Effect of excessive cement in stabilised subbase 2

[defg]

I have cariedout laboratory tests for subbase stabilisation trial test.According to the test results revealed that by using 2.5%cement is adequate to aquire a streght of more than 1MPa including the initial consumption of lime/cement.When the Contractor started to lay the stabilised materials on the road,the quality control of the materials taken from the road during spreading of the stabilised material.This quality control was done in the laboratory by moulding the samples,cured for seven days and soaked for four hours prior to crushing.When crushed the test results ranges from(2.87 to 2.o6)MPa.This seems to be higher than 1MPa minimum given in the specifications.In the specifications no maximum strength has been given,except minimum strengh which is 1MPa.Hopefully this increment may cause cracks of the layers.If no serious cracks will be noted,can I allow the Contractor to proceeds with the base course layer?(The base course layer is expected to be crushed stone-CRR)

 

[kslee1000]

Suggest to check ACI acceptance criteria on deviation of concrete compressive strength, and follow through.

 

[Ron]

defg...this is the answer I posted to your similar question in the "Materials Engineering, Other Topics" forum...

Two common problems exist. The first is that you'll get more shrinkage in the cement stabilized material. This will cause cracks and they will reflect through a flexible pavement layer (asphalt).

The second problem is bonding. You will definitely need to apply a strong tack coat to make sure you don't lose bond between the asphalt and base. Since you mentioned that this is a subbase, you might want to consider some roughening of hard surface prior to adding base layer if it is a granular layer as opposed to an asphalt layer.

Additionally, you have asked if it is OK to allow contractor to go ahead if there are no cracks.

There will be cracks. It is a cement stabilize material, so you can expect cracks. Usually the cracks will occur at regular intervals depending on the strength of the material. Interestingly, up to a point, the stronger the material, the closer the cracks will be together.

Since you are using a crushed stone base material, the cracks will be irrelevant. Don't worry about them. Further, the higher strength material at a level below the aggregate base does not detrimentally affect the pavement section...in fact, it helps. I would be concerned about the consistency of the contractor's operation and achieving consistent results over the entire roadway. If he is using a low cement content and achieving higher strength results, then your either of the following may be true:

1. Your trial batch materials are not representative of the in-situ materials
2. The contractor's inconsistency has led you to test an overly strong mixture, that perhaps doesn't exist everywhere. If that's the case, then there are area of likely lower strengths than the specification allows.

Do some more testing. The samples can and should be molded in the field, then allowed to harden, then cured in the lab and tested. Do this as they are placing and mixing the materials. Site mixed cement stabilization can be very erratic in strength.

 

[VersaK]

defg,
I agree with everything posted so far, having done several roadway rehab programs where FDR (full depth reclamation) cemented base has been used. generally more cement menas more hydration/shrinkage cracking, so we try to use minimum % that still achieves required f'c value for the base. I havent seen it used in subbase application before, but I imagine its going to work the same. you mentioned creating labratory specimens from in-place mixed material; youmay want to try coring the hardened subbase to see what the actual value is of the field compacted subbase. something else to consider; the process of micro-cracking (making passes with vibratory roller prior to placement of surfacing course) may help alleviate some refelction cracking.

 

[defg]

Universalkeith,
Thanks for good reply,let me give you a short history of the source of materials prior to stabilisation.In the project corridor no suitable borrowpits which comply to C1 materials requirements prior to stabilisation.This forced the Contractor to crush soil materials with some rock boulders inorder to meet the requirements prior to stabilisation.When the parameters checked were found within the requirements for C1 prior.When cement added in the laboratory it gave the required stregth,including the initial consumption of lime/cement.The materials spreaded on the road was mixed with cement in the mixing plant.And on the road it was spread by a paver.Samples were taken from the road during spreading for unconfined compressive strength,but the test results obtaied after 7days curing and soaked 4hours were found drastically higher than normal!

 

[VersaK]

again i would say instead of labratory moulded samples, I would try coring in-place material. Usually a 4-inch concrete coring bit will work, then trim the ends and sulfur cap, see what you get for compressive strength. I would recommend taking at least two or three cores for comparison purposes ,and try to stickt ot he top part of the core, as the lower (deeper) you go, the more likely it is you will hit material that hasnt been thoroughly mixed.