Performance and Characteristic Foamed Concrete Mix

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INTERNATIONAL JOURNAL Of ACADEMIC RESEARCH Vol. 3. No. 2. March, 2011, Part IV

PERFORMANCE AND CHARACTERISTIC FOAMED CONCRETE MIX DESIGN WITH SILICA FUME FOR HOUSING DEVELOPMENT

Fahrizal Zulkarnain, Mahyuddin Ramli

School of Housing, Building and Planning, Universiti Sains Malaysia, Penang (MALAYSIA)

ABSTRACT This paper investigated the performance of the properties of foamed concrete in replacing volumes of

cement at around 10% and 20% by weight. A control unit of foamed concrete mixture made with Ordinary Portland Cement (OPC) and 10% and 20% silica fume (SF) was prepared. SF is commonly used to increase the compressive strength of concrete materials and for economic concerns. The foamed concrete in this study was cured at a relative humidity of 75% and ± temperature of 27°C. This study highlighted the compressive strength of concrete under sealed condition, showing no significant reduction in strength. There was little difference in the performance of the sample compared with that of the usual mixture of concrete. An effective water/cement (w/c) ratio was developed to predict the strength (up to 28 days) of foamed concrete made with normal densities and with a different percentage of SF. The calculated results compared well with the experimental results.

Key words: Foamed concrete, concrete mix design, compressive strength, housing construction. 1. INTRODUCTION Lightweight concrete can be prepared either by injecting air in its composition or it can be achieved by

omitting the finer sizes of the aggregate or even replacing them by a hollow, cellular or porous aggregate. Particularly, lightweight concrete can be categorized into three groups:

1. No-fines concrete. 2. Lightweight aggregate concrete. 3. Aerated/Foamed concrete. Foamed concrete is either cement paste or mortar classified as lightweight concrete, where air-voids are

entrapped in mortar by a suitable foaming agent. It possesses high flow ability, low self-weight, minimal consumption of aggregate, controlled low strength, and excellent thermal insulation properties. A wide range of densities (1600–400 kg/m3) of foamed concrete, with proper control in the dosage of the foam, can be obtained for application to structural, partition, insulation, and filling grades [21].

Foamed concrete is a lightweight material consisting of Portland cement paste or cement filler matrix (mortar), with a homogeneous void or pore structure created by introducing air in the form of small bubbles [16].

The properties of its components—the cement paste and the voids—have a measurable effect on the properties of the combined materials [11].

Durability can be defined as the ability of material to withstand the effects of its environment. It may be measured against chemical attack, water absorption, and carbonation. Chemical attack may come through the aggressive seepage of ground-water (particularly those contaminated with sulphate), interaction with polluted air, and spillage of reactive liquids. The chemical aspect of durability is the stability of the material itself, particularly in the presence of moisture [25].

The method of pore-formation in aerated concrete (either through gas release or foaming) can influence the microstructure and, consequently, its properties. The material structure of aerated concrete is characterized by its solid microporous matrix and macropores [18]. The macropores are formed due to the expansion of the mass caused by aeration. Micropores also appear in the walls between the macropores [2]. Macropores have usually been defined as pores with a diameter of more than 60 μm [20].

In 1963, Short and Kinniburgh presented the first comprehensive review on cellular concrete, summarizing the composition, properties, and uses of cellular concrete, irrespective of the method of formation of the cell structure. Jones and McCarthy recently reviewed the history of the uses of foamed concrete and its constituent material, properties, and construction application, including for some projects carried out worldwide. These reviews included an evaluation of its functional properties such as fire resistance, thermal conductivity, and acoustics. However, data on fresh state properties, durability, and air-void systems of foamed concrete are limited [25].

The production of stable foamed concrete mix depends on many factors, such as selection of foaming agent, method of foam preparation and addition of uniform air-voids distribution, material section and mixture design strategies, and production of foamed concrete [21].

One of the oldest applications for composite structures is in the building industry. Glass reinforced plastic (GRP) components have been used in this sector for over three decades. As building materials, composites are suitable for numerous applications, such as in pipes, sills, slabs, formwork, linings, pillars, foundations, tanks, housings, containers, doors, covers, bricks, walls, frames, steps, and gutters. The possibilities are virtually unlimited. Composites innovations continue to find usefulness in building sites [22].

This growing popularity has led to the development of scaffolding boards made of GRP. The idea was to replace the heavy timber boards, relieving the stress on high scaffolding as well as enabling higher transport and erection performance by scaffolders. This objective was achieved. Savings of over 20% (in weight) was achieved over conventional timber scaffolding boards after successful load and weathering tests [22].

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In this paper, studies on and classifications of foamed concrete related to the proportional mix design of foamed concrete for housing construction are also discussed.

2. BACKGROUND Aside from Ordinary Portland Cement (OPC), Rapid Hardening Portland Cement (high alumina and

calcium sulfoaluminate) are used to reduce the setting time and to improve the early strength of foamed concrete. Fly ash and ground granulated blast furnace slag have been used in the range of 3070% and 1050%, respectively, as cement replacement to reduce cost, enhance consistency of mixture, and reduce heat of hydration, while contributing to long-term strength. Silica fume (SF) of up to 10% (by mass) is also added to intensify the strength of cement [21].

The water requirement for a mixture depends on the composition and use of admixtures, and is governed by the consistency and stability requirements of the mixture [9]. The mixture becomes too stiff with lower water content, causing bubbles to break, whereas the mixtures becomes too thin to hold the bubbles with high water content, leading to the separation of bubbles from the mixture and, consequently, segregation [14]. Water-cement (w/c) ratio usually ranges from 0.4–1.25 [26]. Super plasticizers are sometimes used [7], but they can cause instability of the foam [8]. Compatibility of the admixture with the foamed concrete is very important.

Foamed concrete is produced either by pre-foaming method or mixed foaming method. Pre-foaming method involves the separate production of a base mix and a stably preformed aqueous, and then the thorough blending of this foam into a base mix. In mixed foaming, the surface active agent is mixed with the base mixture ingredients. Foam is produced, resulting in a typical cellular structure in concrete during the process of mixing [5]. The foam must be firm and stable to be able to resist the pressure of the mortar until the cement takes its initial set and a strong skeleton of concrete is built up around the void filled with air [12]. The preformed foam can be either wet or dry. Wet foam is produced by spraying a solution of foaming agent over a fine mesh, has bubbles of 2–5 mm in size, and is relatively less. Dry foam is produced by forcing the foaming agent solution through a series of high-density restrictions and simultaneously forcing compressed air into the mixing chamber. Dry foam is extremely stable and has a size smaller than 1 mm. This makes it suitable for easier application in base materials required in producing pumpable foamed concrete [1].

The foaming method (foamed concrete) is considered one of the most economical and controllable pore-forming processes [29, 23] because it does not involve any chemical reactions. The introduction of pores is achieved through mechanical means either by pre-formed foaming (foaming agent mixed with a part of mixing water) or mix foaming (foaming agent mixed with the mortar). The various foaming agents used are detergents, resin soap, glue resins, saponin, and hydrolysed proteins such as keratin and similar materials [18].

3. PROPORTIONING AND PREPARATION OF FOAMED CONCRETE A trial and error process is commonly adopted to achieve foamed concrete with the desired properties

[19]. A rational proportion method based on solid volume calculation for a given mixture proportion and density was proposed by McCormick [13]. ASTM C 796-97 provides a method for the calculation of the foam volume required to make cement material slurry based on a specific w/c ratio and target density. The typical mixture design equations of Nambiar and Ramamurthy [15] determine the mixture constituents for a given 28 d compressive strength, filler-cement ratio, and fresh density. Such equations consider the percentage foam volume, net water content, cement content, and percentage fly ash replacement.

Most of the methods assist in the calculation of batch quantities if the mixture proportions are known. Strength can be increased by changing the constituent materials for a given density even though the strength of foamed concrete depends on its density. Moreover, the foam volume requirement depends on the constituent material [15] for a given density. Therefore, the mixture design strategy, for a given strength and density requirement, should be able to determine the batch quantities.

4. PROPERTIES OF FOAMED CONCRETE 4.1 Fresh density Foamed concrete should have flow and self-compactability because it cannot be subjected to compaction

or vibration. These two properties are evaluated in terms of consistency and stability of the foamed concrete, which is influenced by the water content in the base mix and the amount of foam added along with the other solid ingredients in the mix [17].

4.2 Stability The stability of foamed concrete is measured based on consistency, where the density ratio is nearly one

(the measured fresh density/design density), without any segregation and bleeding [14]. This ratio is higher than unity at both lower and higher consistencies due to stiffer mix or segregation. The stability of test mixes can also be assessed by comparing the (i) calculated and actual quantities of foam required to achieve a plastic density within 50 kg/m3 of a design value; and (ii) calculated and actual w/c ratios. The additional free water content resulting from the foam collapse corresponds to an increase in actual w/c ratio [8]. The consistency of the base mix where foam is added is an important factor because it affects the stability of the mix. This consistency is reduced considerably when foam is added and depends on the filler type.



5. MECHANICAL PROPERTIES 5.1 Compressive strength Compressive strength decreases exponentially with a reduction in the density of foamed concrete [10].

The specimen size and shape, method of pore formation, direction of loading, age, water content, characteristic of ingredients used, and the method of curing are reported to influence the strength of cellular concrete in general [29].

Compressive strength decreases with an increase in void diameter, particularly with a dry density foamed concrete between 500 and 1000 kg/m³. The composition of the paste determines the compressive strength [31] in densities higher than 1000 kg/m³ because the air-voids are far apart to have an influence on the compressive strength. That small change in the w/c ratio does not affect the strength of foamed concrete as in the case of normal weight concrete has been reported [8]. An increase in strength is observed with an increase in w/c ratio within the consistency and stability limit [6, 27]. This finding is opposite to the trend usually noted in conventional concrete/mortar, where the entrapped air content only forms a low percentage of the material by volume. Tam et al. (1987) concluded that (i) the strength of moist-cured foamed concrete depends on the water-cement and air-cement ratios, and (ii) the combined effect should be considered when the volumetric composition of air-voids approaches that of the water voids.

6. MATERIALS Tests have shown that the production of foamed concrete with predictable densities and strengths is only

possible with protein foam [32]. This investigation was then conducted using this type of foaming agent only. All the materials used were produced in Malaysia, and only one source of protein agent, cement, and superplasticizer was used.

Foamed concrete was produced under controlled conditions. The cement, filler, water, and a liquid chemical [30] were diluted with water and aerated to form the foaming agent. The foaming agent used was “NORAITE PA-1”, which was manufactured in Malaysia, and contained an additive agent. The foaming agent was diluted with water at a ratio of 1:33 (by volume) and was aerated with a density of 75–80 g/L.

OPC from the Cement Industries of Malaysia Berhad (CIMA Group), Kangar, Perlis Indera Kayangan, Malaysia, was used. Cement can be classified as MS 522 and BSEN 196. The OPC Type I cement produced by CIMA is packed under the brand name “Blue Lion” cement. The product is available in 50 kg/bag and in bulk form. The chemical properties and particles size distribution of all samples are presented in Table 1 and Figure 1.

Table 1. Chemical properties of the samples

ITEM CLINKER % CEMENT %

Oxide Composition SiO2 21.04 19.98

Al2O3 5.24 5.17 Fe2O3 3.41 3.27 CaO 63.31 63.17 MgO 0.85 0.79 SO2 0.41 2.38

Total Alkalis 0.9 0.9 Insoluble Residue 0 0.2

Loss of Ignition 0.5 2.5 Modulus

Lime Saturation Factor 0.93 0.96 Silica Modulus 2.39 2.37 Iron Modulus 1.9 1.58

Mineral Composition C3S 55.4 59.9 C2S 18.53 12.71 C3A 8.59 8.18

C4AF 10.36 9.94 Free CaO (lime) 1.9 0

Compressive Strength, N/mm2

3 d 38 7 d 46 28 d 56

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Fig. 1. Particle size distribution

7. COMPOSITION OF MIXTURES The types of particle size distribution of sand and SF vary by sieving, based on the BS 812-103.1:1985

method for the determination of particle size distribution. All samples were cast with cement, sand, water, and foam content. The first three mixtures contained cement, sand, water, and superplasticizer only with the same w/c ratios. These mixtures were used to determine the cementing efficiency of the SF. Mixtures numbered 4 to 18 contained cement classified and SF with different percentages (Table 2). One mixture (0.05 m3) only per day was prepared. A little amount of mortar underwent slump test in one mixture to determine the mortar suitable for good bonding and mortar density. A good slump is approximately 18–20 cm (Fig. 2). The volume of mixture design, dry density, wet density, and mixture cement ratios are shown in Table 3. The sand used for this test was approximately 1.0 m3. A storage tank was used after sieving to prevent water and chemical contamination prior to testing.

Table 2. Composition of the mixtures.

Mixture Target Density w/c Composition of the

mixtures (in kg) Added water SF

No (kg/m³) Cement Sand Water (litre) 1 1150 0.45 18.88 28.32 8.5 0 0 2 1150 0.45 18.88 28.32 8.5 0 0 3 1150 0.45 18.88 28.32 8.5 0 0 4 1150 0.45 18.88 28.32 8.5 0.5 10% 5 1150 0.45 18.88 28.32 8.5 0.5 10% 6 1150 0.45 18.88 28.32 8.5 0.5 10% 7 1150 0.45 18.88 28.32 8.5 0 10% 8 1150 0.45 18.88 28.32 8.5 0 10% 9 1150 0.45 18.88 28.32 8.5 0 10%

10 1150 0.45 18.88 28.32 8.5 0.5 10% 11 1150 0.45 18.88 28.32 8.5 0.5 10% 12 1150 0.45 18.88 28.32 8.5 0.5 10% 13 1150 0.45 18.88 28.32 8.5 0 15% 14 1150 0.45 18.88 28.32 8.5 0 15% 15 1150 0.45 18.88 28.32 8.5 0 15% 16 1150 0.45 18.88 28.32 8.5 0.5 15% 17 1150 0.45 18.88 28.32 8.5 0.5 15% 18 1150 0.45 18.88 28.32 8.5 0.5 15%

Particle Size Distribution

0

20

40

60

80

100

0.1 1 10 100Particle diameter (mm)

Perc

enta

ge P

assin

g

Sand

Silica fume



8. TESTS CONDUCTED The proportion of the control mixture with foam was 1:1.5:0.45 by mass of OPC, sand, and water,

respectively. The approximate quantity of the OPC was 18.88 kg. The control concrete was modified using 10% SF as OPC replacement for structural concrete. Table 3 presents the composition of the concrete mixtures produced and tested for normal or control mixture. Mortar density for the control mixture was approximately 2025 kg/m³. The slump workability value was 19.0 cm. Mortar density after using the foam was around 1263 kg/m³. The percentage of the foam with 0% silica foam was approximately 43.21%.

SF of approximately 10% by weight of the cement was used (Table 4). Mortar density was around 2060 kg/m³ after mixing. An increase in mortar density with a normal mortar was observed. The actual density after using the foam also decreased to 1185 kg/m³. The percentage of foam with 10% silica foam was approximately 44.17%.

SF of approximately 20% of the cement by weight was also used (Table 5). Mortar density was around 2085 kg/m³ after the mix design. An increase in mortar density with a normal mortar was observed (Tables 3 and 4). The actual density after using the foam also decreased to 1270 kg/m³. The percentage of foam used with 20% silica foam was approximately 44.84%.

Compressive strength of the foamed concrete was determined from 100 mm cubes. The cubes were cast in steel moulds, demoulded after 24 h, wrapped in polythene wrapping, and stored in a room with a constant room temperature of ± 28°C up to the day of testing. Each cube was unwrapped and weighed before testing.

Fig. 2. Slump test 9. RESULTS AND DISCUSSION The compressive strength of the paste mixtures containing SF is plotted as a function of time in Fig. 3.

This graph clearly shows that the compressive strength of SF mixtures increased over time compared with the mixtures without SF. Gain in strength for 3, 7, and 28 days was 2.81, 3.67, and 4.67 N/mm², respectively. Strength for 3, 7, and 28 days was 2.73, 2.98, and 3.41 N/mm², respectively, for the control samples. This difference in strength was approximately the same for all ages of testing. These results show a trend similar to the observation in the mixtures containing SF.

The graph in Fig. 4 shows that the compressive strength of SF mixtures increased over a significantly longer period than the mixture containing 20% SF. Gain in strength for 3, 7, and 28 days was approximately 3.45, 3.90, and 4.95 N/mm², respectively. Strength for 3, 7, and days was 2.73, 2.98, and 3.41 N/mm², respectively, for the control samples. This difference in strength was approximately the same for all ages of testing. These results show a trend similar to the observation in the mixtures containing SF.

Fig. 3. Compressive strength of paste containing 10% SF

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Fig. 4. Compressive strength of paste containing 20% SF There was no apparent significant difference between the mixture with SF and the control as far as

ultimate compressive strength is concerned. These results indicate that the classification of the SF does not improve its effectiveness with regard to compressive strength. These tests used 10% and 20% of SF to increase the strength. SF should then comprise more than 20% of the total volume to show a significant increase in compressive strength.

Table 3. Mixture design sheet for foamed concrete (normal)

No. Descriptions Value Units Note 1 Volume 0.05 m3 2 Dry density 1050 kg/m3 3 Density difference 100 kg/m3 4 Wet density 1150 kg/m3 5 Solid Mass 57.5 kg 6 Estimated foam mass 1.8 kg 7 Actual Mixture Mass 55.7 kg 8 Mixture cement ratio 1 : 1.5 : 0.45 9 Cement 1

10 Sand 1.5 11 Water 0.45

12 Total ratio 2.95

13 Cement 18.88 kg 14 Sand 28.32 kg 15 Water 8.50 kg 16 Additional water - L 17 Total mortar weight 55.7 kg Slump 19.0 cm

18 Mortar density 2025 kg/m3 As measured 19 Mortar volume 0.028 m3 20 Estimated foamed

volume 0.022 m3

21 Estimated foam volume 22 litres 22 Foam density (Actual) 76.2 g/L 23 Foam weight in mixture 75–80 g/L 24 Actual density 1263 kg/m3 As measured

25 Foam flow rate 4.26 L/s 26 Time of foaming 5.16 s

27 Percentage of foam 43.21 %



Table 4. Mixture design sheet for foamed concrete (with 10% SF)


10 Sand 1.5 11 Water 0.45

12 Total ratio 2.95

13 Cement 18.88 kg 14 Sand 28.32 kg 15 Water 8.50 kg 16 Additional water 0.5 L 17 Total mortar weight 55.7 kg Slump 19.5 cm

18 Mortar density 2060 kg/m3 As measured 19 Mortar volume 0.027 m3 20 Estimated foamed

volume 0.023 m3

21 Estimated foam volume 23 litres 22 Foam density (Actual) 77.6 g/L 23 Foam weight in mixture 75–80 g/L 24 Actual density 1185 kg/m3 As measured


27 Percentage of foam 44.17 %

Table 5. Mix design sheet for foamed concrete (with 20% SF)


10 Sand 1.5 11 Water 0.45

12 Total ratio 2.95

13 Cement 18.88 kg 14 Sand 28.32 kg 15 Water 8.50 kg 16 Additional water 0.5 L 17 Total mortar weight 55.7 kg Slump 18.7 cm

18 Mortar density 2085 kg/m3 As measured 19 Mortar volume 0.027 m3 20 Estimated foamed 0.023 m3

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volume 21 Estimated foam volume 23 litres 22 Foam density (Actual) 76.9 g/L 23 Foam weight in mixture 75–80 g/L 24 Actual density 1270 kg/m3 As measured


27 Percentage of foam 44.84 % 10. CONCLUSIONS Results from the 28-day tests indicate that the compressive strength of foamed concrete is primarily a

function of dry density and is only minimally affected by the percentage of cement replaced by SF. That replacing high proportions of cement with SF does not significantly affect the long-term compressive strength of well-cured foamed concrete (in this case, 28 days) can be concluded. Results can be used to predict the strength of foamed concrete of different densities and ages.

Results also show that this strength was observed in all samples, although the foamed concrete mixtures with high SF content may need a longer period to reach their ultimate strength. Similar results on SF also indicate that the cost of foamed concrete mixtures could be reduced by replacing large volumes of cement without significantly affecting the long-term strength.

ACKNOWLEDGMENTS We would like to extend our gratitude to the Universiti Sains Malaysia (USM) for the Postgraduate

Research Grant Scheme (USM-RU-PRGS) that helped fund this research. We would also like to thank the USM Fellowship, the Dean of the School of Housing, Building, and Planning, USM, and those who gave their timely assistance for the successful completion of this paper.

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Performance and Characteristic Foamed Concrete Mix