Cold recycling

Cold recycling  (eng.  cold deep in-place recycling ; also:  cold regeneration ) is a technology for strengthening (stabilizing) soils, stone materials and asphalt granules obtained by crushing asphalt concrete scrap (FAL), various binders, by pre-milling and mixing on road.

Recycling machines were developed a few years ago by corresponding modifications to road milling machines and soil stabilization machines.

The cold recycling technology of XCMG machines is based on a milling and mixing drum with a large number of special cutters. Rotating, the drum crushes the pavement material.

Soil strengthening, as a rule, is carried out using specialized chemical additives to binders (Portland cement). This method allows you to work 3-5 times faster than traditional methods of soil stabilization.

road construction

During milling, a binder is injected under pressure into the working chamber of the recycler in the form of a water-cement suspension, which is prepared in a mobile mixing plant. Cement and water are mixed in precisely metered amounts, creating a suspension. The amount of slurry is precisely controlled by a microprocessor-controlled pump so that after mixing with the material milled by the milling drum, the moisture content of the resulting mixture is optimal for its compaction.

The composition of a group of recycling machines may vary, depending on the purpose and type of stabilizer used.

In each case, the recycler pushes a mobile cement slurry mixing plant in front of it. After recycling, a layer of the resulting mixture is pre-compacted between the recycler wheels with a roller to create the same density of the material. Then the material is profiled by a motor grader, after which it is finally compacted with vibratory rollers. The freshly laid base is cared for by pouring a bituminous emulsion.

The advantages of cold in-situ recycling technology are:

• No pollution of the environment due to the full use of the material of the old road pavement, there is no need for dump sites, the volume of imported materials is minimal, and transportation is very small. Energy consumption is significantly reduced, as is the destructive impact of vehicles on the road network.
• The quality of the recycled layer due to the successive mixing of locally obtained materials with water and a stabilizer. Liquids are introduced in exactly the right amount thanks to the microprocessor control system of the pumps. Mixing meets the highest requirements, since the components are forcibly mixed in the working chamber.
• Structural integrity of pavement. Cold recycling makes it possible to obtain cohesive layers of great thickness, which are distinguished by the homogeneity of the material. This eliminates the need for liquid binders between thin layers of pavement, which is sometimes necessary in traditional pavement designs.
• Preservation of soil integrity, as there is less damage to poor quality soil during recycling compared to using conventional road construction machines to restore pavement. Typically, cold recycling is performed in one pass by a recycler on pneumatic tires, which exert little pressure on the ground and little deform it.
• Reducing the duration of construction work. Modern recycling machines are characterized by high productivity, which significantly reduces construction time compared to traditional methods of road resurfacing. Shortening the work time is beneficial for road users as it closes the roads to traffic for a shorter period.

These advantages make cold recycling the most attractive technology for pavement restoration in terms of cost/efficiency.

The assessment of the condition of the old pavement and the requirements for the characteristics of the road after restoration are interrelated. In practice, several methods are used to assess the condition of the pavement:

1. Visual evaluation

2. Sampling for laboratory testing

3. Core sampling

4. Deflection measurement.

An important part of the pavement research process is the selection of the composition of the mixture. Preliminary samples are tested to select the composition of the mixture. Samples are prepared so that their material is as close as possible to the material that will be obtained in the actual recycling process. When pre-selecting a stabilizer, the suitability in terms of the type and quality of the material being processed, the required technical characteristics of the mixture to be obtained as a result of recycling are taken into account; preparation of sample parts by mixing the material with varying amounts of water until a mixture with an optimal consistency for compaction is obtained. Typically, at least four blends are prepared, each with a different stabilizer content; preparation of samples using standardized compaction methods; release of samples from forms; testing of specimens after release from molds to evaluate their technical characteristics and sensitivity to humidity. To determine the optimal stabilizer content, the results of these tests are compared with each other, taking into account the content of the stabilizer in each of the mixtures. The content of the stabilizer, which optimizes the properties of the mixture, is regarded as optimal.

From the point of view of the service life of the pavement, the most important characteristics are the quality of the material and the thickness in the finished recycled layer. They are the key parameters needed to predict the service life of a refurbished pavement. Prior to recycling, all aspects of the operation should be analyzed and planned; identification and timely removal of any interference with the operation of a group of recycling machines; material needs assessment; ensuring high readiness of machines for work; appropriate training of drivers and maintenance personnel; work safety issues.

The implementation of recycling requires a high quality of its planning. Before starting work, it is important to think through the steps and operations that must be completed in a day or shift, and fix them in the form of a work plan. The type of machine determines the throughput, width and depth of the layer that can be processed in one pass. The width of the road determines the number of recycler passes required to process the entire width of the road. Tapered sections require special attention to the execution of the work. The shape of the surface (bulge or transverse slope) affects the location of the longitudinal seams between the joined sections. For the duration of the work, a change or a complete stop of traffic is carried out.

In addition to the layer thickness, exact requirements for the result to be obtained at the end of the work must be formulated. This concerns the final levels of the road surface and tolerances for its profile, degree of compaction, surface texture and surface material.

In the material of an existing roadway, the type of material, consistency and moisture content of all its components are important. Changes in the thickness of the existing pavement materials (asphalt concrete layers) can significantly affect the performance of the recycler. Differences may require a change in the rate of stabilizer, an increase in humidity, or even a recycling depth.

Preparatory work preceding the execution of work includes:

• removal of obstacles (hatches),
• installation of new water conduits or other additional drainage equipment,
• pre-milling to prepare the surface of the desired level and profile,
• delivery and distribution of new material on the existing road surface.

When scheduling work for a work shift, consider:

• the recycling sequence, the number of passes required to complete the full width of the road, the overlap data for each longitudinal joint and the effective recycling width for each pass,
• sequence of passes, direction and length of the section,
• volume of imported materials, stabilizer, water,
• sketch of a section of the existing pavement indicating the depths of recycling.

Cold recycling can begin with a full check of all machines and equipment, including rollers and tankers, checking water supplies, stabilizer for the planned passage length, placing a group of recycling machines on the line of the first pass, with the distance between them prescribed for work, with connecting all feeders pipelines to the recycler, complete removal of air from the system, when checking whether all valves are fully open. These pre-checks are carried out at the start of each work shift.

At the beginning of work on the starting section of a new recycled area, it is necessary to evaluate how the material behaves in the existing pavement. Usually the starting section has a length of about 100 m and captures the road along its entire width or half the width. Three of the most important aspects of recycling can be assessed at this site: the material processed by the recycler must be tested to determine if it matches the samples that were used to select the mixture in the laboratory. A quick sieve analysis will show if this selection was correct. The speed of the milling drum and the feed rate of the recycler have an impact on the particle size distribution of the processed material. The WR 2500 is equipped with a crushing plate that can be adjusted to limit the maximum material size. These three parameters must be set so as to find the best combination to achieve the desired material composition. One of the most important characteristics of a finished recycled bed is its degree of compaction. Thick (> 250 mm) layers often require special compaction methods, and the effectiveness of various rolling methods can be evaluated in the starting section. Asphalt concrete layers in old damaged road layers usually have a low content of voids, natural (granular) materials usually compact during operation. Recycling of such materials usually results in an increase in their volume, which affects the levels of the finished layer. One of the most important characteristics of a finished recycled bed is its degree of compaction. Thick (> 250 mm) layers often require special compaction methods, and the effectiveness of various rolling methods can be evaluated in the starting section. Asphalt concrete layers in old damaged road layers usually have a low content of voids, natural (granular) materials usually compact during operation. Recycling of such materials usually results in an increase in their volume, which affects the levels of the finished layer. One of the most important characteristics of a finished recycled bed is its degree of compaction. Thick (> 250 mm) layers often require special compaction methods, and the effectiveness of various rolling methods can be evaluated in the starting section. Asphalt concrete layers in old damaged road layers usually have a low content of voids, natural (granular) materials usually compact during operation. Recycling of such materials usually results in an increase in their volume, which affects the levels of the finished layer. natural (granular) materials are usually compacted during operation. Recycling of such materials usually results in an increase in their volume, which affects the levels of the finished layer. natural (granular) materials are usually compacted during operation. Recycling of such materials usually results in an increase in their volume, which affects the levels of the finished layer.

At the start of recycling, a series of control tests are carried out:

• passage depth on both sides of the recycler,
• the accuracy of the movement of the recycler along the intended line with the required overlap width,
• the humidity of the treated material must be sufficient to guarantee its compaction.

Determining the optimal pass coverage depends on the type of stabilizer used. When working with cement, shorter sections are used to allow sufficient time to process the entire half-width of the road, grading and compacting the surface before the cement sets.

After recycling, the processed material must be profiled and compacted to the required degree.

The amount of work of a motor grader depends on the type of cover layer. If a large layer of asphalt concrete is to be laid, then the surface level tolerances will be wider than with the final layer laid in one pass. Where tolerances are relatively tight, the finished half-width (or full-width) of the road must be graded to remove the bumps (up to 10mm) that often form at longitudinal seams. In addition, the motor grader is useful for correcting longitudinal material misalignments that sometimes occur in transverse seams. Good compaction of recycled material to obtain the required density is one of the most important conditions for the performance of reclaimed pavement. When the material to be stabilized is not properly compacted, the required layer strength is not achieved, which leads to premature destruction of the pavement. Sealing layers? 200mm is now standard practice. To achieve the required quality, the choice of rollers and their mode of operation is important. Currently for sealing layers ? 200 mm, heavy (with a static mass of more than 15 tons) vibratory rollers with a change in the frequency and amplitude of vibration are used. Vibration with high amplitude and low frequency breaks the material of the upper layer, often deforming the surface. The deformation is easily eliminated by a motor grader before rolling with a small amplitude and a high frequency. Moisture is the most critical variable in achieving compaction at the lowest cost. Due to the time delay between recycling and finishing, the surface should always be lightly wetted before final rolling. If too much sealing force is applied, «overconsolidation» occurs. The material crumbles and its density decreases if rolling is continued after the maximum density has been reached.

With regard to the compaction of recycled material, it is important to consider two conditions:

1. The compaction must be uniform across the entire width of the passage before the surface is graded by a motor grader. The rear wheels of the WR 2500 must always be on the surface of the recycled material, on both sides of the aisle. They partially compact the material, but between them the material remains uncompacted. Uncompacted material must first be compacted before leveling to eliminate the difference in compaction in the tracks from and between the recycler wheels;

2. Precisely profiled material with low ductility is prone to shifting under the roller to the sides. The most radical aid in compacting such materials is water. But even with optimal humidity, it is difficult to provide an acceptable surface quality here, which requires an additional pass of the motor grader to eliminate unevenness from the roller.

Finishing the recycled layer requires the creation of a strongly bonded surface structure that will not let water through. This is achieved by appropriate wetting and pneumatic rolling of the layer surface, which brings to the surface a sufficiently fine material that fills the voids between large particles. This operation is usually performed as the final step in the compaction process.

The quality of the finished work is determined by the test results:

The strength of the material of the recycled layer is assessed by laboratory tests on samples of the mixture taken from the recycled layer, or using cores. Free compressive strength is the most widely used test for the evaluation of cemented materials. Free compressive strength is usually determined on prepared specimens aged for 7 days. Some test methods accelerate aging by placing specimens in an oven. Mixing, laying, compacting and finishing should be done in the shortest possible time. A maximum of 4 hours is usually allowed for the treatment of the cement, from its first contact with the material until the end of compaction.

Density of dry compacted material.

The thickness of the finished layer is checked by physical measurements.