AS 3735:2001 (R2017)
Concrete Structures Retaining Liquids
AS 3735:2001 (Reconfirmed 2017) specifies the requirements for the design and construction of concrete structures retaining liquids, including water reservoirs, water-treatment plants, sewage-treatment plants, swimming pools, fire reservoirs, and similar liquid-retaining structures. The standard supplements AS 3600 with provisions specific to the liquid-retaining application: more rigorous crack-control requirements (typically maximum 0.1 to 0.2 mm crack width depending on exposure and liquid type), tighter durability cover and exposure-class requirements, joint-sealing and watertightness provisions, and the explicit treatment of liquid pressure as a permanent action with prescribed serviceability and ultimate-limit-state combinations. AS 3735 covers ground-supported, elevated, and partially-buried liquid-retaining concrete structures, with provisions for dimensional shrinkage, thermal contraction, and prestressing where applicable. It is the deemed-to-satisfy reference for liquid-retaining concrete structures under NCC Volume One Section B and is the controlling design standard for water-utility and waste-water infrastructure in Australia. The 2001 edition with 2017 reconfirmation remains current; minor amendments have been issued covering specific construction-joint provisions.
AS 3735 is decision-controlling on a recurring category of TRSC engagement: existing liquid-retaining concrete infrastructure undergoing continuing-life recertification, leakage-driven remediation design, or change-of-service assessment (e.g. converting a fire reservoir to potable storage). The standard supplements AS 3600 — both apply, with AS 3735 providing the additional liquid-retaining provisions. Three application points matter for existing-asset practice. First, AS 3735 crack-control provisions are the most commonly decision-controlling input for continuing-life certification. The standard requires maximum crack widths of 0.1 mm for severe-exposure or aggressive-liquid retention, 0.2 mm for moderate-exposure water retention, and slightly relaxed limits for occasionally-loaded structures (fire reservoirs, surge tanks). For existing liquid-retaining concrete with measured crack widths exceeding the standard's limits — common in pre-2001 infrastructure designed under earlier standards with relaxed limits — the assessment must determine whether the observed cracking is decision-controlling for liquid containment, durability, or structural capacity. TRSC's standard methodology combines crack-width mapping (typically by photogrammetric survey with calibrated reference scales) with measured chloride profile and carbonation depth to determine whether crack-induced corrosion is active. Where crack width exceeds the AS 3735 limit but corrosion is not active, the certification can support continuing service with documented monitoring; where corrosion is active, intervention is required before re-certification. Second, AS 3735 durability provisions for liquid-retaining concrete are more stringent than AS 3600 building-application provisions because of the chronic moisture exposure on the wet face. The standard requires Class B2 exposure for water-retaining and Class C2 exposure for waste-water-retaining surfaces — both more stringent than the typical building exposure classifications. For existing infrastructure under continuing-life recertification, the assessment compares measured chloride profile and carbonation depth against the AS 3735 durability acceptance criteria, with explicit allowance for the wet-face exposure history. Pre-2001 infrastructure designed under AS 3600 building-exposure provisions frequently has cover depth that meets AS 3600 building requirements but not AS 3735 liquid-retaining requirements — and the residual life is calculated against the more stringent AS 3735 durability framework. Third, AS 3735 joint-sealing and watertightness provisions are decision-controlling for leakage-driven remediation. The standard prescribes construction-joint sealing methodology (typically Sika hydrophilic waterstops, polyurethane backer-rod and sealant systems, or epoxy-injection of cracked construction joints), and the remediation specification must select a system compatible with the retained liquid (potable water requires NSF or AS 4020 certified materials; waste-water can use a wider material range). TRSC's leakage-driven remediation specifications for liquid-retaining infrastructure cite AS 3735 joint-sealing provisions, AS 4020 product compliance, and the AS 3600 underlying capacity provisions as the combined design basis. The Saltwater Creek bridge and several Queensland water-utility reservoir engagements have used this combined framework.
Form 15 RPEQ certifications for liquid-retaining concrete structures reference AS 3735:2001 in conjunction with AS 3600:2018, with both standards governing the structural design basis. The Form 15 declaration is conditional on the structure meeting AS 3735 crack-control, durability, and watertightness provisions in addition to AS 3600 capacity requirements. For continuing-life recertification of pre-2001 liquid-retaining infrastructure, the Form 15 file documents the measured crack widths against AS 3735 acceptance criteria, the measured durability indicators against AS 3735 durability acceptance, and the engineering basis for any reduced-action assessment that relies on documented operating restrictions. Where leakage-driven remediation is part of the certification scope, the Form 15 includes the AS 3735 joint-sealing specification and the AS 4020 product compliance for potable-water applications. The certification is high-stakes for water-utility assets because liquid-containment failure has public-health and operational-continuity consequences.