INSPECTION OF HIGHWAY STRUCTURES

1.  INTRODUCTION:

It is of great importance that any State Ministry of Works have an inspection regime for their fleet of assets. Without this, there would not be a proper and meaningful maintenance of the assets. But before a meaningful inspection regime can be initiated, it is  essential that the data for all the structures must be known. From the data, you will be able to establish the following:

2.  Type of structures

a)  Age of structures

b)  Design calculations for the structures: this will ensure that the structure is fit for purpose especially with increase in vehicular loading.

c)  ‘As -Built’ Drawings of the structures

 

All the above must be collated and kept for record purposes.

 

It is very disheartening that most of our highway structures exhibit a lot of structural defects, and more often than not, they are not attended to, until the defects have reached an unmanageable and alarming state of repair. When the defects are eventually attended to, the cost of remedial works would have skyrocketed. If the structures have suffered severe defects, the remedial works will require closure of the structure to traffic before they can be undertaken. This will disrupt traffic, and cause congestion due to road diversions, as the traffic would have to be diverted to another road network. And in most cases, the bridges in other networks, may have not been designed for such volume of traffic.    

 

It is therefore imperative that inspections of a fleet of structures are regularly carried out in order to capture defects early and undertake remedial works promptly. 

 

It should be mentioned that if these defects are not attended to promptly, the design life of the structures may be shortened. 

 

Bridges are amongst the largest, most expensive and complex structures, which makes them a crucial and valuable transportation asset for modern infrastructure.  Bridge inspection is an essential component of monitoring and maintaining these complex structures. It provides a safety assessment and condition documentation on a regular basis, noting maintenance actions needed to counteract defects like cracks, corrosion and spalling.  

 

Cable Stayed, prestressed and post tensioned bridges are complex structures that are constantly exposed to adversities and dynamic loading. This makes them difficult to inspect. A comprehensive study on the reliability of manual inspection has identified significant drawbacks regarding completeness and objectivity. 

Different inspection levels are defined to fulfil the purpose. They mainly differ in frequency, accuracy and duration. The lowest tier and most often performed inspection is the general inspection, which is done every two years. This inspection collects physical condition data for all visible elements. It is performed by an inspector at visible ranges and requires little traffic control. The third level of inspection is a principal inspection, which is performed every five or six years. It is an in-depth inspection that requires close examination of elements within touching distance, use of service tools and instruments like a hammer and paint thickness analyser, or an instrument to measure crack width.  These three inspection types are performed in an alternating fixed scheme.  Severe findings might require a more rigid scheme. Additional inspections are performed on an irregular basis, or as required. A special inspection provides detailed information on a particular element, damage, or area. An acceptance inspection is performed upon completion of a bridge, which primarily aims to exchange information and agree on current status for approval of a construction contract. Some countries such as Denmark and France, have a flexible inspection scheme that allows an inspector to decide on the duration until the next inspection based on inspector experience and imminent deterioration of a structure.

The different types of highway Structures are the following:

  Road Bridges and footbridges

  Buried structures 

  Subway underpass

  Culvert and any other similar structure

  Earth retaining structures

  Reinforced/strengthened soil/fill structure with hard facings

  Sign gantry and signal gantry

  Mast

  Access gantry

  Road tunnels

 

3.  PLANNING FOR AN INSPECTION:

 

a)  DESK STUDY: 

Before embarking on an inspection of a bridge structure, it is very necessary that desk study is done on the structure being planned for inspection. You need to know all the details of the structure, viz, type of structure; construction form; last inspection date; copy of last inspection; recommendations made in the last inspection; remedial works executed; structure reference number and exact location of the structure.

 

Having done all that, you will also need to have a copy of last inspection report to hand when going out for the inspection. This will help to determine if defects reported during the last inspection have deteriorated, or it the defects have been repaired. If the defects have been repaired, you will be able to determine if some of the defects are recurring. 

 

You will also determine if you will require traffic management in order to carry out the inspection, i.e., whether the road around the bridge or underneath the bridge need to be temporarily closed to be able to carry out inspection within touching distance, this is required if the inspection is a principal inspection.

 

b)  RISK ASSESSMENT:

Risk assessment must be carried out in the office prior to going out on site for the inspection. You will have to list all the risks you will be exposed to during the inspection and mitigate against each of the risk. 

 

c)  HEALTH & SAFETY:

This is one of the most aspect of inspection. You will need to carry with you all the required PPE to site. The following are required:

 

  Hard Hat

  Visibility Vest

  Boots

  Nose Mask, if necessary

  Gloves, etc

 

d)  TOOLS:

Hammer – for tapping spalled concrete to reveal if there is exposed reinforcement, or check extent of looseness of concrete.

Measuring Tape

Camera

Crack width Measure

Vernier Callipers

 

e)  WRITING MATERIALS:

Pen

Pencil

Site Notebook

Eraser

Ruler

Sharpener

 

f)  FIRST AID BOX

Paracetamol

Plaster

Bandage

Spirit

Bottle of water

 

It should be mentioned that, you should not go out alone for the inspection. You must be accompanied by another engineer to assist on site, and for health & safety reason. Lone working should be discouraged. But at times, lone working may be allowed depending on the circumstances, but the engineer must call the lead engineer, or the manager on arrival on site to inform the lead that all is well. And the inspecting engineer must make another call when the inspection is finished and heading back to the office.  

 

Concluding, it is worth mentioning that it is not all engineers that can successfully carry out the inspection of bridges, and other highway structures. You will need to have gone through rigorous training, and at the end, be able to diagnose all structural defects to a bridge structure that is in distress. You should be able to differentiate between superficial defects to structural defects. And most importantly, be able to draw conclusions and make sound recommendations for remedial works required to get the bridge fit for purpose. This is very important, without this, a sound remedial works cannot be reached. When this happens, it will defeat the purpose of inspection.

 

4.  Maintenance inspections 

g)  Inspection types 

The five types of maintenance inspection that shall be used for highway structures are: 

  safety inspection. 

  general inspection. 

  principal inspection. 

  special inspection; and, 

  inspection for assessment.

 

John Martins, [02.05.20 19:02]

NOTE Variations to the five types of maintenance inspection are provided in the National Application Annexes. Safety inspection Definition 

5.  Safety inspection Definition

Safety inspections for highway structures shall be carried out to ensure the safe and efficient identification of safety related defects. 

NOTE 1 A safety inspection provides a check of those parts of a highway structure that are visible from the highway and are usually undertaken as part of the highway safety inspection regime. 

NOTE 2 The purpose of a safety inspection is to identify significant deficiencies or signs of damage which represent, or can lead to, a danger to the public or high maintenance costs and therefore require immediate or urgent attention. For example, collision damage to superstructure or bridge supports, damage to parapets, spalling concrete or insecure expansion joint plates. 

NOTE 3 Safety inspections are not specific to highway structures and generally cover all fixed assets on the highway network, including carriageways, footways, structures, drainage, verges and lighting. 

NOTE 4: 

Variations to safety inspectionsforhighwaystructuresareprovidedintheNationalApplicationAnnexes. 

Safety inspections for highway structures should be scheduled at a frequency according to the risks and significance of a particular route or asset. 

Safety inspections should be carried out by trained highway maintenance staff. 

Safety inspections should be carried out from a slow moving vehicle. 

Alternative methods for carrying out the safety inspection may be agreed with the Overseeing Organisation. 

NOTE In certain circumstances, it is necessary for staff to proceed on foot either to confirm suspected defects or to complete the inspection. For example, some bridges require a weekly or monthly walkover.

Safety inspections of all fixed assets shall be carried out at frequencies not less than the requirements set down by the Overseeing Organisation. 

NOTE Variations to safety inspections of all fixed assets are provided in the National Application Annexes. 

The frequency of safety inspections should give due regard to any special considerations, for example, does the structure form, material, usage or location influence the required frequency and reflect the importance of a particular route or asset. 

Safety inspections may be necessary as a result of notification of a defect by a third party, e.g. police or public.

6.  Maintenance inspections 

Call out inspection 

Maintenance staff carrying out a safety inspection shall inform a member of the inspection organisation of any defects which they consider to require a subsequent call out inspection. 

The need for a call out inspection in response to a concern raised by a safety inspection shall be assessed by a member of the inspection organisation. 

Where the need for a call out inspection is confirmed it shall be undertaken by the inspection organisation with the appropriate priority according to the nature of the concern. 

NOTE A call out inspection is not categorised as a special inspection and as such the agent is not required to agree this activity with the Overseeing Organisation. 

Any resulting actions from the call out inspection shall be dealt with as an immediate risk to public safety or prioritised for action in accordance with asset management procedures.

 

John Martins, [02.05.20 19:02]

7.  General inspection Definition 

A general inspection shall comprise a visual inspection of all parts of the structure that can be inspected without the need for additional access equipment, using safe, ground level viewing positions around the structure. 

NOTE 1 The purpose of a general inspection is to provide information on the physical condition of all visible elements of a highway structure. 

NOTE 2 Variations to general inspections are provided in the National Application Annexes. 

Visual aids may be used to assist identification of defects over a distance, e.g: 1) binoculars; or, 2) cameras with telephoto lenses and a tripod. 

The inspection should include adjacent earthworks and waterways where they can affect the behaviour or stability of the structure. 

NOTE For example, riverbanks in the vicinity of a bridge examined for evidence of scour or flooding or for conditions, such as the deposition of debris or blockages to the waterway, which can lead to scour of bridge supports or flooding. 

Traffic management arrangements should reflect the class of road, traffic volumes, presence of hard shoulder and means of access from off-highway that exist at a structure. 

NOTE Guidance is provided in TSM Chapter 8 [Ref 22.N], the DfT publication ‘Safety at Street Works and Road Works’ ‘ Streetworks CoP [Ref 15.N] and IAN 161 [Ref 16.N]. 3.8.4 The inspection should be undertaken in daylight, enabling the whole structure to be inspected from a number of safe viewing points or through the use of fixed access facilities, e.g. inspection galleries and steps. NOTE Other planned daytime operations, e.g. litter clearance, can afford sufficient daytime traffic management opportunities to provide safe viewing positions for inspection. 

The inspection may take place under street lighting if there is a sufficient level of lighting:

  to show all parts of the structure; and, 

  to allow any defects to be identified from the viewing positions. 

External task lighting may be used subject to suitable controls to prevent dazzling of road users. 

Frequency 

General inspections shall be undertaken at intervals of 24 months. 

Tolerance on the due date for inspection may be permitted for the determination of the scheduled date for inspection in accordance with section

 

John Martins, [02.05.20 19:02]

8.  Principal inspection 

Definition 

A principal inspection shall comprise a close examination, within touching distance, of all accessible parts of a structure. 

NOTE 1 The purpose of a principal inspection is to provide information on the physical condition of all accessible parts of a highway structure. 

NOTE 2 A principal inspection is more comprehensive and provides more detailed information than a general inspection. 

NOTE 3 Variations to principal inspections are provided in the National Application Annexes. 

The inspection should include adjacent earthworks and waterways where they could affect the behaviour or stability of the structure. 

NOTE For example, riverbanks in the vicinity of a bridge examined for evidence of scour or flooding or for conditions, such as the deposition of debris or blockages to the waterway, which could lead to scour of bridge supports or flooding. 

The inspection should include removal of inspection covers and opening of hatches and enclosures to access hidden components. 

NOTE See Section 10 for further advice on confined spaces. 

A principal inspection shall utilise suitable means of safe access and/or traffic management works where required. 

NOTE The agent can propose, for the agreement of the TAA, the use of alternative means of access for areas of difficult or dangerous access in accordance with Section 7, e.g. obscured parts of a structure and/or confined spaces. 

A principal inspection shall utilise suitable inspection techniques including hammer tapping to detect loose concrete cover and repeated thickness measurements of weathering steel in accordance with CD 361 [Ref 23.N]. 

NOTE Intrusive testing is not a requirement for a principal inspection, but can be recommended for further investigation, e.g. during a special inspection, or incorporated into the principal inspection under agreement with the agent as a result of previously identified defects. 

 A principal inspection shall include the measurement of minimum headroom for the following areas, where present, at the down-chain and up-chain edges of the structure for: 

  each structure free zone (verge)

  the paved width including separate measurements for: 

  each running lane; b) each hard strip or hard shoulder.

A principal inspection shall be a replacement of a due general inspection. 

Principal inspections shall be undertaken at intervals of 72 months unless a longer interval has been agreed by the Overseeing Organisation.

Tolerance on the due date for inspection may be permitted for the determination of the scheduled date for inspection in accordance with section 4. 

A longer interval between principal inspections may be agreed with the Overseeing Organisation providing the proposal is supported by a risk assessment in accordance with Section 8. 

9.  Special inspection Definition 

A special inspection shall provide detailed information on a particular element, part, area or defect that is causing concern, or inspection of which is beyond the requirements of the general and principal inspection regime.

Agreement and reviews 

All special inspections shall be agreed by the Technical Approval Authority (TAA) before being implemented by the agent, and their justification recorded on the asset information management system of the Overseeing Organisation. 

NOTE Variations to special inspections are provided in the National Application Annexes. 

Where a series of special inspections is proposed at a particular frequency over a period of time the total number and frequency shall be subject to initial agreement with the Technical Approval Authority (TAA). 

A review of the justification and frequency of a series of special inspections shall be subject to regular agreement with the Technical Approval Authority (TAA), in light of information gathered during the course of the special inspections, at intervals no greater than: 

  12 months; or, 2) after six special inspections since the last review.

10.  Suitable situations

 

John Martins, [02.05.20 19:02]

Once agreed, special inspections shall be tailored to a specific need to gather information on the condition or details of the structure. 

NOTE A special inspection can be a suitable response to: 

  the specific characteristics of the structure, for examples the material or structural form. 

  a recommendation identified by a maintenance inspection. 

  certain events, such as the transit of a significant abnormal load, or a flood; or, 

  a recommendation to consider parts of the structure more closely or at a more frequent interval than the normal general and principal inspection regime. 

Special inspections may be suitable for the following situations: 

  a structure known or suspected to be subject to a rapid change in condition or circumstance. 

  a general inspection is not sufficient to provide the access or information required. 

  visible defects not explained by a single cause or where the extent of a defect needs to be ascertained; periodic or continuous monitoring to check against a specific problem from worsening, e.g. crack growth and deformations.

  cast iron structures, at intervals not exceeding six months. 

  structures strengthened by the use of bonded plates, at intervals of six months for the first two years and thereafter in accordance with the intervals prescribed in the maintenance records. 

  structures that have weight restrictions, or other forms of restriction to reduce traffic loading, at intervals not exceeding six months or as agreed by the Overseeing Organisation. 

  structures that have to carry an abnormal heavy load are to be inspected before, during and after the passage of the load if either:

11.  Maintenance inspections

an assessment has indicated that the margin of safety is below that which could be provided for a design to current standards. 

  similar loads are not known to have been carried. 

  structures in areas of mineral extraction when subsidence occurs. 

  structures where settlement is observed greater than that allowed for in the design. The cause is to be identified and steps taken to monitor the rate of settlement and to assess the urgency of remedial measures. 

  to investigate the damage to structures involved in a major incident, chemical spillage or fire. 

  bridge piers situated in a fast flowing river, Probing of river bridge foundations after flooding. Where probing indicates the possibility of scour, further underwater inspection is to be carried out; 13) permanent access gantries prior to use and at intervals in accordance with The Institution of Structural Engineers report on The Operation and Maintenance of Bridge Access Gantries and Runways; Gantries & Runways. 

  hoists, winches and associated cables are to be inspected in accordance with the relevant chapters of the Factories Act; 15) post-tensioned concrete bridges.

I] NOTE Further examples of when special inspections are recommended are given in the Inspection Manual for Highway Structures IMHS [Ref 5.I]. 3.20.2 A special inspection may comprise the following in any combination:

  a close visual inspection. 

  testing and measurements or. 

  monitoring. 

12.  Special inspections may be planned as: 

  a one-off inspection. 

  a discrete series of inspections; or 

  an ongoing programme of inspections. 

NOTE 1 Refer to CS 470 [Ref 12.N] for further guidance associated with the monitoring and management of substandard structures. 

NOTE 2 Guidance on limited site testing that can be undertaken as part of a special inspection for concrete structures, i.e. half-cell potential, chloride level, cover meter and depth of carbonation, is provided in CD 359 [Ref 3.I]. 

NOTE 3 The wall thickness of steel hollow sections can diminish through internal corrosion and can go unnoticed. The non-destructive measurement of uncorroded thickness of sections at critical areas, e.g. base of parapet posts, rails near ends or joints, is recommended for elements: 1) which exceed or are approaching the end of their design life; or 

  where poor detailing for durability is evident; or

 

John Martins, [02.05.20 19:02]

  where there are visible signs of distress or deterioration. 

NOTE 4 See also NAA’s for exceptions to Note 3 where some items are included in PI coverage.

13.  Underwater inspection 

A programme of underwater inspections shall be implemented for structures where the foundations and parts of the structure are below water. 

NOTE An underwater inspection is a specific type of special inspection concerned with parts of highway structures that are below water level.

An underwater inspection shall record the condition of the structure below water level, the existing stream bed profiles and any evidence of scour in accordance with BD 97 [Ref 18.N]. 

Photographic records provided by remotely operated vehicles or unmanned underwater vehicles shall be in accordance with the requirements of photographic or video records given in Section 5. 

14.  Inspection for assessment 

An inspection for assessment shall provide the information required to undertake a structural assessment. 

Inspection for assessment shall be undertaken following the requirements given in CS 454 [Ref 3.N]. Headroom measurements 

The equipment and method used for headroom measurement shall have a tolerance of not greater than +/- 10mm. 

Any headroom deficiencies shall be managed and notified to the Overseeing Organisation in accordance with CD 127 [Ref 4.N]. 3.27.1 Where there is any evidence of collision by road vehicles or waterborne vessels these should be documented including dimensioned sketches and photographs to show the extent and location of the damage and the members affected. 3.28 Any safety concerns as a result of collisions, e.g. significant deformations or damaged concrete or reinforcement, shall be reported immediately in accordance with the procedure for an immediate risk to public safety. 3.29 The condition of any low headroom warning signs attached to the structure shall be recorded, in accordance with CS 125 [Ref 10.N].

References:

The Deign Manual for Road and Bridges (DMRB): The following DMRB Standards can be downloaded from Highways England Website on Google.

CS 450: Inspection of Highway Structures Standard. 

BD54/15 – Management of Post-Tensioned Concrete Bridges

CS 125 – Inspection of traffic signs

CS 470 – Management of sub-standard highway structures

CD 361 – Weathering steel for highway structures

CD 359 – Design requirements for permanent soffit formwork

Thank You.

Engr. John Akinola Martins.

 

DOWNLOAD REPORT

 

You can click on the above document to have a read through. That is a Principal Report on a road bridge spanning over a Railway Line.

 

The last document just posted, is a review of post tensioned flyover bridge at Hammersmith, London. This bridge reported severe corrosion to the post tensioning strands and more importantly, provided a detailed explanation of the deterioration mechanisms.