Frequently Asked Questions

When should Renderoc DS be specified instead of Renderoc S or a site-batched concrete repair mix for a large volume column reinstatement?

For large volume column reinstatement, three options are typically considered: Renderoc DS (Fosroc pre-bagged deep section mortar), standard repair mortars (Renderoc S or HB40 in multiple thin layers), or a site-batched concrete repair mix (small-aggregate concrete or micro concrete such as Renderoc Microconcrete placed by hand or form-and-pour). The specification decision depends on the volume and depth of repair, the application method, and the quality control requirements. Renderoc DS is specified in preference to standard repair mortars (Renderoc S, HB40) when: the repair depth is greater than 50 mm and multiple thin lifts of standard repair mortar would require many cycles (each lift, wait for set, apply next lift) that would be impractical in terms of time and cost; the repair is in a congested reinforcement zone (column core repair) where the coarser aggregate of Renderoc DS flows better through the congested bar pattern than a standard fine-aggregate repair mortar; and where the lower shrinkage of the coarser formulation of Renderoc DS provides better long-term repair performance than stacked thin layers of fine-aggregate mortar. Renderoc DS is specified in preference to site-batched concrete when: the quality control and consistency of a factory-controlled pre-bagged product is preferred over site-batched concrete with variable mixing and aggregate quality; the repair volume is moderate (a few columns to a few dozen columns) where the unit cost premium of pre-bagged product over site-batched concrete is acceptable; and where the polymer modification of Renderoc DS provides adhesion performance that site-batched concrete without polymer addition cannot match. For very large volume repairs (complete column cross-section reconstruction over many metres of column height), form-and-pour with Renderoc Microconcrete or a similar micro concrete may be more economical than Renderoc DS.

How is Renderoc DS applied in a form-and-pour system for reconstructing a heavily damaged column and what is the procedure for achieving full compaction?

Form-and-pour application of Renderoc DS (or Renderoc Microconcrete) for complete column section reconstruction is the method used when the damage to the column cross-section is so extensive that the entire outer zone or even the full cross-section must be reinstated by forming the column profile with shuttering and pouring the repair mortar into the void between the formwork and the prepared concrete core. The procedure is as follows. Substrate preparation: remove all deteriorated, spalled, and delaminated concrete by hydro-jetting or mechanical breaking to expose sound concrete and all corroded reinforcement; break out concrete behind corroded reinforcement to a minimum clear depth of 15 mm behind the bar perimeter; clean and treat reinforcement with Fosroc Nitoprime Zincrich and Nitoprime Zincrich; if reinforcement section has been significantly reduced by corrosion (more than 20 per cent loss of cross-sectional area), the structural engineer must determine whether additional reinforcement is needed. Formwork: timber or steel formwork is erected to the designed column cross-section profile, supported off the existing sound concrete at the base and tied to the column reinforcement at intervals — the formwork must be leak-tight (joints sealed with foam tape or masking tape) to prevent Renderoc DS cement paste leaking out and leaving aggregate-rich voids at the formwork face. Pour: Renderoc DS is mixed to a pourable consistency (slightly wetter than hand-pack, but not so wet that aggregate settles) and poured into the formwork from the top, rodded or vibrated with an internal vibrator (pencil vibrator) as each layer is placed to ensure compaction and removal of air pockets — particularly important at the formwork face and around reinforcement. Formwork should not be struck until Renderoc DS has achieved adequate early strength for the cross-section to be self-supporting (typically 24 to 48 hours at 25 degrees Celsius). After formwork removal, apply a curing compound or wet hessian curing.

Does Renderoc DS require a bonding agent like Nitobond EP when applied in a deep section repair and how many coats of bonding agent are needed for a large-area substrate?

Yes — Nitobond EP epoxy bonding agent is required for Renderoc DS deep section repairs, and its application is arguably more critical in deep section repairs than in standard shallow repairs because: the dead weight of a deep section repair mortar (50 to 200 mm thickness at a density of approximately 2,200 kg per cubic metre — generating a self-weight of 1.1 to 4.4 kPa on the bond interface) is significantly higher than for a 20 mm thin repair, placing greater tensile demand on the bond interface; and the restrained shrinkage stress at the interface between the Renderoc DS repair and the existing concrete substrate is higher for thick sections than for thin sections (thick repairs shrink more in absolute terms as the cement hydrates and dries), generating a peeling stress at the repair edges that must be resisted by the bond. The Nitobond EP application procedure for Renderoc DS deep section repair is: after substrate preparation, apply the Nitobond EP base coat by brush to the entire prepared concrete substrate including the vertical faces of the repair pocket — one thin, continuous coat (approximately 0.2 to 0.3 kg per square metre) ensuring full coverage of all substrate surfaces that will be in contact with Renderoc DS; immediately follow with the Nitobond EP adhesive coat (second coat applied before the first coat has fully cured, wet-on-wet technique) to all surfaces; apply the first lift of Renderoc DS while the Nitobond EP adhesive coat is still wet and tacky (within the pot life of 30 to 45 minutes at 25 degrees Celsius). For large-area repairs where the Nitobond EP application and the Renderoc DS placement cannot be completed within one pot life, divide the repair area into manageable sections — apply Nitobond EP and immediately follow with Renderoc DS to each section before moving to the next.