Construction and Building Materials 75 (2015) 227–233

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

On entrained pore size distribution of foamed concrete

http://dx.doi.org/10.1016/j.conbuildmat.2014.09.1170950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Addresses: Department of Civil Engineering, Faculty ofEngineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK;Civil Engineering Department, Faculty of Engineering, University of Anbar, Iraq. Tel.:+44 (0) 115 846 8427; fax: +44 (0) 115 951 3909.

E-mail address: evxaah@nottingham.ac.uk (A.A. Hilal).

Ameer A. Hilal a,b,⇑, Nicholas Howard Thom a, Andrew Robert Dawson a

a Department of Civil Engineering, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UKb Civil Engineering Department, Faculty of Engineering, University of Anbar, Iraq

h i g h l i g h t s

� Clear images of preformed foam were captured by treating with bitumen emulsion.� Successful comparison between the foam bubble and concrete void size distributions.� Voids circularity factor was successfully measured and related to added foam volume.� Evidence was found for both bubble loss and bubble merging during the mixing process.

a r t i c l e i n f o

Article history:Received 6 June 2014Received in revised form 18 September 2014Accepted 22 September 2014

Keywords:Foamed concretePore structureCircularity factorOptical microscopeImage processing

a b s t r a c t

The pore structure of foamed concrete is a significant characteristic since it affects properties such asstrength and durability. To investigate these properties, the determination of total air voids content isnot sufficient as the shape, size and distribution of air voids may also be influential. To understand the for-mation of voids after hardening, an investigation of the bubble size distribution of foam (before adding tothe mixture) and the pore size distribution of the foamed concrete mixes (after hardening) is discussed inthis paper. These distributions have been quantified by examining selected size parameters to make a com-parison between them. In addition, void circularity factors have been determined to examine the phenom-enon of voids merging. In order to investigate the foam structure before adding to the mix, it was found thatby treating the foam with bitumen emulsion, a clear image of its structure can be captured using an opticalmicroscope. Using this technique, a significant difference was found between the size distribution of foambubbles and those of air pores within foamed concrete mixes. From circularity factor results, there is evi-dence for increased bubble merging with increased added foam volume (decreased density).

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Foamed concrete is a versatile material consisting of eitherPortland cement paste or cement filler matrix (mortar) withhomogeneous pore structure created by entrained air voids roughly0.1–1.0 mm size [1–4]. Nambiar and Ramamurthy [1], reported thatthe introduction of pores inside foamed concrete can be achievedmechanically either by preformed foaming (forming the foam beforeadding it to the mix) or mix foaming (mixing in a foaming agent). Itshould be noted that the foamed concrete investigated in this studyhas been manufactured using the preformed foaming method.

The pore structure of cementitious material is a very significantcharacteristic since it affects properties such as strength and dura-

bility due to their dependence on material porosity and permeability[2]. However, determination of the total air void content (porosity) isnot sufficient as shape, size and distribution of voids may affect thestrength and durability of concrete [5].

Ramamurthy et al. [2], mentioned that the air-void distributionis one of the most significant micro-properties influencing thestrength of foamed concrete and concluded that foamed concretewith a narrower air-void size distribution shows higher strength.

It seems likely that the pore structure and microstructure offoamed concrete has an important influence on its properties. It isusually classified into gel pores (<10 nm), capillary pores(<10 lm) and air voids (air entrained and entrapped pores).Although the gel pores do not influence the concrete strength, theyare directly related to creep and shrinkage. On the other hand, cap-illary and other large pores are responsible for reduction in strengthand elasticity [1]. In spite of this significant influence, evaluation offoamed concrete pore structure is seldom reported [6].

Nambiar and Ramamurthy [1] and Just and Middendorf [7] bothmentioned that the pores of foamed concrete can be measured by

Natural Foam

Foam with bitumen emulsion

Fig. 1. Image of foam during bitumen emulsion application [15.43 mm � 11.57 mm].

228 A.A. Hilal et al. / Construction and Building Materials 75 (2015) 227–233

several test methods such as: nitrogen gas absorption–desorption,optical microscopy with image processing, mercury porosimetryand X-ray computed tomography with image processing. In addi-tion, for testing the pore structure and microstructure of foamedconcrete, both scanning electron microscopy (SEM) and lightmicroscopy combined with digital imaging were used by Yu et al.[6]. The results from both measurement techniques revealed thatthe pore diameters were mainly in the range of 100–200 lm.

In their investigation into the microstructure of foamed con-crete produced with the inclusion of either classified (pfa) andunclassified (Pozz-fill) fly ash, Kearesely [5] concluded that therewas no obvious difference between the void sizes observed inthe two mixes and that for a 1500 kg/m3 mix, the entrained air voiddiameters varied between approximately 40 and 300 lm.

Nambiar and Ramamurthy [1] also determined the air void sizedistribution of foamed concrete mixes with different added foamvolumes (10%, 30% and 50%) and found that the size of the largervoids increased sharply with an increase in foam volume, whilefor the same foam volume they were smaller for a cement-flyash mix compared to a cement-sand mix.

Thus, although the pore size distribution of foamed concrete hasto some extent been investigated, a great deal remains to be under-stood, so this paper aims to investigate the formation of the voidsduring mixing. This is achieved by:

(1) Determining and comparing the size distributions of airvoids in the foamed concrete mixes (after hardening) tothose of bubbles in the preformed foam based on both num-ber and area of bubbles/voids.

(2) Investigating the circularity of the voids within the mixes.

2. Experimental details

2.1. Constituent materials

The materials used were: ordinary Portland cement CEM I-52,5 N (3.15 S.G.)conforming to BS EN 197-1:2011 [8], natural fine aggregate (sand) (2.65 S.G.) con-forming to BS 882:1992 [9], sieved to remove particles greater than 2.36 mm tohelp improve the flow characteristics and stability of the final product [10,11], pota-ble water and foam. Three mixes of foamed concrete were made with nominal den-sities of 1300, 1600 and 1900 kg/m3, designated FC3, FC6 and FC9. To achieve thesetarget densities, the water cement ratios of these mixes were determined, by trials,ensuring the stability of the wet foamed concrete mix and also that the measureddensity was equal or nearly equal to the design density [12,13]. The materialsrequired per m3 of the selected mixes were calculated using the absolute volumemethod. An ordinary mixer was used to produce foamed concrete in the laboratoryby the addition of preformed foam to a base mortar (sand-cement) mix. Therequired amount of foam was generated and added to the base mix and mixed untilthe foam was uniformly distributed and incorporated into the mix [12]. The mixproportions of the foamed concrete mixes investigated are given in Table 1 perm3 of final concrete.

2.2. Specimen preparation

2.2.1. FoamPre-formed foam (at 45 kg/m3) produced by blending a foaming agent, EABAS-

SOC (1.05 S.G.), water and compressed air at predetermined proportions of 55: 1(water: foaming agent by volume) in a foam generator. A STONEFOAM-4 generatorwas used in this study.

Table 1Mix proportions of selected foamed concrete mixes.

Mixes

FC3 FC6 FC9

Target density (kg/m3) 1300 1600 1900Cement content (kg/m3) 500 500 500W/C ratio 0.475 0.5 0.525Water content (kg/m3) 237.5 249.9 262.5Sand content (kg/m3) 562 850 1137.5Foam (l/m3) 424 295 166Foaming agent (kg/m3) 0.35 0.24 0.14

About a litre of foam has been taken from the foam generator and then put in acylindrical plastic container (50 mm diameter and 20 mm height) for foam surfacemicroscopic investigation. Due to the impossibility of capturing a clear image of thefoam in its natural state using an optical microscope with low magnification, in wasdecided to impregnate it with a very small dose of bitumen emulsion, see Fig. 1.Bitumen emulsion was chosen since it contains carbon which, when using an opti-cal microscope, gives an image of the free surface of the foam with good clarity andcontrast between the edges and surfaces of individual foam bubbles, see Fig. 2. Inaddition, the production process of bitumen emulsion involves a surfactant (emul-sifier) which surrounds individual bitumen droplets (of size <10 lm) within thewater, which is essentially the same mechanism as used in foam production, seeFig. 3. The result is that the bitumen emulsion is compatible with the foam andspread easily through the bubble membranes, giving them colour.

2.2.2. Foamed concreteFor each foamed concrete mix, 3 slices (50 � 50 � 15 mm) were cut from the

centres of three cured specimens, perpendicular to the cast face, and used for poresize investigation.

To make the boundaries between the air voids and the matrix sharp and easilydistinguishable, the specimens were first polished and cleaned to remove any res-idues. Then, to enhance the contrast, the specimen surfaces were treated by apply-ing two coats of permanent marker ink to them. After placing them in an oven at50 �C for 4 h, a white powder (Sodium bicarbonate) with a minimum particle size5 lm was pressed into the surfaces of the specimens and forced into the voids. Thisleft the concrete surface black and the voids white, resulting in specimens withexcellent properties for image analysis. This technique is described more fully inEN 480-1 [14,1].

2.3. Image capture, processing and analysis

A camera connected to an optical microscope and a computer was used to cap-ture the images of both foam and foamed concrete mixes.

Fig. 2. Foam image of natural free top surface [6.14 mm � 4.60 mm].

Fig. 3. The interaction between foam bubbles and bitumen emulsion.

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Bubble diameter (μm)

Frequency %

Cumula�ve Frequency %

Fig. 5. Numeric bubble size distribution and cumulative frequency of foam.

26 μm

Fig. 6. Foam image showing constrained surface bubbles with diameters less than100 lm when sandwiched beneath a microscope glass slide [2.06 mm � 2.30 mm].

A.A. Hilal et al. / Construction and Building Materials 75 (2015) 227–233 229

For the foam investigation, a magnification of (56�) was selected, with a pixelrepresenting 2.34 lm and an image of 28.3 mm2 (6.14 mm � 4.60 mm). However, itproved impossible to derive a binary image suitable for automated analysis (in Ima-geJ) and manual measurements were therefore carried out to determine the voiddiameters (around 200 voids in each image) from the captured foam images.

For mixes, a magnification of (23�) was selected with a pixel representing 6 lmand an image size of 178.52 mm2 (15.43 mm � 11.57 mm). This magnification waschosen in order that air voids with diameters in excess of 20 lm could be easilyidentified, see Section 3.2. Ten images were captured for each mix and then digi-tized, converted into binary form and analysed. For this study, only two phases,air voids and solid, were of interest.

A histogram of grey levels from the optical microscope image was used to selectthe threshold value, below which all pixels were considered voids and above whichthey were considered as solid, creating the final binary image required for analysis.Although the grey-scale histograms did not have a sharp boundary between the twophases (voids and matrix) interface, there was always a minimum in the boundaryregion and this was set as the threshold for analysis of the images in this study.

Although software operations such as dilation, erosion, opening, closing andhole filling have all been suggested as being useful in application to concretemicroscopy [1], in this study, it was found that the simple operation of hole fillingwas sufficient since there is a sharp contrast between the white coloured air voidsand the surrounding black coloured matrix. Typical binary images for the threeinvestigated mixes are shown in Fig. 4.

3. Results and discussion

3.1. Bubble size distribution of foam

The bubble size distribution and the corresponding cumulativefrequencies (on the basis of number of bubbles) for the foamimages are shown in Fig. 5. From this, it can be seen that the min-imum bubble diameter is about 100 lm and the largest is 875 lmwith a median diameter D50 of 325 lm and a 90th percentile (D90)of 600 lm. However, it was observed that the natural surface of thefoam formed in such a way as to conceal some of the smaller bub-bles (see Fig. 2), and a second set of ten images was therefore cap-tured from the same foam samples after sandwiching the foam

Fig. 4. Typical binary images [15.43 mm � 1

under a microscope glass slide, see Fig. 6. From this figure, themembrane thickness between two bubbles is about 100 lm andsince the individual bitumen droplets are less than 10 lm, littleeffect on the observed bubble diameters is anticipated.

The numeric cumulative frequency curves for the foam withand without glass plate application are shown in Fig. 7.

3.2. Pore size distribution of foamed concrete

For each void, an effective diameter was calculated by measur-ing the void area and assuming it to be perfect circle [5].

Fig. 7 shows the resulting pore size distributions for foamedconcrete mixes with nominal densities of 1300, 1600 and1900 kg/m3 (mixes FC3, FC6 and FC9 respectively), where it maybe seen that sizes vary between approximately 20 and 2000 lm.

1.57 mm] (a) FC3, (b) FC6 and (c) FC9.

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(%)

Pore diameter (μm)

FC3

FC6

FC9

Natural foam surface

Foam with glass slide

Fig. 7. Numeric cumulative frequency of bubble/pore diameters of foam andfoamed concrete mixes.

Table 2Parameters of pores sizes and circularity of foam and mixes.

Foam FC3 FC6 FC9

(D or O)50⁄ (lm) 325 180 175 165

(D or O)90⁄ (lm) 600 750 650 525

(D or O)50⁄⁄ (lm) 470 770 685 550

(D or O)90⁄⁄ (lm) 765 1425 1225 990

Fcirc50 0.53 0.59 0.65Fcirc10 0.25 0.29 0.35

Note: Diameter of bubbles (D) and voids (O) derived either from cumulative dis-tribution based on numeric of bubbles/voids(⁄) at each size or on area of bubbles/voids(⁄⁄) at each size.

Fig. 8. SEM images of foamed concrete mixes (a) FC3, (b) FC6 and (c) FC9.

230 A.A. Hilal et al. / Construction and Building Materials 75 (2015) 227–233

It is clear that at higher density, the proportion of the larger voidsdecreases leadin g to a narrower air void size distribution. In orderto quantify and compare the air void distribution of differentmixes, the parameters O50 (median opening pore size) and O90

(90th percentile) were calculated on the basis of number of voids,see Table 2; O50 varied from 165 to 180 lm, O90 from 525 to750 lm, and both O50 and O90 increased with foam volume. Thesmallest air void diameter identified was about 20 lm. To checkthat these smallest pores came from the added foam (entrainedair voids) rather than from the manufacturing process (entrappedair voids), SEM images were captured from mortar mixes both withand without added foam, Figs. 8 and 9. In Fig. 9, it can be seen thatthere are very few entrapped air voids in the 20 lm size range,leading to the conclusion that all pores in excess of 20 lm, clearlyapparent in Fig. 8, are foam pores.

The calculations were repeated this time by calculating the O50

and O90 on the basis of the area contained within each void (seeTable 2). This is discussed in the next section.

3.3. Comparison

Fig. 7 illustrates the cumulative frequency of bubble/pore diam-eters in the foam and the foamed concrete mixes (on the basis ofnumber of bubbles/voids). Two very clear differences are apparent.First, foamed concrete mixes contain some larger sized pores thanthose in the foam itself and the number of such pores increases withthe increase in added foam volume. This is logical due to the com-bining of foam bubbles during and possibly after mixing. However,the second difference is much more substantial. From Fig. 5, thesmallest bubble diameter in the foam was about 100 lm, while inthe foamed concrete mixes there were many voids with sizes lowerthan this value. Even when the microscope glass slide was pressed

into the foam surface, Fig. 6, no more than 20% of bubbles werefound to be smaller than 100 lm (Fig. 7) and it could be argued thatthis technique leads to bubble distension and an overestimation ofbubble diameters. In contrast, 30–40% of voids in the mixes had adiameter less than 100 lm. Looking at the D50 values, that for foamwas 300–325 lm, depending on the observational technique used,compared to 165–185 lm for the mixes.

There are two possible reasons for this. Firstly; merging of largebubbles, by reducing the number of larger voids, reduces the totalnumber of voids compared to that of foam leading to an increasein the numeric proportion of the smallest voids and positioningthe numeric cumulative curve for the mix above the curve for thefoam.

Secondly, from a vacuum saturation test, it was found that theporosities of the mortars (without foam) were 13.6%, 13.1% and

Fig. 9. SEM images for mixes without foam (a) 1300 (b) 1600 and (c) 1900 kg/m3.

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Foam with glass slide

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FC3

Fig. 10. Area cumulative frequency of bubble/pore diameters of foam and foamedconcrete mixes.

A.A. Hilal et al. / Construction and Building Materials 75 (2015) 227–233 231

12.9% for FC3, FC6 and FC9 respectively. While for the FC3, FC6 andFC9 foamed concrete, they were 51.6%, 40.7% and 28.7%. By know-ing the added foam (Table 1) and the difference between foamedconcrete and corresponding mortar porosities, it was found thatthere is foam volume loss of about 4.3%, 1.8% and 0.8% for FC3,FC6 and FC9 respectively. This loss is probably because foam bub-bles collapse or the air in them is lost to the atmosphere, and this ismost likely with the large bubbles. This has the same effect asmerging, leading to the median diameter of foam bubbles (D50)being larger than those of the voids (O50) in the mixes.

Another possible interpretation is that the loss of foam bubbles(by collapse) during the mixing process leaves a solution (foamingagent with water) which works as an air-entraining agent andproduces, during mixing, other smaller bubbles. In this context,the addition of a foam stabilizer could usefully be investigated

and the bubble size distribution in the hardened concreteexamined.

In place of analysis of numbers of bubbles at each diameter, thesame data was considered from the prospective of the area of thebubbles in the foam and the concrete images. Fig. 10 shows thecumulative frequency by area of the bubble/pore diameters offoam and foamed concrete mixes. The area cumulative frequencyof foam may be contrasted with the numeric frequency previouslypresented in Fig. 5. A low number of larger bubbles (Fig. 5) meansthat the area contained within these bubbles comprises a signifi-cant proportion of the space occupied by the foam, as seen inFig. 10 between 550 and 875 lm. This has the effect of increasingthe D50 calculated on the basis of area (470 lm) from the value of325 lm calculated on the basis of number of bubbles (Table 2).Because in concrete the larger bubbles are more implicated inthe development of cracking and, hence, strength reduction, thecharacterisation by bubble area is probably to be preferred. Contin-uing this argument, characterisation by, for example, D90 may bemore germane.

A comparison of foam bubble area and concrete mix pore area isincluded in Table 2. It shows that both median and large character-istic voids are significantly greater in area than in the foam. Thisimplies that there has been significant merging of small voids intoa few larger voids during the concrete mixing process. This behav-iour is most pronounced in the least dense mix.

Considering this observation with the earlier one that medianpore size based on number of pores reduces, comparison of Figs. 7and 10 allows us to deduce that bubble merging is prevalent in allmixes. In the less dense mixes, bubble merging takes place at allsizes (the cumulative area void curve for the concrete is alwaysbeneath that for the foam). In the most dense mix the area con-tained in small pores does not change much at all, indicating thatthe small bubbles result in small pores without much loss tomerged bubbles.

In the most dense mix, since the voids merging of larger voids isless than in the lighter mixes, loss of voids must be more effectivethan their merging in making the mix curve lie above the foamcurve within the small diameter range (Fig. 10).

Considering all the foamed concretes in Fig. 10, the small orabsence of curve increase in the small diameter range indicatesthat bubble splitting/shrinkage does not occur in any mixes or ifit does, bubble merging offsets its effect.

3.4. Pore circularity

The circularity factor (Fcirc) is a function of the perimeter andsurface area of each pore, defined as follows;

Fig. 11. SEM images of foamed concrete mixes showing the bubble merging (a) FC3, (b) FC6 and (c) FC9.

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Fig. 12. Circularity factor of foamed concrete mixes.

232 A.A. Hilal et al. / Construction and Building Materials 75 (2015) 227–233

Fcirc ¼ 4p Area

perimeter2

!ð1Þ

Circularity factor equals (1) for a perfect circular pore and it is smal-ler for irregular shapes [15].

From the SEM images for foamed concrete mixes (Fig. 8), it canbe seen that the voids shape, at high magnification (>500�), isalmost circular (i.e. no apparent merging of small voids) whichmeans that their circularity factor should be near to 1. However,with the optical microscope (at low magnification, <25�), largervoids (omitted from the area studied in Fig. 8) can clearly be seen.These are of irregular shapes due to bubble merging; see Fig. 4.These are more clearly identifiable in low magnification SEMimages (Fig. 11). From image analysis results, Fig. 12 shows thatvoid merging is more evident with increased added foam volume.Therefore, the Fcirc50 (median circularity factor) and Fcirc10 (Circu-larity factor at which 10% are less) for FC9 are higher than thoseof FC3 showing that irregularity of voids for FC9 is less than forFC3; see cumulative frequency curves in Fig. 12 and Table 2. Thiseffect, bubble merging, is likely to be a primary reason that theporosity values (36.71, 23.65 and 15.1 for FC3, FC6 and FC9 respec-tively) calculated by image analysis were lower than the addedfoam volumes (42.4, 29.5 and 16.6), a reason also suggested byNambiar and Ramamurthy [1], and the difference increases withincreased added foam (decrease in density).

4. Conclusion

From the tests presented in this paper and based on the aboveresults and discussion, the following conclusions can be drawn:

– By treating with bitumen emulsion, a clear image, of foam bub-bles shape and distribution, can be captured using an opticalmicroscope.

– Bubble merging in all mixes is relatively significant, the greatermerging being observed in the lowest density mixes. This isobserved directly and on the basis of circularity results.

– Compared to the foam bubble size distribution, some largersized pores were presented in foamed concrete mixes owingto the merging of larger bubbles during mixing.

– In contrast the D50 of foam bubbles is larger than that of the O50

of the voids in the foamed concretes. Taken with the previousconclusions this implies loss of larger foam bubbles from theconcrete prior to hardening.

– Bubble splitting or shrinkage does not appear to be significant.– By virtue of their size, larger bubbles contribute a significant

proportion of the area (and hence volume) of voids in the con-crete mixes and are more implicated in concrete weakness.Therefore, it is recommended that definition of voids on thebasis of area is to be preferred.

This study has suggested a number of avenues for futureresearch including:

– Using different doses of the bitumen emulsion and investigatingtheir effect on the observed bubbles thickness.

– Addition of foam stabilizer and its effect on bubble size distribu-tion in hardened concrete.

Acknowledgements

The authors would like to acknowledge the support of theHigher Committee for Education Development in Iraq (HCED) forthe research scholarship enabling this work to be conducted aspart of a larger research project. The authors also wish to thankDr Daniel Wells (E-A-B Associates Company, UK) for providingthe foaming agent. Finally, the valuable help and comments ofMr Keith Dinsdale (University of Nottingham) during the micros-copy observation and Mr Martin Roe (University of Nottingham)during the SEM test are gratefully acknowledged.

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