Paving Failure

Paving Failure

Please read the following important article in regards to the importance of Paving Maintenance

Copy of extracts published from report to international conference on concrete block paving

Erosion of jointing sand from concrete block paving.

John a. emery, BA(hons), C.Eng,MICE,FIHT,MASCE

The stability of a block paving surface is dependant upon the presense of compacted sands in its joints to provide and maintain interlock. Part of the sand, is taken up from the laying course sand as the blocks are vibrated. The remainder of the joint space is filled with fine sand brushed in from the surface and compacted as the blocks are vibrated again.

One of the fundamental requirements of any block surface layer is to prevent water penetrating the underlying structure and subgrade. Block paving relies upon a naturally occurring surface ‘sealing’ process to stabilize the surface jointing sand and to reduce the ingress of water through its joints. Experience has shown however, that this ‘sealing’ process, which relies upon the presence of materials such as dust, detritus, oil, rubber etc. being in the vicinity of the paved area, is not always effective in meeting these requirements.

In most of its varied applications, block paving relies on the retention of sand in its joints to provide interlock and hence, stability of the surface. It also reduces the ingress of water into the underlying structure. Loss of jointing sand can lead to failure of driveways surfaced with block paving, by increasing the moisture content of sub-grades and reducing their CBR values.

Various materials have been used in attempts to stabilize jointing sand, some of which provided a temporary solution to the problem. However, the most effective means of preventing erosion has been by the use of specially formulated liquid pre-polymer which retains elasticity after polymerization and is thus able to sustain the essential flexure properties of the block paving surface.

This paper also reports on recent joint investigations by British Aerospace PLC and the author into the effectiveness of the polymer in terms of its erosion protection for block subjected to VSTOL aircraft operations. Tests have shown considerable erosion resistance is provided by the polymer. The generally accepted definition of ‘interlock’, in terms of block paving, is the “the development of friction between adjacent and closely spaced blocks”. This friction is developed when sand particles have been vibrated into the joints.

Any agent of erosion of jointing sand is therefore a serious problem which may lead to surface failure.

Since the initial trials and subsequent full scale use of the polymer stabilizer other benefits provided by the materials have been recognized. These are:

  • The appearance of block paving is enhanced by imparting to it a sheen and a more uniform colouring.
  • Surface dusting from blocks is greatly reduced.
  • It provides additional resistance to chemicals which might be harmful to concrete products and reduces incidence of efflorescence.
  • Inhibits weed growth in the joints of block paving.

There is some reduction in skid resistance of the driveway surface immediately after application of the polymer stabilizer. Tests have been made on concrete blocks using the pendulum test apparatus. The average Skid Resistance Value (SRV) was found to be 52. This is an acceptable value for most needs and will improve as the polymer on the surface of the blocks is abraded by trafficking.

The range of applications for which the polymer stabilizer is proving effective is growing rapidly, varying from heavy industrial pavements, such as bus lanes, busy road intersections and large pedestrianised area, to small garden patios. The following two examples illustrate this broad range.

A turning circle at one of the runway ends at Luton International Airport which is subject to frequent movement of aircraft at full take-off thrust. To date, aircraft pavements surfaced with concrete blocks have been treated with the polymer stabilizer, at 10 airports in the UK and overseas.

A promenade area adjacent to the River Mersey at Egremont in North-West England. Clearly visible is the damage resulting from high tides and severe storms. Repairs were made later and the jointing sand stabilized with the polymer despite being inundated by tidal water to a depth of approximately 1m there has been no subsequent damage. Where block paving is subjected to high velocity and turbulent water flow, eg; around gullies, in drainage channels and on steep slopes, the action of this water can remove jointing sand and lead to unstable conditions in the laying course. Water penetration through the joints in block paving trial areas are subject to water flow was seen to produce unstable conditions in the laying course.

A recently recognized problem is that of degradation and liquefaction of certain types of laying sands resulting in failure of a number of block paving areas. The problem appears to be confined to incidents where poor quality sands or certain crushed rocks were used for the laying course. It is believed that the ingress of water through joints and breakdown of soft granular material into finer particle forms slurry which is exuded through the joints under the action of traffic. The loss of fine material by liquefaction causes areas of block paving to settle, typically, into elliptical shapes which he describes as “elephant footprints”.

To date, guidance relating to driveway surfaced with pavers has been fragmented with a bias towards ensuring that the paving units and the structural base are designed and specified accurately, whereas problems have usually been associated with the jointing and bedding sand. The majority of those filed pavements which the author has investigated would have worked had their joints been treated with RESIBLOCK.

A driveway cannot be considered to be fully specified until its maintenance regime has been developed. At one extreme, a statement of Engineering Parameters can be developed which state explicitly how the driveway will be managed through its prescribed life. At the other extreme, the driveway can be allowed to deteriorate progressively so it has zero value at the end of its design life. The reduction in future maintenance spending, even taking into account discount cash flow analysis will be greater than the initial application cost.

When the above projects are analyzed collectively, several conclusions can be drawn. The first is that whereas previous research has frequently focused upon ensuring that the driveway components remain unstressed, in fact, failure rarely occurs as a result of straightforward overloading. A surprisingly common theme which correlates particularly well with performance and quality is the behaviour of the paver joints ceasng to operate. The joints can be too wide, too narrow, unfilled or filled with inappropriate material. In any of these cases, the failure can be dramatic and sudden. The joints need to be considered in conjuction with the bedding material and care needs to taken to ensure that that the jointing material does not drop into the bedding material.
RESIBLOCK will ensure that the jointing material remains in place and would have eliminated all of the defects discussed in this report.

Bedding sands have initiated failure when the material has been too fine to permit the unimpeded flow of water. This has allowed hydrostatic pressure to develop in the bedding sand which has in turn reduced the shear strength of the sand. A total collapse of the bedding sand occurred, resulting in quicksand conditions, in which the sand adopts the rheology of a zero-shear fluid. Bedding sand failure can be prevented by the application of RESIBLOCK.

Prevents paving failure

Any block surface paving often is used time and time again, over years and years, with little or no maintenance. You may have noticed other drives, paths or patios that have started to pit and sag, it gives the impression of a lump or bump in the paving. It may be starting to occur on your block paved area. This is the early signs of PAVING FAILURE and if it worsens it could become costly.

This image illustrates an area of concrete block paving installed over a DTp type 1 sub base. Water has penetrated the paver joints and has become trapped in both the sub-base and the sand laying course material. Subsequent trafficking has led to deformation.

Once saturated, both the sub-base material and the laying course material lost stability and proved insufficiently stable to support traffic. The deformed surface is typical of concrete block paving in which the granular sub-base material has failed. The original sub-base material probably had a CBR of 40% to 60% and this would have been reduced to less than 5% in the saturated condition. Also, the laying course material has pumped to the surface and is now evident in the ruts.

The pavers have cracked as a result of the surface deformation. Pavers are installed “hand tight” and are normally able to accommodate small elastic defluxions and permanent deformations without deterioration. In experience once deflexion or deformation exceeds 10mm, breakage of the pavers is likely. In this instance, deformation was of the order of 50mm, hence the breakage.

Resiblock, Europe’s leading company specializing in providing solutions for the stabalisation of small element flexibility laid paving has over a decade of experience at airport and port side pavements. Resiblocks wealth of expertise and knowledge is now generally recognized as the most definitive in its field.

We have numerous case studies, where projects have successfully benefited from long term paver stabilization, resulting in significantly reduced maintenance, and reduction of whole life costs.

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