Pavement

Cement stabilisation is one method to prevent pavement failures from increased heavy traffic use. The process involves adding cement and water to pavement aggregates to enhance the engineering properties of the pavement. The cement reacts with water in the same way as it does in concrete, and the resulting cement hydration products bind the aggregate particles together.

Cement stabilisation typically increases the pavement’s load bearing capacity and stiffness, and reduces its sensitivity to moisture. Resistance to rutting and other deformations is also improved. It can be used in the construction of new pavements and the rehabilitation of existing pavements.

The drive to minimise waste and to conserve our natural resources has focussed attention on the use of our aggregate resources. Cement stabilisation is an ideal method to improve the properties of marginal materials so that they can be used instead of premium quality aggregates even in the highest stressed areas of a pavement. Both new aggregates and recycled aggregates in existing pavements can be stabilised.

NZ specifications now exist for in-situ stabilisation of modified pavement layers (TNZ B/5), in-situ stabilisation of bound sub-base layers (TNZ B/6), the manufacture and construction of plant mixed modified pavement layers (TNZ/B7) and the manufacture and construction of plant mixed bound sub-base pavement layers (TNZ/B8). These can be found on the NZTA website.

The stabilised material is grouped into two types:

Modified – typically <2% cement. This is normally used as an intermediate option, bridging the gap between an unmodified and structural pavement for improving resistance to deformation and rutting.  It also enables the use of lower quality aggregates or insitu recycled materials. It would be used on State Highways with high traffic volumes.

Bound – typically >3% cement. This is normally used to provide a sub-base with high strength over a weak substrate to support the overlying chip seal surfacing on thick asphalt pavement layers. It would be commonly used under rigid concrete pavements and on high volume or heavily loaded roads.

Pavement types are shown in the table below:

 

Modified Sub Base

Cemented Sub Base

Modified Base and Cemented Sub Base

Base

Modified

Granular

Modified

Sub Base

Granular

Bound

Bound

 

Standard configurations incorporating stabilised pavement layers and subgrades (Austroads 2006) are displayed below:

layers-pavement 

Cement and water can be blended with the aggregate on site (“in-situ stabilisation”) or at a separate processing plant.

In-situ stabilisation is the process of mixing cement and water in place. Cement is placed on top of a pre-dampened granular layer or existing road and then mixed in by hoe to a specified depth. Extra water is then applied from a following truck to provide the water needed to hydrate the cement. Finally, the layers are compacted by several passes of a vibrating roller.

In-situ stabilisation is common for pavement rehabilitation and can be used for new pavements using imported granular materials. Additional coarse aggregate and cement may be needed to allow for breakdown of aggregates. Laboratory testing of the hoed material is recommended to ensure that the cement stabilised material achieves the required strength.

Although the required cement can be spread in front of the stabiliser it tends to be added separately from the water in a self contained mixing chamber.

Plant mix is produced by continuous feed mixing or in a pugmill at static mixing plants. Electronic weighing systems and automated batching improve control over the mix proportions of aggregates, binders, water and additives.

Plant mix provides tighter control of batch quantities than in-situ mixing. Consequently it is more suitable for new modified basecourse layers. Plant mix needs to be placed within two hours of manufacture.

Consistently manufactured EverSure will meet all requirements for cement stabilisation. In addition Golden Bay Cement can blend specific levels of EverPlus Class C Fly Ash to achieve any necessary requirements such as increasing placement times, improving long term bearing capacity and to assist with compaction.

Concrete Roads

Concrete pavements are in extensive use in many parts of the world but have not been utilised for roads in New Zealand. Historically, New Zealand engineers have been constrained to use the cheapest cost design in constructing the national road network in a sparsely populated country and this has prevented the adoption of concrete.

The New Zealand road network has developed with an increase of traffic densities and hence pavement costs have grown considerably.

Economic efficiency in pavement design is still just as vital as ever but achieving it now requires a more sophisticated approach. Instead of simply minimising the initial cost, it is also necessary to consider the long-term user and maintenance benefits of the various alternative pavement types available.

The modern concrete pavement has been improved and now provides significant road user benefits as well as the traditionally recognised advantages of great durability and lower maintenance costs.

The Transfund New Zealand Pavement Evaluation Manual is the accepted framework for economic calculation of the merits of road works.

There is an extensive list of areas associated with the calculation which affect the comparative measures of concrete pavements and the competing flexible pavement options. These are, namely the:

discount rate, analysis period, initial cost, future pavement maintenance assumptions, pavement rigidity effects and surface texture on rolling resistance and fuel consumption, tyre/surface noise generation, carbon dioxide emissions, travel times savings during maintenance activities and early completion.

The construction of concrete roads is split into three categories based on design:

Jointed Plain Concrete Pavement - (JPCP) contains enough joints to control the location of all the expected natural cracks. The concrete cracks at the joints and not elsewhere in the slabs. Jointed plain pavements do not contain any steel reinforcement. However, there may be smooth steel bars at transverse joints and deformed steel bars at longitudinal joints.

Jointed Reinforced Concrete Pavement - (JRCP) contains steel mesh reinforcement (sometimes called distributed steel). In jointed reinforced concrete pavements, designers increase the joint spacing purposely, and include reinforcing steel to hold together intermediate cracks in each slab.

Continuously Reinforced Concrete Pavement - (CRCP) does not require any transverse joints. Transverse cracks are expected and CRCP pavements are designed with enough steel so that cracks are held together tightly. Determining an appropriate spacing between the cracks is part of the design process for this type of pavement.

CRCP designs generally cost more than JPCP or JRCP designs initially due to increased quantities of steel. However, they can demonstrate superior long-term performance and cost-effectiveness.

There are two methods for concrete pavement which are placed on top of a well prepared sub grade and base:

Slip Form Paving is produced by a machine that rides on treads over the area to be paved—similar to a train moving on a set of tracks. Fresh concrete is deposited in front of the paving machine which then spreads, shapes, consolidates, screeds, and float finishes the concrete in one continuous operation.

Fixed Form Paving is produced by stationary metal forms are set and aligned on a solid foundation and staked rigidly. Final preparation and shaping of the subgrade or sub base is completed after the forms are set. Forms are cleaned and oiled first to ensure that they release from the concrete after the concrete hardens. Once concrete is deposited near its final position on the subgrade, spreading is completed by a mechanical spreader riding on top of the preset forms and the concrete. The spreading machine is followed by one or more machines that shape, consolidate, and float finish the concrete.