GB2426994A

GB2426994A - Method for preventing cracks in hydraulically bound materials

Info

Publication number: GB2426994A
Application number: GB0511521A
Authority: GB
Inventors: Mark Andrew Holden; Richard Arthur Barker
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-06-07
Filing date: 2005-06-07
Publication date: 2006-12-13
Also published as: GB0511521D0

Abstract

The method, primarily for use in hydraulically bound support and bearing layers used on their own or beneath flexible or semi-flexible layers, involves de-bonding the aggregate 1 from the binder 2 whilst maintaining the full aggregate interlock so as to form a multitude of fine or micro cracks 4 which allow movement due to thermal expansion etc. without forming any visible cracks or causing any reflective cracking of an upper layer. The aggregate may be de-bonded by reducing the surface roughness of the aggregate by selecting aggregates with minimal surface porosity and low irregularity or by physically polishing aggregates or by using naturally polished aggregates. Alternatively, the de-bonding may be achieved by coating the surface of the aggregate with a de-bonding agent 3 such as silicon or possibly waste oil. Alternatively the de-bonding of the aggregate may be achieved by retarding the hydration process to form a weak joint around the aggregate such that cracks are formed due to the early thermal movements during the hydration process. Any of these approaches may be used in combination. Materials such as sawdust, olive kernels, plastic granules and clay pellets may be blended with the constituents to form weak zones or pockets which may be beneficial in aiding the micro cracking.

Description

I

Prevention of Cracks in Hydraulically Bound Materials This invention relates to Hydraulically Bound support and bearing layers used on their own or below an upper layer of flexible or semi flexible pavement material.

Flexible pavements, also known as asphalt pavements are characterised by their immediate serviceability, good riding quality and absence of joints. They are composed of bitumen bonded materials, of which aggregates hold the greatest proportion.

Semi flexible pavements combine the best characteristics of asphalt and concrete to create a pavement that can be quickly installed, that has a high load bearing capacity, good flexibility and high durability that does not require joints.

Examples of such materials sold under registered names are Salviacirn Betophalt by Alfred Kunz AG, DenSiphaftWby Densit A/S and ConfaltWby Contec ApS. These materials are all combinations of asphalt and concretelike materials, where a high strength micro silica mortar penetrates entirely into the air voids (20 to 40%) of an open textured asphalt structure, thus ensuring a homogenous pavement material with very low porosity and in comparison to regular asphalt, high compressive strength. This type of material is generally described as grouted macadam.

Grouted macadams have 2 - 3 times the Stiffness Modulus of traditional flexible pavement materials such as asphalt; High rutting resistance and very good durability and fatigue life, thus enabling the use of semi flexible pavements in heavy duty loading environments and with a thinner overall pavement construction.

Grouted macadams are used in the wearing and binder (penultimate) course layers for both internal and external applications where special requirements to load bearing capacity and durability must be fulfilled, for example in warehouses, on docks, airports, distribution centres, retail areas, good terminals, industrial floors, production halls, shopping centres, roads and other places where heavy loads and excessive durability is required.

As described above, a pavement containing a grouted macadam layer will exhibit very good durability characteristics and will usually have a longer service life than either concrete or asphalt. However a grouted macadam layer (and to a lesser degree a flexible pavement) is susceptible to reflective cracking when laid on top of a Hydraulically Bound Material base (HBM).

Uncontrolled cracking in a HBM base is mainly the result of its failure in tension due to naturally occurring internally induced stresses associated with hydration shrinkage and thermal shrinkage. Temperature changes cause the opening and closing of cracks in the HBM. The amount of movement depends upon the length of the HBM slab each side of the crack, the thermal properties of the HBM and the frictional restraint between the base and sub-base. These thermal movements in the HBM will impose concentrated strains and stresses in the upper layer of grouted macadam or flexible bituminous material and the crack will propagate through its thickness - reflective cracking.

The common way to minimise or delay this uncontrolled cracking and hence reflective cracking is to deliberately introduce cracks at a closer spacing than those which would naturally occur due to thermal movement. This is achieved by either wet-forming of joints at close centres (2 to 3m) or by de-stressing the HBM using a vibratory roller. With either method, a minimum curing period of 7 days is recommended prior to overlaying with a flexible or semi-flexible pavement material. Neither method is guaranteed to provide an "uncontrolled crack" free HBM.

The present invention therefore addresses the problem of how to provide a hydraulically bound material which may be used on its own or as a support layer to an upper layer of semi flexible or flexible material to create a pavement with a crack free surface course, which can be installed in one continuous operation i.e. without need for pre-cracking or a HBM curing break, prior to trafficking, loading or upper pavement layer installation.

This problem is solved by the present invention, by de-bonding the aggregate from the binder whilst maintaining full aggregate interlock to form a stiff, fine or micro cracked HBM. This not only retains the compressive strength and stiffness of the HBM but distributes a multitude of fine or micro cracks throughout the HBM matrix allowing thermal movement to occur without any visible or detrimental uncontrolled reflective cracking on the surface of the flexible or semi-flexible pavement.

A preferred embodiment of the invention will now be described with reference to the accompanying drawing in which: Figure 1 Shows the aggregate de-bonded / cracked from the binder with fine or micro cracking within the binder itself; Figure 2 Shows weak zones or pockets within the HBM to aid the occurrence of fine or micro cracks at the aggregate / binder interface and within the binder itself.

As shown in figure 1, the Hydraulically Bound Material (HBM) consists of a mixture of soil or aggregate I with binder 2 combinations that set and harden by hydraulic reaction and that has a water content suitable for compaction by rolling.

The binder 2 may be a fast setting and hardening cement such as Portland or a slow setting and hardening material such as lime, pulverised fuel ash (PEA) or granulated blast furnace slag (GBS).

The aggregate I may be a raw previously unused material or a recycled material (previously used in the construction industry) or a secondary aggregate (by products of industrial processes not previously used in construction) or a combination of all or part.

Suitable aggregates include limestone, china clay sand, crushed concrete, Incinerator Bottom Ash Aggregates, un-burnt colliery spoil, basic oxygen slag, road planning's.

The aggregate I is encapsulated by the binder 2.

The aggregate I is de-bonded 3 from the binder 2 such that any movement of the HBM due to internal or external factors, will cause a series of fine or micro cracks 4 to occur at the aggregate / binder interface and within the binder itself, which whilst maintaining full aggregate interlock of the HBM and thus intact properties such as load transfer, compressive strength and stiffness, will not allow cracks to occur at centres not exceeding 3m, or more preferably I m, or still more preferably 100mm or less.

The largest movement may occur at any point in the life of the HBM though most probably at less than one year old, more likely at less than 56 days old, still more likely at less than 28 days old and even more likely at less than 7 days old.

Movement may be caused by externally applied or internally developed stresses which exceed the tensile strength at the aggregate 1 / binder 2 interface and within the binder material itself, resulting in fine or micro cracking 4 at the aggregate / binder interface and within the binder itself. This may occur throughout the HBM's full depth and pavement area.

Externally applied stresses may be due to dynamic loading, static loading and drag resulting from the movement of an adjacent layer (sub-base or sub-grade). Internally induced stresses are associated with primary shrinkage (caused by self-desiccation due to hydration of the binder 2 and drying of the material) and with thermal shrinkage / expansion.

The opening and closing of a crack 4 caused by temperature changes in the HBM can be significant on long lengths of slab and may still be of the range 0.1mm to 2.0mm in a 5m slab. The fine or micro cracked binder 2 presented in this invention will limit crack widths to a maximum of 0. 25mm or less. Thus it is unlikely that any reflective cracking in the upper layers will be visible to the naked eye and will not cause any detrimental effect to or degradation of the bituminous based flexible material / grouted macadam and HBM.

The HBM's interlocked fine or micro cracked 4 matrix will negate any requirement for pre- cracking or de-stressing of the HBM to avoid the effect of reflective cracking in the upper layers.

As shown in figure 2, to aid fine or micro cracking 4 at the aggregate I I binder 2 interface and within the binder itself, materials may be blended with the dry or wet constituents of the HBM prior to installation, to form weak zones or pockets 5. These materials may be new or recycled and their selection is only limited by their elasticity, compressive strength and water expansive qualities, which may be beneficial in inducing internal cracking through expansion, contraction or flexure. These materials could include sawdust, olive kernels, plastic granules, clay pellets etc. The aggregate 1 may be de-bonded 3 from the binder 2 by reducing the surface roughness of the aggregate or by coating the aggregate to increase its slipperiness or by retarding the hydration process at the aggregate / binder interface or maybe through a combination of all or part.

Surface roughness of the aggregate I may be reduced by selecting aggregates with minimal surface porosity and low irregularity or by physically polishing aggregates or using naturally occurring polished aggregates. These materials may include gravels, glass, plastic, road planning's.

Slipperiness of the aggregate I may be ifçeased by coating the aggregate with a de- bonding 3 agent such as silicon, Teflon7or perhaps waste oil. The coating would be applied to the aggregate prior to mixing with the binder and water.

Retarding the hydration process at the aggregate I / binder 2 interface by up to 7 days would form a weak joint around the aggregate, such that any early age thermal movements during the hydration process of the HBM would de-bond 3 the binder and aggregate. Fine or micro-cracking 4 of the binder matrix would ensue through this thermal movement and through any subsequent externally applied load.

Claims

Claims 1. A Hydraulically Bound Material (HBM) in which the crack width

and location has been controlled to create a pavement with a visibly crack free surface, without the need for externally applied induced cracking or de-stressing.
2. A HBM according to claim 1, which will provide support to an upper layer of bituminous based flexible material to create a pavement with a visibly crack free surface course.
3. A HBM according to claim 1, which will provide support to an upper layer of semi- flexible grouted macadam to create a pavement with a visibly crack free surface course.
4. A HBM according to claim 1, which does not need to cure before overlaying with a bituminous based flexible or grouted macadam semiflexible upper layer.
5. A HBM according to claim 1, in which the aggregate and binder are debonded such that any movement of the HBM, will cause a series of fine or micro cracks to occur at the aggregate I binder interface and within the binder itself, whilst maintaining full aggregate interlock and thus intact properties such as durability, load transfer, compressive strength and stiffness.
6. A HBM according to claim 5, in which the fine or micro cracks at the aggregate I binder interface and within the binder itself are caused by externally applied or internally developed stresses which exceed the tensile strength at the aggregate / binder interface and the binder material itself.
7. A HBM according to claim 5, in which new or recycled materials may be blended with the dry or wet constituents of the HBM prior to installation, to form weak zones or pockets which will aid fine or micro cracking at the aggregate / binder interface and within the binder itself, by expansion, contraction or flexure upon exposure to water or compaction or hydration of the binder or thermal shrinkage / expansion or maybe through a combination of all or part.
8. A HBM according to claim 5, which will not allow surface cracks to occur at centres exceeding 3m, or more preferably 1 m, or still more preferably 100mm or less.
9. A HBM according to claim 8, in which surface cracks are limited to a maximum width of 0.25mm or less.
10.A HBM according to claim 5, in which the aggregate may be de-bonded from the binder by reducing the surface roughness of the aggregate or by coating the aggregate to increase its slipperiness or by retarding the hydration process at the aggregate / binder interface or maybe through a combination of all or part.
11. A HBM according to claim 10, in which the surface roughness of the aggregate may be reduced by selecting aggregates with minimal surface porosity and low irregularity or by physically polishing aggregates or using naturally occurring polished aggregates.
12. A HBM according to claim 10, in which the slipperiness of the aggregate may be increased by coating the aggregate with a de-bonding agent such as silicon, Teflont(T'r-) waste oil.
13. A HBM according to claim 10, in which the hydration process at the aggregate / binder interface is retarded to form a weak joint around the aggregate, such that any early age thermal movements during the hydration process of the HBM would de-bond the binder from the aggregate and cause fine or micro cracking at the aggregate / binder interface and within the binder itself.