Concrete rectification using micro-concrete form-and-pour technique
22nd August 2019
Written by Hamid Khan
Accelerated and premature deterioration of some reinforced concrete structures during the initial years of their life cycle is of major concern to asset owners. The premature deterioration results in reduced service life of the structures. The adoption of high standards for design and construction in new concrete structures has become a major focus today. The construction industry has recently been witnessing ever-increasing expectations for concrete’s design life. This article highlights the use of high performance durable micro-concrete repair material to rectify defects in long design life structures during and after the construction phase.
Repair materials need to be able to be used in difficult locations and achieve good compaction around the reinforcing steel without any external aids such as vibration. The finished repair must provide protection against corrosive agents such as chlorides and carbon dioxide and present minimal shrinkage to reduce the risk of cracking.
What is the purpose of using fluid micro-concrete repair mortar?
The primary purpose of using the fluid micro-mortar system is to restore the structure to its original strength and integrity. The repair may include large damaged structural sections of concrete as well as for smaller locations where difficulties of access make hand or trowel applied mortars impractical and have a high associated failure rate.
How is it different from conventional concrete?
If a conventional mix of high strength concrete is used for replacement, small gaps can occur around the reinforcement steel either through poor compaction or settlement providing a potential site to initiate corrosion. To avoid these common pitfalls the repair materials need to be exceptionally fluid to completely eliminate this problem. Pre-bagged fluid micro-concrete such as Fosroc’s Renderoc LA55 Plus typically have a compressive strength of ≥75 MPa and are based on supplementary cement materials making them environmental friendly and sustainable with a low carbon foot print.
The micro-concrete fluid repair materials are produced off site in manageable 20 kg bags so they are simple and practical to use. The content of each bag is simply added to the measured amount of water and mixed to a uniform fluid consistency that can be pumped or poured into the formwork.
Do I need a bonding agent?
Because of the outstanding adhesion property of micro-concrete mortars, a bonding agent is not normally required. When repairs are to be carried out using cementitious mortars, the surfaces must be pre-wetted to achieve a Saturated Surface Dry (SSD) condition after cleaning in order to avoid the host concrete absorbing the moisture from the repair mortar that is required for its hydration. Although the term SSD is somewhat subjective, many experts consider it a ‘safe compromise’ by pre-soaking the concrete. If the concrete is dry and ‘thirsty’, pre-soaking is of utmost importance. The concrete should be thoroughly pre-soaked so that the concrete is ‘saturated’ with water. In the SSD condition, the substrate is damp and saturated but does not contain any free water on the surface. Free water at the surface must be avoided as it can impair the bond at the interface due to increased shrinkage, leading to lower material strength and reduced bond strength.
Do I need to prepare the surface when using micro-concrete repair mortar?
The best of repair materials, despite the best of mixing and application practices, are destined to fail unless the concrete substrate is properly prepared. In European standard (EN 1504- 10:2017), the term bond refers to the adhesion of the applied material or system to the concrete substrate. Hence, adhesion has an underlying importance in the repair of concrete structures. Surface preparation of the concrete substrate is considered to be the most crucial step in a concrete repair project. Surface preparation will often be pivotal in determining the overall performance and durability of a repair. A successful repair means that the resulting multi-layer system acts monolithically, ensuring long service life. Proper attention to surface preparation is essential to achieve a robust bond between repair materials and the existing concrete substrate. Only a strong bond will lead to a strong and durable repair.
How do I prepare the surface?
Regardless of the repair technique, surface preparation is essentially the same. Defective concrete is removed until sound concrete is reached.
Marking out - Mark the perimeter of the repair area. The geometry of the repair area should be in simple square or rectangular shapes.
Sawn Edges: Sawcut the perimeter of the repair. Saw cutting is used to delineate the perimeter of the repair zone. A disc type mechanical grinder is used for saw cutting the edges along the perimeter of the repair area. The right-angled saw cut to a depth of 10-15mm is recommended to avoid any feather edging and it should not be deeper than the reinforced concrete cover.
Removal of Spalled Concrete: Remove unsound and spalled concrete with an appropriate size chipping hammer. Larger hammers may cause damage to the substrate and reinforcement. Defective concrete should be broken back to a sound and dense concrete surface. Prior to the removal of any spalled concrete from a load bearing structure, certified shoring must be provided to the structure. To avoid any micro-cracking of the concrete substrate, hydro-demolition or abrasive sand, shot or water blasting sometimes become the preferred choice for specifiers.
Steel Reinforcement Surface Preparation: Clean the reinforcement with abrasive blasting as necessary. If the steel has lost more than 25 percent of its cross-sectional area due to rusting, reinstatement of reinforcement bars should be carried out by cutting out and butt welding in new bars with backing plates, or splicing and lapping the affected bars with supplemental reinforcement, or by introducing mechanical coupler joints. An unbroken coat of anti-corrosion zinc rich epoxy primer is normally recommended to provide additional protection to the steel reinforcement within repair mortars to reduce the incipient anode effect.
Can it be over-coated with protective decorative finishes?
The form and pour micro concrete mortars are smooth surfaced, well bonded and match the existing concrete structure. These are strong, durable and sustainable and will protect the reinforcement from environmental attack in the repaired areas. But in order to improve the integrity of the total structure the original concrete may need extra protection. This is achieved by preparing the substrate and applying a protective coating system to the whole structure (Figure 3). This not only protects the structure but also enhances its appearance.
Can I use it for structural strengthening?
It is an ideal choice for structural strengthening of columns, beams and other structural elements using a reinforced concrete enlargement system. Form and pour micro-concrete mortar results in structural strength, outstanding bond, low chloride and carbon dioxide diffusion, high resistance to chemicals and extremes of weathering –providing a durable lasting repair.
When do I use form and pour fluid repair mortar?
Fluid form and pour repair mortar is commonly used on vertical surfaces such as structural walls, columns, and other structural elements such as beams. It can also be used to repair slab soffits, depending on the accessibility from the above side to pour the material through holes or openings cut through the slab. It is ideal to reinstate large voids and honeycombing in reinforced concrete structures due to its fluid and self-compacting nature. The example shown in the demonstration model (Figure 6& 7) shows the unique fluidity of Fosroc Renderoc LA55 Plus and its self-compacting properties.
What are the main benefits to asset owners, designers and contractors?
Environmental friendly – The use of supplementary cementitious materials (SCMs) makes it environmental friendly and sustainable product with low carbon foot print and reduced emissions of greenhouse gases.
Enhanced durability – It is ideal for reinstatement of new construction concrete defect reinstatement conforming to the requirements of EN 1504-3 Class R4.
Structural strengthening – It can be used for strengthening of columns, beams and retaining walls using reinforced concrete enlargement system when the project involves change of use and alteration to structure.
Saves time and money – It eliminates extra steps of additional section pours. Improves site productivity due to large volume applications. It is suitable for structural repairs that can be applied in sections from 50mm to 500mm deep. Ideal to be applied in thick sections to a depth outside normal functional range of repair mortars.
Emmons, P. (2011), Field guide to concrete repair application, Surface repair using form-and-pour techniques – ACI RAP-4
Khan, H. (2017), Be Prepared - Is Surface Preparation for Concrete Repairs a Fad? Corrosion and Materials, Vol 42 No 2, pp 34-36
Hamid Khan is the Product Manager for Repairs and Grouts at Parchem, and Australasian Concrete Repair Association (ACRA) President 2017-19. This article was first published in the August 2019 edition of Corrosion Materials, Vol 44 No 3.