Strength Properties of Foamed Concrete Containing Crushed Steel Slag as Partial Replacement of Sand with Specific Gradation

Lightweight construction material, notably foamed concrete, had become more favourable to reduce building weight and cost, accelerate construction process, and ease handling of precast segment. Simultaneously, rapid development had result in price rising of conventional material and environmental issue due to abundant wastes, for instance steel slag. As a consequence, feasibility of steel slag to be incorporated in lightweight foamed concrete for both structural and nonstructural purpose is worth to be investigated. This paper is aimed to evaluate the effects of crushed steel slag, as partial replacement of sand with specific gradation, on performance of lightweight foamed concrete (LFC) with density of 1600 kg/m to 1700 kg/m in terms of compressive and tensile strengths. Different steel slag based LFCs were developed by replacing 0, 25, 50, 75 and 100% of steel slag for sand. Different water to cement ratios (w/c) and dosages of super-plasticizer (sp) were adopted to confirm certain workability, strength properties was then studied for ages of 7 and 28 days. The laboratory results showed that lightweight foamed concrete with incorporation of crushed steel slag has decreased strength; however it still achieves structural strength of 17 MPa when replacement level is less than 25% at density of 1600 kg/m to 1700 kg/m.


Introduction
Lightweight foamed concrete (LFC), as one type of the lightweight concretes (LWC), is a mortar, in which artificial air voids are introduced by suitable foaming agent [1].
LWC possess a density between 300 kg/m³ and 1850 kg/m³ based on its type and usage, and can be classified by its purpose to structural lightweight concrete, concrete masonry unit, and insulating concrete [2].According to ASTM C 330-89 [3], structural lightweight concrete should achieve 28-day cylinder compressive strength (f ck ) of 17 MPa, which approximately equivalent to characteristic cube strength (f cu ) of 21 MPa.
In the review study by Amran et al. [4], foamed concrete has found applied in many area such as structural application, lightweight blocks and pre-cast panel, shock absorbing barriers for traffic, road sub-base, cavity and void filling, repair and rehabilitation, fire, thermal, and acoustic insulation, and many other purposes [1,5,6].Included in this review, annual market size of foamed concrete is estimated 250,000 -300,000 m³ in the UK, 50,000 m³ in Western Canada, and 250,000 m³ just in floor heating system in Korea [7][8][9].
Steel slag is by-product of steel manufacturing.It has been noticed that per year fifty million tons of steel slag is generated from throughout the world [10,11].
From the research carried out in the Asian country, approximately 10% of steel slag was produced from the production of steel, for instance, in Malaysia about 750 tons of steel were produced daily and approximately 7.5 tons of slag were produced [12].
However, the rate of utilization of steel slag in Malaysia is too low compared to some advanced countries.Albeit, steel slag is still used in some application such as asphalt aggregates for road construction, sub-base, cement stabilization, ground improvement and some other miscellaneous usage [12].
Maslehuddin et al. [13] fully replace coarse limestone aggregate (SG=2.51)with coarse steel slag (SG=3.51) in normal concrete and report 5% increment in compressive strength, reduction in absorption and permeability, but reduction in tensile strength.
Kothai and Malathy [14] partially replace fine sand (SG=2.65)with fine steel slag (SG=2.95) in normal concrete and found increment of compressive strength at 5% to 10% when replacement level is between 20% and 40%, but have negligible increment on tensile strength; the gradation show that steel slag used has larger fineness modulus than that of sand.
Falade et al. [15] replace sand (SG=2.66)with steel slag (SG=3.47) in foamed concrete and found optimum replacement level of 30% with 20% increment in compressive strength, but the gradation are not specified.
Based on the above literature review, this study is aimed to study the effects of crushed steel slag on strength properties of lightweight foamed concrete (LFC) with density of 1600 kg/m³ to 1700 kg/m³ in terms of compressive and tensile strengths at ages of 7 and 28 days, where the gradation of steel slag is same as that of sand, and steel slag is replacing the sand at replacement level of 0%, 25%, 50%, 75% and 100%.

Experimental procedures
In this study, strength properties of lightweight foamed concrete with super-plasticizer and crushed steel slag as partial of sand with specific gradation were evaluated.

Materials
Raw materials used for this study included cement, sand, steel slag, water, foam agent and super-plasticizer (sp).
The ordinary Portland cement complying with Type I Portland Cement in accordance with ASTM C 150-05 [16] was sieved through 300 μm to remove all lumps, subsequently store in moisture-proof container indoor.
The natural river sand was oven-dried and sieved to ensure 100% passing rate for 2.36mm opening.The steel slag, obtained from local hot-roll steel manufacturer, was washed, oven-dried, crushed, and sieved to have equivalent gradation with sand, as shown in Fig. 1.The sand has SG of 2.65 while steel slag has SG of 3.8.The water used was tap water from the municipal water supply in accordance with ASTM 1602-06 [17].Petroleum base synthetic detergent was used as foaming agent.Besides, the carboxylic ether type super-plasticizer was used in this study.

Mix proportions
The foamed concrete in this study was produced by using pre-foaming method, where base mix and pre-formed foam are produced separately and then mix together thoroughly [1,18].
The foam used was dry foam that produced by forcing the foaming agent solution through a series of high density restrictions, and compressed air into mixing chamfer; it is very stable and has size of smaller than 1mm [1,19].Foam agent and water was mixed in ratio of 1:30 in foam generator before applying compressed air.
The foamed concrete mix proportion and base mix density for different levels of steel slag replacement to sand is presented in Table 1.The foam volume required is affected by base mix density and target density.In this study, dosage of super-plasticizer and water to cement ratio were varied to obtain equivalent level of base mix flowability based on observation.Base mix density is mainly affected by replacement level of steel slag due to large difference between specific gravity of steel slag and that of sand.The water to cement ratio has very minor effect on base mix density, for instance, when the water to cement ratio for 0% replacement was change to 0.35, the base mix density, instead of 2215 kg/m³, will be 2250 kg/m³ compare to 2525 kg/m³ at 100% replacement level.

Testing methods
Testing method used for this study include inverted slump test, compression, split tensile, and flexural tests.
Inverted slump test was performed as per ASTM C 1611-05 [20] to determine the workability of foamed concrete.The slump cone, as per ASTM C 143-08 [21], was placed on a centre of flat base plate with the smaller opening facing down, and filled with foamed concrete.Immediately, the cone was lifted up vertically with a distance of 225 ± 75 mm in 3 ± 1 seconds.The average spread diameter was measured after the concrete stop flowing.
Compressive test was conducted as per BS EN 12390-3 [22].The concrete cubical specimens with dimension of 100 mm were tested by using a universal compression machine with loading rate of 3 kN/s.
Split tensile test was conducted as per BS EN 12390-6 [23].The cylindrical specimens with 100 mm diameter and 200 mm length were place horizontally with packing strip and tested by universal compression test machine with loading rate of 1.76 kN/s.
Flexural test was performed using centre-point loading method as per BS EN 12390-5 [24].The specimens with 40mm x 40mm cross section and total length of 160mm were tested by using testing machine named INSTRON 5582 with effective length of 120mm and loading rate of 0.008 mm/min.

Workability
Inverted slump test results were shown in Table 2.It was noticed that increment of foam volume has reduced the inverted slump flow of foamed concrete, as shown in Table 2.

Compressive strength
Hardened density, percentage of foam, compressive strength and corresponding compressive strength were tested and evaluated in Table 3.
The relationship between compressive strength and percentage of foam volume, regardless the effect of steel slag, was shown in Fig. 2. Base on the relationship, the corresponding strength at any density between 1600 kg/m³ to 1700 kg/m³ can be calculated.Hence, the corresponding compressive strength at density of 1700 kg/m³ was calculated and shown in Table 3.
The compressive strength, performance index of compressive strength, and corresponding compressive strength were compared, as shown in Fig. 3. Performance index is strength per 1000 kg/m³ density, calculated by strength divided by the density, was used to evaluate the strength performance.
It was noticeable that compressive strength and performance index were influenced by the achieved hardened density, whereas corresponding compressive strength shows an approximate linear transforms, in which, compressive strength was reduced when steel slag replacement level was increased.
The previous researches which indicate increment of strength might have different gradation for sand and steel slag, where steel slag might have larger fineness modulus [14], and have smaller specific gravity [13][14][15].
No doubt certain amount of fine steel slag is able to improve the strength of concrete [14,15], but it was insufficient to compensate the reduction of strength cause by increment of foam volume due to high specific gravity of steel slag used.

Split tensile strength
Hardened density, percentage of foam, split tensile strength and corresponding split tensile strength were tested and evaluated in Table 4.
The relationship between split tensile strength and percentage of foam volume, was shown in Fig. 4, and the corresponding split tensile strength was calculated based on the relationship and shown in Table 4.
The split tensile strength, performance index of split tensile strength, and corresponding split tensile strength were compared in Fig. 5.It was also noticeable that split tensile strength and performance index were influenced by the achieved hardened density.Therefore, corresponding split tensile strength was estimated for comparison and concluded that, similar to compressive strength, split tensile strength was reduced almost linearly when steel slag replacement level was increased.
Split tensile strength is found at 8% to 10% corresponded to compressive strength in this study.Byun et al. had found a similar result [4,18].

Flexural strength
Hardened density, flexural strength and performance index of flexural strength were tested and evaluated in Table 5.
The flexural strength and performance index of flexural strength were compared in Fig. 6.Approximate linear reduction of flexural strength performance index by increment of steel slag replacement was observed.Performance index of flexural strength is found at 32% to 48% corresponded to its performance index of compressive strength in this study, which is very different from split tensile strength.Byun et al. also found similar result, which is, flexural strength is 20% to 40% of compressive strength, whereas split tensile strength is 8% to 11% of compressive strength [4,18].

Conclusions
Strength properties of foamed concrete with density of 1600 kg/m³ to 1700 kg/m³ was studies, in which sand with specific gravity of 2.65 was partially replaced by steel slag with specific gravity of 3.8 and similar gradation, with different replacement level, namely 0%, 25%, 50%, 75%, and 100%.
The results show that the compressive, split tensile and flexural strengths were reduced almost linearly with increment of replacement level, however, with replacement level of 25% and lesser at density of 1600 kg/m³ to 1700 kg/m³, the achieved 28 days strength fulfil the structural strength of 17 MPa set by ASTM C 330-89 [3].
The split tensile strength is found 8% to 10% of compressive strength, whereas flexural strength is found 32% to 48% of compressive strength.
The support provided by Universiti Tunku Abdul Rahman (UTAR) in the form of a research grant (Vote No. 6200/LG3), and Masteel BhD in the form of raw steel slag for this study is very much appreciated.

Fig. 3 .
Fig. 3. Comparison between compressive strength, performance index (PI) of compressive strength, corresponding compressive strength and replacement level of steel slag.

Fig. 5 .
Fig. 5. Comparison between split tensile strength, performance index (PI) of split tensile strength, corresponding split tensile strength and replacement of steel slag

Table 1 .
Mix proportion and details of foamed concrete.

Table 3 .
Compressive strength, hardened density, and percentage of foam volume

Table 4 .
Split tensile strength, hardened density, and percentage of foam in volume Relationship between split tensile strength and foam volume.

Table 5 .
Flexural strength, hardened density, and performance index of flexural strength Comparison between flexural strength, performance index (PI) of flexural strength, and replacement level of steel slag.