New approach for shrinkage prediction of high-strength lightweight aggregate concrete

Construction and Building Materials 35 (2012) 84–91

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New approach for shrinkage prediction of high-strength lightweight aggregate concrete Hugo Costa a,b,⇑, Eduardo Júlio b, Jorge Lourenço a a b

Department of Civil Engineering, Polytechnic Institute of Coimbra, Rua Pedro Nunes – Quinta da Nora, 3030-199 Coimbra, Portugal ICIST, Department of Civil Engineering and Architecture, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

a r t i c l e

i n f o

Article history: Received 4 March 2011 Received in revised form 9 January 2012 Accepted 25 February 2012

Keywords: Shrinkage High-strength Lightweight aggregate concrete (LWAC)

a b s t r a c t The use of saturated lightweight aggregates (LWAs) on concrete provides an efficient internal curing, resulting in reduced shrinkage. Codes predict concrete shrinkage based on concrete strength, type of cement, geometry and curing conditions, being mixture design parameters neglected. Herein, an experimental study is presented, considering several high-strength lightweight aggregate concrete (HSLWAC) mixtures with different densities and compressive strengths. Compared to codes prediction, significantly lower shrinkage values were measured. It was concluded that, besides the parameters considered by codes, HSLWAC shrinkage also depends on the type, dosage and moisture of LWA. A new approach for shrinkage prediction of HSLWAC is presented. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The recent production of lightweight aggregates (LWAs) with high mechanical strength, associated with the use of more efficient binders and third generation superplasticizers, allowed the development of high strength lightweight aggregate concrete (HSLWAC) [1]. Depending on the specified density, generally between 1500 and 2000 kg/m3, it is possible to produce HSLWAC with compressive strength varying from 40 to 90 MPa [2]. The improved high-strength is usually obtained by designing binding paste matrixes with reduced water/binder (W/B) mass ratio and considering pozzolanic additions, e.g. silica fume [3]. However, due to the higher sensitivity to self-desiccation of the binding matrix, these design options lead to higher shrinkage of high performance concrete, produced with normal weight aggregates, and therefore to an increased risk of early-age cracking [4–6]. Nevertheless, this risk can be reduced using an efficient shrinkage-reducing admixture (SRA) [7,8]. When compared to normal weight concrete (NWC), different studies [9–14] proved that shrinkage in HSLWAC is usually lower. This happens when saturated LWA are used, since an efficient internal curing is promoted by an uninterrupted hydration of the binding paste, as a result of a continuous flow of LWA moisture stored in its alveolar internal structure, avoiding self-desiccation ⇑ Corresponding author at: Department of Civil Engineering, Polytechnic Institute of Coimbra, Rua Pedro Nunes – Quinta da Nora, 3030-199 Coimbra, Portugal. Tel.: +351 960 071 786; fax: +351 239 790 311. E-mail addresses: [email protected] (H. Costa), [email protected] (E. Júlio), [email protected] (J. Lourenço). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2012.02.052

[1,9,10,15,16]. This gives rise to expansive autogenous deformations [10,12,15] and thus to reduced total shrinkage. Also improved transition zone between the LWA and the binding matrix is observed, reducing its permeability [1,17,18]. Despite the enhanced internal curing of HSLWAC, there is a remaining risk of superficial micro-cracking at early ages, due to differential shrinkage between the cover mortar matrix and the interior of the concrete element. Therefore, special care on superficial concrete curing is needed and the use of a curing membrane or of an SRA admixture is mostly recommended. The effectiveness of internal curing and shrinkage reduction of HSLWAC is influenced by several factors: LWA proportioning; LWA saturation degree; specimen size; starting age of drying shrinkage and type of cement and additions [10,12,15].

2. Design codes According to Eurocode 2 (EC2) and Model Code 2010 (MC10) [19,20], the total shrinkage of NWC, ecs, is the sum of drying shrinkage, ecd, with autogenous shrinkage, eca. In line with these codes, shrinkage of both LWAC and HSLWAC is predicted based on expressions developed for NWC, adjusted using a corrective coefficient g. However, that sum is a simplification, since both parameters are due to the reduction of the relative humidity (RH) of the concrete, which in the case of drying shrinkage occurs due to evaporation at the concrete’s surface, while in the case of autogenous deformation is caused by partial emptying of the gel pores as a consequence of cement hydration [18,21]. If the concrete has a low RH, due to

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H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

self-desiccation, it has a reduced shrinkage when submitted to drying, but if it has high internal RH due to internal curing (using wet LWA) it will have a reduced shrinkage in autogenous conditions, or even an expansion. This autogenous deformation is uniform in the concrete element, occurring mainly in the first month, while drying shrinkage is non-uniform in the cross-section, with higher shrinkage in the exposed surfaces than inside the element, depending mainly on the moisture transport to the drying surfaces [21]. Therefore, autogenous and drying shrinkage are not independent and their sum does not correspond, in reality, to total shrinkage. For both EC2 and MC10, the evolution of ecd with age depends on concrete strength, cement type, geometry, curing conditions and starting age of drying shrinkage, whereas the evolution of eca with age only depends on the concrete strength, for EC2, and on concrete strength and cement type, for MC10. Although both EC2 and MC10 state that eca is significantly reduced for LWAC produced with saturated LWA, due to internal curing provided by the aggregates’ moisture, this reduction is not quantified. The approach of ACI [22,23] for shrinkage prediction is addressed using a different expression for the evolution of ecd with age. According to this, besides the parameters considered on EC2 and MC10, the evolution of ecd also depends on some properties of the concrete mixture, such as slump, air content, fine aggregates percentage and cement dosage. Design codes do not consider the LWA moisture to predict the shrinkage. However, it is known that to produce an HSLWAC, partially or totally saturated LWA must be used, and that it is essential to accurately quantify and control the LWA absorption water in the mixture design [24]. Dry LWA can also be used, considering additional absorption water, which should be added during the mixing, but this will result on a mixture with partially saturated aggregates. Thus, it is better to previously mix this water with LWA during about 2 min. Otherwise, these will absorb the effective water from the binding matrix, compromising the concrete workability and the specified properties [2]. For this reason, the beneficial effect of the LWA moisture to the internal curing and to the expansive autogenous deformation needs to be considered. Since EC2 and MC10 shrinkage predictions for HSLWAC lead to values higher than those measured in practice, presented in Section 4.2 ahead, and ACI prediction leads, in some cases, to even higher values, there is an obvious need to further investigate this issue. The study herein presented was conducted aiming to improve codes prediction. Given that both the compressive strength and the Young’s modulus of HSLWAC depend essentially on the binding paste and on the LWA type and dosage [2,24–26], an experimental study was conducted considering several HSLWAC mixtures to quantify the influence of these parameters on drying shrinkage and on autogenous deformation.

analysis of these aggregates, the following properties were also characterized (Table 1): dry particle density, qP0; dry bulk density, p0; interior moisture, HP; absorption from natural stocking state, AN; total absorption to saturated state, AS; and crushing strength, fCr. First, saturated particle density was characterized, followed by the total absorption (percentage of water by mass of the dry LWA), and then the dry particle density was calculated. Total absorption corresponds to the total water inside the LWA, in saturated state, and this is the responsible for the internal curing of LWAC. Afterwards, the interior moisture was determined on stocking state, being this used in mixture design to quantify the mass dosage of each LWA. Since LWAs are not usually saturated in stock conditions, absorption from stocking state (percentage of absorption water by mass of LWA in stocking state) was calculated, in order to quantify the absorption water of the mixture [2]. However, the interior moisture of the LWA, as well as the absorption water, is not considered neither for the effective water of the binding paste nor for the W/B ratio. 3.2. Mixture design method for HSLWAC A mixture design method was specifically developed for HSLWAC [28], based on Faury’s method [27] and on Feret’s expression [24]. The desired density for concrete is assured by an appropriate selection of LWA and by adjusting the granulometric curve of the mixture to Faury’s reference curve. The aimed compressive strength, flc,p, is obtained based on Feret´s expression, (1), by first estimating the strength of the binding paste, fc,b, which depends on the paste compactness, c, and on the Feret’s coefficients, kj, and by multiplying this by a corrective coefficient, Cf, that depends on the LWA strength and dosage [2,26]. The expression (2), proposed to predict Cf, was adjusted based on correlations defined between the density, qP0, and the crushing strength, fCr, of the LWA and on the corresponding reduction in strength of the HSLWAC [2,26]

fc;b ¼ kj  c2

Cf ¼

n  Y

1

i¼1

ð1Þ V abs;i q2:9  0:2  0:51 P0;i 100

 ð2Þ

where Cf is the strength reducing coefficient of the LWA blend; Vabs,i is the absolute volume dosage, in dm3, of LWA i; qP0,i is the dry particle density, in kg/dm3, of LWA i; and n is the number of different types of LWA used in the mixture. 3.3. HSLWAC produced mixtures Several mixtures of HSLWAC were produced (Table 2) by considering different aggregates and proportions and by varying the cement dosage, with 5% silica fume addition. First, the mixture compactness (relation between the volume of the solid constituents and the total volume of the HSLWAC) and the water and admixture dosages were adjusted to the wanted density and consistency. Afterwards, these were combined with the remaining parameters, as coarse/fine volumetric relation and as fine and coarse aggregates pre-blends. The main parameters of the HSLWAC mixtures, such as LWA and air proportions, W/B ratio and the predicted strengths (fc,b and flc,p), are presented in Table 2.

4. Shrinkage results and discussion Shrinkage was measured not only for all the produced HSLWAC mixtures, but also for most representative mortar matrixes of HSLWAC, in order to assess the influence of mortar shrinkage on the corresponding HSLWAC shrinkage. Both HSLWAC and mortar specimens were stocked in a thermo-hygrometric chamber, at 20 °C and 50% R.H., 24 h after its production.

3. Materials and methods

4.1. Mortar matrixes According to the wanted properties both in fresh and hardened states, namely, consistency, density and compressive strength, the HSLWAC mixtures were designed by adopting optimized proportions of cement, pozzolanic addition, lightweight and/or normal weight sand and coarse LWA. With this aim, the granulometric adjustment was performed using Faury’s reference curve [24,26,27]. The adequate dosage of water was quantified for each mixture, providing hydration of the binding powder and simultaneously ensuring the requested consistency and workability, taking into account the adopted superplasticizer.

For each of the HSLWAC adopted mixtures (types B and E, with S5 slump class, and types C and F, with S4 slump class), mortar matrix specimens were produced. The constituents’ proportions for each mortar were obtained by multiplying the proportions of the

3.1. Materials

Table 1 Characterized properties of the Leca LWA.

The following constituents were adopted for the binding paste: cement CEM II/ A-L 42.5R, an addition of silica fume, a third generation superplasticizer and water. Two types of normal weight sand, 0/2 mm (FS) and 0/4 mm (MS), and lightweight expanded clay aggregate (Leca) sand, 0.5/3 mm (XS), were used. Three types of Leca were adopted as coarse aggregates: structural Leca 2/4 mm (HD2/4), structural Leca 4/12 mm (HD4/12) and structural Leca 4/10 mm (MD). Besides the granulometric

LWA

qP0 (kg/dm3)

p0 (kg/dm3)

HP (%)

AS (%)

AN (%)

fCr (MPa)

HD2/4 HD4/12 MD XS

1.33 1.09 0.83 1.02

0.74 0.60 0.46 0.58

9.6 19.1 0.6 6.1

11.5 19.7 11.4 12.9

1.6 0.5 10.3 6.0

11.0 6.8 3.7 5.2

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H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

Table 2 Types of produced HSLWAC mixtures. HSLWAC

Density (kg/m3)

Cement (kg/m3)

Fine aggreg.

Coarse LWA 3

W/B

Air (%)

fc,b (MPa)

flc,p (MPa)

Slump class

3

Type

(dm /m )

B500.S4 B500.S5 B425.S5

1950

500 500 425

FS + MS

HD2/4

315 308 309

0.29 0.32 0.36

0.25 0.25 0.25

87 80 81

75 69 61

S4 S5 S5

E425.S4 E500.S5 E425.S5

1800

425 500 425

FS + XS

HD2/4

354 350 338

0.32 0.32 0.36

0.25 0.25 0.25

77 80 81

57 61 53

S4 S5 S5

D425.S4

1800

425

FS + MS

MD

326

0.32

0.20

79

45

S4

C500.S4 C425.S4 C350.S4 C500.S5

1800

500 425 350 500

FS + MS

HD4/12

423 426 431 402

0.29 0.32 0.37 0.33

0.20 0.20 0.20 0.20

89 79 69 79

59 52 46 53

S4 S4 S4 S5

F500.S4 F425.S4 F350.S4 F500.S5

1600

500 425 350 500

FS + XS

HD4/12

423 426 429 415

0.29 0.32 0.37 0.33

0.20 0.20 0.20 0.20

89 79 69 79

51 45 40 46

S4 S4 S4 S5

corresponding concrete mixture by the volumetric proportion of the mortar. The mortar mixtures type E and type F contain LWA sand with the respective absorption water; however, due to its reduced particle size, the interior moisture will fast flow to the binding matrix. The corresponding shrinkage, ems, was assessed by measuring the deformations on three prismatic specimens with 40  40  160 mm3, starting at 24 h of age (after demoulding), until no significant evolution was detected. In Fig. 1, the evolution of ems with age of the produced mortar mixtures is presented. It was registered that the analysed mortars exhibit similar evolution for shrinkage curves, although presenting different amplitudes. Approximately 80% of the measured ems occurred during the first 28 days of age. The obtained results proved that ems increases with the following parameters: (i) increase of cement dosage; (ii) use of LWA sand blended with FS (mixtures E and F), instead of the blend of FS and MS (mixtures B and C); (iii) decrease of sand dosage (mixtures C and F) and, thus, with the increase of coarse LWA dosage. 4.2. HSLWAC mixtures

1000

1200

800

1000

ε ms (× 10-6 )

ε ms (× 10-6 )

Both total shrinkage, ecs, and autogenous deformation, eca, were experimentally assessed using two pairs of prismatic specimens

with 150  150  600 mm3, starting at 24 h of age (after demoulding), according to the standard [28]. The specimens used to measure eca were appropriately sealed with a triple layer of plastic film and hot glue, and no mass variations were registered, assuring that no evaporation occurred. In Fig. 2, the evolution of total shrinkage, ecs, for each HSLWAC mixture, as well as the corresponding EC2 prediction, is presented. The shrinkage curves proposed by MC10 and by ACI are also presented, and compared with the measured values, in Figs. 3 and 4, respectively. Shrinkage expressions proposed by EC2 and MC10, although different, result in similar curves. Neither strength nor density are considered in ACI for shrinkage prediction, but mixture parameters, such as slump, fine aggregates percentage and air content, resulting when compared to EC2 in higher shrinkage values, for types B and E mixtures, and in lower values, for types C and F mixtures. For the mixtures produced with LWA with higher particle density, qP0, (B, C, E and F), the experimental results showed that: (i) mixtures with higher W/B ratio, and consequently within S5 consistency class (fluid), exhibit higher shrinkage, ecs, than those within lower W/B ratio (and S4 consistency class); (ii) measured values of the total shrinkage, ecs, are less than 20% of EC2 predictions, for

600 400 B500.S5

200

800 600 400

E500.S5

200

B425.S5

0

E425.S5

0 0

28

56

84

112

140

168

196

0

28

56

84

112

140

168

196

Age (days)

1000

1200

800

1000

ε ms (× 10-6 )

ε ms (× 10-6 )

Age (days)

600 400

C500.S4 C425.S4

200

800 600 F500.S4

400

F425.S4

200

C350.S4

0

F350.S4

0 0

28

56

84

112

Age (days)

140

168

196

0

28

56

84

112

Age (days)

Fig. 1. Evolution of mortar shrinkage, ems, with age for the tested mixtures.

140

168

196

87

1000 900 800 700 600 500 400 300 200 100 0 -100

B500.S5 B425.S5 B500.S4

B500.S5_EC2 B425.S5_EC 2 B500.S4_EC2

ε cs (× 10 -6)

ε cs (× 10 -6)

H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

0

112

224

336

448

560

672

1000 900 800 700 600 500 400 300 200 100 0 -100

E500.S5 E425.S5 E425.S4 D425.S4

0

112

224

900 800 700 600 500 400 300 200 100 0 -100

C500.S5 C500.S4 C425.S4 C350.S4

0

112

224

C500.S5_EC2 C500.S4_EC2 C425.S4_EC2 C350.S4_EC2

336

336

448

560

672

Age (days)

448

560

ε cs (× 10 -6)

ε cs (× 10 -6)

Age (days)

E500.S5_EC2 E425.S5_EC2 E425.S5_EC2 D425.S4_EC2

900 800 700 600 500 400 300 200 100 0 -100

672

F500.S5 F500.S4 F425.S4 F350.S4

0

112

224

F500.S5_EC2 F500.S4_EC2 F425.S4_EC2 F350.S4_EC2

336

448

560

672

Age (days)

Age (days)

1000 900 800 700 600 500 400 300 200 100 0 -100

B500.S5 B425.S5 B500.S4

B500.S5_MC10 B425.S5_MC10 B500.S4_MC10

ε cs (× 10 -6)

ε cs (× 10 -6)

Fig. 2. Evolution of measured values of total shrinkage, ecs, and corresponding EC2 curves.

0

112

224

336

448

560

672

1000 900 800 700 600 500 400 300 200 100 0 -100

E500.S5 E425.S5 E425.S4 D425.S4

0

112

224

900 800 700 600 500 400 300 200 100 0 -100

C500.S5 C500.S4 C425.S4 C350.S4

0

112

224

C500.S5_MC10 C500.S4_MC10 C425.S4_MC10 C350.S4_MC10

336

448

336

448

560

672

Age (days)

560

672

Age (days)

ε cs (× 10 -6)

ε cs (× 10 -6)

Age (days)

E500.S5_MC1 0 E425.S5_MC10 E425.S5_MC10 D425.S4_MC10

900 800 700 600 500 400 300 200 100 0 -100

F500.S5 F500.S4 F425.S4 F350.S4

0

112

224

F500.S5_MC10 F500.S4_MC10 F425.S4_MC10 F350.S4_MC10

336

448

560

672

Age (days)

Fig. 3. Evolution of measured values of total shrinkage, ecs, and corresponding MC10 curves.

S4 mixtures, and less than 30% of EC2 predictions, for S5 mixtures. For mixture produced with LWA with lower qP0 (mixture D), the measured value of ecs is approximately 40% of the corresponding EC2 prediction. The codes predictions are in average 10 times higher than the measured values, with minimum and maximum ratios of 3 and 16, respectively. 4.2.1. Autogenous deformation From the results analysis, it was possible to assess the efficiency of each LWA to stock internal moisture and to verify its transfer rate with age to the binding paste, thus influencing autogenous

deformation in expansion. Aiming to quantify these deformations, the evolution of autogenous deformation, eca, was measured for most mixtures, being results presented in Fig. 5. Since it is assumed that concrete shrinks, autogenous expansion due to internal curing provided by LWA moisture is assumed as a negative shrinkage. This effect was also observed in other studies [11,15]. Since the development of eca is similar for all mixtures, independently of using saturated or quasi-saturated LWA, it can be concluded that drying shrinkage, ecd, is responsible for the main observed differences in total shrinkage, thus having the type of LWA a significant influence. This influence can be explained taking into account that

1000 900 800 700 600 500 400 300 200 100 0 -100

B500.S5 B425.S5 B500.S4

0

112

224

B500.S5_ACI B425.S5_ACI B500.S4_ACI

336

448

560

ε cs (× 10 -6)

H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

ε cs (× 10 -6)

88

67 2

1000 900 800 700 600 500 400 300 200 100 0 -100

E500.S5 E425.S5 E425.S4 D425.S4

0

112

900 800 700 600 500 400 300 200 100 0 -100

C500.S5 C500.S4 C425.S4 C350.S4

0

112

224

C500.S5_ACI C500.S4_ACI C425.S4_ACI C350.S4_ACI

336

448

224

336

448

560

672

Age (days)

560

ε cs (× 10 -6)

ε cs (× 10 -6)

Age (days)

E500.S5_ACI E425.S5_ACI E425.S5_ACI D425.S4_ACI

900 800 700 600 500 400 300 200 100 0 -100

672

F500.S5 F500.S4 F425.S4 F350.S4

0

112

224

F500.S5_ACI F500.S4_ACI F425.S4_ACI F350.S4_ACI

336

448

560

672

Age (days)

Age (days)

Fig. 4. Evolution of measured values of total shrinkage, ecs, and corresponding ACI curves.

Age (days)

Age (days)

0 14 28 42 56 70 84 98 112 126 140

0 14 28 42 56 70 84 98 112 126 140

0

-40

B500 E500 Ref. curve

B425 E425

-60 -80

-20

ε ca (×10 -6)

ε ca (×10 -6)

-20

0 -40

C500 F500 D425

C425 F425 Ref. curve

-60 -80 -100

-100

-120

-120

-140

Fig. 5. Evolution of autogenous shrinkage, eca, for the HSLWAC and of the estimated reference curve.

LWA with higher density and stiffness has higher capacity to prevent drying shrinkage. Additionally, the alveolar internal structure of the LWA is related with its density and has high influence on its capacity to release water to the binding paste. Thus, and despite the common variance of the LWA’s open porosity, the saturated LWA with high density and strength have high efficiency to slowly hydrate the binding matrix. Consequently, and due to this internal curing, an enhanced transition zone, between the saturated LWA and the surrounding paste, is obtained, also with reduced porosity and permeability [1]. Therefore, the influence of the LWA density on the drying shrinkage is performed: (i) directly, whenever the LWA with higher density oppose, through the higher stiffness, the drying shrinkage; (ii) indirectly, whenever the saturated LWA with higher density promote a more efficient internal curing, reducing the permeability of the binding matrix, and reducing the drying shrinkage. When partially saturated LWA are used [10,13,15], the internal curing is less efficient, and the expansive autogenous deformation is lower, so that parameter must be considered. To conservatively predict the autogenous deformation, eca,p, an envelope was established with expression (3), as a function of both time and parameter cas, the latter depending on the coarse LWA. Two different values were estimated for cas: 110, for mixtures (B and E), and 130, for mixtures (C, F, D), resulting into the reference curves of Fig. 5.

eca;p ¼ cas  ð1  expð0:35  ðt  t0 Þ0:5 ÞÞ

ð3Þ

The production of HSLWAC should not be conducted using dry LWA, since these will absorb effective water from the mixture, therefore changing the concrete properties, both in fresh and hardened states. Even so, if reduced LWA’s moisture is considered in HSLWAC production, expansive autogenous deformation will occur [6,10]. Nevertheless, the advisable situation corresponds to adopt partially or totally saturated LWA, resulting on expansive autogenous deformations. Based on the results of the present study, as well as on the results obtained by other authors [10,13,15], new recommendations to predict the autogenous deformation, depending on those parameters, are presented ahead.

4.2.2. Drying shrinkage Although drying shrinkage, ecd, cannot be measured on real systems, thus being difficult to quantify it individually, as explained in Section 2, a simplified approximation to its evolution was computed for each mixture, by subtracting the estimated values of eca,p, in expression (3), to the measured values of ecs, resulting in the evolution curves presented in Fig. 6. This simplification was considered to follow the approach in codes, allowing to quantify the influence of the design parameters on the shrinkage behaviour of HSLWAC.

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H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

500

B500.S5_cs B425.S5_cs B500.S4_cs

400

E500.S5_cs E425.S5_cs E425.S4_cs D425.S4_cs

600

B500.S5_cd B425.S5_cd B500.S4_cd

500

E500.S5_cd E425.S5_cd E425.S4_cd D425.S4_cd

ε c (× 10 -6)

ε c (× 10 -6)

400

300 200 100

300 200 100

0

0

-100

-100

0

112

224

336

448

560

672

0

112

Age (days) 500

C500.S5_cs C500.S4_cs C425.S4_cs C350.S4_cs

500

C500.S5_cd C500.S4_cd C425.S4_cd C350.S4_cd

300 200 100

448

560

672

F500.S5_cs F500.S4_cs F425.S4_cs F350.S4_cs

400

F500.S5_cd F500.S4_cd F425.S4_cd F350.S4_cd

300 200 100 0

0 -100

336

Age (days)

ε c (× 10 -6)

ε c (× 10 -6)

400

224

0

112

224

336

448

560

672

-100

0

112

Age (days)

224

336

448

560

672

Age (days)

Fig. 6. Evolution of shrinkage, ec (drying, ecd, and total, ecs), of the produced HSLWAC.

Analysing the experimental results and comparing these with codes predictions, three main aspects were identified as having a significant influence on HSLWAC shrinkage: (i) the binding paste strength; (ii) the type and dosage of LWA and its saturation degree, corroborating the conclusions of other studies [6,10]; and (iii) the W/B ratio, which is also related with the HSLWAC’s consistency and workability. Hence, it is proposed that the average value of concrete compressive strength, fcm, currently used to determine the evolution of ecd, be replaced by the value of the compressive strength of the HSLWAC binding paste, fc,b. This is justified by the fact that an HSLWAC with a high-strength binding paste and a high volume of LWA will have reduced density and a significant reduction of compressive strength, although presenting reduced shrinkage. This parameter, fc,b, can be predicted by the Feret’s expression (1). Alternatively, this value can be also obtained by dividing the compressive strength of the HSLWAC, flcm, by the strength reduction coefficient (2), Cf, intrinsic to the type and dosage of the used LWA. Type and dosage of LWA used in each mixture proved to have significant influence on HSLWAC drying shrinkage. In fact, high

dosage of LWA with lower density (and consequently less stiffness) has less efficiency on opposing to the high shrinkage of the mortar matrix (Fig. 1). In order to quantify this behaviour, a coefficient, Csh, presented ahead, was determined by first analysing individually the type and dosage of each LWA with the increase of ecd and then by adjusting the linear functions that can be observed in Fig. 7. Afterwards, a correlation between the module of the derivative of each linear function, msh, and the LWA particle density, qP0, could be obtained (Fig. 8), similarly to the procedure adopted in the study conducted to predict HSLWAC’s strength and modulus of elasticity [2,26]. The consistency of HSLWAC is related mainly with the W/B ratio and with the admixture’s type and dosage. With the decrease of the W/B ratio, the drying shrinkage also decreases, mainly due to water reduction in the binding matrix but also due to the reduction of its permeability. By studying the influence of W/B ratio on drying shrinkage, between mixtures with similar LWA type and dosage and equal cement dosage and air content, a correction coefficient, CW/B, ahead presented, could be obtained.

0.14 1.0

0.10

msh

0.8 0.6

Csh

15

m sh =0.13×0.25 ρP0 (R 2=0.98)

0.12

HD2/4 HD4/12 XS MD M

0.4 0.2

0.08 0.06 0.04 0.02 0.00 0.4

0.0 0

100

200

300

400

500

0.6

0.8

1

1.2

1.4

1.6

ρ P0 (kg/dm 3 )

V (dm 3 /m 3 ) Fig. 7. Coefficient Csh, function of type and dosage of LWA, for shrinkage prediction.

Fig. 8. Correlation between dry particle density, qP0, and the coefficient msh of the LWA.

90

H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

Table 3 Proposal for autogenous deformation of HSLWAC. LWA saturation degree (%)

Autogenous deformation

Recommendation for eca

[0; 30] [30; 40] [40; 60] [60; 100]

Shrinkage Null Expansion Expansion

EC2 expression 0 Expression (3), with cas = 40 Expression (3), with cas = 80

Finally, drying shrinkage can be predicted for HSLWAC, considering the corrections proposed in this study together with EC2 expressions. 5. New approach for shrinkage prediction of HSLWAC Based on the study herein presented, the following changes to EC2 are suggested to improve the prediction of drying shrinkage, ecd,c: (i) To replace the concrete compressive strength, fcm, by the binding paste strength, fc,b, in the expression (Eq. B.11) of Annex B.2.1 used to determine ecd,0. (ii) To replace the coefficient g3, defined according to the Section 11.3.3 (2) of EC2, to correct ecd, by expression (4), where the effect of the W/B ratio and of the type and dosage of LWA are considered, through the respective coefficients CW/B and Csh, given by expressions (5) and (6).

g3 ¼ C W=B =C sh

ð4Þ

C W=B ¼ 1:7  ðW=BÞ0:8

ð5Þ

 n  Y V abs;i q15  0:13  0:25 P0;i 1 100 i¼1

ð6Þ

Based on the results of the present study, as well as on other studies [10,13,15], the recommendations presented in Table 3 can be used to predict autogenous deformation, eca, depending on LWA saturation degree. 500

B500.S5_cd B425.S5_cd B500.S4_cd

Total shrinkage considered by concrete design codes usually comprehends both drying and autogenous shrinkage, which depend essentially on: the concrete compressive strength, the cement type, the geometry and the curing conditions. Shrinkage measured on HSLWAC specimens showed that codes predictions are not accurate in this case, presenting significantly high values, on average 10 times higher than the corresponding experimental results. This is mainly due to the fact that codes do not consider the favourable effect of the internal moisture of LWA on autogenous deformation. In fact, for HSLWAC produced with saturated LWA, the autogenous deformation is expansive, due to internal curing, thus opposing to drying shrinkage. Also drying shrinkage predicted by codes is much higher than measured values in HSLWAC specimens. The study herein presented proves that this is mainly due to neglected factors in shrinkage prediction, namely: the binding paste strength; the water/ binder ratio; the type and dosage of LWA. A new approach is presented, where corrections are proposed, using the factors previously mentioned, to improve EC2 shrinkage prediction. With those corrections, EC2 predictions exhibited a good approximation to measured values, reducing from 10 to 1.5 the ratio between predicted and measured values. However, and since this study was developed using Leca LWA, further research is previewed to adjust corrections to other types of manufactured LWA, such as sintered fly ash or expanded shale. Acknowledgments The authors acknowledge: (i) the financial support of the Portuguese Science and Technology Foundation (FCT) through the PhD Grant number SFRH/BD/44217/2008; (ii) the material supply made by the companies Sain-Gobain Webber, Secil, Sika and Argilis.

B500.S5_cd.c B425.S5_cd.c B500.S4_cd.c

300 200

E425.S4_cd E500.S5_cd E425.S5_cd D425.S4_cd

600 500

ε cd (× 10 -6)

ε cd (× 10 -6)

400

6. Conclusions

100

400 300 200

0

0

112

224

336

448

560

0

672

112

C500.S5_cd C500.S4_cd C425.S4_cd C350.S4_cd

500 400

224

336

448

560

672

Age (days)

Age (days)

ε cd (× 10 -6)

E425.S4_cd.c E500.S5_cd.c E425.S5_cd.c D425.S4_cd.c

100

0

C500.S5_cd.c C500.S4_cd.c C425.S4_cd.c C350.S4_cd.c

300 200

F500.S5_cd F500.S4_cd F425.S4_cd F350.S4_cd

500 400

ε cd (× 10 -6)

C sh ¼

With the proposed corrections, the drying shrinkage prediction presents a good approximation to the experimental results obtained for the HSLWAC produced with Leca LWA (Fig. 9). Comparing to EC2, predictions reduce in average from 10 to just 1.5 times the measured values.

F500.S5_cd.c F500.S4_cd.c F425.S4_cd.c F350.S4_cd.c

300 200 100

100

0

0 0

112

224

336

448

Age (days)

560

672

0

112

224

336

448

560

672

Age (days)

Fig. 9. Evolution of drying shrinkage, ecd, and of the corrected prediction of drying shrinkage, ecd,c, of the produced HSLWAC.

H. Costa et al. / Construction and Building Materials 35 (2012) 84–91

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