1. Introduction
Earthen construction materials and techniques have existed for centuries. Approximately 30% of all the United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Sites utilize earth as a building material; a testament to the durability of earthen structures when properly constructed and maintained [
1]. In spite of the global use of earth as a building material, there are real and perceived limitations to its structural use [
2,
3]. Some of the identified factors limiting the structural use of earthen masonry globally include relatively low strength properties compared to concrete-based blocks and burnt bricks, significant variations in performance depending on soil type used, manufacturing methods, and climatic conditions [
2,
4,
5].
The modern use of earth in the form of compressed earth blocks (CEBs) can be traced back to 1950s [
6]. CEBs are produced by mixing soil with a stabilizer, which is then moistened and compressed either manually or mechanically in a mold to produce blocks [
6,
7]. It has been observed that engineered CEBs can outperform adobe with respect to strength, durability, and embodied carbon [
8]. Considered as an “appropriate technology” by the United Nations, CEB production is small scale, labor intensive, sustainable, and requires simple operating technology and maintenance. This makes CEBs a well-suited material for providing affordable housing units [
8,
9,
10,
11]. Despite the strength improvement achieved through using CEBs over other traditional forms of earthen construction, they are still more brittle and weaker in bending and compression in comparison to concrete masonry units (CMU) and fired bricks [
4]. Their compressive strength values that range between 2.0 to 5.0 MPa, which is significantly lower than concrete masonry units and burnt bricks, which average between 13.10 MPa and 17.24 MPa, respectively [
4,
7,
10].
Some of the disadvantages of CEBs and CEB technology is poor resistance to moisture when not properly constructed and the lack of standardized procedures for production and construction. Where codes and standards exist, they are very limited in scope [
10,
12]. Variability in soils from site to site means optimum mix proportions differ, thus complicating the issue of developing standards of practice and codes [
13]. Obonyo et al. [
2] identified specific properties of earthen blocks/masonry systems that need to be optimized to promote their use for structural applications. The properties identified include optimizing the strength of individual units, geometry of blocks, strength of mortar, deformation characteristics of blocks and mortar, joint thickness, suction of blocks, water retention of mortar, blockwork bonding, and workmanship.
The findings presented in this paper are part of a larger collaborative research effort aimed at developing engineered fiber-reinforced masonry systems for high wind regions. Some of the findings from the larger research effort have been presented in other publications [
4,
14,
15,
16,
17]. Obonyo et al. [
14] and Donkor et al. [
15] reported a diminishing return in the strength of PP fiber reinforced CEBs when certain fiber thresholds are exceeded. The authors also reported that the length of the PP fibers used had an influence on the strength of the CEBs produced. Donkor and Obonyo [
4] reported that PP fiber quantity influences CEB compressive strength, 3-point bending strength, and strain capacity. The authors further proposed a regression model for predicting PP fiber reinforced CEB compressive strength. Matrices used for specimen production in the study were hand mixed. Donkor and Obonyo [
17] focused on the microstructural analysis of failed fiber reinforced CEBs and proposed best practices for PP fiber addition into CEB matrices. The flexural properties of the specimens were also analyzed. The research approach adopted is to improve strength properties at the block level before scaling up to the system level. This current paper analyzes the flexural properties of PP fiber reinforced matrices used for CEB production and focuses on developing an equivalent flexural strength predictive model. In non-conventional walling materials such as CEBs, flexural strength is an important design parameter with respect to lateral loads due to wind, floods, or other load scenarios that can cause out-of-plane bending [
18]. Modulus of rupture, also referred to as flexural strength, is one of the material properties used to assess the durability of earthen building materials [
19]. For example, the State of New Mexico’s Earthen Building Materials Code requires a minimum modulus of rupture of 50 psi for earthen building materials. Earthen building materials as defined under the code include adobe, burned adobe, compressed earth blocks, rammed earth, or terrón [
20].
Research into CEBs have been focused on alternate or supplementary stabilizers [
21,
22,
23,
24,
25,
26]; influence of soil types on performance [
2,
27,
28,
29]; hygroscopicity [
23,
30,
31]; system level performance [
18,
32,
33,
34,
35,
36,
37,
38]; and the use of fibers as reinforcement.
Widely seen as an attractive building material particularly in the global south, the more modern forms of earthen masonry systems are attracting attention from sustainable building movements within developed countries in an attempt at finding more ecologically friendly building materials [
35,
39]. For example, the Mortenson Center in Engineering for Developing Communities at the University of Colorado, USA, collaborated with the Apsaalooke Nation Housing Authority to put up affordable houses using CEBs for the Crow Tribe in the state of Montana, USA [
40,
41].
1.1. Outstanding Issues in the Use of Fiber in Earthen Masonry
Earth, which has been stabilized with a binding agent, such as Portland cement, is an acceptable building material with adequate strength in compression. Unfortunately, it is brittle with low tensile strength, which often leads to poor resistance to bending and the development of tensile cracks in response to external forces [
42,
43] There is historic precedence in ancient civilization of enhancing the tensile strength of unfired earthen masonry units and plaster using naturally occurring fibers such as straws and horsehair [
44]. Such practices have been adopted in modern earthen masonry units. This can be accomplished using naturally occurring plant and animal fibers as well as synthetic fibers to reinforce soils for CEB production [
6]. As observed by [
43], such use of fibers can improve ductility, which in the wake of a disaster will delay catastrophic failure, thereby giving building occupants time to escape, as opposed to facing the risk of injury of even fatality because of sudden failure [
45].
Fibers that have been used for CEB production includes sisal fiber [
21,
46]; coconut fiber [
2,
23,
47,
48]), straw [
49], polyethylene [
29,
43], and jute [
50,
51]. The findings of these studies indicated that desirable structural properties could be realized through incorporating either natural or synthetic fibers in earthen masonry units. It was also observed that untreated natural fibers could degrade when exposed to the highly alkaline environment that is created following the hydration of cement [
44]. This has a direct bearing on the durability of these fibers. Some addition research is necessary to identify the optimal strategies for using these fibers [
23]. When the synthetic fibers in question are derivatives of post–consumer plastic waste products, their performance as secondary reinforcement is subject to variations—this is a known problem associated with the re-use of waste material [
52]. Such material variability coupled with the inherent variation in soil properties begets difficulty with respect to the isolation of CEB performance properties that are directly linked to specific fiber properties/attributes. For this reason, this research used commercially available PP fibers used for concrete production. The authors draw from the use of PP in concrete and geotechnical engineering applications. Historically, CEB advancement has benefited from techniques used in both industries [
6,
53,
54].
1.2. Contributions from This Work
This research evaluated flexural properties such as first-peak strength, peak strength, equivalent flexural strength, residual strength, and toughness of soil-cement matrices used for CEB production. Cementitious matrices are quasi-brittle and have low tensile strength and strain capacities. A practical means of enhancing the performance of such matrices is the inclusion of various types of fibers to enhance ductility, strength, toughness, and resistance to impact loads [
55]. In addition, the strength of fiber-reinforced CEBs is influenced by fiber type and quantity, soil type, stabilizer type and quantity, and level of compaction of the matrix, curing conditions, and testing procedures. Soil type, stabilizer type and quantity, and level of compaction also affect the strength properties and durability of CEBs. Mechanical properties such as compressive strength, flexural strength, and shear strength can therefore be enhanced if an optimal fiber-reinforcing ratio is identified and used [
42,
56]. The paper evaluated equivalent flexural strength (
fe) at different fiber dosages and proposed the model to establish the correlation between equivalent flexural strength (
fe), first–peak (
f1), and fiber mass (
Wf). Equivalent flexural strength (
fe) shows the effectiveness of fibers to bridge cracks and enhance the energy absorption ability of the reinforced matrices under loading. The first–peak (
f1) strength depicts the flexural behavior of beams (both reinforced and unreinforced) up to the onset of cracks. The model is therefore to help predict how different fiber dosages influence how effective the fibers are in bridging cracks after the onset of cracking.
In general, synthetic fibers such as PP are known to improve concrete freeze–thaw resistance, impact resistance, and shrinkage cracking [
57]. In cement-stabilized soils used for geotechnical applications, the inclusion of randomly distributed PP fibers increases unconfined compressive strength, residual strength, absorbed energy, ductility, splitting tensile strength, and flexural toughness of soil mixtures [
58,
59,
60,
61,
62]. This suggests that there is a potential for using PP fibers in CEB production. Due to production differences between soil-cement matrices used in geotechnical applications and those used in CEB production, there is the need for an evaluation of the effect of PP fibers on the flexural performance of matrices for CEB production.
Recent findings from studies carried out on the inclusion of fibers into CEB matrices include: fiber surface conditions influence CEB strength [
43] fiber length, aspect ratio, and quantity influence the strength and failure mode of CEBs [
4,
15,
63] and there is a diminishing return in strength when certain fiber thresholds are exceeded [
15,
30,
63]. Mechanical properties such compressive strength, flexural strength, and shear strength can therefore be enhanced if an optimal fiber-reinforcing ratio is identified and used [
3,
42,
49,
63]. The existing body of knowledge on CEB flexural performance is scarce compared to that of conventional walling materials. The work discussed in this paper is directed at contributing to this body of knowledge. Specimens for flexural performance evaluation were produced with 8% OPC and fiber mass content of 0.2, 0.4, 0.6, 0.8, and 1.0%. Economic feasibility was outside the scope of this study.
Based on UNEP (ND), sixty million plastic drinking bottles are purchased every hour. UNEP also estimates that across the world, we deploy 5 trillion single-use plastic bags. Fifty percent of the existing plastics are usually discarded after a single use. Developing viable pathways for re-using this plastic to enhance the structural properties of CEBs could address the existing environmental sustainability problem of plastic pollution.
4. Conclusions
This research was directed at assessing the extent to which PP fibers could be used to enhance the performance of earthen masonry units. The authors focused specifically on investigating the influence of the incorporation of fibers and fiber content on the flexural behavior of CEB matrices. Unreinforced and PP fiber–fiber reinforced specimens (short flexural beams) were produced with 8% OPC and 0.2, 0.4, 0.6, 0.8, and 1.0% fiber dosages by mass. The specimens were tested according to Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete per ASTM C 1609. The strength of fiber-reinforced CEBs is influenced by fiber type and quantity, soil type, stabilizer type and quantity, and level of compaction of the matrix, curing conditions, and testing procedures. Within the limits of the experimental program used, the main conclusions have been outlined below:
The inclusion of the PP fibers into the soil–cement matrices enhanced flexural performance as seen by the load-deflection response after initial crack, residual strength, flexural toughness, and equivalent flexural strength.
At fiber content of 0.2 and 0.4%, there was a sharp drop in load carrying capacity after initial crack. A rebound in load carrying capacity was observed after the fibers were engaged. This was followed by a deflection-softening behavior.
At fiber content of 0.6% and above, a deflection-hardening behavior was observed after a slight drop in load carrying capacity after the onset of crack.
Fiber inclusion resulted in a high degree of load retention after first crack. The unreinforced matrices were brittle and experienced catastrophic failure.
The fibers prevented catastrophic failure of the reinforced specimens even at large crack widths. The fiber-reinforced specimens deformed significantly after ultimate load without collapse.
The failure patterns and response of the fiber–reinforced specimens during testing suggests an improvement in ductility as a result of PP fiber reinforcement.
An equivalent flexural strength predictive model with first-peak strength and fiber mass content as variables was proposed.
The optimal fiber mass content was determined to 0.6% based on test results. In subsequent research, the social, financial, and economic aspects of implementing the proposed use of PP as secondary reinforcement for engineered earthen masonry units should be investigated further.