Compressive strength-evaluation-of-structural

1. Compressive strength evaluation of structural lightweight concrete
by non-destructive ultrasonic pulse velocity method
J. Alexandre Bogas ⇑
, M. Glória Gomes, Augusto Gomes
DECivil/ICIST, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
a r t i c l e i n f o
Article history:
Received 17 July 2012
Received in revised form 13 December 2012
Accepted 17 December 2012
Available online 3 January 2013
Keywords:
Lightweight aggregate concrete
Non-destructive tests
Ultrasonic pulse velocity
Compressive strength
Admixtures
a b s t r a c t
In this paper the compressive strength of a wide range of structural lightweight aggregate concrete mixes
is evaluated by the non-destructive ultrasonic pulse velocity method. This study involves about 84 differ-
ent compositions tested between 3 and 180 days for compressive strengths ranging from about 30 to
80 MPa. The influence of several factors on the relation between the ultrasonic pulse velocity and com-
pressive strength is examined. These factors include the cement type and content, amount of water, type
of admixture, initial wetting conditions, type and volume of aggregate and the partial replacement of nor-
mal weight coarse and fine aggregates by lightweight aggregates. It is found that lightweight and normal
weight concretes are affected differently by mix design parameters. In addition, the prediction of the con-
crete’s compressive strength by means of the non-destructive ultrasonic pulse velocity test is studied.
Based on the dependence of the ultrasonic pulse velocity on the density and elasticity of concrete, a sim-
plified expression is proposed to estimate the compressive strength, regardless the type of concrete and
its composition. More than 200 results for different types of aggregates and concrete compositions were
analyzed and high correlation coefficients were obtained.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
The non-destructive ultrasonic pulse velocity method has been
widely applied to the investigation of the mechanical properties
and integrity of concrete structures [1–7]. It is easy to use and re-
sults can be quickly achieved on site. The ultrasonic pulse velocity
(UPV) of a homogeneous solid can be easily related to its physical
and mechanical properties. Based on the theory of elasticity ap-
plied to homogeneous and isotropic materials, the pulse velocity
of compressional waves (P-waves) is directly proportional to the
square root of the dynamic modulus of elasticity, Ed, and inversely
proportional to the square root of its density, q, according to Eq. (1)
[7,8]. td is the dynamic Poisson’s ratio. Concrete is heterogeneous
and so these assumptions are not strictly valid. However, the high
attenuation in concrete limits the UPV method to frequencies up to
about 100 kHz [9], which means that compressional waves do not
interact with most concrete inhomogeneities [9,10]. In this case,
concrete can be reasonably regarded as a homogeneous material
[5].
UPV ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Ed
q
Á
ð1 À tdÞ
ð1 þ tdÞ Á ð1 À 2tdÞ
s
ð1Þ
According to Eq. (1), the relevant physical properties of materi-
als that influence pulse velocity are the density, elastic modulus
and td. Thus, correlations between the pulse velocity and the com-
pressive strength of concrete, fc, are based on the indirect relation
between this property and the elastic modulus, Ec. EN 1992-1-1
[11] suggests the expression Eq. (2) to relate Ec and fc, where q is
the oven-dry density.
Ec % 22 Á
fc
10
0:3
Á
q
2200
2
½GPaŠ ð2Þ
However, it is well known that the compressive strength and
elastic modulus may be influenced differently, depending on the
concrete composition. Therefore, the relation between UPV and fc
is not unique and can be affected by factors such as the type and
size of aggregate, physical properties of the cement paste, curing
conditions, mixture composition, concrete age and moisture con-
tent [8,12–17]. Ben-Zeitun [15] and Trtnik et al. [16] achieved bet-
ter correlations when they also took into account other variables
such as the w/c ratio, volume and size of aggregates, concrete
age and curing conditions. Thus, although in situ estimation of fc
from UPV is covered in EN 13791 [18], there is no standard corre-
lation between these properties. So far, the correlation between fc
and UPV must be calibrated for each specific concrete mix
[18,19]. Moreover, the heterogeneous nature of concrete caused
by the introduction of aggregates results in increased scatter, i.e.,
0041-624X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultras.2012.12.012
⇑ Corresponding author. Tel.: +351 218418226; fax: +351 218418380.
E-mail address: abogas@civil.ist.utl.pt (J.A. Bogas).
Ultrasonics 53 (2013) 962–972
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Ultrasonics
journal homepage: www.elsevier.com/locate/ultras

2. dispersive properties. This is why Philippids [20] found that the
ultrasound velocity increased 11% in concrete specimens through-
out the 15–200 kHz band.
Nonetheless, several relationships between UPV and fc have been
proposed, especially for normal density concrete (NWC)
[1,6,13,15,21,22]. Sturrup et al. [21] proposed a logarithmic relation-
ship between UPV and fc, while Price and Haynes [6], Phoon et al.
[13] and Ben-Zeitun [15] suggested linear relationships. However,
exponential relationships are the commonest [1,3,10,13,14,16,23].
The various relations proposed in the literature prove the different
influence of concrete composition on fc and UPV. For example, differ-
ent volumes of normal weight aggregate (NA) affect UPV but have
little, if any, influence on fc. Depending on the mix design, the higher
NA content can even cause a UPV increase and, at the same time, a
loss of compressive strength [14,16].
Most investigations have focused on NWC behavior. Published
studies involving lightweight concrete (LWC) are still limited. Nas-
ser and Al-Manaseer [24] reported expressions of the type fc =
aÁUPVb
for NWC and LWC produced with expanded clay aggregates.
The authors also showed that UPV depends on the concrete density,
which is lower in LWC than in NWC of the same compressive
strength. Chang et al. [10] established exponential relationships
between UPV and fc for LWC with two types of lightweight aggre-
gates. Hamidian et al. [25] found poor correlations when several
LWC mixes were analyzed together. Tanyidizi and Coskun [26]
used the analysis of variance (ANOVA) to study the influence of
curing conditions, maximum size of aggregate, mineral admixtures
and curing time on UPV and the compressive strength of light-
weight concrete. The maximum size of the aggregate was the main
parameter governing UPV and fc.
Expanded clay LWC is almost one hundred years old, and a lot of
old LWC structures that have been built since the 1950s, especially
in North America and Europe, now represent a major issue in terms
of maintenance and rehabilitation. Non-destructive ultrasound
pulse velocity tests have proved to be very helpful in the inspection
of old structures. However, the experience acquired in this field
and the correlations that have been built between the quality of
concrete and its UPV are essentially limited to NWC. Therefore,
due to the specificity of LWC, new correlations must be established
for this type of concrete, regardless the type of LWA. Knowledge of
general correlations between fc and UPV will be a major advance in
the inspection and assessment of existing LWC structures.
This study investigates the use of the non-destructive ultrasonic
pulse velocity method to assess the compressive strength of LWC
produced with different types of expanded clay aggregates. The
experimental work was comprehensive, testing at various ages
several concrete specimens produced from different compositions.
The influence of mix design parameters such as the water/binder
(w/b) ratio, type, volume and initial water content of aggregates
and type and volume of binder was analyzed. Finally, based on
the dependence of UPV on density and elasticity (Eq. (1)) and tak-
ing into account the empirical relationship between fc and Ec (Eq.
(2)), a general simplified expression is proposed and assessed that
relates fc and UPV, irrespective of the type of concrete, mixture
composition and test age.
2. Experimental program
2.1. Materials
Three Iberian expanded clay lightweight aggregates were ana-
lyzed: Leca and Argex from Portugal and Arlita from Spain. Their
total porosity, PT, particle density, qp, bulk density, qb, and 24 h
water absorption, wabs,24h, are indicated in Table 1. They differ in
terms of porosity, geometry and bulk density, which makes it
possible to produce concrete with strengths ranging from about
25 to 70 MPa [27], thereby covering the most common structural
LWC. A more detailed microstructural characterization of these
aggregates can be found elsewhere [28,29].
Normal weight coarse and fine aggregates (NA) were also used.
For the reference NWC, two crushed limestone aggregates of differ-
ent sizes were combined so as to have the same grading curve as
Leca (20% fine and 80% coarse gravel). Fine aggregates consisted
of 2/3 coarse and 1/3 fine sand. Their main properties are listed
in Table 1. The two fractions of Argex were also combined to have
the same grading curve as Leca (35% 2–4 and 65% 3–8F, Table 1).
The maximum aggregate size was 12.5 mm. Cement type I 52.5
R, I 42.5 R, II-A/L 42.5, II-A/D 42.5 (8% of SF by weight), II-A/V
42.5 (20% of FA by weight) and IV-A 42.5 (8% SF and 20% FA)
according to EN 197-1 [30], were considered. Their main physical
and mechanical properties are listed in Table 2. For low w/b ratios,
a polycarboxylate based superplasticizer (SP) was used. A water
dispersed RHEOMAC VMA 350 nanosilica (NS) with an average den-
sity of 1.1 and about 16.1% solids content was also tested.
2.2. Concrete mixing and compositions
Based on an extensive study of the durability and mechanical
characterization of structural lightweight concretes produced with
different types of aggregates that was conducted at the Instituto
Superior Técnico [27], the ultrasonic pulse velocities of about 84
different compositions were measured. The compositions varied
in terms of type, volume (150–450 L/m3
), and initial wetting con-
ditions of aggregates (initially dry, pre-wetted and pre-soaked),
different water/binder (w/b) ratios (0.3–0.65), the types and
amounts of cement (300–525 kg/m3
), the types and volumes of
mineral admixtures (22% and 40% of fly ash (FA), 8% of silica fume
(SF) and 1.3% of nanosilica), the partial replacement of normal
weight coarse aggregates by lightweight aggregate (LWA) and also
the partial replacement of natural sand by lightweight sand (light-
weight sand concrete – LWSC).
The concretes were produced in a vertical shaft mixer with bot-
tom discharge. Except for initially dry or pre-wetted aggregates,
the LWA was pre-soaked for 24 h to better control the workability
and effective water content of the concrete. The aggregates were
then surface dried with absorbent towels and placed in the mixer
with sand and 50% of the total water. After 2 min of mixing, the
binder and the rest of the water were added. When used, the SP
was added slowly with 10% of water, after 1 more minute. The total
mixing time was 7 min.
All the concrete mixtures studied for this paper are listed in de-
tail elsewhere [27]. The main characteristics of each composition
are summarized in Table A1 in the appendix. The w/b ratio signifies
the effective water available for binder hydration. The denomina-
tions ‘NA’, ‘L’, ‘A’ and ‘Argex’ correspond to the mixes with normal
weight aggregate, Leca, Arlita and Argex. These denominations are
usually followed by the volume of binder and then by the w/b ra-
tio, when it differs from 0.35. The prefix ‘V’ refers to different vol-
umes of aggregate. The compositions were basically variations of a
reference mixture with 450 kg/m3
of binder, 158 L/m3
of water (w/
b = 0.35), 350 L/m3
of coarse aggregate (Leca, Arlita, Argex, NA) and
0.5–1.0% of SP. Except for LWSC, natural sand was used in combi-
nation with coarse LWA. For LWSC, the 2/3 coarse natural sand
was replaced by the lightweight sand indicated in Table 1 (Leca
0–3). Modified normal density concretes (MND) were produced
with partial replacement of NA by 35% and 65% of Leca or Arlita.
To study the influence of pre-wetting aggregate, some concrete
specimens with initially dry LWA (PD) or pre-wetted LWA (PW)
were also produced. The PD aggregate is added during mixing
and the PW aggregate is previously wetted for 3 min with 50% of
the total water before mixing.
J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 963

3. 2.3. Specimen preparation and test setup
For each mix at each age, three 150 mm cubic specimens were
tested for ultrasonic pulse velocity and then for compressive
strength according to EN 12390-3 [31]. After demolding at 24 h,
specimens were kept in water until testing, according to EN
12390-3 [31]. UPV measurements were performed on unloaded
wet specimens.
The ultrasonic pulse velocity was obtained by direct transmis-
sion according to EN 12504-4 [17]. The equipment used was the
portable ultrasonic non-destructive digital indicating tester (PUNDIT),
shown in Fig. 1 [8]. In this method an ultrasonic pulse is generated
by a pulse generator and transmitted to the surface of concrete
through the transmitter transducer. The time taken by the pulse
to travel through the concrete, tus, is measured by the receiver
transducer on the opposite side. The 54 kHz transducers were posi-
tioned in the middle of each opposing face, orthogonal to the direc-
tion of concreting. The propagation time of the ultrasonic waves
transmitted through the 150 mm cubic specimens was measured
with accuracy up to 0.1 ls. A digital readout is displayed in a
4-digit LCD. Finally, UPV is the ratio between the length traveled
by the pulse (150 mm) and the measured time, tus. A thin couplant
(solid vaseline) was used on the interface between transducers and
concrete to ensure good contact. Before each measurement the
equipment was calibrated with a cylindrical Perspex bar of known tus.
Three measurements were taken for each test specimen by
switching the position of the transducers between the two oppo-
site faces of the concrete cubes. For all mixes ultrasonic pulse
velocity was measured at 28 days. Tests were also performed at
1, 3, 7, 90 and 180 days on certain selected mixtures (Table A1).
3. Test results and discussion
All the average results of compressive strength, fc, and pulse
velocity, UPV, are listed in Table A1, for each composition at each
age. Fig. 2 summarizes the mean values of UPV and fc obtained
for each mixture, between 3 and 90 days. A total of about 208 aver-
age results were considered, involving different concrete strengths
ranging from about 30–80 MPa and UPV from 3.5 to 5.2 km/s.
Table 1
Aggregate properties.
Property Normal weight aggregates Lightweight aggregates
Fine sand Coarse sand Fine gravel Coarse gravel Leca 0–3 Leca 4–12 Argex 2–4 Argex 3–8F Arlita AF7
Particle dry density, qp (kg/m3
) 2620 2610 2631 2612 1060 1068 865 705 1290
Loose bulk density, qp (kg/m3
) 1416 1530 1343 1377 562 613 423 397 738
24 h water absorption, wabs,24h (%) 0.2 0.5 1.4 1.1 – 12.3 22.9 23.3 12.1
Total porosity, PT (%) – – – – 59 60 67 73 52
Granulometric fraction (di/Di) 0/2 0/4 4/6.3 6.3/12.5 0.5/3 4/11.2 4/8 6.3/12.5 3/10
Los Angeles coefficient (%) – – 33.3 30.5 – – – – –
Table 2
Main characteristics of cement, silica fume and fly ash.
Parameter Standard Fly ash Silica fume Cement I 52.5 R Cement I 42.5 R Cement II/A-L 42.5 R
Residue on the 45 lm sieve (%) EN 451-2 10.2 92.0a
1.1 4.7 8.3
Blaine specific surface (cm2
/g) EN 196-6 – – 5102 3981 4477
Compressive strength of reference mortar (MPa) 2 days
28 days
EN 196-1 – – 40.4 32.8 27.2
– – 62.7 54.9 51.4
Activity index at 28 days (%) EN 196-1 83.7b
106.7c
– – –
Activity index at 90 daysa
(%) EN 196-1 103.1 – – – –
Expansion (mm) EN 196-3 0.5a
– 0.5 0.5 0.5
Loss on ignition (LOI) (%) EN 196-7 6.5 3.7 1.64 3.06 5.34
SiO2 + A12O3 + Fe2O3 (%) EN 196-2 83.0 94.0 29.1 27.6 26.1
CaO (%) – 3.38 0.83 61.6 63.5 61.6
Free CaO (%) EN 451-1 0.36 Not detected 1.45 1.31 1.8
Density (g/cm3
) EN 196-6 2.33 2.25 3.11 3.11 3.05
a
Residue on the 90 lm sieve.
b
Mortar with CEM I42.5 R + 25% fly ash.
c
Mortar with CEM I42.5 R + 10% silica fume.
Fig. 1. Scheme of the ultrasonic pulse velocity measurement in concrete specimens.
y = 3.38e0.62x
R² = 0.61
20
30
40
50
60
70
80
90
3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4
UPV (Km/s)
Fig. 2. Relationship between UPV and fc for different concrete compositions and
different types of aggregate at ages between 3 and 90 days.
964 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

4. The coefficients of variation of UPV, CVUPV, for the specimens
measured at 28 days are also presented in Table A1. For other ages
the CVUPV is of the same order. As it can be seen, the CVUPV obtained
from 3 specimens of each composition at each age (three speci-
mens measured in three directions) was generally lower than 0.5.
This shows the lower variability of the UPV method and also the
homogeneity of the concrete specimens produced.
As expected, when different compositions, types of aggregate
and test ages are considered simultaneously there is a poor corre-
lation between UPV and fc (Fig. 2). Therefore, the influence of the
type and volume of aggregate, age of testing, w/b ratio and type
of binder are analyzed separately in the following sections.
3.1. Influence of type of aggregate
When the mixtures with different types of aggregate, light-
weight sand (LWSC) and the partial replacement of coarse NA by
LWA (MND) are analyzed separately, there is a natural increase
of the correlation coefficient (Fig. 3). Based on Eqs. (1) and (2)
and as documented in [27], the introduction of lightweight aggre-
gate has a greater impact on elasticity than on density, leading to
the reduction of UPV.
For similar values of UPV, the strength is higher in LWC of higher
density. Conversely, the lower the density of the LWA the higher
the UPV for a given compressive strength. This trend is likely to
be primarily related to the: lower proportional increment of UPV
in relation to fc, for higher strength levels; simultaneous reduction
of density and stiffness in LWC, which means a smaller variation of
UPV (Eq. (1)); slight variation of fc for LWC with rich mortars and
more porous aggregates; higher compacity of richer mortars in
more porous LWC of the same strength; small differences between
the ultrasonic pulse velocities of lightweight aggregates, UPVag;
higher water content in LWC with lower density aggregates.
The importance of the aggregate type is highlighted in Fig. 4,
where the UPV in reference mixes with a w/b ratio of 0.35 is com-
pared with that obtained for a mortar with an equivalent composi-
tion (Mortar_0.35 with the same w/c ratio and sand/cement ratio,
Table A1). The absence of coarse aggregates leads to a reduction of
UPV in NWC and the opposite effect in LWC. The difference is high-
er in NWC, which means the aggregate has greater influence on
this type of concrete. Assuming that the aggregate stiffness varies
with the square of its density, q2
ag [32], then the UPVag decreases
more or less in line with q0:5
ag (Eq. (1)).
Taking concrete as a two-phase composite material, let us as-
sume that the ultrasonic pulse velocity in concrete, UPVc, is related
to the ultrasonic velocity of the aggregate, UPVag, and the ultrasonic
velocity of the mortar, UPVm, according to Eq. (3) (series model,
[16]). tag and tm are the respective relative volumes of aggregate
and mortar. The influence of the transition zone paste/aggregate
is neglected.
1
UPVc
¼
tag
UPVag
þ
tm
UPVm
ð3Þ
Based on the UPV average values obtained at 28 days for the
mortar (UPVm = 4.5 km/s) and for the reference concretes A/L/Ar-
gex/NA450 with tag of 0.35 (Table A1 and Fig. 4), the UPVag values
are 3.6, 4.1, 4.1 and 6.3 km/s, respectively for Argex, Leca, Arlita
and normal aggregate (NA). Thus, the UPVag/UPVm ratio is 1.4 for
NA and only 0.9 for Leca and Arlita. This confirms that NWC is af-
fected more by the volume of aggregate. Moreover, the dispersion
effect caused by concrete heterogeneity should be lower in LWC.
On the other hand, since the NWC strength is essentially con-
trolled by the mortar, the UPV decreases with the volume of aggre-
gate, without a significant variation of fc, i.e., the relation between
UPV and fc strongly depends on the proportion of aggregate in the
mix. Thus, the correlation between fc and UPV has to be established
for each type of NWC with a given volume of aggregate. The same
is concluded by Lin et al. [14] and Popovics et al. [12].
LWC behaves differently. The strength is also affected by LWA,
and hence both UPV and fc decrease with the greater volume of
aggregate. Therefore, one would expect the relation between UPV
and fc to be less affected. However, although UPV varies in the same
direction as fc, they may progress differently. Since UPVag/UPVm is
close to unity, the fc variation can be higher than that of UPV. More-
over, the compressive strength of LWC is affected by the strength
level, whereas UPV is not. This is especially noticeable in LWC with
more porous aggregate (Leca and Argex) and higher strength levels,
since fc is limited by the capacity of LWA and cannot follow UPV.
However, this phenomenon occurs later in LWC with less porous
aggregates (Arlita). That is why the regression curves of Fig. 3,
for different types of LWA, diverge from each other with the incre-
ment of fc. The mortar quality has a greater impact on the strength
evolution of the higher density LWC. As expected, UPV and fc de-
crease with the partial replacement of natural sand by lightweight
sand. The simultaneous inclusion of normal and lightweight aggre-
gates leads to values between those obtained for NWC and LWC
(Fig. 3).
Data from Fig. 3 can also be approximated by more common
exponential relationships, with similar correlation coefficients
(Eqs. (4)–(7)). The estimation of fc by means of Eqs. (4)–(7) leads
to an average error of 5.5% for Argex, 4.9% for Leca, 7.3% for Arlita
and 6.3% for normal aggregate. The standard deviations of these er-
rors are respectively 3.4%, 4.6%, 5.3% and 5.8%. There were more
LWC compositions with Arlita, which is why the largest error
was obtained in this type of concrete.
Arlita : fcm ¼ 1:07 Á e0:92ÁUPV
; R2
¼ 0:82 ð4Þ
Leca : fcm ¼ 3:0Á0:63ÁUPV
; R2
¼ 0:82 ð5Þ
R² = 0.84
R² = 0.85
R² = 0.84
R² = 0.91
0
20
40
60
80
100
3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4
UPV (km/s)
Leca
Arlita
Argex
NWC
LWSC
MND(Leca)
MND(Arlita)
Mortar
Fig. 3. Different relationships between UPV and fc for each type of aggregate,
considering different compositions at ages between 3 and 90 days (Table A1).
20
30
40
50
60
70
80
4.0 4.2 4.4 4.6 4.8 5.0 5.2
UPV (km/s)
Leca
Arlita
Argex
NWC
Mortar
Fig. 4. Relationship between UPV and fc in reference concrete and in the respective
mortar of equivalent composition at 7 and 28 days (the same sand/cement ratio and
w/b ratio of 0.35). The volume of coarse aggregate in concrete is 350 L/m3
.
J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 965

5. Argex : fcm ¼ 1:65 Á e0:70ÁUPV
; R2
¼ 0:82 ð6Þ
Normal aggregate : fcm ¼ 0:023 Á e1:6ÁUPV
; R2
¼ 0:88 ð7Þ
3.2. Influence of concrete age
The fc and UPV trend for some illustrative mixes with different
w/b ratios and different types and amounts of aggregate is shown
in Fig. 5. VL250 is a reference mixture with 250 L/m3
of coarse Leca.
As expected, UPV and fc increase with curing time [13,33]. In fact,
since the pulse velocity through voids is lower than that through
solid matter, the greater the paste hydration the lower the volume
of pores and the greater the UPV [33].
High correlations are obtained when each concrete composition
is individually assessed. However, the correlation decreases when
different compositions are analyzed together. For example, there
is a greater dispersion when different w/b ratios are considered
in LWC with Leca (dashed line in Fig. 5). In fact, whereas Vus tends
to increase faster with age than fc, fc increases more with the w/c
ratio than Vus does. Therefore, the simultaneous consideration of
distinct ages and w/c ratios implies different relations between fc
and Vus. However, the relation between fc and UPV seems to be less
affected by the volume of aggregate (VL250 vs L450), contrary to
what is normally reported for NWC [14,16]. As mentioned before,
LWA affects both fc and Vus.
The concrete strength tends to increase faster than UPV, espe-
cially in NWC, where fc is not limited by the strength of the aggre-
gate (Fig. 5 and Table A1). The same is documented in [10,14,21].
The fc trend in LWC is less steep and hence less sensitive to small
changes in UPV. As shown in this study, the influence of each
mix design’s parameters must be analyzed at the same age, and
this is done in the next sections.
3.3. Influence of the w/c ratio
Fig. 6 shows the UPV at 28 days for each type of aggregate and
different w/c ratios. Since only one parameter of the mixture is
changed for each type of cement, the correlations are high. Mixes
with the same volume of coarse aggregate and the same type
and cement content were considered in LWC with Leca or Arlita.
Different w/c ratios were obtained by varying the amount of water
and the respective volume of sand. Mixes with the same volume of
water and coarse aggregate were considered in NWC. Different w/c
ratios were obtained by varying the amount of cement and the
respective volume of sand. This is why the UPV trend with the
w/c ratio is less pronounced in NWC (the higher w/c ratio is
partially offset by the greater volume of sand). Otherwise, the slope
of each aggregate curve should be similar. LWSC mixes are associ-
ated with different amounts of cement, sand and water.
When the regression analysis takes different water and cement
contents into account at the same time, there is a reduction of the
correlation coefficient (Figs. 7 and 8). As shown in Fig. 8, fc is less
sensitive than UPV to the type of w/c, i.e., fc tends to be less affected
by different amounts of water, sand and cement than UPV, for a gi-
ven w/c ratio. For the same w/c ratio and different cement con-
tents, UPV can vary by more than 100 m/s (Fig. 8). Therefore, the
relation between UPV and w/c also depends on how the w/c ratio
is changed. Furthermore, moisture content helps the propagation
velocity in concrete [27,34] but may affect compressive strength
negatively.
3.4. Influence of the volume of aggregate
For LWC, fc and UPV decrease as the volume of LWA increases
(Fig. 9). But UPV increases with the volume of aggregate in NWC.
The NWC compressive strength also increases, albeit only slightly,
with the volume of aggregate. An opposite trend is reported by
other authors [14,16], which may explain the better correlation ob-
tained in this work for NWC (Fig. 3).
As expected, differences are higher when different w/c ratios
and volumes of aggregate are considered at the same time
(Fig. 10). In lower density LWC (Leca), the relation between fc
and UPV seems to be less affected by the w/c ratio and the volume
of aggregate. Since the compressive strength of these concretes is
also affected by the aggregate, the variation of fc with w/c is lower
than in NWC and LWC of higher density.
R² = 0.97
R² = 0.93
R² = 0.90
R² = 0.98
R² = 0.96
R² = 0.95
R² = 0.85
20
30
40
50
60
70
80
90
3.0 3.5 4.0 4.5 5.0 5.5
UPV (Km/s)
Fig. 5. Relationship between UPV and fc at different ages (between 1 and 180 days)
for different w/b ratios (0.35, 0.45, 0.55), types and volumes of aggregate (250 and
350 L/m3
).
R² = 1.00
R² = 0.99R² = 0.97
R² = 0.96
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
0.25 0.35 0.45 0.55 0.65
UPV(km/s)
w/c
NWC
Arlita
Leca
LWSC
Fig. 6. UPV versus the w/c ratio for different types of aggregate at 28 days (w/c ratio
obtained by varying the amount of water – LWC with Leca or Arlita; w/c ratio
obtained by varying the cement content – NWC).
R² = 0.84
R² = 0.83
30
40
50
60
70
80
3.6 4.0 4.4 4.8
UPV (km/s)
Arlita
Leca
Fig. 7. Relationship between fc and UPV at 28 days for different w/c ratios (0.3, 0.35,
0.4, 0.45, 0.55) by varying the amount of cement and water (Arlita and Leca).
966 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

6. Moreover, the strength of LWC is more affected by the volume
of aggregate than that of NWC. In other words, UPV and fc are both
affected by the propagation velocity and the strength of aggregate
and mortar. Therefore, there is a greater interdependence between
UPV and fc in LWC than in NWC. However, when LWC reaches its
ceiling strength the behavior may change. After a given strength
level a further increase of fc is not meaningful, contrary to what
happens with UPV.
The LWC with less porous aggregates exhibits similar behavior
to that of NWC. This is because the limit strength of higher density
LWC, above which the fc is governed by the paste, is much higher
than that of LWC with less porous aggregates. As shown in
[27,35], up to about 60 MPa the compressive behavior of LWC with
Arlita is similar to that of NWC.
3.5. Influence of the type of binder
There is a high correlation between UPV and fc regardless the
type of mineral admixture (Fig. 11). The regression takes into
account LWC produced with different types of admixture (8% of sil-
ica fume – SF; 1.3% of nanosilica – NS; 22% and 40% of fly ash – FA)
tested at ages ranging from 7 to 180 days.
The densification of the porous structure was not detected in
LWC with silica fume or nanosilica, which was less efficient than
expected. It is likely that there was no effective dispersion of such
admixtures. Moreover, the strength limitation imposed by LWA
and the better quality of the aggregate–paste transition zone in
LWC also play a part in the lower efficiency of SF and NS. It is also
shown that the replacement of cement by fly ash leads to less
dense microstructures at early ages. However, this recovers over
time and after some months the microstruture of fly ash concrete
tends to be as dense as the reference LWC without admixtures.
This is more clearly shown in Fig. 12, where both UPV and fc con-
tinuously increased between 28 days and 180 days, due to the pro-
gressive development of the pozolanic reactions. These results
confirm the findings of Ulucan et al. [36] and Demirboga et al.
[23] for fly ash NWC.
The correlation is also high for LWC produced with different types
of cement (Fig. 13). The data in Fig. 13 relates to LWC with Arlita and
different w/b ratios, tested at 28 days. It is thus shown that when a
given type of binder is used without interfering with the other con-
stituents of concrete, there appears to be little effect on the relation-
ship between fc and UPV. Note, however, that SF was ineffective.
3.6. Influence of the initial wetting conditions of LWA
Fig. 14 summarizes the data from LWC produced with LWA pre-
soaked for 24 h and with initially dry (PD) or pre-wetted LWA
(PW).
For ages between 3 and 180 days, the correlation is high in LWC
with Leca but less reasonable in LWC with Arlita, for which differ-
ences from the regression line are up to 5%. Therefore, one can only
conclude that there is no clear distinction between the different
wetting conditions. Contrary to what might be expected, lightweight
concretes with higher initial water content do not show higher
ultrasonic pulse velocities (A450 with pre-soaked LWA, Fig. 14). This
is probably because all the data are very close to each other and
small differences can be masked by the variability of the tests
UPV= -2.27.(w/c) + 5.23
10
20
30
40
50
60
70
3.4
3.8
4.2
4.6
5.0
5.4
0.25 0.35 0.45 0.55
UVP(km/s)
w/c
350 kg/m3 450 kg/m3 525 kg/m3 400 kg/m3
Fig. 8. fc and UPV versus the w/c ratio for LWC with Arlita and different water and
cement contents at 28 days (CEM I52.5).
0
15
30
45
60
75
3.5
4.5
5.5
6.5
7.5
8.5
200 250 300 350 400
Leca-UPV
Arlita-UPV
Argex-UPV
NWC-UPV
Leca-fc
Arlita-fc
Argex-fc
NWC-fc
Fig. 9. UPV and fc for different volumes of aggregate at 28 days.
Fig. 10. Relationship between UPV and fc for different w/c ratios (0.3, 0.35, 0.4, 0.45,
0.55) and volumes of aggregate (150, 250, 300, 350 and 400 L/m3
) at 28 days.
R² = 0.87
R² = 0.93
35
40
45
50
55
60
65
70
3.8 4.0 4.2 4.4 4.6
UPV (km/s)
A450
AFA22
AFA40
ASF8
ANS
L450
LFA22
LFA40
LNS
Fig. 11. Relationship between UPV and fc for LWC produced with different types of
admixtures and tested at different ages (7–180 days).
J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 967

7. themselves. The probably better quality of the interface aggregate–
paste offered by non-pre-soaked LWA [27,37] may also play a part.
4. Proposed expression to estimate LWC compressive strength
from UPV
Taking into account Eq. (1), which relates UPV to Ed and q, and
the expression suggested by EN1992-1-1 [11] that relates Ec with fc
and q (Eq. (2)), the equation Eq. (8) can be obtained. The parame-
ters A, B and KUPV are constants. This is an approximate expression,
since Eq. (8) is given by combining a theoretical formula (Eq. (1))
with an empirical relation obtained from curve fitting analyses
(Eq. (2)). The reasonable accuracy of Eq. (2) applied to LWC is dem-
onstrated in [27,38].
UPV % A Á
ffiffiffiffiffi
Ec
q
s
% A:
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
B Á f0:3
cm Á ð q
2200
Þ2
q
s
% KUPV f0:15
cm Á q0:5
ð8Þ
The constant KUPV can be easily determined from the linear
regression analysis in Fig. 15. Wet density at 28 days was assumed
in Eq. (8). The difference is not significant for other ages because all
the specimens were water-cured until the age of testing. The cor-
relation in Fig. 15 is determined by forcing the regression line to
cross the origin. Although better correlations can be obtained with-
out this condition, the physical meaning is distorted.
If we compare with Fig. 2, the application of Eq. (8) leads to a
significant improvement of the correlation coefficient, even taking
different compositions, types of aggregate and test ages into ac-
count (Fig. 15). The approximation for LWC with more porous
aggregates (Argex) is poorer. This is probably because these con-
cretes work near their ceiling strength. For that reason, the corre-
lation coefficient indicated in Fig. 15 (0.86) only takes into
account the LWA with density above 1000 kg/m3
. Also note that
better correlations should be obtained for concrete dry densities.
In fact, contrary to UPV, the modulus of elasticity is hardly affected
by the water content. However, even for Argex the correlation
coefficient would be 0.81. Therefore, expressions similar to Eq.
(9) allow a better estimation of fc from UPV and are practically
independent of the type of concrete and its composition. In Eq.
(9), UPV is in m/s and q in kg/m3
.
Fc %
UPV
KUPVÁq0:5
!2=3
½MPaŠ ð9Þ
According to the regression analysis of Fig. 15, the KUPV is equal
to 54.6 or 54.3 m2.5
MPaÀ0.15
kgÀ0.5
sÀ1
, depending on whether Ar-
gex is included or not. Note that Eq. (9) is assessed for more than
200 results considering different types, volumes and wetting
conditions of aggregates, types and amounts of cement, types
0 22 40
UPV(km/s)
4.0
4.2
4.4
4.6
4.8
5.0
0
10
20
30
40
50
60
70
% FA
Leca 28d fc
Leca 180d fc
Arlita 28d fc
Arlita 180d fc
Leca 28d UPV
Leca 180d UPV
Arlita 28d UPV
Arlita 180d UPV
Fig. 12. UPV and fc for 0%, 22% and 40% cement replacement by fly ash (by weight)
at 28 and 180 days.
R² = 0.86
30
35
40
45
50
55
60
3.7 3.9 4.1 4.3 4.5
UPV (km/s)
CEM I 42.5
CEM II AL
CEM II AV
CEM II AD
CEM IV A
Fig. 13. Relation between UPV and fc for LWC with Arlita and different types of
cement and w/b ratio (28 days).
R² = 0.64
R² = 0.83
40
45
50
55
60
65
70
4.2 4.3 4.4 4.5 4.6
UVP (km/s)
A450
A450 PW
A450 PD
L450
L450 PW
L450 PD
Fig. 14. Relationship between UPV and fc for LWC with Leca or Arlita with different
initial wetting conditions (3–180 days).
y = 18.43x
R² = 0.86
40
50
60
70
80
90
100
3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4
UPV (km/s)
Leca
Arlita
Argex
NWC
LWSC
MND (Leca)
MND (Arlita)
Fig. 15. UPV as a function of fc and for different concrete compositions and types of
aggregate at ages between 3 and 90 days (Table A1).
968 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

8. and volumes of admixtures, w/b ratios, the partial replacement of
coarse and fine NA by LWA and also a range of test ages between
3 and 90 days (Table A1).
5. Conclusions
The non-destructive ultrasonic pulse velocity method was
used to assess the mechanical compressive strength of LWC.
Based on a comprehensive experimental investigation involving
more than 80 different compositions the main conclusions are:
Calibrating curves for each type of concrete with a given type of
aggregate must be previously established when the compressive
strength, fc, is to be directly estimated from UPV. More specifi-
cally, independent curves have to be established for the same
proportion of aggregate or the same mortar characteristics.
LWCs with less porous aggregates are associated with lower
ultrasonic pulse velocity for a given fc and higher fc for a given
UPV.
The relationship between UPV and fc tends to be less affected
by the aggregate volume in LWC than in NWC. In LWC, the
propagation velocity of aggregate is closer to that of the sur-
rounding mortar, since it is less influenced by a variation in
the proportion of each phase. Moreover, both fc and UPV are
affected by the volume of aggregate, which is not true of
NWC. However, in LWC with more porous aggregates and rich
mortars there is a greater relative variation of UPV than fc.
As expected, in lightweight concrete UPV and fc increase with
age and decrease with the w/c ratio and volume of aggregate.
However, fc is little affected by the type of w/c ratio, unlike
UPV, which also depends on the proportion of mortar constit-
uents. UPV variations of over 100 m/s were obtained for a
given compressive strength.
The relation between UPV and fc was little affected by different
types of cement and additions or by different initial wetting
conditions of the aggregates.
Finally, a new general simplified expression that allows a more
accurate estimate of fc from UPV was defined that was not af-
fected by the type of concrete and its composition. A high corre-
lation coefficient of over 0.85 was obtained for common normal
and lightweight concrete ranging from 30 to 80 MPa and pro-
duced with aggregates of density above 1000 kg/m3
, even taking
into account more than 200 results for different types of aggre-
gate, concrete compositions and test ages.
This study contributes to a better understanding of the non-
destructive ultrasonic pulse velocity method in LWAC, and en-
ables this technique to be used with greater confidence. A more
accurate relation between fc and UPV is provided, regardless the
concrete composition, which improves the rational use of the
UPS method for LWC structures.
Acknowledgements
The authors wish to thank ICIST-IST for funding the research
and the companies Argex, Saint-Gobain Weber Portugal, Soarvamil
and SECIL for supplying the materials used in the experiments.
The first author also would like to acknowledge the financial sup-
port given by the Portuguese Foundation for Science and Technol-
ogy (FCT), under Grant SFRH/BD/27366/2006.
Appendix A. Appendix
See Table A1.
TableA1
Mixproportions,ultrasonicpulsevelocity,compressivestrengthandwetdensity.
Mixturesw/bc.a.d
(L/m3
)CementtypeBinder(kg/m3
)fc,3days(MPa)UPV3d
(km/s)fc,7days(MPa)UPV7d(km/s)fc,28days(MPa)UPV28(km/s)CVUPV(%)fc,90days(MPa)UPV90d(km/s)q28days(kg/m3
)
Leca
L3500.45350I52.5350––––43.14.20.444.4/45.la
4.2/4.3a
1899
L3940.4350I52.5394––––44.94.30.446.4/46.3b
4.4/4.4b
1893
L4500.35350I52.545041.3a
/44.24.2a
/4.346.74.448.64.40.249.8/50.4b
4.4/4.5b
1915
L5250.3350I52.5525––––50.04.30.751.04.31917
L350_0.550.55350I52.535029.53.831.43.835.53.90.437.04.11870
L350_0.350.35350I52.535044.74.444.84.449.14.50.348.54.51913
L450_0.550.55350I52.545028.03.631.73.736.13.70.438.63.81791
L450_0.450.45350I52.545035.13.938.24.041.94.00.244.14.11868
L450_0.300.3350I52.545048.74.349.34.451.84.40.651.84.51927
VL1500.35150I52.5450––53.94.559.34.60.3––2106
VL2500.35250I52.545047.24.348.84.452.44.50.553.74.72000
VL3000.35300I52.545045.44.247.44.350.34.40.249.74.61944
VL4000.35400I52.5450––43.84.045.74.20.746.74.41839
L42.5IIAL0.35350II42.5AL450––––45.34.30.446.34.31913
L450PW0.35350I52.5450––45.14.346.54.40.646.9/48.3b
4.4/4.4b
1827
L450PD0.35350I52.545044.04.345.34.346.54.30.047.34.41854
LFA220.35350I52.5450(22%FA)––––42.44.20.743.6/47.4b
4.3/4.3b
1862
LFA400.35350I52.5450(40%FA)––––37.14.00.240.7/44.4b
4.2/4.3b
1820
LSF80.35350I52.5450(8%SF)––45.84.247.64.30.249.3/51b
4.4/4.4b
1888
LNS0.35350I52.5450(1.3%NS)––45.14.346.74.30.047.5/47.6b
4.4/4.5b
1908
L295_I42.50.65350I42.5295––––29.24.00.3––1801
L345_I42.50.6350I42.5345––––32.44.00.2––1780
(continuedonnextpage)
J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 969

9. TableA1(continued)
Mixturesw/bc.a.d
(L/m3
)CementtypeBinder(kg/m3
)fc,3days(MPa)UPV3d
(km/s)fc,7days(MPa)UPV7d(km/s)fc,28days(MPa)UPV28(km/s)CVUPV(%)fc,90days(MPa)UPV90d(km/s)q28days(kg/m3
)
L345_sat7dc
0.6350I42.5345––––31.83.90.2––1785
L345_satldc
0.6350I42.5345––––32.63.90.5––1696
L35(MND)0.35350I52.5450––––59.84.90.264.25.02209
L65(MND)0.35350I52.5450––––53.34.70.554.74.72077
LWSC
LS4500.35350I52.5450––––37.53.80.537.23.81618
LS295_I42.50.65350I42.5295––––25.23.50.4––1458
LS345_I42.50.6350I42.5345––––27.53.60.1––1487
LS440_I42.50.45350I42.5440––––30.93.70.3––1501
LS460_I42.50.4350I42.5460––––34.83.70.1––1529
Normalweightaggregates(NA)
NA3500.45350I52.5350––––65.85.00.271.45.02396
NA3940.4350I52.5394––––71.65.00.874.75.12387
NA4500.35350I52.5450––71.65.076.25.10.281.1/85.lb
5.1/5.2b
2411
NA5250.3350I52.5525––––81.65.10.289.75.22430
NA42.5AL0.35350II42.5AL450––71.74.975.85.10.778.75.12409
VNA2500.35250I52.5450––69.94.974.25.00.3––2333
VNA3000.35300I52.5450––69.55.073.55.00.5––2382
VNA4000.35400I52.5450––72.65.075.65.20.7––2405
NA295_I42.50.65350I42.5295––––38.04.70.2––2351
NA345_I42.50.6350I42.5345––––41.14.80.2––2353
NA440_I42.50.45350I42.5440––––52.64.80.3––2368
NA460_I42.50.4350I42.5460––––59.24.90.5––2378
NA394JVA0.55350IVA42.5394––––37.84.70.6––2323
NA420IVA0.45350IVA42.5420––––50.34.80.2––2340
Argex
VArgex2500.35250I52.545036.44.337.14.438.74.40.239.24.71924
Argex4500.35350I52.545026.8a
/28.44.1a
/4.130.44.231.24.20.232.84.21776
VArgex4000.35400I52.545025.14.026.24.028.14.00.428.24.21631
Arilita
A3500.45350I52.535047.54.151.14.157.64.20.358.24.31942
A3940.4350I52.539453.14.257.14.262.64.30.262.94.41964
A4500.35350I52.545055.9a
/58.44.2a
/4.361.44.364.64.40.264.9/66.2b
4.4/4.5b
1982
A5250.3350I52.552562.54.365.74.468.54.50.370.34.61995
A350_0.350.35350I52.5350––––65.04.60.2––1995
A450_0.550.55350I52.545029.93.737.03.843.93.90.348.63.91862
A450_0.450.45350I52.545040.14.046.24.154.94.10.255.14.21892
A450_0.300.3350I52.545063.94.570.64.572.14.60.474.74.62014
VA250_I42.50.35250I42.5450––––66.24.60.2––2022
VA400_I42.50.35400I42.5450––––63.84.40.3––1884
A42.5IIAL0.35350II42.5AL450––53.44.360.04.40.264.44.41974
A450PW0.35350I52.545056.94.258.84.463.54.30.367.04.61943
A450PD0.35350I52.5450––62.24.465.14.40.265.04.61956
AFA220.35350I52.5450(22%FA)––54.34.260.04.30.264.9/67.5b
4.3/4.4b
1959
AFA400.35350I52.5450(40%FA)41.24.046.14.054.34.10.661.5/63.9b
4.3/4.3b
1941
ASF80.35350I52.5450(8%SF)––55.74.260.84.20.364.64.41931
ANS0.35350I52.5450(1.3%NS)56.84.360.94.265.54.40.265.9/68b
4.5/4.5b
1976
A295_I42.50.65350I42.5295––––36.74.10.2––1872
A345_I42.50.6350I42.5345––––40.34.10.0––1872
A440_I42.50.45350I42.5440––––50.84.30.3––1901
A460_I42.50.4350I42.5460––––54.64.30.4––1913
A345_JIAL0.6350I42.5AL345––––39.24.10.4––1890
A440_JIAL0.45350I42.5AL440––––51.34.20.2––1896
A460_JIAL0.4350I42.5AL460––––54.14.20.2––1904
A345_JIAV0.6350II42.5AV345––––353.90.3––1882
A394_JIAV0.55350II42.5AV394––––39.13.90.4––1876
970 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

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A420_JIAV0.45350II42.5AV420––––48.14.10.4––1891
A345_JIAD0.6350II42.5AD345––––39.94.00.3––1854
A394_JIAD0.55350II42.5AD394––––41.54.10.2––1833
A420_JIAD0.45350II42.5AD420––––50.14.20.2––1868
A394_JVA0.55350IIVA42.5394––––37.13.90.2––1852
A420_JVA0.45350IIVA42.5420––––52.84.20.3––1886
A35(MND)0.35350I52.5450––––72.35.00.275.94.92243
A65(MND)0.35350I52.5450––––66.54.70.470.64.72115
Mortar0.350.350I52.5702––61.14.564.84.50.271.24.72216
a
Resultsobtainedatlday.
b
Resultsobtainedat180days.
c
Oneorsevendayswater-cured.
d
c.a.–coarseaggregate.
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