In addition to the materials testing offered, Simtec is able to carry out a comprehensive range of monitoring and workmanship testing for completed highway works. This includes surface texture and irregularity, layer thickness, inter-layer bond and air void content.
Delivered Material Temperature Control
The delivery and rolling temperatures are critical in ensuring adequate compaction of hot bituminous material. Failure to achieve this leads to high air voids, cracking and a probable reduction in service life. High mixing and delivery temperatures can lead to “coking” where the bitumen binder becomes oxidised and loses its adhesive properties. Any material in this condition is rejected to avoid inclusion in the works. Material which is below the specified delivery temperature can be difficult to compact and may not bond to either the substrate or any other material laid against it.
Because of the difference between the surface temperature and the material delivery temperature, a monitor of the rolling temperature is needed to ensure compaction takes place before the layer temperature is too low. Simtec technicians are trained and equipped to control both aspects using state of the art equipment including infra-red thermometry.
“Core Samples” Using the Hydraulic Drilling Rig
Road cores are taken for several reasons including quality control of new surfacing works and investigation of existing pavements. They can also be taken to facilitate road stud fitting, fencing and magnetometer installation.
The majority of road cores are taken to verify the compacted layer thickness of new bituminous material and are drilled in accordance with BS EN 12697-27 and the Specification for Highway works. The modern hydraulic drilling rig can drill a road core in just a few minutes and can supply power to a range of satellite tools for carrying our reinstatement of the core holes.
The drilling rig consists of a diesel powered hydraulic motor driving a vertically mounted, water cooled drill with interchangeable barrels. The rig is vehicle towed and can be maneuvered into most locations including footways but additionally the rig can supply power to a free-standing coring drill allowing access to limited and precise locations. Core diameters range from around 50mm to 300mm, but other sizes are available. Coring depth can be increased by the addition of extension rods to the drill which is useful for existing overlaid pavements. For general highway cores a 150mm diameter drill barrel is used and the layers are drilled down to the interface with the underlying unbound stone or sub base. Cores are returned to the laboratory for measurement; (core logging to BS EN 12697-36), assessment of bond, residual air void content and layer density calculation, all of which are indicators of compliance. The results of this assessment can be used to determine the service life of the as built pavement.
Cores can also be used to gain material for retrospective “Compositional Analysis” of the bituminous materials used in a pavement. For this process several cores are taken and returned to the laboratory for re-heating and amalgamation into an acceptable test sample. Additionally, the same system can be used to take vertical or horizontal cores in structural concrete to BS EN12504-1. The recovered cores are often used as samples for retrospective compressive strength testing or other laboratory analysis.
Surface Texture testing using the Volumetric Patch Method
Together with the “Rolling Straight Edge”, the “Sand Patch” test, correctly termed “the volumetric patch method”, is used to establish the acceptability of newly laid Hot Rolled Asphalt and Thin Surface Course systems. Testing is a requirement of the Specification for Highway Works Clause 921-1 and is carried out in accordance with BS EN 13036 part1.
Adequate skidding resistance is essential in any road construction particularly where heavy braking is possible. The surface of Hot Rolled Asphalt (HRA) is spread with highly engineered pre-coated chippings to give the maximum skidding resistance. The rate of spread of these chippings is measured in accordance with BS 598-1 and the resultant texture needs to be monitored. For this reason, a swift, reliable test is required to give confidence that the surfacing is going to deliver that level of resistance. The surface texture has been found to correlate well with the skidding resistance and the Sand Patch test appearing in BS 598-105 (now withdrawn) was developed to measure this attribute.
In this test, traditionally a measured volume of fine sand is tipped onto the surface to be measured and worked into the surface to produce a roughly circular patch hence the common name of this test. In recent years the sand has been commonly replaced by single sized glass bead in accordance with BS EN 13036-1. The target diameter for the patch is when the tops of the surface chippings appear through the sand or glass. In the case of a thin surfacing system this would be the point the coarse aggregate was showing through the sand. The diameter of the patch is measured with a calibrated rule and the average texture depth calculated. The test is repeated at 5m intervals along a string line placed diagonally across the breed being measured over a linear length of 50m. The mean of these tests is the texture depth for that 50m of surfacing and should fall within the parameters set down in the contract.
The texture depth should not be too low, or the skidding resistance will be compromised whereas a very high texture depth can indicate poor chipping embedment leading to aggregate loss in service.
Both test methods are recognised and Simtec are UKAS accredited for both.
Rolling Straight Edge
The “Rolling Straight Edge” is piece of equipment which has been used successfully for many years to determine the ride quality of new highway surfacing and also newly completed utility reinstatements. The requirement for new road surfaces to comply with a minimum standard of ride quality appears in the Specification for Highway Works in Series 700, Clause 702-7 and Table 7/2. The method used complies with TRL report number SR290.
The apparatus consists of a steel frame three metres in length which is mounted on two parallel rows of rubber tyred wheels. Whilst these wheels keep the machine level, a single, centre mounted, freely moving wheel measures the difference in road level with travel. The equipment is pushed at walking pace along the line to be tested and irregularities in the road surface are registered on a dial gauge mounted to the machine.
Invariably a requirement for surface irregularity testing is included in highway contracts and the Rolling Straight Edge is the usual method of identifying these. The format of this requirement is generally a maximum permissible number of irregularities per 300m of test length with a further requirement for the maximum depth of irregularity. Based on the results of the testing a length of road can be accepted or rejected by the Client. Additionally, since the machine logs the distance travelled it is easy to identify particular irregularities for consideration of localised remedial works.
Failure to meet the specification for surface irregularity will result in a poor ride quality although in urban areas mitigation for ironworks is often applied as some shaping around gullies and manholes is sometimes unavoidable.
A further, associated test often requested for both highway works and runways is the “Transverse 3m Beam or straight Edge” This apparatus is used to test the transverse regularity across the carriageway by sliding calibrated wedges under the beam in the location of deformities. These deformities can also be measured with a calibrated ruler. It is particularly suitable for checking the longitudinal joints between existing surfaces and adjacent reinstated repair works such as utility trenches.