Road testing

A good skid resistant surface is extremely important for traffic safety; this applies to dry but especially to wet road surfaces.
Together with the macro texture, the skid resistance forms that complex property of the road surface which is necessary to be able to steer, accelerate, drive and brake a vehicle on the road surface. Unlike roughness and many other surface properties, road users cannot visibly determine the skid resistance quality of a road. A road user therefore cannot adjust his behaviour if the road is more slippery than expected.

Testing of wet slipperiness is therefore the most common of all. After all, the risk of accidents on wet roads greatly outweighs that on dry roads. Kiwa KOAC monitors the wet skid resistance properties with the 86% retarded wheel method described in the Netherlands Standard RAW Conditions of Contract for Works. The measuring speed is 70 or 50 km/h. We also have other monitoring solutions for pavement surfaces or pavement sections where the required measuring speed cannot be attained.

Testing of the skid resistance of dry road surfaces is particularly important for open wearing courses. In sthese layers, the bitumen film around the mineral aggregate has not yet been worn off shortly after construction, so that blocking brakes increase the risk of 'bitu-planing' or melting of the bitumen, therefore slowing down the braking process of the car. Kiwa KOAC deploys a specially-equipped test car for this purpose, to measure the deceleration for a 100% retarded (blocked) wheel. Since 2014 we can mobilise this system for more continuous dry skid resistance monitoring, without any form of traffic hindrance.

Testing at very small locations can take place using small monitoring systems such as the Skid Resistance Tester or the Floor Slide Control 2000.

Texture is the name given to the micro roughness of a road surface. A road surface needs texture for ensuring that functional properties such as skid resistance and noise reduction can be garanteed. The texture also influences the rolling resistance somewhat. The texture is determined through choices made in the mix composition (macro texture) and type of aggregate (micro texture), and by the quality of the material application process (macro and mega texture). The durability of the required texture is a question of quality. The micro texture has a wavelength of maximum 0.5 mm, micro texture between 0.5 and 50 mm and mega texture between 50 and 500 mm.

In years gone by, the sand patch method was used, a time-consuming test which could not be carried out under normal traffic conditions. The test resulted in the mean texture depth MTD.
Kiwa KOAC can nowadays measure texture in the wavelength range from 2.5 to 200 mm at normal traffic speeds. We do so by using texture lasers mounted to or under the test car, such as for example the ARAN (Automatic Road Analyser). It is possible to combine the measurement with roughness and skid resistance measurements. We can present the results as complete texture profiles and in the form of texture parameters MPD (mean profile depth) and RMS (average roughness). In size, the parameters MPD (two-dimensional) and MTD (three-dimensional) are approximately equal.

Using the texture laser, we can quickly and objectively monitor the type of pavement surface and the extent of any ravelling (loss of mineral aggregate out of the road surface). Unlike the visual assessment of ravelling, this is an objective and reproducible measuring system.

When open to traffic, a road surface must be flat, for transport taking place comfortably and without risk of damage to the vehicle or loss of load. Damage can occur over the years however, leading to a loss in roughness, due to increasing cracking and reduced load distribution in the construction. A lack of evenness can also be the result of settlement in the underlying subgrade, especially in areas with a compressible soil type. This type of roughness is characterised by long waves, and forms thresholds or bumps upon the transition of the road from bridges to the local subgrade. Short-wave roughness problems are generally due to an non-optimal construction process. The variables in this case are the logistics, trimming of the asphalt spreader and the compaction regime.

The roughness can be objectively measured. Visual assessment of roughness is possible for short waves, but is not the ideal method for long waves. Kiwa KOAC can measure the roughness using the HSRP (High Speed Road Profiler) or the ARAN (Automatic Road Analyser). All HSRP systems operated by Kiwa KOAC carry a valid CROW-certificate. For cycle paths, we have developed the so-called FCM (bicycle comfort metre). All our monitoring systems make use of one or more laser measuring systems. These instruments record the road profile up to a set maximum wavelength. This maximum wavelength becomes larger as the measuring speed increases.

One main advantage of measuring the profile is that the raw test data can be used to calculate all standard (and future) roughness indicators. The measurements provide input for the C5 value as commonly used in the Netherlands, but also data for the International Roughness Index.

Rutting, or transverse unevenness, is the permanent deformation in the road caused by great repetition of heavy trucks leaving ruts in the asphalt or block paving. Passenger cars experience great hindrance from rutting, because they do not quite fit in the ruts, which influences the steering behaviour of the car. Rutting is hazardous in rainy weather, due to large puddles forming in the tracks, which can cause aquaplaning at high speeds. If the foundation or deeper lying layers become deformed, this becomes visible on the surface in broad ruts with shallow flanks. The higher the deformation in the construction, the narrower the ruts and steeper the flanks.

Kiwa KOAC can visually assess the rutting. We can measure the rut depth objectively per 5 m, at normal traffic speeds, using the ARAN (Automatic Road Analyser). Rut depths can be calculated under a 1.20 m beam, in the transverse profile measured over a traffic lane, but so too can water layer depths.

For a more detailed study of the causes of rutting, we take a measurement using the Flatmate. The Flatmate records the road profile over 2.25 m, at the project level. This test is generally combined with sequential drilling for allocation of the source of the deformation.

Ride comfort is mainly determined by the properties of the vehicle, the ride speed and the road. The latter includes sharp bends, unclear situations but also the condition of the road and the pavement surface. The presence of roughness is apparently the main factor affecting the ride comfort of motor vehicles and cyclists. Besides this roughness, road users' comfort is also affected by hindrance from water splashes and spray, tyre noise in the vehicle and visibility of road markings.

Kiwa KOAC can measure the roughness of the pavement surface at normal traffic speeds, using the HSRP (High Speed Road Profiler) and the ARAN (Automatic Road Analyser). The drainage capacity of the pavement surface can be measured stationary using the Becker device, a process which is carried out periodically on the hard shoulders of the Dutch motorway network for determination of the degree of clogging of the (very) porous top layer and the possibilities for cleaning. The day and night visibility of markings can be measured both dynamically and stationary. The measurement of the whiteness and the retro-reflection plays an important role here.

Kiwa KOAC deploys the FCM bicycle comfort meter for cycle paths. Using the laser sensors and accelerometers on the rear of the small test car, we determine the ride comfort in terms of roughness. The co-pilot registers other quality factors by means of a visual inspection.

Practice has shown the speed hump to be an effective measure in supporting a required speed regime. The speed control achieved by the measure contributes to the prevention of hazardous traffic situations. However, the measure can also result in undesirable side-effects, including vibrations which cause hindrance to residents in the vicinity of the hump location.

Road network managers are increasingly confronted with complaints or even damage claims due to vibration hindrance. Kiwa KOAC has many years' experience with such problems and is often called in by municipal and provincial authorities and water boards for research and consultation. We have developed a computer supported monitoring system (TRIMA) for vibration research according to the guidelines of the SBR (Foundation for Construction Research). This monitoring process involves monitoring traffic vibrations at critical points in buildings along the road. In doing so, we regularly check which driving speeds and which types of trucks are problematic.

'Prevention is better than a cure', and Kiwa KOAC is therefore fully equipped to map out the vibration consequences of the proposed construction of a speed hump or table via a prognosis, in critical cases. We do so by making use of the Vibra Prediction software developed by TNO. We can then formulate alternative solutions in the event of vibration problems.

Ravelling is the damage formed when mineral aggregate is forced out of the road surface due to the combination of weather conditions and traffic load. As soon as a single stone is lost, the support for the adjacent stones is also lost. If the bitumen does not provide enough adhesion for the adjacent stones, they too will be lost, followed by the adjacent stones again, etc. The process then soon accelerates. This is a particularly common form of damage in very porous types of asphalt in particular, due to the large voids in the asphalt mix.

Ravelling, also called fretting, results in disintegration of the road surface, and all undesirable consequences for durability, ride comfort, traffic safety and noise hindrance. Because the development of ravelling is correlated to a change in the texture of the pavement surface, Kiwa KOAC users texture lasers to assess the ravelling on busy roads. On less busy roads, we generally visually assess ravelling and register it as a percentage of stone loss in the total assessed surface.

Kiwa KOAC can measure texture in the wavelength range from 2.5 to 200 mm at normal traffic speeds. We do so using texture lasers mounted to or under the test car, such as for example the ARAN (Automatic Road Analyser) and other test vwehicles. The test can be combined with roughness and skid resistance measurements. Using the texture laser, we can quickly and objectively monitor the type of road surface and the extent of any ravelling. Unlike the visual assessment of ravelling, the texture based condition mapping method is an objective and reproducible measuring system.

Approximately 25% of Dutch people experience serious traffic noise hindrance. One solution for this problem is the application of noise-reducing road surfaces, which have a fine granular composition and open texture, and sometimes also a porous surface. Porous wearing courses minimise the splash and spray water during rainfall. Dutch road users apparently appreciate these properties most of all.

The success of the water permeability depends on the self-cleaning effect of fast traffic and, when this is lacking, the periodic cleaning process by the road network manager. Noise-reducing properties are lost when the asphalt pores become clogged and when ravelling of the road surface has developed.

Kiwa KOAC and its partners conduct Statistical Pass-By measurements (SPB) and Close Proximity (CPX) measurements. The SPB method measures the noise reduction of a road surface at a single point along the road. The CPX method measures the noise generated on a road surface, from a trailer in which microphones are fitted close to the tyres. These measurements are then used to determine whether road surfaces meet the set noise requirements over their entire length. A disadvantage of the CPX method is that it is not possible to directly link the requirements to the measured level. The measured CPX level must therefore be translated into an SPB level and a noise reduction. The pavement surface texture, important for rolling noise, is measured by texture lasers.

The various types of striping, symbols and markings inform road users how to use the road, and therefore play an important role in traffic. Curbside stripes indicate the direction of travel, and lane striping indicates a road user's position on the road. The pavement surface communicates with road users via marking, providing clarity and safety.

For safety reasons, markings must be easily visible day and night. They must also not be slippery. Kiwa KOAC therefore tests the suitability of marking materials both on a laboratory scale and in test sections on the roads. We can test all types of markings, such as a layer of paint or a cast layer of thermoplastic or cold plastic, in terms of its composition, skid resistance and functional properties. We assess the day and night visibility at both normal traffic speeds and stationary, in combination with skid resistance testing.

Kiwa KOAC has the only laboratory in the Netherlands, in Groningen, fitted with state-of-the-art equipment and ISO/IEC 17025 accredited by the Dutch Accreditation Council (RvA) under number L007 on road surface testing and marking testing. At this laboratory, we test paint types, dilutions, glass beads, thermoplastic and cold plastics not only for wear, but also aspects such as colour stability and retro-reflection.

The load-bearing capacity of a road or site pavement is one of the most important areas of attention both during design of new roads and redesign of existing roads. Falling Weight Deflectometer (FWD) testing allows us to assess the strength of the total construction and determine the stiffness of the individual layers. For this type of analysis, we require information on the layer thickness and the types of materials used.

Our measurements are generally deployed in assessment of the residual life of an existing road. We analyse the data to determine the required strengthening thickness, if the residual life is too short. Falling Weight Deflectometer testing is increasingly used for mapping out the structural condition of the road shortly after its construction and at the end of the contract period. It allows us to check whether the construction deteriorates according to a 'normal' pattern. Kiwa KOAC deploys Falling Weight Deflectometer testing more and more to determine the so-called zero condition, especially on behalf of the Department of public works. This is the condition of a road section where maintenance is essential and whereby prospective contractors can gain an impression of the expected work, based on reporting of the zero condition.

Kiwa KOAC also uses the Falling Weight Deflectometer to assess the safety of asphalt dike revetments. We have a HWD (Heavy Weight Deflectometer) for testing thick and stiff pavements, and two FWDs, allowing us to use multiple devices during one and the same traffic diversion process.

The foundation of a road plays an important role in two phases: in the construction phase and in the usage phase.
In the construction phase, the stiffness of the foundation provides a sounding board which enables good-quality construction and compaction of the upper structural layers. In the usage phase, the foundation must withstand long-term traffic loads, together with the other constituent structural layers, without suffering excessive damage. The load-bearing capacity of any foundation can be determined using Falling Weight Deflectometer testing.

An impression of whether the foundation stiffness suffices during the construction phase can be gained by conducting normal Falling Weight Deflectometer testing (FWD). We can also deploy lightweight, manually operated equipment (Light Weight Deflectometer). Using the FWD and LWD data, we can measure the surface modulus on the basis of the centre deflection and impact pulse, which gives the equivalent stiffness of the entire system of layers. This type of measurement is also known as a dynamic plate bearing test. In FWD tests, we can determine the stiffness of the individual layers because we measure the entire deflection profile per test station at 9 to 15 offsets from the load centre.

On request, we can also conduct static plate bearing tests. This is not standard procedure in the Netherlands, though it is used in Belgium and Germany. The test results give a combined indication of the stiffness and the resistance to permanent deformation. Due to this test being extremely time-consuming, Kiwa KOAC has developed correlation equations with which we can predict the values of the static plate bearing test, from the quicker dynamic plate bearing test.

The underlying capping layer of a pavement structure mainly comprises a sand-filled bank. Lightweight filling materials are more likely to be used in areas with a poor structural support subgrade.. Kiwa KOAC can measure the load-bearing capacity of the subgrade and capping layer using the Light Weight Deflectometer (LWD), but also by means of shallow manual penetration testing, dynamic cone penetrometer testing and the Penetrologger. Testing with determination of cone penetration resistance and the use of a friction sleeve is more suitable for determination of the stiffness pattern over the depth of thick underlying banks. The stiffness and density of the subgrade play an important role in finding solutions for settlement reduction.

With the penetrologger and impact penetration testing, we can quickly give you an impression of the variation of penetration resistance over the first metre of a sand bed or filled bank. However, the test results do not give the stiffness modulus as commonly used in design procedures. This stiffness can be determined using the LWD.

Kiwa KOAC has developed a model to predict how the stiffness of the capping layer changes during the process of pavement completion. The weight exerted on the capping layer changes during this process, but at the same time the stiffer upper layers distribute the load more effectively. Layer thickness, density and stiffness of all the top layers jointly provide the input required to determine whether the stiffness of the capping layer meets the requirements during each phase of the construction process.

The layer thickness of all constituent structural layers of a pavement construction plays an important role in the structural assessment of roads, and of airport and industrial site pavements. Drilling has traditionally been the method of determining the layer thickness down to 1 metre depth, and for description of the materials. Ground Penetrating Radar (GPR) or continuous layer thickness measurement are the most suitable methods in large projects, which particularly require information on the variation of the layer thickness over the length of the road. We can conduct these tests at low speed and at normal traffic speed. Layer thickness and construction details are essential for delineation of roads in homogeneous sections. Non-destructive testing can also map out the nature, extent and cause of all kinds of defects.

Together with its partners, Kiwa KOAC conducts Ground Penetrating Radar (GPR) testing for quickly gaining an impression of the layer thickness and the bond between the bound layers. Such measurements can only be conducted reliably if the layers to be tested are all above the groundwater level. GPR testing is regularly applied for the structural assessment of asphalt.

GPR testing is also useful when voids spaces must be detected under the asphalt layers. This may be the case in buffer plates adjacent to engineering structures such as bridges. For improvement of the accuracy of the GPR testing, it is strongly recommended that reference cores be drilled at a number of characteristic locations.

Over the years, cracks can occur in all the bound construction layers, which either begin at the road surface or which develop up towards the road surface from below. These cracks negatively influence the residual value of the road and lead to accelerated deterioration. Visual inspection, on foot or from a moving car, offers the possibility of objective mapping out of the damage. Such inspection can also be used to trace the source and cause of the damage and to gain an impression of possible maintenance measures.

Pavement cracking does not normally have any real consequences for road users, as it does not affect the ride comfort too much. It is however a signal to the road network manager, that the condition of the pavement structure is deteriorating and that maintenance will be required in due course to retain usability of the road as efficiently as possible. Inventory of the location and degree of cracking is one of the main activities when formulating advice on pavement quality.

On busy roads, automatic analysis systems are preferable for safety reasons. Kiwa KOAC deploys the ARAN (Automatic Road Analyser) for this purpose, whose camera and laser scanning systems map out any cracking in the surface.

The resistance to permanent deformation is one of the most important indicators for the structural behaviour of a pavement structure. There are no generally accepted testing methods operational as yet, which can directly measure this resistance to permanent deformation. The degree of compaction is therefore usually measured instead. In the countries around the Netherlands, the static plate bearing test is applied for the unbound construction layers.

Kiwa KOAC can measure the degree of compaction in various ways, via nuclear methods but also using traditional methods such as the sand method and grit method. We can also arrange for plate bearing testing if required.

The degree of compaction shows the ratio of compaction measured in the field versus the reference value determined under laboratory conditions. For foundation materials and materials from the underlying layers, we use the Proctor test to determine the maximum density. This density is defined as the density at optimum moisture content. The higher the density, the better the aggregate particles are packed, the more limited the risk of permanent deformation in the layer tested. For this reason, there is always a minimum requirement for the degree of compaction. A higher density almost always results in greater stiffness.

The traffic load plays a very important role in the design of the layer compilation and materials to be used in a pavement. This applies both in the design of new roads and in the redesign of existing roads. The volume of passenger cars is unimportant for the designing of a pavement. While they determine the number of lanes required, they have no effect on the nature and thickness of the pavement structure. The weight and intensity of the heavy goods vehicle flow is the deciding factor. On industrial sites, the straddle carriers, reach stackers, etc., are what counts for the load.

Kiwa KOAC can install monitoring stations for you, to count the volume of trucks and axle passes. At the same time, we measure the dynamic axle load per passing axle, without the traffic needing to leave the road for that purpose. Axle load measuring systems can be more or less permanent, and can be built in to a certain monitoring location for a few years. The systems can also be attached to a road surface as a 'mobile' version for a shorter period of time.

Our measurements inform you whether the actual traffic load matches the assumed traffic load. This information is very important in contracts whereby the contractor must guarantee a certain quality level during the contract period. Our monitoring data can be directly input in the new generation of design programmes such as OIA.