Inspection and Assessment
An inspection and assessment is normally undertaken to determine the cause(s) of the cracks in order to determine the effective remedial strategies. If the assessment on why the crack has formed is incorrect, the selected method of repair may not work. Hence, it is important to have the cause diagnosed correctly.
Guides and References
BS EN 1504 — Products and Systems for the Protection and Repair of Concrete Structures 3 4) is mainly a comprehensive set of standards for
material requirement for various concrete repair techniques, it also covers the selection of repair methods based on principle(s) ie. the purpose to which the repair method will address the deterioration issue, right through the quality control during the repairs and maintenance thereafter.
Therefore, even BS EN 1504 calls for the proper assessment of defects before deciding on the appropriate materials and techniques used for the repairs. BS EN 1504 is broken down to the following parts:
In the context of cracks in concrete, perhaps Parts 2, 3 and 5 would be more relevant in terms of material characteristics and requirements to be used. The challenge is to produce a range of products that satisfy the repair principles and performance. Part 9 is the key document for the Engineer as it provides a structured approach to the selection of repair method after the cause of the cracks is determined.
The various steps in the process of assessment, choosing repair option, selection of product and systems, site execution and monitoring of structures described in BS EN 1504 Part 9 are illustrated below:
Site application and the associated quality control are covered in Part 10 of the Standard which gives general guidance for the preparation, application, and quality control of the selected system. Ultimately, Engineers, Material Suppliers and Contractors will benefit from the cross consultation with each other using the Standard as a reference to ensure success in a project.
Carry out Inspection and Assessment
We will take a detailed look into Steps 1, 2 and 3 here. Steps 4 to 7 will be discussed in the next Section as the topics fall appropriately under materials, repair methods and quality control during repairs.
Step 1 — Assess Structure
The objective here is to determine what has caused the cracking. It is likely that the following background information would be useful in any assessment:
Historical Information on the Cracks Lines.
Information such as when the structure was completed and when the cracks were first noticed. Environmental conditions such as weather (rain or hot weather). External influences such as premature dismantling of formwork and loading of a newly cast slab may lead to the cracks. It is important to keep records such as sequence of casting, time of casting, duration of casting and when curing commenced and stopped.
Mappinq of Crack Lines.
The pattern of the crack lines should be drawn (known as mapping) for record keeping. Often, the mapping of the crack lines may be able to provide preliminary information as to the type and nature of cracks that have been formed. Moreover, the mapping may be used later as records highlighting the quantities and locations of the repairs. It is useful at this stage to measure the widths of the crack lines and indicate it on the mapping. The UK codes which limit widths make no attempt to define the measuring techniques to be used. The most common methods currently available use graduated magnifying devices, templates or feeler gauges. These can be accurate for wide cracks, but when two people measure the width of a crack of the order of 0.1mm to 0.2mm (ie. BS 8007 limits) it is unlikely that they will obtain similar or even repeatable results. This is probably because cracks are not simple, parallel sided “canyons”. The width of a crack line would usually be wider on the surface of the concrete compared to width within the concrete and cracks may:
Cracks may be regarded as unacceptable if they
However, in the case of a structure such as an elevated water tower, it is probable that no leakage would be acceptable because the stains produced by water or calcium carbonate deposits are bound to be unsightly on the outside of the tank. In order to satisfy this onerous requirement — which is a combination of watertightness and aesthetics — it will be necessary to design the tank in one of two ways. The first would be to ensure that the tank has no racks and that all contraction and shrinkage are accommodated at movement joints fully protected by waterstops. The second and more secure solution would be to line the tank internally so that water cannot reach the cracks or joints;
Affect the durability of the structure (If cracks allow the ingress of carbon dioxide (and chlorides if present)) and this will break down the passive state of the steel surface, an electrolytic cell will be established at or near the crack and thus corrosion of the steel will take place. Various studies(3 3 (38a have revealed different views of the impact of crack width and the direction of crack on the impact of corrosion of the reinforcement. Still, the fact remains when favourable conditions exist, ie. the presence of supply of oxygen and water, cracks that intersect to the reinforcement (whether cracks which follow the line of reinforcement (coincident cracks) or cracks which cross the line of reinforcement (intersecting cracks) will initiate reinforcement corrosion.
Are structurally significant (not considered here)
Photographs of Crack Lines
Photographs of the crack lines, both general and close-ups, should be taken as much as possible for record keeping, also, the pattern of the crack lines may provide information of the possible causes.
In the above photograph, slanted crack lines were noted to on the column and wall. Core samples were extracted on the cracks itself. The cracks were found to have penetrated the plaster layer into the surface of the concrete as shown in the photograph below.
Extraction of Samples
Quite often, core samples are taken at pre-determined locations of the crack lines after reviewing the historical information and results of the mapping survey. Extraction is usually in the form of taking core samples. The diameter and depth of the core will vary depending on the physical dimensions of the structure such as thickness of the component (beam or slab) and the width of crack. One of the most useful information that can be obtained from the core samples is the depth of the crack from the surface of the oonwete.
Testing would provide information such as depth of concrete cover to reinforcement, depth of carbonated concrete from the concrete surface, amount of chloride content in concrete, likelihood of corrosion of reinforcement using half-cell potential measurement, petrographic examination to determine alkali-silica reaction.
Measuring depth of carbonation
The depth of carbonated concrete can be measured by spraying a phenolphalein solution on the freshly extracted core sample. The carbonated zone will remain colourless while the non-carbonated zone will remain fuscia in colour. This test is covered by BS EN 14630, Products ance systems for the protection and repair of concrete structures, Test IVlethods, Determination of carbonation depth in hardened concrete.
Where the depth of carbonated concrete has exceeded the depth of concrete cover, the passive oxide layer surrouncling the steel bar is de- passivated, allowing the initiation of corrosion of the steel bar to take place The application of an anti-carbonation coating will not be of use at this stage.
Measurinq chloride content
The chloride content in the concrete sample can be measured using the titration method in the laboratory covered in BS 1881-124: 2015 Testing concrete, Methods of analysis of harclened concrete.
In chloride-free content concrete the steel is protected by a passive oxide film that is formed in the highly alkaline environment of cement paste (> pH 13). This passive film is being continuously corroded, but also continuously reinstated by the hydroxyl ions in the pore water. However, when chloi ides reach the steel, they act as a catalyst to accelerate the breakdown of the passive layers. This process begins at very low chloride contents, but it is only when the chloride ions reach a critical level in relation to the hydroxyl ions in the pore water that the rate of breakdown of the passive film exceeds the rate of replenishment, and significant corrosion begins. The chloride content at which this occurs is known as the ‘threshold’ level.
A threshold value of 0.4% chloride (by binder weight) has evolved in many countries, but it is believed to err on the side of safety. Reported threshold levels vary from 0.25% to 2.5% (a tenfold difference) indicating that there is unlikely to be a fixed value for a given concrete. Experts have suggested that the chloride threshold level is best considered in terms of corrosion risk. This approach was suggested by Browne, who proposed the risk classification in the below table:
|Chloride (% by weiqht of binder)||Risk of corrosion|
|0.4 to 1.0||Possible|
|1.0 to 20||Probable|
|Greate than 20||Certain|
The concrete coi e is typically cut in equal increment of thickness starting front the surface reaching the depth of concrete cover. Hence, a concrete with a concrete cover of 50mm may but cut in 20miv increments. Each 20mm increment would then be tested for the chloride content.
A chloride content exceeding the threshold value at the steel location would suggest the increased risk of corrosion.
When excessive chlorides are present, it must be remembered that the corrosive process cannot be simply chemically arrested by, for example, sealing the surface; chloride attack can continue slowly with the presence of oxygen and moisture. All the concrete adjacent to the cracks and surrounding the affected steel must be removed. The steel must be thoroughly de-rusted and protected from further attack, and concrete replaced with layers of repair mortar. However, it is quite likely that some further areas of the concrete will subsequently crack due to the incipient anode effect; thus a decision will have to be made to determine how much apparently yet unaffected concrete should also be removed. This decision is usually governed by financial restraints, and pei iodic monitoring and repair is sometimes more economical than complete removal.
Measuring concrete cover and diameter of steel bar
The instrument shown in the abo\ie photograph is a covermeter. It measures the depth of the concrete cover as well as the diameter of the rebar. A more advanced type of covermeter is shown below.
Measurinq the chance of corrosion of steel bars
The instrument shown in the below photograph is a half-cell potential meter with the probe. It is used to measure the degree of reinforcement corrosion (readout in mV with reference to Cu/CuSO‹ electrode). ASTM C876-09 Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete gives the test method.
The criteria for determining degree of corrosion is shown on the table below:
|mV (Cu/CuSO4)||Chance of Corrosion|
|Greater than – 200||5%|
|– 200 tO – 350||500/«|
|– 350 to – 500||95’/o|
|Lesser than – 500||Visible evidence|
The photograph below shows a screenshot taken from the software used in Canin+ corrosion analysis manufactured by Proceq.
Measurement of uniformity / strenqth / depth of crack of concrete
The instrument shown below is the ultrasonic pulse velocity (UPV) tester. It is used to measure the uniformity of concrete and the applications can also be used to measure the strength of concrete, and in some cases the depth of cracks in concrete.
From the historical information , mapping of crack lines and photographs, extraction of samples and testing, an assessment is made on the nature and possible cause of the crack line. Other useful information if a’vailabIe, would assist in the assessment and deciding the repair options. These include:
Step 2 — Choose Options
The following options shall be taken into account in deciding the appropriate action to meet the future requirements for the life of the structure:
Step 3 — Select Repair Principles
In BS EN 1504 Part 9, the Principles governing repair of deteriorated concrete structures are set out below. For each type of deterioration discussed below, a suitable repair principle (or principles) from the list is suggested.
|Principles related to defects in concrete|
|Principle 1||Protection against ingress|
|Principle 2||Moisture control|
|Principle 3||Concrete restoration|
|Principle 4||Structural strengthening|
|Principle 5||Increasing physical resistance|
|Principle 6||Increasing resistance to chemicals|
|Principles related to reinforcement corrosion|
|Principle 7||Preserving or restoring passivity|
|Principle 8||Cathodic control|
|Principle 9||Cathodic protection|
|Control of anodic areas|
Again, in the context of cracks in concrete, perhaps Principles 1, 3, 4, 7 and 10 would be more relevant in terms of choosing the appropriate method of spam based on the principle. The methods based on the latter Principles are discussed further in the next section “Rectification of Cracks”.
Residual Structural Capacit’y
When the residual structural capacity is in doubt, always consult Engineers experienced in the strengthening design. Weakening at the point of repair (eg. due to the loss of concrete section in a compression member, or loss in cross-section of reinforcement bar due to corrosion) can be calculated through a structural design check. (Note — In a structure suffering from cracks, spalling that has been damaged by fire, the strength of both the concrete and the reinforcement bars can be significantly reduced, refer to CSTR 68, Assessment, design and repair of fire-damaged concrete structures).
An assessment to determine the cause(s) of cracks will typically involve an inspection, non-invasive (or invasive) testing and review of the defect history.
Testing may include measuring of crack widths, sampling by coring, testing of physical properties of concrete such as strength and cover of reinforcement and chemical testing of hardened concrete such as depth of carbonation and chloride content.
It is important to identify the cause(s) of the crack lines or defects. The principle (purpose) will guide the selection of the repair method as in the case of BS EN 1504 — Part 9. For CSTR No. 22, guidance to the selection of the repair method is given based on the type of crack ie. li\ie, fine, wide, fracture or multiple cracks (Table 3).
Other considerations that would influence the selection of approach and repair method are:
Rectification of Cracks
This section covers the different types of repair methods, selection of materials and monitoring of site repairs after an assessment on the cause of cracking is determined, making reference to both CSTR No. 22 and BS EN 1504. There is also a brief section discussing how the repair works carried out should be checked for effectiveness and on- going maintenance after the repair works are completed.
Guidance and Reference
Re ai s sin CSTR No 22 as a Reference Guide
GSTR No. 22 separates dormant cracks and live cracks when determining the type of repair. The following is suggested:
Dormant cracks which are unlikely to widen, close or extend further. These cracks are subdivided as follows:
Li\ie cracks which may be subject to further movement
The traditional approach to cracks considered to be structurally significant was to cut out and repair with fresh mortar or concrete. These cracks may now be injected with various materials either to rebond the substrate or to act as void fillers.
Materials used may be:
Materials used may be:
Riqid Fillers (for Dormant Cracks)
To close a crack against surface water penetration, it may be sufficient to brush in a cementitious slurry. The crack should be three to four times wider than the largest aggregate particle. The grout should be modified with a styrene butadiene or styrene acrylate polymer to increase adhesion and reduce permeability.
Cement grouts can be injected to seal cracks such as construction joints in mass concrete structures. For fine cracks, from about 0.5mm to 1.0mm wide, a specially ground, air- separated cement (all finer than 40 micron) can be used.
Sodium silicate is suitable for brushing into fine cracks on horizontal surfaces. It has also been used for injecting into fine cracks but has now been superseded by synthetic resin materials.
During the past three decades, epoxy resins have become the most used materials for injecting dormant cracks and it is claimed that some formulations can penetrate as fine as 0.01mm*3 *. Epoxies are preferred materials because:
Polyester resins are also used for injection repairs though lesser used nowadays. These materials generally cost less than epoxies and can have lower viscosity, thus achieving better penetrations. Care should be exercised when using these reins in wet conditions as bond strength can be reduced.
Synthetic latexes, such as styrene butadiene, acrylic, polyvinyl acetate, or co-polymers of these, are cheaper still than polyesters, but have considerably less strength than the resins. Care should be taken in continuous wet conditions as these latexes rely on air curing to harden. Polyvinyl acetate (PVA) should not be used under damp conditions as it is soluble. These should only be considered perhaps mainly to fill gaps or voids and where adhesion strength is not critical.
Flexible Sealants (for Live Cracks)
Plasto-elastic resin fillers are now available for sealing cracks subject to differential movements and standing water pressures.
A wide range of sealants for movement joints are available. The most commonly used are bitumen-based compounds, polysulphides and polyurethanes. There is a much wider choice of coatings and membrane materials which can be used for surface sealing of fine cracks. These and the sealant types are reviewed in under the section Repairs to live cracks.
Repairs to Dormant Cracks
Fine cracks will most commonly be repaired by injecting with an epoxy resin or other suitable synthetic resin. If the crack is structurally significant, it should always be repaired in this manner. Cracks which extend through a structure are treated by fixing injection points at intervals along the crack at the surface. The distance between the injection points is generally about two-thirds of the depth of penetration required.
Between these 2 points, the surface of the crack is sealed on both sides of the structure; epoxy and polyester resins have been used for the surface sealing. Care must be taken when fixing the injection points to ensure that they are not blocked by detritus.
Several devices are used to inject resins into cracks; the common type is are modified grease guns or sealant guns, pressure pots actuated by elastic bands to give the pumping pressure and special-purpose twin-line metering pumps. The guns and enclosed pressure pots have the limitation that the resin will begin to increase in viscosity soon after mixing and will soon harden so that it cannot be injected and may block and damage the equipment. This method is wasteful, as a proportion of the mixed resin is often unused because the equipment must be cleaned out before the hardening is too far advanced.
Twin-line metering pumps deliver resin and hardener, in correct proportions, through delivery lines to a point close to the injection nipple. The materials are thoroughly mixed continuously in a mixing head, and pumped directly into the crack. Only a small amount of the resin is retained in the mixing head and, when injection stops, this is flushed out with solVent from a tank which is part of the equipment.
Repair of a wide crack on a vertical surface will often be most practical by the injection method. Cracks on a horizontal surface can be repaired by injection but simpler methods can also be used. The crack can be treated by sealing the underside, where this is accessible. The repair material is then poured into the crack, either starting in the middle and working towards the ends, or starting at one end.
The materials used can be epoxy resins or cement grouts, the latter with or without a synthetic latex added to the mix. Cracks with a surface width of about 1 to 2mm will probably be easier to fill using an epoxy material because of the lower viscosity. Ordinary Portland cement grout added with hydraulic binders are available and hardening time reduced.
Fractures can be repaired using epoxy resin mortar or proprietary grout, noting that epoxy resin mortar are more costlier than grouts. The ad’vantages are the epoxy resin mortar are better bond to the substrate and cure more quickly.
Repairs to Live Cracks
Cracks subjected to movement even when after repairs should be treated carefully. Suffice to say, the repaired cracks will still be expanding and contracting after the repairs. CSTR No. 22 suggests the following options:
Repairs Using BS EN 1504 as a Reference
As mentioned earlier in the section under “Inspection and Assessmen1”, Principles 1, 3, 4 and 7 would be appropriate when dealing with repairs of cracks in concrete. The table below is taken from Part 9 of BS EN 1504, Tables 1& 2.
|Principle No.||Principle and its
|Methods based on the
|Principle 1 [PI]||Protection against
Reducing or preventing the ingress of adverse agents, eg. Water, other liquids, vapour, gas, chemicals and biological agents
|Applying liquid products
which penetrate the concrete and block the pore system
|1.2 Surface coating with
and without crack bridqinq ability
|1.3 Locally bandaged
|1.4 Filling cracks|
|1.5 Transferring cracks into joints*1|
|1.6 Erecting external
panels 1 (2)
|Principle 2 [MC]||Moisture Control
Adjusting and maintaining the moisture content in the concrete within a specified range of values
|2.2 Surface coating|
|2.3 Sheltering or overcIadding *1 (2)|
|Applying a potential
difference across parts of the concrete to assist or resist the passage of water through the concrete. (Not for reinforced concrete without assessment of the risk of inducing
|Principle 3 [CR]||Concrete Restoration
Restoring the original concrete of an element of the structure to the originally specified shape and function.
Restoring the concrete structure by replacing part of it
|3.1 Applying mortar by
|3.2 Recasting with
|3.3 Spraying concrete or
|3.4 Replacing elements|
|Principle 4 [SS]||Structural Strengthening
Increasing or restoring the structural load bearing capacity of an element of the concrete structure
|4.1 Adding or replacing
embedded or external reinforcing steel bars
|4.2 Installing bonded
rebars in preformed or drilled holes in the concrete
|4.3 Plate bonding|
|4.4 Adding mortar to
|4.5 Injecting cracks,
voids or interstices
|4.6 Filling cracks, voids
|4.7 Prestressing — (post-
|Principle 5 [PR]||Physical Resistance
Increasing resistance to physical or mechanical
|5.1 Overlays or coatings|
|Principle 6 [RC]||Resistance to
Increasing resistance to the concrete surface to deteriorations by chemical attack
|6.1 Overlays or coatings|
|Principle 7 [RP]||Preserving or restoring
Creating chemical conditions in which the surface of the reinforcement is
|7.1 Increasing cover to
reinforcement with additional cementitious
mortar or concrete
|maintained in or is
returned to a passive condition
|7.4 Realkalisation of
carbonated concrete by
|Principle 8 [IR]||Increasing Resistivity
Increasing the electrical resistivity of the concrete
|8.1 Limiting moisture
content by surface treatments, coatings or sheltering
|Principle 9 [CC]||Cathodic Control
Creating conditions in which potentially cathodic areas of reinforcement are unable to drive an anodic reaction.
|9.1 Limiting oxygen
content (at the cathode) by saturation or surface coating)*2*
|Principle 10 [CP]||Cathodic Protection||10.1 Applying electrical
|Principle 11 [CA]||Control of Anodic Areas
Creating conditions in which potentially anodic areas of reinforcement are unable to take part in the corrosion reaction
reinforcement with coatings containing active pigments
|11.3 Applying inhibitors to the concrete*1) 2)|
|1 These methods may make use of products and systems not covered
by the EN 1504 series
*2* Inclusion of methods in this Prestandard does not imply their approval.
Example on the use of BS EN 1504 on the condition of reinforced concrete exposed to chlorides
Reinforced concrete exposed to chlorides in sea water
How to use of BS EN 1504 to determine the proposed treatment to the steel and concrete
Photo below showing effect of water on surfaces treated with impregnation hydrophobic coating (method 2.1)
Sketch below showing crack induced in a slab by creating a cut or by installing a pressed metal prior to casting (method 1.5)
Photo below showing applying of mortar onto concrete by hand (method 3.1)
Photo below showing spraying concrete (method 3.3)
Photo below showing installed bonded rebars in drilled holes in the concrete (method 4.2)
Photo below showing structural strengthening using plate bonding (method 4.3)
Photo below showing injecting of cracks (method 4.5)
Sketch below showing various layers of a chemical resistant coating on concrete (method)
Sketch below showing the schematic process of electrochemical re-alkalisation of carbonated concrete(method 7.3)
Sketch below showing the schematic process of electrochemical chloride extraction (method 7.5)
Sketch below showing the schematic process of applying electrical potential (method 10.1) using Impressed Current Cathodic Protection
Sketch below showing the schematic process of applying electrical potential (method 10.1) using Galvanic Anodes Cathodic Protection
Photo below showing the painting of reinforcement with coating containing active pigments (method 11.1)
Photo below showing the structural strengthening of vehicle viaducts using post tensioning (method 4.)
New Concrete Repair Specifications.
ACT announces the availability of ACI 563-18: Specifications for Repair of Concrete in Buildings. ACI 563-18. Specifications for Repair of Concrete in Buildings is a reference specification that the Architect/Engineer can apply to any construction repair and rehabilitation project involving structural concrete by citing it in the Project Specifications. Mandatory requirements and optional requirements checklists are provided to assist tile Architect/Engineer in supplementing the provisions of this Specification, as required or needed, by designating or specifying individual project requirements.
The document covers general construction requirements for all repair work; shoring and bracing of the structure or member to be repaired. concrete removal and prepai ation of the concrete substrate for repair and defines common equipment and methods; materials ancl proportioning of concrete; proprietary cementitious and polymer repair materials; reinforcement; production, placing, finishing, and cui ing of repair materials; formwork performance criteria and construction; treatment of joints; embedded items; repair of surface defects; mockups; and finishing of formed and unformed surfaces. Provisions governing testing, evaluation, and acceptance of repair materials as well as acceptance of the repair worl‹ are included. Sections 9 and 10 incorporate by reference two other specifications—ACI 503.7 and ACT 506.2—into this ACI Standard to cover crack repair by epoxy injection and shotcrete, respectively.
ACI 563-18 complements ACI 562 Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures. The ACI 562 code requirements combine the Institute’s historical knowledge with state-of-the-art resources on the evaluation, repair, and rehabilitation of concrete buildings. ACI 562 provides minimum performance requirements that address the unique nature of existing building construction.
Step 4 — Choosinq Repair Method
From the above table, it can be seen that the recommended methods based on the principle are:
Step 5 — Speci*vinq Material Performance
Having chosen the method of repair based on the principle, the next step is to determine the properties of product and systems required to comply with the principles of protection and repair, refer to Annex A, Table A1 in BS EN 1504, Part 9. The table indicates the various properties to be met in the product.
The materials typically used are epoxies and cementitious based grouts or mortars for the repairs of crack lines.
The common performance characteristics and requirement for cementitious repair materials (BS EN 1504 — Part 3 : Structural and non-structural repairs) are:
The common performance characteristics and requirement for injectable materials (BS EN 1504 — Part 5 : Concrete Injection) are:
Step 6 — Carry out Repair Works
All preparation works on the concrete required for the chosen method of repair is given in BS EN 1504 Part 10, clause 7.2. The preparation works here means cleaning, roughening and concrete removal. The reinforcement may require derusting and primer application under clause 7.3.
The method statement for the chosen method of repair is not given in BS EN 1504 Part 10. However, it gi\/es pertinent points regarding the method of site application for the Engineer, Specifier or Contractor to consider. The following are to be considered for the different methods of repair:
Hand applied mortar and concrete (clause 8.2.2)
Cracks and joints (clause 8.2.6)
Surface coatings and other treatments (clause 8.2.7)
Treatinq Rusted Reinforcement (clause 3.1)
The treatment of reinforcement to prevent corrosion should comply with EN 1504 Part 7
Step 7 — Set Out On-qoinq Requirements
Before, during and after repairs, there are various observations or tests that can be carried out to ensure integrity of the repairs. BS EN 1504 — Part 10 : Table 4 list the available tests and observations for quality control. In the context of crack lines in concrete, I have highlighted relevant test or observations below:
Maintenance Followinq Completion of Protection or Repair
Useful information to be kept after the repairs are completed are:
Repairs of Crack Lines with Reference to CSTR No. 22
Repairs of Crack Lines with Reference to BS EN 1504
The following Figures and Tables were extracted from the Concrete Society Technical Report No. 22, Non-structural cracks in concrete, 1992:
Figures 1-7, 11-14, 16-21, 24-25. Tables 1-3
The following Figures were extracted from Ascent Facilities Engineering Pte Ltd, Job No. AJ195/00: Figures 8-10