A Comprehensive Range of Specialist Assessments

The Highways sector have had standardised protocols for the ‘Inspection’ of all of their structures, for many years. In our opinion, all buildings and structures, particularly those that interface with 3rd parties, should also be subjected to a regime of inspections, similar to highways structures.

CRL Surveys have over 20 years’ experience undertaking inspections for Local, Regional and National Authorities and their Supply Chain Partners, for Engineering Consultants and private owners.

The UK Bridges Board with the support of the department of Transport has recently introduced a Bridge Inspector Certification Scheme as a key part of the management process. Our Engineers and Technicians are currently going through the Certification process to ensure we are able to satisfy these new industry standards.

We undertake:

Preliminary Inspection: To assess the structure for specific requirements, such as access and traffic management, to enable a higher level Inspection.

Safety Inspection: To identify obvious deficiencies which represent, or might lead to, a danger to the public and, therefore, require immediate or urgent attention.

A structure will be inspected from appropriate, safe, vantage points, for the purposes of identifying obvious deficiencies or signs of damage and deterioration that may require urgent attention or may lead to accidents or high maintenance costs, e.g. collision damage to superstructure or bridge supports, damage to parapets, spalling concrete and insecure expansion plates and the like.

General Inspection (conventionally at 24month intervals): To provide information on the physical condition of all visible elements on a highway structure.

As such an inspection of all visible parts of the structure will be undertaken, but without special access equipment or traffic management.

NB:  We will require details of the structure, its form and construction, together with copies of all previous inspection reports. We will also need details of any maintenance, repair and modifications undertaken since the last inspection of the structure.

We will undertake a desk study of all available information, to familiarise ourselves with the structure and its condition at its last inspection, prior to undertaking this inspection.

A General Inspection may result in recommendations for a subsequent Special Inspection, if specific parts of the structure require more frequent inspection to ascertain condition.

A General Inspection may also result in recommendations for subsequent monitoring, either periodic or continuous, if specific defects, e.g. cracks or deflections, need to be checked for progressive worsening.

Principal Inspection (conventionally at nominal 6yearly intervals): To provide information on the physical condition of all inspectable parts a highway structure. A principal Inspection is more comprehensive and provides more detailed information than a General Inspection.

As such a tactile inspection, at touching distance, of all inspectable parts of the structure will be undertaken, with all necessary inspection techniques employed, together with any special access equipment and traffic management required.

NB: We will require details of the structure, its form and construction, together with copies of all previous inspection reports. We will also need details of any maintenance, repair and modifications undertaken since the last inspection of the structure.

We will undertake a desk study of all available information, to familiarise ourselves with the structure and its condition at its last inspection, prior to undertaking this inspection.

A Principal Inspection may also result in recommendations for a subsequent Special Inspection, if specific parts of the structure require more frequent inspection to ascertain condition.

A Principal Inspection may also result in recommendations for subsequent monitoring, either periodic or continuous, if specific defects, e.g. cracks or deflections, need to be checked for progressive worsening.

CRL Surveys assessments follow the guidance given in Concrete Society Technical Report 68.

Unless we have undertaken a Stage 1 Preliminary Inspection, in conjunction with a Structural Engineer, and the structure deemed safe to enter, for the purposes of carrying out our work, we will assume that this has been done by others. A Stage 1 Preliminary Inspection should be undertaken by appropriate specialists, including a Structural Engineer experienced with fire damaged structures, with all necessary temporary works (propping) done prior to the following.

We can undertake a “Stage 2 Assessment of damage”, with some elements of the “Stage 3 Testing and detailed assessment” and “Stage 4 Design of repairs to structural elements”, as described in Table 1 of Concrete Society Technical Report 68.

Notwithstanding, we would point out that the following proposals are concerned only with the assessment/s of material condition and the extent of repairs required. They will therefore not constitute an appraisal of structural stability, or competence, of either individual elements forming the structure/s concerned, relevant portions of the structure/s concerned or of the whole of the structure/s concerned.

Also, the following proposals generally follow the guidance given for “the first methodology” described within Concrete Society Technical Report 68, i.e. in Section 2, “test the fire damaged concrete to directly assess concrete quality”.

To this end we propose to:

1. Carry out visual inspection and hammer testing.
2. Carry out core sampling and / or dust sampling of the concrete, with the extraction of samples of the reinforcement and the subsequent Laboratory analyses and testing of same. Analyses and testing to include: determinations of compressive strength and density (of concrete), petrographical analysis (of Concrete) and tensile strength testing (of reinforcement).
3. As the intention will presumably be to carry out concrete repairs, assuming that the damage to the structure/s is repairable (both structurally and economically), in our opinion, the assessment should include an assessment of underlying concrete condition, so that a concrete repairs strategy can be formulated with optimum life-to-first-maintenance. To this end, we also recommend:
   • Assessments of original depths of cover to reinforcement, i.e. likely cover to reformed surfaces.
   • Assessments of carbonation, beneath residual original cast surfaces.
   • Screening for chloride.

Stage 2 Assessment of Damage (Essentially Non-Destructive)

We will undertake a detailed, close-quarters tactile visual inspection and hammer test survey, as described elsewhere, but additionally focussing on Macroscopical Changes / Effects in ‘2D’. The concrete surfaces and associated debris will be assessed for evidence of severity of damage. This will include an assessment of discolouration, sooting and invasive damage to surface finishes, in addition to discolouration of the concrete surface zones. Concrete Society Technical Report 68 gives guidance on potential evidence that may be available for the assessment of the temperature attained by the fire and the severity of the exposure of the concrete. The results of the visual inspection and hammer testing will be used to prepare record drawings, onto which the condition of the surfaces will be assessed and classified, in general accordance with the guidance provided in Table 7 and Figure 20 of Concrete Society Technical Report 68. In addition, wherever possible, the depth and extent of surface loss due to spalling / delamination will be recorded.

Stage 3 Testing and Detailed Assessment (Partially Destructive)

NB: Unless otherwise requested we do not normally undertake thermal modelling or dimensional surveys for the assessment of deflections and / or lateral movements.

Macroscopical Changes / Effects at Depth:- The results of the Stage 2 Assessment of damage, and in particular the drawings prepared, will be used to focus further investigations.

In a limited number of representative locations, i.e. representative of the various elements and damage classifications identified (including, for reference un-damaged, ‘as-built’ concrete), nominal 30mm diameter concrete core samples will be prepared using a rigidly mounted rotary drill fitted with a diamond-tipped core bit.

The core samples will each be subjected to a ‘simple’ visual inspection, with record photographs prepared, to show an overview of each core and any salient features.

A representative selection of the cores will then be submitted to a UKAS Accredited laboratory for preparation and detailed petrographical analysis in accordance with the procedures described within ASTM C856.

The concrete will be analysed for general composition and condition, with attention also focussed on any features potentially indicative or suggestive of exposure to elevated temperatures.

The results of these investigations would be used to resolve in more detail the extent of concrete repairs required.

Residual Strength of Concrete: The residual strength of the concrete can be assessed by either the determination of rebound number in accordance with BS EN 12504 or by the testing of additional core samples in accordance with BS EN 12504: Part 1: 2000, with testing for saturated density in accordance with BS 1881: Part 114: 1983.

Residual Strength of Reinforcement: Selected, representative samples of the reinforcement will be cut from the structure/s concerned and submitted to a UKAS Accredited Laboratory, for preparation and testing for tensile strength in general accordance with the procedures described within BS EN 10002. In summary, nominal 600mm lengths of reinforcement would be disc-cut from the structure, subjected to testing at ambient temperature and the results then compared with the values given variously within the age related issues of BS 4449.

Stage 4 Design of Repairs to Structural Elements

NB: We generally evaluate the repairs required, in terms of probable form and approximate size, so that approximate quantities may be estimated.

Assessment of Concrete Patch-Repairs Required: Visual inspection and hammer testing, as described elsewhere will be carried out with due regard to guidance provided within BS EN 1504 and other industry guidance documents.

The Underlying Condition of the Concrete:

NB: As the intention of our investigations will be to enable concrete repairs, assuming that the damage to the structure/s is repairable, in our opinion, an assessment should include an assessment of underlying concrete condition, so that the concrete repairs strategy can be formulated with optimum life-to-first-maintenance.

We would therefore recommend assessments of; Depths of cover to the reinforcement, screening drilled dust samples for Chloride and in-situ testing for depths of carbonation

The Form of Previous or Existing Repairs

Where concrete surfaces have been subjected previously to repair, in our opinion, the repairs should be investigated and the condition and methodologies employed assessed. These investigations may provide valuable information concerning the failure of the concrete surfaces leading to the need for repair. Such investigations will also enable a future repair strategy to be formulated, to employ the successes, but avoid the failures of the past.

Furthermore, if the previous or existing repairs are to remain in-situ somebody may be required to accept liability for them and essentially warranty them for a further period, in which case the data resulting from our assessments will be essential.

Previous or existing repairs will be subjected to close visual scrutiny and hammer tested. Notes concerning the following will be made:-

1. Associations, i.e. are the repairs associated with anything in particular, e.g. structural edges, joints, or other such features
2. Relative colour/s, parent substrate vs repairs
3. Surface finish (trowelled, brushed, as-cast etc.)
4. Evidence of curing
5. Slumping and other plastic deformation
6. Displacement, or ‘as-cast’ miss-alignments
7. Cracking, micro-cracking and crazing, and any associated rust or other staining
8. Delamination / hollowness

A limited selection of representative repairs will then be broken out, at least in part, in order to assess, as far as possible, the extent of such concrete patch-repair fundamentals as:-

9. Preparatory breaking out (defects to repairs), feather-edging and squaring-off etc.
10. If present (record if not), reinforcement cleaning and priming (Cautionary Note:- Older repairs may have been applied following treatment of the reinforcement with a paint, commonly red, but sometimes others colours such as orange or white, which may be lead-based paint.
11. Concrete cleaning and priming
12. Repair material type (cementitious vs resin / proprietary vs site mixed sand: cement vs site batched concrete / gunite)
13. Repair depths and layering
14. Repair material robustness, compaction and voids (record where any voids are, e.g. behind reinforcement and / or along layer interfaces

Detailed notes and record photographs will be prepared.

Our aging, and in some instances 'new', buildings and structures commonly do not have any 'as-built' information, or if they do, it may not be correct. The maintenance, repair, refurbishment and strengthening of existing structures requires detailed and accurate 'as-built' information if projects are to progress as efficiently and safely as possible. CRL Surveys can assist in the clarification of 'as-built' details, as follows:

At selected locations the form and type of construction, together with the relationships between adjacent, discrete, elements can variously be investigated using a combination of non-destructive direct / indirect measurement, the removal of internal, finishing-panels (with due regard and safety protocols for asbestos, lead-paint, historic plasters and other potentially hazardous materials and zoonoses), together with remote scanning using various covermeters and related scanners, ultrasonics and radar.

However, where necessary, increasingly damaging and intrusive techniques, including remote inspection using various borescopes /endoscopes and breaking-out will be employed.

NB: Our intensions will be to maximise the information gathered, whilst minimising the extent of disruptive and damaging intrusion.

For Pre-cast Concrete (cladding fixings and connections), Brickwork and Other Elements with inherent cavities and voids:- Fixings, connections and ties have been available in a wide range of types, manufactured in a wide range of materials, including plastic / nylon, mild steel / galvanized mild-steel, alloy steels and stainless steel. The above instruments, other than a proprietary wall tie detector, fitted with an appropriate detector-head, will be inappropriate for the detection of fixings, other than those manufactured from mild-steel / galvanized mild-steel.

Furthermore, even where fixings were manufactured from mild-steel / galvanized mild-steel detection limits, with respect to both depth and accurate positioning, vary significantly from instrument to instrument and generally, in our experience, resolutions deteriorate significantly with depths greater than around 100mm. Furthermore, in some cases, congested reinforcement /contamination of the concrete with magnetic constituents can result in erroneous responses which can be, at best, difficult and misleading to resolve.

Therefore, at selected locations cavities or voids can be investigated using fibre-optical borescopes / endoscopes, inserted through small drilled holes.

NB: Low or poor lighting within the cavities and the limited fields of view, together with contamination of the cavities with rubble and other debris will collectively limit the resolution of the fixings.

In cases where the above cannot satisfactorily resolve the required detail, or where further investigations are considered appropriate and safe, such investigations will be carried out, using small drills / breakers and hand-held tools used carefully to breakout and expose the hidden details.

To minimise damage and unnecessary breaking-out the results from the various covermeters and related scanners, ultrasonics, radar and borescopes / endoscopes can be interrogated and used to focus on particular 'features', which can then carefully be exposed by 'keyhole' breaking-out. Once exposed, to the point where a 'feature can be satisfactorily resolved, breaking-out can be halted, the detail subjected to direct inspection and measurement and the break-out made-good.

NB: Intrusions into a structural fabric although limited, both in number and size, to maximize the amount of information gathered, whilst minimizing the amount of disruption and damage caused will represent potentially 'vulnerable-points' going forward, regardless of making-good, until the structure has been subjected to appropriate repair and maintenance.

Scheduled defect dimensions are provided for guidance purposes only and should not be used in isolation for costing purposes. The processes involved in concrete patch-repair include the preparation of some defects by cutting-out. Cutting-out is undertaken to both prepare the defects to accommodate repair materials and also to ensure that all of the defective concrete is removed and all deteriorated reinforcement is treated. Concrete patch-repairs could, therefore, be significantly different in both size and shape when compared to the defects from which they were derived. Limited exploratory cutting-out may be carried out, on some 'typical' defects in order to evaluate potential over-cut, defect to repair, but we would, nevertheless, point out that the only truly and fully accurate measure of repair quantities is that carried out once all defects have been cut-out, ready for repair.

To provide data concerning the likely extent of cutting out required we undertake the visual inspection and hammer testing, described elsewhere with due regard to guidance provided within BS EN 1504 and an array of references from the Concrete Society, CIRIA, Building Research Establishment, the Institution of Civil Engineers and the International Concrete Repair Institute.

Reinforcement at selected, representative existing spalled locations should also be chased back into sound concrete. Exposed and corroded reinforcement will be further exposed in order to assess both the extent of corrosion, in relation to the depth of carbonation and depth of chloride contamination at that location, but also to determine reinforcement bar type/s, which would be identified using the classifications described within CIRIA Special Publication 118. Reinforcement bar diameters and, if applicable, any loss of cross-section will be recorded.

In addition, for guidance, the relative dimensions and form, of potential repairs, compared to the visual defects, will be assessed, to provide, as far as practicable, data for the subsequent preparation of concrete repair bills of quantity, as described within the Concrete Repair Association, Standard Method of Measurement and Concrete Society Technical Report No.38.

In some locations, the reinforcement within selected sound areas, at progressively greater depths of cover, may be exposed and inspected for evidence of deterioration and corrosion in order to assess the likely depth at which the reinforcement is potentially at risk from deterioration and corrosion.

NB: In the case of pre-cast cladding panels, and potentially other forms of construction, an assessment of Form and Fixing may be advisable, or even necessary. Deterioration and distress may focus around particular areas of constructional detail, such as fixings, which should be resolved in order to assess reparability.

The Extents of Dilapidations, Defects and Remedial Works Required In many cases, an important part of our work is to evaluate requirements for remedial works in terms of the quantities of repair required. The following procedures are used as a means of identifying and scheduling these requirements, obviously undertaken by suitably trained and experienced Technicians.

Visual Inspection:- We undertake, as far as practicable, a full close-quarters visual inspection following the procedure described within our Documented In-house Procedure CRLS STP01.

This procedure is covered by CRL Surveys UKAS Accreditation, UKAS Ref: 2728. For further details please visit www.ukas.com

Sounding or Hammer Testing:- Concrete surfaces will, as far as practicable be subjected fully to light sounding using a lump hammer following the procedure described within our Documented In-house Procedure CRLS STP02.

This procedure is covered by CRL Surveys UKAS Accreditation, UKAS Ref: 2728. For further details please visit www.ukas.com

A lump hammer should be drawn over the concrete surfaces or used lightly to tap the surfaces. However, although a 'simple' test, it is extremely useful, but easily flawed by poor practice and Operatives should therefore consider:-

1. Thin structures / elements, such as pre-cast cladding panels and parts thereof, which may inherently sound 'hollow', and should be tested accordingly. Is the hollowness identified a detail of the panel make-up, rather than damage or distress (also consider 4, below)?
2. Drawing a hammer across or over surfaces will highlight relatively 'shallow' delamination / surface skims, with the latter either de-bonded or masking / hiding 'faults', which should be explored and resolved.
3. 'Heavier' tapping or hammering may not resolve 'shallow' hollowness, but will resolve 'deeper' hollowness, but consider 1, above.
4. Excessive tapping or hammering may cause unnecessary damage, particularly to exposed aggregate surfaces, thin finishes and edges, e.g. the edges of some pre-cast panels were cast with recesses to accept baffle / sealing strips.

Exploratory Breaking Out (concrete only):- The reinforcement at selected, representative existing spalled locations will be chased back into sound concrete. Exposed and corroded reinforcement will be further exposed in order to assess both the extent of corrosion, in relation to the depth of carbonation and depth of chloride contamination at that location, but also to determine reinforcement bar type/s, which would be identified using the classifications described within CIRIA Special Publication 118. Reinforcement bar diameters and, if applicable, any loss of cross-section will be recorded.In addition, for guidance, the relative dimensions and form, of potential repairs, compared to the visual defects, will be assessed, to provide, as far as practicable, data for the subsequent preparation of concrete repair bills of quantity, as described within the Concrete Repair Association (CRA), Standard Method of Measurement and Concrete Society Technical Report No.38.

In some locations, the reinforcement within selected sound areas, at progressively greater depths of cover, may be exposed and inspected for evidence of deterioration and corrosion in order to assess the likely depth at which the reinforcement is potentially at risk from deterioration and corrosion.

NB: In the case of pre-cast cladding panels, and potentially other forms of construction, an assessment of Form and Fixing may be advisable, or even necessary. Deterioration and distress may focus around particular areas of constructional detail, such as fixings, which should be resolved in order to assess reparability.

The potential for reinforcement corrosion will be assessed using the measurement of half-cell potential, as detailed within American Standards for Testing Materials (ASTM) C876, with the rate of reinforcement corrosion assessed by the measurement of resistivity in general accordance with the procedures described within BRE Digest 434 and Concrete Society Technical Report 60.

This procedure is covered by CRL Surveys UKAS Accreditation, UKAS Ref: 2728. For further details please visit www.ukas.com

Half-cell potential surveying will generally be carried out using either a copper/copper sulphate, or a silver/silver chloride half-cell, the latter with values recorded as equivalent copper/copper sulphate.Further information concerning these procedures can be found in BRE Digest 434, Transport and Road Research Laboratory (TRRL) Application Guide (AG) 9 and The Concrete Society's Concrete Bridge Development Group, Technical Guide No.2.

The continuity of the reinforcement across the area to be surveyed will be checked and electrical resistance measured. Connections to the reinforcement, suitable for the areas of continuity will then be made.

Measurement node points, arranged in an orthogonal grid, to suit the subject, will be treated with an appropriate wetting agent and the potential values will then be recorded.

The potential values, where appropriate, will be tabulated and then used to plot colour coded, iso-potential, contour maps.

Resistivity surveying will be carried out using a Wenner type, four-electrode resistivity meter.

The electrodes will be placed on the concrete surface, using the same arrangement of node points as for the half-cell potential survey above, but only using the most 'anodic' and 'cathodic' nodes, and the resistivity values will be recorded.

In addition to the above and currently outside our UKAS Schedule of Accreditation, Linear Polarisation Resistance can be measured, using a BAC Corrosion Control Limited Portable LPR kit.

Measurements would be made on the concrete surfaces at the most 'anodic' and 'cathodic' nodes highlighted during the half-cell potential survey and the corrosion rate values would be recorded.

In addition to the above and currently outside our UKAS Schedule of Accreditation, Site-specific Validation can be undertaken, specifically to check and balance the instrumental data. The reinforcement within selected 'hot' or anodic and 'cold' or cathodic areas can be exposed and inspected for evidence of deterioration and corrosion. The reinforcement, at progressively greater depths of cover could be exposed and inspected in order to assess the likely depth at which the reinforcement is potentially at risk.

Reinforcement bar types could be identified using the classifications described within CIRIA, Special Publication 118.

With over 20years experience undertaking inspections of bridges, including “Special Inspections” of post-tensioned structures CRL Surveys were well placed to assist customers with other, broadly related pre-stressed and post-tensioned structures. Consequently, following the partial collapse of a pre-stressed, wire-wound ‘Preload’ tank in 1999 and a subsequent total collapse of a post-tensioned tank in 2006 we have undertaken over 30No. “Special Inspections” of various Tank designs, from various manufactures, to both assess their structural form, long after ‘as-built’ details had been lost, together with assessing material and structural condition.

The ‘Preload’ and related systems originated in the USA during the 1940’s, with thousands of structures built under licence around the world and including the UK. Structures included ground bearing tanks, some with and some without domed roofs, water towers and bund walls. In the UK, designs differ from site to site, with structures in many cases approaching 50years old. Many UK structures were reported to have suffered corrosion of the wire-windings, critical to structural stability, early in their lives, with some re-wound and others remedially post-tensioned, some on several occasions. Consequently, potential problems and vulnerabilities can be encountered dating back to design and construction, or any of the subsequent remedial works phases, with potentially significant distress, corrosion of wires and tendons, hidden under layers of ‘repair’ and paint.

The post-tensioned tanks investigated were of later construction, many less than 10years old, comprising thin pre-cast wall panels threaded with post-tensioned cables, which were anchored within thicker wall panels or anchor-blocks. Tanks have been encountered with construction faults, where poorly aligned panels have caused problems threading tendons, causing stripping of their protective sheaths. In many cases water (in some cases seawater) has been found within the tendon ducts, with others containing effluent from inside the tanks. Corrosion of the anchors is commonly found, particularly when the outer ends have not periodically been re-greased.

Working directly for Water Authorities, or with their Supply Chain Partners, we have developed bespoke methodologies, based upon our experience undertaking Special Inspections of post-tensioned bridges, to ensure these potentially sensitive and vulnerable structures are approached in a cautious, methodical and safe manner, to optimise the data gathered and minimise damage, which could represent further vulnerabilities.

Based upon Highways Agency, Design Manual For Roads and Bridges guidance, specifically for the “Special Inspection” of post-tensioned bridges, our approach is staged, commencing with a preliminary inspection, to assess general form and overall condition, followed by a ‘desk study’ of previous inspection reports and any available design and ‘as-built’ information. In many cases historical, site-specific information is scarce.

If deemed safe to continue, bespoke, site-specific Method Statements and Risk Assessments are then prepared for a full and thorough, close-quarters, tactile visual inspection, with hammer testing and in some cases preliminary, non-destructive scanning to provide preliminary information concerning some pre-stressing or post-tensioning details. The results of this inspection are used to enable the focus of the next stage, i.e. a targeted intrusive investigation of a representative number of the structures vulnerable locations

Again, bespoke Method Statements and Risk Assessments are prepared, where required including the installation of temporary protection works. This stage of our investigations involves pin-pointing target details, by remote scanning to identify hidden components and then very careful and controlled ‘keyhole’ breaking out to expose and assess the target details.

Important note

In our opinion, any investigation of attacked, eroded or weathered surfaces, where the principal aims are to formulate a solution involving a durable repair and subsequent protection, must involve:

1. A detailed study of the environment of exposure, e.g. water, effluent and treatment process chemistry, with reference study of unaffected, protected or sheltered surfaces, so that the processes of degradation can be fully understood and factored into the design of the solution.
2. A qualitative investigation of the affected surfaces, to assess the extents of penetration of any aggressive agents, beyond the obviously degraded surface zones.
3. A quantitative investigation of the affected structures and surfaces, to assess the practicalities of undertaking the preparatory, repair and protective coatings procedures, together with an evaluation of the quantities involved (surface areas and depths).

Raw / Clean Water Processing

The combination of an increasing population, climate change and associated water shortages, together with modern requirements for drinking water quality, require some Water Authorities to use more aggressive chemistry, to adjust the composition of at least some raw waters. Raw water abstracted from some rivers, reservoirs and boreholes may be contaminated with pollutants and / or dissolved solids and the chemical treatments or dosing both facilitate efficient processing, the quality of the final product and the supply of sufficient drinking water to satisfy demand.

However, in some cases, the water passing through treatment plants has been found to be aggressive towards both the concrete forming the plant, protective coatings applied to the concrete surfaces and the various metal sluices, pipes and other mechanical services.

In our experience some affected plants had experienced upwards of 25mm surface losses in less than 5years, with protective coatings, applied to protect against degradation of the concrete substrate softening in less than 1year and completely dissolving within 2years, despite being “resistant to low pH and chemical attack”.

In our opinion, affected works, including the water passing through them, from raw inlet to final supply, and every dosing stage in between and the affected concrete surfaces should be subjected to a detailed investigation. The concrete surfaces should be assessed, in detail, to quantify the surface areas affected, the depths of degradation and penetration of any aggressive agents into the ‘sound’ concrete behind.

The water passing through the plant, at every dosing stage, should be analysed for general composition, but focussing on constituents potentially deleterious to concrete and other plant infrastructure and assessed for ‘aggressiveness’.

From this repair methodologies, such as methods of cutting-back, and accurate quantities for both cutting-back and reinstatement can be evaluated. The extents of any reinforcement corrosion can also be evaluated, together with quantities of replacement reinforcement required.

Once the chemistry of the ‘system’ is understood and the aggressiveness of the water established suitable protective coating products and systems can be considered, although considering the cost implications of shutdowns and subsequent failures, trials of various coatings could be undertaken in-situ.

Dirty Water / Effluent Processing (sewage and chemical wastes)

Essentially, the chemical and / or bacteriological degradation of effluents produces hydrogen sulphide and other gases, which would normally vent to atmosphere. However, when covered, for health, safety and odour / environmental control the gases condense on concrete surfaces above the effluents, with the condensate comprising various acids and other potentially deleterious agents. The cement matrix, binding the concrete and any vulnerable aggregate constituents, when exposed to acids, will degrade and dissolve, leaving the insoluble concrete residue nominally in-situ, unless there is a mechanism to ‘wash’ or ‘abrade’ the residue away. Once degradation or dissolution of the surfaces reaches the reinforcement it will also variously corrode and dissolve the steel. This can lead to significant section loss through walls and roofs, with the potential for structural failure, particularly when degradation occurs hidden within enclosed chambers and channels.

Although experience suggests that the rate of surface degradation outstrips the rates of penetration of potentially aggressive agents, into the ‘sound’ concrete behind, in our opinion, this should additionally be checked. Aggressive agents such as sulphate and chloride should be included, but a review of the composition of chemical wastes should be undertaken to ensure all possibilities are resolved.

An investigation, to assess, in detail, to quantify the surface areas affected, the depths of degradation and penetration of aggressive agents into the ‘sound’ concrete behind should be undertaken. From this repair methodologies, such as methods of cutting-back, and accurate quantities for both cutting-back and reinstatement can be evaluated. The extents of any reinforcement corrosion can also be evaluated, together with quantities of replacement reinforcement required.

Suitable protective coating products and systems can then be considered, although considering the cost implications of shutdowns and subsequent failures, trials of various coatings could be undertaken, in-situ.

Subsequent Surface Protection

The degradation of concrete surfaces and protective coatings exposed to aggressive waters and atmospheres is a relatively recent phenomena, in terms of the understanding of the detailed processes involved and, more importantly, the formulation, application and durability of protective coatings with sufficient resistance to attack, within what must be considered as an extremely aggressive environment. We would therefore suggest that the Customer and all other parties involved need to understand that due to the aggressive nature of the environment of exposure, both physically and chemically, applied coatings will need regular monitoring and timely maintenance, to ensure durability of the system.

There have been coatings ‘failures’ (failures and perceived failures) in these environments and we would suggest that these must be taken into consideration as a part of the design process for remediation.

Repairing and protecting concrete surfaces that will be subjected to high levels of H2S and possibly also exposure to effluents with high and / or low pH, and / or variable aggressiveness indices, is extremely difficult to achieve. Inlet works are particularly difficult because of the potential for high rates of flow and the abrasive nature of effluents.

The channels and chambers in inlet works are also particular tight which means that the usual methods for cutting back, washing down, surface preparation and the application of coatings, e.g. applying coatings by hot spray, are not practicable, with other methods potentially time consuming to ensure the quality required for success.

In order to cope with fluctuations in pH and chemical aggressiveness, we would normally recommend a Polyurea coating. However these have low adhesion to concrete (typically 0.5 MPa) which makes them unsuitable for high flow, abrasive, situations such as inlet works. On the other hand two pack epoxies have excellent bond/adhesion to concrete, although they may not be able to cope with high/low pH, or chemical aggressiveness. The latter are also rigid and any cracking in the substrate will be mirrored in the coating leading to a risk of failure.

Whatever coating is proposed we would expect to see at least some blistering, due to osmosis, which should not necessarily be considered as a defect. Such coatings can still work as effective barriers.

Whilst we are happy to offer advice on coatings based upon our experience, we are not prepared to accept design responsibility which we see as lying with a specialist Consultant. It is also worth pointing out that we can only apply coatings that are readily available in the market and whilst we will select the best available coatings, the Customer must accept that there is a risk of failure even when it has been installed 100% correctly, with extensive QA/QC records to prove it.