Humboldt H-4140 GeoGauge - Manual (Page 24)

24
For decades, cement-amending a roadway
base has been a way of employing
economical local materials in place of
imported, high cost aggregate. These
bases have the strength and low water
permeability to potentially out perform
flexible aggregate bases. Unfortunately,
cement-amended bases are prone
to significant shrinkage cracking that
concentrate and reflect traffic stress
resulting in premature pavement cracking
sufficient to warrant repair or rehabilitation. Consequently, cement-amended
bases have seen limited use over the last twenty years.
A simple and economical construction method has been developed in Europe
and implemented in the United States that minimizes shrinkage cracking in
these bases. Dr. Tom Scullion of Texas A&M University accomplished the
implementation in response to a request from the City of College Station,
Texas 1 . The method included aspects that have been under investigation
for many years such as controlling the amount of cement used (3% to 9% by
weight), the water content, the quality of the subgrade and the type & time of
final surfacing. What was new in the method was the controlled induction of
micro-cracks into the base. The micro-cracks prevent the development of the
larger shrinkage cracks without significantly impacting stiffness of the base.
The performance of the mix was first verified through a series of unconfined
compressive strength tests in the laboratory. A strength of 300 psi to 500 psi was
required with around 300 psi considered optimal.
During the first two days of life, the stiffness of the base was evaluated to
assure sufficient ultimate strength. The 6” to 12” lifts addressed by the method
had to achieve a stiffness of about 50 to 60 Mega Newtons per meter (MN/m)
during this period. At the end of this period, limited vibratory rolling (1 to 4
passes) using a 12-ton, steel wheel roller was conducted to lower base stiffness
by about 40% and thereby induce a network of micro-cracks. Stiffness was
directly measured in real-time without penetrating the base using the Humboldt
GeoGauge.
CASE STUDY:
Micro-Cracking
A Cement Treated Roadway Base
To Minimize Shrinkage Cracks
Texas A&M University
The Citites of Bryan & College Station, TX
The Cities of La Quinta & Santa Rosa, CA
CEMEX
1 Field Investigation: Pre-Cracking of Soil-Cement Bases to Reduce Reflection Cracking, 2001, Tom Scullion,
Texas A&M University, College Station, TX 77843