Be Prepared - Is Surface Preparation for Concrete Repairs a FAD?

Prior to the commencement of concrete repairs, there are three most fundamental things to consider; surface preparation, surface preparation and surface preparation. Strong and lasting bond between a repair material and the host concrete substrate is one of the crucial aspects of durability of concrete repairs. The potential performance of the repair can be completely undermined if the surface on which it is applied has not been properly prepared.

Repair Material to Concrete Substrate – An Alien or a Monolithic Bond: In European standard (EN1504-10:2004), 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. A poorly prepared surface will result in the weak association to the repair zone, no matter how proficient and expensive the repair material might be. The repair material when applied, should not act as ‘an alien body’ to the host concrete substrate, rather, it should become an integral part of the existing concrete restoring the structure to its original monolithic strength. Lukovic et al., (2012), in their paper “Reliable Concrete Repair – A Critical Review”, highlighted that the composite system by the integration of the repair material with the existing concrete, forming a monolithic bond, would allow uniform transfer of stresses in the system.

The quality of the surface preparation of the substrate is a strong determinant of the success or failure of a project regardless of the repair material cost and quality of application technique employed. It is pointless to exert efforts to achieve good adhesion to a weak friable substrate as failure of the concrete surface is eminent in such cases. Similarly, a sound surface might result in poor adhesion if the surface is not properly prepared. The good bonding of repair material to the existing substrate predominantly relies firstly upon, the mechanical bond of a well prepared substrate and secondly, upon the chemical bond amid the repair materials. Several other factors determining the bond strength of the repair system, include exposure conditions, properties of the repair materials and concrete substrate to name a few.

Sawn Edges – Doing It Right the First Time: 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. Saw cut squared edges help contain the repair material. The saw cut edges should be roughened slightly by needle gun or hacking as polished vertical sawed face may result in poor bonding.

The geometry of the repair area should be in simple square or rectangular shapes. Sharp acute angles and re-entrant corners should be avoided (Figure 1). Some concrete repair field installers usually form excessive or tortuous edge conditions as they try to closely follow the geometry of the distressed concrete. Such complex and zigzag edge conditions often result in shrinkage stresses leading to cracking. Where saw cutting is not possible due to smaller areas, chipping tools should be used to remove concrete ensuring that the edges of the repair area are cut perpendicular to the substrate.

Figure 1: Concrete Repair Geometry. Source: ACI Webinar, 2013

Removal of Spalled Concrete: Most of the repairs require surface preparation comprising of roughening, exposure of the aggregates or removal of the damaged, delaminated and loose concrete. Regardless of the type of deterioration, all weak, flaky, unsound and disintegrated concrete must be removed. 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. The removal of concrete usually starts with saw cutting the repair boundaries. The deteriorated unsound concrete in the centre of the repair area is then removed. Breaking out and the removal of concrete progresses from the centre to the outwards towards the edges. The next step is to remove the concrete near the edges without damaging the sound concrete at the interface.

The extent of concrete removal depends on the extent of damage. Concrete may be removed by impacting methods using power tools or by hydro-demolition such as water blasting and water jetting. The most commonly used concrete removal techniques are impacting methods such as hand held percussive equipment, pneumatic breakers, chipping hammers and scabblers where repeated striking of a concrete surface with a high power tool to break the concrete is employed. Whenever unsound concrete is removed using impacting methods such as percussive power tools, the surface of the concrete might exhibit micro-cracking or bruising that will form a weak plane acting as ‘bond breakers’. It is recommended that the remaining concrete should therefore receive additional cleaning and preparation using wet sandblasting or water jetting. To avoid any micro-cracking of the concrete substrate, hydro-demolition or abrasive sand, shot or water blasting sometimes become the preferred choice for contractors. 

Concrete Surface Cleaning – Avoid Bond Breakers: After removal of the deteriorated concrete the exposed concrete substrate must be cleaned with a high pressure water washing, oil free air compressor or other appropriate methods. Normal high water washing pressure of 15 MPa or lower water jetting/water blasting pressure of 35 MPa or less can usually be adopted to clean concrete surfaces that have already been prepared by impacting concrete removal methods. Though, some might consider it a redundant step, surface cleaning is crucial to attain the robust bond between the repair material and the substrate.  Surface cleanliness is a critical step in surface preparation after the concrete removal process and prior to the commencement of repair materials application, as any dirt, debris or loose particles can act as ‘bond breakers’. Surface cleaning facilitates repair materials to have direct contact with the host concrete substrate, increasing the surface contact area and roughness of the surface, resulting in enhanced anchorage of the applied repair material.

Steel Reinforcement Surface Preparation – Reaching Behind and Between Corroded Rebars: On steel substrates there is a problem of corrosion. This normally takes the form of rust. Initiation of corrosion and de-passivation of reinforcement is only possible in the presence of water, oxygen and corrosive agents such as chlorides and carbon dioxide. The rust layer is mechanically weak, poorly bonded to the surface and must be removed prior to any application. According to the American Concrete Institute (ACI 546), all weak, spalled, severely cracked, damaged, and easily removable concrete should be chipped away from corroded reinforcement steel. All corroded steel in the repair area should be fully exposed to full circumference and thoroughly cleaned of all loose scale, corrosion deposits and other contaminants. An old rule of thumb is that at least 20- 25mm of clearance around and behind rebar is required to ensure proper cleaning, encasement and bond of repair materials that also complies to the requirements of ACI, AS, EN and other standards.

If the deterioration of concrete has been caused by corrosion of reinforcement, the products of corrosion must be removed prior to the application of the repair material, or else the repair will be fugacious. If the structural capacity of the reinforcement is compromised due to chloride contamination, it is essential to remove all rust from the steel before proceeding. Steel reinforcement should be cleaned to achieve a surface preparation equivalent to AS1627 Part 4, Class 2.5 or Part 2, Class 2. The preferred method is abrasive blasting (SSPC-SP 10/NACE No. 2) or water jetting (Vaughn O’Dea, 2011).

Exposed reinforcement in smaller repair sections can be cleaned manually by using hand or mechanical wire brush and emery paper to reach and clean behind and between the rebars. Exposure of steel reinforcement must also continue along its length until non-corroded steel is reached and continued at least 50mm beyond to show sound rust-free steel. If the steel has lost more than 25 percent of its cross-sectional area due to rusting, splicing of reinforcement bars should be carried out by butt welding the bars with backing plates, lapping the effected bars with supplemental reinforcement or by introducing coupler mechanical joints. The reinforcement bars used in repairs shall conform to the requirements of AS4671. An unbroken coat of anti-corrosion zinc rich epoxy primer is normally recommended to protect the steel reinforcement within repair mortars.

Bonding Agents – Bond Aiders or Bond Breakers: There are number of repair failures recorded when concrete surface preparation prior to repair is neglected due to a false assumption that poor surface preparation can be compensated by using a bonding agent (Bissonnette et al., 2012). Engineers specify bonding agents as a ‘belt and braces’ measure to enhance the bond at the repair interface, but it should not be considered by any means a replacement of the surface preparation. Bonding agents provide an additional step and a layer that can create a weak plane if proper instructions are not followed. If the bonding agent is allowed to cure prior to the application of the repair mortar, it would rather act as a ‘bond breaker’ than a ‘bond aider’, causing failure of the repair. Sprayed repair mortars, in particular, do not require bonding agents as the shotcrete process exhibit excellent bonding characteristics by itself.

Drunken Concrete – A Safe Compromise: 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 host concrete absorbing the moisture from the repair mortar that is in fact required for its hydration. Although, the term saturated surface dry (SSD) is somewhat subjective, yet many experts consider it a ‘safe compromise’ for 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 ‘drunk’. If the substrate is not pre-soaked thoroughly, the rate of movement of water from the repair mortar to the host concrete will be high due to the moisture imbalance between the adherent ‘substrate’ and the adhesive ‘repair mortar’. In 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 shrinkage leading to lower material strength and reduced bond strength.

Surface Preparation Safety – Be in Control of Potential Hazards: The effect of the concrete removal on the structural integrity prior to the commencement of removal of existing deteriorated concrete, must be thoroughly assessed. In case of removal of spalled concrete or damaged reinforcement of structural elements, precautionary measures must be employed by providing temporary support. During the concrete breakout and removal process, dust and debris should be contained as not to pose any hazard to the stakeholders. The areas of repair should be examined to ensure there are no electric conduits, sockets or utility connection lines embedded that might get damaged during concrete removal. All effective measures should be adopted to ensure the safety of the structure is not compromised by repair activities.

Surface Preparation Testing: The tensile pull-off adhesion test of the existing concrete should be conducted as part of the condition evaluation report. To ensure that the surface preparation procedures were followed as per the specifications, the pull-off strength of the prepared surface prior to repair application is carried out. ICRI Guideline No. 210.3-2004, “Guide to Using In-Situ Tensile Pull-off Tests to Evaluate Bond of Concrete Surface Materials” is followed by most Engineers.  In case of a significant deviation of the pull-off strength of the prepared surface from the tensile strength of the existing concrete, the result should be examined by the Engineer for additional surface preparation. Such benchmark criteria would allow the Engineer to establish and specify the realistic adhesion strength requirements for the on-site repair condition.

To prequalify the quality of a repair it is vital to evaluate the quality of surface preparation and eventually the durability of bond. This is done by conducting the direct pull off test on a representative sample area for the cured in-situ repair material. This step of surface preparation testing would verify the tensile bond strength of the repair material and the existing host concrete. During the course of the project, surface preparation need to be periodically validated using tensile pull-off test method, benchmarking Engineer’s specifications and the values obtained during prequalification of the reference sample. Vicroads, standard specifications, section 689 suggest that the mean adhesion or pull off strength to concrete substrate at 7 days should not be less than 0.75 MPa, with no individual result less than 0.65 MPa for substrate mode of tensile failure within existing concrete substrate. Bond values for shotcrete and form-and-pour repairs typically exceed 0.75MPa and, in most cases exceed 1.0 MPa. ACI 503R and VicRoads Test Method RC 252.02 are commonly used standards for pull-off testing.

Conclusion: The best of repair materials despite the best of mixing and application practices are destined to fail unless the concrete substrate is properly prepared. The intent of this article is to promote precise specifications for surface preparation rather than taking a broad generic approach. The conventional approach of surface preparation for concrete repairs such as ‘clean and sound’ should be avoided. This commonly used phrase is too ambivalent to specify the correct level of surface preparation. There is a need to go beyond the boundaries of ‘clean and sound’ approach. Field technicians and installers are the cornerstone in any concrete repair project. They must be provided with thorough technical training to enhance their skills. 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 monolithic, 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 would lead to a strong and durable repair. If you want to get the most out of the repair materials, then be prepared to prepare!

References

1-    Reny, S., 2013. Surface Preparation for Shotcrete Repair.

2-    Bissonnette, B., Vaysburd, A.M. and von Fay, K.F., 2012. Best practices for preparing concrete surfaces prior to repairs and overlays (No. MERL 12-17).

3-    Lukovic, M., Ye, G. and Van Breugel, K., 2012, July. Reliable concrete repair: A critical review. In 14th International Conference Structural Faults and Repair, Edinburgh, Scotland, UK, 3-5 July 2012.

4-    Vicroads, 2010, “Cementitious Patch Repair of Concrete”, section 689 standard specifications.

5-    Vaughn O’Dea, 2011, “How to Handle Exposed Rebar when Repairing Concrete”, Surface Preparation of Concrete Substrates A JPCL eBook

6-    American Concrete Institute, 2013, “Field Guide to Concrete Repair Application Procedures”, ACI webinar.

About the Author: Hamid Khan working presently as Product Segment Manager – Repairs and Grouts at Parchem (DuluxGroup), Australasia, holds a bachelor degree in Civil Engineering discipline. He also holds a double Master in Business and Strategy from the University of Wollongong. Hamid is certified in Concrete Technology and Construction, by City & Guilds of London Institute (UK) and is a qualified expert in concrete repair & refurbishment with 19 years of experience in the industry. Hamid is a regular speaker at various industry related National and International Conferences and Seminars. Hamid contributed to articles for Australasian Concrete Repair Association (ACRA)- Concrete Connection, Concrete Institute of Australia (CIA) – Concrete in Australia, Australasian Corrosion Association (ACA) – Corrosion and materials, The Australian Institute of Building (AIB) – Construct for Chartered Building Professionals and other leading construction magazines.

Hamid is also an active board member of Australasian Concrete Repair Association (ACRA). He was associated with Fosroc International in Dubai for 14 years taking up various roles in technical and management. Hamid’s experience comes from the Gulf, Middle East, Europe, East Asia and Central Asia.  

This paper has been published in ACRA Concrete Connections, March 2017 edition. To view full article with picture illustrations, please visit the link below:

ACRA Concrete Connections, March 2017