Executive Summary
Cementitious stabilisation offers a cost-effective method of improving long-term performance and reducing whole-of-life costs of modern, heavily-trafficked pavements. Cracking is the primary and predominant distress type of cementitiously-bound materials. There are two principal forms of cracking:
• cracking from hydration and drying shrinkage
• fatigue cracking.
The shrinkage cracking dilemma severely impedes the utility of cementitious stabilisation in pavements. Conventional approaches to mitigating shrinkage cracking result in substantial performance and design constraints.
Renolith nanotechnology is highly effective at preventing cracking distress. This enables a greater range of options in pavement design and a practical means to substantially improve the longevity of road pavements and reduce the triple-bottom-line cost.
Cementitious stabilisation – Pro/Con
Austroads Guide to Pavement Technology Part 4D, Stabilised Materials AGPT04D-19 states:
The use of stabilisation technology for stabilising and recycling materials for pavement construction and maintenance is widely accepted as a cost-effective method of improving long-term performance and reducing whole-of-life costs of modern, heavily-trafficked pavements.
Cementitiously-bound pavement materials are produced by the addition of stabilising binders to granular materials in sufficient quantities to produce a material which has tensile strength, this being significantly higher for bound materials compared to lightly-bound materials. Binders commonly used include cement and cementitious materials.
A bound material acts like a ‘beam’ in the pavement to resist traffic loading and has significantly increased structural capacity compared with unbound granular and modified materials. However, shrinkage and premature fatigue cracking in the pavement layers needs to be controlled.
Cracking is the primary and predominant distress type of cementitiously-bound materials. There are two principal forms of cracking:
- cracking from hydration and drying shrinkage
- fatigue cracking.
A combination of subgrade restraint, high shrinkage and tensile strength in cementitiously-bound materials can cause widely-spaced (commonly 0.5–5.0 m) transverse and/or block cracking to occur.
Shrinkage cracking
Austroads Guide to Pavement Technology Part 2 – Pavement Structural Design (AGPT02-24) states:
Cracking in cemented materials is normally due to thermal and shrinkage stresses resulting from hydration of the binder.
Shrinkage cracking of cemented materials tends to be unavoidable. Cracks which propagate to the pavement surface provide pathways for the infiltration of moisture which can lead to debonding of layer interfaces within the pavement and/or weakening of granular layers and subgrade. The extent and severity of cracking is influenced by factors such as binder type and content, material type, initial moisture content and drying and curing conditions.
The shrinkage cracking challenge is summarized below.
In Shrinkage Cracking Of Soils And Cementitiously-Stabilized Soils: Mechanisms And Modeling, Li explores the problem further. Excerpts below:
Abstract
Shrinkage cracking of soil or cement soil can cause the infiltration or seepage of water into the material and lead to reflective cracking in the structure above it. Shrinkage cracking greatly limits the use of cementitious stabilization.2.6.4 Shrinkage Cracking
The risk of shrinkage cracking in any given structure is related to many factors, including the magnitude and distribution of free shrinkage, rate of shrinkage, degree of stress relaxation, degree of structural restraint, and how the material strength and stiffness develop (Weiss and Shah 2002).2.7 CONCLUSIONS OF LITERATURE REVIEW
Based on the literature review, the major conclusions are listed as below.
- Cementitious stabilization can substantially improve the engineering properties of unbound materials, such as the strength, stiffness, durability, and stability, etc. But it has its own engineering problems. The primary distresses associated with the use of CSM in pavement are load induced fatigue cracking and shrinkage cracking.
- Many factors have effect on the CSM properties, including host material, binder type, binder content, moisture content, curing condition, age, etc.
- The shrinkage of CSM can be separated into four categories: autogeneous shrinkage, drying shrinkage, thermal shrinkage, and carbonation shrinkage. Autogenous shrinkage is relatively small. Temperature shrinkage can be conveniently superposed with drying shrinkage. Carbonation shrinkage is a long term reaction. Drying shrinkage dominates the shrinkage strain. It is directly related to moisture loss and caused by matric suction.
- Moisture loss from any porous material depends mainly on the surface area, the lengths of the moisture migration pathways, and the drying environment. Nonlinear moisture diffusion theory should be used to describe the drying process of CSM. Although there are many studies about the moisture diffusion in concrete, few study focus on CSM.
- The shrinkage coefficient could be used to express the change in shrinkage strain as a function of moisture or relative humidity change. There are models developed for shrinkage coefficient for concrete. But for CSM, the model is not well established.
- The shrinkage stress is caused by restraint of shrinkage strain. If the restraint condition is known, the shrinkage stress can be solved based on equilibrium and compatibility equations. However, during this procedure, the stiffness change of the material due to moisture loss and cementitious hydration should be considered.
- Shrinkage of stabilized materials is coupled with relaxation. Relaxation of stabilized material reduces the shrinkage stresses and mitigates the occurrence of shrinkage cracking.
- Different shrinkage cracking criteria are used in literatures, such as the criterion based on empirical, tensile strength, fracture mechanics, and energy minimization principle. The shrinkage crack prediction model are not sufficiently verified by tests for CSM;
Shrinkage cracking – implications
In concrete pavements, shrinkage cracking can be accommodated via control joints and reinforcement. This is not appropriate for cementitiously-stabilised flexible pavements.
This results in a dilemma:
- Design models struggle to predict shrinkage cracking.
- Conventional methods can only reduce or mitigate (not prevent) cracking.
- Minimising the cementitious binder content yields a weaker, less useful material.
Renolith crack-free pavements
The shrinkage cracking dilemma severely impedes the utility of cementitious stabilisation in pavements.
The conventional approach to manage cracking is to impose constraints on the design. Some soils aren’t considered viable. Binder content is typically limited to the lightly-bound range to reduce the severity of shrinkage cracking, which means limits on the performance of the material and how that material is used in a pavement. Moreover, it is difficult to be confident that the pavement will actually achieve the design life because of the combined effects of drying shrinkage, traffic and environmental effects. This typically results in either an underperforming pavement, or an overengineered (and expensive) pavement.
Renolith nanotechnology solves the dilemma and removes the constraints.
Renolith significantly improves the engineering properties of bound materials and greatly reduces the probability and severity of shrinkage cracking. This enables much more flexibility in design and construction and results in better pavements. It also significantly reduces the risk of premature failure because unpredictable drying shrinkage is removed from the equation.
The Renolith 2.0 product may be used in any cementitiously-stabilised pavement or concrete application. The optimal use is in bound or heavily bound (UCS>2MPa) flexible pavement base layers. This achieves concrete-like performance advantages whilst retaining the simplicity and low-cost of cementitiously-stabilised flexible pavements.
Renolith admixtures have been used in roads for over 30 years, totalling more than 70,000,000m2 of cementitiously bound pavement bases. There are no reported instances of shrinkage or fatigue cracking. This phenomenon is explained by the Renolith crack prevention model below. This ‘crack-free’ behaviour greatly improves the longevity of road pavements and provides an opportunity to substantially reduce the triple-bottom-line cost.
For example, the picture below shows a trial road built in the Emilia-Romagna region of Italy. The pavement construction was a cementitiously bound base with a thin bituminous wearing course. The right-hand lane (control – no admixture) base formed severe shrinkage cracks which propagated through the wearing course. The left-hand lane (Cement + Renolith admixture) did not crack.
Fatigue cracking
Austroads Guide to Pavement Technology Part 2 – Pavement Structural Design (AGPT02-24) states:
- Material fatigue relationships are key inputs for the design of modern, heavily trafficked pavements.
- The fatigue relationships of cemented materials may be estimated from the flexural strength and elastic modulus.
Renolith admixture improves the flexural strength and elastic modulus of cemented materials. The degree of improvement depends on the material and binder dose; typically falling in the range 20%-70%.
In a typical scenario assuming a 30% improvement in flexural strength and elastic modulus compared to cement-only stabilisation, a massive increase in the calculated fatigue life (i.e. traffic capacity) can be achieved. Further, the actual life of the pavement is likely to far exceed the calculated life because of the self-healing attributes of Renolith-enhanced materials. Self-healing is not considered in the Austroads design model.
Conclusion
Cementitious stabilisation offers a cost-effective method of improving long-term performance and reducing whole-of-life costs of road pavements. Shrinkage cracking and fatigue cracking are the primary and predominant distress types of cementitiously-bound materials. Conventional approaches to mitigating shrinkage cracking result in substantial design and performance constraints.
Renolith nanotechnology is highly effective at preventing cracking distress. Renolith provides a practical method to greatly improve the longevity of road pavements and reduce the triple-bottom-line cost.