Scholarly article on topic 'Water-binder Ratio Influence on De-icing Salt Scaling of Fly Ash Concretes'

Water-binder Ratio Influence on De-icing Salt Scaling of Fly Ash Concretes Academic research paper on "Civil engineering"

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{"air-entrained concrete" / scaling / "freeze and thaw resistance" / "de-icing salt" / "siliceous fly ash" / "loss on ignition"}

Abstract of research paper on Civil engineering, author of scientific article — Aneta Nowak-Michta

Abstract The influence of water-binder ratio on de-icing salt scaling of hardened concretes with siliceous fly ash addition is analysed in the paper. The analysis made on the basis of 56 cycles scaling Borås test of air-entrained concretes. The levels of water-binder ratio (w/b): 0.45 and 0.38, the partial replacement of cement with fly ash (0, 20, 35, 50% mass of cement), and kind of siliceous fly ash in three categories of loss on ignition A, B and C were variable in concrete mixes. The results show that apart from air-entraining, which was the sufficient condition to ensure freeze-thaw resistance, also the lowest value of water-binder ratio is necessarily to ensure de-icing salt scaling resistance of fly ash concretes. Loss on ignition of fly ash has significant influence on resistance to scaling only in air-entrained concretes with 0.45 water-binder ratio.

Academic research paper on topic "Water-binder Ratio Influence on De-icing Salt Scaling of Fly Ash Concretes"

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Procedía Engineering 57 (2013) 823 - 829

Procedía Engineering

www.elsevier.com/locate/procedia

11th International Conference on Modern Building Materials, Structures and Techniques,

MBMST 2013

Water-binder Ratio Influence on De-icing Salt Scaling of Fly Ash Concretes

Aneta Nowak-Michta *

Institute of Building Materials and Structures, Faculty of Civil Engineering, Cracow University of Technology, ul. Warszawska 24, 31-155 Krakow, Poland

Abstract

The influence of water-binder ratio on de-icing salt scaling of hardened concretes with siliceous fly ash addition is analysed in the paper. The analysis made on the basis of 56 cycles scaling Boras test of air-entrained concretes. The levels of water-binder ratio (w/b): 0.45 and 0.38, the partial replacement of cement with fly ash (0, 20, 35, 50% mass of cement), and kind of siliceous fly ash in three categories of loss on ignition A, B and C were variable in concrete mixes. The results show that apart from air-entraining, which was the sufficient condition to ensure freeze-thaw resistance, also the lowest value of water-binder ratio is necessarily to ensure de-icing salt scaling resistance of fly ash concretes. Loss on ignition of fly ash has significant influence on resistance to scaling only in air-entrained concretes with 0.45 water-binder ratio.

© 2013 The Authors. Published by Else vier Ltd.

Selection and peer-review underresponsibility of" the Vilniu s Gediminas Technical University Keywords: air-entrained concrete, scaling, freeze and thaw resistance, de-icing salt, siliceous fly ash, loss on ignition.

1. Introduction

Fly ash is ons of ths most popular mineral additions to concrsts. Howsvsr, ths durability assurance of concrstss with their addition makss plsnty of problsms [1]. Commonly used air-sntraining of concrsts to ensure thsir freezs-thaw durability, in ths cass of fly ash application causes a lot of controvsrsy [2-4]. Ths rsssarch results ars divsrgsnt in this arsa. Many rsssarchss show that propsrly air-sntrainsd concrsts with ths addition of "good quality" fly ash ars rssistant to cyclic frsszing and thawing [2-4]. Howsvsr, thsrs ars also rsssarchss [1, 5] showing that durability of fly ash concrstss dscrsasss by frsszing and thawing.

As main qualitative paramstsr of fly ash dstsrmining frsszs-thaw rssistancs concrsts with thsir addition, ths contsnt of ths loss on ignition is indicatsd. Effscts of ths loss on ignition (2.2-20.5%) and fly ash contsnt (0-20%) for reducing strsngth aftsr frsszing and thawing wsrs studied by Lushr [6]. Ths obtainsd results clsarly confirm ths nsgativs impact of loss on ignition in ths ashss on ths frost rssistancs of concrsts with thsir addition. Rsfsrring to an accsptabls 20% dscrsass in strsngth critsrion PN-88/B 06250 did not msst ths requirement to only mix with ths ashss of a loss on ignition of mors than 10%.

Ths revised standard in 2006 PN-EN 450-1 „Fly ash to concrsts - Part 1: Definition, specifications and conformity criteria." introduced three categories of loss on ignition (LOI): A: LOI<5%, B: LOI=2-7% & C: LOI=4-9%. In addition to ths standard terms of loss on ignition thsrs is a note that ths amount of loss on ignition can affect ths efficiency of airsntraining admixtures used to produce frost-resistant concrsts. Ths defined three categories of loss on ignition allow ths user to take this into account by selecting ths appropriate category for application and exposure class.

Air-sntraining and correspondingly low value of water-binder ratio is indicatsd as basic parameters influencing rssistancs of concrsts to de-icing salt scaling [7-10].

*E-mail address: a_nowak@pk.edu.pl

1877-7058 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the Vilnius Gediminas Technical University doi:10.1016/j.proeng.2013.04.104

Air-entraining of concrete with siliceous fly ash in three categories of loss on ignition was a sufficient condition to ensure their frost resistance of F150 according to PN-88/B 06250. The value of water-binder ratio in the range 0.55 to 0.38 did not play a role [11].

Surface freezing effect involving de-icing is more intense than in the case of volume freezing. In case of surface interaction with de-icing not only air-entraining plays an important role, but also the structure of the concrete surface, which is significantly shaped by the value of water-binder [7-10]. Therefore, it seems appropriate to recognize its impact on the de-icing salt scaling resistance of concrete with the fly ash addition.

2. Experimental section

2.1. Purpose and scope of research

The research program aims to assess the influence of water-binder ratio for resistance to de-icing salt scaling of concrete with siliceous fly ash. Recipe mixes were designed with the following assumptions:

• four quantities of added fly ash 9: 0, 20, 35 and 50% of weight of cement (using a simple method to replace the cement with fly ash),

• two levels of water-binder ratio w/b: 0.38 and 0.45 (binder = cement + fly ash),

• three types of siliceous fly ash compatible with PN-EN 450-1: 2012, in terms of loss on ignition A (LOI = 1.9%), B (LOI = 5.1%) & C (LOI = 9.0%),

• air-entraining with admixture based on modified wood resins (concrete mixes without the addition of fly ash: non-air-entraining and air-entraining of 4.5%, air-entraining concrete mixes with the addition of fly ash of 4.5%),

• fixed consistency of mix concrete S3 (100 ^ 150 mm slump) adjusted by the superplasticizer based on polycarboxylates. Mixtures are made of cement CEM I 32,5R and natural aggregate of sand point 35% and the maximum grain D = 16 mm.

The amounts of fly ashes additive are in the range of 0^50% due to the widespread use in practice [12, 13]. The main criterion used in choosing the fly ash was to generate a representative domestic production of loss on ignition in three categories A, B and C, while having a similar chemical composition and fineness. The analysis of the ashes selected three of the physical and chemical properties shown in Table 1.

In accordance with PN-EN 206-1 for durability of concrete subjected to freezing with salt scaling - exposure classes XF2 and XF4 are required, inter alia, the maximum value of water-binder ratio of 0.45. Among the tested air-entraining concretes with water-binder ratios of 0.55, 0.45 and 0.38, which showed the frost resistance F150 according to PN-88/B 06250 [12], the requirement for water-binder ratio exposure classes XF2 and XF4 meet the requirement of a series with w/b = 0.45 and 0.38 thus their selection for testing resistance to scaling.

Table 1. Properties of fly ashes

Fly ash A Fly ash B Fly ash C Requirements of PN-EN 450-1

SiO2 53,49% 48,71% 50,50% SiO2 < 25% SiO2 + AI2O3 + Fe2O3 < 70%

A12O3 25,80% 25,29% 29,98%

Fe2O3 7,30% 5,36% 4,74%

CaO total 3,08% 3,26% 1,65% CaO < 10%

CaO free 0,10% 0,11% 0,10% < 1%

Loss on ignition 1,9% 5,1% 9,0% Category: A < 5,0% B 2,0 - 7,0% C 4,0 - 9,0%

SO3 0,40% 0,40% 0,35% < 3,0%

Fineness remains on sieve 45 pm 36 39 27 Category: N < 40% S < 12%

Activity index at 28 days at 90 days 97% 117% 102% 124% 94% 105% > 75% > 85%

Density [g/cm3] 2,08 2,17 2,39

Table 2. Compositions and properties of concrete with w/b = 0.38

Type of ash without Fly ash A Fly ash B Fly ash C

Series U0 U0N U2AN U3AN U5AN U2BN U3BN U5BN U2CN U3CN

Share of ash in the binder 9 [%] 0 0 20 35 50 20 35 50 20 35

Water-binder ratio w/b 0,38

Cement [kg/m3] 450 450 360 293 225 360 293 225 360 293

Fly ash [kg/m3] 0 0 90 158 225 90 158 225 90 158

Water [kg/m3] 171

Sand 1 [kg/m3] 151 170 115 87 115 87 96 72 96 112

Sand 2 [kg/m3] 431

Gravel 2/8 [kg/m3] 712

Gravel 8/16 [kg/m3] 518

Superplasticizer [% m.s.] 1,3 1,2 1,3 1,4 1,1 1,1 1,3 1,2 1,3 1,6

Air-entraining admixture [% m.s.] 1,0 1,0 1,1 1,0 0,95 1,0 1,0 1,0 1,2

Compressive strength at 90 days [MPa] 74,7 64,1 63,2 44,6 37,0 61,8 49,4 42,8 66,9 59,7

Table 3. Compositions and properties of concrete with w/b = 0.45

Type of ash without Fly ash A Fly ash B Fly ash C

Series S0N S2AN S3 AN S5AN S2BN S3BN S5BN S2CN S3CN S5CN

Share of ash in the binder 9 [%] 0 20 35 50 20 35 50 20 35 35

Water-binder ratio w/b 0,45

Cement [kg/m3] 400 320 260 200 320 260 200 320 260 200

Fly ash [kg/m3] 0 80 140 200 80 140 200 80 140 200

Water [kg/m3] 180

Sand 1 [kg/m3] 170 137 113 89 142 121 100 150 135 120

Sand 2 [kg/m3] 431

Gravel 2/8 [kg/m3] 712

Gravel 8/16 [kg/m3] 518

Superplasticizer [% m.s.] 0,7 0,7 0,65 0,7 0,7 0,75 0,75 0,8 1,0 1,0

Air-entraining admixture [% m.s.] 0,3 0,35 0,45 0,4 0,5 0,6 0,6 0,65 0,75 0,75

Compressive strength at 90 days [MPa] 59,6 55,7 37,5 34,7 46,7 41,4 38,9 52,8 50,9 46,3

3. Test samples

Concrete was performed in the laboratory at 20°C and relative humidity above 60%. The quantities of additives, both air-entraining and superplasticizer were adjusted to achieve consistency in the class S3 (100^150 mm slump) and the required level of air-entraining. The amounts of admixtures were used within the ranges recommended by the manufacturer.

Test samples were demolded after 24 hours and stored in a chamber at 20±2 °C and humidity of 95±5% in accordance with PN EN 12390-3. Scaling resistance test was started after 90 days.

Compositions and the compressive strength of concretes with w/b = 0.38, after 90 days of maturation are given in Table 2, while the concrete with w/b = 0.45 in Table 3.

4. Scaling test

Determination of scaling resistance was carried out with an automatic chamber for freezing and thawing of samples, using Slab test, according to PKN-CEN/TS 12390-9: 2007 Testing hardened concrete - Part 9: Freeze-thaw resistance -Scaling. For each of a series of concrete the test was conducted on four samples of 140x72x50 mm, which were subjected to impact of 56 freeze-thaw cycles in the presence of 3% NaCl.

To assess the resistance of concrete to scaling the criteria of the Swedish standard for Boras method SS 137244 were used. The Standard category of scaling resistance in presence of 3% NaCl:

• very good quality concrete: m56<0,1 kg/m2,

• good quality concrete: m56<0,2 kg/m2 or m56<0,5 kg/m2 and m56/m28<2 or m112<0,5 kg/m2,

• acceptable quality concrete: m56<1,0kg/m2 and m56/m28<2 or m112<1,0 kg/m2,

• unacceptable quality concrete: m56>1,0 kg/m2 and m56/m28>2 or m112>1,0 kg/m2.

5. The results of scaling resistance of concrete with the fly ash addition

Average losses of mass after 56 cycles of freezing and thawing in the presence of 3% NaCl, expressed in kg/m2 are presented in Tables 4 and 5, respectively for concrete of w/b = 0.38 and 0.45.

Among the tested concretes with w/b = 0.38 only control concrete, without air-sntrainment and without addition of fly ash, in light of the evaluation according to Swedish Standard SS 113 72 44 (Boras method) is a concrete of unacceptable quality. All air-sntraining concrstss, both without and with ths addition of fly ash ars concrstss complying with ths standards for allowable loss of mass.

Among the tested concretes with w/b = 0.45, only control concrete without the fly ash addition is a concrete of acceptable quality, whereas all of the concretes with 20, 35 and 50% fly ash addition, whose the loss on ignition are in the range 1.9-9.0% are not meeting the standards requirements for allowable loss of mass.

Table 4. The results of scaling resistance of concrete with w/b = 0.38

Series Fly ash 9 [%] Loss of mass [kg/m2] Assessment of concrete quality

U0 U0N without 0 0 117,51 0,05 unacceptable very good

U2AN 20 0,05 very good

U3AN Fly ash A 35 0,94 acceptable

U5AN 50 0,99 acceptable

U2BN 20 0,15 very good

U3BN Fly ash B 35 0,64 acceptable

U5BN 50 0,25 very good

U2CN U3CN Fly ash C 20 35 0,15 0,15 good good

Table 5. The results of scaling resistance of concrete with w/b = 0.45

Series Fly ash 9 [%] Loss of mass [kg/m2] Assessment of concrete quality

S0N without 0 0,64 acceptable

S2AN 20 1,60 unacceptable

S3AN Fly ash A 35 1,53 unacceptable

S5AN 50 1,23 unacceptable

S2BN 20 1,69 unacceptable

S3BN Fly ash B 35 2,28 unacceptable

S5BN 50 2,95 unacceptable

S2CN 20 1,53 unacceptable

S3CN Fly ash C 35 3,05 unacceptable

S5CN 50 6,06 unacceptable

In fly ash concretes with w/b = 0.38 losses of mass varied depending on the content and size of the loss on ignition in the ashes. In the case of concrete without the fly ash addition and concrete with 20% fly ash addition of A and B category of loss on ignition, losses of mass did not exceed 0.1 kg/m2, therefore, they are of very good quality. Concretes with 20 and 35% addition of fly ash C and added with 50% fly ash B showed a loss of mass 0.1 to 0.2 kg/m2, which classifies them as a good quality concrete. However, in concretes with 35 and 50% addition of fly ash A and 35% fly ash B, losses of mass ranged from 0.2-1.0 kg/m2, which are concretes of acceptable quality.

In fly ash concretes with w/b = 0.45 losses of mass depend on the content and size of the loss on ignition in the ashes. In case of 20% fly ash losses are comparable for all concretes with all three ashes. At 35 and 50% fly ash additive clearly points out the influence of the size of loss on ignition on losses of masses. Losses of masses increase with an increase in the loss on ignition, reaching the level of 6.06 kg/m2 for 50% fly ash C, content of 9.0% loss on ignition.

6. Analysis of test results

Water-binder ratio formed porosity of the cement paste (both in the structure and in the transition zone paste-aggregate), and consequently all of the concrete properties. Salt scaling resistance of concrete increases with a decrease in w/b ratio [710]. Many laboratories have confirmed that the use of fly ash air-entraining concretes significantly reduces their resistance to salt scaling [14-16, 3, 13, 17, 18].

The attempt to clarify the issues of lowered salt scaling resistance of air-entraining concrete with high fly ash content was made by Zhang et al. [17]. On the basis of these studies and analyses they hypothesized that in concrete with the fly ash addition subjected to salt scaling destruction occurs in micro cracks running at the contact zone between fly ash and paste. The a15, uthors explain the hypothesis as follows: the zone around grains of fly ash is usually more porous than its hydration products. With time, the thickness of the coating decreases, the density yielding reaction with the development of pozzolanic fly ash. When fly ash concrete is exposed to scaling, particularly in the early period, the salt solution penetrates into the porous structure of the concrete thorough coating around particles of ash. During freezing, the water volume increases by 9%. Depending on the available volume it can cause significant hydraulic pressure resulting in the formation of cracks and chips. Also, the subsequent changes in temperature can generate significant internal stresses in the present coating as a result of differences in the coefficients of thermal deformability of the fly ash particles, ice and matrix hydration products. Even at a later time, when the thickness of shell around the particles is reduced to zero, the temperature may continue to generate considerable internal stresses in this area. Thus, the structure of porous cement paste containing fly ash on the surface of concrete during freezing and thawing in the presence of de-icing, has a significant effect on the resistance of concrete to scaling [18].

The increase in number of chips in concrete with high fly ash content is the result of a large amount of fly ash particles, which usually run around microcraks. With a large number of ash particles these zones partially overlap and are weak spots in the concrete, the more susceptible to cracks [14, 15].

The research programme carried out confirmed literature data for concretes with w/b = 0.45. Those fly ash concretes show a lack of resistance to frost with de-icing. At the same time the impact of loss on ignition is clearly seen, especially at large, 35 and 50% of fly ash content in the binder (Fig. 1). It should be noted that all air-entraining, concretes on the level of at least 4% were resistant to 150 freeze-thaw cycles according to PN-88/B 06250. They did not show any defects of mass or strength decreases. Because of that, the thesis that air-entraining concrete protection against destruction by freezing and thawing without the use of salt is confirmed. It is not sufficient to protect the concrete by scaling. In the case involving the effects of scaling is required in the surface layer of concrete, particularly subjected to destructive effects of frost to produce a very tight structure of the yeast, the yeast are contact-aggregate, which is the most susceptible to the destructive effects of frost with de-icing [15, 17, 18].

At 20% of fly ash content in the binder is marked by virtually no impact of the size of loss on ignition on losses of masses, while at 35 and 50% fly ash the mass of chips significantly increases with an increase in the loss on ignition.

The hypothesis posed by Zhang et al. [17] was confirmed in the light of the research results. Furthermore, it appears that the high resistance of the fly ash on the surface of concrete with frost effect de-icing decreases with an increase in the loss on ignition due to the high content of harmful carbon particles focused on the grain surface of the ash. Unburned carbon particles have a large surface area, in the absence of a vitreous phase, which is responsible for pozzolanic reaction of fly ash [18]. Thus, the circle of ash particles with a high content of unburned carbon is a highly porous, weak point in the structure of the paste, and at the same time a source of destruction involving frost de-icing.

Fig. 1. Dependence of average loss of mass and the fly ash content in the binder for concretes with addition of A, B and C categories of fly ash after 56 cycles of scaling

The situation changes significantly for concretes with w/b = 0.38. When used in the research program of sufficiently low water-binder ratio, at 0.38, all tested air-entraining concretes; despite the simple substitution method used in the design of the ash cement concretes have shown resistance to scaling. It should be noted that in the subjects tested binder in concretes with w/b = 0.38, with 50% addition of fly ash is the sum of 225 kg/m3 cement and 225 kg/m3 fly ash, the water-cement ratio is 0.76. No air-entraining concrete without the addition of fly ash, with w/c = 0.38 has scattered during the test. It confirms that both the proper air-entraining and low coefficient of water to binder are necessary for resistance to scaling.

In the light of the test results for concretes with w/b = 0.38, the impact of quantitative and qualitative parameters of fly ash on resistance to scaling is not be indicated (Fig. 1). On the basis of losses of mass, which are the criterion according to the Swedish Standard SS 113 72 44 resistance of concrete to scaling, it can be observed (Fig. 1) that for each fly ash losses of mass (except for concrete with B fly ash content of 35%) grow with the increase of the fly ash content.

Fig. 2. Dependence of average losses of mass and compressive strength at 90 days for concretes with addition of A, B and C categories of fly ash after 56 cycles of scaling

Fig. 2 shows the relationship between compressive strength and the average losses of mass caused by scaling after 56 cycles of freeze and thaw with de-icing. Chart analysis leads to the conclusion that the air-entrained concrete with a strength of more than 60 MPa, regardless of the type and quantity of the fly ash in it are resistant to frost with de-icing. In terms of strength from 35 to 60 MPa the lowest resistances to scaling have concretes with fly ash C, but there is no relationship between strength and resistance to scaling.

Commonly used to evaluate frost resistance concrete is the criterion of air void parameters: spacing factor L<0,200 mm and micro air content A300>1.8% to ensure frost resistance [8-10] in case of concrete with the addition of fly ash in the light of research conducted by the author is not applicable [11]. Air content in hardened concretes with fly ash diagnosed by an automated system RapidAir 457, increase with the increase in the percentage replacement of the cement with fly ash. As the analysis shows [11], pores in the hardened concrete with the addition of fly ash are the sum of the pore cavities in the air-entraining and ash grains whose dimensions are consistent with the pore dimensions of air-entraining.

7. Conclusions

The carried out research programme and analysis of the obtained results were the basis to the following conclusions:

• Air-entraining of concrete mix with fly ash addition, which protects it from the freeze-thaw destruction without the de-icing salt during 150 cycles, independently of the value of the water-binder ratio, does not sufficiently protect concrete from freeze-deicing salt. To achieve the freeze-deicing salt resistance of fly ash concrete, not only air-entraining is necessary, but also appropriate minimum value of w/b = 0.38.

• In every tested air-entrained fly ash concrete with the water-binder ratio of 0.38, subjected to 56 freeze-thaw cycles in presence of 3% NaCl, the mass loss did not exceed 1,0 kg/m2. Therefore such concretes, in accordance to SS 137244 classification, meet a criterion allowing to apply them at the freeze-deicing salt exposition.

• In all air-entraining fly ash concretes with water to binder ratio 0.45, subjected to 56 cycles of freezing with 3% NaCl, losses of mass exceeded 1.0 kg/m2, which, according to the classification of SS 137244 states that they are not meeting the criterion of exposure with the use of the de-icing salt.

• In concretes with w/b = 0.45 with 35 and 50% fly ash content the impact of loss on ignition in the fly ashes on the mass loss of scaling is emphasized. These masses grow with the increase in the total loss on ignition. The mass loss of the concrete with the addition of fly ash C (LOI = 9.0%) is almost five times higher than in the concrete with the addition of fly ash A (LOI = 1.9%).

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