Scholarly article on topic 'Influence of Superplasticizer on Porosity Structures in Hardened Concretes'

Influence of Superplasticizer on Porosity Structures in Hardened Concretes Academic research paper on "Civil engineering"

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{"Air-entrained Concrete" / Superplasticizer / "Air Entraining Admixture" / "Image Analysis" / "Pore Size Distribution" / Consistency}

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

Abstract One of the criteria used to achieve frost resistant concrete is to obtain appropriate parameters of porosity structure in the hardened concrete. Polish requirements in this regard are not defined by standards. There is only a requirement in terms of the Spacing Factor for air-entraining admixtures contained in EN 934-2. The standard EN 206 to ensure frost resistance of concretes requires in exposure classes XF2-XF4 air content in fresh concretes at least 4% without verification of the porosity structure in the hardened concrete. The technical specifications for paving concretes and bridges usually contain requirements for parameters of porosity structures based on international requirements. Air entraining process of concrete mixtures is complex and depends on many parameters. The analysis of porosity structures in hardened concretes, despite the required frost resistance of concrete, often do not give satisfactory results. Analysis of the results indicates that the reason is inadequate pores sizes in the concretes. Thus, an attempt of explanation of influences of one of the parameters affecting the air entraining process – consistencyof concrete mixes (quantity of superplasticizer) on the porosity structure in the hardened concrete. The research programme was performed on eight series of air-entraining and non-air-entraining concretes with a variable content of superplasticizer. The basic composition and air-entraining admixtures content in air-entraining concrete mixtures were constant. The results showed that with the increase of the content of superplasticizer (with increasing fluidity) air content decreases and the pores sizes changed.

Academic research paper on topic "Influence of Superplasticizer on Porosity Structures in Hardened Concretes"

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ScienceDirect Procedía

Engineering

ELSEVIER Procedía Engineering 108 (2015) 262 - 269

www.elsevier.com/locate/procedia

7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD'2015)

Influence of superplasticizer on porosity structures in hardened

concretes

Aneta Nowak-Michtaa*

aCracow University of Technology, Warszawska 24, 31-155 Krakow, Poland

Abstract

One of the criteria used to achieve frost resistant concrete is to obtain appropriate parameters of porosity structure in the hardened concrete. Polish requirements in this regard are not defined by standards. There is only a requirement in terms of the Spacing Factor for air-entraining admixtures contained in EN 934-2. The standard EN 206 to ensure frost resistance of concretes requires in exposure classes XF2-XF4 air content in fresh concretes at least 4% without verification of the porosity structure in the hardened concrete. The technical specifications for paving concretes and bridges usually contain requirements for parameters of porosity structures based on international requirements. Air entraining process of concrete mixtures is complex and depends on many parameters. The analysis of porosity structures in hardened concretes, despite the required frost resistance of concrete, often do not give satisfactory results. Analysis of the results indicates that the reason is inadequate pores sizes in the concretes. Thus, an attempt of explanation of influences of one of the parameters affecting the air entraining process - consistency of concrete mixes (quantity of superplasticizer) on the porosity structure in the hardened concrete. The research programme was performed on eight series of air-entraining and non-air-entraining concretes with a variable content of superplasticizer. The basic composition and air-entraining admixtures content in air-entraining concrete mixtures were constant. The results showed that with the increase of the content of superplasticizer (with increasing fluidity) air content decreases and the pores sizes changed.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-reviewunderresponsibilityof organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering Keywords: Air-entrained Concrete; Superplasticizer; Air Entraining Admixture; Image Analysis; Pore Size Distribution; Consistency

CrossMarl

* Corresponding author. Tel.: +48 12 628-23-63. E-mail address: a_nowak@pk.edu.pl

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering doi: 10. 1016/j .proeng .2015.06.146

1. Introduction

As the main parameters influencing the frost resistance of concrete are indicated air entraining and correspondingly low value of the water-cement ratio [1-9]. The standard EN 206 [10] to ensure frost resistance of concretes in exposure classes XF2-XF4 requires (apart from a maximum water-cement ratio, minimum strength class, minimum cement content) minimum air content of 4%, without verification of PPS. The requirements for PPS in hardened concrete are not specified by Polish standards. There is only a standard requirement specified in the Table. 5 of EN 934-2 + A1: 2012 [11] in terms of the size of L in the reference concrete with tested AEA. Neither the standard EN 13877-2: 2013 [12] for the pavement concrete nor the standard for bridge concrete specifies the requirements for PPS. The requirement for the maximum value of L (<0.200mm) in the paving concrete is included in point 5.3 of Specification D-05.03.04 [13], which also determines the procedure of indication in accordance with EN 480-11 [14].

In Poland, there are commonly used requirements for PPS of concrete in exposure class XF4 included in both the specification D-05.03.04 [13] as well as German standard ZTV Beton - StB 01 [15], which determines the required value of L <0.200mm and A300^1.8% in hardened concrete. Test results obtained by the author, as well as the ones presented by Glinicki and Zielinski [16] in terms of the diagnosis of AVS of embedded concrete structures and pavement concretes showed that although obtained the required air content in concrete mix, there are many irregularities in the AVS. The following abnormalities were observed:

• insufficient or excessive A,

• unfavorable AVD, too few bubbles with diameters of less than 300^m and at the same time too many 'larger' pores with diameters up to 4 mm,

• too high values of L .

Nomenclature

A total content of air in hardened concretes (%)

A300 micro air content (%)

AE air entraining

AEA air entraining admixture

AEC air entrained concretes

AVD air void distribution in hardened concretes

AVS air void system in hardened concretes

a specific surface (mm-1)

L spacing factor (mm)

NAEC non-air entrained concretes

PPS parameters of porosity structure

SP superplasicizer

Vp total air content in concrete mix (%)

These irregularities are due to the fact that obtaining proper AVD depends on many material and technological factors [6, 17-19]. Among a number of parameters affecting A and AVS, as most often used in current concretes production the following factors are indicated [20-22]: (1) the consistency of concrete mix, (2) compatibility between AEA and SP, (3) kind of AEA and (4) kind of SP.

2. Experimental section

2.1. Purpose and scope of research

AEA and SP interact with each other [7]. On the one hand the dispersion effect of SP affects the dispersion of air in the water, on the other hand, air entraining improves workability of the concrete mix. Therefore, the effectiveness of SP may depend, among others, on the presence of AEA and vice versa. Thus, as the aim of the research programme was adopted to determine the effect of SP and the consequent change of consistency on the PPS in AEC and NAEC.

The basic compositions of concrete mixes were constant (Table. 1), the content of paste in all concretes was 30.71%. The water-cement ratio was also constant and its value has been determined experimentally and it was 0.46. The pre-assumed highest value of the w/c ratio will be in the range of 0.45-0.55, for which, with the quantity of cement not exceeding 400kg/m3, consistency class S1 will be obtained. The main distinguishing feature of the composition of the concrete mixes were the contents of SP and AEA, which are given in the Table 2.

Table 1. Basic compositions of concrete mixes.

Component Cement Water Sand 0/2 Basalt 2/8 Basalt 8/16

(kg/m3) 391 181 572 795 635

As part of the research programme 8 sets of concrete mixture were performed:

• S1 - control mix (non air-entrained, without SP) in consistency class S1 (slump 10 to 40mm),

• SN1 - air-entrained mix of 4% without SP,

• S2, S3, S4 - non air-entrained mixes with basic composition like mix S1 with SP permitting respectively consistency classes: S2 (slump 50 to 90mm), S3 (slump 100 to 150mm) & S4 (slump 160 to 210mm) according to EN 206,

• SN2, SN3, SN4 - air-entrained mixes with quantity of AEA like in SN1, basic compositions and quantity of SP like in S2, S3 & S4.

Table 2. Content of SP and AEA in concrete mixes.

Admixture/symbol Contents (% S1 m.c.) S2 S3 S4 SN1 SN2 SN3 SN4

SP - 0.20 0.35 0.50 - 0.20 0.35 0.50

AEA - - - - 0.20 0.20 0.20 0.20

2.2. Materials

SP was a concentrated water solution of modified polycarboxylates. Among a wide range of available AEA [19] admixture based on modified wood resins was used, which in the tests by Barfield and Ghafoori [20] showed the best efficiency.

In order to eliminate a possible increase in air content resulting from a lack of compatibility of cement and SP [21] admixtures, which had already been used in the studies [23] were used. In addition, during the analysis of porosity structures in these hardened, both AEC and NAEC 'large pores' were observed, which became a starting point to approach this issue. Also Lazniewska-Piekarczyk [22] in her studies confirmed the side effect (air-entraining) of SP on the base of polycarboxylates.

Portland cement CEM I 42.5R used in the tests meets the requirements of standard EN 480-1 [24] in the field of cement used in reference concrete. The main parameter taken into account in the selection of coarse aggregate, was

to select aggregate without porous grains which require repainting during the samples preparation for testing AVS in hardened concretes [23]. The coarse aggregate fractional (2/8 and 8/16), basalt with a maximum grain D=16mm and natural fine aggregate (0/2) were applied in the research programme. Mix of aggregate was composed by iterations and verified by its particle size in the range of requirements for reference concrete according to the standard EN 480-1 [24].

2.3. Test samples

Concrete mixes were performed in a laboratory at 20°C and a relative humidity above 60%. Concrete mixtures with a volume of 40dm3 were made in a positive mixer with a volume of 100dm3. The following procedures of mixing the ingredients were used: (1) mixing for about 0.5 minute to homogenize the dry ingredients: aggregate + cement, (2) adding a portion of the mixing water and mixing for another 0.5 minute, (3) adding SP with 2dm3 of water and mixing for 1 minute, (4) for air-entraining mixtures adding AEA with 2dm3 of water and mixing for 2 minutes.

After mixing the components, the testing of concrete mixes was run parallely: the consistency, density and air content. Then the samples were moulded and compacted. For each of a series of concrete 5 cubes with a side of 15cm were formed. The test samples were removed from moulds after 24 hours and stored in a chamber at 20±2°C and humidity greater than 95% according to EN 12390-2 [25].

2.4. The scope of research

The following tests of concrete were determined: PPS in accordance with EN 480-11 [14] for two cubes with the side of 150mm, compressive strength after 28 days in accordance with EN 12390-3 [26] for three cubes with a side of 150mm, density after 28 days in accordance with EN 12390-7 [27] in a state of real, for five cubes with a side of 150mm. The tests of concrete mixtures included determination: consistency by slump in accordance with EN 12350-2 [28], density in accordance with EN 12350-6 [29] and Vp in accordance with EN 12350-7 [30].

2.5. Methodology of PPS determination in hardened concrete

Determination of the PPS in the hardened concrete was carried out in accordance with EN 480-11 [14] using an automated image analysis system for voids in the hardened concrete RapidAir 457. For each of a series of concretes in accordance with the recommendations of the EN 480-11 [14] the test was carried out for two samples. PPS were determined for a selected lightest and heaviest sample among the formed five cubes. The following PPS were obtained for each tested sample: total content of air in hardened concretes A, specific surface a, spacing factor L , air void distribution AVD, micro air content A300.

Table 3. Test results of concrete mixes.

Property/symbol S1 S2 S3 S4 SN1 SN2 SN3 SN4

Slump (mm) 30 70 170 210 10 110 180 210

Density (kg/m3) 2567 2593 2613 2610 2470 2515 2543 2570

Vp (%) 2.9 1.7 1.2 1.4 6.0 4.6 3.8 2.7

Table 4. Test results of concretes.

Prop erty/symbol S1 S2 S3 S4 SN1 SN2 SN3 SN4

Compressive strength(MPa) Density (kg/m3) 51.7 58.6 59.0 59.0 49.3 49.5 52.5 56.0

2532 2586 2556 2567 2511 2498 2501 2525

A (%) 3.36 2.61 2.62 1.76 4.66 3.61 3.95 3.66

a (mm-1) 16.09 12.03 11.89 10.30 43.46 39.98 31.47 17.45

L (mm) 0.382 0.573 0.585 0.820 0.122 0.149 0.181 0.338

A300 (%) 0.88 0.55 0.43 0.23 2.50 1.69 1.84 1.00

2.6. Tests results

The test results of concrete mixes are given in the Table 3, and the average tests results of concretes are shown in the Table 4.

Figure 1 shows the average AVD in the NAEC, while in Figure 2 in the AEC. PPS for each series of concrete depending on the maximum chord taken into account: 0.5; 1.0 and 4.0 mm are presented in Tables 5 and 6, respectively for NAEC and AEC.

1.2 g 1.0 ■ S 5 o.s ■ ■j C3 'Z- 0.6 ■ s E 3- ■ 3 « ' 0.0 ■ XAIC □ SI BS2 »S3 BS4 U II ll i J ,1

J? J» <S> # <P sfi» / & * * * * * * 4>* ^ ^ /V ^^ /V" Void diameter (microns) / ^ # / / ^ 4" # #

Fig. 1. AVD according to EN 480-1 in NAEC.

1.2 G 0.5 ■ C3 Eh «0.6 -e § 3.4 ■ u < 02 ■ AEC □ SN1 C3SN2 HSN3 ■ SN4 .LkLLklkLihiiiiJjjiriib i Ilk L i

„-? J' # # # -r? # ^ 4s # ¿P # # # .# » SS * * * ^ ^ / / / / /// / / // //y/^y Void diameter (microns)

Fig. 2. AVD according to EN 480-1 in AEC.

Table 5. PPS in NAEC in dependence of considered the maximum chord.

Symbol S1 S2 S3 S4

Chords (mm) < 0.5 < 1.0 < 4.0 < 0.5 < 1.0 < 4.0 < 0.5 < 1.0 < 4.0 < 0.5 < 1.0 < 4.0

A (%) a (mm-1) L (mm) 1.41 30.24 0.268 2.31 19.44 0.370 3.36 16.09 0.382 0.83 31.88 0.354 1.32 21.92 0.422 2.61 12.03 0.573 1.07 24.53 0.410 1.40 20.22 0.444 2.62 11.89 0.585 0.57 26.39 0.517 0.78 20.90 0.572 1.76 10.30 0.820

Table 6. PPS in AEC in dependence of considered the maximum chord.

Symbol SN1 SN2 SN3 SN4

Chords (mm) < 0.5 < 1.0 < 4.0 < 0.5 < 1.0 < 4.0 < 0.5 < 1.0 < 4.0 < 0.5 < 1.0 < 4.0

A (%) a (mm- ) L (mm) 3.47 57.00 0.106 3.82 52.33 0.110 4.66 43.46 0.122 2.33 60.17 0.120 2.63 53.88 0.127 3.61 39.98 0.149 2.58 47.13 0.149 3.45 36.45 0.169 3.95 31.47 0.181 1.65 33.51 0.249 2.71 22.52 0.298 3.66 17.45 0.338

3. Analysis of test results

Average total air content in concrete mixes Vp (Table. 3) and in hardened concretes A (Table. 4) differed from -1.34 to +1.42%. Whereas in the all NAEC A was greater than Vp, and the difference between A and Vp increased with increasing content of the SP, with the exception of a series of S4 in which this difference was smaller, and was 0.36%. There was a decline in air content by 1.34 and 0.99% in AEC concretes marked as SN1 and SN2, while the difference in the concretes SN3 and SN4 was positive and equal of 0.15 and 0.96%. Comparing the differences with the differences obtained by Nowak-Michta [23], Lazniewska-Piekarczyk [22] and Glinicki and Zielinski [16], the results can be considered correct, within the range of accuracy of measurements and not constituting the basis for determining the effect of the loss, or additional air entraining of concrete.

By the action of a constant amount of AEA, Vp (Table. 3), depending on the consistency of the concrete mix increased by 3.1% for the S1 consistency, 2.9% for the S2 consistency, 2.6% for the S3 consistency and 1.3% for the S4 consistency. One can observe a decline in the efficiency of air entraining with increasing fluidity of concrete mix. Change in consistency was obtained in each case with a suitable dose of SP, so this decrease in effectiveness of AEA is due to the liquefaction, SP action, interaction AEA and SP or overlap of these effects. Quantitatively A, as a result of the air entraining, increased less than Vp. In contrast to Vp, the differences in the individual series of concretes were similar and ranged from 1% for the S1 consistency to 1.9% for S4 consistency.

Significant differences appear in case of AVD (Fig. 1 and 2). In NAEC, we can see the residual air content in fractions of 0 to 500 ^m, and a substantial air content in fractions of 0.5 to 4 mm. In AEC proportions change, there is a significant amount of air in each of the fractions in the range of 0 to 500 ^m, pore content in the fraction 500 to 1000 ^m increased. Air content in the fractions of 1-1.5 and 1.5-2 mm does not change, whereas most pores disappear in fractions of 2.5-3 and 3-4 mm.

There are two types of air pores introduced into the concrete mix: 'entrained air' - the effect of intentionally entrained air, the effect of AEA, which is a beneficial effect and 'entrapped air' - the air accidentally caught during the mixing of the concrete mix, which is a side effect [17, 19]. In the examined AEC (Fig. 2) one can clearly observed 'entrained air' and a decrease in 'entrapped air' content in the fractions of 2.5-3 and 3-4 mm, which in NAEC (Fig. 1) cannot observed.

In NAEC (Fig. 1), the air content in the individual fractions 0-2mm decreases with the liquefaction of concrete mix (content SP), while in fractions 2-4mm proportions changed. In the AEC, with increase in the liquefaction concrete mix (content SP) air content in the individual fractions in the range of 0-160 ^m decreases, but it increases of the range of 0.5-1mm, and ranges of 160-500 ^m and in the range of 1-4mm remain at comparable levels.

In all series of concretes (Fig. 3-6) the air content by the action of AEA in each of the fractions up to 300 ^m is greater in AEC than NAEC, which quantitatively confirmed contents of A300 in these concretes listed in Table 4. With the liquefaction of concrete mixes (content SP) in both AEC and NAEC the content of A300 decreases. Furthermore, with the increase in liquefaction (content SP) the proportions of the contents of macropores change. In the concretes without SP (Fig. 3) the content of macropore in individual fractions are greater in NAEC than in AEC. However, in the concretes with SP (Fig. 4-6) the content of macropores in the individual fractions in the range of 0.5-2mm is greater in AEC than in NAEC and their contents increases with increasing SP content, particularly in fraction 0.5-1mm. These observations in the scope of AVD confirm PPS presented in Table 4. The specific surface of pores (a), which is a measure of pore size in both AEC and NAEC decreases with the increase in liquefaction (SP content) and in AEC (SN1-SN3) is about three times higher than in the NAEC, and in concrete with the highest dose of SP (SN4) is 1.7 times greater. Consequently, the spacing factors L (Table. 4), which are a measure of the spacing of the pores, in both AEC and NAEC grow with the increase in liquefaction (SP content)

and in AEC take values over three times smaller, with the exception of concrete with the highest dose of SP, in which the value is 2.4 times smaller.

2 R S § 5j

ô »A i/, i/, iA

—> (N ci T

^ iA, — — — — (N Ol n (N (N f: f, 7 Î O O ""i C C

8 ¡S A * >A "A 1/, <A «A «A *

'' R ri R

Void diameter (microns)

Fig. 3. AVD in SI and SN1.

288SSÏ8888S8888S888 S 8 §

- n Cl t 1

l/, l/, l/, l/| l/, l/, l/, l/, l/, l/, W*, l/, l/, l/,

---i'iiMOn-i'iMO^giij

Void diameter (microns)

Fig. 4. AVD in S2 and SN2.

d IT| in l/| ¡A

-h n m f

.A —' — — — — O n n ("■) Ol C, f: 7 ^ V^I it! <A IA "A i/| i/| i/| i/| i/| «A iA ij^j i/| iA iA iA iA OOOCM^T'OOOOfMTr'OQCOW-.QVj

Void diameter (microns)

' " K O i/-. O vi G

— — rJ

— ra (

. _ ' C M c t . ... — —---rfl n t^ n n c

Void diameter (microns)

Fig. 5. AVD in S3 and SN3. Fig. 6. AVD in S4 and SN4.

The standard EN 480-11 [14] provides for the calculation of the PPS taking into account all the analysed pores, which is up to 4.0mm. Preferred pores in order to ensure resistance to frost are entrained air - pores with diameters up to 0.5 or 1.0mm. In Tables 5 and 6 are set, for all series of concrete, PPS calculated taking into account the pore chord up to 0.5; 1.0 and 4.0 mm. For each of a series of concrete one can observe beneficial effect of the elimination of entrapped air - "large" pores of the concrete consisting of depressing A, as well as significant increase in a and reduced L . We can easily count these parameters, but the question remaining, how to eliminate them from the concrete, in order to obtain the preferred PPS realignment. The analysis of the obtained results clearly showed that the air entraining of the concrete without SP reduces the content of 'large' pores in the concrete. However, in the concretes with SP their content increases with the increase in the SP content. So, the first clue as to the proper development of the AVD in the AEC is to find a proper system of additives, which does not increase the contents of large pores. This issue in terms of concrete SCC was dealt by Lazniewska-Piekarczyk [22].

4. Conclusions

The tests results showed that with liquefaction of concrete mix (with increasing SP contents) the efficiency of a constant amount of AEA lowers, which is the result of the liquefaction, SP action, interaction AEA and SP or overlap of these effects.

As a result of the air entraining A changes slightly, but AVD undergo a significant change. In the AEC content of pores of disadvantaged size - entrapped air decreases and content of pores with favorable sizes - entrained air grows.

With the liquefaction of concrete mix (with increasing SP content) in both NAEC and AEC, air content decreases in small fractions, while it increases in the coarse fraction.

Air entraining of concrete without SP reduces the content of 'large' pores in the concrete. However, in the concrete with the SP their content increases with an increase in SP.

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[25] EN 12390-2: 2009 Testing hardened concrete - Part 2: Marking and curing specimens for strength tests.

[26] EN 12390-3:2009 Testing hardened concrete - Part 3: Compressive strength of test specimens.

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