Assessments

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

Stage 1 Preliminary Inspection

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.

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 / 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: Part 1: 2000 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.

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.

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  CRLS Schedule of Accreditation

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.

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:

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. A qualitative investigation of the affected surfaces, to assess the extents of penetration of any aggressive agents, beyond the obviously degraded surface zones. 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 /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

Essentially, the chemical / 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 / low pH / 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.