Impact of aggregate grading and air-entrainment on the properties of fresh and hardened mortars

Construction and Building Materials 82 (2015) 219–226

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Impact of aggregate grading and air-entrainment on the properties of fresh and hardened mortars Roberto Cesar de Oliveira Romano ⇑, Danilo dos Reis Torres, Rafael Giuliano Pileggi Polytechnic School of the University of São Paulo, Department of Civil Construction Engineering, Av. Prof. Almeida Prado, trav. 2, n° 83 – cep: 05424-970, São Paulo (SP), Brazil

h i g h l i g h t s  Fresh and hardened properties of rendering mortars were evaluated.  The aggregate grading has considerable impact on the fresh state.  The air-entrainment can lessen the impact of grading variation.  Hardened properties was governed by the porosity.  Porosity was mainly influenced by the air-entrainment in the fresh state.

a r t i c l e

i n f o

Article history: Received 16 September 2014 Received in revised form 4 February 2015 Accepted 23 February 2015 Available online 10 March 2015 Keywords: Air-entrainment Size distribution and mortar

a b s t r a c t The great variance in the properties of mortars in the fresh and hardened state or during performance in use may be associated with a lack of control over materials and admixtures. Thus, the main purpose of this work is to evaluate the impacts that aggregates size distribution, the proportion of entrained air and the type of air-entraining admixtures have on the fresh and hardened properties of mortars. Mortar mixing behavior and air-entrainment were monitored to understand the changes in the fresh properties while the hardened properties were quantified by the porosity, mechanical strength, modulus of elasticity, adhesion and air-permeability. The results show that variations in the aggregates size distribution cause considerable impact during the mixing stage but these impacts are lessened by the use of air-entraining admixtures. Contrarily, in the hardened state, only the air-permeability was influenced by the size distribution, although, the other properties were impacted by the air-entrainment. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The solid particles in mortar range a great deal in particle size, from submicron up to millimeters. The large volume of coarse particles used in the formulations (60 vol.%) exerts significant contribution in all fresh and hardened properties of mortars. Nevertheless, the resulting behavior depends on the complete particle size distribution [1–3]. Robustness [4,5] is a big challenge in large-scale mortar and concrete production. This quest arises because the flow behavior during the fresh state and the properties on the hardened state may be distinctly affected by the variations in particle size distribution, the proportion of fines-aggregates, the water content and other factors, which are often uncontrolled [1].

⇑ Corresponding author. E-mail address: [email protected] (R.C.d.O. Romano). http://dx.doi.org/10.1016/j.conbuildmat.2015.02.067 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

The use of air-entraining admixtures (AEA) in the formulation of mortars is an even more common option to improve properties in the fresh state, like density decrease, workability increase. After hardening, a reduction in the modulus of elasticity may also occur. Another aspect regarding AEA is its potential to attenuate errors caused by the variations in mortars particle size distribution, since the air bubbles plays great influence in both fresh and hardened states. However, AEA requests careful use by itself, since the volume of entrained air is very sensitive to the amount and quality of admixture. Several studies in literature [4–10] reports the individual effects of particle size distribution, water content, air-entrained agents in fresh and hardened state. However, less attention has been dedicated to their combined effects, which are necessary for better understanding and improvement of robustness in mortars production. Therefore, the main purpose of the present work was to evaluate the associated impacts of aggregate size distribution

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and air-entraining admixtures in the fresh and hardened properties of mortars. 2. Materials and methods 2.1. Materials used The mortars were formulated using Portland cement blended with 20% limestone filler, Brazilian type-F – 32 MPa, and ground limestone sand (medium and fine). All raw materials were provided by mortar producers. The particle size distributions are presented in Fig. 1, and the solid density and specific surface area (SSA) are presented in Table 1. Two air-entraining admixtures (AEA) based on anionic sodium lauryl sulfate based molecules were tested. However, one of them is 100% organic molecules and the other is 67%. So, they are designated as AEA-100 and AEA-67, respectively. This difference may impact the air entrainment, the air-volume generated and the admixture consumption, because organic material is the active agent for producing foams [13]. Just for comparison, the admixtures with distinct quantities of entrained air were defined as distinct PRODUCTS. However, it must be clear that both have the same molecular basis. Regardless, the reference mortar was mixed without air-entraining admixture and the other mortars were mixed using 0.2 or 2.0 g/L of AEA (or 0.012% and 0.12% in function of cement weight). 2.2. Methods The methods used in this work are described below. All hardened tests were done after cure for 7 days at 25 °C and 98% of relative humidity. Mixing rheometry: all dry powder was placed in a planetary rheometer cup and the water was added controlling the flow at 45 g/s. The mix was monitored for 150 s while maintaining the rotational speed at 500 rpm. The result was the equivalent torque in function of time of mix. Air-entrainment: the tests were performed according to gravimetric method, using a cup of 400 ml volume, and quantifying the mass needed to fill it. The values of air entrainment were calculated based on the mortar’s water content and the real density of the dry powder. Porosity: measured according the Archimedes immersion method, based on the dry, wet and immersed mass. Initially, the dry mass of each sample was estimated, then the samples were completely immersed in water and stays under vacuum for 2.5 h. After this time, the wet and immersed mass were measured. The total porosity was calculated according the Eq. (1), where is qREL is the relative density of mortar:

Total porosity ð%Þ ¼ ð1  qREL Þ  100%

ð1Þ

Mechanical strength: the tensile strength was determined using the ‘Brazillian test’. The tests were carried out in a universal testing machine, Instron, model 5569, controlling the load at 0.05 ± 0.02 MPa/s and using samples with 50 mm diameter and 20 mm thickness.

14

Discrete distribution

Portland cement

12

Fine sand

10

Medium sand

8 6

Modulus of elasticity: measured according to Brazilian standard NBR 15630/08 using equipment with frequency transducers of 200 kHz, and a circular transversal section with 20 mm diameter. Pull-out test – adhesion: the tests were performed using a digital dynamometer, Imada, model ATX-500 DPU, with a load cell of 5 kN, Dynatest, accuracy 1.0 N (Fig. 2a). The samples were cast in a different manner than the conventional method, to try to reduce the high variability caused during preparation in accordance to the Brazilian standard (where the sample is cut, generating stresses). This methodology was developed by Romano et al. [13] and consists of: i. washing the standardized substrate with water and drying for 48 h; ii. placing the molds with diameters of 50 mm and 20 mm thickness on the slab (8 per slab) as shown in Fig. 2b; iii. casting the mortar in two stages: fill half the mold and give 20 scams using a metallic pestle, then fill the other half and give 20 more scams using a metallic pestle; iv. leveling the surface using a metallic spatula. As observed in previous studies variations due to the standardized substrate are very high, so different compositions of mortar were cast on the same substrate to try to reduce its variations even more. After removing the samples from the molds, aluminum plates were fixed onto the samples using epoxy cement, as illustrated in Fig. 2c, and then waiting for about 3 h to complete drying and begin testing. Air-permeability: measured according to the vacuum-decay method [11–13]. The apparatus employed was a vacuum pump connected to a suction chamber that is in contact with the surface of the mortar. When the vacuum pump is turned on a transducer registers the pressure variations in function of time until the pressure stabilizes. The test starts when the vacuum is turned off and the time it takes for the pressure to subside is quantified. The permeability (expressed in k1 (m2) values) is calculated using the Forchheimer equation (Eq. (2)), considering two basic hypotheses: negligible air-compressibility and using just the linear part of the equation [16].

DP l q ¼ v s þ v 2s k1 k2 L

ð2Þ

L is the sample thickness, l and q are, respectively, the fluid viscosity and density, vs is the speed of air-percolation and DP is the pressure variation, for which vs, l and q are measured or calculated. The term lvs/k1 shows the viscous effect of fluid–solid interaction, while the term qvs2/k2 represents the inertial effects. The terms k1 and k2 are thus known as Darcian and non-Darcian permeability constants, in reference to Darcy’s law, a simpler and earlier empirical model for permeability description. However, k2 was not used to compare the results in this work [16]. 2.3. Compositions evaluated The mortars were formulated using different sand proportions (illustrated by the Table 2) fixing the cement-to-aggregates ratio, 1:3 in mass, and resulting in the particle size distribution shown in Fig. 3. The resulting packing porosity (estimated according the Westman and Hugill model [17]) and specific surface area are presented in Table 3. Up to 75 lm the particle size distribution was the same for all mortars and the differences between them is due to the aggregate particle size distribution: C1 was formulated only with medium sand, resulting in a particle size distribution with greater gap between fine and coarse particles and worse packing than C2, which was formulated using just fine sand, and C3 was formulated with 50% fine sand and 25% medium sand. These changes directly affect the water demand, but in this work the water content was fixed at 15% in relation to the dry powder and the changes in consistency were evaluated.

4

3. Results and discussions

2 0 0,1

1

10 100 Diâmetro (µm)

1000

Fig. 1. Particle size distribution, real density and specific surface area of the sands and cement used in the mortar formulations.

Table 1 Solid density and specific surface area (SSA) of raw materials. Raw material

SSA (m2/g)

Real density (g/cm3)

Portland cement Fine sand Medium sand

1.75 0.32 0.19

3.01 2.79 2.79

3.1. Air-entrainment This effect of air-entraining admixture on the air-incorporation is observed in Fig. 4. All mortars were mixed for 150 s ant the tests were carried out 2 min after stop the mix. The air-entrainment level presented indicates the absolute value and is equivalent to an average of three measurements. This kind of evaluation was adopted to show the sensibility of the mortar to the mix process, using the same equipment but in distinct batches. Air-entrainment was observed even in the mortars without admixture due to the shear history, and the values were from 5% to 10%, without any dependency on the aggregate size distribution.

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(a)

(b)

(c)

Fig. 2. In (a) is presented the dynamometer used in the tests and in (b) and (c) the stages used to cast and prepare the samples for testing.

However, using an excessive quantity of AEA, the air-entrainment caused by the AEA-67 was around 20% higher than that caused by AEA-100, showing a better interaction with the cement particles, even though with a lower quantity of effective sodium lauryl sulfate molecules. It can be inferred that the admixture blended with inorganic material may fix better the air-bubbles intentionally incorporated in the mortars. This content, 2.0 g/L or 0.12% in function of cement weight, is a very high addition in mortar compositions because it can make the system very sensitive to the mix process. However, the application of quantities many times higher than this is common in the compositions of industrialized mortars, making, in many cases, the control of the characteristics in the fresh state difficult and causing deterioration of some properties in the hardened state [1,15–17].

Table 2 Percentage of sands and cement used in the mortar formulations. Cement

Fine sand

Medium sand

C1 C2 C3

25 25 25

– 75 50

75 – 25

Discrete distribution (%)

Mortar

10 9 8 7 6 5 4 3 2 1 0 0,1

C1 C2 C3

3.2. Mixing behavior

1

10 100 Diameter (µm)

After the water addition the changes in the mixing torque were monitored in function of time and the results are shown in Fig. 5. In (a) is presented the results for the composition C1, in (b) C2 and in (c) C3. Evaluating only the mortars without the air entraining admixtures, it became clear the great differences on the torque in function of aggregate size distribution: the torque for the composition C2 was higher than to the other compositions, because this mortar was formulated with higher aggregates packing porosity, lower gap sized between fine and coarse particles and, consequently, the lowest MPT (Maximum Paste Thickness, calculated according Eq. (3), follow [9]). In this mortar, even after mixing for 150 s, the stability of torque had not yet been reached, owing to the ongoing descending high torque values. This illustrates the greater difficulty in processing. The result is different from that observed for the mortar with composition C1, which has a smaller specific surface area, was formulated with gap between fine and coarse particles, resulting in a higher MPT. In this case, the mixing torque was almost stable after 120 s shear.

1000

Fig. 3. Mortars particle size distribution.

Table 3 Packing porosity and specific surface area resulted in the mortars. Mortar

Packing porosity (%)

SSA (m2/g)

C1 C2 C3

18.2 15.6 8.9

0.58 0.67 0.63

For the compositions formulated with AEA-100, the maximum volume of air was incorporated using just 0.2 g/L of admixture (or 0.012% in function of cement weight). On the other hand, for AEA-67, the air volume was 30% lower for the same admixture content. This was a predictable result because AEA-67 has 33% less organic material which is responsible for foaming.

20 15 10

C1 C2

5

C3 0 0.0

0.5

1.0 1.5 AEA content (g/L)

(b)

(a)

AEA-100

25

Ar incorporado (%)

Air-incorporation (%)

30

2.0

C1 C2 C3

AEA-67 2.5 0.0

0.5

1.0 1.5 AEA content (g/L)

2.0

Fig. 4. Air-entrainment in the mortars. In (a) is shown the results for the AEA-100 and in (b) for AEA-67.

2.5

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R.C.d.O. Romano et al. / Construction and Building Materials 82 (2015) 219–226 Table 4 Summary of statistical analysis of variance evaluating the impact of particle size distribution on air-entrainment.

3.0

(a) C1

Torque (N.m)

2.5

without AEA 0.2-AEA-100 2.0-AEA-100 0.2-AEA-67 2.0-AEA-67

2.0 1.5 1.0 0.5 0.0 3.0

(b) C2

Torque (N.m)

2.5

Group

Count

Sum

Average

Variance

C1 C2 C3

4 4 4

89.24 86.49 78.68

22.31 21.62 19.67

15.63 7.57 22.47

Source of variation

SQ

gl

MQ

Fcalc

P-value

Fcritic

Between groups In the groups Total

15.03 137.02 152.04

2.00 9.00 11.00

7.51 15.22

0.49

0.63

4.26

Tempo (s)

2.0

Table 5 Z-Test for evaluation of the PRODUCT impact.

1.5 1.0

without AEA 0.2-AEA-100 2.0-AEA-100 0.2-AEA-67 2.0-AEA-67

0.5 0.0 3.0

(c) C3

Torque (N.m)

2.5

Tempo (s)

2.0

without AEA 0.2-AEA-100 2.0-AEA-100 0.2-AEA-67 2.0-AEA-67

Product

AEA-100

AEA-67

Average Known variance Observations Hypothesis of difference between the average Z P (Z 6 z) one-tailed z critic one-tailed P (Z 6 z) two-tailed z critic two-tailed

20.83 5.50 6 0 0.32 0.37 1.64 0.75 ±1.96

20.10 25.17 6

1.5 1.0

Table 6 Z-Test for evaluation of the admixture CONTENT impact, on the air-entrainment.

0.5

Admixture content (g/L)

0.0 0

30

60 90 Time (sec)

120

150

Fig. 5. Mortar mixing behavior: at the top are the results for the mortar without AEA, in the middle are the results for the mortars mixed with 0.2 g/L of AEA and below are the results for the compositions with 2.0 g/L of AEA.

Energy of mixing (N.m/s)

400 AEA-100 AEA-67

350 C2

300 200 150

C3

100 50

R² = 0.94

C1

0 0

5 10 Maximum Paste Thickness (µm)

15

Fig. 6. Relationship between the energy of mixing and the maximum paste thickness.

For the composition C3 an intermediate behavior was obtained. Substantial changes on the pattern of curve, on the final torque of mix, or even on the maximum torque, were observed when the air-entraining admixtures were used. In order to better understanding the combined effect of air-entrained agents and aggregate size distribution on mixing, the equivalent mixing energy (area below torque versus time) should be related to a microstructural parameter affected by them. Considering that only air content and coarse particle size distribution changed, the suitable parameter is the mean distance among aggregates, calculated by the Maximum Paste Thickness (MPT) equation (Eq. (3)) as defined by Pileggi [9].

2.0

IA-100

IA-67

IA-100

IA-67

22.64 1.23 3 0

17.36 5.86 3

18.95 2.43 3 0

25.85 1.24 3

3.44 2.95e04 1.64 5.90e04 ±1.96

" !# 2 1 1 MPT ¼   VSAagg V sagg 1  P ofagg

Without air-entrained admixture

250

Average Known variance Observations Hypothesis of difference between the average Z P (Z 6 z) one-tailed z critic one-tailed P (Z 6 z) two-tailed z critic two-tailed

0.2

6.29 2.30e10 1.64 4.59e10 ±1.96

ð3Þ

This equation originally considers the volumetric surface area of aggregates (VSAagg), the volumetric fraction of coarse particles (Vsagg) and the porous fraction (Pofagg) when all particles are in contact during maximum packing calculated by the Westman and Hugill model [17]. However, from the moment that the airbubbles are placed in the system, the volumetric fraction of coarse particles (Vsagg) is reduced by adding the volume of air into the mortar paste volume. Thus, the relationship between the energy of mixing and MPT was plotted in Fig. 6. The use of air-entraining admixtures results in lower mixing energy due to the greater distance between aggregates caused by the air-bubbles. However, the particle size distribution was the variable that produced the most impact during the mortar mixing process [18-20]. Thus, it can be inferred that the use of AEA is very conducive in reducing the impact of eventual granulometric failures or variations on raw materials during the production of mortars. Since the aggregate size distribution and the air-entrainment affects the levels of torque during mixing, and consequently, the energy required for processing the mortar, a statistical analysis of variance (ANOVA) and z-test were performed to check the individual influence of each variable.

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on the air-entrainment. However, the data presented in Table 6 allows concluding that the admixture CONTENT had considerable impact on the foaming: for both concentrations, Z > zcritic two-tailed (with significance of 5%).

50 C1

Total porosity (%)

45

C2

C3

40 35 30

R² = 0.90

3.3. Impact of air-entrainment on the porosity

25 20 AEA-100 AEA-67

15 10 10

20 30 40 Expected void volume (%)

50

Fig. 7. Relationship between the expected voids volume and the total porosity resulted after hardening.

Modulus of elasticity (GPa)

Tensile strength (MPa)

4.5 C1

4.0

C2

C3

3.5 3.0 2.5 R² = 0.82

2.0 1.5

(a)

1.0 40 Porosidade Total (%)

35

3.4. Tensile strength and modulus of elasticity

AEA-100 AEA-67

30 25 20 15

(b)

R² = 0.85

10 20

25

30 35 Total porosity (%)

40

Fig. 8. Relationship between porosity and tensile strength (a) or modulus of elasticity (b).

Assuming the initial hypothesis that the average air-entrainment levels were the same, ANOVA was applied to 3 averages by comparative analysis. The data presented in Table 4 confirms that aggregate size distribution DID NOT HAVE significant influence on the air-entrainment (Fcalc < Fcritic) [21]. In the same manner, the z-test was used to evaluate the impact of PRODUCT or admixture CONTENT by comparing two averages. The results are shown in Tables 5 and 6. A comparative analysis of the averages points out the value of Z < zcritic two-tailed, indicating that the PRODUCT had negligible impact

0.30

Variations in the tensile strength and modulus of elasticity are presented in Fig. 8, in function of the total porosity of each sample. As expected, the increase in porosity resulted in a decrease in absolute values for both properties. The voids act as points which concentrate tension, thus, facilitating the onset of cracking and decreasing mechanical strength, at the same time in which lets the mortar to have greater deformation before complete rupture. Furthermore, the higher the pore volume, the smaller the section that resists [2,11,14–16,22]. As these mortars do not have a structural function tensile strength may be considered a secondary parameter when evaluating performance of rendering. The modulus of elasticity, however, is very important when evaluating performance, because it must be capable of withstanding the dimensional heterogeneity generated in the buildings. 3.5. Air-permeability Fig. 9 shows the correlation between air-permeability (k1) and the total porosity (a) and with the maximum paste thickness – MPT (b). Is clear, evaluating Fig. 9a, that the total porosity was not the only variable which changed the air-permeability. For the same

(a)

0.25 0.20

(b)

C1 C2 C3

R² = 0.58 k1 (x10-13 m²)

k1 (x10-13 m²)

As was expected, the variations on the fresh state impacting directly on the microstructure formation after hardening, as is shown in Fig. 7, from the relationship between the expected voids volume (air-volume + water content + combined water as cement hydrates) and the total porosity resulted after hardening [2]. As this relationship was anticipated nothing additional could be observed in function of this tendency except that there was a higher loss of air during hardening of the mortars with higher air-bubble volume (more than 30%), because the expected porosity (air + evaporable water) was slightly lower than that measured. Another important observation is that in the mortars without airentraining admixture the total porosity was higher than the expected, possible due to packing failures during the casting. As structural voids may have consider impact on the hardened properties, the results of mechanical strength, modulus of elasticity, air-permeability and adhesion in function of the total porosity are presented following.

R² = 0.44 0.15

R² = 0.71 0.10

R² = 0.82

AEA-100 AEA-67

0.05 0.00 20

25

30 Total porosity (%)

35

40

0

5 10 Maximum Paste Thickness (µm)

15

Fig. 9. Relationship between the air-permeability and the total porosity (a), and with the Maximum Paste Thickness – MPT (b).

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Fig. 10. Micrographs of mortars formulated with composition C1, which show the structural differences due to the granulometric aggregate distribution. On the right is presented a petrographic assessment, showing the high air-bubble concentration on the paste.

Fig. 11. Micrographs of mortars formulated with composition C2, which show the structural differences due to the granulometric aggregate distribution. On the right is presented a petrographic assessment, showing the lower air-bubble concentration on the paste.

porosity, was obtained different k1, depending on the aggregate size distribution. As in the mortars with high porosity, the air-percolation occurs by the cementitious matrix (not by the aggregate–matrix interface), this process suffered heavy influence from the changes in maximum paste thickness [13]. Just to illustrate this observation is presented in Figs. 10 and 11, taken by an optical microscope, pictures showing the likely route of air percolation into the mortar, for the compositions C1 and C2, respectively. For both figures is illustrated on the right

two petrographic assessments, showing the concentration of air-bubbles on the pastes. In mortar C2, formulated just with fine sand and cement, resulting in a lower particle packing than C1, the air-percolation by way of the microstructure was more difficult than in mortar C1 which had greater interconnected porosity and a higher volume of porous paste around the aggregates. The air-permeability is one of the parameters used for evaluating durability and time-life of cementitious materials, in that it is related to the conditions of the structural voids and can indicate

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1.8 C1 C2 C3

Adhesion (MPa)

1.6 1.4 1.2 1.0 0.8

R² = 0.49

0.6 0.4 AEA-100 AEA-67

0.2 0.0 20

25

30 Total porosity (%)

35

40

225

On the other hand, the rheological properties or the hardened characteristics were influenced by the air-entrainment and by the aggregate size distribution. More specifically, the porosity had impact on the mechanical strength, modulus of elasticity and adhesion, while the behavior of mortars during the mixing and the air-permeability were affected by the air-entrainment and by the aggregate size distribution. The impact of both variables, could be inferred by calculating the mean distance among the aggregates (MPT) considering the volume of air entrained in the system. At this way maybe is possible to infer some hardened behavior of mortars from the knowledge of raw materials characteristics and fresh properties evaluation.

Fig. 12. Adhesion of mortars in function of total porosity.

Acknowledgements the structural condition of the mortar’s porosity during testing [2,14]. So, it can be inferred that even with no differences in the mechanical strength and modulus of elasticity (in function of aggregate size distribution) in the mortars evaluated in this work, the higher is the MPT, the higher the air-permeability will be, showing that the either the aggregate size distribution or the airentrainment together, affect the air percolation. 3.6. Adhesion The air-permeability, mechanical strength and modulus of elasticity are properties that result from the intrinsic characteristics of the formulations and the air-entrainment during the mixing stage. However, as when the mortars are in use, it must be adhered to a substrate so as to evaluate its performance by an adhesion test (pull-out test). Fig. 12 shows the results of adhesion in function of the total porosity of mortar. The highest values were observed for the mortars with no additives and the tests of adhesion revealed proportionally inverse relationships with total porosity, even when there was a low correlation coefficient. However, it must be emphasized that the results of the adhesion tests varied widely: according Gonçalves and Bauer [23], adhesion presents an intrinsic variability of 52%, while the test method, per se, presents a variation of 19%. In this work, the average variation coefficient was around 16%, with the minimum value being 7.5% and the maximum 30%. So, it can be affirmed that the form of molding was efficient in reducing the test variability. The correlation between porosity and adhesion follows the same pattern as observed for the relation between porosity and tensile strength, but with lower R2 (0.49). This indicates that the results of pull-out test have influence of substrate and that the tensile strength is the maximum value for adhesion. Nonetheless, the statistical evaluations revealed no relationship between the results and the aggregate size distribution, type of product or concentration of AEA. 4. Conclusions According the results obtained, the air-entrainment and final porosity were mainly influenced by the admixture content and did not have a relationship with the aggregate size distribution. The air-entrainment changed from 5% to 10%, on the compositions mixed without air-entrained admixtures, to higher than 15% on the additived compositions. Consequently, the total porosity on the hardened state, was a consequence of the air-entrainment on the fresh state + water content + combined water as cement hydrates.

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