Influence of cracking on the capillary absorption and carbonation of structural lightweight aggregate concrete

Cement and Concrete Composites 104 (2019) 103382

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Influence of cracking on the capillary absorption and carbonation of structural lightweight aggregate concrete

T

J.A. Bogas*, A. Carriço, J. Pontes CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal

A R T I C LE I N FO

A B S T R A C T

Keywords: Lightweight aggregate concrete Artificial cracking Natural cracking Capillary absorption Carbonation resistance

This paper aims to study the influence of the presence of cracks on the capillary absorption and carbonation resistance of structural lightweight aggregate concrete (LWAC) and to compare its behaviour to normal weight concrete. For this purpose, the effect of artificial and natural cracks on LWAC with different w/c and types of aggregates was analysed. Artificial and natural cracks in the range 0.1–0.3 mm were induced by the notch method and bending tests, respectively. Results show that the influence of artificial cracks on the studied durability properties was not significantly affected by the aggregate type. However, when LWAC was subjected to natural cracks, there was greater participation of the more porous aggregates. For crack widths greater than 0.1 mm, the resistance to CO2 diffusion was less affected by the crack characteristics. Depending on the relationship between carbonation depth and crack length, the estimated carbonation rate under real exposure conditions can increase more than 80% in cracked concrete.

1. Introduction Concrete is a quasi-brittle material that is highly susceptible to cracking during its designed service life. Cracks in concrete can appear for different reasons. On the one hand, mechanical actions and poor designing can induce structural cracks. On the other hand, hygrothermal variations, restrained shrinkage and concrete deterioration mechanisms, such as freezing and thawing and other expansive reactions (reinforcement corrosion, alkali-silica reaction) can lead to nonstructural cracks [1]. When the local tensile stresses in concrete exceed its maximum tensile strength, cracks are formed, and new external and internal paths are established [2]. These new paths should increase concrete permeability and may favour the penetration of deleterious species, inevitably affecting its durability. Various studies have been conducted on an experimental and numerical simulation basis, regarding the influence of cracks on the transport properties of conventional normal weight concrete (NWC) [2–11]. Studies focused on the permeability of cracked concrete have shown converging results. Earlier studies, such as those by Wang et al. [4] and Aldea et al. [6], using feedback-controlled splitting tensile tests concluded that water permeability was little affected for crack widths lower than 0.05 mm and steeply increased within a range that varied from 0.05 mm to about 0.1–0.2 mm [3,4]. Beyond the upper bound of this range permeability also increased, but at a steadier rate with

*

increasing crack width. Recently, Shin et al. [10] investigated the water permeability of concrete specimens with both artificial (straight smooth surfaces) and natural (tortuous and rough surfaces) cracks with 0.1–0.5 mm widths. In both cases, the water permeability coefficient increased parabolically with crack width. Over the critical width of 0.3 mm, for artificial cracks, and 0.27 mm, for natural cracks, the permeability coefficient deviated from the parabolic behaviour and linearly increased with increasing crack width. Yang et al. [12] studied the influence of mechanically and freeze-thaw induced cracks on concrete sorptivity. The former crack inducing method created discreetly distributed cracks that influenced local water absorption. The freeze-thaw action led to a linear increase of sorptivity due to the higher connectivity and more uniform distribution of the crack pattern. Zhang et al. [7] concluded that the presence of cracks on the concrete surface, even the finest microcracks, are immediately filled with water as soon as the cracked surface is put into contact with water. Although it seems relatively intuitive that the presence of cracks increases the penetrability of concrete, different geometry, orientation, depth, and density of cracks may provide distinct outcomes on experimental test results. These factors have also made it a challenging task to accurately determine the influence of cracking on the overall durability of concrete. To achieve comparable results, various authors have established critical crack widths to quantify the effects of this parameter on the deterioration mechanisms of corrosion induced by

Corresponding author E-mail address: [email protected] (J.A. Bogas).

https://doi.org/10.1016/j.cemconcomp.2019.103382 Received 11 July 2018; Received in revised form 25 July 2019; Accepted 29 July 2019 Available online 30 July 2019 0958-9465/ © 2019 Elsevier Ltd. All rights reserved.

Cement and Concrete Composites 104 (2019) 103382

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in CO2 diffusion [32,33]. Especially in more porous LWA, the carbonation front tends to circumvent the aggregate, confirming higher CO2 diffusion through LWA [34]. Therefore, the adequate protection provided by high-quality cement pastes is considered a primary condition for the high carbonation resistance of LWAC [33,35]. Bogas [36] performed a comprehensive study on the carbonation resistance of LWAC produced with different types of expanded clay aggregates and pastes with 0.35–0.65 w/c ratios. For w/c lower than 0.4, LWAC exhibited similar carbonation resistance than NWC. However, when the w/c was increased the carbonation depth was not negligible, and LWAC showed lower carbonation resistance than NWC of equal composition. From accelerated carbonation tests of concretes with water/binder ratios between 0.3 and 0.7, Lo et al. [37] found that LWAC had higher carbonation resistance than NWC of equal strength, but slightly lower carbonation resistance than NWC of equal composition, especially for high w/c ratios. In a recent study of Bogas et al. [21], the carbonation mechanism in LWAC was described through a biphasic behaviour model in which two distinct carbonation coefficients, associated to different stages of carbonation, are defined. According to the authors, due to the wall-effect in moulded concrete, a protective boundary layer of a few millimetres is formed near the concrete surface. Therefore, it was suggested that until the carbonation front overcomes this thin protective layer of about Dmax/2-Dmax (maximum diameter of coarse aggregate), LWA does not significantly participate in CO2 diffusion, and the carbonation coefficient of LWAC is similar to that of NWC of the same composition. After this stage, which may take a long period in concretes with low w/c, the carbonation rate is also affected by the LWA, and the carbonation coefficient depends on the paste characteristics and LWA porosity. This may explain some contradictory trends reported in the literature, in particular when results from accelerated tests are compared with those from real exposure conditions, in which the carbonation depth is usually low [21]. In fact, when compared to NWC, similar or better performance of LWAC in real environments have been reported [16,37,38]. In brief, although some studies [2–11,39,40] have focused on the transport properties and deterioration mechanisms of cracked NWC and uncracked LWAC, to the best of the author's knowledge no studies on the performance of LWAC under cracked conditions have been put forward. In this context, this paper aims to study the influence of cracking on the capillary absorption and carbonation resistance of LWAC. For this purpose, the effect of inducing artificial and natural cracks on LWAC produced with different types of aggregates and w/c ratios is analysed, as well as the relative performance when compared to cracked NWC of the same composition.

carbonation and chloride attack [5,8,9,11,13]. The critical crack width serves as a threshold value, above which a further increase in the crack opening has no significant influence on the diffusion rate, at least no more than the one accountable to uncracked conditions. Djerbi et al. [14] and Jang et al. [9] reported 0.05–0.08 mm as the threshold crack width in chloride diffusion of cracked concrete. Expressions relating the crack width with the depth of carbonation and chloride penetration were proposed by Schutter et al. [3], based on small-scale experiments performed on mortar specimens. A crack influence factor was determined, which was not significantly affected by the mortar composition. Alahmad et al. [11] studied the effect of crack opening, ranging from 0.009 to 0.4 mm, on the ability of carbon dioxide to diffuse in cracked mortars. The authors concluded that even for small crack widths (0.009 mm), CO2 could better diffuse along the crack path than through the surrounding paste. However, significant diffusion perpendicular to the crack length only occurred for crack widths on the upper range (0.4 mm). Green-Sullivan [5] described the relationship between the crack width and the carbonation depth as an "S" shaped curve, i.e. a lower bound connected to an upper bound by a transitioning curve. For small crack openings (< 0.5 mm) there was no significant increase in the measured property. Over 0.5 mm, the carbonation rate increased linearly with crack width up to a threshold value. After this limit, the carbonation rate stabilised, and diffusion was no longer influenced by further increasing crack width. According to the author, in this upper range, differences in carbonation rate should be mainly dominated by other effects, such as surface conditions, internal humidity and the carbonation products that are eventually formed. In recent decades, structural lightweight aggregate concrete (LWAC) has been used for many applications such as building frames and floors, offshore oil platforms, precast modules and rehabilitation works. From its earlier use on concrete ships built during World War I [15], various examples of application in existing structures have shown the adequate durability of reinforced LWAC [16]. However, knowledge on the durability behaviour of LWAC, namely regarding the understanding of the deterioration mechanisms of concrete, is still less consolidated than that of NWC. Nevertheless, various laboratory studies concerning the durability characterisation of LWAC have been carried out, especially regarding the transport properties [17,18], freeze-thaw [19,20], carbonation [21] and chloride penetration [22] resistance of LWAC. Although contradictory results have been reported in the literature, it is concluded that the durability performance of uncracked LWAC dramatically depends on the paste composition and porosity of the lightweight aggregate (LWA) [22,23]. In fact, since LWA exhibit an internal highly porous microstructure, they possess higher penetrability than the surrounding paste [24]. Therefore, the durability of LWAC strongly depends on the effective dispersion of LWA in high-quality cement pastes [25]. Otherwise, the establishment of diffusion bridges between the concrete surface and the steel reinforcement through the LWA particles occurs and the penetrability may be considerably increased [26,27]. Other relevant aspects in the durability behaviour of LWAC are the better elastic compatibility between LWA and mortar [28], the higher quality of the aggregate-paste interfacial transition zone in LWAC compared to NWC [27,30] and the internal curing provided by LWA on the surrounding mortar [[29],31]. Also, LWAC is usually produced with higher cement content and lower w/c ratio than NWC of equal strength. Taking into account a wide range of concrete compressive strength and density classes, Bogas et al. [17], found that the capillary absorption of LWAC was little affected by the type of LWA, regardless of the quality and drying degree of the cement matrix. Only for LWA of high open porosity, the capillary absorption was slightly higher in LWAC. It was concluded that the coarser porosity of LWA than that of the surrounding paste leads to a relevant reduction of the capillary action and an expected increase of permeability. Few researchers have addressed the carbonation behaviour of uncracked LWAC, showing the higher participation of more porous LWA

2. Experimental program 2.1. Materials and mix proportions Two types of coarse lightweight aggregate of very different porosity were selected to produce LWAC; one expanded clay aggregate from Portugal (Leca) and one expanded slate aggregate of higher bulk density from the USA (Stalite). NWC was produced with three crushed limestones of different grain size, namely fine, coarse gravel 1 and 2. The same coarse and fine natural siliceous sand were used in both LWAC and NWC. The main properties of the aggregates employed in this study are listed in Table 1. A type I 42.5R cement was the selected binder for all compositions. For low w/c concrete, a polycarboxylate based superplasticiser (SP) was also used. For each type of coarse aggregate, two w/c ratios (0.35 and 0.55) were considered to cover concretes with pastes of different quality. Concretes were designed according to the methodology proposed by Bogas and Gomes [41]. The mixture compositions are indicated in Table 2. For comparison purposes, NWC and LWAC were produced with the same paste volume and composition. 2

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Table 1 Main properties of the aggregates. Property

3

Dry density (kg/m ) Dry bulk density (kg/m3) Absorption at 24 h (%) Granulometric fraction (di/Di) Shape index

Natural siliceous sand

Natural limestone aggregates

Lightweight aggregates

Fine sand

Coarse sand

Fine gravel

Coarse gravel 1

Coarse gravel 2

Leca

Stalite

2605 1569 0,19 0.25/1 –

2617 1708 0,26 0.5/3.35 –

2646 1309 0.73 3.35/5.6 34 (SI40)

2683 1346 0.35 6.3/10 20 (SI20)

2618 1325 1.05 11.2/20 15 (SI15)

969 632 16.28 4/10 1 (SI15)

1483 760 3.57 8/12.5 10 (SI15)

2.2. Concrete mixing and curing All concretes were produced in a vertical shaft mixer with bottom discharge. The lightweight aggregates were pre-saturated for 24 h and were subsequently surface dried using absorbent towels. The mixing procedure began with the incorporation of the coarse and fine aggregates and 50% of the mixing water. After mixing for 2 min, the mixture was left to rest for 1 min, before adding the cement and part of the remaining water. When used, the SP was slowly added with 10% of water, after one more minute. The total mixing time was about 7 min. For each mix, 150 mm cubic specimens were cast for compressive strength tests according to EN 12390–1 [42]. After demoulding at 24 h, the specimens were kept in water until testing. The dry density was measured in 100 mm cubic specimens and followed the same curing procedure. Cylindrical specimens for capillarity absorption were demoulded after 24 h and water cured for 7 days. Afterwards, the samples were sawn into φ150 × 5 mm specimens, which were kept in a controlled chamber (T = 22 ± 2 °C; RH = 50 ± 5%) for 7 days. An aluminum tape was used to cover the lateral surface of the specimens that were then oven-dried at 60 °C for 3 days, followed by 10 days without moisture exchange. This pre-conditioning procedure follows the recommendations of the document RILEM TC116-PCD [43], which minimises the water content variability and allows approximately uniform moisture distributions throughout concrete specimens without severe drying conditions. Then, the specimens were left in ambient temperature for one day before testing. The carbonation test was performed on cylindrical specimens. Specimens with artificial cracks were produced from moulded cylinders, while specimens with natural cracks were drilled from 60 × 15 × 15 cm prisms (section 2.3). Both types of specimens were water cured for 7 days and then placed in the controlled chamber for 21 days before testing.

Fig. 1. Artificial pre-cracking of concrete specimens with insertion of brass plates with different thickness (0.1, 0.2 and 0.3 mm).

considering the benefits and limitations of each method. Artificial cracks were obtained using the notch method, in which a thin brass plate with a given thickness was inserted in the sample, while concrete was still in the fresh state (Fig. 1). This method was also used by other authors [44,45] since it is easy to simulate the intended crack width accurately. Then, after some concrete hardening, the plates were carefully removed leaving a crack with the desired width and depth. Based on preliminary tests on mortar specimens of the same composition, the time necessary to guaranty that the plates could be easily removed without crack closure was 4 and 5 h, for concrete with w/c of 0.55 and 0.35, respectively. In this study, brass plates with 100 mm width and 0.1, 0.2 and 0.3 mm thickness were inserted 20 mm deep into concrete, which corresponded to a final crack depth of about 10 mm, after sawing the trowelled surface. This method was applied for cracked samples used both in capillary absorption and carbonation tests (Fig. 1). The main disadvantage of this method is that the metallic plate induces wall-effect and smooth cracked surface, which deviates from the real behaviour of cracked concrete. A three-point controlled bending method was used to obtain natural cracks on beam specimens with 600 × 150 × 150 mm. In order to avoid a sudden failure and control the crack width, beams were reinforced with two φ 6 A500 NR steel rods. Only NWC and LWAC with Leca, for both w/c of 0.35 and 0.55 were subjected to this method. Three beams per each composition were tested at three distinct placements, as illustrated in Fig. 2. Afterwards, cracking was induced in each place with localised bending tests.

2.3. Pre-cracking concrete There are mainly two types of cracks that can be induced for experimental testing and evaluation of crack influence in cementitious materials: artificial and natural cracks. Artificial cracks have uniform characteristics and the crack width is easier to control. Natural cracks provide a closer simulation to reality than artificial cracks, with tortuous and non-uniform geometry, but the crack width and repeatability are challenging to control. Both types of cracks were studied Table 2 Concrete mix composition for NWC and LWAC. Type of concrete

w/c

Cement content (kg/m3)

Vpaste (L/m3)

Vcoarse aggregate (L/m3)

Vsand (L/m3)

Vwater (L/m3)

Fine

Coarse

NWA LWAC with Stalite LWAC with Leca

0.35

450

330

435 355 355

80 100 114

154 214 201

158

NWA LWAC with Stalite LWAC with Leca

0.55

350

330

402 355 355

114 114 134

154 201 181

193

3

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The accelerated carbonation tests were carried out according to LNEC E391 [49]. After curing, all surfaces except the cracked one were sealed with an epoxy-based paint so that the diffusion of CO2 would only occur through the cracked surface. The specimens were then placed in a carbonation chamber at 23 ± 3 °C, 60 ± 5% RH and 3 ± 0.1% CO2. Specimens with artificial cracks were tested at 28, 56 and 91 days. For each composition, age and type of crack (uncracked or 0.1–0.3 mm crack width) two specimens were tested. Only compositions with w/c of 0.55 were considered for this test because carbonation is not significant in low w/c concrete [21,50]. The carbonation depth was measured after spraying a phenolphthalein solution on the freshly broken surfaces. Two distinct measurements were taken in the uncracked region and along the crack path, as shown in Fig. 3. Specimens with natural cracks were tested at 7 and 28 days. For each composition, three specimens with crack widths varying between about 0.10 and 0.15 mm were tested. After 7 days in the carbonation chamber, the specimens were sectioned longitudinally, crossing the length of the crack perpendicularly. The specimens were then air dried for 24 h and placed back in the carbonation chamber. At 28 days, the same procedure was repeated. One cut was done at each testing age, as exemplified in Fig. 3. The carbonation depth, for the samples with artificial cracks, was measured in the uncracked zone and along the crack path. The carbonation coefficient, Kc, was obtained from the linear regression between the carbonation depth, xc, and the square root of time, t1/2, by applying Fick's first law of diffusion through Eq. (1).

Fig. 2. Localized bending tests to induce natural cracks.

A hydraulic INSTRON press with 250 KN load capacity was adopted. In each pre-cracked section, the crack width was controlled with linear variable differential transformer (LVDT) from TML, model CDP-25 with a 25 mm stroke placed on both sides of the sample (Fig. 2). The values of stress and strain were registered using an HBM Spider8 data logger. Various loading and unloading cycles were carried out to obtain the desired crack width after unloading. Three cores with φ 9.5 × 15 (cm) were drilled from the cracked zones of each beam using a HILTI core drill (Fig. 2). Each core was then wrapped with a tight tape and sectioned into φ 9.5 × 5 (cm) samples destined for accelerated carbonation tests. Due to the inherent difficulty of controlling this method, the final crack width was on averaged about 0.15 and 0.10 mm, for NWC and LWAC with Leca, respectively. This was measured using a microscope from DINO LITE model AM7915 MZT with image treatment.

x C = K c t 1/2 [mm/year1/2]

(1)

3. Results and discussion 3.1. Fresh properties, compressive strength and structural efficiency The average slump, fresh (ρf) and dry density (ρd), compressive strength (fcm,28d) and structural efficiency (fcm,28d/ρd) are presented in Table 3. All concretes were produced with slumps varying between 10 and 15 cm, aiming at an S3 slump class, according to EN 206 [51]. Even knowing that NWC was produced with coarser aggregates than LWAC, for the same paste composition (Table 1), the slump was slightly higher in LWAC. This higher slump may be attributed to the rounder shape and lower inertia of LWA, which have contributed to the higher workability of LWAC compared to NWC [52,53]. As expected, the fresh density increased with the decrease of LWA porosity. The compressive strength and the dry density varied between 26.8 and 74.7 MPa, and between 1622 and 2341 kg/m3, respectively. These values correspond to LWAC with a strength class of LC20/22-LC55/60 and a density class of D1.8 - D2.0, and to NWC with a strength class of C40/50 to C55/67, thus covering a wide range of common structural LWAC and NWC. As expected, regardless of the type of LWA, the compressive strength of LWAC was lower than that of NWC of equal composition. The strength reduction compared to NWC increased with the increase of LWA porosity and the reduction of the w/c ratio (Table 3). In fact, for these conditions, the compressive strength of LWAC becomes more

2.4. Test procedures The compressive strength and dry density were measured at 28 days, according to EN12390-3 [46] and EN12390-7 [47], respectively. For the compressive strength, A TONI PACT 3000 with a load capacity of 3000 KN was used with a loading rate of 13.5 kN/s. Capillary absorption was determined at 28 days in compliance with LNEC E393 [48]. The test consists in measuring the water uptake of the specimen by measuring the mass increase due to water absorption as a function of time when one surface of the specimen is immersed in a 5 ± 1 mm film of water. The mass of the specimen was recorded at 10, 20, 30 min and 1, 3, 6, 24 and 72 h, after initial contact with water. For each composition, three reference uncracked specimens and three specimens with artificial cracks of 0.3 mm width were tested.

Fig. 3. Carbonation measurement of specimens with artificial cracks (left) and natural cracks (rigth). 4

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Table 3 Fresh properties, density, compressive strength and capillary absorption of uncracked concrete and concrete with artificial cracks. Type of concrete

NWC

LWAC with Stalite

LWAC with Leca

w/c

Crack width wc

Slump

Fresh density, ρf 3

Dry density, ρd 3

fcm,28d

CVfcm,28d

fcm,28d/ρd

Capillary absorption abs (kg/m2)

(mm)

(cm)

(kg/m )

(kg/m )

(MPa)

(%)

(x 10

0.35

0 0.3

13

2420

2341

74.7

4.3

0.55

0 0.3

10

2337

2270

51.2

0.35

0 0.3

16

2006

1881

0.55

0 0.3

13

1948

0.35

0 0.3

14

0.55

0 0.3

11

−3

m)

Cabs −6

0.5

m/min

)

RH

LC

(%)

(mm)

10 min

6h

72 h

(x10

31.91

0.27 0.36

1.16 1.43

2.47 2.78

52.3 64.2

63.7 53.8

– 9

1.6

22.56

0.45 0.6

2.94 3.04

4.66 4.64

149.8 148.8

60.0 59.1

61.6

0.7

32.75

0.32 0.45

1.51 1.74

3.13 3.5

72.4 77.0

1802

41.2

4.9

22.86

0.64 0.78

3.57 3.73

5.5 5.37

1909

1702

35.1

4.3

20.62

0.39 0.53

1.44 1.64

1861

1622

26.8

4.0

16.52

0.75 0.91

3.83 4.16

βC

havg/LC 180 min

360 min

– 1.099

– 2.4

– 3

– 9.8

– 0.886

– 2.4

– 3.2

54.9 56.7

– 15.5

– 0.913

– 1.5

– 1.8

183.8 186.3

53.2 52.9

– 14.9

– 0.834

– 1.7

– 2.3

2.93 3.19

62.9 66.5

70.4 61.8

– 16.2

– 0.890

– 1.8

– 2.2

6.58 6.75

188.7 197.9

58.8 62.1

– 14.8

– 0.878

– 1.7

– 2.2

and, with less relevance, on the type of aggregate. From previous studies, it would be expected similar coefficients of absorption in uncracked NWC and LWAC [17,22]. In fact, according to Bogas et al. [17], this is attributed to the high quality of the aggregate-paste interface in LWAC and the lower capillary action of the LWA with a coarser porosity than the surrounding paste (section 1). However, in this study, concretes with LWA showed slightly higher coefficients of absorption than NWC of equal w/c. On the one hand, this may be related to the low moisture content of the specimens tested in this study (50–60%, Table 3), which increased the participation of the LWA porosity in water absorption, especially near the specimen's surface. On the other hand, for the same paste composition, NWC was produced with a higher amount of coarse aggregate than LWAC, which reduces the volume of porous interfacial transition zones. Besides, the vibration tends to be less effective in LWAC than in NWC, increasing the air content. Nevertheless, in general, the coefficient of absorption was not significantly affected by the type of aggregate. In fact, as shown in Table 1, although LWAC with Leca presented more than 5 times higher water absorption than with Stalite, the coefficients of absorption of concretes produced with these aggregates were very similar (Fig. 4). Concerning the influence of artificial cracking in the capillary absorption, as expected cracked concrete exhibited higher water absorption and coefficient of absorption than uncracked concrete (Table 3). However, these differences were not significant, because the area of the

influence by the aggregate properties and the difference of mechanical strength to NWC increases [52]. Nevertheless, LWAC with less porous aggregates (Stalite) was able to achieve equal to higher structural efficiency than NWC of equal composition. In this case, LWAC with Stalite can reach of moderate to high strength, while keeping structural efficiency levels at least equal to those of NWC. Only for high w/c, the structural efficiency of LWAC with more porous aggregates (Leca) could be closer to that of other concretes, showing that this aggregate is adequate for low to moderate strength concrete.

3.2. Capillary absorption The capillary water absorption at 10 min, 6 and 72 h (abs10m, abs6h and abs72h) are indicated in Table 3, as well as the relative humidity of concrete (RH), the crack width (wc) and length (Lc) for each concrete composition. The coefficients of absorption (Cabs) showed in Table 3 and Fig. 4 correspond to the slope of the regression line obtained from the water absorption vs the square root of time plot between 20 min and 6 h. In general, the coefficients of variation averaged 5%, regardless of the type of aggregate and should be primarily related to the differences in porosity among the tested specimens, as well as possible variations in their water content. The coefficients of absorption varied between 52.3 × 10−6 and 197.9 × 10−6 m/min0.5 (Fig. 4), essentially depending on the w/c ratio

Fig. 4. Coefficients of absorption (Cabs) between √20 min and √6 h of absorption for NWC and LWAC with different types of lightweight aggregates. 5

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crack was relatively small compared to the remaining uncracked area. In addition, the coefficients of absorption were measured after 20 min, minimising the effect of the rapid initial absorption caused by the crack. In fact, it was found that the difference in water absorption between cracked and uncracked specimens gradually reduced after the first 10 min (Table 3). The average values of crack length are presented in Table 3. In general, the three specimens tested per each composition presented similar values of Lc. However, due to execution difficulties, Lc varied between different concretes. Therefore, the direct comparison of the absorption coefficients between concretes is not possible. In order to compare different concretes and to study the influence of the crack, a crack coefficient (βC) was defined according to Eq. (2). The crack coefficient establishes the relationship between the additional water absorption in cracked concrete (due to the presence of the crack) and the water absorption in reference uncracked concrete with the same composition. Due to the variability in crack length among the specimens, Eq. (2) was derived in order to remove the dependence of the water absorption (absc) on crack length (Lc), thus allowing the comparison between concrete samples. This was done by considering that for the cracked specimens, the surface in contact with water, A0, is increased by the area of the crack walls (2Lc.D). In Eq. (2), absc and absref are the water absorption at 6 h (kg/m2) of cracked and reference concrete, respectively. Table 3 shows the average values of βC for each composition.

βC =

Fig. 5. Coefficient of absorption in the cracked region (CabsCR) as a function of the square root of time.

stabilization periods are explained by the higher Lc in LWAC than in NWC. In parallel, due to the different water rise in the cracked and uncracked region of the specimen during the absorption test, the equivalent water height (havg) can be determined according to Eq. (5), where ΔmH2O is the maximum water absorption per unit volume of concrete, which corresponds to the water accessible porosity of concrete. The values of havg/Lc for the stabilisation times of 180 and 360 min are presented in Table 3. In general, it is found that the influence of cracking is less significant for havg/Lc higher than 2, regardless of the type of concrete.

absc × A0 A0 + (2Lc × D)

absref

(2)

havg =

From Table 3, it is possible to conclude that the influence of cracking is higher for concrete with lower w/c and less porous aggregates. This may be attributed to the fact that a more open pore structure in the uncracked zone leads to a lower difference of water absorption between this region and the cracked region. Therefore, it is not confirmed a more significant increase of absorption in LWAC than in NWC, at least for artificial cracks of 0.3 mm. Values of βC lower than 1 indicate that the influence of the crack on the absorption behaviour was lower than that of the effect of increasing the area of absorption by the crack area. Although the crack depths varied among the tested compositions, there was one case where the average crack depths were similar, and the results could be directly compared, which occurred for LWAC with Stalite and Leca, with w/c of 0.55 (Table 3). In this case, absc/absref was 1.05 for LWAC with Stalite and 1.09 for LWAC with Leca. These results show the little influence of the type of aggregate on the absorption behaviour of cracked concrete. To better understand the influence of cracking, the absorption coefficient for the cracked region (CabsCR) was determined according to Eq. (3). This coefficient represents the additional rate of water absorption due to the crack. The water absorption in the cracked region (absCR) is obtained from Eq. (4), where A0, absc and absref were defined in Eq. (2) considering that this additional water only penetrates through the cracked area, AC, which corresponds to the product of the crack width with the diameter of the specimen.

CabsCR =

absCR Ac × t

absCR = (absc × A 0 ) − absref × (A 0 − A c )

absc ΔmH 2O

(5)

3.3. Carbonation resistance The main results obtained in carbonation tests are presented in Table 4 (artificial cracks) and Table 5 (natural cracks), namely the carbonation depth in the reference specimens (xc) and in the cracked specimens, at the uncracked (xc,UR) and cracked (xc,C) region. The uncracked region may be defined as the zone where the carbonation depth is not significantly affected by the crack (at a distance from the crack approximately equivalent to the carbonation depth in uncracked concrete). The coefficient of carbonation of uncracked concrete (Kc) and of the uncracked zone of cracked concrete (Kc,UR) are also presented in Tables 4 and 5, as well as the average crack width (wc) and length (Lc). According to Table 4 the carbonation coefficient of the uncracked reference concrete, Kc, varied between 16.5 and 25.9 mm/year0.5. The R2 between the carbonation depth and the square root of time was always higher than 0.97, which confirms that the accelerated carbonation tests approximately followed Eq. (1). As expected, LWAC with more porous aggregates (Leca) had the highest CO2 diffusion rate. For w/c of 0.55, LWAC with Stalite and Leca showed Kc values 13% and 57% higher than those obtained for NWC, respectively. The Kc values obtained for LWAC with Leca were about 27% lower than those reported by Bogas et al. [21,36,50,54]. in concretes with the same composition. This difference might be associated with variations in Leca properties and the pre-conditioning procedure. Moreover, the study performed by Bogas et al. [21] considered more extended periods of carbonation, involving higher carbonation depths, which allowed for greater participation of LWA.

(3) (4)

Fig. 5 shows the variation of CabsCR over time for the studied compositions. It is confirmed that the influence of the crack is more significant in the first minutes of the test, exponentially reducing over time. The CabsCR was three orders of magnitude higher than the absorption coefficient of the uncracked zone, confirming that artificial cracks with 0.3 mm have little resistance to capillary absorption. From Fig. 5, it is also found that after about 180 min, for NWC, and 360 min, for LWAC, the reduction of CabsCR tends to stabilise. The different

3.3.1. Artificial cracks As for the capillary absorption test, it was not possible to guaranty the same crack lengths in different concretes. Therefore, a new methodology was implemented to allow the comparison between compositions, regardless of the Lc. To this end, the carbonation coefficient through the crack (Kc,C) was determined by assuming that for carbonation depths higher than Lc, the carbonation front progresses at the 6

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Table 4 Main results obtained in the accelerated carbonation test for uncracked reference concrete and concrete with artificial cracks. Type of concrete

w/c

wc (mm)

Accelerated carbonation xc/xc,UR (mm)

xc,C (mm)

LC (mm)

R2

Kc/Kc,UR

28d

28d

28d

(mm/year

Kc,C

0.5

(mm/year0.5)

)

NWC

0.55

0 0.1 0.2 0.3

5.4 5.2 5.5 5.2

– 11.7 13.5 15.3

– 9.8 10.9 12.6

16.5 16.3 16.8 16.3

0.97 0.97 0.97 0.98

– 60.8 87.0 113.3

LWAC with Stalite

0.55

0 0.1 0.2 0.3

5.9 5.3 6.2 6.1

– 12.0 15.5 18.3

– 9.9 13.0 16.1

18.6 18.2 19.2 18.8

0.97 0.98 0.96 0.96

– 64.2 92.0 115.7

LWAC with Leca

0.55

0 0.1 0.2 0.3

7.9 6.8 8.3 8.2

– 12.9 16.5 17.1

– 11.0 14.0 13.0

25.9 24.9 24.8 25.9

0.98 0.97 0.95 0.97

– 56.6 78.6 108.0

same rate of that found in the current uncracked zone. Based on this assumption and taking into account Eq. (1), it is possible to estimate the time taken by the carbonation front to reach Lc (tc) according to Eq. (6), where td is the total duration of the test (28 days) and xc,28d is the carbonation depth at 28 days. Note that for all measured ages, the carbonation front progressed beyond the full crack depth. Therefore, any measured carbonation depth, between 28 and 90 days, could be considered in Eq. (6). However, to minimize the effects of increased water content in the concrete depth, tc was estimated based on the carbonation results at 28 days. In fact, for later ages, the carbonation behaviour would likely deviate from the assumption that beyond Lc the carbonation progresses similarly to the outer concrete surface of the specimens. Finally, Kc,C can be estimated from Eq. (7).

Fig. 6. Coefficients of carbonation in reference uncraked concrete and through the crack (Kc,C), for NWC and LWAC with different types of lightweight aggregates.

2

x c,28d − Lc ⎞ tc = ⎜⎛ td − ⎟ K c, UR ⎠ ⎝ K c, C =

(6)

Lc tc

(7)

Fig. 6 shows the values of Kc,C for all studied compositions and crack widths. As expected, Kc,C increased with crack width, highlighting the easier diffusion of CO2 when wider cracks are present. In general, the highest increase in the carbonation rate was found for cracks up to 0.1 mm width. For greater crack widths, the diffusion increased steadily but at a lower rate. In this case, a linear relationship was found between wc and Kc,C. This phenomenon was also reported by other authors (section 1) and can be explained by the fact that for wc over 0.1 mm, the CO2 diffusion is not as sensitive to a further increase in wc. Even considering the test variability, Kc,C was similar among the different compositions, regardless of the type of aggregate (Fig. 6). This suggests that artificial cracks with 0.1–0.3 mm width have the same diffusion characteristics for different types of concrete. Comparing the values of Kc,C with Kc of reference uncracked specimens, for different crack widths, it is found that the influence of the crack decreases with

Fig. 7. Relationship between Kc,C and Kc for NWC and LWAC with different types of lightweight aggregates.

Table 5 Main results obtained in the accelerated carbonation test for concrete with natural cracks. Type of concrete

NWC LWAC with Leca

w/c

0.55

wc (mm)

0.15 0.1

R2

Uncracked region

R2

Cracked region

xc,UR,7d

xc,UR,28d

Kc,UR

xc,C,7d

xc,C,28d

Kc,C

(mm)

(mm)

(mm/year0.5)

(mm)

(mm)

(mm/year0.5)

2.7 3.5

4.9 7.5

18.0 26.8

7.1 7.9

13.8 16

50.0 57.5

7

0.99 1

1 1

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cracks, as suggested in Fig. 8. On the one hand, in the region of the crack where aggregates were intercepted, the diffusion of CO2 to the surrounding mortar is facilitated (Fig. 9). On the other hand, the crack increases the connectivity between the aggregate particles, which are no longer dispersed throughout the paste (Fig. 9). Therefore, the gas permeability of concrete should be increased, which may explain the higher Kc,C in LWAC than in NWC. Comparing with concrete with artificial cracks of 0.10 mm (section 3.3.1), NWC with natural cracks of 0.15 mm showed lower Kc,C. Therefore, it can be concluded that the induction of tortuous and rugged natural cracks in NWC led to lower carbonation rates than artificial cracks of equivalent width. Also note that induced cracks from bending tests are usually V-shaped, being narrower in depth. On the other hand, LWAC with natural cracks of 0.1 mm presented slightly higher Kc,C than that with vertical artificial cracks with the same width. In this case, the higher tortuosity of natural cracks did not compensate for the effect of the crack interception of porous LWA. As shown in Fig. 9, the artificial cracks benefit from the wall-effect provided by the metal plate inserted during concrete moulding, avoiding the crack development through LWA particles. Naturally, natural cracks are more representative of the real service behaviour of LWAC.

the increasing porosity of aggregates (Fig. 7). In fact, for example for 0.1 mm crack width, Kc,C/Kc was 3.7, 3.5 and 2.2 for NWC and LWAC with Stalite and Leca, respectively. In this case, it can be concluded that the same type of cracking is more relevant in NWC than in LWAC. This is mostly related to the fact that Kc,C was similar in all concretes, while the Kc was higher in LWAC. Therefore, due to the higher porosity of the LWA and following higher diffusion properties of LWAC, the cracking in these concretes has a lower impact on the final carbonation rate than it has in NWC. Naturally, as discussed above Kc,C/Kc increased with the crack width (Fig. 7). There was no clear relationship between Kc,C and the type of aggregate. Nevertheless, Kc,C was slightly higher in LWAC with dense LWA (Stalite) and lower in LWAC with more porous aggregates (Leca). Differences between concretes may be related to the different behaviour of concrete beyond the crack tip and in the uncracked zone, as assumed in the calculation of Kc,C. For example, if the water content of concrete after the crack is higher than that of the uncracked region, the carbonation rate becomes lower and Kc,C tends to be underestimated. In this case, the differences between LWAC and NWC may increase, because the drying time of LWAC tends to be higher than that of NWC. This is attributed to the usually higher initial water content of LWAC and the internal curing provided by LWA [50,55]. In addition, contrary to what occurs in the specimen surface, concrete near the crack tends to present a higher concentration of mortar, because of the wall-effect provided by the brass plate during the induction of artificial cracks. Therefore, in a small layer of some few millimetres, lower than the maximum dimension of the aggregate, the CO2 diffusion is controlled by the mortar, and the carbonation rate of LWAC should be similar to that of NWC [21] (section 1). In this situation, the carbonation rate after the crack is lower and Kc,C is also underestimated. The above-mentioned aspects are more relevant the higher the LWA porosity, which may partly explain the lower values of Kc,C found in LWAC with Leca. In short, given the small differences between concretes, for the range of crack widths analysed in this study, the influence of artificial cracks on the carbonation behaviour of concrete was not significantly affected by the type of aggregate. From the obtained results, it was not possible to confirm the possible contribution of porous LWA in increasing the diffusion of CO2 in these regions near the crack walls. This aspect is discussed later in section 3.3.2.

3.3.3. Estimation of the carbonation rate of cracked concrete under real exposure conditions From Eq. (1), the carbonation coefficients listed in Tables 4 and 5 can be used to roughly estimate the carbonation rate under real exposure conditions, by assuming that carbonation coefficients obtained from real exposure conditions (Kc,real) are related to those obtained from laboratory tests (Kc), according to Eq. (8) [50,54]. In this equation, cc,accel corresponds to the concentration of CO2 in the accelerated carbonation chamber (54 × 10−3 kg/m3), and cc,real is the concentration of CO2 in the real environment (0.7 × 10−3 kg/m3 is assumed). It is assumed that the carbonation occurs in the same hygrothermal conditions considered in accelerated tests and that the binding capacity and diffusion of CO2 are similar in real and accelerated conditions.

K c, real = K c ×

Cc, real Cc, accel

(8)

Taking into account the accelerated coefficients of carbonation in the uncracked (Kc) and cracked (Kc,C) region, and taking into account Eq. (1) and Eq. (7), it is possible to roughly estimate the carbonation depth of concrete under real conditions from Eq. (9), where tc is the time at which the cracking length is reached (Lc) and xc is the carbonation depth at a given time t. It is assumed that the carbonation depth is always higher than Lc.

3.3.2. Natural cracks As for concrete with artificial cracks, the carbonation behaviour in the uncracked region of specimens with natural cracks was similar to that found in uncracked concrete (Table 5). The direct analysis of the influence of natural cracking was valid because the carbonation front was always within the crack depth (Lc). In addition, as the carbonation at different ages was measured in the same specimen, it was possible to directly determine the carbonation coefficient in the cracked zone (Kc,C) from the slope of the linear regression between the carbonation depth and the square root of time. The Kc,C and the carbonation coefficient in the uncracked region (Kc,UR) for the tested compositions are indicated in Table 5. The direct comparison between concretes was not possible, because specimens were tested with different crack widths. As mentioned, it was difficult to ensure natural cracks with a predetermined width. In NWC, the average crack width slightly deviated from the initial aimed target of 0.10 mm. As shown in Fig. 8, the crack width may vary along the crack, especially when porous LWA are intercepted. In this case, the crack is widened in the more porous regions of the aggregate. Therefore, the crack width tends to be higher in LWA than in the surrounding mortar. This suggests that cracking through the LWA should have a more significant impact on the carbonation behaviour of LWAC. Despite the slightly larger crack width of NWC, the carbonation rate was higher in LWAC. These results do not corroborate the similar values of Kc,C found in specimens with artificial cracks, regardless of the type of aggregate. Therefore, LWA had a higher participation in natural

x c = Lc + K c ×

cc, real × cc, accel

t −

K c × Lc K c, C

(9)

The estimated average time for the carbonation front to reach depths between 10 and 40 mm is indicated in Table 6 for different types of concrete with natural or artificial cracks of 0.1 mm. The estimation was carried out for crack lengths equal to the carbonation depth (xc) or half of the carbonation depth (xc/2). Concerning the natural cracks, only the scenario of LWAC with Leca for Lc = xc/2 was considered. As shown in Table 6, the concrete carbonation can be significantly affected by cracking. It is estimated that a carbonation depth of 30 mm may only be attained after more than 255 years in uncracked NWC. However, the same carbonation depth may be reached 2.5 times faster in the same concrete with cracks of 15 mm in length and 0.1 mm in width. If the crack length is equal to the carbonation depth, the estimated time is further reduced by about 82%. The carbonation behaviour of LWAC with Stalite was similar to that of NWC. For the same conditions mentioned above, the reduction of the estimated time was 59% and 92% for cracking lengths of 15 and 30 mm, respectively. For LWAC with Leca, the reduction was 47% and 8

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Fig. 8. Natural crack crossing through the aggregate in NWC (left) and LWAC (right).

Fig. 9. Schematic illustration of the influence of natural (left) and artificial cracks (right) on the gas permeability of LWAC.

main conclusions have been drawn:

79%, respectively. As discussed in section 3.3.1, the lower reduction in concretes of lower density is explained by their higher carbonation rate in the uncracked region. Regarding the influence of the natural cracking, it was found that for LWAC with Leca and Lc = xc/2, the time estimated to reach a carbonation depth higher than Lc was only about 6% lower than that obtained for artificial cracks. As suggested from the results obtained in section 3.3.2, the carbonation behaviour of LWAC was not significantly affected by the type of crack.

• The capillary absorption of uncracked concrete was not significantly

4. Conclusions

•

The present paper aimed at analysing the influence of cracking on the capillarity absorption and carbonation behaviour of LWAC. The study involved the analysis of common structural LWAC with compressive strength classes from LC20/22 to LC55/60. NWC of equal composition was also analysed for comparison purposes. The following

•

affected by the type of aggregate. Artificial cracks of 0.3 mm showed little resistance to capillary absorption, increasing the water absorption in cracked concrete. Suggesting a new crack influencing factor, it was possible to compare the absorption behaviour between different concretes, regardless of the crack length. It is concluded that the influence of cracking tends to be lower in LWAC with more porous aggregates and higher w/c. The influence of cracking was less significant for equivalent water depths higher than about 2 times the cracking length, regardless of the type of aggregate. The carbonation resistance of uncracked concrete decreased with increasing porosity of aggregates. For concrete with w/c of 0.55, the carbonation coefficient of LWAC with more porous aggregates was 57% higher than that of NWC. For the range of crack widths analysed in this study, the influence of

Table 6 Estimated average time for uncracked and cracked concrete with cracks of 0.1 mm to reach different carbonation depths. Type of concrete

Crack type

Crack depth (mm)

Average time to reach a given carbonation depth (years) xc = 10 mm

xc = 20 mm

xc = 30 mm

xc = 40 mm

NWC

uncracked artificial (0.1 mm)

0 xc/2 xc

28 11 2

113 46 8

255 103 19

453 183 33

LWAC with Stalite

uncracked artificial (0.1 mm)

0 xc/2 xc

22 9 2

89 37 7

201 83 17

357 148 30

LWAC with Leca

uncracked artificial (0.1 mm)

0 xc/2 xc

11 6 2

46 24 10

103 55 22

184 98 39

natural (0.1 mm)

xc/2

6

23

52

92

9

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• •

•

artificial cracks on the carbonation behaviour of concrete was not significantly affected by aggregate type, regardless of the crack width. However, the influence of cracking decreased with the increasing porosity of aggregates. It is thus concluded that artificial cracking is more relevant in NWC than in LWAC. The highest increase of carbonation rate occurred for crack widths up to 0.1 mm. For greater crack widths, the resistance to CO2 diffusion was less affected by the crack characteristics. Natural cracking had a more significant impact on the carbonation rate of LWAC than in NWC. When lightweight aggregates are intercepted by the natural cracks, the gas permeability should increase, and they can better participate in the carbonation mechanism. Because of this effect, the influence of natural cracks in the carbonation rate of LWAC was not higher than that found with artificial cracks of the same width, contrary to what happened in NWC. The carbonation rate under real exposure conditions was roughly estimated. The concrete carbonation may be greatly affected by cracking, regardless of the type of aggregate. Depending on the relationship between the carbonation depth and the crack length, the reduction of the estimated time to reach a certain carbonation depth, compared to uncracked reference concrete, can be higher than 80%. This reduction tends to be lower in LWAC than in NWC, due to the higher carbonation rate of these concretes in the uncracked region. The estimated carbonation behaviour of LWAC was not significantly affected by the type of crack.

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