Ultrasonics 53 (2013) 962–972

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Ultrasonics

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Compressive strength evaluation of structural lightweight concreteby non-destructive ultrasonic pulse velocity method

J. Alexandre Bogas ⇑, M. Glória Gomes, Augusto GomesDECivil/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 2012Received in revised form 13 December 2012Accepted 17 December 2012Available online 3 January 2013

Keywords:Lightweight aggregate concreteNon-destructive testsUltrasonic pulse velocityCompressive strengthAdmixtures

0041-624X/$ - see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.ultras.2012.12.012

⇑ Corresponding author. Tel.: +351 218418226; faxE-mail address: abogas@civil.ist.utl.pt (J.A. Bogas).

a b s t r a c t

In this paper the compressive strength of a wide range of structural lightweight aggregate concrete mixesis 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 to80 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, typeof 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 normalweight 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 andits composition. More than 200 results for different types of aggregates and concrete compositions wereanalyzed and high correlation coefficients were obtained.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The non-destructive ultrasonic pulse velocity method has beenwidely applied to the investigation of the mechanical propertiesand 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 physicaland mechanical properties. Based on the theory of elasticity ap-plied to homogeneous and isotropic materials, the pulse velocityof compressional waves (P-waves) is directly proportional to thesquare root of the dynamic modulus of elasticity, Ed, and inverselyproportional to the square root of its density, q, according to Eq. (1)[7,8]. td is the dynamic Poisson’s ratio. Concrete is heterogeneousand so these assumptions are not strictly valid. However, the highattenuation in concrete limits the UPV method to frequencies up toabout 100 kHz [9], which means that compressional waves do notinteract with most concrete inhomogeneities [9,10]. In this case,concrete can be reasonably regarded as a homogeneous material[5].

UPV ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiEd

q� ð1� tdÞð1þ tdÞ � ð1� 2tdÞ

sð1Þ

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: +351 218418380.

According to Eq. (1), the relevant physical properties of materi-als that influence pulse velocity are the density, elastic modulusand td. Thus, correlations between the pulse velocity and the com-pressive strength of concrete, fc, are based on the indirect relationbetween this property and the elastic modulus, Ec. EN 1992-1-1[11] suggests the expression Eq. (2) to relate Ec and fc, where q isthe oven-dry density.

Ec � 22 � fc

10

� �0:3

� q2200

� �2½GPa� ð2Þ

However, it is well known that the compressive strength andelastic modulus may be influenced differently, depending on theconcrete composition. Therefore, the relation between UPV and fc

is not unique and can be affected by factors such as the type andsize of aggregate, physical properties of the cement paste, curingconditions, 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 variablessuch as the w/c ratio, volume and size of aggregates, concreteage 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 causedby the introduction of aggregates results in increased scatter, i.e.,

J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 963

dispersive properties. This is why Philippids [20] found that theultrasound velocity increased 11% in concrete specimens through-out the 15–200 kHz band.

Nonetheless, several relationships between UPV and fc have beenproposed, 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 differentinfluence of concrete composition on fc and UPV. For example, differ-ent volumes of normal weight aggregate (NA) affect UPV but havelittle, if any, influence on fc. Depending on the mix design, the higherNA content can even cause a UPV increase and, at the same time, aloss of compressive strength [14,16].

Most investigations have focused on NWC behavior. Publishedstudies involving lightweight concrete (LWC) are still limited. Nas-ser and Al-Manaseer [24] reported expressions of the type fc =a�UPVb 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 compressivestrength. Chang et al. [10] established exponential relationshipsbetween UPV and fc for LWC with two types of lightweight aggre-gates. Hamidian et al. [25] found poor correlations when severalLWC mixes were analyzed together. Tanyidizi and Coskun [26]used the analysis of variance (ANOVA) to study the influence ofcuring conditions, maximum size of aggregate, mineral admixturesand curing time on UPV and the compressive strength of light-weight concrete. The maximum size of the aggregate was the mainparameter governing UPV and fc.

Expanded clay LWC is almost one hundred years old, and a lot ofold LWC structures that have been built since the 1950s, especiallyin North America and Europe, now represent a major issue in termsof maintenance and rehabilitation. Non-destructive ultrasoundpulse velocity tests have proved to be very helpful in the inspectionof old structures. However, the experience acquired in this fieldand the correlations that have been built between the quality ofconcrete and its UPV are essentially limited to NWC. Therefore,due to the specificity of LWC, new correlations must be establishedfor this type of concrete, regardless the type of LWA. Knowledge ofgeneral correlations between fc and UPV will be a major advance inthe inspection and assessment of existing LWC structures.

This study investigates the use of the non-destructive ultrasonicpulse velocity method to assess the compressive strength of LWCproduced with different types of expanded clay aggregates. Theexperimental work was comprehensive, testing at various agesseveral 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 aggregatesand type and volume of binder was analyzed. Finally, based onthe 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 thatrelates fc and UPV, irrespective of the type of concrete, mixturecomposition 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. Theirtotal porosity, PT, particle density, qp, bulk density, qb, and 24 hwater absorption, wabs,24h, are indicated in Table 1. They differ interms of porosity, geometry and bulk density, which makes it

possible to produce concrete with strengths ranging from about25 to 70 MPa [27], thereby covering the most common structuralLWC. A more detailed microstructural characterization of theseaggregates 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 asLeca (20% fine and 80% coarse gravel). Fine aggregates consistedof 2/3 coarse and 1/3 fine sand. Their main properties are listedin Table 1. The two fractions of Argex were also combined to havethe 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 52.5R, I 42.5 R, II-A/L 42.5, II-A/D 42.5 (8% of SF by weight), II-A/V42.5 (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 physicaland mechanical properties are listed in Table 2. For low w/b ratios,a polycarboxylate based superplasticizer (SP) was used. A waterdispersed 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 mechanicalcharacterization of structural lightweight concretes produced withdifferent types of aggregates that was conducted at the InstitutoSuperior Técnico [27], the ultrasonic pulse velocities of about 84different compositions were measured. The compositions variedin 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 andamounts of cement (300–525 kg/m3), the types and volumes ofmineral admixtures (22% and 40% of fly ash (FA), 8% of silica fume(SF) and 1.3% of nanosilica), the partial replacement of normalweight coarse aggregates by lightweight aggregate (LWA) and alsothe 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 workabilityand effective water content of the concrete. The aggregates werethen surface dried with absorbent towels and placed in the mixerwith sand and 50% of the total water. After 2 min of mixing, thebinder and the rest of the water were added. When used, the SPwas added slowly with 10% of water, after 1 more minute. The totalmixing time was 7 min.

All the concrete mixtures studied for this paper are listed in de-tail elsewhere [27]. The main characteristics of each compositionare summarized in Table A1 in the appendix. The w/b ratio signifiesthe effective water available for binder hydration. The denomina-tions ‘NA’, ‘L’, ‘A’ and ‘Argex’ correspond to the mixes with normalweight aggregate, Leca, Arlita and Argex. These denominations areusually 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 areference 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) and0.5–1.0% of SP. Except for LWSC, natural sand was used in combi-nation with coarse LWA. For LWSC, the 2/3 coarse natural sandwas replaced by the lightweight sand indicated in Table 1 (Leca0–3). Modified normal density concretes (MND) were producedwith partial replacement of NA by 35% and 65% of Leca or Arlita.

To study the influence of pre-wetting aggregate, some concretespecimens with initially dry LWA (PD) or pre-wetted LWA (PW)were also produced. The PD aggregate is added during mixingand the PW aggregate is previously wetted for 3 min with 50% ofthe total water before mixing.

Table 1Aggregate 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 1290Loose bulk density, qp (kg/m3) 1416 1530 1343 1377 562 613 423 397 73824 h water absorption, wabs,24h (%) 0.2 0.5 1.4 1.1 – 12.3 22.9 23.3 12.1Total porosity, PT (%) – – – – 59 60 67 73 52Granulometric 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 3/10Los Angeles coefficient (%) – – 33.3 30.5 – – – – –

Table 2Main 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 8.3Blaine specific surface (cm2/g) EN 196-6 – – 5102 3981 4477Compressive strength of reference mortar (MPa) 2 days

28 daysEN 196-1 – – 40.4 32.8 27.2

– – 62.7 54.9 51.4Activity 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 0.5Loss on ignition (LOI) (%) EN 196-7 6.5 3.7 1.64 3.06 5.34SiO2 + A12O3 + Fe2O3 (%) EN 196-2 83.0 94.0 29.1 27.6 26.1CaO (%) – 3.38 0.83 61.6 63.5 61.6Free CaO (%) EN 451-1 0.36 Not detected 1.45 1.31 1.8Density (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.

964 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

2.3. Specimen preparation and test setup

For each mix at each age, three 150 mm cubic specimens weretested for ultrasonic pulse velocity and then for compressivestrength according to EN 12390-3 [31]. After demolding at 24 h,specimens were kept in water until testing, according to EN12390-3 [31]. UPV measurements were performed on unloadedwet specimens.

The ultrasonic pulse velocity was obtained by direct transmis-sion according to EN 12504-4 [17]. The equipment used was theportable ultrasonic non-destructive digital indicating tester (PUNDIT),shown in Fig. 1 [8]. In this method an ultrasonic pulse is generatedby a pulse generator and transmitted to the surface of concretethrough the transmitter transducer. The time taken by the pulseto travel through the concrete, tus, is measured by the receivertransducer 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 wavestransmitted through the 150 mm cubic specimens was measuredwith accuracy up to 0.1 ls. A digital readout is displayed in a

Fig. 1. Scheme of the ultrasonic pulse velocity measurement in concrete specimens.

4-digit LCD. Finally, UPV is the ratio between the length traveledby the pulse (150 mm) and the measured time, tus. A thin couplant(solid vaseline) was used on the interface between transducers andconcrete to ensure good contact. Before each measurement theequipment was calibrated with a cylindrical Perspex bar of known tus.

Three measurements were taken for each test specimen byswitching the position of the transducers between the two oppo-site faces of the concrete cubes. For all mixes ultrasonic pulsevelocity was measured at 28 days. Tests were also performed at1, 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 pulsevelocity, UPV, are listed in Table A1, for each composition at eachage. Fig. 2 summarizes the mean values of UPV and fc obtainedfor each mixture, between 3 and 90 days. A total of about 208 aver-age results were considered, involving different concrete strengthsranging from about 30–80 MPa and UPV from 3.5 to 5.2 km/s.

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 anddifferent types of aggregate at ages between 3 and 90 days.

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 respectivemortar of equivalent composition at 7 and 28 days (the same sand/cement ratio andw/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

The coefficients of variation of UPV, CVUPV, for the specimensmeasured at 28 days are also presented in Table A1. For other agesthe CVUPV is of the same order. As it can be seen, the CVUPV obtainedfrom 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 thehomogeneity of the concrete specimens produced.

As expected, when different compositions, types of aggregateand test ages are considered simultaneously there is a poor corre-lation between UPV and fc (Fig. 2). Therefore, the influence of thetype and volume of aggregate, age of testing, w/b ratio and typeof 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 byLWA (MND) are analyzed separately, there is a natural increaseof 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 tothe reduction of UPV.

For similar values of UPV, the strength is higher in LWC of higherdensity. Conversely, the lower the density of the LWA the higherthe UPV for a given compressive strength. This trend is likely tobe primarily related to the: lower proportional increment of UPVin relation to fc, for higher strength levels; simultaneous reductionof density and stiffness in LWC, which means a smaller variation ofUPV (Eq. (1)); slight variation of fc for LWC with rich mortars andmore porous aggregates; higher compacity of richer mortars inmore porous LWC of the same strength; small differences betweenthe 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 ofUPV in NWC and the opposite effect in LWC. The difference is high-er in NWC, which means the aggregate has greater influence onthis type of concrete. Assuming that the aggregate stiffness varieswith the square of its density, q2

ag [32], then the UPVag decreasesmore 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 relatedto the ultrasonic velocity of the aggregate, UPVag, and the ultrasonicvelocity of the mortar, UPVm, according to Eq. (3) (series model,[16]). tag and tm are the respective relative volumes of aggregateand mortar. The influence of the transition zone paste/aggregateis neglected.

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).

1UPVc

¼ tag

UPVagþ tm

UPVmð3Þ

Based on the UPV average values obtained at 28 days for themortar (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 valuesare 3.6, 4.1, 4.1 and 6.3 km/s, respectively for Argex, Leca, Arlitaand normal aggregate (NA). Thus, the UPVag/UPVm ratio is 1.4 forNA and only 0.9 for Leca and Arlita. This confirms that NWC is af-fected more by the volume of aggregate. Moreover, the dispersioneffect 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 betweenUPV and fc strongly depends on the proportion of aggregate in themix. Thus, the correlation between fc and UPV has to be establishedfor each type of NWC with a given volume of aggregate. The sameis 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 ofaggregate. Therefore, one would expect the relation between UPVand fc to be less affected. However, although UPV varies in the samedirection as fc, they may progress differently. Since UPVag/UPVm isclose to unity, the fc variation can be higher than that of UPV. More-over, the compressive strength of LWC is affected by the strengthlevel, whereas UPV is not. This is especially noticeable in LWC withmore 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 porousaggregates (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 strengthevolution of the higher density LWC. As expected, UPV and fc de-crease with the partial replacement of natural sand by lightweightsand. 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 commonexponential relationships, with similar correlation coefficients(Eqs. (4)–(7)). The estimation of fc by means of Eqs. (4)–(7) leadsto an average error of 5.5% for Argex, 4.9% for Leca, 7.3% for Arlitaand 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 moreLWC compositions with Arlita, which is why the largest errorwas obtained in this type of concrete.

Arlita : fcm ¼ 1:07 � e0:92�UPV ; R2 ¼ 0:82 ð4Þ

Leca : fcm ¼ 3:0�0:63�UPV ; R2 ¼ 0:82 ð5Þ

R² = 1.004.8

5.0

5.2

966 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

Argex : fcm ¼ 1:65 � e0:70�UPV ; R2 ¼ 0:82 ð6Þ

Normal aggregate : fcm ¼ 0:023 � e1:6�UPV ; R2 ¼ 0:88 ð7Þ

R² = 0.99R² = 0.97

R² = 0.963.4

3.6

3.8

4.0

4.2

4.4

4.6

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 ratioobtained by varying the amount of water – LWC with Leca or Arlita; w/c ratioobtained by varying the cement content – NWC).

3.2. Influence of concrete age

The fc and UPV trend for some illustrative mixes with differentw/b ratios and different types and amounts of aggregate is shownin 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 throughsolid matter, the greater the paste hydration the lower the volumeof pores and the greater the UPV [33].

High correlations are obtained when each concrete compositionis individually assessed. However, the correlation decreases whendifferent compositions are analyzed together. For example, thereis a greater dispersion when different w/b ratios are consideredin LWC with Leca (dashed line in Fig. 5). In fact, whereas Vus tendsto increase faster with age than fc, fc increases more with the w/cratio than Vus does. Therefore, the simultaneous consideration ofdistinct ages and w/c ratios implies different relations between fc

and Vus. However, the relation between fc and UPV seems to be lessaffected by the volume of aggregate (VL250 vs L450), contrary towhat 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 smallchanges in UPV. As shown in this study, the influence of eachmix design’s parameters must be analyzed at the same age, andthis 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 anddifferent w/c ratios. Since only one parameter of the mixture ischanged for each type of cement, the correlations are high. Mixeswith the same volume of coarse aggregate and the same typeand cement content were considered in LWC with Leca or Arlita.Different w/c ratios were obtained by varying the amount of waterand the respective volume of sand. Mixes with the same volume ofwater and coarse aggregate were considered in NWC. Different w/cratios were obtained by varying the amount of cement and therespective volume of sand. This is why the UPV trend with thew/c ratio is less pronounced in NWC (the higher w/c ratio ispartially offset by the greater volume of sand). Otherwise, the slope

R² = 0.97

R² = 0.93

R² = 0.90

R² = 0.98R² = 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 and350 L/m3).

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 cementcontents into account at the same time, there is a reduction of thecorrelation coefficient (Figs. 7 and 8). As shown in Fig. 8, fc is lesssensitive than UPV to the type of w/c, i.e., fc tends to be less affectedby 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, therelation between UPV and w/c also depends on how the w/c ratiois changed. Furthermore, moisture content helps the propagationvelocity in concrete [27,34] but may affect compressive strengthnegatively.

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 byother 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 ratiosand 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 volumeof aggregate. Since the compressive strength of these concretes isalso affected by the aggregate, the variation of fc with w/c is lowerthan in NWC and LWC of higher density.

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).

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

UV

P (k

m/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 andcement 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 ofadmixtures and tested at different ages (7–180 days).

J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 967

Moreover, the strength of LWC is more affected by the volumeof aggregate than that of NWC. In other words, UPV and fc are bothaffected by the propagation velocity and the strength of aggregateand mortar. Therefore, there is a greater interdependence betweenUPV and fc in LWC than in NWC. However, when LWC reaches itsceiling strength the behavior may change. After a given strengthlevel a further increase of fc is not meaningful, contrary to whathappens with UPV.

The LWC with less porous aggregates exhibits similar behaviorto that of NWC. This is because the limit strength of higher density

LWC, above which the fc is governed by the paste, is much higherthan that of LWC with less porous aggregates. As shown in[27,35], up to about 60 MPa the compressive behavior of LWC withArlita is similar to that of NWC.

3.5. Influence of the type of binder

There is a high correlation between UPV and fc regardless thetype of mineral admixture (Fig. 11). The regression takes intoaccount 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 inLWC with silica fume or nanosilica, which was less efficient thanexpected. It is likely that there was no effective dispersion of suchadmixtures. Moreover, the strength limitation imposed by LWAand the better quality of the aggregate–paste transition zone inLWC also play a part in the lower efficiency of SF and NS. It is alsoshown that the replacement of cement by fly ash leads to lessdense microstructures at early ages. However, this recovers overtime and after some months the microstruture of fly ash concretetends 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 resultsconfirm 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 typesof cement (Fig. 13). The data in Fig. 13 relates to LWC with Arlita anddifferent w/b ratios, tested at 28 days. It is thus shown that when agiven 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 LWCwith Leca but less reasonable in LWC with Arlita, for which differ-ences from the regression line are up to 5%. Therefore, one can onlyconclude that there is no clear distinction between the differentwetting conditions. Contrary to what might be expected, lightweightconcretes with higher initial water content do not show higherultrasonic pulse velocities (A450 with pre-soaked LWA, Fig. 14). Thisis probably because all the data are very close to each other andsmall differences can be masked by the variability of the tests

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 ofcement 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 differentinitial wetting conditions (3–180 days).

y = 18.43xR² = 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 ofaggregate at ages between 3 and 90 days (Table A1).

968 J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972

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 strengthfrom UPV

Taking into account Eq. (1), which relates UPV to Ed and q, andthe 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 �

ffiffiffiffiffiEc

q

s� A:

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiB � f 0:3

cm � ðq

2200 Þ2

q

s� KUPV f 0:15

cm � q0:5 ð8Þ

The constant KUPV can be easily determined from the linearregression analysis in Fig. 15. Wet density at 28 days was assumedin Eq. (8). The difference is not significant for other ages because allthe specimens were water-cured until the age of testing. The cor-relation in Fig. 15 is determined by forcing the regression line tocross 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 asignificant improvement of the correlation coefficient, even takingdifferent compositions, types of aggregate and test ages into ac-count (Fig. 15). The approximation for LWC with more porousaggregates (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 intoaccount the LWA with density above 1000 kg/m3. Also note thatbetter correlations should be obtained for concrete dry densities.In fact, contrary to UPV, the modulus of elasticity is hardly affectedby the water content. However, even for Argex the correlationcoefficient would be 0.81. Therefore, expressions similar to Eq.(9) allow a better estimation of fc from UPV and are practicallyindependent of the type of concrete and its composition. In Eq.(9), UPV is in m/s and q in kg/m3.

Fc �UPV

KUPV �q0:5

!2=3

½MPa� ð9Þ

According to the regression analysis of Fig. 15, the KUPV is equalto 54.6 or 54.3 m2.5 MPa�0.15 kg�0.5 s�1, depending on whether Ar-gex is included or not. Note that Eq. (9) is assessed for more than200 results considering different types, volumes and wettingconditions of aggregates, types and amounts of cement, types

(kg/

m3)

next

page

)

J.A. Bogas et al. / Ultrasonics 53 (2013) 962–972 969

and volumes of admixtures, w/b ratios, the partial replacement ofcoarse and fine NA by LWA and also a range of test ages between3 and 90 days (Table A1).

ssiv

est

reng

than

dw

etde

nsit

y.

peB

inde

r(k

g/m

3)

f c,3

day

s(M

Pa)

UPV

3d

(km

/s)

f c,7

day

s(M

Pa)

UPV

7d

(km

/s)

f c,2

8d

ays

(MPa

)U

PV2

8(k

m/s

)C

VU

PV

(%)

f c,9

0d

ays

(MPa

)U

PV9

0d

(km

/s)

q2

8d

ays

350

––

––

43.1

4.2

0.4

44.4

/45.

la4.

2/4.

3a18

9939

4–

––

–44

.94.

30.

446

.4/4

6.3b

4.4/

4.4b

1893

450

41.3

a /44.

24.

2a /4.3

46.7

4.4

48.6

4.4

0.2

49.8

/50.

4b4.

4/4.

5b19

1552

5–

––

–50

.04.

30.

751

.04.

319

1735

029

.53.

831

.43.

835

.53.

90.

437

.04.

118

7035

044

.74.

444

.84.

449

.14.

50.

348

.54.

519

1345

028

.03.

631

.73.

736

.13.

70.

438

.63.

817

9145

035

.13.

938

.24.

041

.94.

00.

244

.14.

118

6845

048

.74.

349

.34.

451

.84.

40.

651

.84.

519

2745

0–

–53

.94.

559

.34.

60.

3–

–21

0645

047

.24.

348

.84.

452

.44.

50.

553

.74.

720

0045

045

.44.

247

.44.

350

.34.

40.

249

.74.

619

4445

0–

–43

.84.

045

.74.

20.

746

.74.

418

3945

0–

––

–45

.34.

30.

446

.34.

319

1345

0–

–45

.14.

346

.54.

40.

646

.9/4

8.3b

4.4/

4.4b

1827

450

44.0

4.3

45.3

4.3

46.5

4.3

0.0

47.3

4.4

1854

450(

22%

FA)

––

––

42.4

4.2

0.7

43.6

/47.

4b4.

3/4.

3b18

6245

0(40

%FA

)–

––

–37

.14.

00.

240

.7/4

4.4b

4.2/

4.3b

1820

450(

8%SF

)–

–45

.84.

247

.64.

30.

249

.3/5

1b4.

4/4.

4b18

8845

0(1.

3%N

S)–

–45

.14.

346

.74.

30.

047

.5/4

7.6b

4.4/

4.5b

1908

295

––

––

29.2

4.0

0.3

––

1801

345

––

––

32.4

4.0

0.2

––

1780

(con

tinu

edon

5. Conclusions

The non-destructive ultrasonic pulse velocity method wasused to assess the mechanical compressive strength of LWC.Based on a comprehensive experimental investigation involvingmore than 80 different compositions the main conclusions are:

� Calibrating curves for each type of concrete with a given type ofaggregate must be previously established when the compressivestrength, fc, is to be directly estimated from UPV. More specifi-cally, independent curves have to be established for the sameproportion 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 givenUPV.� The relationship between UPV and fc tends to be less affected

by the aggregate volume in LWC than in NWC. In LWC, thepropagation velocity of aggregate is closer to that of the sur-rounding mortar, since it is less influenced by a variation inthe proportion of each phase. Moreover, both fc and UPV areaffected by the volume of aggregate, which is not true ofNWC. However, in LWC with more porous aggregates and richmortars 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, unlikeUPV, which also depends on the proportion of mortar constit-uents. UPV variations of over 100 m/s were obtained for agiven compressive strength.� The relation between UPV and fc was little affected by different

types of cement and additions or by different initial wettingconditions of the aggregates.

Finally, a new general simplified expression that allows a moreaccurate 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 normaland lightweight concrete ranging from 30 to 80 MPa and pro-duced with aggregates of density above 1000 kg/m3, even takinginto 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 moreaccurate relation between fc and UPV is provided, regardless theconcrete composition, which improves the rational use of theUPS method for LWC structures.

,ult

raso

nic

puls

eve

loci

ty,c

ompr

e

w/b

c.a.

d(L

/m3)

Cem

ent

ty

0.45

350

I52.

50.

435

0I5

2.5

0.35

350

I52.

50.

335

0I5

2.5

0.55

350

I52.

50.

3535

0I5

2.5

0.55

350

I52.

50.

4535

0I5

2.5

0.3

350

I52.

50.

3515

0I5

2.5

0.35

250

I52.

50.

3530

0I5

2.5

0.35

400

I52.

50.

3535

0II

42.5

AL

0.35

350

I52.

50.

3535

0I5

2.5

0.35

350

I52.

50.

3535

0I5

2.5

0.35

350

I52.

50.

3535

0I5

2.5

0.65

350

I42.

50.

635

0I4

2.5

Acknowledgements

The authors wish to thank ICIST-IST for funding the researchand the companies Argex, Saint-Gobain Weber Portugal, Soarvamiland 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.

Tabl

eA

1M

ixpr

opor

tion

s

Mix

ture

s

Leca

L350

L394

L450

L525

L350

_0.5

5L3

50_0

.35

L450

_0.5

5L4

50_0

.45

L450

_0.3

0V

L150

VL2

50V

L300

VL4

00L4

2.5I

IAL

L450

PWL4

50PD

LFA

22LF

A40

LSF8

LNS

L295

_I42

.5L3

45_I

42.5

Appendix A. Appendix

See Table A1.

Tabl

eA

1(c

onti

nued

)

Mix

ture

sw

/bc.

a.d

(L/m

3)

Cem

ent

type

Bin

der

(kg/

m3)

f c,3

day

s(M

Pa)

UPV

3d

(km

/s)

f c,7

day

s(M

Pa)

UPV

7d

(km

/s)

f c,2

8d

ays

(MPa

)U

PV2

8(k

m/s

)C

VU

PV

(%)

f c,9

0d

ays

(MPa

)U

PV9

0d

(km

/s)

q2

8d

ays

(kg/

m3)

L345

_sat

7dc

0.6

350

I42.

534

5–

––

–31

.83.

90.

2–

–17

85L3

45_s

atld

c0.

635

0I4

2.5

345

––

––

32.6

3.9

0.5

––

1696

L35

(MN

D)

0.35

350

I52.

545

0–

––

–59

.84.

90.

264

.25.

022

09L6

5(M

ND

)0.

3535

0I5

2.5

450

––

––

53.3

4.7

0.5

54.7

4.7

2077

LWSC

LS45

00.

3535

0I5

2.5

450

––

––

37.5

3.8

0.5

37.2

3.8

1618

LS29

5_I4

2.5

0.65

350

I42.

529

5–

––

–25

.23.

50.

4–

–14

58LS

345_

I42.

50.

635

0I4

2.5

345

––

––

27.5

3.6

0.1

––

1487

LS44

0_I4

2.5

0.45

350

I42.

544

0–

––

–30

.93.

70.

3–

–15

01LS

460_

I42.

50.

435

0I4

2.5

460

––

––

34.8

3.7

0.1

––

1529

Nor

mal

wei

ght

aggr

egat

es(N

A)

NA

350

0.45

350

I52.

535

0–

––

–65

.85.

00.

271

.45.

023

96N

A39

40.

435

0I5

2.5

394

––

––

71.6

5.0

0.8

74.7

5.1

2387

NA

450

0.35

350

I52.

545

0–

–71

.65.

076

.25.

10.

281

.1/8

5.lb

5.1/

5.2b

2411

NA

525

0.3

350

I52.

552

5–

––

–81

.65.

10.

289

.75.

224

30N

A42

.5A

L0.

3535

0II

42.5

AL

450

––

71.7

4.9

75.8

5.1

0.7

78.7

5.1

2409

VN

A25

00.

3525

0I5

2.5

450

––

69.9

4.9

74.2

5.0

0.3

––

2333

VN

A30

00.

3530

0I5

2.5

450

––

69.5

5.0

73.5

5.0

0.5

––

2382

VN

A40

00.

3540

0I5

2.5

450

––

72.6

5.0

75.6

5.2

0.7

––

2405

NA

295_

I42.

50.

6535

0I4

2.5

295

––

––

38.0

4.7

0.2

––

2351

NA

345_

I42.

50.

635

0I4

2.5

345

––

––

41.1

4.8

0.2

––

2353

NA

440_

I42.

50.

4535

0I4

2.5

440

––

––

52.6

4.8

0.3

––

2368

NA

460_

I42.

50.

435

0I4

2.5

460

––

––

59.2

4.9

0.5

––

2378

NA

394

JVA

0.55

350

IVA

42.5

394

––

––

37.8

4.7

0.6

––

2323

NA

420

IVA

0.45

350

IVA

42.5

420

––

––

50.3

4.8

0.2

––

2340

Arg

exV

Arg

ex25

00.

3525

0I5

2.5

450

36.4

4.3

37.1

4.4

38.7

4.4

0.2

39.2

4.7

1924

Arg

ex45

00.

3535

0I5

2.5

450

26.8

a /28.

44.

1a /4.1

30.4

4.2

31.2

4.2

0.2

32.8

4.2

1776

VA

rgex

400

0.35

400

I52.

545

025

.14.

026

.24.

028

.14.

00.

428

.24.

216

31A

rilit

aA

350

0.45

350

I52.

535

047

.54.

151

.14.

157

.64.

20.

358

.24.

319

42A

394

0.4

350

I52.

539

453

.14.

257

.14.

262

.64.

30.

262

.94.

419

64A

450

0.35

350

I52.

545

055

.9a /5

8.4

4.2a /4

.361

.44.

364

.64.

40.

264

.9/6

6.2b

4.4/

4.5b

1982

A52

50.

335

0I5

2.5

525

62.5

4.3

65.7

4.4

68.5

4.5

0.3

70.3

4.6

1995

A35

0_0.

350.

3535

0I5

2.5

350

––

––

65.0

4.6

0.2

––

1995

A45

0_0.

550.

5535

0I5

2.5

450

29.9

3.7

37.0

3.8

43.9

3.9

0.3

48.6

3.9

1862

A45

0_0.

450.

4535

0I5

2.5

450

40.1

4.0

46.2

4.1

54.9

4.1

0.2

55.1

4.2

1892

A45

0_0.

300.

335

0I5

2.5

450

63.9

4.5

70.6

4.5

72.1

4.6

0.4

74.7

4.6

2014

VA

250_

I42.

50.

3525

0I4

2.5

450

––

––

66.2

4.6

0.2

––

2022

VA

400_

I42.

50.

3540

0I4

2.5

450

––

––

63.8

4.4

0.3

––

1884

A42

.5II

AL

0.35

350

II42

.5A

L45

0–

–53

.44.

360

.04.

40.

264

.44.

419

74A

450

PW0.

3535

0I5

2.5

450

56.9

4.2

58.8

4.4

63.5

4.3

0.3

67.0

4.6

1943

A45

0PD

0.35

350

I52.

545

0–

–62

.24.

465

.14.

40.

265

.04.

619

56A

FA22

0.35

350

I52.

545

0(22

%FA

)–

–54

.34.

260

.04.

30.

264

.9/6

7.5b

4.3/

4.4b

1959

AFA

400.

3535

0I5

2.5

450(

40%

FA)

41.2

4.0

46.1

4.0

54.3

4.1

0.6

61.5

/63.

9b4.

3/4.

3b19

41A

SF8

0.35

350

I52.

545

0(8%

SF)

––

55.7

4.2

60.8

4.2

0.3

64.6

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