Technology

Base of Excellence

June 2010

Nilesh Chiroda, Assistant Manager - Quality & Innovation, Mahindra Lifespaces Developers Ltd, tells us how enzyme technology and high volume fly ash concrete can strengthen the sub base of roads as well as save time, money, effort, and the environment.

Internal roads for a housing complex are generally made of asphalt or concrete. Being an internal road, less importance is given to compaction or owing to time constraints the compaction prior to laying concrete/asphalt is restricted. Quality issues such as surface s on concrete, settlement or de-bonding of asphalt coat are observed. While these are ascribed to improper concrete being poured or asphalt coat thickness not being provided as per required specification, the most important reason for failure is the sub base, which requires soil stabilisation and proper compaction. To avoid settlement, investment in soling or subsequent material is mandatory, which requires time and money.

There is a huge requirement of murum, soling material and granular sub base for preparation of roads. Local site generated from the site after excavation, which is generally dumped, can be effectively re-used for the construction of sub-base of roads. Enzyme technology helps soil stabilisation and provides proper compaction prior to the top coat: the concrete/asphalt layer. Enzymes are natural, non-toxic liquids, formulated by using vegetable extracts and sugar molasses. They act as a bio enzymatic soil stabiliser. The enzyme enables utilisation of local soil, thereby resulting in savings of time, cost and effort. An enzyme transforms the soil into road base raw material. Enzymes are ecologically friendly products that can be used in the construction of highways, rural roads, township roads, secondary roads, airport roads, recreation paths and parking lots.

Enzyme-rich soil replaces conventional granular base (murum) and granular sub base (GSB). It also enables strength, performance and higher resistance towards deformation, which is a major bugbear from the maintenance perspective. The cementation process of enzymes with soil in depth of 200 mm is enough to sustain traffic load up to 70 tonne.

High-volume fly ash concrete (HVFAC) or high-performance concrete is concrete where fly ash content is more than 50 per cent. Large-scale concrete where cement is replaced by such by-products is highly advantageous from the point of economy, energy efficiency, durability and environmental benefits. Because of low water-cement ratio in HVFAC, high-end water-reducing admixture (superplasticizer) is used. In cases where high early strength and high slump are not required, the use of superplasticizer is not necessary.

The concept

• Elimination of GSB and murum by use of enzymes in the local soil.
• Stabilisation of ground soil and thus reduced settlement by adding enzymes.
• Reduction in plastic shrinkage and s in concrete owing to better sub base prepared from enzyme-rich local soil.
• Maximum use of available ground soil for preparation of base for road, which will lead to cost reduction in basic material.
• Reduction in time cycle for preparation of base for concrete/asphalt layer.
• Use of enzyme-rich soil stabilises sub-base prepared for laying of concrete/asphalt for the road.
• Maximum use of fly ash enables carbon credit points as per green building requirements (HVFAC consists of 55 per cent of fly ash).
• HVFAC is an M25 grade design that provides the concrete strength of M40 at the end of 56 days.

Uses of equipment

• Grader is used for proper alignment as per required gradient and to level the road.
• Compactor (40 T vibratory) is used to compact the enzyme-mixed soil to achieve compaction results. The use of static roller is also permissible, but it will not be able to give the results that a vibratory roller can.
• Water truck mixes the enzymes with water and ensures proper spraying of water through the local soil before compaction.
• Disc plough and plough are used to loosen the area where the road is being prepared prior to mixing of enzymes.
• Crumble plough enables proper mixing of soil after adding enzymes to it.
• Sand replacement apparatus is used to check the field density of compacted soil.

How does an enzyme act?

Enzymes act upon organic fines contained in the soil through a catalytic bonding process, which causes soil to bond during compaction into a dense permanent base that resists water penetration, weathering and wear and tear of the base.

Methodology for construction of HVFAC with enzymes

1. Original ground level: Filling or cutting as per ground profile. Use of site waste/local soil can be introduced for backfilling.

2. Sub base construction by enzyme dosing (200 mm): Murum or local soil (without any boulders) to be used for preparing sub base. The murum and existing soil should be tested to determine chemical contaminations if any, dry and wet density, optimum moisture content, and humidity. The formulation/dosage of enzyme will be determined based on the soil test report. All debris, boulders need to be removed from the existing ground level and the murum should be levelled in required slope before enzyme dosing. Enzyme to be dozed in the water tanker as per the dosages determined. The enzyme-dozed water shall be sprinkled over the levelled murum by perforated sprinklers attached to the water tanker. After sprinkling, the soil shall be scrapped to ensure proper mixing of the enzyme in the soil. The soil shall then be levelled using the scrapper. After levelling, vibratory rollers shall be used to compact the prepared sub-base layer and achieve minimum 98 per cent of the laboratory proctor density.

3. Antifriction layer: Waterproof polythene sheet/polypropylene plastic sheet 125 µm thick shall be laid over GSB layer. This will act as an antifriction layer and facilitate unrestricted movement of concrete slab over the sub base. Lap length of minimum 100 mm shall be provided for overlapping portions.

4. HVFAC (150 mm) (after sub base): Maximum size of course aggregates for concrete shall be 20 mm and should be natural material complying with IS: 383. Coarse aggregate shall consist of clean, hard, strong, dense, non-porous and durable pieces of crushed stone or crushed gravel free from deleterious material. For Grade I fly ash, testing shall be done once to satisfy requirement as per IS: 3812-1981.

Mix design considered is according to the details above; mixing of concrete shall not be done at site without a batching plant.

Slump at site should be 50-60 mm manual placing of concrete.

The inspection and testing for concrete shall be carried out as per the inspection and testing plan.

5. Construction joint and contraction joint: Construction of pavement shall be carried out in panels of 3 m (width) × 24 m (length) in a single pour. Transversal contraction joint shall be provided at every 4-m interval. Dowel bars shall be provided in the contraction joint. They shall be placed in a re-bar chair to hold dowels in proper position. Use 25 mm diameter scrap steel of 600 mm length as dowel bars. These shall be placed in concrete at the middle depth at 600 mm c/c. Half the length of these dowel bars shall be embedded in the concrete and the remaining half of the length of dowel bars shall be covered with plastic sleeves/ wrapped with plastic sheet/greased in the construction joint. This is done to break the bond between concrete and steel and provide unrestricted movement of slabs owing to thermal effects.

Tie bars from scrap steel of 600 mm length, 12 mm diameter at 500 mm spacing shall be placed between two lanes at middle of concrete thickening.

6. Groove cutting:

Option 1: Groove width of 3-4 mm and 30 mm deep shall be 'saw cut' after concrete has finally set (say within 72 hours). Interval for cutting in transversal direction is 4 mt c/c. Joints shall be sealed with approved bituminous filler.

Option 2: The contraction joint shall also be provided by placing a 3-mm-thick aluminium/MS strip for 30 mm deep immediately after placing the concreting at every 4m c/c and removing the strip after two hours of placing. The strip shall be greased before placing for smooth de-bonding. Utmost care shall be taken while removing to prevent any minor s getting induced in concrete and breakage of the edges. The removed strip can be reused further. In case of any doubt or if this is not possible, Option 1 should be adopted.

Dimension of the metal strip: 3,200 mm × 50 mm × 3 mm.

7. Broom finish for roads:

Surface texture: After the final set of the concrete and before starting curing, the surface of concrete slab shall be brush-textured in the transverse direction of the road (i.e. along the width of the road). Brush texture shall be applied evenly across the slab in one direction by the use of a wire brush. After the application of the brushed texture, the surface of the slab shall have a uniform appearance. The depth of the brush texture shall be approximately 1 mm.

Wire brush details: The wire brush shall be not less than 450 mm wide. The brush shall be made of 32 gauge tape wires grouped together in tufts spaced at 10 mm centres. The tufts shall contain an average of 14 wires and initially be 100 mm long. The brush shall have two rows of tufts. The rows shall be 20 mm apart and the tufts in one row shall be opposite the centre of the gap between tufts in the other row. The brush shall be replaced when the shortest tuft wears down to 90 mm long.

8. Curing: Minimum 14 days of curing (by ponding) shall be carried out.

9. Traffic movement: Human traffic shall be allowed after final set of concrete (for curing purpose). Light motor vehicles (LMV) can be allowed after 14 days of wet curing and three days of air curing.

Conclusion

• By introduction of enzyme technology, we can use the available soil/local soil on site that can lead to the elimination of GSB, murum and rubble soling.
• Reduction in cost, time and effort is possible through this technology.
• Reuse of construction waste and local excavated earth is made possible.
• Quality issues such as s on surface of concrete roads, settlement and de-bonding of top coat will be eliminated.
• Cost of enzyme technology along with HVFAC is comparatively less than the standard procedure used.
• Use of enzymes in the construction of base for road will enhance ecological and environmental parameters.
• Replacing cement with fly ash in a maximum percentage in concrete pro-vides advantages in cost, economy, energy-efficiency, durability and environmental aspects.

HVFAC Strength Results:

The average compressive strength for HVFAC road at different ages is as mentioned below:

• 7 days compressive strength: 16.76 mpa
• 28 days compressive strength: 32.05 mpa
• 56 days compressive strength: 40.23 mpa
• 90 days compressive strength: 47.36 mpa
(Courtesy: Mahindra Royale Cube Test Register)