Concrete from Alternative and Waste Materials

Abstract

The manuscript discusses alternative and cheaper concrete types developed through research and practice that attract worldwide attention, especially those that contain components, such as waste materials from industry, agriculture, mining, and domestic. These concretes include post-concrete and geopolymer concrete. Others are synthetic concrete, pozzolana concrete, fiber concrete, and mortar. This alternative concrete has been found to possess improved mechanical strength and durability, especially higher tensile, flexural, shockproof, hardness, crack relief, and resistance to acid and sulfate attacks. The choice of alternative constituent materials has become a potential waste material that can completely or partially replace these mixture components. The production of alternative concrete is essential in the reduction of costs, carbon dioxide emissions, and for environmental sustainability. The purpose of this manuscript is to present to stakeholders in the construction industry some alternative concretes that are economical and environmentally friendly but yet compete with conventional concretes in strength and surpass durability in most cases.

1. Introduction

Concrete is a brittle composite material that is good in compression but poor in tension used for construction purposes and consists of hard aggregates that are bonded together by cement and water. Most normal or conventional concrete is produced from ordinary portland cement (OPC). The aggregates are usually sand, referred to as fine aggregates, and gravels referred to as coarse aggregates. The bonding agent is a slurry of cement with water (referred to as cement paste) which reacts together in a process called hydration. The concrete sets and hardens over time due to curing.

As an important construction material, concrete is in high demand for use in the built environment to erect buildings and construct civil engineering structures like highways, railways, bridges, harbors, and dams. Other structural applications include its use in the manufacture of concrete kerbs, pipes, and open channel drainage system elements.

Concrete is considered the most widely used substance on earth after water [1]. It is considered the base of modern built environment development by providing roofing systems, protection from natural disasters, and providing structures for human endeavors. However, the constituent materials that make up the concrete composite are expensive. The cost of cement is not only high but its production also contributes immensely to carbon dioxide (CO₂) emissions leading to negative climate change. In many parts of the world, fine and coarse aggregates are not only expensive but are scarce commodities. The option for alternative component materials from possibly waste materials that can either fully or partially replace these mixture components to produce alternative concrete has become imperative.

Alternative concretes from cheaper, lighter, and less dangerous component materials are being researched and used to either totally or partially replace the components of conventional concrete. They include geopolymer, synthetic plastic, laterite, and natural fiber materials. These materials in most cases were waste agricultural, industrial, and mining materials before they are incorporated into mixtures to produce alternative concretes. The impregnation of these materials to form the alternative concretes is found to improve the mechanical strengths and durability of normal concrete in the same way steel-reinforced concrete has. The areas of concrete property improvements include tension, flexure, crack mitigation, toughness, impact resistance, acid, sulfate, and chloride attacks. Others like compression and alkaline attack may require the addition of admixtures for any significant improvements, while they generally may not improve workability and modulus of elasticity except further research efforts are invested.

The purpose of this manuscript is to report to construction stakeholders about the research, efforts, and practices of the past years on the alternatives to conventional concrete, and to show that alternative concrete from cheaper, lighter, and less hazardous components made largely from waste materials are available for use when the components are either fully or partially introduced to replace the conventional components of cement, sand, and gravel.

2. Alternative concrete composites

2.1 Cement and alternative concretes

Generally, cement or ordinary portland cement (OPC) is the most common type of cement around the world as a basic ingredient of concrete and mortar. This gray-colored (and sometimes white) powder material is a product of a fossil fuel energy-consuming manufacturing process with mainly limestone and some clay composites as raw materials. It is estimated that 2.8 billion tonnes produced annually in the world, and its production is responsible for about 8% of world carbon dioxide (CO₂) emissions [1, 2].

OPC is produced in a process called calcination, in which limestone (calcium carbonate) along with clay of silico-aluminous material is fed into a rotary kiln and heated at very high temperatures of about 1500°C. The calcination chemical equation is as given thus;

$$5\mathit{CaC}{O}_3+2S_i{O}_2\to \left(3\mathit{CaO}.S_i{O}_2\right)+5C{O}_2$$E1

While the CO₂ gas is emitted into the atmosphere, the 3CaO.SiO₂ (oxides of calcium and silicon, sometimes referred to as calcium silicates and oxides) compounds constitute the major constituent of cement. Minor constituents include aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃) that form part of the complex cement chemical composition. This 5CO₂ exhaust in Eq. (1) combined with the fueling of the cement kiln is mostly responsible for its position as a major contributor to climate change.

When the cement is mixed with sand and gravel in the presence of water, it binds these components to produce the concrete composite. The hydration reaction that produces the binding gel is roughly described with the equations;

$$2C_{3}S+6H\to C_3-S_2-H_3+3\mathit{CH}$$E2

$$2C_{2}S+4H\to C_3-S_2-H_3+\mathit{CH}$$E3

where $C_{3}S$ and $C_{2}S$ denote the calcium silicates and oxides, $H$ denotes water, $C_3-S_2-H_3$ denote the calcium-silicate hydrate gel binder and $\mathit{CH}$ denote calcium hydroxide.

The trouble with cement, at least in Nigeria, is not only about the greenhouse effect and the environmental pollution associated with its production but the high cost of procuring it due to the high prices that had kept increasing over the years. For example, Table 1 shows the average nationwide prices in 5 years from market surveys, with the rates of increase that stood at 12.34% over the period.

Year	2018	2019	2020	2021	2022	Ave. rate of increase (%)
Prices (NGN)	2,675	2,650	3,000	4,000	4,150	12.34
Rate of price increase (%)		−0.93	13.21	33.33	3.75

Table 1.

Retail prices of cement for 50 kg bags in Nigerian Naira (NGN) [3].

The spiraling inflationary rate in the last 3 years is therefore a wake-up call for research and development in cheap and environmentally-friendly alternative materials.

2.1.1 Geopolymer concrete

Geopolymer concrete is a type of concrete in which the cement component is replaced with a material that is referred to as Geopolymer cement (GPC). In order to protect the environment, alternatives to OPC are required. Geopolymer cement is one of the most important alternatives though it may not be able to replace it completely. Credible research sources [4] have established that when cured at a temperature, GPC develops high strength and can be used for various applications.

GPC is an innovative and durable alternative binder to OPC. It is made from simply processed natural materials or industrial waste by-products. The natural materials include calcined clays, kaolinitic clays or metakaolin, lateritic clays, volcanic rocks, and mine tailings. The regular industrial waste by-products include ground granulated blast furnace slag (GGBS), which is a waste from Iron and steel production, and pulverized fly ash (PFA), which is a waste from coal and thermal plants. GGBS and PFA are rich in silica (oxides of silicon) and alumina (oxides of aluminum), which react with alkali activating solution by a process called polymerization to form molecular chains and networks of aluminosilicate gel that acts as the hard binding material for the concrete. This binding material is the GPC, and with this, the binding property serves as a complete or partial replacement for cement in concrete. The polymerization procedure is shown in Figure 1.

Figure 1.

The polymerization process.

Extensive research elaboration has shown that the existing abundant raw materials resources, mainly industrial wastes, and the simple preparation technique that saves energy and environment accord GPC advantages over OPC. Also, the additional significant benefit of geopolymer cement represents their properties which include high early strength, low shrinkage, freeze–thaw resistance, sulfate resistance, and corrosion resistance. These properties make them the material advisable for application in various branches of industry besides the binder region [5].

The concrete so produced is referred to as geopolymer concrete. It is considered an eco-friendly alternative to conventional concrete because of the significant reduction of CO₂ footprint in producing the binding gel. The production process reduces emissions by 80−90%. Another attraction to GPC, making it gain importance and acceptance is that it ensures sustainability as most of the constituents are wastes that are recyclable. Figure 2 is a diagram showing the benefits of geopolymer concrete.

Figure 2.

Benefits of geopolymer concrete.

Additionally, the improved engineering properties based on findings of research efforts confer some advantages on GPCs. These include the fact that they cure more rapidly than Portland-based cements and gain most of their strength within 24 hours to as much as 25 N/mm², and 70 N/mm² reported for 28 days [6]. However, they are set slowly enough so that they can be mixed at a batch plant and delivered in a concrete mixer. They also have the ability to form a strong chemical bond with all kinds of rock-based aggregates [7]. In some cases, GGBS replacements can be as high as 70% with 30% OPC for precast concrete manufacturing with reported increased resistance to chemical and chloride attacks, improved long-term strength gain, and lower heat of hydration enabling large volume pours due to reduced risk of cracking [8]. The use of newly developed accelerators as additives is making it possible to manufacture GGBS-based concrete without OPC with lower cost, increased strength, longer life, smoother finish, and white color it gives to the concrete. Figure 3 shows a typical process of producing geopolymer concrete that has no OPC content.

Figure 3.

Process of producing geopolymer concrete.

2.1.2 Laterite concrete

Laterite is a type of soil found in hot and wet tropical and sub-tropical regions of the world, particularly in sub-Saharan Africa and Asia. It is formed from rocks that had weathered over many years under high temperatures and rainfall with wet and dry spells. Unlike geopolymer materials, it has none or very little silica content because the high rainfall leached away the silica and thereby leaving it rich in aluminates and iron oxides. The iron oxide constituent gives it various colors that range from red to brown and yellow. It becomes hard when exposed to the atmosphere, and its ease of abstraction makes its use in construction purposes desirable.

The laterite soil is a popular building material utilized in the regions of the world aforementioned because of its availability, relatively low – cost, and economical benefit compared to other natural earth materials. It is far cheaper than river sand used as fine aggregates in concrete and sandcrete building blocks and has no health hazard associated with silica dust since it leached away under tropical rainfall. In addition to its cost-effectiveness, laterite is also considered to possess better energy efficiency when compared to conventional modern building materials in the tropical regions of the world [9].

Laterite concrete or laterized concrete is a type of concrete, in which the fine aggregate of sand is partially or fully replaced with laterite soil. This alternative concrete composite has shown comparable engineering properties with the conventional concrete. It has been known to compare in strength with conventional concrete, though with less workability at fresh stage. This can be overcome with the use of admixtures. A review effort showed that laterite soil has proven to possess good structural properties and can enhance some properties of concrete depending on the nature of the laterite and blended material [10].

An engineering consideration in the use of laterite in concrete among others is particle size distribution that cuts across the three main constituents of engineering soils, that is, clay (15−30%), silt (10−30%), and sand (40−75%). The gap-graded nature of river-washed sand, which is generally used in production of normal concrete, therefore accommodates it in a concrete mix. It is observed that better parking of aggregate materials within the concrete matrix due to a robust particle distribution resulted in enhanced shear resistance of laterite concrete beam.

According to a research report [11], the inclusion of laterite in concrete increased the shear resistance of concrete from 2.25 to 18.49 N/mm² for M20 concrete grade and from 5.17 to 13.14 N/mm² for M25 concrete grade at 15% replacement of river sand with laterite. Table 2 shows some of the strength characterization results from concrete of M25 grade with improvements in compressive strength, flexural strength, density, and elastic modulus over the plain normal concrete (with 0% laterite). For this grade of concrete, there is clear evidence of the increased toughness of the concrete beam, peaking at 15% replacement.

Sample tag	% Laterite	Compressive strength (N/mm^2)	Flexural strength (N/mm^2)	Density (kg/m^3)	Elastic modulus (GPa)	Shear strength (N/mm^2)
C0	0	22.27	9.883	2266.7	27.97	5.17
C5	5	27.40	9.925	2431.1	29.77	9.90
C10	10	21.24	9.870	2367.4	27.58	3.95
C15	15	26.32	12.693	2336.3	29.41	13.14
C20	20	24.46	10.800	2503.7	28.77	10.94
C25	25	19.79	9.199	2494.8	27.00	8.44

Table 2.

Some engineering properties of laterite concrete [9].

Since the compressive strength of concrete is considered a most important engineering property because of its basic function of resisting compressive load, laterized concrete has shown to provide adequate strength that equaled and exceeded 20 N/mm². Reports that show 40% sand replacements that produced 20 N/mm² [12] and a 100% laterite replacement with a 26.89 N/mm² strength [13] after 28 days of curing have been recorded.

Compressive strength results have generally ranged between 17 and 30 N/mm². This confers engineering viability on laterite concrete as a construction and building material.

Sandcrete blocks (i.e., often hollow blocks made from sand and cement) are very common building blocks. Because, just like concrete, they are not environmentally friendly and expensive to procure, stabilized laterite blocks are viable alternatives. Due to the relatively higher naturally occurring sand fraction in laterites, they bond well with cement in the presence of water to form strong and durable building blocks. Research results have shown that at 3−7% cement replacement of laterite, laterite-cement blocks improved in compressive strengths for up to 3.48 N/mm² after 28 days of curing, surpassing the minimum compressive strengths of 2.5 N/mm² specified by the national standards. Laterite concrete is also found to possess durability qualities, such as in water absorption capacity, surpassing sandcrete block at 4.88%, while the latter had 9.26% [9, 14, 15].

2.1.3 Pozzolana concrete

A pozzolana (or pozzolan) is a material with very high silica content or combination of silica and alumina that does not, in itself, have binding cementitious value or possesses a little of it but will, if finely pulverized into ash or dust form in the presence of water or moisture will chemically react with calcium hydroxide at room temperatures to form compounds that have binding and cementitious properties. Pozzolanas can both be naturally occurring or artificially made. Descriptions of various kinds of pozzolanas and their specifications are given in ASTM C618 and ASTM C1240 [16].

The reaction of silica (S) in pozzolana with calcium hydroxide (CH) produced from the reaction of OPC with water; it is a continuation of the OPC hydration reaction equation shown in Eqs. (2) and (3) and presented in Eq. (4);

$$\mathit{\text{Silica in pozzolana}}\left(S\right)+\mathit{CH}+H\to C-S-H$$E4

The resulting production of more calcium silicate hydrate ($C-S-H$) binder makes pozzolana an asset in concrete production. Because pozzolanas have to react with the calcium hydroxide produced from the hydration of OPC, partial replacement of OPC with a pozzolan then becomes perhaps the only relevant way of its use as a concrete/mortar material.

Natural pozzolanas are of volcanic origin. There are also artificial pozzolanas; this includes GGBS, pulverized fly ash (PFA), calcined clay, red brick dust and glass powder from industrial wastes, and agricultural wastes, such as rice husk ash (RHA), guinea corn husk ash (GCHA), and fonio husk ash (FHA).

These agricultural or post-harvest waste (sometimes referred to as agro-waste) materials that had constituted environmental nuisance over the years are found to exhibit pozzolanic properties when incinerated and converted to ash due to the very high silica and alumina content in the ashes. Other ashes include groundnut husk ash (GHA), millet husk ash (MHA), sugarcane bagasse ash (SCBA) and locust beam pod ash (LBPA), among others. An ash is considered a pozzolana according to ACI if the combined silica, alumina, and iron oxide components sum up to more than 70% of the oxide composition, that is, ${\mathit{SiO}}_2+{\mathit{Al}}_2{O}_3+{\mathit{Fe}}_2{O}_3>70\%$ [17].

An example of experimented ash analysis result that formed part of a concrete constituent is the GCHA shown in Table 3 that has high content of silicate (78.192%) and the pozzolana criteria exceeding 70% at 80.374% from an atomic absorption spectrometer analysis [18].

Oxide	SiO2	Al2O3	Fe2O3	K2O	MgO	CaO	P2O5	SO3	Others
% Content	78.192	1.345	0.837	7.674	3.757	3.338	2.946	0.494	1.417
Total for Pozzolana	80.374>70%

Table 3.

Oxides composition of GCHA.

Table 4 shows oxides composition for other ashes from agro-wastes. Ranges in values where they exist show different values from different authors. The slight differences are not unusual as varieties of crops are bound to differ in chemical combination ratios. The presented results show that all are pozzolanas and RHA along with GCHA stand out. The results also show that cheap and available agricultural wastes that constitute environmental nuisance and hazards over the years can readily be converted to concrete and mortar component materials for construction and building purposes.

Pozzolana	RHA (Range)	GCHA (Range)	GHA	FHA	SCBA
Oxide
SiO2	67.30–88.32	78.2−85.4	51.54	59.05	54.9
Al2O3	4.90	0–1.35	22.45	7.89	7.8
Fe2O3	0.95	0.64–0.84	2.40	3.12	10.0
SiO2+ Al2O3+ Fe2O3	73.15–94.17	78.84–87.59	76.39	70.06	72.70
CaO	1.36	2.04–3.34	15.63	2.63	4.9
SO3	2.70	0−0.49	0.94	0.30	1.2
Na2O	—	0.25−0.98	—	0.90	0.2
K2O	—	4.01−7.67	—	4.80	3.0
P2O5	—	0−2.95	0.60	0.80	—
MgO	1.81	0.01−3.76	1.20	1.73	2.5

Table 4.

Oxide composition of some pozzolanas from agro-wastes [19, 20, 21, 22, 23, 24, 25].

In most of the results of research efforts on pozzolanas, the concretes and mortars made from the partial replacements of OPC with these ashes have resulted in appreciable strengths for the concretes depending on the percent replacement levels and the curing period. They set and harden more slowly than normal concrete, thereby making them more attractive and useful in mass concreting works. Usually, beyond 10% replacement, the strengths are reduced considerably. Table 5 shows some results of the compressive strengths from various ashes.

Pozzolana	RHA	GCHA	GHA	LBPA	FHA	Neem seed Ash concrete
% Replacement; of OPC
0	25.5−41.0	22.9–25.5	21.8	22.2	25.1–26.1	40.1
5	34.6	23.5–26.3	—	20.9	27.0	—
10	26.1−28.1	23.51	18.4	13.7	20.0–25.6	41.9

Table 5.

Compressive strength of concretes from some agro-waste pozzolanas in N/mm² [21, 24, 26, 27, 28, 29, 30].

In addition to competing strengths with cement and concrete composites made from it, the benefits of pozzolanas may be summarized as follows;

1. The economic gain obtained by partial replacement of OPC with cheaper pozzolanas, especially from ashes of agricultural wastes.

2. Massive reduction of negative environmental implications associated with the carbon dioxide gases emitted during OPC production and concrete manufacture.

3. Improved durability of the concrete and mortar made from pozzolanas without reducing the compressive strength or other characterized performance indices in a significant way. The durability indices include water absorption, permeability, and resistance to aggressive environments that have sulfates, acids, and chlorides.

4. The recycling process of converting wastes into durable building materials and thereby enhancing environmental sustainability.

2.1.4 Polymer concrete

Polymer concrete (PC) is a special type of concrete in which polymer is used as an alternative to OPC as a binder. In some cases, the polymer may be used along with OPC to make the composite, which is referred to as polymer cement concrete (PCC) or polymer modified concrete (PMC). Polymers are used for making shopping bags, water bottles, vulcanized rubbers, food packaging, and auto parts. Their tendency to end up as after-use waste materials makes them attractive as cheap and environmentally sustainable materials for building purposes.

To produce the PC, the polymer or monomer is mixed with fine and coarse aggregates under heating, then placed in position before it is polymerized. Polymerization can be carried out by a number of means. This includes the methods of elevated thermal catalytic reaction, catalyst promoter reaction, or radiation.

The American Concrete Institute (ACI) [31] postulates that for a composite to qualify as PC, it must possess a unique combination of properties. The properties include fast curing at temperatures between −18 to +40°C; high strengths in tension, flexure, and compression; good adhesion characteristics on surfaces; durability over a long period of time through low permeability to water and aggressive solutions and thawing and freezing cycles and chemical resistance, in addition to being light-weight. The durability properties of corrosion resistance and impermeability of PC an advantage in its use in building build swimming pools, making sewerage and drainage channel systems, and other structures containing liquids or corrosive chemicals.

The compressive strength obtained with PC can be as high as 140 N/mm² within a curing period that is short. However, these characteristic properties depend on the type and quantity of polymers used in the concrete. Types of polymers include polyester resin, which is used in construction, such as in fiberglass, general waterproofing, and repairs; vinylidene chloride, which is used to make plastics for food wraps and in packaging; epoxy; styrene, which is used as a strong adhesive in automobiles and aircrafts, among others; and acrylics, which are used in makeups, particularly with nails.

The advantages of PC as an alternative concrete against the normal conventional OPC concrete include the following:

1. It possesses better compressive, flexural and tensile strengths, and abrasion resistance.

2. It has a very good adhesion on most surfaces and can be applied in very thin cross-sections.

3. It can attain up to 70% strength after 24 hours of curing at room temperature, therefore requiring fast treatment, whereas OPC concrete may attain 20% strength after 28 days.

4. It offers long-term durability as earlier discussed, reduces shrinkage, and has lightweight.

5. It reduces the infiltration of carbon dioxide and protects the PC from carbonation.

6. It is useful in repairing existing structures due to the fast-setting binder and its resistance to weathering effects.

It also has the following disadvantages:

1. It costs more to produce, however, this is ameliorated or minimized if the polymer is an after-use waste material.

2. It requires very high skills and precision work during production, especially in proportioning and mixing.

3. It is very important to make use of masks and hand gloves for skin safety because of the type of dangerous chemicals used in PC production.

2.1.5 Expanded polystyrene (EPS) concrete

The expanded polystyrene (EPS) material (also called Styrofoam) is a rigid, tough, hydroscopic, and light-weight foam made from a pre-expanded polystyrene bead. It is used as food containers, insulations in buildings, and for packaging of fragile items inside box containers. According to some research and industry sources [32, 33] the world produces over 14 tons of polystyrene every year, much of which after use is often discarded as no-cost domestic and industrial waste materials. Since they are non-degradable, they constitute environmental hazards and nuisance in landfills, if not converted for good.

EPS concrete (EPSC) or EPScrete is made from cement and balls of EPS with little or without aggregates. it is not usually as strong as normal concrete. Its possible preference over normal concrete is based on its lightweight, excellent heat preservation, and sound insulation. Others, according to some findings [34] are its hydrophobic properties, lower cost, environmental friendliness and increased thermal insulation properties. It is increasingly being used in various applications in constructing environmentally “green” homes. The global green building movement, an already huge factor in the real estate development sector for residential housing is at the forefront of this.

2.1.6 Fiber reinforced concrete (FRC)

Fiber-reinforced concrete (FRC) is an alternative and cost-effective composite material to conventional steel-reinforced concrete (CSRC). It usually consists of fibrous materials that improve the structural integrity of the concrete or mortar. FRC may also be described as concrete that contains slender, and most times, short discrete fibers that are uniformly distributed and randomly oriented in the concrete mixture for the purpose of improving the structural performance. The fibers can be obtained in natural or synthetic forms with each providing varying properties to the concrete.

Important advantages of FRC apart from cost-effectiveness include time-saving as a result of adding fiber and mixing offsite. When this ready-to-use concrete is delivered, it allows for faster placement and eliminates handling time in contrast with steel-reinforced concrete works, where a lot of effort and time are expended in bar scheduling, bar bending, and bar installation. The solution provided by FRC also has the potential to reduce carbon dioxide emissions when compared to the provisions of reinforcing steel bars.

The general attraction of fibers in concrete is to control cracking due to plastic and drying shrinkages. Others are to reduce the permeability of concrete leading to reduction of the bleeding of water and increased shear and strain capacities.

The fibers in FRC can be classified into two categories; Microfibers and Macrofibers. While the earlier are usually shorter and thinner, the latter are longer and may be thicker. In some constructions, both may be combined to form composite that includes mixtures of cement, mortar, or concrete and discontinuous, discrete, uniformly dispersed suitable fibers for improved properties. In other cases where the load bearing is very high, they may be combined with the regular steel bar reinforcements.

Micro fiber reinforced concrete (MiFRC) has many individual fibers that are dispersed throughout the concrete as evenly as possible during the mixing process. The matrix-like structure so created provides long-term durability to concrete while reducing the frequency of plastic shrinkage cracks in fresh concrete after placement. Another benefit is the tougher surface it possesses, which makes it have more resistance to impact stress.

Macro fiber reinforced concrete (MaFRC) has individual fibers arranged and spread in the concrete matrix. They improve the performance characteristics of the concrete by increasing toughness and ductility of hardened concrete. They also improve post-crack performance by allowing concrete to retain the load-carrying capability after cracking has occurred. Other general benefits of macro fibers in concrete include increased tensile and flexural strengths, improved impact and abrasion resistance.

Over the years MiFRC and MaFRC have found engineering applications in the construction of floors, car parks, airports, bridges, domestic, and agricultural infrastructure projects. They are also applied in places and instances when conventional reinforcement placement is very difficult or not possible like in very thin structures.

2.1.7 Types of fiber-reinforced concrete (FRC)

The major factors that generally affect the performance characteristics of FRC are the water/cement ratio, the quantity or percentage content, diameter, and length of fibers or the aspect ratio. Figure 4 gives the different types of FRCs used in construction with brief discussions of them.

Figure 4.

Types of fiber reinforced concrete (FRC).

2.2 Steel fiber reinforced concrete (SFRC)

The steel fiber is a product of steel metal of a specified geometry with an aspect ratio (ratio between length and diameter) between 20 and 100. Steel is one of the most important materials in the construction industry. It provides tensile resistance to concrete since concrete is known to be weak in tension and brittle. Therefore, the presence of its fiber in concrete can cause changes in the quality of the concrete’s physical property by greatly increasing resistance to tension, cracking, impact, fatigue, bending, and durability. It also improves other mechanical strengths like toughness and stress resistance in concrete.

Compressive strength of SFRC is affected mainly by the quantity of fibers, fiber sizes, and bonding with the matrix. Micro fibers mitigate the development of microcracks in the concrete, resulting in improved compressive strengths. On the other hand, macro fibers help to control crack openings, which increases the concrete energy absorption capacity. However, increased fiber addition may cause some distress in the matrix, which can result in creation of more voids. The designer, therefore, will need to create a balance that aims at an optimum fiber quantity and fiber sizes for comparable compressive strength values. Previous research results [35, 36, 37] show that SFRC significantly improved in flexural and axial strengths with overall toughness compared to the normal reinforced concrete. Other properties like compressive and shear strength increased at up to 1.0–1.5% optimum fiber content by weight after which there was reduction in strength, impact resistance improved up to 10 times over plain concrete and there was higher resistance to corrosion. However, modulus of elasticity decreased with an increase in fiber content.

SFRC has found structural applications in the design and construction of floors, buildings, precast concrete, concrete bridges, tunneling, and heavy-duty rigid pavements. The types of steel fibers used in SFRC are specified by ASTM A820/A820M [38] which for example, specifies that the average tensile strength of fiber should not be less than 345 MPa. It also has specifications on fiber dimensions and workmanship, among others.

2.3 Polypropylene fiber reinforced concrete (PpFRC)

Polypropylene Fiber (PpF) is a synthetic fiber made from propylene with similar properties to polyethylene, though harder and more heat resistant. It displays good heat-insulating properties, is highly resistant to acids, alkalis, and organic solvents, and is used in a variety of applications.

Research results [39] have shown that adding PpF to concrete improved its compressive and tensile strengths without regard to the water-cement (w/c) ratio. For example, adding 0.5% of PpF by volume to concrete with w/c ratios of 0.46 and 0.36 increased compressive strengths by 12 and 6%, respectively, while for tensile strengths it increased by 17 and 8%, respectively. The impact strengths increased 5 and 2.1 times at first cracks for the two w/c ratios, respectively, which translates to higher energy absorption capacity in the concrete.

Also, in a hybrid FRC involving the combination of steel and PpF at constant 0.75% by volume [40], an attractive solution for enhancing the post-cracking behavior of concrete was found in compression and flexure though without a significant effect on their strength values. The even compressive and bending strength values might have been due to the relatively low fiber percent in the mix as optimum fractions from many works stand at 1.0–1.5%. This is further attested to by Blazy and Blazy [41], where it was generally mentioned that PpF improves concrete properties, but until a certain dosage (i.e., optimum value) after which if exceeded will have reduced performance.

In summary, the effect of PpF on the properties of concrete can be described as very improved in toughness, crack mitigation and limitations, impact and spalling resistance, anti-freeze–thaw cycles and durability; improved in flexural and tensile strengths, abrasion resistance, eco-friendliness, and economics; requiring further assessment in water absorption, porosity, and permeability; slightly neutral in compressive strength and modulus of elasticity and totally neutral in workability.

2.4 Polyester fber reinforced concrete (PeFRC)

Polyester fiber is a synthetic polymer that is manufactured when there is a chemical reaction between an acid and alcohol. It is usually presented as non-biodegradable industrial waste. The raw materials are usually petroleum, coal, water, and air. It has appreciable elasticity, toughness, and sound absorption properties. It is traditionally used in the textile industry and sometimes blended with cotton to produce clothing.

Research results [42] on PeFRC indicate follows: an increase in impact resistance (166.7%); an increase in tensile (65.5%), flexural (66.7%), and compressive (21.5%) strengths; and decreases in workability and modulus of elasticity (7.14%). The above results came from optimum fiber content of 0.075 and 10% cement replacement with fly ash. However, in another result without fly ash component [43], the, tensile strength, flexural strength, and compressive strength of the PeFRC increased by 24.3, 15.13, and 36.2%, respectively compared to normal OPC concrete after 28 days of curing with an optimum fiber content value steadying at 3.5%. This shows that admixtures have positive effects on the strength and the quantity of fibers used in concrete by increasing strength and reducing fiber content.

Other results in Ref. [44] indicate that in addition to flexural and compressive strengths improvements, there was resistance to abrasion, alkali resistance and reduced drying shrinkage when compared to normal OPC concrete.

The fiber is used to reinforce concrete which has found applications in the construction of industrial and warehouse floors, pavements, and precast concrete. Like with the other FRCs they provide adequate resistance to the formation of plastic shrinkage cracks and enhance toughness.

2.5 Glass fiber reinforced concrete (GFRC)

Glass fiber compares well with polymers and carbon fiber in mechanical strength. It is much cheaper and less brittle when used in composites. These characteristics make it useful as a reinforcing component in many polymer composite products. The resulting products called glass-reinforced plastic (GRP) or fiberglass are very strong and relatively lightweight. However, when, rather than reinforcing polymer with glass, OPC and sand are used to replace polymer, then, the resulting material will be GFRC.

Compressive review of glass fiber in concrete [45] shows that it impacted negatively on workability of GFRC with an increase in fiber content. While it enhanced the tensile and flexural strengths, it did not significantly improve the compressive strength within the optimum fiber contents of 1−2%. It resisted cracking but did not significantly improve surface hardness. It has significant improvement in durability properties by reducing permeability of GFRC to chloride-ions because of its ability to mitigate against cracks; it improved performance in marine environments under acid and sulfate exposures. It is suggested that further research that incorporate pozzolanas be investigated to improve compressive strength of GFRC in view of the importance of this property in load bearing function of concretes.

GFRC has been used for many years to produce different and varied products.

The products include thin architectural cladding panels, ornamental concrete, such as domes, statues, fountains, decorative panels, concrete countertops, and artificial rock work.

The challenge with using glass fibers in GFRC is that since glass is primarily a silica material it breaks down in the alkaline concrete environment due to alkali-silica reactivity (ASR) when there is reactive silica in the concrete aggregate. However, innovations in alkali-resistant (AR) glass fibers including impregnation of pozzolanas have helped in overcoming the alkaline attack leading to a rapid increase in GFRC use.

2.6 Carbon fiber reinforced concrete (CFRC)

Carbon fibers are mostly made up of carbon atoms that in physical appearance are thinner than even human hair, measuring about 5–10 micrometers in diameter. They can be combined to form a yarn or processed to make a grid structure and then coated. The benefits of carbon fibers include high stiffness, high tensile strength, lightweight, high chemical resistance, high-temperature tolerance, and low thermal expansion. When compared to steel, carbon fiber is four times lighter and yet possesses six times more load-bearing capacity. When the fiber is integrated into a plastic resin under heat it becomes a composite called carbon fiber reinforced polymer or commonly referred to as carbon fiber, possessing very rigid property and high strength/weight ratio, though brittle.

Carbon fiber reinforced concrete is an innovative combination of carbon fiber fabrics or bars with fine-grained concrete composite material. This type of concrete can be formed into varied shapes including slimmer and delicate structures and yet with a high load-bearing capacity. CFRC can be used for repairs of existing structures and for sustainable less- material-intensive and lightweight construction.

According to a research result [46], a concrete mix with 0, 0.5, and 0.75% carbon fiber contents produced compressive strengths of 40, 45 and 48.7 N/mm², respectively, and flexural strengths (measured as modulus of rupture) of 4.8, 5.9 and 6.1 N/mm², respectively. The results were determined after 28 days of curing and admixtures were added to the concrete mix. These results compare significantly M40 concrete in compression and exceed considerably the flexural values of the same grade of concrete.

In another research output [47], it was found that by including carbon fibers in concretes of different compressive strengths, the fibers provided for the concrete of low strength (M20 grade) an efficiency factor of 258.6, the medium strength (M30 grade) had an efficiency factor of 62.1 and for high strength (M40 grade) concrete it provided 2.7 as efficiency factor. What this implies is that carbon fiber not only improves strength but also can provide the most effective reinforcement for the concretes of low strength.

Yet in related work [48], fiber content was varied at 0, 0.75, 1, and 1.25% in an M25 concrete and though workability decreased, compressive strength increased by 46.8, 59.9, and 32.4% respectively with optimum content value at 1%. It was also observed that split-tensile strength increased at 28.1, 56.3, and 9.4% over values of normal concrete again at 1% optimum fiber content. The values for flexural strength increased by 88.5, 107.69, and 78.46% respectively at 1% optimum content. The same result showed that CFRC resisted acid and sulfate attacks far better than normal concrete, especially at 1% optimum fiber content. The use of GFRC as an alternative to conventional concrete is well established.

2.7 Natural fiber reinforced concrete (NFRC)

Synthetic fibers manufacture is relatively expensive and their production process demands considerable energy. An alternative is the use of low-modulus natural fibers in the production of building composites. Natural fibers are usually cheap, available in many parts of the developing world as waste materials constituting environmental nuisance which can be deployed for sustainable use as building material components.

Natural fibers are made up of combination of cells in which the thickness is almost negligible when compared with the length, that is, with a high aspect ratio. They can broadly be classified as being from vegetable or animal origins. Fibers of vegetable origin include coconut husk or coir, sisal, sugarcane bagasse, bamboo, jute, palm fruit husk, wood fibers, and straw, and those of animal origin include silk and wool. Natural fibers of vegetable origin have a huge history of use as reinforcing and stabilizing materials for soil-based composites like mud, clay, and laterites to improve the properties of bricks, roofing and plaster materials in buildings [49].

Natural fibers are potential choices for reinforcing concrete because of their availability and cheapness, particularly in developing countries, and improvement in mechanical properties. A research result [50] on the use of sisal fibers found that improvements in fresh concrete and hardened concrete with significant increases were recorded for compressive strength at optimum fiber content of 1.5%, split tensile strength increased even at 0.5% fiber content, and flexural strength improved at 1% fiber content over normal concrete.

A comprehensive review of coir, sisal, jute, and bamboo fibers in concrete [51] shows that the usage of the fibers improved the concrete’s compressive, tensile, and flexural strengths, durability, and the load-carrying capacity. Another finding was that coir and sisal not only delayed and controlled cracking due to tension but presented the best improvements in the concrete properties with optimum fiber contents averaging 1.5%. However, admixtures like micro silica assisted in strength and durability enhancements.

In another study [52], the conclusions were similar to the ones mentioned above with enhanced fracture toughness, impact strength and crack resistance except that there was no significant increase in compressive strength values and workability values were higher. These results are good, economical, and beneficial in rural communities where the fibers are freely and easily available.

The use of coir in cement mortar and mathematical optimization of the mortar mixture was investigated [53] and the results confirmed the superiority of the fiber mortar over plain mortar with the optimized compressive strength exceeding the laboratory value by 42.8%.

A major disadvantage with natural fibers in concrete is that as biodegradable materials they are susceptible to decay over time resulting in to loss of strength and subsequent brittle failure. However, this can be avoided by pre-treatment of the fibers with resins and gums before mixing and inclusion of admixtures like pozzolanas (rice husk ash, silica fume, etc.).

3. Conclusion

It is established that research efforts and practices over the years have found credible alternatives to conventional concrete and reinforced concrete, as highlighted in this chapter. The general benefits include the following:

1. Alternative concretes produced from geopolymers, laterites, pozzolanas, polymers, expanded polystyrene, and fibers constituents possess adequate mechanical strengths and durability qualities respectively to compete with conventional plain concrete and even steel bar reinforced concrete.

2. These alternative concrete materials are mostly derived from industrial, agricultural, mining, and domestic wastes thereby making them very cheap, readily available, and economical. They also make for environmental sustainability when used for building and construction purposes.

3. The use of these materials over cement and steel will reduce the incidence of carbon dioxide emissions associated with the manufacture of cement and steel and thereby curtail negative climate change.

4. Since concrete is brittle and very deficient in tensile strength, these alternative concretes have significantly provided improved tensile and flexural strengths for concrete, and compressive strength to some extent. Other property enhancements include impact resistance, toughness, cracks mitigation; acid, chloride, sulfate resistance, and other durability properties.

5. In addition to competing use in conventional constructions, these alternative concrete have especially found applications in the construction of thin delicate structures often of architectural importance, in the repairs of existing structures, and in mass concreting of heavy-duty floors.

6. Nevertheless, further investigations are suggested in improving their workability, compressive strength, alkaline attack in glass fiber reinforced concrete, permeability, and water absorption, though these properties are presently being treated with incorporation of admixtures.

Conflict of interest

The author declares no conflict of interest. All the figures were made by the author and so did not warrant copyright permission.