Foam concrete (FC) is a concrete composite with enclosed-air voids to reduce its self-weight. The light-weight characteristics are caused by introducing bubbles of air using suitable preformed foam into cement paste. The foam is formed using water solution, expanded foaming agent with pressurized air. The density of FC is varied from 300 to 1800 kg/m3
]. The presence of air bubbles in FC produces unique features compared with ordinary concrete, e.g., acoustic absorption, low self-weight, resistance to fire, thermal insulation, high porosity, high flowability, and required compressive strength [2
]. FC is used effectively in various countries, such as Turkey, Thailand, Philippines, Germany, and UK [4
]. FC is a lightweight construction material that highly depends on its density and constituents of mixture [1
], and it can be used in various construction applications (Figure 1
). The non-structural use of FC has been found actively in construction [5
The cellular concrete is categorized into two types based on the pore-formation method: FC and air-entrained concrete [6
]. In the method of air-entraining, chemicals, such as gas-forming, are mixed uniformly into the cement paste. As a result, the porous structure is formed by generating gas due to chemical reaction action while mixing [7
]. The aerating agents, namely hydrogen peroxide, calcium carbide, and aluminum powder, are widely used. Two approaches are in everyday use to form the pores in FC; (i) the pre-foaming process, which includes the appropriate amount of preformed foam, is mixed with a water solution. (ii) Process of mixed foaming consisting of the cement and water is mixing to obtain the uniform paste; subsequently, the prepared foam is poured into the cement paste [8
]. According to the curing method and its density [9
], aerated concrete is categorized as autoclave (cured under temperature and pressure in a sealed device) and non-autoclaved concrete. For insulating and filling purposes, the density ranged from 300 to 600 kg/m3
is used extensively. The density varied between 600 and 1200 kg/m3
, its application can be extended to non-load-bearing structures (soundproofing screeds, thermal insulation, partition wall, outer leaf building panel, and precast block). For the load-bearing structures, a high-density FC in the range of 1200–1900 kg/m3
is widely used [10
Apart from providing good thermal [15
], acoustic, insulation properties [15
], and less depleting resource consumption with the replacement of industrial and agricultural waste as building materials [17
], are the current research ventures towards low carbon footprint. Thereby, FC is also beneficial to the pre-fabrication industry by minimizing transportation frequency and machinery. FC is conventionally cast-in-situ, and its construction also brings the advantage of reducing temporary supports or props with smaller formwork pressures [7
]. A higher strength-to-weight ratio can also be attained with the lightweight features, resulting in a smaller foundation, few columns, or longer beam span [3
]. FC alternatively becomes one of the lightweight construction materials, as lightweight aggregate concrete competitively gives a narrow range of density reduction. In contrast, aerated concrete needs a high temperature at the curing stage to chemically induce the voids [12
] (Figure 1
The Romans first noted the significant improvement of workability in lime mortar mixtures. This improvement is observed by agitating animals’ blood to introduce bubbles in the lime mortar [1
]. During 1923, the FC was placed in service for the first time as an insulative material [21
]. In the 1950s and 1960s, FC’s production and its composition and physical properties were studied extensively first-ever. It is initially applied for ground stabilization and filling of void and afterward driven to more frequent usage as a material for building. For building performance, the consumed quantities of fossil fuels can be reduced for energy-efficient building by minimizing the amount of carbon dioxide emitted [22
]. Reportedly, FC exhibited sufficient strength to meet the usage limits of material for construction and an industrial building [23
]. No compaction and vibration are needed to fill the voids and cavities over a long-range. It offers excellent thermal insulation, good resistance to fire, freeze/thawing properties, and settlement-free and quick construction [24
]. FC has found many applications in recent years; ground stabilization, maintenance of road sub-bases for bridge abutments, monolithic low-rise building, single dwelling building, thermal insulation, wall panels, and building blocks production, masonry grouting, cavity filling, well backfilling, double-pitched roofs, mono-pitched roofs, thermal protection of flat, acoustical barrier floors [25
]. Although FC holds excellent properties as a suitable material for construction, its emergence was about 5.6, 33.3, 5.6, and 5.6% for countries like Africa, Europe, Australia, and North America, respectively.
Henceforth, it is to be noted that FC’s role in the building industry is not widely accepted due to a lack of knowledge, material sureness, and obtainability of required technology [28
]. However, there is an occasional use of reinforced foam concrete (RFC) in the structural concrete system. This is due to a lack of knowledge on the fabrication of concrete structural applications. To this end, this paper reviews the design efficiency, characteristics, and utilization of RFC. It may have a different performance than ordinary concrete, with a relatively low tensile strength of FC. Material properties of FC were critically reviewed and the feasibility of structural use was also discussed according to the specifications of current codes of practice. For reinforced concrete design, the theoretical background for reinforced concrete was also studied and the adaption of codes of practice with reduction factors and the future direction of lightweight FC applications were also highlighted.
5. Reinforced Foamed Concrete (RFC)
Before applying into the reinforced FC system, some requirements that need to be achieved for the FC properties. The structural use of concrete should satisfy the minimum concrete grade of C20, according to the Buildings Department of Hong Kong [143
], which is a minimum value of 20 MPa at a 28-day concrete age. Minimum grades from BS 8110 [144
] for reinforced concrete are C15 for lightweight aggregate concrete and C25 for normal weight concrete. As Eurocode 2 [145
] does not specify the minimum compressive strength, each density class’s minimum requirement should be fulfilled for good design and building condition. Moreover, ACI 318-14 Section 19 [146
] specifies that the structural use of normal or lightweight concrete would satisfy a minimum strength of 17 MPa for a 28-day concrete age. For different moment frames and structural walls, the recommended minimum and maximum compressive strengths are 21 and 35 MPa, respectively. Therefore, the FC’s ability, mentioned above, has a compressive strength requirement that makes it successful for the first screening process. ASTM C330 [116
] provides the lowest tensile strength of 2 MPa and FC is able to achieve this tensile strength with polypropylene fibers.
For durability, it is not essential for the concrete to be incredibly durable, since covers can be applied to resist the potential environmental hazards and chemical attacks. As the pores are not continuous in FC, it gives an advantage to the durability as the hazards may not penetrate into reinforcement bars to reduce its serviceability duration. However, Eurocode 2 and ACI 318 categorize the concrete into several categories for potential threats, such as sulfate attack, chloride exposure, etc. It was proven by [147
] with SEM images of the formation of 50–100 μm pores and the maximum dosage of liberated foams was 0.6% by weight of the binder. When exceeding 0.6%, the pores’ size was increased thanks to the formation of interconnected pores.
Shrinkage and creep are other variables in structural concrete design. As there is a lack of information for shrinkage and creep, theoretical models, such as Pickett’s and Hansen’s equations, have failed to calculate FC’s reliable predictions [124
]. Lightweight aggregate, sand, and fibers make up the mixture of materials that potentially improve these effects. Therefore, a new prediction model should be developed for structural concrete design. At this stage, shrinkage and creep behavior has been identified, where they have more significant effects than normal weight concrete. A prediction model is yet to be developed.
Therefore, after all these related properties have been investigated and prediction models have been developed, FC can be applied to a reinforced concrete structure design. Although there are no mandatory requirements, like creep and shrinkage, it is advisable to include reinforced foamed concrete design. It has more significant effects than normal weight concrete, and has a closed correlation with concrete elastic modulus, tensile strength, and flexural capacity.
There are four identified basic actions for reinforced concrete structures: bending, axial load, shear, and torsion. These actions can be designed solely or in any combination to fulfill Navier’s three principles: stress equilibrium, strain compatibility, and material constitutive law. Metal reinforcement has been introduced to the concrete to take the applied splitting strength where concrete is weak at the tension zone. The stresses redistribution of reinforced concrete elements due to the problem of strain compatibility between reinforcement steel and concrete may induce excessive deflections and lower the member’s structural performance [148
]. Therefore, bond strength is one of the reinforced design concrete parameters due to the strain compatibility problem.
5.1. Bond Characteristics
Several factors influence the bond characteristics between concrete and steel bars, namely aggregate size, water–binder ratio, type of reinforcement bar, bond length, bar diameter, and confinement. Pull-out tests are usually used to determine the bond strength and bond stress-slip relationship. Previous related research data have been summarized in Table 8
. Research of [148
] discovered that bond strength for lightweight concrete was twice the code equation prediction than experimentally. Generally, FC has a lower bond strength compared to normal weight concrete. Researchers from Stellenbosch University [102
] found that material improvement can be done by the addition of aggregate to improve cracking tortuosity. The additive of polypropylene fibers improves the bond between FC and reinforcement bar, and increment of fracture energy, material brittleness, and reinforcement bond (Table 9
]. Durability has been reported in [102
] and more complete results will be reported in the future. Figure 7
shows the failure modes of FC with steel strips that obtained from previous investigations [152
5.2. Previous Research on RFC
Several research has been conducted for reinforced foamed concrete members and summarized in the following contents.
5.2.1. Steel and FRP RFC
Jones and McCarthy [109
] were preliminarily studying the possibility of applying FC to flexural beams with gradual decreases in density. It has been concluded that the characteristics of FC, including stiffness performance, comparatively low tensile strength, and excellent drying shrinkage strain, ref [109
] may result in different structural behaviors than normal weight concrete. The previous investigation on the FC beam reinforced by wire mesh as reinforcements achieved greater flexural and compressive strengths than those FC with no reinforcement or reinforced by plastic mesh [162
]. For FC beams with oil palm shells, the shear behavior has been investigated [163
]. With about 25% of the modulus of elastic of normal weight concrete, FC beams exhibited 50% higher deflections, and twice the number of cracks was found [163
Moreover, Kum also studied FC beams, and design equations have been proposed [115
]. The cracking mode and shear strength were investigated in this research. Normal weight concrete can resist shearing until the flexural mode after the onset of diagonal cracking, while lightweight aggregate concrete was incapable of improving enough resistance and materially ruined in a brittle shear manner. It was also found that FC beams had diagonal cracks at lower loads. This is due to their low tensile strength and ability to resist significant amounts of shearing after the onset of diagonal cracking before the angular and irregular cracking planes at the macro level compared with the smooth crack surface at the micro level [115
Furthermore, precast FC walls have been investigated through experimental and numerical studies [164
]. The capacity of lateral force was majorly affected by the degree of dowel reinforcement crossing horizontal connections. These walls responded structurally ductile and predictable with preventing brittle failure with the connection placement. Nevertheless, the flexural behavior of slab with RFC was investigated [142
]. A proposal calculation of a new type floor slab system has been suggested by [165
], where normal concrete is bonded with FC with steel reinforcement. The flexural performance of FC slabs with a cold-shaped steel frame also was investigated.
Moreover, lightweight FC strengthened with glass fibre reinforced polymer (GFRP) bars was investigated and associated with normal concrete beams [166
]. It was discovered that lightweight FC has a 3.6% increment of load capacity without reinforcement and an 11.54% increase in GFRP than steel reinforcement. The experimental results correlated well with the ACI model for deflection and crack width predictions. Many researchers proved that lightweight concrete members have a similar performance fundamentally to normal weight concrete [167
]. Therefore, according to recommendations in codes of practice, reduction factors should be introduced to normal weight concrete design equations while being adapted to lightweight concrete members’ design.
5.2.2. Numerical Study of RFC
Three-points flexural test configuration for the notched beam was modeled to study FC facture behavior using the extended finite element method [169
]. The increment of density was found a benefit to its stiffness, maximum tensile stress and fracture energy. The finite-discrete element technique was used to estimate the fracture energy for FC [118
]. The heat transfer model was simulated for FC and the temperature fields were recorded for further analysis [172
]. Finite element analysis also was used to model heat transfer through pixelated microstructure [173
]. Pore shapes of non-circular and circular (square, hexagon, and pentagon) and the ellipse’s aspect ratio were studied.
6. Design Specifications for RFC
BS 8110, Eurocode 2, and ACI 318 are the reference codes in this section. Strength classes below LC20/22 should not be applied for reinforced concrete, as illustrated in BS 8110. The design shear resistance, torsional resistance, and deflection of a beam should be included with stated coefficients in BS 8110. For Eurocode 2, design specifications have been recommended for lightweight aggregate concrete structures. Coefficients are introduced for the calculations of modulus of elasticity, tensile strength, creep determination, and drying shrinkage. The details of the lightweight concrete design are as stated in ACI 318-14, by presenting reduction factors when adapting traditional weight concrete design equations.
6.1. Summary of FC for Structural Use
Before entering the RFC design, the structural use of FC should be achieved. Table 10
suggests some mix design that can achieve a load bearing characteristic concrete. The requirements are, minimum of 17 MPa of compressive strength, while 2 MPa of tensile strength according to ACI and ASTM specifications. Other future FC mixes that can achieve the stated requirements are also recommended to be applied in RFC design. Pozzolans, such as silica fume and fly ash, can be added into concrete mix in order to achieve higher strength, and polypropylene fires also can be applied to increase the tensile and flexural strengths of FC.
The strength of FC is highly depending on its density where there is an exponential correlation between both parameters. As long as FC is able to achieve ACI and ASTM structural requirements, it is acceptable to be applied in RFC. Moreover, British Standard suggests to use concrete class of C15 for lightweight concrete where Hong Kong code recommends C20 for the structural use. It is advised to follow the stated specifications if there is no reference in one’s own country.
In order to achieve structural usage, the density is recommended to be at least 1500 kg/m3, while for non-structural application, it can be controlled in a lower range of FC density. Silica fume and fly ash should be added to FC to increase its compressive strength and polypropylene is suggested to incorporate in concrete to enhance its tensile and flexural strengths. For non-structural application, for acoustic and fire resistances, the foams should be as many as possible, where the air-bubbles in the concrete act as a barrier for sound and thermal conductivity.
There are several factors governing in FC strength development, namely pozzolans and aggregate grading, as they are altering the water–binder ratio. Pozzolans may significantly increase the concrete strength and also requires more water for workability. In this circumstance, water reducing agent is the solution to enhance its strength. Finer aggregate also will increase FC strength. However, it may impractical in situ casting where factory precast solution may suit to this condition. Synthetic fibers, such as polypropylene, also enhance its flexural and tensile strengths. Therefore, FC can be used for structural or non-structural elements.
6.2. Ultimate Limit State
Structural members with lightweight concrete showed similar performances than normal weight concrete performances, but to different degrees of performances [161
]. Hence, it requires design modifications where reduction factors are introduced to ordinary weight concrete design equations in lightweight concrete design. A flexure beam’s failure is determined by the reinforcement conditions, which are balanced, over-reinforced, and under-reinforced. Furthermore, a concrete strain of 0.003 is suggested for the flexural member at extreme compression fiber for normal concrete [176
]. According to BS 8110, the flexural prediction is valid for FC without and with pulverized bone [108
] based on rectangular stress idealization for normal concrete. Another research [177
] also proved that BS 8110 is safe for application in the RFC beam using stress block analysis. The FC’s noticeable feature is the lower tensile strength compared with equivalent strength of those of normal weight concrete. Shear friction, the early focused interest in lightweight concrete beams, is assumed to have a predominant contribution to the member shear capacity, as it was observed that tensile cracks spread through the aggregates [178
] as these aggregates have lower strength. FC without coarse aggregates was also found to agree well with BS 8110 in developing shear capacities [179
]. However, the shear tests data remain statically scattered, which slows down the development of a reliable design for foamed or lightweight concrete. From previous research [115
], it was found that diagonal cracks were formed at much lower loadings in comparison with the ordinary weight concrete, due to its smaller tensile strength. The irregular and angular cracking at a macro level is significant in resisting shearing after diagonal cracking.
6.3. Serviceability Limit State
It is essential to perform deflection checks due to the deflection of reinforced FC being more than normal weight concrete, quantitatively. Although deflection checking, and crack controlling are not available for FC, adopting design specifications of normal weight concrete into the design has been suggested. The calculation of the span–depth ratio in Eurocode 2 is to control deflection to a maximum of span/250 to avoid excessive deflection. Cracks were also found at least twice at the ultimate limit state. The cracks during the serviceability limit state should be considered in the design, as cracks are found predominantly in FC flexural beams. Therefore, crack control should be performed. The crack width should be limited to the prediction formulations under a quasi-permanent combination of loads, according to Eurocode 2 Section 7
6.4. Design Treatment
The code modifications for reinforced lightweight concrete are limited to those lightweight aggregate concretes, where lightweight FC design is rarely to be found in the code of practice. To date, the code treatments for flexural and shear reinforced lightweight concrete design are described in the following sub-sections.
Comparative studies have been carried out between normal and lightweight aggregate concretes through experimental beam flexural tests [180
]. It was concluded that lightweight concrete achieved 92% of moment capacity for normal weight concrete while exhibiting a 40% larger deflection, and the density was not identified. For FC beams, experimental results showed a 22 to 24% lower ultimate load than normal weight concrete, and 13 to 20% more deflection. Deflection checks for lightweight concrete from BS 8110-2 should be limited by the span/effective depth ratio and multiplied by a reduction factor of 0.85 if the imposed load exceeds 4 kN/m2
. Eurocode does not specify for the flexural beam design. Table 11
shows the comparison between experimental results with BS 8110-2 prediction.
For shear members, ACI 318 addressed two methods in the design treatment for lightweight aggregate concrete. Here, the square root relationship of compressive and tensile strengths is replaced by cylinder splitting values, with reduction factors of 0.75 and 0.85 for all-lightweight concrete and sand-lightweight concrete, respectively. ACI 318 also limits these methods for the concrete with a strength of not more than 41 MPa. For high-strength lightweight concrete between 41 to 69 MPa, the reduction factor of 0.85 for sand-lightweight concrete was found imprecise in predicting its shear capacity [181
]. A reduction factor of 0.8 was introduced to all types of lightweight concrete in BS 8110. The maximum limits of shear stress (0.63fcu
or 4 MPa) and compressive strength (40 MPa) are applied to the design (Table 11
). The computed shear strength is in an adequate safety margin for the concrete strength that exceeds 40 MPa [182
]. Eurocode 2 also provides a coefficient in determining tensile strength. Eurocode 2 and BS 8110 are not specified in the design of FC. Only composite columns have been found for compression members in previous research where FC was an infill material for hollow cold-formed steel sections under fire tests [183
]. A record of reinforced FC column was rarely found in the previous investigation. However, there is a recommended factor from ACI 318.
6.5. Design Summary
Structural members for reinforced design are divided into flexural, shear, torsion, and compression members. The current study was finding no relevant research has been conducted for reinforced compression and torsion members. These behaviors have been observed from previous research on other FC members, namely sandwich panels, while it is not included in this paper’s scope. For beam flexural design, stress block analysis is suggested as it gives a reliable prediction for RFC beams, according to BS 8110 [108
]. For beam shear behavior, the summary and design are described in Table 12
, according to ACI 318, BS 8110, and Eurocode 2.
From previous investigations and codes review, it is suggested to reduce factors of 0.75 for shear, torsion, and compression, 0.70 for deflection control, and 0.60 for flexural members (Table 13
). For flexural treatment, it was found that reinforced FC has 24% lower than normal concrete for beam and 0.60 for slab; therefore, a factor of 0.60 is proposed. For deflection control, only BS 8110-2 suggests using a safety factor of 0.85 for lightweight aggregate concrete, but previous research showed RFC beams deflected more than predicted, and it is suggested to replace by 0.70. It is essential to perform the deflection and crack width, as they are significant for the structural behavior of FC, according to Eurocode 2. It is suggested to design with experimental results at the current stage, which increases the design reliability.
As the creep and shrinkage may arise as one of the major issues in design for long-term effect, it is suggested to restrict compression members’ usage, such as columns and walls. Non-load bearing walls, namely brick wall, can be replaced by FC for better thermal comfort experiences, as the loads can be transferred through structural frames. However, BS 8110 clause 5.7 and 5.8 describe the column and wall design for lightweight aggregate reinforced concrete design where the slender column and wall are emphasized in this context. For compression and torsion, the reduction factors are proposed for 0.75. Research is needed for design verification for the use of the long-term effect.
Although the FC’s properties have been rigorously investigated, the application for reinforced foamed concrete is yet to be exposed. During the comprehensive review, FC properties have been summarized to determine its feasibility on the application in various types of structural use. Other properties were evaluated and equated with the normal weight concrete; however, its prediction models have yet to be developed. The properties of FC can be summarized accordingly, where the compressive strength values were greater than 40 MPa at 28 days, but was marginally achievable for structural use with >17 MPa.
Elastic modulus: four-times lower than normal concrete, which justified that there are more cracks during serviceability state;
Splitting tensile strength: non-loading cracks from pore formation may induce lower tensile strength, minimum permissible strength of 2 MPa is suggested by ASTM C330;
Time dependency properties: all prediction models, GL2000, ACI 209, SAK and CEB MC90, failed to estimate the drying shrinkage and specific creep of FC without aggregate;
Bond strength: generally lower than normal concrete, but able to be applied in RFC design.
Due to limitations in design specifications, this review paper also summarizes the nuances of designing structural members with RFC. It can be concluded that, in this stage, for RFC design, only ACI 318 offers suggestions for reduction factors in ultimate limit design for beam shear capacity. For deflection and crack control, Eurocode 2 should be adopted for FC design. Both designs, through ultimate and serviceability limit states, should be performed, as cracks may be found often in FC. Proposed reduction factors should be adapted to the RFC design for safety consideration. Compression members are suggested to be investigated, especially creep and shrinkage, before confident design can be obtained. Currently, reduction factors of 0.75 are proposed for shear, torsion, and compression, while 0.60 and 0.70 for flexural and deflection control respectively.
Some issues need to be concentrated in future research direction for securing more reliable design. In revealing RFC’s behavior, it is suggested to perform bond properties of reinforcement bar and FC with various densities, which are rarely found in the current research trend. Comprehensive environmental research on RFC should also be performed as it is potentially reducing the carbon footprint in the construction industry. However, the environmental impacts should be identified and possibly quantified.