GB2515831A

GB2515831A - Apparatus & method for monitoring strength development of concrete

Info

Publication number: GB2515831A
Application number: GB1312136.3A
Authority: GB
Inventors: Benoit Jones
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2013-07-05
Filing date: 2013-07-05
Publication date: 2015-01-07
Also published as: GB201312136D0

Abstract

A method of monitoring strength development of a sprayed concrete lining comprises: determining the position of a thermal imaging camera 410 within a concrete lined tunnel, the spatial position derived from sighting of fixed reference markers located in the tunnel; cyclically measuring surface temperatures (hydration heat)412 at a multiplicity of identified locations on the concrete-lined tunnel with respect to time, the identified locations resolved relative to the spatial position of the thermal imaging camera; using the measured surface temperatures to construct thermal profiles 414 of the concrete-lined tunnel at the identified locations; comparing the constructed thermal profiles with a reference profile 420 that associates temperature profiles to concrete strength development; and providing a time-varying output corresponding to an assessed level of strength 418 developed at each of the identified locations, the assessed level of strength determined with reference to the comparison. Also defined is a computer program and apparatus for monitoring strength development of sprayed concrete. The non-destructive method allows each excavated tunnel advance to be checked for strength to allow the next advance to be excavated.

Description

APPARATUS & METHOD FOR MONITORING STRENGTH DEVELOPMENT

OF CONCRETE

This invention relates to apparatus and a method for monitoring strength development of concrete, particularly but not exclusively of sprayed concrete, commonly referred to as shotcrete.

As a building material, sprayed concrete is increasingly used to form shaft and tunnel linings to support the ground to create an underground space. Sprayed concrete linings are usually 100mm to 400 mm thick. In this respect, sprayed concrete ("shotcrete" or "gunite", varied depending upon the nature of the mix but generally considered as equivalent and interchangeable terms) is usually applied to form structures, such as tunnel walls, having overhanging surfaces, although it is also applied to other surfaces in which rapid strength development is important.

Once set or "cured", a sprayed concrete lining provides a support structure in intimate contact with the substrate (whether earth or rock or a mixture), thereby sealing and holding back the ground. Limiting settlement and preventing cave-in. Once cured, the concrete can be loaded at an early stage to permit advancement of the tunnel. It is therefore of vital importance to monitor early age strength gain of sprayed concrete so that an excavation can proceed at a maximum safe rate.

In order to limit the potential for failure due to early over-stressing, construction engineers currently adopt a test strategy assessing early material strength. This test uses a combination of penetration needles and nail gun penetration tests to assess the strength of the concrete. Particularly, there is a correlation between depth of penetration and compressive strength. This form of testing unfortunately gives a high scatter of measured results. In addition, only small areas of concrete are tested, and the tests furthermore cause direct damage (e.g. microscopic cracking) to the concrete's structure notwithstanding that the nail penetration test is viewed as "non-destructive". The resulting amage should, ideally, be repaired in another processing step, although (in reality) repair is frequently overlooked if not ignored.

The current sporadic test regime inevitably means that test areas do not necessarily coincide with areas of relative concrete liner weakness (especially because concrete is considered to be heterogeneous). Indeed, due to the inherent variability in composition and depth of sprayed concrete and!or the possibility of variable accelerator dosage, localised penetration-based assessment cannot guarantee the strength in the lining at other locations.

Dangerously, penetration results may not therefore identify areas of potential failure (in either the short or longer terms).

Moreover, nail gun penetration tests use explosive cartridges that are classified as is hazardous waste, with the testing engineer needing specialist training (and specialist safety attire, e.g. impact-resistant goggles) to ensure the nail gun is used safely.

A further difficulty arises from the point of access. Specifically, access to the test arealsurface is required in order to operate the nail gun; this may require use of a mobile platform or scaffolding. Furthermore, the engineer and any lifting equipment may be exposed to risk in the event that an overhanging body of concrete becomes dislodged or is materially flawed. Therefore, these tests are usually performed on easily accessible areas, perhaps one metre above of the tunnel or otherwise on test panels sprayed specifically for testing purposes. Conversely, the crown of the tunnel is never tested.

Other tests on concrete are described in "Quality Control of Concrete Structures", Proceedings of the Second International RILEM/CEB Symposium (ISBN 0419 15800 6 (IIB)), pages 371 to 374. For example, the ultrasonic pulse velocity test measures wave velocity between transducers, although this form of testing has greater applicability in old ao (rather than young "green") concrete.

The article "Monitoring concrete temperature during construction of the Confederation Bridge" by James A. Gallard and W.H. Dilger (16 June 1997) and published through the NRC Canada [Can. J. Civ. Eng. Vol. 24, 1997, pages 941 to 950] describes temperature changes in large concrete members and discusses the effects of the heat of hydration.

According to a first aspect of the present invention there is provided a method of monitoring strength development of a sprayed concrete lining, the method comprising: determining a spatial position of a thermal imaging camera within a concrete-lined tumid, the spatial position derived from sighting fixed reference markers located in the tunnel; cyclically measuring surface temperatures at a multiplicity of identified locations on the concrete-lined tunnel with respect to time, the identified locations resolved relative to the spatial position of a thermal imaging camera; using the measured surface temperatures to construct thermal profiles of the concrete-lined tunnel at the identified locations; comparing the constructed thermal profiles wilh a reference profile that associates temperature profiles to concrete strength development; and providing a time-varying output corresponding to an assessed level of strength developed at each of the identified locations, the assessed level of strength determined with reference to the comparison.

In a second aspect of the invention there is provided a computer program product containing code that, when run of a computer, causes the computer perform the steps of: resolving a spatial position of a thermal imaging camera within a concrete-lined tunnel, the spatial position derived from sighting of fixed reference markers located in the tunnel; controlling cyclic measurement of surface temperatures at a multiplicity of identified locations on the concrete-lined tunnel with respect to time, the identified locations resolved relative to the spatial position of a thermal imaging camera; using the measured surface temperatures to construct thermal profiles of the concrete-lined tunnel at the identified locations; comparing the constructed thermal profiles with a reference profile that associates temperature profiles to concrete strength development; and providing a time-varying output corresponding to an assessed level of strength developed at each of the identified locations, the assessed level of strength determined with reference to the comparison.

In yet another aspect of the invention there is provided apparatus for monitoring strength development of a sprayed concrete lining, the apparatus comprising: a thermal imaging camera; a position locator; a clock for marking periods of time; a processor; and a memory storing a reference profile that associates temperature profiles to concrete strength development; wherein the position locator is configured to resolve, in use, a relative position of the thermal imaging camera relative to a body of sprayed concrete under investigation, thereby permitting the thermal imaging camera to monitor temperature measurements at one or more locations on a surface of the body of concrete at particular time intervals, the monitored temperatures recorded in the memory, and wherein the processor is configured: to assemble one or more thermal profiles for the body of sprayed concrete under investigation; to contrast said one or more thermal profiles against the reference profile to identify whether a minimum cured strength has been acquired at said one or more locations of the surface of the body of concrete; and to provide an output indicative of strength development at said one or more locations of the surface of the body of concrete.

The inventive approach is based on the fact that the hydration of cement in concrete is an exothermic reaction. If the temperature profile over time is known, the degree of hydration can be back-calculated using thermodynamic equations. The degree of hydration is directly proportional to concrete strength; thus the strength can be deduced.

Advantageously, the present invention provides an apparatus and methodology for assessing the curing strength of concrete through a non-evasive, non-destructive assessment protocol. Moreover, the system requires minimal set-up and can be automated.

Apparatus in accordance with this invention confers several additional benefits, such as the facts that a whole area, i.e. lining of the tunnel, may be surveyed without the need for lifting equipment and remote locations may be surveyed without difficulty. The approach undertaken in the present invention therefore contrasts with prior known penetration testing methods that are undertaken only at an easy-to-reach location.

The apparatus is lightweight and portable, avoiding any hazard from transportation and deployment of bulky or heavy equipment in a confined space. Particularly, the thermal imaging camera can be used to scan the tunnel lining at a safe distance from the face of the tunnel, removing the risk of injury or death due to falls of green' sprayed concrete or falls of loose ground or blocks of soil or rock from the face.

Furthermore, since sprayed concrete tunnels are usually constructed in consecutive sections, known as "advances", a first section may be excavated into the ground and a layer of sprayed concrete applied. Using the apparatus and methodology of the preferred embodiments, a second section (contiguous to the first section) can be excavated as soon as the thermal results yield a positive finding on attained strength. Consequently, the tunnel can progress more rapidly and with a higher degree of confidence than conventional approaches.

Use of the thermal imaging camera arrangement of the present invention to monitor the strength gain of the tunnel lining therefore results in a step-change in safety and quality is control.

The present invention is further described by means of example but not in any limitative sense with reference to the accompanying drawings, of which: FIG. 1 is a diagranmrntic representation of the construction of a typical tunnel; FIG. 2 is a section view through a longitudinal axis of the tunnel 10 of FIG. 1, with the tunnel further including preferred apparatus for monitoring concrete curing according to the present invention; FIG. 3 is a schematic representation of a preferred arrangement of an apparatus for monitoring concrete curing; FIG. 4 is a flow diagram illustrating the method of a preferred embodiment of the present invention; and FIG. 5 is a graph of temperature versus time for curing of a typical sprayed concrete.

Having regard to FIG. 1 there is shown a typical construction for a tunnel 10 through, for example, a hillside 12. The tunnel 10 is excavated through the rock or soil strata 14, with the tunnel typically taking a generally arc-shaped profile. An inner surface of the tunnel 10 is supported by a concrete lining 16 (as described above) which has a varying thickness (dycd2<d0, where n is a positive integer) because of the nature of the tunnel's excavation with a boring machine, drill or the like. The concrete lining 16 typically has a stipulated s minimum thickness d1 along both its longitudinal and perpendicular axes. The concrete lining 16 therefore has an internal surface finish that generally has the form of an arc.

FIG. 2 is a section view through a longitudinal axis of the tunnel 10 of FIG. 1 and in which an apparatus 30 for assessing the state of concrete curing, according to the present invention, is located. FIG. 2 also shows a distribution of reference markers 32 that permit the apparatus 30 to assess its relative position within the tunnel. For acquisition of a spatial position within the tunnel, it will be appreciated that at least three reference markers are used. The reference markers 32 may be realised by passive reflectors (such as a mirror) of electromagnetic radiation. The reference marker 32 is preferably realised by a reflective disc or plate or a reflective prism that can be fixed to the surface 18 of the concrete lining 16 for a period of time, but it may also be realized by a hard boundary or structure, such as a pillar 34 at an entrance of the tunnel 10. The reference marker 32 (which in essence is a position locator) may be a marking on the concrete surface.

Turning to FIG. 3, a schematic representation of a preferred arrangement of the apparatus for monitoring concrete curing ("cure monitoring apparatus") is shown. The cure monitoring apparatus 30 is based around a processor 40 that control overall system operation and which is coupled to a memory or accessible database 42 in which is stored curing profiles 44. The memory 42 is preferably local to the cure monitoring apparatus, but equally it could be physically remote (as is typically the case with a server-based database). The memory 42 fbrther typically includes code modules 46 that are called upon by the processor in providing a determination on concrete curing in an environment.

Memory 42 will also include a storage area for storing accumulated data for processing, although this storage may be realized by cache associated with the processor 40.

The cure monitoring apparatus 30 further includes a timer (or clock) 48 coupled to the processor and arranged to provide a time base reference for operation of the system. The cure monitoring apparatus 30 further includes an output 49 coupled to the processor. The output can be an interface to an external or internal display, or may be a digital bit output that is communicated to a remote device for flirther processing, recording and assessment.

The cure monitoring apparatus 30 includes a thermal imaging camera 50 operationally responsive to the processor. The cure monitoring apparatus 30 also includes a power supply 52, such as a battery. Additionally, the cure monitoring apparatus 30 includes a position locator 60 that cooperates with the processor 40 to evaluate the relative position of the cure monitoring apparatus 30 to a reference point, such as provided by the reference marker 32. The position locator 60 may be based on the principles of a laser-theodolite F since a theodolite is a precision instrument for measuring angles in the horizontal and vertical planes (relative to an initial datum). The laser aspect of the theodolite provides an ability to measure displacement of the theodolite from a fixed reference point, thereby providing a three-dimensional location system for the cure monitoring apparatus 30, with an accuracy typically within a sphere of radius five millimetres. The TopcontM DT-209L High-Performance Laser Theodolite may adapted for the purposes explained herein since this device has range that is better than fifty metres in direct sunlight.

Other forms of position locator can be employed, as will be appreciated. For example, rather than to use the passive reflectors of FIG. 2 to provide a direct measurement to the cure monitoring apparatus 30, an alternative arrangement makes use of a base station relay arrangement that sights at least three reference markers 32 (having known positions) to permit resolution (through calculation) of its own position. An JR camera may therefore be detached from the base station, with the JR camera mounted to a tripod or adjustably levelled bracket equipped with orientation articulators and horizontal and vertical angle measurement capabilities (as are well known to the skilled addressee). Additionally, the IR camera includes reflectors that are sighted on the base station to permit both the position of the JR camera and the direction that it is pointing to be calculated. This configuration allows the JR camera to resolve what part of the sprayed concrete lining is being so observed.assessed The thermal imaging camera 50 and laser theodolite may, in fact, be discrete units that plug into an interface of the cure monitoring apparatus 30 to form a multi-device system; this would allow the cure monitoring apparatus to be based on a laptop computer programmed with appropriate instructional code.

The thennal imaging camera and laser may be mounted to a processor-controlled mount that can pan and tilt to allow for automated collection of data across a multiplicity of points. Thermal resolution is typically in the region of about 0.1 degrees over a typical test region covering about one square metre. Test areas may, however, be of varying size and are typically up to about ten square metres.

In overview, the cure monitoring apparatus 30 is configured to sense and then assemble time-based thermal profiles from specific point/regions in the face of an applied concrete lining and then to evaluate concrete liner curing based on the changing thermal surface profile, properties of the concrete and physical characteristics of the liner at each specific measured point in the tunnel.

It is noted that a computer-based system with a position location input is commonly used to map the interior of a tunnel before and after application of a sprayed concrete layer.

Cure monitoring apparatus in accordance with the invention may be coupled to such a system to augment the base system to monitor concrete curing and strength development.

The apparatus of this invention may be linked to a map or positional co-ordinates of the substrate surface and concrete layer applied to the surface.

The inventors have appreciated that the curing of concrete involves an exothermic reaction. Moreover, while not wishing to be bound by theory, the strength developed by the concrete appears to be linearly proportional to the extent of hydration.

During curing, heat is released by hydration of a concrete mixture, with a body of sprayed concrete typically reaching a maximum temperature of 30°-60°C after about twelve hours.

In contrast, a defective and thus dangerous concrete may not produce any significant exothermic reaction at all, or the curing stage for defective concrete yields a noticeably different thermal profile.

Therefore, according to the invention, a change in temperature and a change in thermal 3s profile with time of the concrete surface are used to provide an indication of the extent of hydration and hence the strength developed at the surface of the concrete. This measure is conservative as higher temperatures will exist within the body of the concrete and therefore the surface strength will develop more slowly. In a preferred embodiment, the assessment and calculation process functions to back-calculate the temperature profile across the thickness of the sprayed concrete lining given an appreciation of: i) the thermal conductivity and specific heat capacity of both the concrete and the ground; and ii) the rate io of heat loss to the air within the tunnel (which is dependent upon the ambient temperature.

It is further understood that a variation in attained maximum temperature as a function of time is influenced by certain physical and/or environmental factors, including mix composition, lining thickness, ambient temperature and the concrete's temperature at the time when it is sprayed. However, recorded thermal profile can take these variants into consideration since effective curing will still exhibit a particular and noticeable trend.

In contrast with an approach based on the use of embedded thermocouples or thermistors to measure the temperature across a section of concrete, the use of a thermal imaging camera presents complications in the back-calculation of concrete strength because (1) it measures only the surface temperature and (2) the temperature profile across the thickness of the sprayed concrete must be deduced.

Since the temperature within any concrete section will always be higher than the surface temperature due to heat loss to the air, it has been appreciated that a robust result and approach can be based on using only the surface temperature to calculate the degree of hydration and hence concrete strength. The degree of hydration may be calculated, for a given concrete mix, through an understanding of the relationship between normalised affinity and the degree of hydration, and by knowing the time-temperature history.

so Therefore, the concrete strength at the surface may be deduced, with the safe assumption that concrete within the section will experience higher temperatures and therefore will gain strength more quickly.

Returning briefly to FIG. 2, the cure monitoring apparatus 30 is configured relative to a marker so that the relative position of the cure monitoring apparatus 30 can be determined within the context of the tunnel 12; this is shown by outward and reflected laser beam path 80. Once the position within the tunnel is understood, regions 82, 84, 86 for inspection -only three are shown for reasons of drawing clarity -can be located by ranging 88 supported by the laser theodolite 60, and those regions 82, 84, 86 subsequently targeted by the thermal imaging camera 50. Thermal radiation 92 that radiates from the each selectable and identifiable test region is collected by the thermal imaging camera 60 over a range of rn minutes and hours to permit the constructions of curing profiles for each test region. Data points for each test region are stored in memory 42, with the processor 40 using program code to compensate for concrete mix, potential result attenuation arising from angular displacement of the thermal imaging camera relative to the test region, concrete thickness and other variables that affect the curing process.

Thermal imaging of a test region is, however, conducted generally perpendicular to the test region 82, 84, 86, with this meaning that the cure monitoring apparatus 30 is moved and the distance measuring system (e.g. laser) used to calculate the absolute position of the cure monitoring apparatus 30 in the tunnel 10 by relative position of the cure monitoring apparatus to the reference marker 32. Movement may therefore be in discrete selectable distances, such as 1 metre intervals. At each point, the thermal imaging camera 50 may sweep an arc to therefore assess a section of the tunnel 16 perpendicular to the major axis (reference numeral 100 in FIG. 2) of the tunnel 10.

As these curing profiles are assembled, they are actively contrasted against pre-stored curing reference profiles that reflect at least one of an acceptable curing regime, but preferably include a range of profiles that define varying degrees of acceptable concrete curing (based on the heat of hydration) and particularly, by implication, corresponding levels of attained strength within the concrete lining.

It is noted that the system may undertake a pre-scan of the excavation profile in advance of the application of the concrete lining 16. This pre-scan looks to map the profile of the rock surface and therefore, post application, to calculate an approximate depth of the concrete lining 16 at each point along the tunnel. The pre-scan therefore makes use of a known reference point, such as that provided by the reference marker 32, and the distance measuring system, such as realised by the laser theodolite 60 to build an initial tunnel

-II-

profile. After application of the concrete lining 16, an intermediate scan (relative to the reference marker) dimensions the surface 18, thereby allowing the processor to estimate applied thickness. The thickness of the concrete can therefore be taken into consideration in the assessment of curing and related concrete strength.

io Placing the cure monitoring apparatus 30 on a portable chassis, tripod or other moveable vehicle permits movement of the cure monitoring apparatus 30 within the tunnel 10 over time. Movement may allow for an averaging of readings from target regions through processor-controlled active pan and tilt of the thermal camera. Movement of the portable chassis may allow for averaging of results and possible mitigation of parallax and/or attenuation errors.

In terms of assembling the initial curing reference profiles that are used as a basis for assessment, the instantiated algorithm recognizes that the temperature profile of the surface of a body of concrete during curing generally depends on several factors, including: (1) the composition of the concrete formulation; (2) the heat absorption properties of aggregates in the concrete; (3) the nature and amount of accelerator employed; (4) the initial temperature of the concrete mixture given ambient temperature; (5) the thickness (d) of the applied layer; (6) the temperature, specific heat capacity and thermal conductivity of the underlying substrate; (7) the ambient temperature; and (8) the specific heat capacity and thermal conductivity of the concrete mix. The reference profiles may therefore be modelled or empirically assessed from a local test conditions or similar environmental conditions.

In terms of curing assessment, the processing algorithms assembles the curing profiles having regard to at least one (and generally multiple) measured or inferred thermal characteristic, namely: (1) the rate of temperature increase and/or decrease; (2) the maximum temperature attained at the test region under assessment; (3) the time required to reach the maximum temperature; (4) the measured decrease in temperature from the maximum value after a selected time interval; (5) the time taken for the temperature to decrease from a maximum to an intermediate point above ambient. Generally, the profiles are assembled from a sufficient number of time-temperature points that permit stepwise calculations. More particularly, the rate of hydration is dependent upon normalized affinity (which itself depends on the degree of hydration) and temperature, so the assessment fUnction calculates rate of hydration sufficiently frequently so as not to introduce WHAT????? -your comment was missing from the bottom of page 10.

Once the reference profiles are assembled and the thermal (and positioning) data acquired, the processor 40 can apply point-by-point or comparative curve profile analysis to yield an assessment of curing and acquired strength. Results from the comparative analysis may be displayed in a number of alternative forms on the output (reference numeral 49 of FIG.3), including hard data points or a graphical representation. A visual representation of instantaneous measurements (and position) may furthermore be presented as the output, with this representation covering sections or the entire lining surface that is under construction. A map of the strength development of an entire concreted area may therefore be provided. After taking into consideration any influencing factors, a variation from the reference profiles by a safety margin of, say, 5% may be used as an indicator of possible issues with structural integrity of the concrete lining that warrant further investigation.

Preferably, the Austrian Sprayed Concrete Guideline J2' curve is used as minimum strength development required in the lining, so inferred strength development that falls below the J2 curve then the system is configured to flag a problem at that point of the lining. An alternative reference curve specified by the lining contractor/engineer may alternatively be used to reflect a minimum attained strength acceptable for tunnelling advancement; this could reflect the so-called "re-entry time" used in mining to denote when it it presumed safe to work under a sprayed concrete ceiling, vault or the like.

Curing In terms of general theory, the hydration of clinkers in concrete is an exothermic reaction, i.e. there is a net production of heat. The reaction, however, requires heat to be put in before it can occur; this initial heat injection requirement is known as the activation energy. The heat produced by the first reactions will enable more reactions to take place, which explains the acceleration of heat liberation. The ambient temperature also has an as effect. -J3

s Furthermore, the higher the temperature, the faster the reaction rate. This is Arrhenius' law as explained by Laplante & Boulay, 1994; Hellmich et cii., 200 Ia:

E

ç=Aexp(-) Where: is the degree of hydration or reaction extent', thus is the rate of hydration, * A is the normalised affinity, which is the driving force of hydration, taking account to of the permeability' of the diffusion process during hydration, * T is the absolute temperature, * E is the activation energy, and * R is the ideal gas constant.

EIR has been found to be practically constant for calcium silicate clinkers.

Affinity is to chemical reactions as the Newtonian concept of force is to motion. For concrete, affinity represents the thermodynamic imbalance between free water and water combined in the solid phase. In a closed system not initially in chemical equilibrium (such as fresh sprayed concrete), chemical reactions drive the system to a state of equilibrium in which the affinities of the reactions vanish.

The concept that ageing may be defined by a thermodynamic state variable is based on the hypothesis that stress and temperature variations do not affect thermodynamic imbalance, i.e. affinity. For this to hold true, the latent heat released to the exterior of the concrete (e.g. liner) by deformation at constant temperature and constant degree of hydration, and the heat produced by chemical dissipation, must be negligible with respect to the latent heat of hydration. Therefore, the rate of hydration accelerates from a slow start point, then eventually slows as the quantity of uthydrated cement diminishes and diffusion paths increase, reducing the rate of hydration and hence the rate of heat production. When the rate of heat production reduces to less than the rate of heat diffusion, the temperature will begin to drop. The shape of a cumulative heat liberation curve is therefore similar to the shape of a strength development curve for concrete.

For a given concrete mix, it has been established that the strength development as a function of degree of hydration will be the same regardless of the curing temperature. The Arrhenius function can therefore be used to predict the strength development with time for a given concrete mix with any temperature history. At low ambient temperatures, the rate of strength gain may be significantly retarded at early age; this could occur locally in a io sprayed concrete tunnel lining. For instance, a significant temperature variation may be experienced at a tunnel entrance or at a point close to a ventilation duet where fresh air is brought into the turmel from the outside. This is important because strength development with time is highly dependent on temperature. The inventors have recognised that, to date, these facts have been ignored in both codes of practice and specifications.

In accordance with operating methodologies of the present invention, there are three (3) possible approaches to monitoring and assessing early age strength of sprayed concrete with a thermal imaging camera: 1. Development of an empirical correlation between the surface temperature history and the strength. This correlation can be found by measuring surface temperatures for a particular concrete mix at a variety of ambient temperatures and section thicknesses to develop a set of curves from which to calculate the strength.

2. Determination of the relationship between the degree of hydration and normalised affinity, derived from a series of isothennal compressive strength tests or adiabatic tests. An example of how to determine the relationship between the degree of hydration and normalised affinity is given in Hellmich ci cii. (1999) for isothermal strength measurements, and in Cervera et cii. (1999) for adiabatic tests. It has been appreciated that a single curve of A against can calculate strength for any time-temperature history.

3. Deducing the temperature profile across the thickness of the concrete from a series of measurements of surface temperature. As well as requiring the same information as for the second method, this approach also requires knowledge of the thermal conductivity and specific heat capacity of the concrete and the ground behind the lining, the thickness of the lining and the ambient temperature in the tunnel.

Although a preferred embodiment makes use of thermal imaging to determine the temperature at the surface of the concrete, it is envisioned that an alternative (more elaborate embodiment) may model the thermal profile through the thickness of thc concrete liner. Since the hydration rate is related to the temperature of the concrete at that time and an outer surface will present a lower temperature because of external cooling io influences and heat migrations delays, use of the surface temperature builds in a safety factor for determined early strengths.

Turning to the flow diagram of FIG. 4, a cured concrete strength evaluation process begins at step 400. Optionally, prior to spraying the concrete lining, the laser theodolite (or the like) scans 402 the tunnel excavation to assemble 404 a profile of the excavation. Shotcrete is then applied 406 to the excavated surface. A further scan of the tunnel (having regard to the relative position of the laser theodolite within the tunnel) permits a determination 408 to be made with respect to a depth of the concrete lining along the tunnel. The process could, however, start at step 410 where a determination of the spatial position of a thermal imaging camera within a concrete-lined tunnel is made, the spatial position derived from sighting of fixed reference markers located in the tunnel in a step conducted either before scanning step 400 or determination step 410. The system then cyclically measures 412 surface temperatures at a multiplicity of identified locations on the concrete-lined tunnel with respect to time, the identified locations resolved relative to the spatial position of a thermal imaging camera. Using the measured surface temperatures, thermal profiles of the concrete-lined tunnel are constructed 414 for each of the identified locations; comparing the constructed thermal profiles with a reference profile that associates temperature profiles to concrete strength development. Once sufficient data point are available (with respect to location and/or time) 416, the system is configured to generate a time-varying output 418 corresponding to an assessed level of strength developed at each of the identified locations, the assessed level of strength determined with reference to the comparison 420 of the compiled thennal profile with a reference profile. The output may be in form of a printout, display or a sensory alarm.

The assessment process may be configured to time out 422 based on elapsed time and heat measured at one of more evaluation areas. For example, if the surface temperature were not to exceed, say, fifty degree Celsius after a determined period of time, then this could infer that the concrete strength would not (and could not) reach the required level of strength Clearly, the assessment of strength based on monitored surface temperatures would take into account influencing factors as outlined above. Alternatively, hydration may cause the surface temperature to exceed a given minimum temperature deemed safe with respect to cured strength, with this allowing a positive sensory output to be recorded or announced. In the event that neither a positive strength result 426 nor a time out has occurred, then the assessment process continues 430 with the collection of more data pothts and corresponding refinement of the constructed thermal profiles for the test point/areas.

It will be understood that the process flow in FIG. 4 may be implemented in a different fashion and at a finer level of process granularity, so FIG. 4 is illustrative of the underlying process from the perspective of major or significant process steps.

The methodology employed to measure a multiplicity of points or areas of sprayed concrete relative to an established spatial position for the thermal imaging (or IR camera) can therefore extend the surface under investigation to substantially the entire area of the tunnel. Of course, fewer measurements could be undertaken to reduce sampling time whilst still covering larger areas (say at least lO& and preferably 20% to 50% if not more) of sprayed concrete that, hitherto, could not be effectively assessed for local strength using

the pre-existing techniques of the prior art.

FIG. 5 is a typical graph of surface temperature versus time for curing concrete having a nominal thickness of 125mm, and reflects the described characteristics of curing in which there is a relatively rapid initial increase in surface temperature (from the point of pouring) followed by a relatively slow loss of heat from a maximum attained temperature.

It will be further understood that unless features in the particular preferred embodiments are expressly identified as incompatible with one another or the surrounding context as implies that they are mutually exclusive and not readily combinable in a complementary and/or supportive sense, the totality of this disclosure contemplates and envisions that specific features of those complementary embodiments can be selectively combined to provide one or more comprehensive, but slightly different, technical solutions.

The present invention, and especially the processing aspect to determine the rate of hydration and cured concrete strength, may be supported as a computer program supplied io on a computer readable memory (or as a download) for execution by a local computer linked to system hardware (as shown in FIG. 3).

It will, of course, be appreciated that the above description has been given by way of example only and that modifications in details may be made within the scope of the present invention. For example, it will be understood that the tunnel 10 of FIG. I is used in the context of an exemplary construction environment that warrants deployment of a concrete lining 16 and that other construction situations will equally make the use of a structural concrete lining 16, such as a shaft, chamber or conduit. These other environments are therefore to be understood as falling within the generic term "tunnel" even though those environments may have only a single entry point and the function of those other constructions may be for access, storage or a void. The term "tunnel" should therefore be understood to include any construction in which a concrete lining is applied for support purposes.

The term "body of concrete" and similar such terms in the specification should be understood to relate to a coating sprayed onto a wall of a tunnel or other cavity, although the invention may be applied to other bodies of concrete, particularly applied to inclined or overhanging surfaces, such as arches or cylindrical vaulted sections. The term "tunnel" should therefore be understood to encompass all excavations to which a concrete lining is so sprayed or otherwise applied.