Mix Design Process – Insitu Stabilized Materials

Mix Design Process : In a similar nature to the majority of engineered materials used for road building, mix design is paramount to ensure the anticipated performance is achieved from a short and long term perspective.

With particular reference to Polymer Modified Materials (PCM’s) and Cement Modified Base (CMB) materials, determining the binder dosages and the additional virgin material quantities (if ordered) is paramount prior to undertaking insitu stabilization for a cost-effective and well performing outcome.

Prior to commencing the insitu stabilization process, common practice includes undertaking a comprehensive geotechnical investigation and extracting the representative material throughout the job to undertake laboratory testing for:

• Particle Size Distribution – To determine the grading of the material, particularly helpful in determining the percentage of fines and the consequence of the ‘soaking’ affect if the mix is fines rich.
• Atterberg limits – Used to determine the characteristics of the material and are particularly helpful in determining the plasticity of the material, further providing guidance on the suitability of the mix.
• California Bearing Ratio – Not always paramount, however CBR testing provides an indication of the material strength (pre-binder) to determine if the material is suitable.

After the initial ‘suitability’ testing is undertaken, consideration is typically given to the desired performance outcomes, including:

• Modification – whether or not the stabilizing agent is only required to increase the material properties (i.e. reduce moisture sensitivity) or:
• Strength Gain – whether or not the addition of the binder is to increase the strength of the existing pavement material to achieve a desired unconfined compressive strength (UCS)

If strength gain is desired, further testing is required to determine the required binder dosages with consideration given to the required strength properties, generally UCS testing is undertaken at 3-5 binder dosages (for 7 or 28 day curing) and linear regression is then undertaken to determine the desired dosage.

Whilst not considered as high risk in PCM materials given the increased flexibility provided by the polymer binder, the desired strength gain must be balanced against the risk of a) reflective shrinkage cracking and b) the risk of premature fatigue cracking (particularly if the stabilization depth is <150mm)

If the material is deemed unsuitable from strength perspective (i.e. required binder dosages are too high, or the desired UCS isn’t achieved at all) consideration is also commonly given to blending in virgin materials at a nominal depth to increase the existing properties prior to stabilization.

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