potential issues with generation and stability of air-void

Paleti, Olek and Nantung

Potential Issues with Generation and Stability of Air-Void System due to Incompatibility of 1 Components in Plain and Fly Ash Cementitious Mixtures 2

3 TRB Paper # 13-xxxx 4 5 Chaitanya Paleti* 6

Graduate Student 7 School of Civil Engineering 8 Purdue University 9 550 Stadium Mall Drive 10 West Lafayette, IN 47907-2051 11 Tel : (765) 237-2288 12 Fax: (765) 494-0395 13 Email: [email protected] 14 15 Jan Olek 16 Professor 17 School of Civil Engineering 18 Purdue University 19 550 Stadium Mall Drive 20 West Lafayette, IN 47907-2051 21 Tel : (765) 494-5015 22 Fax: (765) 494-0395 23 Email: [email protected] 24 25 Tommy E. Nantung 26 Section Manager 27

Office of Research and Development 28

Indiana Department of Transportation 29

1205 Montgomery Road 30

West Lafayette, 47906 IN, USA 31

Phone: (765)-463-1521 ext. 248 32

Fax: (765)-497-1665 33

Email: [email protected] 34 35 36 *Corresponding author 37 38 39

40 41 Word count: 42 43 Words : 6396 44 Tables : 3 x 250 = 750 45 Figures: 7 x 250 = 1750 46 47 Total : 8896 48

TRB 2013 Annual Meeting Paper revised from original submittal.


ABSTRACT 1

Continuing desire to create more sustainable and durable infrastructure leads to the increased 2 usage of various mineral and chemical admixtures as components of present-day concrete mixtures. 3 Unexpected incompatibility problems may arise in certain mixtures as a result of complex interactions that 4 frequently take place between these components. This paper presents an investigation on identifying 5 combinations of component materials which can result in problems related to generation and stability of 6 air-void system (AVS) in both plain and fly ash mixtures. 7

A low (0.3%) alkali Type I cement, a class F fly ash and four chemical admixtures were used in 8 this study. Two out of these four admixtures included different types of air entraining agents (AEA): a) 9 vinsol resin based air entraining agent and b) synthetic type air entraining agent. The other two admixtures 10 included the lignin based Type A water reducing agent (WRA) and polycarboxylate type superplasticizer. 11 In addition to plain cementitious mixtures prepared from various combinations of these components, fly 12 ash mixtures (with 20 and 60% weight replacement of cement by Class F fly) were also studied. The 13 potential degree of difficulty in generating air-voids in fresh concrete was evaluated using the foam index 14 tests (performed on paste slurries) and by measuring the air content of mortar mixtures (performed 15 according to the AASHTO T137). Stability of air-void system in concretes was evaluated using the foam 16 drainage test (performed on paste slurries). 17

The results showed that the amount of the AEA required to obtain the target air percentage 18 increased with the increase in the fly ash content in the mixture. The lignin based WRA had, in general, a 19 higher air entraining effect than the super-plasticizer when used in combination with air entrainers. In 20 general mixtures prepared with synthetic air entraining agent exhibited more stable foam system than 21 mixtures prepared with vinsol resin based air entraining agent. 22 23

24



INTRODUCTION 1

Selection of materials and proportions for present-day concrete mixtures is becoming a complex task due 2 to the increasing durability concerns and for introduction of performance requirements. Often, in order to 3 meet these requirements, concrete mixtures are designed and produced using combination of different 4 cementitious materials and chemical admixtures. The probability of developing undesirable interactions 5 (incompatibilities) between various ingredients of the mixture increases as the number of materials and 6 their combination increases (i.e. concrete becomes “less forgiving”). The term “incompatibility” has been 7 applied to various types of abnormal performance of concrete in both plastic and hardened stages, 8 including setting and strength gain issues, excessive slump losses and generation of deficient (i.e. either 9 unstable or ineffective) air-void system. However, for the purposes of this paper, the term incompatibility 10 refers exclusively to problems related to generation and stability of air-void system in the fresh concrete. 11 The amount and the composition of cement, the use of supplementary cementitious materials and the 12 presence of chemical admixtures all can have significant effect on the amount and the quality of the air in 13 the concrete. 14

The alkali content of the cement has been reported to have a significant effect on the air-void 15 system (AVS) in concrete. Concretes with low alkali content were reported to be prone to problems 16 related to AVS stability (1), (2). Also, alkali content of the cement was found to influence the amount of 17 air entraining agent (AEA) required to attain required air content. It was reported by Dubovoy et al. (1) 18 that the amount of AEA required to produce 6 ± 1% air in concrete mixtures with moderate and high 19 cement alkali level (0.60% or greater) was about 50% lower than that required for concrete mixtures 20 containing low (0.21%) alkali cement. In addition, they reported that concretes with low (0.21%) alkali 21 cement and vinsol resin based air entraining agent cast 90 minutes after mixing exhibited very high 22 spacing factor (0.53mm (0.021in.)) when compared to mixtures placed 10 minutes after mixing (0.15mm 23 (0.006 in.)). Also, concretes prepared with high-(1.2%) alkali cement and air-entraining admixture based 24 on salts of wood resins (NVR) were less prone to scaling than mixtures prepared with low (0.21%) alkali 25 cements. 26

Pigeon et al. (2) observed similar results in their study on the influence of soluble alkalis on air-27 void system stability. They reported significant increase in the spacing factor in mixes with low alkali 28 cements after addition of superplasticizers. In a separate study by Plante et al. (3), it was suspected that 29 high alkali content of cement helped to produce a stable air-void system. 30

The use of supplementary cementitious materials (i.e. silica fume or fly ash) often requires higher 31 dosages of air entraining agents to obtain the satisfactory air-void system. The fine silica fume particles 32 have a very high surface area (compared to that of the cement particles) and hence can adsorb higher 33 dosages of admixtures. Therefore, a higher dosages of AEA are typically required in concrete systems 34 with silica fume. Unburned carbon particles in fly ash were found to be major reason for variable AEA 35 dosage requirement. Gebler and Klieger (4) observed as much as 59% reduction in air content after 90 36 minutes (from completion of mixing) in concretes made with class F ash. It was also observed that 37 concrete with class C ashes had typically lost less air when compared to concretes with class F ashes. 38 Zhang (5) studied the effect of fly ashes (loss on ignition values ranging from 1.3% to 4.4%) on air 39 entrainment and found that the inclusion of fly ash in concrete increases the AEA dosages from one to five 40 times of that required in plain portland cement concrete. Data from this study showed that concrete mixes 41 with vinsol resin AEAs (VRAEA) gave the lowest retention of air content and highest variability with 42 given source of fly ash. 43

The carbon content of fly ash is usually thought to have higher adsorptive surface areas than 44 portland cement grains. Hurt and others attributed the higher dosage requirements of AEAs to the 45 following properties of unburned carbon particles in fly ash: (i) the amount; (ii) the specific surface area; 46 (iii) the accessibility of the surface area; (iv) the chemical nature of the surface (6), (7), (8). Although 47 Class C fly ashes were found to contain, in general, carbon particles with much higher specific surface 48 areas than Class F ash, their potentially negative influence on the required dosage of the AEA is offset by 49 the fact that Class C fly ashes typically have much lower values of the LOI than Class F ashes (6). 50



In the past, the VRAEAs were traditionally used for air entrainment purposes. However, due to 1 limited supply (and thus increased costs) of natural vinsol resins, various other types of AEAs were 2 introduced on the market. In their 2007 study Nagi et al. (9) found that the type of AEA admixture had 3 statistically significant effect on the spacing factor and hence the durability of concrete. It was also 4 reported by the same authors that compressive and flexural strengths of mixtures prepared with non 5 VRAEA were less than that of those prepared with VRAEA. 6

Cross et al. (10) observed that the use of synthetic AEAs with low alkali cements resulted in low 7 compressive strength of concrete. The authors concluded that use of synthetic AEAs in place of VRAEA 8 resulted in clustering of air voids around the aggregates which, in turn, resulted in de-bonding of 9 aggregates and low compressive strengths. Similar conclusions were drawn from a field study conducted 10 for New Jersey Department of Transportation (DOT) which reported that the use of synthetic AEA was 11 the predominate factor responsible for the low compressive strengths (11) mostly due to creation of larger 12 air voids by these admixtures. 13

Bedard and Mailvaganam (12) reported that the simultaneous use of lignin based water reducers 14 (WRA) and AEAs resulted in substantial increase in the air content of concretes when compared to those 15 concretes which used AEA alone. In addition, even though the air content of the former concretes was 16 sometimes almost twice as high as that of concretes without the WRA, the specific surface of the bubbles 17 was substantially reduced. Finally, the same authors also reported that when the addition of the WRA was 18 delayed, the stability of the air-void system was further reduced. These adverse effects are mainly 19 attributed to the presence of sugars and other contaminants in commercial lignosulphonates. 20

Other factors, such as elevated mixing and/or placement temperatures, pumping operations and 21 excessive vibration may also affect the air content of concrete. The undesirable interactions between 22 various components of the mixtures are certainly not surfacing in everyday applications. However, when 23 they arise, concrete may exhibit deficient air-void system and thus develop durability problems. 24

Main objective of the present work was to evaluate the effects of various components of the 25 mixture on problems related to air-void production and stability in fresh concrete. In addition, this paper 26 discusses various methods – AASHTO T 137, foam index and foam drainage tests, which can be used to 27 identify potential incompatibility problems related to the air-void system and provides guidance on 28 selection of the corresponding limiting criteria. 29

It should be noted that the data presented in this paper are limited to the issue of generation and 30 stability of the air void system in the fresh paste system. Experiments were also performed on both fresh 31 and hardened concretes (including determination of air-void generation and stability, setting time and 32 strength) but these results are not presented here due to space limitation. More details can be found in the 33 full report by Paleti (13). 34

35

EXPERIMENTAL PLAN 36

The materials used in this study were purposely selected to maximize the potential for creating problems 37 related to generation and stability of air-void system in the fresh concrete. Also, all the materials (except 38 for the class F ash) used in this study were selected from Indiana department of transportation’s (INDOT) 39 approved list of materials. The class F fly ash used in this study is from an approved source by other 40 DOTs like Michigan, Ohio state DOTs. Based on the literature review, low alkali cements and class F fly 41 ashes with elevated carbon content were identified as the major binder-related causes of problems with 42 air-void system. These binders were then used with various combinations of water reducing and air 43 entraining admixtures to explore potentially incompatible combinations of components. The properties of 44 the low (0.29%) total alkali content cement (C) along with that of the fly ash (LOI 3.98%) used in this 45 study are listed in the Table 1. The cement used in the study was classified as AASHTO M85 Type I 46 cement while the fly ash confirmed to the AASHTO M295 standard for Class F fly ash. Two water 47 reducing agents and two air entraining agents (AEA) which confirmed to the AASHTO M194 were also 48 used. The two air entraining agents (AEA) were vinsol resin based air entraining agent and synthetic type 49 air entrainer whereas, the two water reducing agents were lignin based Type A water reducing agent (WL) 50 and polycarboxylate type superplasticizer (WP). 51



Both plain and fly ash mixtures were used in the study (the latter containing 20 and 60% of fly ash 1 as a replacement for equal weight of cement). The testing included determining air content in mortars and 2 performing foam index and foam drainage experiments on slurries of binders, water and admixtures. Air 3 content in mortar samples was determined following the AASHTO T137. 4

The following nomenclature was used throughout this paper to refer a particular combination of 5 materials to prepare the mixtures. The presence of cement and fly ash in a mixture is denoted by using 6 letters C and F in the designation while the letters W and A were used to represent the water reducer and 7 air entrainer respectively. The presence of lignin based water reducer or poly-carboxylate type 8 superplasticizer in a mixture is denoted by WL or WP, respectively. Similarly, AS represents synthetic air 9 entrainer while AV denotes Vinsol resin based air entrainer in the mixture. The amount of fly ash in the fly 10 ash cementitious mixtures is indicated by the number in the subscript like in the case of CF20WPAS 11 mixture which was prepared with 20% fly ash replacing the cement. 12

TABLE 1 Properties of Cement and Fly Ash 13

Percentage of Mass

Analyte Cement (C) Class F ash (F)

SiO2 20.59 49.72

Al2O3 4.76 25.89

Fe2O3 1.96 14.13

CaO 63.77 3.53

MgO 2.68 1.16

SO3 3 0.69

Na2O 0.13 0.87

K2O 0.25 2.04

TiO2 0.35 -

P2O5 0.11 -

Mn2O3 0.13 -

SrO 0.03 -

Moisture Content - 0.24

Total Alkali Na2Oeq 0.29 2.21

Free CaO 1.02 -

L.O.I(950deg) 2.61 3.98

Compounds (AASHTO M85):

C3S 60 -

C2S 14 -

C3A 9 -

C4AF 6 -

Fineness

Blaine surface area (m2/kg) 379 -

Density (g/cc) 3.15 2.44

-325 Sieve (%) - 20.2 14

In this research, the robustness of various mixtures (in terms of compatibility of their components) 15 was quantified by comparing test results from the multi-component mixtures with the test results obtained 16 from the base mixture. The base mixture is defined as the cementitious system without any admixtures. In 17 the case of simple fly ash cementitious systems (CFx), the plain (cement only) mixture (C) is used as base 18 mixture. Similarly, CFx was used as the base mixture for fly ash mixtures which also included water 19



reducing and air entraining admixtures. The subscript “x” by the letter “F” indicates the percent of cement 1 replacement by the fly ash. 2

TEST METHODS 3

Foam Index Test 4

Foam index test was used to study the effect of supplementary cementitious materials and complex 5 interactions between various components of the mixture on the generation of stable foam system. In this 6 test method, the amount of air entraining agent (AEA) dosage required to form a stable uniform layer of 7 foam on the surface of the liquid was estimated. Abnormal changes in the required dosage of AEA with 8 respect to (w.r.t.) the base mix indicate potential incompatibility problem. This test was performed on 9 paste slurries according to draft procedure outlined by Taylor et al. (14). Although the test was performed 10 on paste slurries, there exits good correlation between foam index test results and the AEA dosage 11 required to attain 6 1% air content in concrete mixtures (15). 12

20 grams of binder and 50 ml water were used to prepare paste slurries. In addition to plain 13 cementitious systems, fly ash cementitious systems (with either 20 or 60% of fly ash used as a 14 replacement for equivalent weight of cement) were also evaluated. Before being used in preparation of the 15 slurries the commercial admixtures were first diluted to a ratio of 1:8 (by volume) by adding de-ionized 16 water. In case of some of the mixtures, the diluted AEA solutions were further diluted to 1:5 (by volume) 17 to ensure that the amount admixtures that were to be added can be measured easily and accurately. 18

Recommendations provided by Taylor et al. (14) were used to establish the limiting criterion for 19 assessing the stability of foam. A change in the amount of air-entraining admixture required to achieve a 20 stable and complete coverage of the surface with foam of more than 30% (with respect to the base 21 mixture) was considered significant. 22

Air Content in Mortars (AASHTO T137) 23

Determination of air content in mortars was performed according to AASHTO T137 standard. This test 24 was performed to evaluate the effect of various mixture components on generation of the system of air-25 voids in mortar samples. This experiment was performed on 18 mixtures to estimate the amount of AEA 26 which will be required to achieve 18±2% air content in mortar samples. The mortar samples were 27 prepared using 350 grams of binder, 1400 grams of the ASTM C778 standard 20 - 30 graded sand, and the 28 amount of chemical admixtures and water needed to obtain a flow of 87.5±7.5 %. Mixing of mortar was 29 performed according to AASHTO T162. Once prepared, the mortar was placed and compacted in a 400ml 30 brass cup. The weight of these 400 ml mortar was then measured and this information was combined with 31 the data on specific gravity of the components and mixture proportions to calculate the air content of the 32 mortars. In addition to plain (cement only) mixtures, fly ash mixtures with two different levels of cement 33 replacement (20% & 60% (by weight)) were also studied. 34

The criterion adopted in this study as an indicator of potential incompatibility of the components 35 was higher than 20% (compared to base mixture) change in the amount of air-entraining admixture 36 required to achieve 18±2% air in the mortars. The 18% air content in the mortars was selected following 37 the recommendations of the AASHTO M295 standard. In addition, Lashley (15) reported that a good 38 correlation was found between the amount of air entrainer needed to attain 18% of air in mortars and 6.5% 39 air in fresh concrete. Limiting value of 20% was adopted from the optional uniformity specifications 40 mentioned in AASHTO M295, which require that the amount of AEA needed to produce 18% air in 41 mortar samples shall not vary from the preceding tests by more than 20%. 42

Foam Drainage Test 43

Foam drainage test was performed to study the stability of air-voids generated in paste slurries which were 44 prepared by mixing 5 grams of binder and 300 ml of water (in the case of mixtures without water reducers) 45 and 5 grams of binder and 250 ml of water (in case of mixtures with water reducers). The amount of AEA 46 in all the mixtures was 10 ml and the amount of water reducer was 50 ml. A hand-held blender, operating 47



at medium speed (7000 rpm) was used to produce the foam. This experiment was performed according to 1 the draft standard recommended by Taylor et al. (16). Brief explanation of the experimental procedure is 2 provided below. 3

Required amount of water was added to a cylindrical jar of 500 ml capacity. Water reducing agent, 4 if any, was then added followed by the addition of air entrainer. Immediately after the addition of the air 5 entrainer, a 5g of binder was added and the mixture was blended for 10s at medium speed. The resulting 6 foamed mixture was transferred to 1000 ml graduated cylinder and the level of foam-liquid interface (Vd 7 measured in ml) was monitored (with respect to time) for a period of 60 minutes. 8

The analysis of data from the foam index test requires preparation of the plot of the instantaneous 9 level of the foam (Vd) versus the inverse of time t (in minutes) as shown in Figure 1. The linear equation 10 of the form Vd = V0-(1/k)*(1/t) is then fitted into the data and used to determine the values of V0 (the 11 intercept on y axis) and -1/k (the slope of the line). 12

13

FIGURE 1 Typical plot of the measurements from the foam drainage test 14

The value of V0 represents the amount of liquid drained from the foam after a long time (at time 15 t= infinity) and 1/k represents the rate of drainage of liquid from the foam. In addition, the data collected 16 in the foam drainage test are also used to determine the percent of foam drainage (% drainage). In this 17 study, the value of % drainage was calculated by a method modified from that previously used by Taylor 18 et al. (14). In their work, the percent of foam drainage was calculated according to the formula given 19 Equation 1. 20

% drainage = 100 – [100*(310 – V0)/310] ….(1)

These researchers further proposed that foams with high % drainage values, calculated based on 21 the above formula, are less stable. However, this formula fails to differentiate between foam systems 22 which have different initial foam contents. As an example let us consider two foam systems with initial 23 foam-liquid level ((Vd) initial) at 200 and 300, respectively, and the final levels (V0) at 308 each. According 24 to the formula used by Taylor et al., each of the two systems will have the same (99.4%) drainage value 25 which is not correct. Therefore, in the present study, the percent (%) of foam drainage was calculated 26 using Equation 2. 27

% foam drainage = 100* [V0 – (Vd) initial] / [300-(Vd)initial] ….(2)

Where, (Vd)initial is the foam-liquid level at the beginning of the experiment. 28 The higher values of % foam drainage represent unstable foam characteristics compared to those 29

with lower values of % foam drainage. Similarly, foams with higher values of V0 are less stable than those 30

y = -112.29x + 241.55

R² = 0.8617

0

50

100

150

200

250

300

0 0.5 1 1.5 2 2.5

Vd

(m

l)

--

->

1/t (1/min)



with lower V0 value. Likewise, larger value of 1/k implies lower stability of foam. The values of 1/k and 1 % foam drainage were used in this research to quantify the stability of the foam system. 2

RESULTS AND DISCUSSIONS 3

Incompatibility Problems Related to Generation of Air-voids 4

Figures 2 and 3 summarize the results of AASHTO T137 test method while Figures 4 and 5 summarize 5 the foam index test results. Letters on the y-axis of these figures represent the admixture combinations 6 present in the mixtures. 7

The results of the air void content in mortars from the AASHTO T137 test are shown in Figures 2 8 and 3. By analyzing these figures, it can be observed that the combination of either of the two water 9 reducing admixtures with either of the two air entrainers reduced the amount of AEA, required to attain 10 18±2 % air content, when compared to the mixes without water reducer. 11

Very similar trends were observed from foam index test results (shown in Figures 4 and 5) 12 especially for mixtures containing combination of WP with either of the AEAs or WL and AV. However, 13 these trends were less clear in the case of mixtures which contained combination of WL and AS. 14

It was also found out (based on findings from AASHTO T137 test) that in mixtures with no water 15 reducing agents, the amount of air entraining agents (both AS and AV), required to attain 18±2 % air 16 content, increased with the increase in fly ash content. Similar results were also observed when either of 17 the AEA agents were added along with lignin based water reducing agent, WL. These results were 18 consistent with the results from the foam index test except in the case of mixtures containing the WL and 19 AS combination. 20

21

FIGURE 2 The amount of air entrainer to obtain 18±2 % in mortars containing synthetic AEA 22 (AS); the amount of WL and WP are fixed at 390 and 260 ml/100kg, respectively 23

The AASHTO T137 tests on mortars indicated that the amount of fly ash in the system did not 24 have significant effect on the amount of AEA required in cases of mixes with WP. Similar results were 25 observed from foam index test of mixtures prepared with WP and AV. This was thought to be because of 26 the tendency of poly-carboxylate type superplasticizer to be preferentially adsorbed on the fly ash particles 27 and not on silica fume particles (17). Based on this, it was thought that the WP molecules get adsorbed on 28 the carbon particles that come along with the class F ash more readily than the molecules of the AEA, thus 29 leaving more of the air entrainer available for the purposes of generation the air voids. 30

0 100 200 300 400 500 600 700

Dosage of AEA - AS (ml /100kg)

Ad

mix

ture

s in

th

e M

ixtu

re

60% fly ash mixes 20% fly ash mixes 0% fly ash mixes

WP + AS

WL + AS

AS



Finally, using the results shown in Figure 2 it can be observed that the mixtures with WL water 1 reducer required lower dosages of both (AS and AV) air entrainers compared to mixtures with WP (with the 2 exception of CF60WPAS mixture). It can be thus inferred that WL had higher air entraining effect compared 3 to that of WP. These results are consistent with findings from the literature regarding the air entraining 4 nature of the lignin based water reducing agent (12). Similar results were observed from the foam index 5 test performed on mixtures prepared with VR based AEA (AV). It is important to take note that 6 superplasticizers sometimes contain de-air entrainers to subdue the inherent air entraining nature of 7 superplasticizers. However, the material data sheet (obtained from the manufacturer) of the 8 superplasticizer used, did not have any mention of such additives or inherent air entraining nature. 9

10

FIGURE 3 The amount of air entrainer to obtain 18±2 % in mortars containing Vinsol resin based 11 AEA (AV); the amount of WL and WP are fixed at 390 and 260 ml/100kg, respectively 12

13

FIGURE 4 Foam index test results for mixtures containing vinsol resin based AEA (AV); the 14 amount of WL and WP are fixed at 390 and 260 ml/100kg, respectively 15

0 100 200 300 400 500

Dosage of AEA - AV (ml /100kg)

Ad

mix

ture

s in

th

e M

ixtu

re


WP + AV

WL + AV

AV

0 50 100 150 200 250 300

Dosage of AEA - AV (ml /100kg)

Ad

mix

ture

s in

th

e M

ixtu

re


WP + AV

WL + AV

AV



1

FIGURE 5 Foam index test results for mixtures containing synthetic AEA (AS); the amount of WL 2 and WP are fixed at 390 and 260 ml/100kg, respectively 3

Table 2 summarizes and compares all results obtained from the foam index tests and from the 4 AASHTO T137 tests for both plain and fly ash cementitious systems. The air entrainer requirements of 5 various combinations of materials were compared with that of the base mixture and the combination of 6 materials which resulted in significant (30% for the foam index tests and 20% for the AASHTO T137 test) 7 differences were considered incompatible. All such combinations are highlighted (in yellow) in Table 2. 8

It can be seen that 75% of the 18 different combinations studied were identified as incompatible 9 based on the foam index test and 87% of the 18 different combinations studied were found to be 10 incompatible based on the AASHTO T137 test. Overall, four different combinations of materials 11 (CWPAS, CF20AS, CF20WLAS and CF60WLAS) were found to be compatible based on the foam index 12 method while only two combinations (CF20AS and CWPAV) were found to be compatible based on the 13 AASHTO T137 method. 14

In general, dosage requirement of AS, as determined by foam index test, was greater than that of 15 the AV AEA with exception one combination of the fly ash cementitious mixture prepared with 20% 16 replacement of cement by class F ash and PCSP (WP). However, no clear trends can be observed in the 17 results of AASHTO T137 test. 18

Finally, when comparing the foam index and mortar tests as a measure of incompatibility in can 19 be seen that, in general, both indicated similar trends. However, in some cases there were some 20 discrepancies in identifying possible incompatibility problems between these two tests. The probable 21 reasons for these discrepancies are discussed in the next section. 22

Comparison of Test Methods Used to Study Problems with Air-Void Generation 23

Figure 6(a) illustrates the correlation between the results of both (foam index and AASHTO T137) 24 experiments taking into consideration all the data points while Figure 6(b) illustrates the same relations but 25 without the three outliers (circled points in Figure 6(a)). These three points were identified based on 26 assumptions of linear regression. That is assuming that the residuals (difference in the values of predicted 27 and measured dependent variable) are normally distributed with average around zero. 28

It can be observed from Figure 6(b) that after the removal of the three outlier points there exists a 29 reasonably good correlation (R

2 value of 0.76) between the results obtained from AASHTO T137 and 30

foam index tests. The material combinations resulting in the three outliers all contained WL (lignin water 31

0 100 200 300 400 500

Dosage of AEA - AS (ml /100kg)

Ad

mix

ture

s in

th

e M

ixtu

re


WP + AS

WL + AS

AS



reducer) and AS (synthetic air entrainer) and included the following mixtures: CWLAS, CF20WLAS and 1 CF60WLAS. 2

When performing the foam index test it was observed that the combination of the WL and AS 3 admixtures always resulted in smaller-size air-voids compared to the other combinations. It was therefore 4 difficult to determine if the thin layer of smaller voids was stable and if it completely covered the liquid 5 surface. This was henceforth concluded to be the reason for the high dosage requirements and thus the 6 deviation from the regular trends. 7

8 TABLE 2 Summary of Results Obtained from Foam Index and AASHTO T137 Tests 9

Mix #

Foam Index Results Amount of Air Entrainer to Obtain 18 ±2 % in

Mortars

AEA dosage

ml/100kg % change

w.r.t base mix AEA dosage

ml/100kg Flow % Air content

%

% change

w.r.t base

mix

Plain Cementitious Mixes

CAS 122.2 - 342.9 80 17.4 -

CAV 44.4 - 128.6 85 18.6 -

CWLAS 194.4 59

2.9 82.5 20.6 -99

CWLAV 4.6 -90 5.7 75 19.5 -96

CWPAS 88.9 27 42.9 80 17.6 -88

CWPAV 27.8 -38 142.9 82.5 19.9 11

20% Fly ash Mixes

CF20AS 150.0 23 400.0 87.5 18.6 17

CF20AV 94.4 113 200.0 80 19.1 56

CF20WLAS 188.9 26 28.6 80 18.9 -86

CF20WLAV 13.9 -85 28.6 80 19.5 -93

CF20WPAS 16.7 -89 57.1 85 19.2 -79

CF20WPAV 27.8 -71 142.9 81.25 18.5 -29

60% Fly ash mixes

CF60AS 405.6 232 571.4 85 18.2 67

CF60AV 255.6 475 400.0 82.5 18.2 211

CF60WLAS 294.4 -27 128.6 87.5 19.02 -71.7

CF60WLAV 33.3 -87 114.3 87.5 17.8 -71

CF60WPAS 127.8 68.5 57.1 87.5 18.3 -90

CF60WPAV 44.4 -89 157.1 87.5 19.6 -73 10



1

FIGURE 6 Correlation between the results of mortar testing and foam index testing: (a) Plot with 2 all the data points (b) Plot with 3 data points excluded 3

Stability of Air-Voids 4

This section summarizes the results of foam drainage test performed to evaluate the stability of air-voids 5 in paste slurries. Table 3 summarizes the results of foam index results of mixtures containing both 6 synthetic (AS) and vinsol resin based air entrainers (AV). The values of 1/k and the percentage (%) foam 7 drainage estimated from the foam drainage test were used to quantify the stability of the foam produced by 8 the various combinations of materials. 9 10

TABLE 3 Foam Drainage Results 11

Mix # 1/k (min) % Foam Drainage

Mixtures Containing Synthetic AEA, AS

CAS 36.09 46.25

CWLAS 5.28 6

CWPAS 23.25 87.3

CF20AS 38.57 70.5

CF20WLAS 1.92 10

CF20WPAS 27.72 38

CF60WLAS 0.88 13.33

CF60WPAS 21.3 73.64

Mixtures Containing Vinsol Resin based AEA, AV

CAV 155.69 68.44

CWLAV 201.95 83.04

CWPAV 62.28 90

CF20AV 122.22 62.58

CF20WLAV 199.55 56.48

CF20WPAV 53. 3 80.67

CF60WLAV 216.57 66.27

CF60WPAV 29.56 56.65

y = 0.8655x + 62.073

R² = 0.37

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

0 200 400 600 AE

A d

osa

ge

- A

AS

HT

O T

13

7

Foam index results

y = 1.3318x + 57.614

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Foam test index results

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In general, it was observed that mixtures prepared with synthetic AEA (AS) had a more stable 1 (lower 1/k and lower % foam drainage values) foam system compared to the mixtures prepared with 2 vinsol resin based AEA (AV) with an exception of % foam drainage results of two mixtures: CF20AS and 3 CF60WPAS. It was also observed that mixtures prepared with AS and WL had higher stability of air-voids 4 compared to those prepared with AS and WP. 5

Mixtures containing 60% class F ash and poly carboxylate super plasticizer (WP) had stable foam 6 compared to the corresponding plain cementitious mixtures without any fly ash. Also, with respect to 1/k 7 values, mixtures prepared with AS and 60% class F ash exhibited stable foam characteristics compared to 8 those prepared with AS and 20% replacement of cement by fly ash, irrespective of the kind of water 9 reducer used. However no such clear trends were observed from the % foam drainage values. 10

SUMMARY 11

Incompatibility problems related to generation and stability of air-voids were investigated in this paper 12 using low (0.29%) alkali cement, Class F ash and four chemical admixtures. Foam index and AASHTO 13 T137 test methods were used to study problems with generation of desirable air void system while foam 14 drainage test was used to evaluate the stability of air-void system. 15

With respect to the ability to obtain the desirable air-void system, the incompatibility problems 16 manifested themselves in the form of an increased demand for the quantity of air entrainer. AASHTO 17 T137 tests indicated that addition of either of the two water reducing agents significantly reduced the 18 amount of AEA (AS or AV) dosage required to produce a stable foam system. Additionally, mixtures 19 prepared with lignin based WRA (WL) required lower AEA dosages compared to that of mixes with WP. 20 As the fly ash content increased, the amount of AS or AV (required to attain 18±2 % air content) also 21 increased. However, variation of required AEA dosage with respect to fly ash content was little in 22 mixtures containing poly-carboxylate type superplasticizer (WP). Incompatibility problems associated with 23 significant increase of AEA dosage were discussed mainly because as it can be observed from Table 2, all 24 the incompatible mixtures had higher requirement of AEA dosage in comparison with base mixture except 25 for one case (CFWPAS). In general, it was found that the demand (dosage requirement) of vinsol resin 26 based AEA was greater than that of the synthetic AEA. The results of foam index testing were in good 27 correlation with that of the results from the AASHTO T137 test method. 28

Foam drainage test method identified that mixtures prepared with synthetic AEA (AS) were, in 29 general, more stable compared to mixtures prepared with vinsol resin based AEA (AV). Also mixtures 30 with WL and AS combinations of admixtures resulted in stable foam system compared to mixtures with WP 31 and AS. 32

CONCLUSIONS 33

1. Majority of the mixtures prepared with low (0.29%) alkali and high (3.98%) LOI content Class F 34

ash resulted in incompatibility problems related to generation and stability of air void system. 35

Also, literature review indicated problems with attaining desirable strength in low alkali cement 36

mixtures prepared with certain admixture combinations. Therefore, it is recommended that in 37

situations when the properties of the materials intended for particular application are similar to 38

those indicated as potentially incompatible, these should be tested with different set of admixtures 39

to identify appropriate combinations to avoid the problem. 40

2. The amount of AEA dosage required to attain desirable air-void content increased with an 41 increase in fly content in mixtures that did not contain any water reducers, confirming previous 42 findings from the literature. Similar trends were found in mixtures prepared with lignin based 43 water reducer. However the fly ash content did not have significant effect on the dosage of AEA 44 required in mixtures prepared with polycarboxylate type superplasticizer. It can thus be inferred 45 that required dosage of AEA depends not only the fly ash content but also the type of water 46 reducer used in the mixture. 47



3. The reduced effect of fly ash content on AEA dosage in mixtures containing WP appears to be due 1 to preferential adsorption of superplasticizer on the carbon particles present in the fly ash. Since 2 this adsorption will reduce the effective amount of superplasticizer available for workability 3 control, such mixes may require higher dosages of this admixtures. 4

4. Water reducers, in particular lignin based water reducers, were found to exhibit air entraining 5 characteristics. Therefore, one should account for this additional air entraining effect to avoid 6 excessive air entrainment and there by prevent concretes with lower strengths. In addition, the 7 quality of air-void system generated by water reducers may be inferior to that generated by AEA. 8

5. The foam index test was found to be an easy and quick method for identification of problems with 9 generation of air voids. The results from this method were in good correlation with those obtained 10 from the AASHTO T137 test. However, foam index test may not yield accurate results while 11 evaluating combinations which result in fine foam system and hence one may need to confirm the 12 findings using the AASHTO T 137 method. 13

ACKNOWLEDGEMENT 14

This work was supported by the Joint Transportation Research Program administered by the Indiana 15 Department of Transportation and Purdue University. The content of this paper reflect the views of the 16 authors, who are responsible for the facts and the accuracy of the data presented herein, and does not 17 necessarily reflect the official views or policies of the Federal Highway Administration and the Indiana 18 Department of transportation, nor do the content constitutes a standard, specification, or regulation. 19

20

REFERENCES 21

1. Dubovoy, V.S., Gebler, S.H. and Kleiger, P. Cement-alkali level as it affects air-void stability, freeze-22 thaw resistance and deicer scaling resistance of concrete. Portland Cement Association, Skokie, Illiniois. 23 S. 24. RD128, 2002. 24 2. Pigeon, M., Plante, P., Pleau, R., Banthia, N. The influence of soluble alkalies on the production and 25 stability of the air-void system in superplasticized and non-superplasticized concrete, ACI Materials 26 Journal, Bd. 89. 24-31, 1992. 27 3. Plante, P., Pigeon, M., Foy, C. The influence of water reducers on the production and stability of the 28 air-void system in the concrete. Cement and Concrete Research, Bd. 19, S. 621-633, 1989. 29 4. Gebler, S.H., Klieger, P. Effect of fly ash on air-void stability of concrete. Portland Cement association, 30 Skokie, Illinois. RD085.01T,1983. 31 5. Zhang, D.S. Air entrainment in pozzalonic fly ashes. Cement and Concrete Composites, 1996, 409-416. 32 6. Laots, K.I., Hurt, H. R., Suuberg, M.E. Size distribution of unburned carbon in coal fly ash and its 33 implications. Fuel, S. 223-230, 2004. 34 7. Helmuth, R. Fly ash in cement and concrete. Portland Cement Association, Skokie, Illinois. 1st ed., 35 1987. 36 8. Hill, R.L., Sarkar, S.L., Rathbone, R.F., Hower, J.C. An examination of fly ash carbon and its 37 interacations with air entraining agent. Cement Concrete and Research, Bd. 27, S. 193-204, 1997. 38 9. Nagi, A.M., Okamoto, A.P., Kozikowski, L.R., Hover, K. Evaluating air entraining admixtures for 39 highway concrete. NCHRP. ISSN 0077-5614, 2007. 40 10. Cross, W., Duke, E., Kellar, J., Johnson, D. Investigation of low compressive strengths of concrete 41 paving, precast and structural concrete. South Dakota department of transportation. SD98-03-F, 2000. 42 11. Ansari, F., Zhang, Z. Maher, A., and Szary, P. Effect of synthetic air entraining agents on compressive 43 strength of Portland cement concrete mechanism of interaction and remediation strategy. FHWA. NJ-44 2002-025, 2002. 45 12. Bedard, C., Mailvaganam, P.N. The use of chemical admixtures in concrete. PartII: Admixture - 46 admixture compatibility and practical problems. J.Perform. Constr. Facil., ASCE, Feb, 2006. 47 13. Paleti, C. Workability and Air Void Related Incompatibility Problems in Plain and Fly Ash 48 Cementitious Systems. Dissertation. : ProQuest/UMI , purdue13957, 2011. 49



14. Taylor, C.P., Graf, A.L., Zemajtis, Z.J., Johansen, C.V., Kozikowski, L.R., Ferraris, C.F.Identifying 1 incompatible combinations of concrete materials: Volume II - Test Protocol. Federal Highway 2 Administration. HRT-06-080, 2006. 3 15. Lashley, L. The compatibility and performance of cementitious materials and chemical admixtures. 4 Department of Civil Engineering, University of Toronto. Portland Cement Association. Thesis, Master of 5 Applied Science,. SN 3106, 2009. 6 16. Taylor, C.P., Graf, A.L., Zemajtis, Z.J., Johansen, C.V., Kozikowski, L.R., Ferraris, C.F. Identifying 7 incompatible combinations of concrete materials: Vol I - Final report. Federal Highway Administration 8 (FHWA), 2006. 9 17. Perche, F., Adsorption of lignosulfonates and polycarboxylates on model and cement powder. Science 10 thesis, EPF Lausanne, Science and Technology Faculty of Engineering STI Section of Materials 11 (Materials Institute). 10.5075/epfl-thesis-3041, 2004. 12 13 14 15



Reply to Reviewers comments 1 2

REVIEWER 1: 3 1. Is the foam index test a standard test? if not need to describe and show a schematic or a photo 4 of the test. Has this test been related to foam generated in concrete mixes since its development in 5 2006? 6 Foam index test is not a standard test method yet. Detailed description of various test methods was not 7 provided because of the constraints on the length of the paper. However, the details of the foam index test 8 can be obtained from the reference by Taylor et al., 2006 (as mentioned in the paper on page 6 – Ls 9 9 and 10). 10 Foam index test method was reported as one that has a history of being useful in quickly indicating 11 variability in cementitious materials and a potential test for field materials acceptance (Taylor et al., 2006). 12 One such research work that established correlation (shown in Figure 1 below) of foam index test results 13 and the AEA dosage required in concrete mixtures to attain the desired air content (6 1%) is the work by 14 Lashley, 2009. Text was added in the modified paper about the correlation between foam index test 15 results and the AEA dosage required in concrete mixtures (Page 6 lines 10-12). 16

17 Figure 1: Relation between foam index and AEA dosages required for fresh concrete to obtain 6.5% air 18

content (Lashley,2009) 19 20 2. What does w.r.t stand for? "With respect to?" if so, say so. 21 Incorporated the suggestion in the final paper (page 6, Ls 8-22) 22 23 3. Did you use cements and Fly ash sources that are conventionally used in ready mixed 24 concrete? Most specs don’t allow high LOI FA sources. 25 Although all materials used in the study were selected from sources approved for use by various DOTs’, 26 they were also chosen with the idea of maximizing the potential for creating the incompatibility problems 27 (the worst case scenario). Specifically, , all materials (except for the class F ash) used in this study were 28 chosen from the Indiana Department of Transportation (INDOT) approved list of materials. The class F fly 29 ash used in this project was from the source approved by other DOTs (Michigan and Ohio). The text of 30 the paper was modified (page 4 Ls 38 -41) to make this issue more clear. 31 32 4. I was hoping that the study would attempt to prepare concrete mixes to test the stability of Air 33 content. 34 The current paper is part of a major project which does include tests on concrete samples. In that study 35 the air content in fresh concrete (measured using the pressure meter) was monitored for 60 minutes to 36 study the stability of air void system. However, the results and subsequent discussions were not 37 presented here because of the constraints on length of the paper. Interested readers can look at the 38 reference (Paleti, 2011) for the results and subsequent discussion on tests performed on the concrete 39 mixtures. 40



1 REVIEWER 2: No comments 2 3 4 REVIEWER 3: 5 The reviewer thanks the authors for submitting this paper for consideration. Please note that the following 6 comments are intended to assist the authors in making the paper better. 7 8 1. The objectives are stated well, but I would have liked to see the specific tests named in the last 9 paragraph of the introduction on Page 4. 10 Introduction was modified to incorporate this suggestion (Page 4: Ls 26-27) 11 12 2. You note on Page 6 the mixture proportions and the dilution of the admixtures. Please expound 13 upon why you chose to dilute the admixtures. 14 The amount of admixtures used in the study (unless otherwise specified) is equal to the maximum dosage 15 recommended by the Indiana Department of Transportation ( INDOT). The upper limit of the INDOT 16 recommended range of dosages were used to maximize the potential for the occurrence of incompatibility. 17 The upper range of the dosage of air entraining admixtures (AEAs) and super plasticizer (WP) 18 recommended by INDOT is equal to 260ml/100kg while that of the lignin based water reducer (WL) is 19 equal to 390ml/100kg. The amount of binder used in foam index test methods is small (20 grams) which 20 correspond to 0.052 and 0.078ml of WP and WL respectively. Admixture solutions were therefore diluted 21 using de-ionized water so that such small dosages of admixtures that should be added can be measured 22 easily and accurately. Text was added in the modified paper (on page 6 Ls 17-18) to address this issue. 23 24 3. Please detail the 'measured amount' of water added to the cylindrical jar in the foam drainage 25 test. 26 This information was already presented on the page 6 (Ls 44-46) (see the quote below):. 27 “Foam drainage test was performed to study the stability of air-voids generated in paste slurries which 28 were prepared by mixing 5 grams of binder and 300 ml of water (in the case of mixtures without water 29 reducers) and 5 grams of binder and 250 grams of water (in case of mixtures with water reducers).” 30 However, the text on Page 7 L4 was also modified to improve readability. 31 32 4. The authors come to the conclusion in the text and in the conclusions section that the lignin 33 based WR had better air entraining qualities than the SP as noted in the literature. It may be 34 appropriate to note that the SP's sometimes get into air entraining issues due to them usually 35 having a de-air entrainer in their chemical makeup due to their inherent air entraining natures. 36 We agree with the idea that some super plasticizers may have additives to suppress the inherent air 37 entraining nature. However the material data sheet obtained from the manufacturers did not have any 38 mention of such additives or inherent air entraining nature. Text was added in the modified paper (on page 39 9 (Ls 6-9)) indicating the possibility of existence of de-air entrainer. 40 41 5. On page 10, you note the results with and without the three outliers. Please detail how these 42 three points were determined to be outliers. 43 These are the data points that were substantially far from the regression line and differ substantially from 44 other observations. One of the underlying assumptions in linear regression analysis is that the residuals 45 (difference in the values of predicted and measured dependent variable) are normally distributed with 46 average around zero. If this is a case, the average (avg.) and standard deviation (S.D) values of the 47 distribution of residuals were used to identify and eliminate outliers to improve the regression fit. Assuming 48 that the residuals are distributed normally, 68% of their values should lie within the range of avg. +/- S.D. 49 The three extreme data points that were eliminated in the Figure 6(a) had their residuals outside the range 50 of avg. S.D. Text was added on page 10 (Ls 26-28) in the modified paper to address this issue. 51 52 6. Editorial Comments: Relevant changes were made in the modified version. 53 pg. 4, ln. 1: Define 'VRAEA' – Defined on page 3 (L 42) 54 pg. 4, ln. 25: Delete 'as already mentioned – Incorporated the suggestion (Page 4 L 25) 55 pg. 4, ln. 47: Indent the paragraph is it is intended to be a new paragraph- Made the modification 56 (Page 5 L5) 57



pg. 6, ln. 2: define 'w.r.t.' incorporated the suggestion in the final paper (page 6, L 8) 1 2 REVIEWER 5: 3 Good paper 4 5 REVIEWER 6: 6 1. Page 6, lines 47-49 + P 7. Ls 1-25: In P.7, L47, Vd is defined as the level of foam-liquid interface. 7 In which units is it expressed?: in/mm?. 8 Foam drainage experiments were performed using a 1000ml graduated (in ml) cylindrical test tube. Vd is 9 the instantaneous level of liquid foam interface measured using the”ml” markings on the graduated 10 cylinder. The units of Vd are therefore milliliters (mentioned on page 7 L7 in the modified paper). 11 12 2. In P7, L6, Vo (having the same unit as Vd) is defined as the "amount of liquid drained.." (at t= 13 infinite) that would insinuate it is a volume; however the chart in Fig. 1 indicate it has no units. 14 V0 is estimated from the Figure 1 as the intercept of the linear regression line on the y-axis. It has the 15 same units as Vd (ml) and it represents the volume of liquid drained from the foam after a long time 16 (t=infinity). Units of Vd on the y-axis that were not initially presented were included for improved readability 17 (Page 7, Figure 1). 18 19 3. From Fig. 1 it seems that the initial value at t=0 would be Vd = 0. 20 Extrapolation of the plot shown in Figure 1 may indicate that for high values of 1/t (for values close to t=0) 21 the value of Vd tends to a zero value. This can be interpreted as the case where the liquid foam interface 22 is at level zero immediately after transferring the foamed solution to the graduated cylinder. Occurrence of 23 this phenomenon depends on the dosage of the air entrainer and the nature of its air entraining 24 characteristics. Although this phenomenon is theoretically possible, none of the mixtures that were tested 25 showed such characteristics. Furthermore, even if this phenomenon were to occur, it should not be of 26 great concern because the slope and the intercepts values of the regression line will reflect such 27 occurrences and therefore will affect the results of the foam drainage test method. 28 29 4. Moreover, in Table 3 there is again a units problem. Variable 1/k is expressed in units [1/min]. 30 However, according to the formula of P.7 Line 1 it should be in [min]. This should be clarified 31 better with numbered equations and a clear description and meaning of the variables and their 32 units. This applies as well for the formulae in P7, Ls 12 and 20. 33 Yes, the units of 1/k should be minutes. Corrections were made to account for this (Page12, Table 3). 34 Authors also agree with the suggestion of numbering equations. Methods of estimation and physical 35 meaning of 1/k, % foam drainage and V0 values were described on the Page 7, L6 to page 8 L2. 36 37 5. It is not clear what is the criterion to define a combination as "incompatible". In some 38 paragraphs (e.g. P10-L7-8) it is indicated that incompatibility results when the difference in air-39 entrainment requirements were significant (depending on the test applied), apparently in positive 40 or negative sense. I.e. a combination is considered "incompatible" if the AEA dosage was 41 significantly higher or lower than the reference case. This criterion was used to identify the 42 "incompatible" combinations in Table 2. However, in the Summary, p.13-Ls18-19, "incompatibility" 43 is associated only to a significant increase of the AEA dosage. 44 The authors were consistent with using the limiting criteria to identify incompatible combinations. A 45 mixture is considered incompatible if the AEA dosage was significantly higher or lower than the reference 46 case (base mixture) when tested using the foam index test and the AASHTO 137 test methods. However 47 as it can be observed from the Figures 2-5 or the Table 2, all the incompatible mixtures had higher 48 requirement of AEA dosage (with respect to the base mixture) except for one case(CFWPAS in Figure 5). 49 The discussion presented in the summary section focuses on the problems that were observed which, in 50 this case, were associated with significant increase in the dosage of AEA. Text was added in the summary 51 section of the modified paper (Page 13 Ls 23-26) to make this issue more clear. 52 53 6. The removal of the "outliers" in Fig. 6 seems, on a first reading, a bit arbitrary. Perhaps the fact 54 that the three points correspond to a specific combination of materials, as explained in P11-Ls 3-55 12. I would recommend to include these comments before the one in P.10-Ls25-27. 56



Relevant modifications were made according to reviewer’s suggestion. The text was re-arranged on Page 1 10 line 24 to Page 11 line 7) 2 3 7. There are some problems with the designation of the materials in P.10, Ls13-14 and an S too 4 many in AASSHTO in that page. 5 Relevant modifications were made to account for these problems (Page 10 Ls 11-29). 6 7 In "Conclusions": 8 1: the effect of the components on strength was not investigated in this research and is an 9 important aspect missing. 10 The current paper is part of a major project which includes compressive strength testing of concrete 11 samples. However, the results and subsequent discussions were not presented here because of the 12 constraints on length of the paper. Interested readers can look at Paleti, 2011 for the results and 13 subsequent discussion on tests performed on the concrete mixtures. 14 15

The word "avoid" seems too strong since the paper shows that an appropriate selection of 16 admixtures can solve the problem; perhaps a warning on the need to find the right combination of 17 admixtures should be more appropriate and would add value to the research presented 18 The authors agree with the reviewer that the use of the word “avoid” is too strong. Appropriate 19 modifications were made in pt.1 of the Conclusions section of the paper 20 21 3: There is no evidence provided to support the claim that the effect of fly ash content on AEA 22 dosage is due to preferential adsorption of SP on C particles of the FA. If the authors want to leave 23 it I would rephrase it as "based on other investigations .... could be attributed ....." 24 This was what was written on page 8 (lines 24-28) and the reference used to this was by Perch, 2004 25 (reference #17 in the paper) 26 27 4: Another important aspect missing in this research (one was the effect on strength) is that no 28 information is provided on the quality of the AVS generated by the different combinations. This 29 deprives the paper of value both for researchers and practitioners. 30 The current paper is part of a major project which includes quality and stability of air void system (AVS) in 31 the concrete samples. Work related to microscopic observation of hardened concrete samples to study 32 the quality of air void system (AVS) is an ongoing work. However, the results and subsequent discussions 33 were not presented here because of the constraints on length of the paper. Interested readers can look at 34 the full report (Paleti, 2011) for the results and subsequent discussion on tests performed on the concrete 35 mixtures. 36 37


potential issues with generation and stability of air-void

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