Presentations Repository

Abstract

Bitumen Rubber binders have been used in Chip seals in South Africa since the early 1980’s. Over the past 3 decades the use thereof has extended to southern Africa, including Namibia where it has been extensively incorporated into the recent reseal program of approximately 3,200km of roads, of which approximately 700km roads have been surfaced using Bitumen rubber between 2016 and 2019. A Total of 340km of these rubber seals were done using the S-R2 bitumen binder, instead of the conventional S-R1 bitumen rubber.

According to the TRH3 design manual of 2007, no data for the adjustment for the non-homogenous S-R2 binder to conventional binder is available. The common practice was to use the S-R1 conversion factors. This use of the S-R1 conversion factor led to extensive bleeding in some cases on Namibian roads. One of the key benefits of using rubber bitumen is that due to the stiffness of the binder which provides resistance to the orientation of the surfacing stone, higher bitumen application rates can be applied compared with conventional binders, normally in the range of 1.8 – 2.3 l/m² hot for a 14mm seal. It was observed that the stone orientations rates differed between the conventional S-R1 and S-R2 bitumen rubbers resulting in early bleeding, subsequently justifying different binder application rates and conversation factors.

The rate of stone orientation was observed using two different S-R2 products sprayed at different application rates in the same direction of travel on a trial section. The use of two SR-2 products from different suppliers was used to established if there is any significant difference in the behavior in the SR-2 binders itself and to established the rate of stone orientation. Application rates ranged from 1.8 l/m² to 2.1 l/m².

Texture depth measurements were taken directly after surfacing prior to traffic and rechecked every day for a week, thereafter weekly for a month and finally monthly over a period of four months. The location of the texture depth measurements was strategically placed in and between the wheel tracks to observe the sensitivity of the S-R2 bitumen rubber to early stone orientation due to traffic. No marked difference in rate of texture loss due to traffic compaction was observed between the two S-R2 products, but it was noted that S-R2 has a lower viscosity specification at a lower temperature than S-R1 which leads to a less stiff bitumen at the time of application. Higher application rates can thus be achieved with S-R1 than with S-R2 binder which reverts to a stable condition at around 5000 vehicle passes. The outcome of the S-R2 trials in Namibia lead to the provisional conversion factors for S-R2 binders, as published in the recently released SABITA Manual 40 / TRH3 2021

Abstract

The purpose of this paper is to illustrate the process of selecting and prioritizing surface treatments such as reseal, in Namibia. Roads Authority of Namibia has an IRMS (Integrated Road Management System) of which the PMS (Pavement Management System) is a subsystem. PMS is a software tool that assists decision makers in identifying and prioritizing potential projects and optimum maintenance strategies. Once potential projects for reseal are identified by the system, the system generates a reseal priority list of projects at network level, which is then circulated to Regional Engineers, PMS Manager and appointed Consulting Engineers (responsible for reseal projects) for scrutiny. The team then evaluates the PMS recommended action, priority and confirms the project start and end. Results of these panel inspections form the basis of a three-year reseal work program.

The data that feed the PMS is collected through standardised Visual Assessments, conducted with a Cloud-based Tablet application; mechanical surveillance measurements that collect road roughness and rut depth using a laser road surface profiler and; Pavement strength measurements using a Falling Weight Deflectometer. The PMS module uses the pavement structural condition, pavement performance and prediction method through historical deterioration to determine both the Surfacing and Structural remaining life.

The reseal type selection process uses the type of distress, their degrees and extents, traffic volume and turning actions, the coarseness or variation of the existing macro texture and road importance, using decision trees. The cost-effectiveness of different reseal types are then evaluated using costs and benefits through an “area under the curve” approach.

During the panel inspections, Regional Engineers share their experience of previous work under various condition in their areas and in some cases, question the selection of correct pre-treatment and reseal types. These comments are then fed back into the PMS and used to improve the PMS models through its adjustable rule sets.

Panel inspections, facilitated by experienced practitioners, are also used for capacity building, allowing young engineers to carry out ball penetration and sand patch testing with explanation of how all gathered information is used to select and prioritise appropriate actions.

Abstract

The video, Chip Seal Construction Process, summarizes the construction process of a triple seal in South Africa. A triple seal has three layers of aggregate; 20mm + 7mm + 7mm; and three spray coats of binder, typically but not restricted to; S-R1 + S-E1 + Cat65%. The second video, Abaqus 3D Chip Seal Model, is a digital copy of the chip seal construction process. The chip seal model was developed in Abaqus, has a load consisting of a 20 kN truck wheel inflated to 800 kPa, rolling at 10 km/h. Stress and strain results were obtained at the aggregate-binder interface as an approach to quantify the environment conducive for ravelling.

Chip Seal Construction Process:

Abaqus 3D Chip Seal Model:

Abstract

In South Africa, the Marvil apparatus (SANS 3001-BT12:2012) is the primary method used to determine the permeability of a seal. The Marvil apparatus, however, does not consider the influence of the magnitude of pressure generated between the wet road and the wheels of the traffic, which led to this research study with three objectives. Firstly, design and build an apparatus able to test the permeability of seals under a pressure of up to 300 kPa. Secondly, introduce a preliminary test protocol, and thirdly conduct pressurised permeability testing on seals.

The objectives were achieved based on the successful testing of 26 cored seal specimens with newly designed High Pressure Permeability (HPP) test apparatus. The permeability was calculated by means of measuring the amount of water seeping through the 100mm seal core over a set time, under a constant pressure. Five different seal types were tested and permeability within the wheel tracks were compared to permeability between the wheel tracks and on the shoulder.
The data from the study suggests the following:

• Permeability increases with the increase in applied pressure; In comparing traffic data to permeability, a trend is recognised which suggest permeability increases as traffic loading increase. There are however exceptions within the data, that does not follow the suggested trend and more testing is recommended;
• Isolated defects, such as cracked stone or small holes, greatly influences the permeability which highlights the negative influence such as aggregate crushing or cracked stones found in seals;
• Aged cores recovered are more permeable in the wheel tracks as opposed to between the wheel tracks, and on the shoulder. This suggests that ageing combined with fatigue damage caused by traffic increases the permeability of the seal;
• Permeability was also compared to binder content, but there is no clear relationship between bitumen content and the permeability.
  An overview of permeability rates measured for each seal type is presented in the paper. It is however important to highlight that each seal functioned under different circumstances and that the number of cores tested per seal type varied. These permeability values should be viewed as preliminary test values and not exact seal type specific values, as more testing is recommended.

Cape Seals had the highest average permeability of 31.54 ml/min at 300 kPa. The other three seals were found to have an average permeability close to each other. The Single Seals was found least permeable with 2.50 ml/min at 300 kPa. The Double Seal had a permeability of 2.75 ml/min and the Multiple Seal a permeability of 2.81 ml/min at 300 kPa. For each seal type these values are the average values found within the wheel track only.

This study did not result in an ultimate measurement for permeability of seals under pressure. It did however produce a meaningful test apparatus and method which will serve as platform for further investigation into permeability of seals and asphalt under pressure.

Abstract

A major behavioural characteristic of surfacing seals includes aggregate re-orientation which involves the re-orientation of seal aggregates, onto their respective least dimensions, during and after construction, to ensure a lower risk of aggregate loss and adhesion problems.

The purpose of this investigation is to gain an understanding of the effect that traffic has on the re-orientation of surfacing seal aggregate. Variables that affect aggregate re-orientation include different binder types, rheological properties, different pavement temperatures and different aggregate matrices. The combination of these variables is measured and discussed to determine the optimal binder adjustment factors and conditions for aggregate re-orientation. This investigation tests eight different binders including 70/100 penetration grade bitumen, Cat65 emulsion, SC-E1, SC-E2, S-E1, S-E2, S-R1 and S-R2 at three different trafficking temperatures (10°C, 20°C and 30°C), two different aggregate matrices (shoulder-to-shoulder and over application) and simulated traffic going up to five thousand load repetitions per combination of variables. To simulate traffic, the Model Mobile Load Simulator Mk. 3 is used, after which a Laser Profilometer is used to record texture readings. The time between construction and testing was kept constant throughout. This study provides a better understanding of different factors that influence aggregate re-orientation, which could be implemented in practice immediately.

Abstract

Infrastructure maintenance and preservation are important activities for any developing city. Microsurfacing is a well known slurry seal surface maintenance technique that leverages the use of cationic quick setting slurry. The technique is considered by some practitioners to be more economic than other conventional surface treatment methods such as asphalt and chip seal. Micro surfacing seals could correct rutting defects, restore skid resistance, reduce pavement oxidation, help preserve water ingress, and in turn extend the surface life span. Quick setting slurries are preferred when working on urban roads where the specialised modified emulsion allows the seal to set quickly, building good cohesion and allowing opening to traffic within 2hrs (weather dependent). Work performed in urban areas during winter poses various challenges for micro surfacing seals due to shades on residential streets (low surface temperature). These conditions are usually mitigated through adjustment of the emulsion formulation to allow different setting/breaking times of the quick set product.

The success of a microsurfacing seals depends on careful pre-planning (using digital logging), surface preparation (sealing of cracks, potholes repairs, etc.); raw materials selection (emulsion and crusher dust); workmanship and fit for purpose equipment. The system can only be placed with a purpose designed microsurfacing machine and spreader box capable of continuously feeding accurate raw material proportions to ensure a consistent, high quality surfacing layer. The application process of quick set slurries is a complex and specialised operation requiring skilled supervision, specialised machine operators and skilled squeegee operators who understand quality requirements, on site trouble shooting, breaking times and time constraints with regards to workability and hand work.

Careful seal design & application techniques are critical for successfully achieving a quick setting slurry seal capable of accommodating light to medium traffic. This presentation addresses onsite challenges & solutions for laying microsurfacing systems.