extendedabstract_tiagogomes_69038

7/23/2019 ExtendedAbstract_TiagoGomes_69038

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Structural Lightweight Concrete Produced with Volcanic Scoria from

So Miguel Island

Tiago Joo de Medeiros Gomes

Instituto Superior Tcnico

Abstract

Acronyms

- LWA - Lightweight aggregates;

- LWSA - Lightweight scoria aggregates;

- LWC - Lightweight concrete;

- NA - Normal weight aggregates;

- NWC - Normal weight concrete;

- LWCSC - Lightweight concrete produced with coarse scoria aggregates and natural sand;

- LWCFSC - Lightweight concrete produced with coarse and fine scoria aggregates;

- GA1 - Coarse gravel;- GA2 - Fine gravel;

- CS - Coarse natural sand;

- FS - Fine natural sand;

- CBA - Coarse bagacina aggregate;

- FBA - Fine bagacina aggregate;

- fL - Limit strength;

- fcs - Ceiling strength.

Introducion

The incorporation of lightweight aggregates (LWA) in concrete is done since the classical antiquity.

However, the production of lightweight concrete, as it is known today, only appeared in the second half

of the XX century. Known for their high thermal conductivity, usually above 1.0 W/mC, and for theirlow density, under 2000 kg/m

3, the lightweight concrete have been used in large span bridges, high-rise

buildings, rehabilitation works and other solutions where the self-weight is relevant. Among the

aggregates able to produce lightweight concrete, the artificial aggregates are the most competitive to

attain structural concrete. Nevertheless, the production of these aggregates is associated with a high cost

due to their energy consumption during the manufacture process. In these terms, more economical

solutions based on natural lightweight aggregates may be advantageous.

Several authors have studied the physical, mechanical and durability properties of concrete produced

with natural lightweight aggregates, especially volcanic scoria and pumice. In general, natural lightweightaggregates usually have more porosity, less density and less crushing strength than normal weight

aggregates (NA). Therefore, it is expected that lightweight concrete produced with natural LWA has

lower density, compression strength, tensile strength and elasticity modulus than concrete with NA.

Yasar et al. (2003) reported reductions of 20% in the concrete dry density of LWC produced with

volcanic scoria when compared to normal weight concrete of equal composition. Reductions of 30-45%

in the compression strength when NA was replaced by volcanic scoria aggregates were reported by

Hossain (2006) and Kili et al.(2009). According to Kili et al.(2009) the introduction of volcanic scoria

and pumice aggregate led to a tensile strength reduction of 7-58% when compared to normal concrete

with identical cement content. In concrete produced with scoria aggregates, reductions of 21-42% in the

elasticity modulus were documented by Hossain et al. (2010). The authors justify this high reduction by

the low stiffness of the pumice.


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According to several authors, the higher shrinkage of concrete with natural LWA is a result of the

high deformability of aggregates (Holm 2000, Chandra and Berntsson 2003). Hossain (2006) found a

shrinkage increase of 14-26%, after 12 weeks of exposure at 50 5% RH, when concrete with natural

weight aggregates were replaced by scoria aggregates.

To the best of the authors knowledge there are only some few published studies about the durability

of concrete made with natural lightweight aggregates, and the information about their comparison with

normal weight concrete is still scarce.

Parhizkar et al. (2001) reported higher capillary absorption in normal weight concrete than in

lightweight concrete with pumice stone of equal w/c. According to the authors, since lightweight pumice

aggregates have a porous structure, the absorbed water is stored in the lightweight aggregates instead of

rising in the cement paste. Several authors refers that the higher quality of the transition zone in LWC

contributes to a lower permeability, leading to a decrease in the capillary absorption of water (Hossain et

al.2010, Holm and Bremner 2000, Bogas 2011).

Gndz and Uur (2004) found an increase of 15-55% in the carbonation depth of concrete with

pumice aggregates, for w/c variations of 0.14-0.59, which highlights the importance of the paste in the

carbonation resistance. Al-Khaiat e Haque (199a) reports decreases in carbonation resistance by replacing

normal weight sand by lightweight fine aggregates. The author justified as being due to the global higherporosity associated. Bremner, Holm and Stepanova (1994) documented lower carbonations on LWAC

with identical resistance level. That fact can be justified by the internal curing effect that delayed the

carbonation.

Assas (2012) reported reductions of 81% in the chloride penetration of concrete with 75% of natural

sand in comparison with a concrete made from pumice sand, exclusively. The author emphasizes the

importance of thepasteshigh permeability in pastes of high w/c. The author also refers that the pumice

sand porosity leads to increased paste permeability. The results that were previously mentioned are

identical to the results obtained by the study done by EuroLightCon (2000).

This paper aims to study and characterize the mechanical and long-term behavior of lightweight

structural concrete with volcanic scoria aggregates originated from the Azores, Portugal. The dry density,

compression strength, tensile strength, elasticity modulus, shrinkage, capillary absorption, carbonation

resistance and chlorides penetration resistance were analyzed and the results were compared to those

obtained for normal aggregate concrete of similar composition.

1. Experimental programme

1.1. Materials and methods

The experimental campaign involved the characterization of concrete made with coarse and fine

volcanic scoria aggregates. Two types of concretes of different w/c ratio were produced: common

structural concrete of w/c=0.56; high performance structural concrete of w/c=0.35. In parallel, reference

concrete with only normal weight aggregates were also produced (NWC). In sum, the following

compositions were analyzed for each type of w/c: normal weight concrete produced with crushed lime

and siliceous sand (NWC); Lightweight concrete produced with coarse scoria aggregates and natural sand

(LWCSC); lightweight concrete produced with coarse and fine scoria aggregates (LWCFSC).

The scoria aggregates from So Miguel (Azores) were obtained through the screening, milling and

separation of material taken from natural deposits. The values of particle dry density, p, crushed strength

and 24 hours water absorption, wabs,24h, of the slag aggregates are show in Table 1. This aggregate,

commercially designated as bagacina is composed by coarse aggregate (CBA) and fine aggregate

(FBA). Normal aggregates (NA) consisted on crushed limestone composed by coarse gravel (GA1) and

fine gravel (GA2). The natural sand was composed by 1/3 fine sand (FS) and 2/3 coarse sand (CS).


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Table 1 - Aggregate properties.

1.2. Mix proportions, concrete mixing and tests

Six mixtures were produced from three families of concrete, as indicated in Table 2. The maximum

size of the aggregate was 12.5 mm. All the concrete compositions are listed in Table 2. The effective

water/cement ratio (w/c) concerns the effective water available for cement hydration, which means that

does not include the water absorbed by aggregates. The Sp/c is the percentage of superplasticizer by

cement weight. In NWC, GA1 and GA2 were combined in order to ensure a grading curve close to the

one of coarse bagacina (CBA). For LWCFSC, the fine bagacina (FBA) and the natural sand (FS) werecombined to have the same grading of the natural sand in NWC and LWCSC.

The concrete was produced in a horizontal axis mixer. First, the aggregates were introduced in the

mixer and then moistened for 3 minutes with 50% of the mixing water. The absorption of lightweight

aggregates was estimated according to the method suggested by Bogas (2012a). Then, the cement, the

remaining water and, when needed, the superplasticizer, were added to the mixture. The final mixture

time was about 7 minutes.

For the physical and mechanical characterization of concrete the following specimens were produced

for each mix: two 100 mm cubic specimens for concrete density at 28 days; seventeen 150 mm cubic

specimens for compressive strength tests at 1, 3, 7, 28 and 90 days, according to EN 12390-3 (2003);

three 150x150 mm cylindrical specimens for splitting strength tests at 28 days according to EN 12390-6(2003); three 150x150 mm cylindrical specimens for the elasticity modulus at 28 days according to EN375 (2003); two prisms of 150x150x300mm for shrinkage tests according to LNEC E398 (1993). For the

durability tests the following specimens were also produced: three sawed 100x50 mm cylindricalspecimens for the rapid chloride migration test, according to NTbuild492 (1999); eight sawed 100x40mm cylindrical specimens for the accelerated carbonation test, according to LNEC E391 (1993); three

sawed 150x100 mm cylindrical specimens for the capillary absorption test, according to LNEC E393(1993) and TC116- PCD (1999).

Table 2 - Mix proportions and concrete density.

1.2.1. Curing process

After demoulding at 24 hours, except for the shrinkage, capillarity absorption, carbonation andchloride tests, the specimens were water cured until testing. The specimens for the shrinkage were stored

at 23 2 C and at a relative humidity of 50 5% since the first day after demoulding. The specimens for

CS FS GA1 GA2 CBA FBA

Particle dry density, p (kg/m) 2577 2584 2622 2641 1556 1963

Loose bulk density, b (kg /m) 1519 1593 1376 1370 705 1015

24 h water absorption, wabs,24h (%) 0.5 0.2 1.4 1.3 13.7 10.4

Crushing strenght (Mpa) - - - - - -

Sieve size fraction (di/Di) 0.5/4 0.25/0.5 8/12.5 4/8 0.063/11.2 0.125/4

LightweightNormal aggregatesNatural sandProperty

NWC 56 734 246 - 457 294 - 350 196 0.56 - 120 2288 2151

LWCSC 56 - - 590 457 294 - 350 196 0.56 - 95 1962 1762

LWCFSC 56 - - 590 - 301 340 350 196 0.56 - 95 1942 1651

NWC 35 736 246 - 447 336 - 440 154 0.35 0.5 120 2332 2293

LWCSC 35 - - 592 447 336 - 440 154 0.35 0.5 140 2010 1903

LWCFSC 35 - - 592 - 314 354 440 154 0.35 0.5 140 1989 1788

density

dry, d

(kg/m)

fresh, f

(kg/m)

Mixes

slump

(mm)SP/c(%)

FBA(kg/m)

FS(kg/m)

CS(kg/m)

CBA(kg/m)

GA2(kg/m)

GA1(kg/m)

effect.

water(L/m)

cement(kg/m)

effect.w/c


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the capillary absorption test were pre-conditioned according to LNEC E464 (1993): water cured for 7

days; then oven dried for 3 days at 50 C, followed by 17 days at 50 C and 1 day at 20 C without

moisture exchange. This procedure allows the redistribution of the water content across the specimen.

The specimens for carbonation and chloride tests were water cured for 7 days then placed in a controlled

chamber at a temperature of 22 2 C and relative humidity of 50 5% until testing.

1.2.2. Drying shrinkage

The total axial shrinkage was monitored by a demountable mechanical strain gauge (DEMEC) with a

precision of 1 m and a gauge length of 5 mm. The DEMEC was placed over two steel pins, 200 mm

apart, which had been glued onto one of the concretes moulded surfaces (Figure 1). The total shrinkageof each specimen was measured between 24 hours and 91 days.

Figure 1 - Scheme for measuring the shrinkage.

1.2.3. Capillary absorption

The absorption tests were carried out according to E393 (1993). This test basically consists of

determining the water absorption rate (sorptivity) of concrete by measuring the increase in the mass of aspecimen due to absorption of water as a function of time when only one surface of the specimen is

exposed to water. The exposed surface of the specimen was immersed in 5 1 mm of water and the massof the specimen was recorded 10, 20, 30, 60 minutes and 2, 6, 24 and 72 hours after the initial contact

with water. For each composition, three specimens were tested at 28 days old.

During the test, the specimens were covered with a bell-glass in order to avoid the water evaporation.

The water absorption and the absorption coefficient were calculated for each age. The absorption

coefficient was obtained from the slope of the linear regression line between min and hours.1.2.4. Carbonation resistance

After curing the top and the bottom surfaces of the sawed cylindrical specimens were painted so that

only lateral diffusion of CO2was possible. Then the specimens were exposed in a controlled chamber at

23 3 C, 60 5% relative humidity and 5 0.1 of CO2, according to LNEC E391 (1993). The

specimens were subjected to accelerated carbonation for 7, 28, 56 and 90 days. The carbonation depths

were measured in half-broken parts by spraying a phenolphthalein solution on the broken surfaces. Two

specimens were tested for each composition at a given age. The carbonation depth, , over time can bedetermined according to Eq. (1), where Kcrefers to the carbonation coefficient obtained from the linear

regression betweenXc and tn, n is usually assumed to be 0.5, especially if the test conditions are constant

over time (Bogas 2011).

(1)


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1.2.5. Chloride penetration resistance

The chloride penetration resistance was assessed by means of the non-steady state rapid chloride

migration test (RCMT) specified in NTDbuild492 (1999). An external electrical potential was applied,

and the chlorides ions were forced to migrate into the specimen. The time of the test depends on the

applied voltage. Then the specimen was half-split and sprayed with a silver nitrate solution. The chloride

penetration depth corresponds to the visible limit of the white silver chloride precipitation. The non-

steady-state chloride migration coefficient of diffusion (Dnscm) was calculated from Eq.2, where Trepresents the average value of the initial and final temperature of the anodic solution (C), L is the

specimens thickness (mm), U is the absolute value of the applied voltage (V), t is the test duration

(hours) andXdis the average value of the measured chloride penetration depth (mm).

nscm..(T).L(U).t d..(t).L.dU [11ms] (2)

2. Results and discussion

The dry density, d, the compressive strength, fcm, the structural efficiency (fcm/s), the tensile

strength, fctm, the elasticity modulus,Ec, the total drying shrinkage at 90 days, cs,90d, the 24 hours capillary

absorption, abs24h, the coefficient of capillary absorption, Cabs, the carbonation depth at 90 days,Xc,90d, the

carbonation coefficient,Kc, and the coefficient of chlorides diffusion, Dnscm, are listed in Table 3 for each

composition.

Table 3 - Dry density, compressive strength, modulus of elasticity, dry shrinkage, capillary absorption,

carbonation and chloride diffusion.

2.1. Concrete dry density

As expected, the concrete dry density decreased with the replacement of normal weight aggregates by

scoria aggregates (CBA and FBA) (Table 3). The incorporation of CBA lead to a density reduction of

18%, and the additional incorporation of FBA contributed to a further 5% reduction of the concrete

density. From Table 3, it is shown that depending on the w/c ratio, the introduction of CBA and FBA

allowed the production of structural lightweight concrete with density classes of D1.8 - D2.0, according

to EN 1992-1-1 (2010).

2.2. Compressive strength

Taking into account the obtained results, the production of common LWC led to a reduction of 18%

(LWSCS) and 26% (LWCFSC) in the compressive strength and the production of high strength LWC led

to a reduction of 20% (LWSCS) and 28% (LWCFSC), when compared to NWC. As expected, the

reduction was greater for high strength concrete.

As it is well recognized (Chen et al. 1995, Bogas et al. 2013a), the compressive behavior of

lightweight concrete is strongly dependent on the limit strength, fL, and the ceiling strength, fcs. fL

Shrinkage chloride diffusion

cs,90days Abs24h Cabs Xc,90d Kc Dnscm,28d

(10m/m) (Kg/m) (x10mm/min) (mm) (mm/year) (x10 m/s)

NWC 56 2151 44.9 2.1 3.9 35.4 418 5.61 0.155 13.0 26.5 16.67

LWCSC 56 1762 36.8 2.1 3.3 21.7 458 6.53 0.176 22.5 42.5 17.36

LWCFSC 56 1651 33.2 2.0 2.7 20.0 514 8.06 0.237 25.5 50.4 25.59

NWC 35 2293 72.2 3.1 5.7 46.2 384 1.79 0.050 1.5 3.0 7.55

LWCSC 35 1903 57.5 3.0 3.9 31.5 500 2.66 0.084 7.0 12.5 11.30

LWCFSC 35 1788 51.9 2.9 3.3 24.4 450 2.85 0.098 11.0 20.3 11.85

Mixes

fcm,28d

(Mpa)

fctm,28d

(Mpa)

Ec,28d

(Gpa)

fcm,dd

(10m)

Capilary absorption carbon.resistanceDry

density,

d (kg/m)


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corresponds to the strength for which the modulus of elasticity is similar to that of the aggregate. Above

fLthe strength of concrete is also affected by LWA and is lower than the mortar strength. fcscorresponds

to the highest bearing strength of LWAC, beyond which an increment of the mortar strength has little

influence on the concrete strength.

The limit strength, fL, can be obtained graphically from the relationship between the strength

development of the concrete and the corresponding mortar (Chen et al.1995, Bogas et al. 2012). This

procedure is shown in Figure 2, where fLwas estimated as being about 36 MPa, based on the mixturesLWSC0.56/0.35 and in the corresponding mortars of equal w/c and sand/cement ratios. From the same

Figure 2 it is possible to conclude that above about 50 MPa, the increment of compressive strength is not

meaningful.

Figure 2 - Limit strength of LWC with coarse bagacina.

From this study we may conclude that for compressive levels up to about 35 MPa, LWC with

bagacina has a similar behavior to the NWC. In other words, the decrease of concrete density does not

affect much the mechanical properties of LWC with bagacina. This means that the structural efficiency

(fcm/s) of concrete with bagacina should be higher than that of NWC, in this strength range.

Above 36 MPa, the strength of LWC with bagacina also strongly depends on the characteristics of

lightweight aggregates and the structural efficiency decreases. As it is shown in Table 3, the structural

efficiency is similar to slight lower in LWC with bagacina than in NWC. In this case, the use of structural

LWC produced with bagacina is more competitive where the permanent load is a relevant factor for the

structural design.

Above 50 MPa, the structural efficiency of LWC with bagacina strongly decreases and its production

is not economically justifiable. The reduction attained in density is not compensated by the important loss

of concrete strength. Therefore, the production of high-strength LWC with bagacina is only justified

when the concrete density and durability are the main factors for the structural design, such as in some

particular rehabilitation works.

From Figure 2, and based on the biphasic model suggested by Chandra and Berntsson (2003), Eq. (3),

the strength of SLWAC as a function of the volume and strength of the mortar and the LWA can be easily

estimated. In Eq. (3), fcm is the mean concrete compressive strength and fm is the mean compressive

strength of the mortar of the same composition as that used for the concrete; vmand vLWAare the relative

volumes of cement and LWA in the mix; fLWAis the strength of the aggregate in the concrete. From theexperimental results,fLWAis estimated as 38-39 MPa, according to Eq.3. The reasonable accuracy of the

suggested expression was shown by Bogas et al.(2013a).

y = 0.9854x - 2.6769

R = 0.9926

y = 0.6633x + 10.362

R = 0.8738

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90

CompressivestrengthLWCSC(MPa)

Compressive strength mortar (MPa)

LWSCS

mortarft = 37.2 MPa


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log(fcm) LA.log(fLA) m.log(fm) (3)The development of the compressive strength with time in LWC with CBA and FBA is shown in

Figure 3. As expected, the compressive strength development of LWC with bagacina tends to be lower

than that of NWC, because the strength of LWC at later ages is limited by the aggregate capacity. This is

more relevant the higher the mortar strength (w/c=0.35). Therefore, the development curve estimated

according to EN 1992-1 (2010) was adequate for conventional concrete of high w/c, but less appropriate

for low w/c lightweight concrete.

Figure 3 - Compressive strength development of LWC and the development curve suggested by EN 1992-1-1 (2010):w/c = 0.56 (top); w/c = 0.35 (below).

2.3.Tensile strength

As for the compressive strength, the tensile strength decreased with the incorporation of bagacina

aggregates. Depending on the w/c ratio, the strength decrease was 15% and 31% in LWC of 0.56 and

0.35, respectively. As expected, the reduction is lower in concrete with higher w/c, where the strength is

less governed by the aggregate. The further partial replacement of sand by FBA led to an additional

decrease of 16% (w/c=0.56) to 11% (w/c=0.35). Nevertheless, the obtained tensile strength was always

higher than the minimum recommended by ASTM C330 (2004) for structural LWC (2 MPa). When

compared to NWC, the reduction in LWC was higher in tensile strength than in compressive strength,

which means that the structural efficiency for tensile strength tends to be lower. This is in accordance

with the lower limit strength,fL, usually reported for tensile strength (Bogas et al, 2011, Holm 2000).

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

1,2

0 10 20 30 40 50 60 70 80 90

Hardnesscoefficient(%)

Age (days)

NWC 56 LWCSC 56 LWCFSC 56 EN 1992-1-1 (2010)

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

1,2

0 10 20 30 40 50 60 70 80 90

Hardnessefficient(%)

Age (days)

NWC 35 LWCSC 35 LWCFSC 35 EN 1992-1-1 (2010)


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As shown in Figure 4, there is a good correlation between fctm,spandfcm, and the expression Eq.(3) is

obtained. The relation between fctm,spandfcmwas about 6.4 to 9%, which is within the range documented

by other authors (e.g. Hoff, 1992; Haque et al., 2004; Curcio el al.,1998; Coquilat et al.,1986).

The expression suggested by EN 1992-1-1 (2010) that relates the compressive strength with the

tensile strength leads to conservative estimates of the tensile strength of concrete with bagacina (Figure

4). The obtained values were generally 5 to 10 % higher than those estimated by the EN 1992-1 (2010).

Figure 4 - Relation between tensile strength and compressive strength.

2.4. Modulus of elasticity

The incorporation of bagacina aggregates (CBA) led to an important reduction of 32% (w/c=0.56) to

39% (w/c=0.35) in the modulus of elasticity. The incorporation of FBA led to an additional reduction of

5% (w/c=0.56) and 15% (w/c=0.35), respectively. The high reduction is easily explained by the lower

stiffness of aggregates, being higher for greater volumes of aggregates in concrete. When compared to

NWC, the reduction tends to be higher for low w/c concrete. This can be partly explained by the

reduction of the elastic compatibility between bagacina and mortar for high strength levels.

The expression suggested by the EN 1992-1-1 (2010) that relates the modulus of elasticity with the

compressive strength and concrete density leads to a conservative estimate of the elasticity modulus of

concrete with bagacina (Figure 5). Differences lower than 13% were obtained between the experimental

and the theoretical values estimated by the EN 1992-1 (2010).

Figure 5 - Relation between the elasticity modulus and the compressive strength .

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

30 40 50 60 70 80

Tensilestrenght,28days(Mpa)

Compressive strenght, 28 days (MPa)

present study EN 1992-1-1 2010

R = 0.809

R = 0.966

0

5

10

15

20

25

30

35

40

45

50

20 25 30 35 40 45 50 55 60 65 70

Modulusofelasticity,28

days(Gpa)

Compressive strenght, 28 days (MPa)

present study EN 1992-1-1 2010

R = 0.864

R = 0.756


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2.5. Drying shrinkage

As expected, the incorporation of porous bagacina aggregates (CBA and FBA) leads to a higher long-

term shrinkage than that of NWC of equal composition. For common concrete, the lower stiffness of

bagacina contributed to a shrinkage increase of 10% (LWCSC) and 23% (LWCFSC), after 91 days.

However, the shrinkage at early ages was lower in LWC, especially when only the coarse normal

aggregate was replaced by CBA. This can be explained by the internal curing effect provided by the

porous aggregate that can act as internal water reservoirs during the early ages (Holm 2000, Neville1995). In fact, these water pockets can compensate the water lost by evaporation and hydration reactions,

and the total shrinkage is delayed (Figure 6 and 7). However, at later ages the internal curing is ceased

and the less stiffness lightweight concrete has higher shrinkage.

The water stored in coarse bagacina seems to be enough to compensate the water lost during the early

ages (Figure 6 and 7). This can explains the higher early shrinkage obtained for LWCFSC produced with

coarse and fine bagacina and hence with a lowered restriction effect.

The expression suggested by EN 1992-1-1 (2010) leads to conservative values of the total shrinkage,

by a factor of about 1.55 to LWCSC 56 and 1.23 for LWCSC 35 (Figure 8). The suggested curve cannot

accurately predict the shrinkage evolution in these types of lightweight concretes, especially at early ages.

The same is concluded by Bogas et al.(2014) for concrete with expanded clay lightweight aggregates.

Figure 6 - Drying shrinkage until 91 days for series w/c = 0.56.

Figure 7 - Drying shrinkage until 91 days for series w/c = 0.35.

-600E-06

-500E-06

-400E-06

-300E-06

-200E-06

-100E-06

000E+00days (log scale)

NWC 56

LWCSC 56

LWCFSC 56

3 91287

-600E-06

-500E-06

-400E-06

-300E-06

-200E-06

-100E-06

000E+00

days (log scale)

NWC 35

LWCSC 35

LWCFSC 35

7 28 913


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Figure 8 - Experimental shrinkage of LWC versus the estimated shrinkage according to EN 1992-1-1 (2010).

2.6. Capillary absorption

Depending on the w/c ratio of concrete, the incorporation of bagacina aggregates led to an increment

of about 14-68% (LWCSC) and 53-96% (LWCFSC) on the coefficient of absorption (Table 3). Thiscould be expected since LWC has higher open porosity than NWC. This is in accordance with the results

reported in Eurolightcon (2000). However, different trends are reported by some authors, where LWC

presents similar to lower capillary absorptions (Bogas 2014, Hammer e Hansen 2000, Parhizkar et al.

2011). This is attributed to the better interface transition zone (ITZ), prolonged internal curing and higher

water content of LWC.

Although the quality of the paste and ITZ can have a great influence on the capillary absorption, the

uptake of water was higher in concrete with more porous aggregates. On one hand, the inexistence of an

external layer in bagacina aggregates, as it happens with expanded clay aggregates, contributes for an

easier participation of the porous structure of aggregates. On the other hand, the higher initial absorption

of LWC, as shown in Figure 9, can greatly affect the rate of capillary absorption. In fact, contrary to

normal weight aggregates, the lightweight aggregates exposed in the bottom surface of the specimens

allow the easy access of water through the whole section of the concrete sample, i.e., more area of the

specimen participates in the absorption mechanism. In opposition, the absorption in NWC essentially

occurs through the paste. This surface phenomenon can partly explain the higher differences obtained

between LWC and NWC for low w/c mixtures. In fact, for low w/c pastes the water penetration is lower

and the surface phenomena can assumes more relevance.

Figure 9 - Capillary water absorption up to 72h.

-800E-06

-700E-06

-600E-06

-500E-06

-400E-06

-300E-06

-200E-06

-100E-06

000E+00

days (log scale)

LWCSC 56

LWCSC 35

EN 1992-1-1 (2010), (w/c = 0.56)

EN 1992-1-1 (2010), (w/c = 0.35)

3 91287

0,0

2,0

4,0

6,0

8,0

10,0

12,0

0 10 20 30 40 50 60 70

Wate

rabsorption(Kg/m)

Time (min)

NWC 56

LWCSC 56

LWCFSC 56

NWC 35

LWCSC 35

LWCFSC 35


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As expected, the capillary absorption increased with the incorporation of lightweight sands (FBA)

and strongly decreased in concrete of lower w/c ratio. The results show that the w/c factor has greater

importance on capillary absorption than the type of aggregate. Nevertheless, in mixes of low w/c (w/c =

0.35) it was possible to attain lower coefficients of absorption than 0.1 mm/min0.5

, which corresponds to

high-durable concrete according to the Brownes classification (11).

2.7. Carbonation resistance

As mentioned, the carbonation depth can be roughly related to the root of time (2.2.4), which is

why the carbonation depth in Figures 10-11 is shown as a function of t. Except for LWC of w/c=0.35,the correlation coefficients are higher than 0.98, which means that Eq. (4) is adequate and the coefficient

of carbonation, Kc, can be determined directly. Due to the higher open porosity of bagacina, the

carbonation depth is higher in LWC than in NWC, regardless the w/c ratio (Table 3). The same is

concluded by Eurolightcon R18 (2000) for concrete with natural lightweight aggregates. In fact, the more

porous aggregates can contribute to a higher diffusion of CO2 through concrete. Therefore, the further

partial replacement of natural sand by FBA also increases the mortar porosity and hence the carbonation

depth. The same was found by Assas (2012) in LWC with pumice aggregates and Bogas (2011) in LWC

with expanded clay aggregates.

However, Figure 11 shows that the carbonation depth until 28 days was similar in NWC andLWC of low w/c ratio. This can be explained by the low carbonation depths attained until this age, being

less than 2-4 mm, which means that aggregate particles are not reached. However, as soon as the

carbonation depth reaches the first aggregate particles, the diffusion of CO2 strongly increases. The

phenomenon is particularly relevant in bagacina aggregates, which do not present a dense outer shell.

This also explains why the curves of LWC are not linear and the regression coefficients are lower. Based

on the Kc values listed in Table 3, it is possible to roughly estimate the carbonation rate under real

exposure conditions. For this, we may simply assume that the carbonation coefficients obtained from

laboratory tests, Kc,lab, are related to those obtained from real exposure conditions, Kc, real, according to

Eq. (4), where cc,realis the concentration of CO2in real environment (0.7x10-3

kg/m3is assumed) and cc,lab

corresponds to the concentration of CO2 in the accelerated chamber (90x10-3

kg/m3). The binding

capacity and diffusion of CO2are simply assumed to be similar in the real and accelerated environment.

(4)

From Eq. (1) and Eq. (4), it is estimated that even in LWSC of high w/c a carbonation depth of

25 mm is only attained after about 44 years, on average. Moreover, in mixtures of low w/c the same

carbonation depth can be only achieved after about 500 years of real exposition. Therefore, despite the

differences between mixtures, it may be concluded that lightweight concrete with bagacina can be a

durable solution and the corrosion induced by carbonation should not be a relevant degradation

mechanism. Also note that the accelerated tests were performed under extremely severe and conservative

environmental conditions, with a constant relative humidity of 65%.

As shown in Figure 12, there is only a reasonable exponential correlation between the

compressive strength and Kc(Table 3). Actually, for high strength levels the durability is more affected

by the quality of the paste and the mechanical strength is more affected by the characteristics of bagacina.

According to the results reported by Ho and Lewis (1987) in normal weight concrete subjected to 4%

CO2and 50% RH, average Kc,realvalues of about 28.7 mm/day0.5

are estimated for a w/c ratio of 0.55,

after the conversion to 5% CO2, which are of the same order of magnitude as those obtained in our study

for NWC56.


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Figure 10 - Accelerated carbonation depth versus the square root of time until 90 days - w/c=0.56.

Figure 11 - Accelerated carbonation depth versus the square root of time until 90 days - w/c=0.35.

Figure 12 - carbonation coefficient versus the compressive strength.

2.8.Chloride penetration resistance

From the obtained results it may be concluded that the incorporation of bagacina led to a reduced

chloride penetration resistance (Figure 13). Since the chloride penetration resistance depends essentially

on the paste quality, a more similar behavior was expected between NWC and LWC, especially for highstrength concrete. However, this was only valid for the less compact concrete (w/c=0.56). In sum, the

diffusion coefficient was 4% (w/c=0.56) and 33% (w/c=0.35) higher in LWCSC and 35% (w/c=0.56) and

0

5

10

15

20

25

30

Carbonationdepth(mm)

time (days)

NWC 56 LWCSC 56 LWCFSC 56

7 28 56 90

0

2

4

6

8

10

12

Carbonationdepth(mm)

time (days)

NWC 35 LWCSC 35 LWCFSC 35

7 28 56 90

0

10

20

30

40

50

60

70

30 40 50 60 70 80

Kc1(mm/ano

)

fcm,28 (Mpa)


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36% (w/c=0.35) higher in LWCFSC, when compared with NWC. Assas (2012) reported an increase from

4 to 12x10-12

m2/s, when 50% of natural sand was replaced by fine pumice aggregate. As expected, the

coefficients of diffusion were lower in low w/c concrete.

The higher chloride penetration in LWC can be partly explained by the surface effect, as mentioned

for capillary absorption. This is more relevant in low w/c concrete where the penetration depth is lower

than in concrete of w/c=0.56. The better elastic compatibility and higher quality of the ITZ in the high

w/c lightweight concrete can also contribute to its better performance. Nevertheless, the diffusioncoefficients obtained for LWCSC35 were abnormally higher than expected, especially when compared

with concrete with fine bagacina (LWCFSC). Also note, that the specimens were previously soaked

before testing, which increases the participation of the saturated porous aggregates. However, in real

cases aggregates are usually not saturated after a short period of drying.

As shown in Figure 14 there is a high correlation between the coefficient of diffusion and the

coefficient of capillary absorption. Bogas (2011) also found high correlations between these properties in

LWC produced with expanded clay aggregates. However, unlike the present study, the author found a

similar behavior between NWC and LWC for both properties. Note, that each analyzed property is

governed by a different penetration mechanism (capillary absorption versus diffusion). However, the

porous system is the same, although the diffusion mechanism is less affected by the porous size. The high

correlation is also related to the fact that both properties were affected by the mentioned surface effects.

As shown for carbonation, there is also a reasonable exponential correlation between the diffusion

coefficients and the compressive strength (Table 3).

Figure 13 - Coefficient diffusion.

Figure 14 - coefficient of diffusion versus coefficient of capillary absorption.

16,717,4

25,6

7,5

11,3 11,8

0

5

10

15

20

25

30

NWC 56 LWCSC 56 LWCFSC 56 NWC 35 LWCSC 35 LWCFSC 35

Dnscm

(x10m/s)

0

5

10

15

20

25

30

0 0,05 0,1 0,15 0,2 0,25

Dnscm

(x10m/s)

Coef. of absorption (x10g/(mmxmin)

NWC

LWCSC

LWCFSC


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3. Conclusions

In the present study, the mechanical and durability behavior of structural lightweight concrete

produced with natural scoria from Azores (bagacina) were analyzed. The following main conclusions

have been drawn:

- It was possible to produce structural LWC with natural scoria aggregates with strength

classes ranging from LC25/28 to LC40/44 and density classes from D1.8 to D2.0.

-

The mechanical behavior and the range of strength levels for the production of efficient

LWC with bagacina were analyzed. For compressive strengths up to about 35 MPa, the

behavior of LWC with bagacina is similar to that of NWC and the structural efficiency

attains the maximum value. In this range, LWC produced with bagacina can be a very

effective alternative solution to NWC, especially when the permanent load is a major factor

for the structural design. For compressive strength levels above 50 MPa, the density

reduction is not compensated by the high loss of compressive strength and the structural

efficiency is strongly reduced

- As expected, the incorporation of bagacina led to a proportional reduction of the tensile

strength and modulus of elasticity. The structural efficiency for tensile strength tends to be

lower than that for compressive strength

-

Conservative differences of about 10%, for tensile strength, and about 13%, for modulus of

elasticity, were obtained between the experimental and theoretical values estimated by EN

1992-1 (2010)

- The long-term shrinkage increased when normal weight aggregate was replaced by LWSA.

However, the internal curing provided by LWSA contributes to the shrinkage delay at early

ages

- All the durability properties analyzed were negatively affected by the incorporation of the

porous scoria in concrete. Even though, high durable properties were attained in lightweight

concrete produced with bagacina, especially in low w/c mixtures

- The coefficient of absorption was as high as 96%, when normal weight aggregate was

replaced by coarse and fine bagacina. The absence of a dense outer-shell in scoria

aggregates and the higher initial absorption of LWC specimens due to surface effects canpartly contribute to the obtained results.

- The more porous aggregates can contribute to a higher diffusion of CO2through concrete.

This is only relevant when the aggregates are attained by the carbonation front, which can

take too long in low w/c concrete. Moreover, it can be concluded from this study that even

with pastes of low to moderate quality the mechanism of carbonation is not determinant to

durability. For low w/c concretes, it was roughly estimated that a carbonation depth of about

25 mm is attained only after 500 year of real exposure.

- A high linear correlation is obtained between the chloride diffusion coefficient and the

coefficient of capillary absorption. The compressive strength is exponentially correlated

with the carbonation and chloride diffusion coefficient. For high strength levels, the

durability is more affected by the quality of the paste and mechanical strength is moreaffected by the properties of LWSA.

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