From failure to recovery: CFRP strengthening of a near-collapse reinforced concrete wastewater structure with root cause diagnosis,failure mechanisms,and structural implications under aggressive environmental conditions

Visual inspection of the tank

Operation of Compartment A [Figure 1(g)] was temporarily suspended in 2015 following the observation of excessive deflections and cracking in the interior roof slab panels. At mid span, three of the panels exhibited deflections exceeding 250 mm, the severity of which was evident from the stagnation of rainwater on the roof slab [Figure 2(a-1)]. With reference to the original design span of 8000 mm, this corresponds to a deflection to span ratio of approximately span/32, far in excess of the serviceability limit of span/250, indicative of severe structural distress rather than a serviceability concern. Pronounced mid span deflections, accompanied by visible cracking, were also observed in several adjacent panels [Figure 2(a-2) to (a-3)].OPEN IN VIEWER

At the time of the initial inspection, the tank compartments remained in operation and internal access was not feasible. Based on the external observations, it was initially hypothesised that settlement of an intermediate column had contributed to the slab deflections. To verify this, a borehole investigation was undertaken at three locations adjacent to the tank. The subsurface profile revealed a 2.2 m thick layer of weak fill near the surface; however, this was not considered structurally critical, as the foundation formation level lay beneath the fill and was supported by relatively stiff underlying strata. The hypothesis of foundation settlement was therefore discounted.

Non-destructive testing was subsequently carried out to assess the extent of structural deformation and material degradation. The results indicated that the structural system could remain in service provided that appropriate strengthening measures were implemented. Two strengthening priorities were identified: first, enhancing the flexural, shear, and torsional capacity of the slabs, beams, and wall slab intersections; and second, improving the axial confinement of the columns so that the structure could safely re-establish its intended load path. Crucially, the strengthening strategy was required to avoid any significant addition of self-weight, given the limited reserve capacity of the existing foundation and the practical difficulty of strengthening it from below an operational tank.

To facilitate a detailed internal inspection, the influent inlets to Compartment A were closed and the wastewater drained through the outlets. Suction fans were then deployed to extract sewer gases and ensure safe working conditions for the inspection team. Internal inspection revealed that an intermediate column had fractured into two distinct sections, with the exposed reinforcement exhibiting significant corrosion [Figure 2(b-1)]. The adjacent columns also displayed slight lateral deflections, and lateral displacement of the vertical pipelines provided further evidence of a disturbed load path resulting from the column failure and the associated slab deflections [Figure 2(b-2)]. As a consequence, the roof beams supporting the slab had also undergone noticeable deformation.

Evidence of concrete corrosion was observed both in the failed column and in the surrounding walls. The wall corrosion was most severe near the roof slab and diminished progressively with depth [Figure 2(c)], a pattern consistent with attack driven by sewer gas accumulation in the headspace above the wastewater. To prevent further structural deterioration and the possibility of progressive collapse, galvanised steel (GI) pipe props were installed adjacent to the damaged column as a temporary measure. The operation of Compartment A was thereafter fully suspended pending renovation. The plan location of the deteriorated elements is indicated in Figure 3.OPEN IN VIEWER

Crack survey on the top of the slab

A crack survey was carried out during the on-site inspection to evaluate the extent of structural deterioration on the upper surface of the roof slab. Cracks with a width greater than 0.3 mm were recorded, in line with the threshold below which cracking is generally considered cosmetic rather than structurally significant. A total of 16 locations across the roof slab were identified for inclusion in the survey, as illustrated in Figure 3. A detailed discussion of the observed crack patterns and their implications is presented in the Discussion section.

SEM analysis

Concrete samples extracted from the corroded surfaces of the beam and slab were examined using a SEM operated at an accelerating voltage of 10 kV. The investigation was carried out to identify microstructural features associated with the principal deterioration mechanisms anticipated in this environment namely, sulphate attack and microbially induced corrosion.

Structural analysis of the tank

A three-dimensional finite element model of the tank was developed and analysed using the commercial structural analysis software Midas Gen (Midas Co, 2024). Two cases were considered.

The first represented the structure in its existing deteriorated state, with the failed intermediate column and the deflected slab modelled as observed on site. The second represented the post-rehabilitation structure following reconstruction of the failed column and reinstatement of the slab to a level profile, with a lightweight infill material varying in thickness from 20 mm to 250 mm applied to compensate for the residual deformation. The numerical models for both cases are illustrated in Figure 4(a) and (b).OPEN IN VIEWER

To account for material degradation, the compressive strength of all concrete members was reduced by 30%, giving an assumed in-situ strength of 28 MPa, and the yield strength of the existing reinforcement was reduced by 20%–50% depending on the location and extent of observed corrosion, giving an assumed in-situ yield strength of 370 MPa. These reductions were informed by the visual inspection, NDT results, and SEM analysis described in the preceding sections.

Beams and columns were idealised as line (frame) elements, while slabs and walls were represented by two-dimensional plate elements. The predicted member forces and moments were independently verified through hand calculations using simplified beam and slab idealisations. Existing and required design capacities were computed in accordance with BS 8110-1(1997), the code under which the original structure was designed. On the basis of these calculations, CFRP strengthening was selected to enhance the load bearing capacity of deficient members, with CFRP sheets and plates chosen according to the specific capacity requirement and the geometry and accessibility of each member. The mechanical properties of the CFRP and epoxy resin used in the project are summarised in the Supplemental Material [Annex-A: Table-1]. The strengthening design was carried out in accordance with ACI 440.2R (2008).

Crack survey

The crack survey revealed extensive cracking distributed across the 16 inspection locations identified in Figure 3, with crack widths ranging from approximately 0.75 mm to 1.25 mm. The observed patterns and their interpretation are summarised below, with the principal observations illustrated in Figure 5.OPEN IN VIEWER

The most severe cracking was recorded at locations A and B, immediately adjacent to the wall-slab joint, where parallel cracks of up to 1.25 mm width were identified. These cracks were oriented parallel to the joint, indicating partial separation between the wall and the slab. The pattern is consistent with sagging of slab panels E-D/1-3 and D-C/1-3 above the failed intermediate column, which has imposed tensile stresses on the slab-wall junction beyond the tensile capacity of the concrete.

At locations C and E, two cracks of approximately 0.75 mm width were observed on either side of beams E/1-3 and C/1-3 respectively. Both locations are adjacent to the most heavily deflected panel, E-C/1-3. The cracking is attributed to tensile stresses generated by the excessive deflection of this panel, exceeding the tensile strength of the cover concrete and producing flexural cracks aligned with the principal moment direction. Location D, in the central region of the deflected panel, exhibited no visible cracking consistent with the predominantly compressive stress state at this location under positive bending; should the deflection progress further, the compressive stresses could approach the concrete crushing strength and initiate compressive surface damage.

Cracking at location F was identified along beam B/1-3 and is attributed to negative bending moments induced in the supporting beam by slab deflection the same mechanism responsible for cracking at C and E. The lower crack intensity at F reflects the smaller negative bending moments developed in this region.

Locations G, H, and I exhibited multiple parallel cracks aligned with the longitudinal direction of the supporting wall. The crack density was noticeably higher than at A and B, again attributed to negative bending moments induced by deflection of the adjacent panels. The higher density relative to A and B reflects the greater rotational restraint provided by the continuous wall along this edge, which concentrates the negative bending moment region and produces more numerous but finer cracks rather than fewer, wider ones.

The crack patterns at locations J and O, and at K and P, were broadly comparable to those at A and B, though the widths were smaller. Locations L, M, and N exhibited fewer and narrower cracks than G, H, and I, but their orientation and distribution were consistent with the same underlying flexural mechanism, indicating that the deflection driven stress field extends across a wide region of the slab beyond the immediate vicinity of the failed column.

Taken together, the survey indicates that the dominant cause of cracking is excessive deflection arising from the failure of the intermediate column, which redistributed the load path through the surrounding frame and generated tensile stresses exceeding the tensile capacity of the concrete in regions of both positive and negative bending. The combination of cracking parallel to the wall-slab joints, transverse cracking along the beam lines, and longitudinal cracking adjacent to the supporting walls is characteristic of a structural system that has progressively adapted to the loss of one of its principal vertical supports consistent with the behaviour expected following column loss in a continuous reinforced concrete frame, with the slab and beam system mobilising secondary load paths through flexure, two-way action, and rotational restraint at the supports.

SEM analysis results

The SEM-EDX analysis of samples extracted from the soffit of the slab and beam is presented in Figure 6 (a)-(c). The micrographs revealed distinct needle-like crystalline formations within the deteriorated concrete matrix, with morphology consistent with ettringite as reported in previous studies and confirmed by the elemental peaks observed in the EDX spectra (Mirvalad, 2013; Mittermayr et al., 2012).OPEN IN VIEWER

The stability of ettringite is strongly influenced by the alkalinity of the surrounding pore solution. Day (1992) reported that ettringite crystals remain stable at pH 11.5–11.8, whereas at pH 12.5–12.8 the same composition tends to occur in a non-crystalline phase, and Hadigheh et al. (2017) similarly observed ettringite formation in the range pH 12.0–12.5. Although ettringite rarely develops on the external surfaces of sewer structures, where surface pH can fall to as low as 3 owing to biofilms of sulphur-oxidising bacteria, the pore solution within sound concrete typically maintains a pH close to 13 (Justin et al., 2025; Wang et al., 2017). This high internal alkalinity favours the crystallisation of ettringite deeper within the matrix, even where the external surface has been acidified.

The formation of ettringite from the reaction of calcium aluminate phases (C₃A) with calcium hydroxide and sulphate ions is well known to cause significant volumetric expansion. Lafuma (1929) noted that calcium aluminate phases are difficult to distinguish when coexisting with calcium hydroxide, but that their solid-state conversion to ettringite can produce a volume increase of up to eight-fold (Mehta, 1973). The internal tensile stresses generated can exceed the tensile strength of the cement matrix, leading to micro-cracking, mechanical degradation, and ultimately loss of structural integrity (Hadigheh et al., 2017; Mehta, 1973).

In the present case, the operational environment of the equalisation tank is highly conducive to sulphate attack of the type associated with microbially induced concrete corrosion (MICC), shown schematically in Figure 7(a) and (b). Hydrogen sulphide (H₂S) gas, generated within the headspace by the metabolic activity of sulphate-reducing bacteria in the wastewater, condenses on the soffits of the slab and beams, where it is oxidised by sulphur-oxidising bacteria to biogenic sulphuric acid (H₂SO₄). The acid attacks the cementitious matrix, depletes surface alkalinity, and drives sulphate ions into the concrete, where they react with calcium aluminate phases to form expansive ettringite. This biogenic mechanism is significantly more aggressive than conventional liquid-phase sulphate attack, owing to the localised acidic environment and continuous regeneration of reactants at the gas-concrete interface (Woyciechowski et al., 2021).OPEN IN VIEWER

The identification of ettringite in the SEM samples therefore confirms that the structural members of the tank, particularly the soffits of the beams and slabs, continuously exposed to the H₂S-laden headspace have undergone sulphate induced microstructural deterioration. This finding is consistent with the visible distress observed during the site inspection: surface concrete deterioration on the wall and column elements, most pronounced near the roof slab as described in the visual inspection of the tank section, and the development of cracking in the soffit regions of the beams and slabs.

Structural analysis results

The results of the finite element analysis for Case 1 (deteriorated pre-rehabilitation condition) and Case 2 (post-rehabilitation condition incorporating the reconstructed column and CFRP strengthening) are summarised in Table 1. The numerical predictions are in close agreement with the crack patterns observed during the on-site survey. Elevated hogging moments were predicted at locations C and E, coinciding with the regions of severe transverse cracking (up to 0.75 mm wide) recorded along the X direction. Slab panels E-D/1-3 and D-C/1-3 exhibited pronounced sagging in both the X and Y directions, producing large positive bending moments and a maximum predicted deflection of approximately 356 mm in the most heavily distressed panel.

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Table 1.

Comparison of slab bending moments, shear forces, and deflections before and after rehabilitation.

Bending moments in top of the roof slab - X direction (pre and post rehabilitation) • Higher and more localised negative bending moments in the X-direction at the slab top were reduced and became more uniformly distributed after rehabilitation	Bending moment in top of the slab - Y direction (pre and post rehabilitation) • Pronounced and irregular negative bending moments in the Y-direction at the slab top were significantly reduced and redistributed more uniformly after rehabilitation
Bending moment in bottom of the slab - X direction (pre and post rehabilitation) • Severe localised positive bending moments in the X-direction at the slab bottom were mitigated after intervention, with stresses redistributed across adjacent panels, reflecting improved stiffness and load continuity	Bending moment in bottom of the slab - Y direction (pre and post rehabilitation) • High and highly localised positive bending moments in the X-direction at the slab soffit were significantly reduced and redistributed more uniformly after rehabilitation
Slab deflection (pre and post rehabilitation) • The existing condition exhibited severe and uneven slab deflections concentrated near the failed column, whereas column reconstruction and CFRP strengthening substantially reduced deflections and produced a more uniform deformation profile, indicating effective stiffness restoration and global structural stabilisation	Shear in slab (pre and post rehabilitation) • High and irregular shear demand in the slab under the existing condition was reduced and became more uniformly distributed after column reconstruction and CFRP strengthening, indicating improved load transfer and reduced stress concentration

The predicted deflection of 356 mm exceeds the field measured value of 250 mm. This difference is attributed to the use of reduced material properties that conservatively account for ageing and corrosion induced degradation, the application of ultimate limit state load combinations, and the model’s inability to capture partial composite action with adjacent slabs that may have contributed to load redistribution and reduced actual deflection. The model therefore provides a conservative envelope of structural demand, appropriate for the design of the rehabilitation strengthening. A detailed comparison of the finite element results for both configurations, including the influence of the lightweight levelling overlay, is provided in the Supplemental Material [Annex-B: Table-2 to 3].

Following reinstatement of the failed column in Case 2, the bending moments in slab panel C-D/1-3 reduced to values comparable with those in the adjacent panels. The peak negative bending moments along the Y direction at the soffit of panel C-D/1-3 were significantly reduced, indicating that reconstruction of the column had restored the vertical load transfer path and substantially relieved the localised stress concentrations in the distressed region.

Although the present study did not involve laboratory load deflection testing of isolated slab specimens, the pre and post rehabilitation finite element results provide a system level indication of stiffness recovery. Under comparable loading conditions, the rehabilitated model exhibited improved load sharing among the slab panels, a reduction in localised moment concentration, and lower predicted incremental deformation demand. These effects correspond to an increase in the effective stiffness of the slab system, achieved through the combined effect of three interventions: reinstatement of the failed column, restoration of the vertical load path, and CFRP strengthening of the deficient slab and beam regions.

The analysis further demonstrated that the columns did not require CFRP strengthening for either axial load or flexural capacity, as their reduced capacities calculated using the degraded material properties described in Section 4.2 remained sufficient under the ultimate limit state load combinations. Similarly, shear strengthening of the beams was not required, with the exception of the most heavily deflected beam adjacent to the failed column. By contrast, the severely deflected slab panels and their associated beams required CFRP strengthening to enhance both their positive and negative bending moment capacities, and to restore the effective stiffness and load carrying capacity of the roof slab system.

Direct field measurement of early strain development at the slab wall and slab beam junctions was not feasible during the inspection, owing to restricted access within the operational tank and the hazardous internal environment. Strain demand was therefore inferred through combined interpretation of crack widths, crack orientations, and the tensile moment regions predicted by the FE model. The most severe tensile cracking was observed at the wall slab and beam slab junctions, with maximum recorded crack widths of 1.25 mm and 0.75 mm respectively, coinciding closely with the zones of elevated hogging moments predicted by the analysis. Following the modelled column reconstruction and CFRP strengthening in Case 2, bending moments redistributed away from these previously distressed junction regions, consistent with reduced tensile strain concentration and improved continuity of force transfer. Although direct strain measurements were not available, the close correspondence between the crack survey observations and the FE predicted tensile demand provides a quantitative basis for identifying the critical strain concentration zones.

The structural implications of the lightweight levelling overlay were also explicitly considered in the post-rehabilitation model. Since the original slab had already developed permanent deflection, the overlay thickness was varied across the affected panels from approximately 20 mm to 250 mm to restore the surface to a level profile and prevent future rainwater ponding. The overlay was modelled in Case 2 as an additional permanent dead load applied alongside the reconstructed column and CFRP strengthened slab system. The resulting bending moments, shear forces, and deflection demand were checked against the available member capacities, accounting for the degraded material properties and CFRP strengthening contributions. Although the overlay increased the gravity load on the roof slab, the reinstated column restored the vertical load-transfer path and the CFRP system provided the additional flexural capacity required. The analysis confirmed that the rehabilitated configuration satisfied all required capacity checks, indicating that the overlay did not compromise the structural safety of the restored tank.

Root cause for the failure

The separation observed in the failed column occurred at its construction joint, whereas no comparable deterioration was identified at the corresponding joints of the other interior columns. This selective failure pattern strongly indicates that the construction joint of this particular column had not been executed to the same standard of workmanship as the others, and that the ingress of wastewater through this defective joint was the primary trigger for the subsequent deterioration.

The corrosion mechanism that developed at the joint is illustrated schematically in Figure 8. Wastewater first penetrated the column through the poorly executed joint [Figure 8(a)]. On reaching the embedded steel reinforcement, the infiltrating water initiated electrochemical corrosion of the steel [Figure 8(b)]. The volumetric expansion associated with the formation of corrosion products is well documented, with reported expansion ratios ranging from 1.2 to 6.9 depending on factors such as the local pH, oxygen availability, corrosion rate, and chloride concentration in the surrounding pore solution (Justin et al., 2025). This expansive pressure generated internal tensile stresses within the surrounding concrete [Figure 8(c)]. Once these stresses exceeded the tensile strength of the concrete, splitting cracks developed along the line of the reinforcement [Figure 8(d)]. Continued corrosion progressively widened these cracks and dislodged coarse aggregate particles from the matrix, creating localised voids in the cover concrete [Figure 8(e-f)]. These voids further increased the permeability of the section, providing an enlarged ingress path for additional wastewater and exposing a greater surface area of reinforcement to the corrosive environment [Figure 8(g)]. The resulting feedback loop corrosion driving cracking, and cracking accelerating corrosion led to the progressive disintegration of the cementitious matrix at the joint, and ultimately to the complete separation of the column into two distinct sections [Figure 8(h)].OPEN IN VIEWER

Construction joints are an essentially unavoidable feature of reinforced concrete construction, arising from practical interruptions to casting operations, delays in material supply, or deliberate design measures intended to control thermal contraction and the development of restrained shrinkage stresses. However, if such joints are not properly executed, they can form critical planes of weakness within the structure particularly in liquid retaining elements continuously exposed to water and chemically aggressive agents, where the joint represents the most vulnerable point in the load bearing path.

To ensure adequate bond strength and long-term durability across construction joints in reinforced concrete, well established surface preparation and concreting procedures must be followed when fresh concrete is placed against partially hardened (green) concrete. The joint surface should be cleaned thoroughly to remove laitance, loose particles, and surface contaminants prior to placement, and fresh concrete should be properly compacted by vibration to achieve full intimate contact with the underlying layer. In liquid retaining structures such as equalisation tanks, additional precautions are required: construction joints must be sealed using appropriate waterproof sealants or caulking compounds, and continuous PVC or rubber water stops should be embedded across the joint during construction to prevent the seepage of liquid through the interface. The case presented in this study illustrates the consequences of failing to implement these measures: a single inadequately executed joint, in an otherwise comparable structural element, became the initiation point for a corrosion-driven failure mechanism that ultimately compromised the load-bearing capacity of the entire column and led to the structural distress observed throughout the surrounding frame.

Selection of retrofitting material

The selection of fiber type for FRP strengthening is governed primarily by considerations of cost, mechanical performance, and long-term durability under the anticipated service environment. Glass fiber reinforced polymer (GFRP) composites are typically less expensive than CFRP composites and have been used extensively in conventional retrofitting applications. However, GFRP is considerably more susceptible to long-term degradation under aggressive environmental conditions particularly alkaline and acidic exposure, prolonged moisture, and combined chemical thermal action which introduces significant uncertainty regarding its sustained performance over the design service life (Ebrahimzadeh et al., 2025; He et al., 2017; Ji and Kim, 2017; Ouyang and Wan, 2009).

In the present case, FRP strengthening was required for the reconstructed column, and for the soffits of the deflected beams and slab panels. During normal tank operation, the column remains partially submerged in wastewater, while the soffits of the beams and slabs are continuously exposed to a moisture-laden headspace containing biogenic sulphuric acid produced by the microbially induced corrosion mechanism described in Section 5.2. Externally, the upper surface of the tank is exposed to direct precipitation, fluctuating temperatures, and ultraviolet radiation, with soffit temperatures previously recorded to reach approximately 42°C during the daytime (Section 2). The combination of submerged conditions, biogenic acid attack in the headspace, and external climatic exposure represents a particularly demanding service environment for any polymer-based composite.

Based on these combined exposure conditions, CFRP composites were selected as the rehabilitation material. CFRP exhibits superior resistance to alkaline, acidic, and humid environments compared with GFRP and AFRP, and its long-term durability has been demonstrated in a range of aggressive industrial settings (Hadigheh et al., 2017; Thushanthan et al., 2022). The environmental reduction factor (C = 0.85) recommended by ACI 440.2R (2017) for CFRP in aggressive exposure conditions is also less penalising than the corresponding factors for GFRP (C_E = 0.50) and AFRP (C = 0.70), reflecting the relative robustness of CFRP under such conditions. The selection of CFRP was therefore driven not only by its mechanical performance but also by its proven durability characteristics in environments comparable to that of the equalisation tank, ensuring a more reliable and longer-term rehabilitation outcome despite the higher initial material cost.

Durability and bond performance of CFRP system under aggressive exposure

The long-term effectiveness of externally bonded CFRP strengthening in wastewater infrastructure depends both on the tensile durability of the CFRP laminate and on the integrity of the concrete-CFRP bond interface. Direct durability and bond-performance evidence for the materials selected was therefore considered essential to justify the rehabilitation strategy adopted in this study.

The durability behaviour of the impregnated CFRP sheets used here was evaluated in a previously published accelerated-ageing investigation by Thushanthan and Gamage (2025). In that study, unidirectional CFRP coupons were exposed to deionised water, a highly acidic solution (pH 1), and an alkaline solution (pH 12.6) for immersion periods of up to 12 weeks at controlled temperature. After 12 weeks of conditioning, the CFRP sheets retained 84.1 % of their initial tensile strength in water, 73.8 % in the acidic solution, and 66.7 % in the alkaline solution, demonstrating that substantial residual strength was maintained even under severe accelerated conditioning. The same study proposed environment specific reduction factors of 0.90, 0.82, and 0.80 for water, acidic, and alkaline exposure respectively, broadly comparable with the single value of 0.85 recommended by ACI 440.2R (2017) for CFRP systems in aggressive environments.

In addition to the published coupon data, CFRP-concrete single-lap shear specimens were tested to evaluate the residual bond performance of the concrete-CFRP interface. These were immersed in a pH 5 acidic solution, selected to represent the moisture condensation and biogenic acidic dripping conditions experienced at the bonded concrete surface within the tank headspace. The conditioning duration of 12 weeks was considerably longer than the period during which the field-applied CFRP system remained directly exposed to the tank atmosphere prior to application of the waterproof and protective finishing layers, providing a conservative indication of bond sensitivity. The strength retention trends for the CFRP coupons and lap shear specimens are presented in Figure 9. After 12 weeks of acidic conditioning, the lap shear specimens retained approximately 88.7 % of their initial bond strength, indicating that the interface maintained substantial residual bond capacity under sustained accelerated acidic exposure.OPEN IN VIEWER

Based on these data, an environmental reduction factor of 0.80 was adopted in the CFRP strengthening design, to account for the potential long-term degradation of both the laminate and the bond interface. This value is more conservative than the 0.85 recommended by ACI 440.2R (2017), is consistent with the environment-specific factors reported by Thushanthan and Gamage (2025), and is in close agreement with Ji and Kim (2017), who proposed a reduction factor of 0.80 for CFRP following exposure to sulphuric acid. The factor was applied throughout the numerical assessment and strengthening design.

To extend the long-term performance of the CFRP system, the cured laminates were over-coated with a chemical-resistant waterproof coating, followed by a plaster protection layer, providing additional barriers against moisture and biogenic acid ingress. Elevated-temperature post-curing of the epoxy adhesive was also specified to enhance cross-linking and chemical resistance of the bond line, consistent with the recommendations of Bernaczyk et al. (2023) and Gamage et al. (2015, 2016).

It should be acknowledged that accelerated conditioning cannot reproduce every aspect of long-term field exposure, particularly sustained structural loading, cyclic stress fluctuations, microbial activity at the laminate surface, and progressive ageing of the protective coatings. Nevertheless, the experimental evidence above provides direct quantitative support for the residual tensile and bond performance of the CFRP system under severe aqueous and acidic conditioning and substantiates the use of the adopted reduction factors in the design. Long-term in-service monitoring will provide further verification over the design life of the rehabilitated structure.

Crack repair

The slab deflections had produced cracking on both the upper and lower surfaces of the roof slab [Figure 10(a)]. These cracks required repair prior to CFRP application, both to prevent further water ingress and reinforcement corrosion and to re-establish the structural continuity of the substrate, which is essential for effective force transfer into the CFRP laminate. All cracks of 0.3 mm width or greater were selected for repair.OPEN IN VIEWER

Repair was carried out by pressure-injected epoxy, selected for its ability to restore both watertightness and structural continuity across the cracked region. Specialised injection pegs (or ports) were installed along each crack at approximately 160 mm spacing [Figure 10(a)]. Each peg was configured so that one end could be connected to the injection gun nozzle while the other was inserted into the crack to provide a sealed pathway for resin flow. The pegs were temporarily anchored using a fast-curing surface sealant, which was also applied to the crack faces between adjacent pegs to prevent resin escape during injection.

A low-viscosity, two-component crack-repair epoxy was then injected through the pegs at approximately 60 psi (∼0.4 MPa), beginning at the lowest peg and progressing upwards to ensure complete penetration and filling of the crack profile to its terminus [Figure 10(b) and (c)]. After sufficient cure, the surface sealant and any residual cured epoxy were removed by mechanical grinding to restore a smooth, clean substrate suitable for CFRP application.

Epoxy injection was selected over cementitious grouting or surface sealing based on its superior performance in restoring structural integrity. Cementitious grouts are generally unsuitable for narrow cracks (less than ∼0.5 mm) owing to their inability to penetrate fine apertures and are prone to drying shrinkage that can result in incomplete bond and reopening of the repaired crack. Surface sealants, while effective in limiting water ingress, do not re-establish monolithic behaviour and cannot transfer tensile or shear stresses across the crack faces. Epoxy injection, by contrast, both prevents further ingress of aggressive agents and restores tensile and shear transfer capacity across the cracked plane; a critical requirement, since the CFRP laminate relies on the integrity of the underlying substrate for effective stress transfer.

Reconstruction of the deteriorated column

The failed column was partially demolished, with the upper 1.0 m and lower 1.5 m of the original column section retained in place [Figure 11(a) and (b)]. The retained sections were selected to preserve sound, undamaged concrete at the column-slab and column-base connections, while allowing the deteriorated central portion which contained the failed construction joint and the severely corroded reinforcement to be removed entirely. The remaining sections were inspected to confirm that the surrounding concrete was visibly sound and that the embedded reinforcement was free from significant corrosion new reinforcement bars were then chemically anchored into these remaining sections. The anchoring process was carried out in accordance with the manufacturer’s specifications, including the recommended drilling depths and installation procedures. Following the anchoring of reinforcement, the column was reconstructed using sulphate resistant cement concrete to enhance durability against the aggressive environmental conditions present in the equalisation tank (Figure 11(c)).OPEN IN VIEWER

Surface preparation

Preparation of epoxy resin and CFRP fabric

The impregnating epoxy was a two-component, ambient-cure structural system comprising a base resin (Part A) and a hardener (Part B). The components were measured by weight in the manufacturer-specified ratio of 2:1 (A: B) and mixed using a low-speed paddle mixer until a uniform colour and consistency were achieved, with no visible streaking. Mixing was carried out in batches sized to allow complete application within the manufacturer’s pot life at the prevailing site temperature, ensuring adequate workability and bond performance throughout the application period.

The prepared resin was applied to the substrate using brushes or short-nap rollers to produce a continuous wet film. CFRP fabric, pre-cut to the required size, was then laid onto the resin-coated substrate with the principal fiber direction aligned to the design tensile-stress direction, and a second resin layer applied to the exposed face to complete the wet lay-up. This technique is one of the most widely adopted approaches in CFRP strengthening, owing to its low cost and suitability for in-situ works on members of varying geometry (Thushanthan et al., 2022). Ribbed plastic rollers were then passed firmly over the surface in the direction of the fibers to expel entrapped air and ensure complete fabric impregnation [Figure 12(d)], with particular attention to wrapped corners and laminate overlap zones where air entrapment is most likely. Coarse silica sand was sprinkled lightly over the wet outer epoxy while it remained tacky, providing surface roughness for mechanical interlock with the subsequent protective coating layers [Figure 12(e)]. Application proceeded continuously to avoid the formation of cold joints within the laminate.

For the roof slab, pre-impregnated CFRP laminates were used in place of wet lay-up fabrics. The slab surface offered sufficient working space for laminate placement, and pre-impregnated laminates provide more uniform resin distribution, better quality control, and higher tensile efficiency than site-impregnated fabrics. Their use ensured consistent strengthening performance in the critical regions of the roof slab while complementing the wet lay-up method applied elsewhere.

CFRP strengthening