The production and delivery of bituminous mixes is closely controlled at source; however, this control is not infallible, the material on arrival may be affected by the distance hauled or the length of time since mixing. For these reasons a regime of sampling and testing is necessary to ensure compliance with the material’s specification.
The temperature of the mix at the point of delivery is important as it has a direct effect on the compactability of the layer placed. For this reason, the material is tested at discharge and at the point of compaction. Simtec is able to provide both electronic and infra-red measurements
The compositional analysis is carried out to ensure the material delivered is, in fact, the material ordered, to confirm the bitumen content and to check the plant’s quality control in respect of aggregate constituents. Our technicians are trained to take samples in a compliant manner and return them to the laboratory for UKAS accredited testing to be carried out.
Bituminous Material Sampling
Correct sampling is essential in order to gain truly representative samples for test and for bituminous materials BS EN 12697-27 details the acceptable methods. Simtec technicians are fully conversant with this specification and can obtain good quality representative samples free from contamination and segregation.
Delivered loads can be sampled directly from the truck, from a paving machine hopper or augers or from workable heaps tipped on site. Recovered samples are returned to our laboratory in Leicester for further examination/testing as required.
The temperature of delivered bituminous material Is also usually tested when sampling to show adequate heat is retained for placement and compaction. Traditionally an electronic thermometer with a bi-metal thermocouple was used for this purpose but an alternative is now available in the infra-red apparatus. This method allows for accurate measurement at a greater distance and is therefore much safer for the technician. Simtec is one of the few operators to be UKAS accredited for both of these methods appearing in BS EN 12697-13.
Bituminous compaction control using the “Pavement Quality Indicator” (PQI)
The ability of a layer or layers of bituminous material to support the anticipated design load due to trafficking is a requirement of all highway contracts and needs to be checked on a regular basis. This aspect is usually determined by taking core samples from the various layers, but the results can only be evaluated by laboratory testing of the core samples which causes an often unacceptable delay. The compaction control of bituminous materials on site can however effectively be controlled by the use of a “Pavement Quality Indicator” or PQI as it is commonly known, The PQI is an electromagnetic instrument with the ability to calculate the density of a layer of material by means of its effect on an induced magnetic field. The density can then be compared to the maximum density of the material and thus produce a percentage compaction. There is an option to calculate the compaction or the estimated air void content and we are now testing to the Specification for Highway Works requirement which is for a rolling mean of six values.
The equipment is small and easily portable making it invaluable for on the spot monitoring. The gauge, previously supplied with an offset to represent the relationship to actual density, is placed on the surface to be tested and is turned on; the machine immediately displays a density value for the layer. This is recorded and the process repeated several times at the same location. In this way a mean value can be found for the spot. The mean value is reported as the density for that location.
This method of measurement is accepted by the Specification for Highway works based on the use of either a Nuclear Density Gauge or a Pavement Quality Indicator, both of which require a calibration against actual physical measurements for the material under test. The calibration factor is fed into the machine and is used to “trim” the output values allowing a more accurate reading. The corrected readings are generally accepted as an accurate density which can be verified at a later date with core samples. However, it should be noted that the offset applied to the machine will have been derived from correlation on other sites and therefore will not be verified until the core samples are completed. Variations in the supplied material can have an effect on the possible maximum density and so the PQI must be viewed as a good guide rather than an absolute measure of the site density. In spite of this the PQI gives a reliable indication of the level of compaction achieved with the equipment available on site. Both the Nuclear Density Gauge method and the PQI method of measurement are defined in BS EN 594987 Annex I
Bond / Tack Coat Rate of Spread
As with surface dressing binders, tack coats and bond coats are sprayed at a constant fixed rate according to the individual site requirements. Simtec are able to supply technical staff to monitor the rate of spread using the “carpet tile” method defined in BS EN 12272-1. Here, squares of carpet-like material of known area and weight are laid in the path of the spreading apparatus. The now coated tiles are weighed and the difference used to calculate the spread rate.
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 forHighway 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
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.
Bituminous Material Laboratory Testing – Compositional Analysis
One of the standard quality control methods for bituminous mixtures is the “Compositional Analysis” (CA) which is used to confirm a mixture complies to the requisite specification.
In this test the bitumen binder is dissolved and washed off of the aggregate particles by means of rolling a weighed sub sample with a calculated volume of solvent, normally methylene chloride. This process produces a clean aggregate sample which can then be subjected to a grading analysis and its compliance checked. Additionally, the bitumen dissolved in the solvent can be extracted and the weight calculated as a percentage of the original sample weight.
Based on the outcome of this combined test the acceptance of the material and its likely performance on site though generally placement is already complete before the results are available.
This test routine is also used at the mixing plants for quality control of batch mixing. On occasions this test is used retrospectively on samples removed from laid material. These samples can be either cut squares from trial excavations or amalgamated core samples which have a compliant combined weight.
Softening Point and Residual Penetration
Another retrospective test often requested is the “Residual Penetration” value of a bituminous binder. This test is usually carried out together with the “Softening Point” test on recovered binder and can aid the estimation of the original penetration grade of the bituminous mixture.
The penetration test involving the insertion of a needle under controlled time and temperature conditions gives a numerical value to the softness of the bitumen. The softening point is determined by the “Ring and Ball” apparatus where the time taken for a steel ball to fall through a film of bitumen under controlled conditions is determined. The values for both of these aspects change over time as the bitumen oxidises and stiffens.
The penetration grade of the binder is important to know as the stiffness and workability of the whole mixture is a function of this property. The common grades are 50pen and 100pen, but others are often used. 35pen bitumen is very stiff whilst 125pen is quite elastic and produces an easily workable material. On site, for hand laid work a material with a higher penetration grade would be ordered as this gives greater workability but for machine laid work a stiffer material is more appropriate as this gives greater resilience and often a longer wear life in stressful applications.
The relevant tests and the corresponding standard numbers are as follows, Simtec are UKAS accredited for all of these tests:
Needle Penetration. BS EN1426:2015
Softening Point. BS EN1427:2015
Bitumen Recovery (Rotary). BS EN12697-3:2013+A1 2018
Binder Content BS EN12697-1:2012
Particle Size Distribution. BS EN12697-2:2015
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