Effect of alkaline-resistant glass fibre on compressive strength of lightweight foamed concrete

Hanizam Awang*, Universiti Sains Malaysia, Malaysia

N M Noordin, Universiti Sains Malaysia, Malaysia  

27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 29 - 30 August 2002, Singapore

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27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 29 - 30 August 2002, Singapore

Effect of alkaline-resistant glass fibre on compressive strength of lightweight foamed concrete

Hanizam Awang*, Universiti Sains Malaysia, Malaysia N M Noordin, Universiti Sains Malaysia, Malaysia

Abstract

This paper presents the finding of an experiment conducted to investigate the effect of glass fibre on compressive strength of foamed concrete which can be used as decorative architectural component for non-load bearing structure. Chopped Strand Alkali-Resistant glass was used in the study with size of 24mm. A cement-to-sand ratio of 1 : 1 was used as a typical design mix with two targeted dry densities of 900 kg/m3 and 1000 kg/m3 were used in the study. Specimens consisting of 126 glass fibre reinforcement concrete cubes (100mm x 100mm x 100mm) were made and subjected to compressive tests at age of 7 and 28 days. Foamed concrete containing fiber glass with compressive strength between 1.3 N/mm2 to 3.1 N/mm2 was produced. The results indicated that there is a significant correlation between strength of concrete and percentage of fibre used. From the different percentage of glass fibre added into foamed concrete, we were able to identify the strength for lightweight foamed concrete made with a 0.45 water-to-cement ratio. The experimental 'findings indicate that the increase of 'fibre content can produce stronger foamed concrete. The results of this study suggest that with further refinements in the mixture designs, there are possibilities of producing foamed concrete using glass fibre.

Keywords : Foamed, concrete, Alkali- Resistance glass fibre, compressive strength

1. Introduction.

Lightweight foamed (or cellular) concrete is normally made from mixing stable foam to slurry of mortar. This action incorporates small-enclosed air bubbles within the mortar thereby making it lighter and possessing special properties such as low thermal conductivity and high fire resistance. Foamed concrete may have densities ranging from as low as 500kg/m3 to as high as 1600kg/m3

. It therefore has a wide range of applications such as material for wall blocks or panels, floor & roof screeds, trench reinstatement, road foundations and also void filling.

Several important materials are normally used in the production of Lightweight Foamed Concrete (LFC) namely cement, sand, water and stable foam. The type of sand used to make foamed concrete is fine (river) sand which is less than 2mm in size. Ordinary Portland cement is used as a binder material in the production of foamed concrete.

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Stable foam that is used in the production of foamed concrete is made by expanding a suitable amount foaming agent (in this case Fine Foam 707) by way of a foaming generator ( Portafoam) developed at the School of Housing, Building and Planning, Universiti Sains Malaysia. The dilution ratio for such a foam agent is 1 :19 and expansion from the foam generator is 25. The pre-formed foam is one of the most important ingredients in the making of foamed concrete and acts as "light" aggregates in the concrete.

The reduction in dry density of the mortar generally results in lower compressive strength compared to the normal sand-cement mortar and this may range from as low as 0.5 N/mm2 to 12 N/mm2 measured at 28 days for the density range mentioned above.

As can be seen above the compressive strength of normal foamed concrete is quite low and it would be advantageous if this can be improved. One possibility is by way of addition of fibers in the mix. It is known from previous studies that the inclusion of fibers in a cement mortar can result in increased compressive strength [1]. Raju et af2] also found that the cube compressive strength of concrete increased linearly with the addition of fibers. Similarly applied to foamed concrete it is hoped that the glass fiber would contribute to the load carrying capacity of the material by shear deformation at the fiber - matrix interface thereby contributing to increased strength.

The type of fiber chosen is alkali-resistant glass fiber because of its suitability, availability and high tensile strength [3]. AR glass fibre in the form of chopped strands was added as one of the ingredients in making foamed concrete in order to improve its compressive strength.

The objective of this study is therefore to investigate the effect of various proportions of alkali ­resistant glass fibre on the cube compressive strength of foamed concrete of different density.

2. Experimental details.

The study would examine the effectiveness of glass fibre in the improvement of LFC. ASTM Type 1 Ordinary Portland cement (OPC) was used for the experiment. The fine aggregate used was natural sand, that was obtained from a local riverbed . A sieve analysis was carried out to see the suitability of the sand to be used and the percentage passing 5 mm sieve size. The sand falls in zone 3 in accordance with British Standard BS 882: Part 2: 1973 . Norizal [4] have mentioned that the appropriate size of fine aggregate used should be between 0 to 2 mm. In addition, 20% of the total quantity of sand used should preferably be of size less than 0.5 mm. Before the test materials were mixed, the sand was in a saturated surface-dry condition . However a moisture test made on the sand sample showed total water content to be 1.1 'Yo. Figure 1 shows the particles sizes distribution of the river sand used in this study.

The dry mix (cement and sand) was blended with water which had been fixed using a water-to­cement ratio of 0.45. Alkali-Resistant glass fibre was added in stages to this mix using three different percentage (0.2%, 0.4%, 0.6%) and mixing was continued for 5 minutes. The alkali resistant glass which was imported from China was in the form of chopped strand 24 mm in length. The alkali resistance of the glass fiber has been confirmed and approved by the Pre-stressed Concrete Institute (PU) and International GRC Institution

The wet mortar containing glass fibre was then tested for viscosity using a flow table similar to the ASTM C 230-68 test for mortar. The flow diameter using this test was 20 cm after 3 turns of the handle. Stable foam which had a weight of 40 g/liter was then added to the mix and the mixing process continued thoroughly until a homogenous and smooth texture of foamed mortar was achieved. The foam produced was from using a foam agent (Fine Foam) which diluted with 19 parts of water and expanded using a portable foam generator (known as "Portafoam"). This foam generator uses compressed air to generate foam and was coupled to the holding tank containing the foam chemicals mentioned earlier. A specially designed lance unit connected to the foam generator also ensured smooth stable foam for producing the foamed concrete. The same mixing procedure continued with the other mixes containing different percentages of glass fibre . The design mixes are shown in Table 1.

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Figure 1 : Particles Size Distribution of Sand

<0.15 0.15 0.3 0.6 1.18 2.36 5 10

Sieve size (mm)

120

100

~ CI 80r::: 'iii III as ~ 60 C) CI as-r::: 40C) !oJ... C)

~ 20

0

--+- % Passing test --- Sand zone BS 882 .. Sand zone BS 882

Table 1: Mix Proportioning of Lightweight Foamed Concrete with Glass Fibre.

Fibre glass content 0.2 % 0.4 % 0.6 %

Target (kg/m~

dry density 1000 900 1000 900 1000 900

A Cement (kg) 41.5 37.5 41.5 37.5 41.5 37.5

B Sand (kg) 41.5 37.5 41.5 37.5 41.5 37.5

C Water (kg) w/c=0.45 18.68 16.88 18.7 16.9 18.7 16.9

0 Glass fibre (g) 203.35 183.75 406.7 367.5 610.1 551.3

E Weight of wet mortar (kg)

101.88 92.06 102.08 92.24 102.29 92.43

F Mortar density (kg/mj 2096.68 2063.80 2195.13 2003.96 2167.2 2139.54

G Volume of mortar mix (pre foamed) (m1

0.0486 0.0446 0.0465 0.0460 0.0472 0.0432

H Volume of foam) (m1

mix (after 0.1019 0.1023 01021 0.1025 0.1023 0.1027

I Volume of foam (m) 0.0533 0.0577 0.0556 0.0565 0.0551 0.0595

J Density of wet foamed concrete (kg/m1

1149.50 975.00 1120.85 1063.56 1055.18 1015.03

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Several specimens were cast from a typical mixture and they consisted mainly of the following:

Cubes of 100 mm x 1 OOmm x 1 OOmm for compression test Cylinders of 334mm x 150mm x 150mm for tensile tests. Prisms of 1 OOmm x 500mm x 1 OOmm for flexural strength tests.

The foamed concrete specimens were de-moulded after one day and stored in a water tank at 20° C until its testing date at the age of 7 and 28 days. Three cubes were tested for the first group which consisted of specimens with a target dry density of 900 kg/m3 and another three from the group with a target dry density of 1000kg/m3 to determine the average strength of the specimens. All tests were performed according to the relevant standards (BS 6073 : Part 2 : 1981). The compressive, tensile and flexural strength of the concrete were determined both at 7 and 28 days, whereas the water absorption test was performed at 28 days. The results presented in this paper are the average compressive strength from three tests.

3. Test Results and Discussion.

The result of 7 and 28 days compressive tests are given in Table 2 and Table 3.

Table 2: The result of compressive strength with different percentage of fibre at 900kg/m3 targeted dry density .

Percentage of fibre (%) 0.2 0.4 0.6

Cube compressive strength (N/mm2) days 7 1.3 1.6 2.4 28 1.9 2.1 2.9

Cylinder tensile strength (N/mm2) days 7 0.2 0.4 0.6 28 0.3 0.5 0.8

Prism flexural strength ( N/mm2) days 7 1.2 1.8 .6 28 1.6 2.5 3.6

Water absorption (%) days 28 11.9 18.85 29.78

Actual dry density (kg/m3) days 28 924 930 912

Table 3: The result of compressive strength with different percentage of fibre at 1000kg/m3 targeted dry density

Percentage of fibre (%) 0.2 0.4 0.6

Cube compressive strength (N/mm2) days 7 28

1.6 2.25

2.4 2.8

2.8 3.0

Cylinder tensile strength (N/mm2) days 7 28

0.3 0.5

0.6 0.7

0.8 0.9

Prism flexural strength ( N/mm2)

days 7 28

2.2 2.6

3.0 3.3

3.4 4.3

Water absorption (%) days 28 14.67 19.52 31.57

Actual dry density (kg/m3) days 28 1030 1015 1070

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0.2 0.4 0.6

Fibre (%)

-+- Compressive strength at 900kg/m3 density

-It- Compressive strength at 1000kg/m3 density

The slump / workability (measured according to ASTM C 230-68 test) varied between 18cm to 20 cm for the various mixes mentioned above. The foamed concrete derived from the above mixes generally showed very good workabili~ . The actual dry density obtained from a target value of 900kg/m3 varied from 912 kg/m3 to 924 kg/m whereas for a target value of 1000kg/m3 the density varied from 1015 kg/m3 to 1070 kg/m3

.

Figure 2 : Relationship between 28 days compessive strength and percentage of fibre used.

N 3.5 E

---E 3 z :; 2.5-m ii 2 .... iii CD 1.5 > ~ 1 CD ....E0.5 o o 0

It is known that compressive strength of foamed concrete is mainly dependent on cement content and fine aggregate grading. In additon, it is shown in this study that fiber content in foamed concrete too can have an effect on its compressive strength.

Figure 2 shows results of tests for compressive strength of glass fibre reinforced foamed concrete . An increase in compressive strength of foamed concrete was seen as a result of an increase in fibre content.

Past research using similar type of foaming agent, sand and mix ratio has shown that their compressive strength was in the region of 1.5 N/mm2 for a density of about 926 to kg/m3

. The above study however show that the average compressive strength of foamed concrete with glass fibre is was 1.9 N/mm2 for a closely similar (and lower) density (924 kg/m3

) . A similar finding was also noted in a higher density of 1030 kg/m3 having a strength of 2.25 N/mm2 (with 0.2% fiber) compared to 1.7 N/mm2 for a slightly higher density of 1048 kg/m3 (without fiber) .

An increase of strength is definitely noticed with the increase of glass fibre used. It is found that, the suitable percentage of fibre to be used are in the range of 0.2 % and 0.4 % since higher percentages of fiber do not result in appreciable strength increase. In additon, the water absorption also increases with the increase in the amount of fibre used. The more fibre glass used, the more the concrete tend to absorb water. Therefore it is desirable to control the amount of fiber used in the foamed concrete mix in order to limit water absorbtion.

It has always been thought that the main use of Alkali Resistance glass fibre in concrete was to provide tensile or flexural strength rather than compressive strength. This study has shown however that glass fibers have also been able to increase the compressive strength of foamed concrete.

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Another factor to consider is that the method and time of mixing should be controlled in order to prevent from over mixing. Over mixing can cause the bubbles to be destroyed and result in the increase in density of the lightweight foamed concrete. Over mixing can also cause the glass fibre used to disperse into smaller filaments thereby increasing the water absorption of the concrete . One way to overcome this problem is to provide suitable protective rendering when exposing this type of foamed concrete to weather.

Glass fibre reinforced foamed concrete has great potential with respect to its use as non-load bearing decorative / architectural components in building. This means that a better surface texture may be needed. This can be achieved through the use of finer aggregates in order to obtain a smoother surface finish and also better compressive strength [ 5]. In additon to strength, decorative components may also need crack resistance, tougher and more resistance to impact [6]. Since research has shown that fiber reinforced cement can attain these improved properties [7], it is highly probable that foamed concrete mixes which are high in cement content may behave similarly.

Even though the used of glass fibre will increase the compressive strength of LFC, the inclusion of the material is only recommended if it is locally available. This is because the added material will result in increasing the cost of the final product. However if desirable properties such as toughness and impact resistance are required , then the addition of glass fibres to foamed concrete would be advantageous and have to be considered.

One of the ways to increase compressive strength of LFC is by using glass fibre reinforcement. Increasing the content of glass fibre will increase the compressive strength of the foamed concrete. The incorporation of glass fibre in foamed concrete has in no way affected other working properties of LFC such as being easily placed , no compaction required, no vibrating or leveling during casting works. Further research is planned at the School of Housing, Building & Planning to study other properties of LFC such as thermal conductivity, fire resistance and shrinkage. Glass fibre foamed concrete is envisaged as a potential material that may be used in housing and building decorative components of the future.

References

[1] R.N. Swamy and P.S. Mangat, Cem Concr. Res. 4(3), 451 (1974) [2] N.K. Raju, B.S. Basavarajaiah and K.J. Rao, Indian Can cr. f. 51(6), 183 (1977) [3] V.S, Ramachandran, R.F. Feldman, 1.1. Beaudon, Concrete Science (treatise on current research) Heydon &

Son Ltd 1981. [4] Norizal Md. Noordin, "DeveLopment ofA Lightweight Concrete Wall paneL System for the Housing

Industry " Thesis for Phd in Building Technology, School of Housing, Building and Planning, Universiti Sains Malaysia (2000)

[5] Norizal Md. Noordin [6 ]1. Cook, J.E. Gordon, Proc. R. Soc. London.Ser. A, 282,508(1964) [7 ]A.Kelly, Proc. R. Soc. London Ser. A 319,95(1970).

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