The first step involved the collection and preparation of materials. Waste rubber tires are sourced and processed into pulverized form using a mechanical shredder. The tires are classified into three categories: chipped, crumb, and ground rubber, with the crumb rubber variant being chosen for its ability to substitute for fine aggregates in concrete. The rubber is milled to an appropriate size (0.425 - 4.75 mm), following industry standards for crumb rubber production [23] .

Ordinary Portland cement (OPC) was used as the binder in the concrete mixture, with natural sand and gravel as the control aggregates. The rubberized concrete mixes are designed by partially replacing the fine aggregates (sand) and coarse aggregates (gravel) with different proportions of PWTR, typically at replacement levels of 10%, 20%, and 30%.

The concrete mix design follows standard proportions, aiming to achieve workability, strength, and durability comparable to conventional concrete. For each mix, the water-to-cement ratio is maintained at 0.45 to ensure consistent hydration. The rubberized concrete mixes are prepared using a mechanical mixer, where the PWTR is blended with cement, sand, and gravel. The mix proportions are tested for consistency, and the fresh concrete is evaluated for workability using the slump test.

The prepared concrete mixes are cast into molds designed to simulate the dimensions of power cable trenches, with the typical trench dimensions being 500 mm in width, 800 mm in height, and 1000 mm in length. After casting, the samples are cured in a controlled environment for 7, 14, and 28 days. The curing is done under standard conditions of 23˚C and 95% humidity to ensure proper hydration of the cementitious matrix.

The mechanical and physical characteristics of concrete power cable trenches entail the workability of fresh batch mix compressive strength test, flexural strength test, water absorption and durability.

Workability is crucial for power cable trenches, affecting concrete placement and compaction around cables. Slump test was performed using the standard slump cone (100 mm top diameter × 200 mm bottom diameter × 300 mm high) per BS 12350 Part 2 (2009). Figure 2 shows samples of the slump test of one of the batches in the laboratory.

Compressive strength test was done according to BS881: Part III 1983, where a Uniaxial compressive strength test ( Figure 3 ) was carried out for all the blocks as in the following diagram.

A flexural strength test was done according to BS EN 12390-5:2019, as shown in the setup in Figure 4 .

The durability test entailed procedures for water absorption and acidity resistance.

Water Absorption

The procedure for water absorption determination was done according to BS EN 1338 The water absorption was calculated using the formula

Water absorption $\alpha +\beta =x$

$\text{Water absorption}\left(\%\right)=\left[\left(w\_2-w\_1\right)/w\_1\right]\times 100$(1)

Acidity Resistance

The evaluation of CPCT block acid resistance was conducted after 3 days of immersion in a 3% H₂SO₄ solution, following the guidelines outlined in BS 6717:1986. Subsequently, the CPCT blocks were removed after 56 days, washed with tap water, and left outdoors until they reached a stable weight. The compressive and flexural strength of the CPCT blocks will then be assessed.

There was a total of 11 preliminary batches before determination of optimum design mix. These included 2 control, 1 extreme, where rubber was used to totally replace sand. 8 other preliminary batches entailed 4 partial replacements of sand with rubber and 4 partial replacements of cement with sand. The preliminary decision was instigated by the existing studies outlined in Table 1 .

Figure 5 indicates that partial replacement of sand (P) with rubber generally reduces the slump, making the concrete less workable. This is due to rubber’s lower density and different surface texture compared to sand, which impairs the cohesion of the mix. In contrast, replacing cement (C) with rubber tends to maintain or slightly increase the slump, as the rubber’s elasticity may enhance flowability. However, the use of rubber in both scenarios compromises the concrete’s overall strength and bonding capacity, making the material more flexible but less durable. Thus, rubber replacement affects both workability and long-term performance of concrete.

A summary of compressive strength test for preliminary laboratory work is presented in Figure 6 .

Figure 6 shows the compressive strength of concrete mixtures with varying percentages of rubber replacing sand (P) and cement (C) over 28 days. Mixtures where rubber replaces sand (P3%, P6%, P7%, P9%) exhibit relatively low compressive strength throughout the curing period. The strength gain is slower, and the values remain below those of the control mixture (C) and those with cement replacement. This indicates that rubber’s presence in sand replacement weakens the bond between concrete particles, hindering the concrete’s ability to develop strength. In contrast, when rubber replaces cement (C3%, C5%, C7%, C10%, C20%), the mixtures show a more consistent and steady increase in compressive strength, especially after the 7th and 14th days. These mixtures achieve higher compressive strengths compared to sand replacement mixtures, as cement plays the primary role in binding the concrete particles together. Though replacing cement with rubber initially reduces bonding, this effect diminishes over time, allowing strength to increase gradually, particularly at higher replacement levels. The “Extreme” mixture, which likely contains a high rubber content, shows very low compressive strength, both initially and over time. This suggests that excess rubber, regardless of whether it replaces sand or cement, severely hampers the concrete’s ability to strengthen, likely by preventing proper cement hydration and bond formation. Rubber reduces bonding because it is non-adhesive compared to sand or cement. Replacing sand with rubber weakens the cohesiveness of the mix, lowering its strength. In contrast, partial cement replacement still allows some bonding, resulting in better strength progression over time. Although rubber impedes cement hydration, it does not prevent it completely, leading to more favorable long-term strength gains when used as a cement replacement. Additionally, rubber’s flexibility improves workability but compromises strength, especially when used in excess, explaining why higher cement replacement levels eventually yield more stable compressive strength.

A summary of preliminary laboratory flexural strength test is presented in Figure 7 .

Figure 7 shows that replacing sand with rubber (P) reduces flexural strength, with slow gains over time. However, replacing cement with rubber (C) results in more consistent improvement in flexural strength, especially at higher replacement levels. Excessive rubber in the Extreme mixture hampers hydration and bond formation, leading to poor flexural properties. Rubber reduces the bonding in concrete due to its non-adhesive nature compared to sand or cement, weakening flexural strength when used as a sand replacement. Replacing cement with rubber slows the hydration process, reducing early strength, but over time, the remaining cement allows for improved flexural strength, especially at higher replacement levels. Rubber’s flexibility enhances workability but compromises the concrete’s resistance to bending and flexural stresses, particularly at higher rubber content. This explains why concrete with higher cement replacement (C20%) shows a stable flexural strength over time, despite slower early strength development.

A summary of preliminary laboratory water absorption test is presented in Figure 8 .

Figure 8 results show higher water absorption with increasing rubber content, especially when cement is replaced (C). Replacing cement reduces active binder content, weakens hydration, and increases porosity due to rubber’s hydrophobicity. Sand replacement (P) has less impact, as sand doesn’t participate in hydration, but excessive rubber still raises porosity. Extreme mixes show maximum absorption due to disrupted compactness and hydration. High water absorption indicates reduced durability, making these mixes unsuitable for structural applications. However, low rubber percentages for sand replacement could be feasible for non-structural uses like lightweight or insulating concrete. Cement replacement with rubber should be minimized for durability.

A summary of preliminary laboratory durability test is presented in Figure 9 .

Results presented in Figure 9 highlight the impact of replacing cement (C) and sand (P) with rubber on concrete durability in acidic environments. Partial sand replacement with rubber (P-series) improves acid resistance at lower levels (P (3%) and P (6%)) due to rubber’s inertness, which limits acid attack. However, excessive replacement (P (9%)) weakens the structure, as rubber lacks the strength and rigidity of sand. In contrast, partial cement replacement (C-series) reduces durability significantly, especially at higher levels (C (10%) and C (20%)), because rubber disrupts hydration reactions critical for strength. Optimal acid resistance is achieved with small sand replacement, while cement replacement with rubber is detrimental.

Ablation study was used to estimate the design ratio of optimal design mix. The ablation table is summarized in Table 2 .

The ablation table indicate $P\_4$ marked $\left(3/5\right)$√ and $R\_2$ marked $\left(4/5\right)$√. Thus, the determination of optimum design mix is based on $\left(P\_4,R\_2\right)$. These were done by having 23 different batches whose summary of results are presented in Table 3 .

Top five best result for each variable is selected from Table 3 to construct Table 4 .

Table 4 indicates that replacing sand with rubber shows lower compressive strength and slightly higher water absorption percentages. The negative impact becomes more evident at higher percentages of sand replacement. Replacing cement with rubber generally results in greater water absorption compared to sand replacement but achieves relatively higher compressive strength. The compressive strength gain is more consistent over time compared to sand replacement.

Sand provides density and compaction in concrete, playing a critical role in the concrete’s microstructure. Replacing sand with rubber introduces a lightweight, hydrophobic material, increasing voids and porosity. This weakens inter-particle bonding, lowering compressive strength. Higher water absorption is observed due to poor packing efficiency and increased porosity.

Cement governs hydration reactions and the formation of calcium silicate hydrates (C-S-H), which contribute to strength development. Rubber replacement reduces active binder content, slowing hydration but allowing better strength retention than sand replacement due to improved bonding. Increased water absorption results from hydrophobic rubber disrupting the hydration matrix and creating voids. High water absorption suggests reduced durability, especially for cement replacement. Compressive strength is more affected by sand replacement due to its role in compaction. A comparison of preliminary durability test results in Figure 9 indicates that the extreme had a compressive change of $\simeq 5\%$ after acid immersion, while the best optimal design mix for durability, OP 9 in Table 4 , indicates 2.55%. Thus, we conclude that the partial replacement of rubber yields an improvement of $\simeq 2\%$ acid resistance.

An ablation study based on normalization of results systematically evaluates the impact of each performance metric to identify the best-performing batch for constructing pre-cast concrete power cable trenches using Table 5 . Normalization scales all data to a range between 0 and 1 to ensure comparability. The formula used is:

$\text{Normalized value}=\left(\text{Actual value}-\text{minimum value}\right)/\left(\text{Maximum value}-\text{minimum value}\right)$ (2)

For water absorption, where lower values are better, the normalization is calculated:

$\text{Normalized}\text{Value}=1-\left(\text{Actual}\text{Value}-\text{Minimum}\text{Value}\right)/\left(\text{Maximum}\text{Value}-\text{Minimum}\text{Value}\right)$ (3)

Equations (2) and (3) are used to prepare Table 5 and Table 6 .

The composite score is calculated as the average of all normalized metrics. For example, for OP1:

$\text{Composite Score}=\left(0.417+0.152+0.202+0.913+0.292\right)/5=0.395$(4)

Equation (4) is used to perform a similar calculation to prepare the composite score (last column) in Table 6 . Based on composite scores, the best-performing batch is OP14 (Composite Score: 0.848), as it demonstrates the highest overall balance across all metrics. The second-best batch is OP7, with a composite score of 0.746, offering excellent workability and strength alongside moderate water absorption and durability. The third best is OP13, with a composite score of 0.691, featuring exceptional durability and good strength but higher water absorption. Thus, OP14 is the top choice due to its superior compressive and flexural strengths, low water absorption, and excellent durability, making it ideal for balanced performance. OP7 and OP13 are viable alternatives based on specific priorities like workability or durability.