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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