FDR Technology in Road Construction: Full Depth Reclamation Explained

What is FDR technology in road construction?

FDR technology in road construction is a cold recycling process that turns a failed road into the base layer of a new one. IRC:120 describes it as a cold mix process in which additives are added, mostly in place, and compacted to obtain an improved base.

Two words in that sentence matter to a contractor. Cold means nothing is heated; the old bitumen is crushed with the stone, not melted. In place, or in-situ, means the mixing happens on the road itself, so the old material never leaves the site.

IRC:120 lists four main steps:

1. Pulverisation: the old surface and base are ground into loose, graded material.
2. Adding the stabiliser, the binder that will hold the material together.
3. Mixing and compaction into one dense layer.
4. Laying a new surface or wearing course on top.

If the old road does not hold enough material for the base thickness the design asks for, new aggregate is brought in and mixed with it. New aggregate can also be added just to correct the grading, which is the spread of stone sizes in the mix.

The finished FDR layer is a base, not a road surface. It always gets a bituminous course, or a surface dressing on a light road, on top.

When does a road need FDR instead of a simple overlay?

A road needs FDR when its damage has reached the granular base, the layer of compacted stone under the bitumen. If only the top layer is cracked, recycling or replacing that layer is enough. If the cracks reach the stone base, a new top layer sits on a weak foundation and fails early.

IRC:120 sets out how to find out which case you have. Engineers cut cores, round samples 150 mm across, through the bitumen at 0.75 m, 3.85 m and 6.95 m from the inside edge of the shoulder, at a rate of one core every 300 m of road. Where a core shows the crack running through the whole bituminous layer, the investigation goes down into the base with more cores or a test pit to see whether full depth reclamation is needed.

The code’s decision chart then works on three questions:

Roughness is measured with a bump integrator, a wheel towed along the road that adds up every bump in millimetres over a kilometre. The five-year test comes from a structural check of how many more heavy axle loads the road can carry.

The code adds one more rule that is easy to forget: recycle to the depth of the damage. A road that is only cracked on top does not need FDR, and FDR on it wastes cement and machine hours.

How is full depth reclamation done on site?

FDR is done by a small train of machines that moves down the road together. IRC:120 describes it as a recycling machine hooked to a water tanker, followed by a steel drum roller with a pad foot shell. The work normally goes to a depth of 100 mm to 300 mm.

The sequence on a typical job runs like this:

1. Mark the stretch and remove anything in the way: manhole covers are lowered, kerbs and culvert heads protected.
2. Pulverise. The recycler’s rotating drum, fitted with hard cutting teeth, grinds the old bitumen and base together. IRC:120 asks for material of minus 40 mm size, meaning every piece passes a 40 mm sieve, graded to suit stabilisation under IRC:37.
3. Spread the stabiliser. Cement or lime is spread over the pulverised layer in measured bags or by a spreader truck. Emulsion or foamed bitumen is sprayed through the recycler’s own spray bar.
4. Mix again with water. The recycler makes a second pass, and the water tanker hooked to it feeds water so the mix reaches the moisture the design asks for.
5. Compact with a pad foot roller first. The pads, rows of blunt feet on the drum, knead the thick layer from the bottom up.
6. Shape with a motor grader to the line, level and camber (the cross slope that sheds rain water).
7. Finish with a smooth drum roller, then cure the layer before the surface goes on.

For small jobs where a recycler cannot be brought in, IRC:120 allows the material to be mixed in a WMM plant, the pugmill plant normally used for wet mix macadam, and laid back.

With cement, the clock matters. Once water meets cement the mix starts to set, so mixing, grading and rolling have to finish within the time the specification allows. How long, and how the layer is cured afterwards, is set out in the guide to CTSB and CTB in road construction, the cement-treated layers an FDR base most resembles.

Which stabilisers are used in FDR?

IRC:120 names two families of stabiliser. Bituminous ones are foamed bitumen and bitumen emulsion. Chemical ones are cement, fly ash and lime, plus commercially available cementitious stabilisers, which are factory blends sold for this job.

Which one suits a road depends on the soil and the old material:

• Cement suits sandy and gravelly material with little clay. It gives the highest early strength but can crack as it dries, so the surface above must be designed for that.
• Lime suits clayey material. It reacts with the clay and stops it swelling and shrinking with the seasons.
• Fly ash is usually used with lime or cement, not alone, and uses up a power-station waste product.
• Bitumen emulsion and foamed bitumen give a more flexible base that cracks less, and suit a layer with plenty of old bitumen in it.

The code also sets checks the mix must pass before the design is accepted:

Sand equivalent is a quick test of how much clay and dust the material carries; a low number means too much fine clay for the binder to work. Indirect tensile strength is how hard a small cylinder of the mix resists being split when it is squeezed on its side. It is tested dry and again after soaking, because a road base has to hold up in the monsoon. How the bitumen itself is graded is covered in the guide to bitumen grades.

How is FDR different from other road recycling methods?

FDR is one of five recycling methods in IRC:120, and the only one that goes into the stone base. The other four recycle only the bituminous layers, either in place on the road or in a plant, and either hot or cold.

The table shows why FDR is the answer for a road failing at the base. The other methods stop above the stone. Only FDR mixes bitumen, stone base and sometimes the top of the sub-base into one new layer.

IRC:120 also notes a choice worth asking about on a big job. If it pays, the old bitumen can be milled off first and kept for a bituminous mix, and only the base below is reclaimed and stabilised. That is two stages instead of one, but it saves the most valuable material.

Which machines are needed for FDR?

An FDR job needs four kinds of machine: a recycler or soil stabiliser, a water tanker, rollers and a motor grader. The recycler is the specialist piece and is usually hired with an operator from a recycling contractor. The rollers and the grader are the same machines that build any road base, so a contractor who already owns them is most of the way there.

Single drum soil compactors in the 11 to 13 tonne class do the rolling. The JCB VM117 weighs 11,525 kg with a 2,100 mm drum, the L&T 1190D 11,150 kg with a 2,100 mm drum, and the Dynapac CA305D 12,600 kg with a 2,130 mm drum, each with a high-amplitude setting of 1.8 mm for thick layers. A thick, freshly mixed FDR layer is first rolled with a pad foot drum, which works from the bottom of the layer up; ask the dealer whether the model you are looking at takes a pad foot shell. How pad foot and sheepsfoot drums knead a layer is explained in the sheepsfoot roller guide.

You can compare soil compactor models and specifications and talk to a dealer about availability in your state before the job is bid.

The grader cuts the mixed layer to level and camber before final rolling. On a village road a small grader such as the Mahindra G9595, 95 HP (69.9 kW) with a 2,600 mm blade, fits the width; on a highway a machine like the Cat 120, 156 HP (116 kW) with a 3,700 mm blade, covers it in fewer passes. Where each of these machines fits in the wider road job is set out in road construction equipment in India.

What are the benefits and limits of FDR?

IRC:120 lists five advantages of full depth reclamation: most pavement distresses are treated, hauling is cut to a minimum, the base can be made much stronger, there is no old material to dump, and the ride improves. For a hill road or a village road far from a quarry, the hauling saving alone often decides it, because stone that does not have to be trucked in costs nothing to move.

The limits are just as real:

• It needs a laboratory. The mix has to be designed from the road’s own material, and that material varies from one stretch to the next. IRC:120 calls this variability the designer’s nightmare.
• It needs a recycler. Few contractors own one, so the job depends on hiring the machine and a trained crew.
• A cement-stabilised base cracks as it cures. The cracks are fine and expected, but the bituminous surface on top has to be thick enough, or treated, so they do not show through.
• Cement leaves a short working window. A breakdown in the middle of a stretch can leave mixed material setting before it is rolled.
• The layer must cure before traffic runs on it, which means diversions on a road people use every day.

FDR is not the answer for a road that is only worn on top, or one whose subgrade, the natural soil at the bottom, is waterlogged. Water in the subgrade has to be drained first, or the new base fails the same way the old one did.

Where is FDR used in India?

FDR is used in India mainly to rebuild worn rural and state roads, and on some national highway stretches. IRC:120’s own list of Indian recycling projects includes the main carriageway of the Tumkur to Honnavar highway (NH-206) near Tumkur, strengthened by full depth reclamation with a commercial cementitious stabiliser and checked afterwards by falling weight deflectometer and ground penetrating radar tests.

Rural road works under PMGSY, the Pradhan Mantri Gram Sadak Yojana, have since used FDR with cement and additives in states including Himachal Pradesh, Nagaland and Kerala. Village roads suit it well: the quarry is often far away, and the old road already holds good stone. How village roads differ from state and national highways is set out in types of roads in India.

How each layer fits together, from subgrade to surface, is in the road construction layers guide. What the job costs per kilometre, and how a recycled base changes that, belongs in road construction cost per km in India. Every other road topic sits under the road construction guide.

The depths, sizes and test limits on this page are from IRC:120-2015, Recommended Practice for Recycling of Bituminous Pavements, Sections 1, 3, 4, 6 and 10 and Annex 2. Before you bid or hire for a reclamation job, compare soil compactors and motor graders and connect with a dealer.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication. Recycling requirements quoted here follow IRC:120-2015 and are not a substitute for the contract specification or a pavement design by a qualified engineer; machine figures are taken from our listings at the time of writing.