamic.
Results and Discussions
The results of the basic properties tests on reinforced concrete beams are
summarized in Table 4. Table 5 summarizes frequency and damping of all
beams.
The mechanical properties of concrete
As shown in Table 4, the concrete compressive strength of LWAC was
varied between 27.4 MPa and 43.5 MPa, and varied between 23.1 MPa and 43.7
MPa for NWC. Although the actual compressive strength is not complete
4
correspond with design strength (20 MPa level and 40 MPa level), the
compressive strength of LWAC is close to NWC both at 20 MPa level and 40
MPa level. It can accept that LWAC and NWC have similar compressive
strength and can proceed to compare other characteristics under the same
strength level.
The modulus of elasticity of LWAC is increasing with increase the concrete
compressive strength. Moreover, at the same concrete strength level, the
modulus of elasticity of LWAC is smaller than NWC.
The results of unit weight of reinforced beam are also given in Table 4. The
unit weight of reinforced LWAC beam was varied between 1800 kg/m3 and
2000 kg/m3, and was about 20% lower than that of reinforced NWC beam.
The stiffness of reinforced beam
This test measured the stiffness of the simple beams; the beams were loaded
with a central point load. The central point load does not exceed the
proportional limit. The stiffness (K) was defined as the slope of the
load-deflection curve (P-Δ curve), K=P/Δ. The results of the stiffness are
summarized in Table 4. Analyses of the test data can obtain many varied graph,
as shown in Figures 4~6.
Figure 4 shows the unit weight of reinforced LWAC beam is lower than
reinforced NWC beam as concrete strength at 20 MPa level, but the stiffness of
reinforced LWAC beam is similar to reinforced NWC beam and the test values
are between 18000kN/m and 27000kN/m. As the concrete strength attains to
40MPa level, for reinforced NWC beams, as unit weight increases, are effective
in improving stiffness of simple beams. For reinforced LWAC beams, are
ineffective in improve stiffness by increasing unit weight.
From the Figure 5 it can be seen that the stiffness of reinforced LWAC beam
5
is similar to reinforced NWC beam under the same lower concrete strength (20
MPa level). As the concrete strength is higher (at 40 MPa), the stiffness of
reinforced NWC beam is 20% higher than that of reinforced LWAC beam. From
the two regression lines (in Fig. 5) it can be found that the slope of NWC beam
stiffness is steeper than that of LWAC beam. It means that the stiffness of
reinforced NWC beam increases effectively with increasing the concrete
strength. However, it is ineffective for reinforced LWAC beam.
The elastic formula of simple beam defection is given follows:  
PL3
48EI
From the formula it can be known that the stiffness (K=P/Δ) is function of
the material property (modulus of elasticity E), span of beam (L) and
cross-section (the beam’s moment of inertia I). When the simple beams have the
same cross-section and equal span, the stiffness of simple beam will mainly
relate to modulus of elasticity. Using different materials in reinforced beams
will change their stiffness; such as using lightweight aggregate concrete and
normal weight concrete because their moduli of elasticity are different.
Table 4 shows modulus of elasticity of LWAC is 16300 MPa and 21400
MPa for NWC when the concrete strength at 20 MPa. As the concrete strength
attains to 40MPa level, modulus of elasticity of LWAC is 19800 MPa and 25500
MPa for NWC. The relationship between the stiffness of reinforced beam and
modulus of elasticity of pure concrete beam is shown in Figure 6. The stiffness
of reinforced NWC beam increases effectively with increasing the modulus of
elasticity of concrete. The stiffness of reinforced LWAC beam does not have the
same behavior. As the concrete strength is lower (at 20MPa level), the ratio of
the modulus of elasticity of LWAC to that of NWC is 1.32, it is higher than the
ratio of the stiffness of reinforced LWAC beam to the stiffness of reinforced
6
NWC beam (which is about 0.97). As the concrete strength at 40 MPa level, the
ratio of the modulus of elasticity (about 1.29) is similar to the ratio of the
stiffness (about 1.19). Therefore, infer the ratio of the stiffness from the ratio of
the modulus of elasticity is suitable at high strength concrete, but it may be fail
at low strength concrete.
The frequency and damping ratio of beams
The frequency and damping ratio are the basic material property in seismic
assistance analysis. An accelerometer was used to measure the variation of
acceleration to investigate the frequency and damping ratio of the free vibrated
beams. Every specimen tested ten times on the frequency test and damping ratio
test respectively. The experimental results are shown in Table 5.
The analysis of frequency
At low strength concrete level (20 MPa level), the average frequency of
LWAC and NWC beam without reinforcement are 831 rad/sec and 821 rad/sec
that were obtained from average of ten tests of two specimens. As the concrete
strength is higher, the average frequency of LWAC beam is 925 rad/sec and 967
rad/sec for NWC beam. The results mean that increasing the concrete strength
increases the frequency of concrete beam. The main reason is high strength
concrete possess more dense texture than low strength concrete.
For the reinforced concrete beams, the average frequency of reinforced
LWAC beam is 748 rad/sec that is higher than the average frequency of
reinforced NWC beams (701 rad/sec) at lower concrete strength level (20MPa).
As the concrete strength is higher, the average frequency of reinforced LWAC
beam is 746 rad/sec and 723 rad/sec for reinforced NWC beam.
Based on the dynamic theory, the frequency (ω) is function of the mass (m)
7
and stiffness (K) of member ( ω 
K
). The frequency increases with increasing
m
the stiffness of structure or decreasing the mass of member. As the concrete
strength is lower (at 20MPa level), the stiffness of reinforced LWAC beam is
similar to reinforced NWC beam, and the unit weight of LWAC beam is lower
than NWC beam. Therefore, the frequency of reinforced LWAC beam is higher
than the frequency of reinforced NWC beam. As the concrete strength attains to
40MPa level, the stiffness and unit weight of reinforced LWAC beam are both
lower than reinforced NWC beam. So, they have a similar frequency.
It can be seen that the frequency of reinforced concrete beam seem to have
no relationship with space of web reinforcement and amount of tensile
reinforcement. However, the frequency of reinforced concrete beam is about
10%~25% lower than pure concrete beam because installed reinforcement.
The analysis of damping ratio
As the concrete strength is lower (at 20MPa level), the damping ratio of
LWAC beams is 8.11% and 6.89% for NWC beams. LWAC is better than NWC
in damping characteristic. As the concrete strength at 40MPa level, LWAC
beams and NWC beams have damping ratios of 7.13% and 7.43% respectively.
For the reinforced concrete beams, the damping ratio of reinforced LWAC
beams is varied between 2.52% and 4.22%. As well as reinforced NWC beams,
is varied between 1.73% and 3.85%. At lower concrete strength level (20MPa)
reinforced LWAC beam is better than reinforced NWC beams in damping ratio.
As the concrete strength attains to 40MPa level, they posses a similar behavior.
From Table 6 it can also be seen that the damping ratio of reinforced
concrete beam seem to have no relationship with space of web reinforcement
and reinforcement ratio. However, the damping ratio of reinforced concrete
8
beam is lower than pure concrete beam because installed reinforcement.
Conclusions
Experimental results of seismic resistant properties of lightweight aggregate
concrete are presented. On the basis of results obtained in this study the
following conclusions can be drawn.
1.
The stiffness of reinforced NWC beam increases with increasing concrete
strength and modulus of elasticity of concrete. For reinforced LWAC beam
is ineffective. At lower concrete strength level (20MPa), the stiffness of
reinforced LWAC beam is close to that for reinforced NWC beam. As the
concrete strength attains to 40MPa level, the stiffness of reinforced NWC
beam is 20% higher than that of reinforced LWAC beam.
2.
For plain concrete beams, increasing concrete strength increases the
frequency. At lower concrete strength level (20MPa level), the frequency of
LWAC beam is near that for NWC beam. As the concrete strength attains to
40MPa level, the frequency of LWAC beam is lower than that of NWC
beam. Moreover, at lower concrete strength level (20MPa level), the
damping ratio of pure LWAC beam is better than that for NWC beam. As
the concrete strength attains to 40MPa level, the damping ratio of LWAC
beam is close to that for NWC beam.
3.
For reinforced concrete beams, at lower concrete strength level (20MPa
level), the frequency and damping ratio of reinforced LWAC beam are
higher than those for reinforced NWC beam. As the concrete strength
attains to 40MPa level, the frequency and damping ratio of LWAC beam are
close to those for NWC beam.
4.
Comparing the seismic resistance properties between LWAC and NWC,
9
unit weight of lightweight aggregate concrete is about 20% lower than that
for NWC at the same concrete strength. As the concrete strength is at
20MPa level, LWAC is similar to NWC in stiffness and possesses better
damping characteristic. At higher concrete strength level (at 40MPa level),
LWAC is similar to NWC in damping characteristic, but LWAC is lower in
stiffness.
References
1. Bertero, V. V., and Popov, E. P., "Seismic Behavior of Ductile Moment -Resisting
Reinforced Concrete Frames," Reinforced Concrete Structures in Seismic Zones, SP-53,
American Concrete Institute, Detroit, PP. 247-291(1977).
2. Viwathanatepa, S., Popov, E. P., and Bertero, V. V., "Seismic Behavior of R. C. Interior
Beam-Column Subassemblage," EERC Report No. UCB/EERC-79/14, Earthquake
Engineering Research Center, University of California, Berkeley (1979).
3. Forzani, B., Popov, E. P., and Bertero, V. V., "Hysteretic Behavior of Lightweight
Reinforced Concrete Beam-Column Subassemblages," EER Report No. UCB/EERC
79/01, Earthquake Engineering Research Center, University of California, Berkeley
(1979).
4. V. V. Bertero, E. P. Popov and B. Forzani, "Seismic Behavior of Lightweight Concrete
Beam-Column Subassemblages," ACI Journal, PP. 44-52 (1980).
5. Sekhniakshivile, E. A., "On the Effective Use of Light Concrete and Reinforced Concrete
10
in Construction in Seismic Regions," Proceedings, Sixth World Conference on Earthquake
Engineering, V. 3, PP. 2034-2024(New Delhi, Jan. 1977).
6. Paramzim, A. M., and Gorovitz, I. G., "Analysis of Light-weight Concrete Use in
Seismic-Resistant Multistory Buildings," Proceedings, Sixth World Conference on
Earthquake Engineering, V. 3, PP. 2124-2125(New Delhi, Jan. 1977).
11
Table 1 The summary of the specimen quantity
LWAC beams
NWC beams
Design
Design
Beam
Tensile
Beam
Tensile
strength
Stirrups quantity
strength
Stirrups quantity
number
steel
number
steel
(MPa)
(MPa)
RLC20410
4-#4 #3@100mm
2
RC20410
4-#4 #3@100mm
2
RLC20610
4-#6 #3@100mm
2
RC20610
4-#6 #3@100mm
2
RLC20415 20
4-#4 #3@150mm
2
RC20415
4-#4 #3@150mm
2
20
RLC20615
4-#6 #3@150mm
2
RC20615
4-#6 #3@150mm
2
LC20
0
0
3
NC20
0
0
3
RLC40410
RLC40610
RLC40415
RLC40615
LC40
40
4-#4
4-#6
4-#4
4-#4
0
#3@100mm
#3@100mm
#3@150mm
#3@150mm
0
2
2
2
2
3
RC40410
RC40610
RC40415
RC40615
NC40
40
4-#4
4-#6
4-#4
4-#4
0
#3@100mm
#3@100mm
#3@150mm
#3@150mm
0
2
2
2
2
3
Illustration of Symbol：For example RLC20410, RLC: reinforced lightweight aggregate
concrete beam. 20: design strength 20MPa. 4: #4 longitudinal reinforcement，10: the space of
web reinforcement is 100mm。
Table 2 Mixt proportions of lightweight aggregate concrete
Design
30min
strength Cement Water Absorption Sand 3/4"~1/2" 1/2"~3/8" 3/8"~#4
(MPa)
(%)
20
297
194
29
734
179
213
175
40
480
194
29
664
166
197
162
Table 3 Mixt proportions of normal weight concrete
Design
strength Cement Water
(MPa)
20
280
197
40
410
196
12
Sand
Coarse
aggregate
781
675
1056
1056
Table 4 The basic properties of the specimens
LWAC beams
Com.
Elastic
strength modulus
(MPa) E(MPa)
27.4
43.5
16300
19800
Beam
number
NWC beams
Unit
Elastic
Stiffness Com.
strength modulus
weight
K (Ton/m) (MPa) E(MPa)
(kg/m3)
RLC20410
RLC20410
RLC20610
RLC20610
RLC20415
RLC20415
RLC20615
RLC20615
RLC40410
1790
1800
1940
1950
1780
1790
1910
1910
1910
1900
1770
2770
2480
2600
2050
2710
2260
2300
RLC40410
RLC40610
RLC40610
RLC40415
RLC40415
RLC40615
RLC40615
1910
2020
2020
1850
1860
1990
1980
2280
2530
2780
2220
2200
2430
2600
23.1
43.7
13
21400
25500
Beam
number
Unit
Stiffness
weight
K
(kg/m3) (Ton/m)
RC20410
RC20410
RC20610
RC20610
RC20415
RC20415
RC20615
RC20615
RC40410
2320
2310
2420
2420
2320
2330
2390
2380
2360
2070
1890
2470
2260
2520
1920
2490
2310
2800
RC40410
RC40610
RC40610
RC40415
RC40415
RC40615
RC40615
2380
2440
2450
2350
2320
2420
2420
2580
2970
2880
2820
2750
3080
3210
Table 5 The results of the frequency and damping ratio
fc'
(MPa)
LWAC
Frequency Damping
Beam
ω
ratioζ
number
(rad/sec)
(%)
865
7.59
LWAC20
797 (831) 8.61 (8.11)
830
RLC20410
27.4
RLC20610
RLC20415
RLC20615
712
733
810
821
710
764
703
732
983
LWAC40
43.5
fc'
(MPa)
NWC
Frequency Damping
Beam
ω
ratioζ
number
(rad/sec)
(%)
781
6.84
NC20
8.12
(723)
(816)
(737)
(718)
4.02
4.41
2.61
4.11
2.85
3.32
3.88
2.67
861 (821) 7.00 (6.89)
796
(4.22)
(3.36)
RC20410
23.1
RC20610
(3.09)
RC20415
(3.28)
RC20615
6.46
616
621
727
731
707
710
768
730
954
891 (925) 7.23 (7.13)
901
7.71
753
2.70
RLC40410
(728)
(2.60)
702
2.50
783
2.63
RLC40610
(760)
(2.52)
737
2.40
727
2.70
RLC40415
(742)
(2.70)
757
2.69
755
3.69
RLC40615
(756)
(4.17)
756
4.64
14
NC40
43.7
6.82
(619)
(729)
(709)
(749)
2.66
2.48
2.56
2.93
1.63
1.83
2.60
2.48
(2.57)
(2.75)
(1.73)
(2.54)
7.76
980 (967) 7.77 (7.43)
966
6.77
769
2.38
RC40410
(745)
(2.42)
721
2.45
706
2.30
RC40610
(712)
(2.55)
718
2.79
744
3.90
RC40415
(727)
(3.85)
710
3.79
720
2.30
RC40615
(708)
(2.48)
696
2.66
0.15m
0.15m
#3
#3
0.13m
0.13m
0.2m
0.2m
4-#4
4-#6
0.08m
0.08m
Figure 1. Specimen details
MTS
Load
Load cell
Specimen
Support
LVDT
0.71m
0.71m
Figure 2 Testing setup for stiff test
Accelerometer
Specimen
0.71m
0.71m
Figure 3 Testing setup for the test of frequency and damping ratio
15
34000
34000
(a) f'c = 20 MPa Level
32000
30000
RLC (40MPa)
RC (40MPa)
30000
28000
28000
K (kn/m)
K (kn/m)
(b) f'c=40 MPa Level
32000
RLC (20MPa)
RC (20MPa)
26000
24000
26000
24000
22000
22000
20000
20000
18000
18000
16000
1600
1800
2000
2200
2400
16000
1600
2600
The unit weight of reinforced concrete (kg/m3)
1800
2000
2200
2400
2600
The unit weight of reinforced concrete (kg/m3)
Figure 4 Unit weight of reinforced concrete effects on the stiffness
(K)
34000
RLC(20MPa)
RLC(40MPa)
RC(20MPa)
RC(40MPa)
32000
30000
K (kn/m)
28000
The growing line of K value of RLC
The growing line of K value of RC
26000
24000
22000
20000
18000
16000
20
25
30
35
40
45
f'c (MPa)
Figure 5 Compressive concrete strength effects on the stiffness (K)
34000
RLC(20MPa)
RLC(40MPa)
RC(20MPa)
RC(40MPa)
32000
30000
K (kn/m)
28000
The growing line of the stiffness of RLC
The growing line of the stiffness of RC
26000
24000
22000
20000
18000
16000
16000
18000
20000
22000
24000
26000
E (MPa)
Figure 6 Modulus of elasticity effects on the stiffness (K)
16
LWAC beams
NWC beams
Design
Design
Beam
Tensile
Beam
Tensile
Stirrups quantity
Stirrups quantity
strength
strength
number
steel
number
steel
(MPa)
(MPa)
RLC20410
4-#4 #3@100mm
2
RC20410
4-#4 #3@100mm
2
RLC20610
4-#6 #3@100mm
2
RC20610
4-#6 #3@100mm
2
20
20
RLC20415
4-#4 #3@150mm
2
RC20415
4-#4 #3@150mm
2
RLC20615
4-#6 #3@150mm
2
RC20615
4-#6 #3@150mm
2
LC20
0
0
3
NC20
0
0
3
RLC40410
RLC40610
RLC40415
RLC40615
LC40
40
4-#4
4-#6
4-#4
4-#4
0
#3@100mm
#3@100mm
#3@150mm
#3@150mm
Design 0
2
RC40410
2
RC40610
2
RC40415
2
RC40615
30min
3
NC40
40
4-#4
4-#6
4-#4
4-#4
0
#3@100mm
#3@100mm
#3@150mm
#3@150mm
0
strength Cement Water Absorption Sand 3/4"~1/2" 1/2"~3/8" 3/8"~#4
(MPa)
(%)
20
297 194
29
734
179
213
175
40
480 194
29
664
166
197
162
Design
30min
strength Cement Water Absorption Sand 3/4"~1/2" 1/2"~3/8" 3/8"~#4
(MPa)
(%)
20
297
194
29
734
179
213
175
40
480
194
29
664
166
197
162
Design
strength Cement Water
(MPa)
20
280
197
40
410
196
17
Sand
Coarse
aggregate
781
675
1056
1056
2
2
2
2
3