Frequently Asked Questions

How is Renderoc Microconcrete placed for a complete column reconstruction where the column has suffered severe corrosion and large-area concrete spalling?

Reconstructing a severely corroded reinforced concrete column with Renderoc Microconcrete by the form-and-pour method is a complete structural repair operation requiring the following sequence of works. Assessment: the structural engineer assesses the residual cross-section of the column reinforcement (inspecting each bar to measure the residual diameter and area after corrosion loss); if the reinforcement section loss is more than 20 per cent of the original bar diameter, additional reinforcement bars (lacing bars or ties) are specified by the structural engineer to be added to the column before reconstruction. Concrete removal: all deteriorated, spalled, and delaminated concrete is removed from the column by hydro-jetting or mechanical breaking — the removal must extend to sound concrete on all faces and at least 15 mm behind all corroded reinforcement bars (to allow the repair material to flow behind and bond to the full bar perimeter). Reinforcement treatment: all exposed bars are cleaned to bright metal by wire brushing or abrasive blasting, treated with Fosroc Nitoprime Zincrich (zinc-rich epoxy primer, two coats) to arrest corrosion and provide a non-reactive surface for the repair material. Substrate preparation: the existing concrete surface (the core of the column that remains sound after removal of deteriorated concrete) is prepared by grinding or hydro-blasting to expose the aggregate and remove all laitance. Formwork: full-height plywood or steel shuttering is erected and fixed to the column reinforcement at the correct cover to achieve the original column dimensions; all form joints are taped to prevent grout leakage; a small inspection window is left at the top of one face to observe the rise of the Renderoc Microconcrete during pour. Nitobond EP: apply Nitobond EP bonding agent wet-on-wet to the prepared concrete substrate immediately before the Renderoc Microconcrete pour. Pour: Renderoc Microconcrete is mixed at the grout consistency (low water:powder ratio for maximum strength — around 0.14 to 0.16 by weight) and poured continuously into the formwork from a hopper at the top, filling the full height in one continuous operation without stopping.

What is the difference between Renderoc Microconcrete and standard site-batched concrete for column repair and which provides better long-term durability?

The fundamental differences between Renderoc Microconcrete (pre-bagged polymer-modified micro concrete) and standard site-batched concrete for column repair determine their relative long-term durability in the specific aggressive conditions that caused the original column deterioration. Standard site-batched concrete for column repair (either standard M30 or M35 grade concrete from a transit mixer, or site-batched aggregate concrete) has: aggregate size up to 20 mm — often too coarse to flow through congested reinforcement in heavily damaged columns without segregation and bridging; no polymer modification — the repair concrete relies on mechanical bond only to the existing substrate, which is typically weaker than the cohesive strength of the concrete and the most common location of long-term repair failure; water:cement ratio controlled on site — but with the variability inherent in site batching (different water additions by different workers, variable aggregate moisture content), the in-place w:c ratio and the resulting concrete permeability and compressive strength are less consistent than factory-controlled pre-bagged material; and no inherent admixture to improve durability beyond standard concrete parameters. Renderoc Microconcrete addresses each of these limitations: fine aggregate (max 5 to 6 mm) flows through congested reinforcement without bridging; polymer modification provides significantly better adhesion to the existing concrete substrate than plain concrete; factory-controlled proportions ensure consistent w:c ratio, compressive strength, and permeability batch to batch; and the specifically formulated admixture package provides accelerated early strength (reducing the exposure time before the reconstructed column can accept structural load) and a corrosion inhibitor (in some grades) that migrates to the reinforcement surface and provides additional protection. For critical structural repairs where the column is load-bearing and the repair is expected to last 30 to 50 years, Renderoc Microconcrete is the more durable and reliable specification than standard site-batched concrete.

How much Renderoc Microconcrete is needed for a typical column reconstruction and how many bags are required per metre height of column reinstatement?

The volume of Renderoc Microconcrete required depends on the cross-sectional area of the repair zone (the volume of concrete that has been removed and must be replaced) and the height of the column requiring reconstruction. For a simple calculation example: a square column of 400 mm x 400 mm cross-section (total cross-sectional area of 0.16 square metres) where the outer 50 mm of concrete has been removed from all four faces — the original column face area was 0.4 x 0.4 = 0.16 m², the remaining core after 50 mm removal from all sides is 0.3 x 0.3 = 0.09 m², and the volume of removed concrete per metre of column height is (0.16 – 0.09) x 1.0 = 0.07 cubic metres per metre height. With the reinforcement cage volume subtracted (estimated at 5 to 10 per cent of the repair volume for a moderately reinforced column), the actual void volume to be filled with Renderoc Microconcrete is approximately 0.063 to 0.067 cubic metres per metre height. Renderoc Microconcrete is typically supplied in 25 kg bags with a mixed volume (at the correct water:powder ratio) of approximately 12 to 14 litres per bag (yield varies — check the specific product TDS for exact yield). To fill 0.065 cubic metres (65 litres) per metre of column height: 65 litres divided by approximately 13 litres per bag = approximately 5 bags per metre height for a 400 mm x 400 mm column with 50 mm repair depth on all faces. For larger columns or deeper damage (100 mm repair depth on all faces), the bag count scales proportionally. It is prudent to order 15 to 20 per cent more material than the calculated volume to account for wastage, overfill at the formwork top, and variations in the actual void volume.