Field of the Invention

The present invention relates to a method and apparatus for assessing strength development in cementitious materials, including, but not limited to shear strength development in sprayed cementitious materials. Background of the Invention

Strength development is often a very important characteristic of cementitious materials used in civil and mining engineering. Cementitious materials comprise a combination of cement, sand, and water. Depending on the application at hand, aggregate and additives such as hardeners, accelerators, fillers and reinforcing fibres may also be incorporated. The provision of water in the cementitious material causes a reaction known as "hydration". Hydration involves many different chemical reactions resulting in the bonding together of individual sand and aggregate particles to form a solid mass. The process of strength growth during hydration is largely due to the formation of calcium silicate hydrate as the cementitious material continues to hydrate.

The hardening of cementitious material continues over a long period of time stretching to months and even years. One well known method of testing strength for cementitious material is to take a sample of the material being used for a particular application to form cylinders which are cured for specific times, for example 2, 7, 28 and 90 days at a set temperature such as 20 C. After reaching the required age for testing the cylinders are crushed in a press. The crushing pressure, measured in MPa, provides an indication of strength development after initial hardening.

In the context of civil and mining engineering excavations, it is common to spray cementitious materials (typically known as shotcrete) onto an exposed rock face to form a protective canopy for personnel and equipment. A given level of strength is required before personnel can safely enter a shotcrete supported excavation. The shotcrete must be able to support its own mass and the mass of any small, loose rocks that may have not been removed, for example by hydro scaling, prior to the application of the shotcrete. The wait time required for a given level of hydration to be achieved places productivity and logistical constraints on the further development of the excavation. It is particularly advantageous for shotcrete to have early or very early strength development characteristics. Shotcrete strength development is often tested indirectly on samples within the excavation. This in itself may disregard early safe re-entry time rules for shotcreted excavations.

The present invention was developed in the context of considering the requirements for strength development in sprayed cementitious material (i.e. shotcrete). However embodiments of the present invention are not limited by the manner of application or placement of cementitious material and may, for example be applied equally to pumped or poured cementitious material. Summary of the Invention

Aspects of the present invention are based on an observed correlation or relationship between electrical resistance and shear strength development of hydrating cementitious materials. Thus, in one aspect the invention may be considered as a method of assessing shear strength development in cementitious material by firstly obtaining empirical data correlating electrical resistance with shear strength development of a hydrating sample of the cementitious material; applying a batch of the cementitious material as required; and, during hydrating taking measurements of electrical resistance which can then be correlated to an estimated shear strength on the basis of the empirical data. This shear strength may also be related to a predetermined safe re-entry time in the event of the application of the cementitious material as shotcrete to excavation surfaces. In an inter-related aspect, the present invention provides an electronic instrument for making electrical resistance measurements of hydrating cementitious material.

One aspect of the invention provides a method of assessing shear strength development in cementitious material comprising:

measuring electrical resistance of a sample of cementitious material during hydration of the cementitious material to establish a correlation between electrical resistance and shear strength of the hydrating sample ;

applying a batch of the cementitious material in accordance with a desired purpose;

measuring electrical resistance of the applied cementitious material in situ during hydration; and,

using the measured electrical resistance of the applied cementitious material and the established correlation to assess the shear strength of the applied hydrating cementitious material. In another aspect the invention provides a method of determining safe access time to excavation surfaces on which a cementitious material is sprayed, the method comprising:

measuring electrical resistance of a sample of sprayed cementitious material during hydration of the sprayed cementitious material to establish a correlation between electrical resistance and shear strength of the hydrating sample;

applying a batch of the cementitious material by spraying onto the excavation surfaces;

measuring the electrical resistance of the applied cementitious material in situ during hydration; and,

using the measured electrical resistance of the applied cementitious material and the established correlation to determine when the shear strength of the applied cementitious has reached a level deemed safe to allow re-entry to, or a location adjacent, the excavation.

In one embodiment, measuring the electrical resistance of the applied cementitious material comprises: measuring the electrical resistance between two or more electrical current paths in the cementitious material. In one embodiment, measuring electrical resistance comprises: using an electrical resistance measurement probe with a first electrode of one polarity and two or more second electrodes of an opposite polarity where the second electrodes are spaced an equal distance from the first electrode to measure the electrical resistance.

In one embodiment the method comprises using a probe wherein the two or more second electrodes are evenly spaced about the first electrode. In one embodiment the method measuring the electrical resistance of the applied cementitious material comprises embedding the first and second electrodes in the applied cementitious material.

In one embodiment the determination of when the shear strength has reached the level deemed safe is made when the measured electrical resistance along each of the two or more paths correlate to the shear strength having reached the level deemed safe.

In one embodiment measuring the electrical resistance of the applied cementitious material comprises transmitting electrical resistance

measurements to a location remote from that to which the cementitious material is applied.

A further aspect of the invention provides an electrical resistance measurement probe comprising:

a first electrode;

two or more second electrodes, the second electrodes being spaced an equal distance from the first electrodes; and,

an electrical power source connected between the first electrode and each of the second electrodes.

In one embodiment the second electrodes are spaced evenly about the first electrode.

In one embodiment the probe comprises three second electrodes.

In one embodiment the probe comprises a body supporting the electrodes wherein the electrodes extend from a founding surface of the body.

In one embodiment the electrodes extend an equal distance from the founding surface. In one embodiment the first electrode comprises one of the group comprising: a pin, a post and a rod. In one embodiment at least one of the second electrodes comprises a blade or plate like member.

In one embodiment each of the second electrodes comprises a blade or plate like member.

In one embodiment a voltmeter is arranged to measure voltage drop between the first electrode and one or more of the second electrodes, and an ammeter is arranged to measure current flow between the first electrode and one or more of the second electrodes; wherein electrical resistance measured by the probe is determined by dividing the measured voltage by the measured current.

In one embodiment the probe comprises a communications circuit arranged to communicate resistance measured by the probe to a location remote from the electrodes.

In one embodiment the communications circuit is a wireless communications circuit. Brief Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 a is an elevation view of an electrical resistance measuring device in accordance with an embodiment of the present invention;

Figure 1 b is a plan view of the electrical resistance measurement probe; Figure 1 c is an isometric view of the electrical resistance probe;

Figure 1 d is an inverted isometric view of the electrical resistance probe;

Figure 2 is an electrical diagram of the electrical resistance measurement probe circuit;

Figure 3 is a graph illustrating a relationship between electrical resistance and shear strength development during the first eight hours of hydration of a cementitious material;

Figure 4 is a re-scaled version of the graph shown in Figure 3 in which the shear strength scale is magnified by a factor of 500;

Figure 5 is a graph showing a relationship between electrical resistance and shear strength development during the first four hours of hydration of a cementitious material for a given layer thickness of a different composition to that represented in Figure 3 and 4; and,

Figure 6 is a graph of minimum shear strength required to develop in a cementitious material of a given thickness in order to support a regular tetrahedral block of rock with one metre side lengths having a mass of approximately 400kg.

Detailed Description of Preferred Embodiments

The present embodiments are described in the context of preparation and use of shotcrete (i.e. sprayed cementitious material). Sprayed cementitious materials are based on hydraulic cements and pozzolanic materials that with time hydrate in the presence of water, transitioning from an initial fluid gel that may be sprayed, to a hardened solid that forms a structural or protective layer. As hydration proceeds water encapsulated in cementitious material is consumed in the chemical reactions that convert the fluid gel to a hardened solid. As the encapsulated water is consumed the electrical resistivity increases. Embodiments of the present invention correlate the increase in electrical resistivity to shear strength of sprayed cementitious material.

Furthermore, as described herein below, an aspect of the present invention provides an electrical resistance probe to enable electrical resistance measurement.

An instrument that may be used to measure the resistance of steel reinforced concrete for the purposes of assessing corrosion, rate of corrosion, and probability of corrosion is known as the Wenner linear 4-point instrument (Wenner instrument). The Wenner instrument was originally developed to measure resistivity of soil and, comprises four electrodes arranged along a common line. When the Wenner instrument is used to measure resistivity of steel reinforced concrete, it incorporates a mould into which the electrodes extend. Concrete is poured into the mould embedding the electrodes. Current is supplied to the concrete through each of the end electrodes and a potential difference is measured between the two inner electrodes. Knowing the potential difference and the current supplied, resistance can be calculated using Ohm's law and the "Wenner geometrical factor". The Wenner geometrical factor assumes a homogeneous, infinite, isotropic material; symmetrical current flows; and, uniform current density.

Typical shotcrete layer thickness for ground support ranges from 50mm to 200mm. Further, shotcrete is a heterogeneous mixture and electrical current flows are asymmetrical with non-uniform current density. Research conducted by the inventors shows that electrical current is greatly dissipated in a typical shotcrete layer rendering the Wenner probe unsuitable for practical use in assessing strength development of the cementitious materials and in particular in situ measurement of electrical resistance for the purposes of assessing shear strength development in sprayed cementitious materials. Figures 1a - 2 illustrate an embodiment of an electrical resistance

measurement probe 10 (hereinafter referred to in general as "probe 10"). The probe 10 is particularly well suited to in situ electrical resistance measurement of hydrating cementitious material. When used in relation to shotcrete one or more of the probes may be inserted into shotcrete sprayed within a standard mould set up within the excavation or alternately inserted into shotcrete sprayed onto the wall of the excavation itself.

The probe 10 comprises a hollow cylindrical body 12, a first electrode 14, and two or more (in this instance three) second electrodes 16a, 16b and 16c (herein after referred to in general as "second electrodes 16"). Each of the second electrodes 16 is spaced in equal distance D from the first electrode 14.

Moreover, the second electrodes 16 are evenly spaced about the first electrode 14. In the present instance, where the probe 10 comprises three second electrodes 16, the electrodes 16 are angularly spaced 120 ^° apart about the first electrode 14.

The first electrode 14 is in the form of a pin, post or rod. Each of the second electrodes 16 however is in the form of a plate or blade like electrode.

Electrodes 16 are orientated so that a corresponding plane in which they lie is substantially perpendicular to a line between the first electrode 14 and a midpoint of the corresponding second electrode 16. Also, each of the electrodes 14 and 16 extends for the same distance from a founding surface 18 of the body 12. Thus by maintaining the founding surface 18 substantially parallel to a surface of the body of cementitious material into which the probe 10 is pushed, each of the electrodes 14 and 16 will be embedded to

substantially the same depth in a cementitious material.

Figure 2 illustrates a power source such as a battery 20; ammeter 22 and voltmeter 24 which are associated with the probe 10. A positive terminal 26 of battery 20 is connected to the second electrode 16a. However electrode 16a is also electrically connected to the electrode 16c and subsequently 16b via conductors 28a and 28b. Thus the second electrodes 16 are coupled in series to the positive terminal 26. Negative terminal 30 of the battery 20 is connected via the ammeter 22 to the first electrode 14. By virtue of this arrangement, the potential difference between any one of the second electrodes 16 and the first electrode 14 is the same. Voltmeter 24 is connected between the first electrode 14 and second electrode 16b.

The battery 20 may be disposed inside or outside of the body 12. It is however envisaged that the battery 20, ammeter 22 and voltmeter 24 are all disposed outside of the body 12. In one example they may all be demountably supported on the probe or held on or in a plug that can be selectively plugged onto and removed from the probe 10. A benefit of this arrangement is that when probe 10 is in use in situ it generally cannot be, or in any event is not, removed until after hydration. However the battery 20, ammeter 22 and voltmeter 24 can be unplugged and thus removed and reused in association with other probes 10.

Optionally, the probe 10 may also incorporate a temperature sensor or probe and a transmitter or transponder. The temperature probe may be in the form of a thermocouple attached to or embedded in the first electrode 14. The transmitter or transponder may be arranged to transmit resistance data measured by the probe 10 to a remote location. However in an alternate arrangement, the temperature sensor or probe and a transmitter or transponder may be demountably supported on the body 12 as described above in relation to the battery 20, ammeter 22 and voltmeter 24.

Due to the configuration of the electrodes 14 and 16, the probe 10 provides electrical resistance measurement in multiple paths in a hydrating cementitious material. That is, when probe 10 has its electrodes 14 and 16 embedded in a hydrating cementitious material, current can flow along a path between the first electrode 14 and each of the electrodes 16a, 16b and 16c. Accordingly resistance measurement can be obtained for a substantial volume of the cementitious material bound by the second electrodes 16.

It is envisaged that in one embodiment the body 12 is made from a non- conductive polymeric material such as polyvinyl chloride and have a diameter of 130mm and height of 1 10mm. Body 12 comprises a base 32 and a

demountable lid or water tight cap 34. Various components, parts or sensors of the probe may also be encapsulated within the body.

Each of the electrodes 14 and 16 may extend for a length of 50mm from the surface 18. It is further envisaged that the electrodes 14 and 16 are made from the same highly conductive metallic material such as but not limited to, copper. The electrode 14 may have a diameter of 5mm, and is provided with a screw thread at a proximal end to facilitate coupling to the body 12.

Electrodes 16 are illustrated as being of an L-shape configuration with a thickness T of about 1 mm and width of 30mm. A foot portion 36 of each electrode 16 is attached to the housing 12 via respective electrically conducting screws 38 which also enable electrical connection with electronic components of the probe 10 such as conductors 28a and 28b. In one example of the probe 10, the distance D between the electrode 14 and each second electrode 16 is approximately 50mm.

As shown in Figures 1 c and 1 d a braid of wires 40 passes through the housing 12 via a grommet 42. The wires 40 may be used to connect various

components of the probe 10 such as the ammeter and voltmeter to a remote data logger or processor. Thus once the probe 10 has been put in place, monitoring of the electrical resistance and thus the development of shear strength can be monitored at a safe location. This avoids the need for people to enter an excavation at different times to take measurements. In a variation measurements and data derived from electronic components of the probe 10 may be communicated wirelessly to a data logger or processor by incorporation of a transmitter or transponder within the body 12. In that event, the braid of wires 40 and grommet 42 may not be required.

One or more probes 10 can be inserted to a shotcreted surface by the people spraying the shotcrete. Alternately the probes 10 may be inserted by use of an elongated boom or a telescopic wand from outside of the excavation. The probes may be inserted into shotcrete sprayed within a standard mould set up within the excavation or alternately inserted into shotcrete sprayed onto the excavation surface.

In order to correlate the resistance of hydrating shotcrete to strength development and in particular shear strength development, tests are initially conducted on samples of cementitious material. This provides empirical data that are subsequently used to estimate strength of the hydrating shotcrete and thus estimate safe re-entry time. Figure 3 illustrates a typical relationship between electrical resistance and shear strength over time for a hydrating cementitious material. Resistance in Ohms is plotted on the left hand side vertical axis, time in hours is plotted on the horizontal axis, and shear strength in MPa is plotted on the right hand side vertical axis. Resistance is measured using the probe 10 inserted into a sample of hydrating cementitious material. Shear strength of a cementitious material when initially in a "fluid gel" state is measured by a rotational vane shear viscometer ("vane test"). The shear strength of a solidifying material "solid gel" is measured on multiple samples using triaxial tests. There is however a mix dependent transition period after the commencement of hydration in which the intrinsic shear strength of the cementitious material exceeds the capability of vane rotational shearing but is insufficient to maintain a free standing cylindrical form required for triaxial testing.

For the particular composition of the cementitious material measured in Figure 3, it is apparent that during the first four hours of hydration electrical resistance of the material increases by approximately 150%; and, by over 250% in the first eight hours. The non-linear rate of change of resistance and the significant magnitude of the change in time enable resistance to be used as a sensitive measure of shear strength development in time for that mix.

Shotcrete, like other cementitious materials, shows strength gain with hydration. Complete curing results in material with considerably greater strength than the 2.0 MPa maximum indicated by the scale of Figure 3. However, in application of the present invention to the determination of the safe re-entry time for shotcreted excavations, it is the shear strength development within the first few hours of hydration that is of interest.

Figure 4 shows the same data as Figure 3 but with the scale of the shear strength magnified by a factor of 500 with maximum shear strength of 100kPa (i.e. 0.1 MPa). From Figure 4 it can be seen for example that for the particular composition of the cementitious material tested, a resistance of 100 Ohms correlates to a shear strength of approximately 65kPa.

It must be recognised that the above empirical data is specific to the particular composition of cementitious material tested as well as the conditions under which hydration is occurring, e.g. temperature and humidity. For example, Figure 5 illustrates a further graph of resistance versus shear strength development over time but under different curing conditions. It should be noted that in Figure 5, the resistance scale on the left hand side is magnified by a factor of ten in comparison with that of Figures 3 and 4 and the time period is magnified by a factor of two.

In embodiments of the present invention samples of a cementitious material to be applied are first tested using the probe 10 and standard shear strength tests. The relationship between the resistance and shear strength in time is recorded and may be represented as a graph like Figures 3-5 or held as look up tables in a data file. The tests conducted to obtain these data are performed in conditions which replicate the conditions under which the same cementitious material (i.e. cementitious material with the same composition) is to be applied. With these data in hand, cementitious material is now applied as required, e.g. pumped, poured or sprayed. One or more of the probes 10 are embedded in the cementitious material and readings of resistance are continuously logged. This resistance is then used together with the previously derived empirical data to provide an indication of the shear strength of the hydrating cementitious material at any particular point in time. In relation to the application of shotcrete and the requirement to re-enter an excavation, the shear strength can be used to estimate a safe re-entry time to the excavation. There are two structural requirements of a freshly sprayed shotcrete layer:

1 . It must support its own mass immediately after being applied to the

surface. 2. It must support the super-incumbent mass of an estimated unstable volume of rock.

In the first instance, the shotcrete supports its own mass by development of a bond strength (comprising adhesion and mechanical interlock) between itself and the substrate and by development of an intrinsic shear strength. Consider a one metre square slab of shotcrete (unit weight of γ in kN/m ³ and of thickness't' in mm). In the case of zero shear strength, the bond strength must typically reach about (yt/1000) kPa (or about 0.025t kPa). In the case of zero bond strength, the shear strength must typically reach (γ/4) kPa (or about 6.25 kPa). In almost all cases, where bond and shear strength development initiate simultaneously after spraying, both laboratory investigations and in situ experience have shown that the required strength levels for shotcrete to support itself are easily achieved. In the second instance, the shotcrete must be capable of supporting the mass of loose rock blocks that may become unstable and represent a risk to personnel that enter the excavation. The volume of loose rock that may become unstable is minimised during blasting by waves that vibrate the excavation surfaces and by subsequent scaling (e.g. hydro-scaling) procedures that remove loose rocks from the excavation surfaces.

The specific arrangement of excavation span, stress and structural geology associated with each excavation will be different and the specification of a single or standard unstable volume of rock is not possible. However, an example calculation shows that within hours of spraying, shotcrete is quite capable of supporting a significant volume of unstable rock and that this volume or mass of rock may be calculated as a function of layer thickness and time after spraying.

Consider, the case of the minimum shear strength required to be developed within the shotcrete to support a regular tetrahedral block of rock with 1.0m side lengths. If this block existed in the roof of an underground excavation, its face triangle (i.e. the expression of its shape visible in the roof when viewed from beneath) would be an equilateral triangle with side-lengths of 1 m.

This example calculation is illustrated in Figure 6 which shows the shear strength required to be developed for different thickness layers of shotcrete and is computed for a rock with an average unit weight 27 kN/m ³ (this is a common assumption for most rock types) which results in a total mass of the block of 400 kg (i.e. equivalent to twenty, 20 kg bags of cement). Figure 6 shows an average shotcrete layer of about 100mm thickness needs to develop a shear strength of about 7 kPa in addition to supporting its own mass. A longer re-entry is required for thinner layers of shotcrete.

Figure 6 also gives an indication of the time required to wait for a given layer thickness to develop the required shear strength to support the block, for a standard proportioned mix of cementitious material without accelerator or other admixtures, that has been placed on a granite rock slab and hydrated under laboratory conditions of temperature and humidity. It is known that some combinations of additives and admixtures delay hydration and desensitise the resistance change in the first hours after mixing is complete. Consequently, the exact relationships between time, resistance and shear strength for a shotcrete mix comprising various additives and admixtures and sprayed onto various surfaces in various environmental conditions will be different.

Embodiments of the method and probe may be used to assist in the

optimisation of a shotcrete or other cementitious material mix to address in situ conditions (such as rock substrate type, substrate dampness), environmental conditions (such as temperature and humidity) and logistic conditions (such as workability and strength development) specific to the site. A simple mix will comprise hydraulic cement, water and graded fractions of aggregates. However there are various additives and admixtures that can be used to assist in mixing, spraying or pumping, bonding, curing and improving strength and stiffness of the cementitious material. By using embodiments of the method and probe it is possible to assess the in situ rate of hydration (and strength development) as a consequence or function of varying the proportions of the mix constituents to suit a particular site.

For example a number of test shotcrete mixes can be prepared with known variations in mix composition and/or cured in different environmental conditions; with a probe 10 embedded in each batch. Resistance readings may be made at common time intervals for each batch with vane or triaxial tests performed at those time intervals to establish respective correlations between resistance and hydration for each batch. One can then assess the significance of the changes or differences in mix and/or environment conditions on hydration.

Modifications and variations that would be obvious to persons of ordinary skill in the art are deemed to be within the scope of the present invention the nature of which is to be determined from the above description and the appended claims.