Cross-Reference

[0001] The present application claims priority from Australian Provisional Patent Application No 2019904894 filed on 23 December 2019, the contents of which are incorporated herein by reference in their entirety.

Technical Field

[0002] The present disclosure relates, generally, to methods of entraining carbon dioxide in concrete aggregate, and, more particularly, to a method of producing carbon dioxide entrained recycled aggregate for use in a concrete mix, a method of formulating a concrete mix based on a desired compressive strength of concrete to be formed from the concrete mix or of determining a compressive strength of concrete formed from a concrete mix, the concrete mix comprising carbon dioxide entrained recycled aggregate, and a method of producing a concrete mix comprising carbon dioxide entrained recycled aggregate.

Background

[0003] Concrete is generally formed from at least aggregate, water and cement in varying ratios. Aggregate is used as a filler for concrete and can comprise coarse aggregate, such as gravel, crushed stone, and slag, as well as fine aggregate such as sand.

[0004] Construction and demolition waste produced, for example, from the demolition of concrete structures, which also include non-concrete masonry material, tile, and organic material, contributes to landfill.

[0005] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims.

Summary

[0006] In an aspect of the present disclosure, there is provided a method of producing a concrete mix, the method comprising: entraining carbon dioxide in recycled aggregate by exposing the recycled aggregate to the carbon dioxide in a carbonation chamber at a predetermined carbonation pressure and for a predetermined carbonation time; removing the carbon dioxide entrained recycled aggregate from the carbonation chamber; and producing the concrete mix by combining the carbon dioxide entrained recycled aggregate with at least cement, fine aggregate and water before the carbon dioxide entrained recycled aggregate has been exposed to ambient pressure for longer than a predetermined exposure time.

[0007] The method described above may further comprise maintaining the carbon dioxide entrained recycled aggregate under pressure, preferably of around 25 kPa, during at least a part of a period between the end of the predetermined carbonation time and its being combined with the cement, the fine aggregate and the water.

[0008] The predetermined carbonation pressure may be up to approximately 75kPa, preferably between approximately 25kPa and 75kPa, and more preferably approximately 25kPa.

[0009] The predetermined carbonation time may be at least 30 minutes, preferably at least 60 minutes, and more preferably at least approximately 120 minutes.

[0010] The predetermined exposure time may be approximately 120 minutes.

[0011] The recycled aggregate may comprise hardened concrete.

[0012] The recycled aggregate may comprise construction waste. The construction waste may comprise hardened concrete and at least one of: non-concrete masonry material; tile; and organic material.

[0013] In another aspect of the present disclosure, there is provided a method of obtaining carbon dioxide entrained recycled aggregate for use in a concrete mix, the method comprising: entraining carbon dioxide in recycled aggregate by exposing the recycled aggregate to the carbon dioxide in a carbonation chamber at a predetermined carbonation pressure and for a predetermined carbonation time; maintaining the carbon dioxide entrained recycled aggregate under pressure after the carbonation time and prior to its use in the concrete mix.

[0014] The predetermined carbonation pressure may be up to approximately 75kPa, preferably between approximately 25kPa and 75kPa, and more preferably approximately 25kPa.

[0015] The predetermined carbonation time may be at least 30 minutes, preferably at least 60 minutes, and more preferably at least approximately 120 minutes.

[0016] The recycled aggregate may comprise comminuted hardened concrete.

[0017] The recycled aggregate may comprise comminuted construction waste. The comminuted construction waste may comprise hardened concrete and at least one of: non concrete masonry material; tile; and organic material.

[0018] In yet another aspect of the present disclosure, there is provided a method of producing carbon dioxide entrained recycled aggregate for use in a concrete mix, the method comprising: providing comminuted construction waste comprising hardened concrete, the comminuted construction waste having a nominal particle size of less than approximately 20mm; entraining carbon dioxide in the comminuted construction waste by exposing the comminuted construction waste to the carbon dioxide in a carbonation chamber at a predetermined carbonation pressure and for a predetermined carbonation time.

[0019] The nominal particle size may be approximately 10mm.

[0020] Said providing comminuted construction waste may comprise comminuting the construction waste to the nominal particle size.

[0021] The comminuted construction waste may further comprise at least one of: non-concrete masonry material; tile; and organic material.

[0022] The method described above may further comprise maintaining the carbon dioxide entrained recycled aggregate under pressure after the carbonation time and prior to its use in the concrete mix.

[0023] The predetermined carbonation pressure may be up to approximately 75kPa, preferably between approximately 25kPa and 75kPa, and more preferably approximately 25kPa.

[0024] The predetermined carbonation time may be at least 30 minutes, preferably at least 60 minutes, and more preferably at least approximately 120 minutes.

[0025] In yet another aspect of the present disclosure, there is provided a method of producing carbon dioxide entrained recycled aggregate for use in a concrete mix, the method comprising: entraining carbon dioxide in recycled aggregate by exposing the recycled aggregate to the carbon dioxide in a carbonation chamber at a predetermined carbonation pressure and for a predetermined carbonation time, wherein the predetermined carbonation pressure is up to approximately 75kPa, preferably between approximately 25kPa and 75kPa, and more preferably approximately 25kPa; and wherein the predetermined carbonation time is at least 30 minutes, preferably at least 60 minutes, and more preferably at least approximately 120 minutes.

[0026] The method described above may further comprise maintaining the carbon dioxide entrained recycled aggregate under pressure after the carbonation time and prior to its use in the concrete mix.

[0027] In yet another aspect of the present disclosure, there is provided a method of formulating a concrete mix based on a desired compressive strength of concrete to be formed from the concrete mix or of determining a compressive strength of concrete formed from a concrete mix, the concrete mix comprising cement, coarse aggregate comprising at least carbon dioxide entrained recycled aggregate, fine aggregate, and water, the method comprising: providing a model interrelating concrete mix variables of the concrete mix to compressive strength of concrete to be formed from the concrete mix, the concrete mix variables comprising: a proportion of the coarse aggregate comprising the carbon dioxide entrained recycled aggregate; at least one parameter relating to the carbon dioxide entrained recycled aggregate; two or more of: a water to cement ratio of the concrete mix, a mass of the cement and a mass of the water; and a mass of the fine aggregate; and using the model to determine a set of said concrete mix variables associated with the desired compressive strength, thereby to formulate a concrete mix for producing concrete having that compressive strength; or using the model to determine a compressive strength of concrete formed from a concrete mix having a predetermined set of said concrete mix variables.

[0028] The model may be a neural network, the neural network comprising: an input layer comprising at least one input node; at least one hidden layer comprising at least one hidden node configured to receive data from the input layer; and an output layer comprising at least one output node configured to receive data from the at least one hidden layer. The input layer may comprise a plurality of input nodes, each of the plurality of input nodes corresponding to one of the concrete mix variables of the predetermined set of said concrete mix variables; and the output layer may comprise one output node corresponding to the compressive strength of concrete formed from the concrete mix having the predetermined set of said concrete mix variables. Alternatively, the input layer may comprise one input node corresponding to the desired compressive strength; and the output layer may comprise a plurality of output nodes, each of the plurality of output nodes corresponding to one of the concrete mix variables of the set of said concrete mix variables associated with the desired compressive strength.

[0029] The neural network may be configured using a data set, the data set being obtained by: forming a plurality of concrete structures, each of the plurality of concrete structures being formed using a respective concrete mix, each said respective concrete mix comprising cement, coarse aggregate comprising at least carbon dioxide entrained recycled aggregate, fine aggregate, and water, and each said respective concrete mix having an associated set of the concrete mix variables; testing compressive strength of each of the plurality of concrete structures; and recording the compressive strength of each of the plurality of concrete structures, wherein the data set comprises, for each of the concrete structures and its respective concrete mix: the associated set of the concrete mix variables; and the recorded compressive strength. [0030] The model may be a multiple linear regression model. The multiple linear regression model may be: where: a, b, c, d, e, g, and h, each represent a predetermined constant;

F _c ' represents the compressive strength of concrete to be formed from the concrete mix; R _wc represents the water to cement ratio of the concrete mix;

R _rca represents the proportion of the coarse aggregate comprising the carbon dioxide entrained recycled aggregate;

C _pi represents a first parameter relating to the carbon dioxide entrained recycled aggregate;

C _p 2 represents a second parameter relating to the carbon dioxide entrained recycled aggregate;

Q _w represents the mass of the water; and Qf _a represents the mass of the fine aggregate.

[0031] Each of the predetermined constants a, b, c, d, e, g, and h, may be determined using a data set, the data set being obtained by: forming a plurality of concrete structures, each of the plurality of concrete structures being formed using a respective concrete mix, each said respective concrete mix comprising cement, coarse aggregate comprising at least carbon dioxide entrained recycled aggregate, fine aggregate, and water, and each said respective concrete mix having an associated set of the concrete mix variables; testing compressive strength of each of the plurality of concrete structures; and recording the compressive strength of each of the plurality of concrete structures, wherein the data set comprises, for each of the concrete structures and its respective concrete mix: the associated set of the concrete mix variables; and the recorded compressive strength. The predetermined constants may be: a = -10089.538; b = 33.332; c = 8.290; d = 0.020; e = 0.001; g = 1369.79; and h = 0.652. [0032] The at least one parameter relating to the carbon dioxide entrained recycled aggregate may comprise at least one of: a carbonation pressure at which carbon dioxide is entrained in recycled aggregate by exposing the recycled aggregate to the carbon dioxide in a carbonation chamber to produce the carbon dioxide entrained recycled aggregate; and a carbonation time during which the recycled aggregate is being exposed to the carbon dioxide in the carbonation chamber to entrain the carbon dioxide in the recycled aggregate to produce the carbon dioxide entrained recycled aggregate.

[0033] The compressive strength may be 28 day compressive strength.

[0034] The fine aggregate may comprise sand.

[0035] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Brief Description of Drawings

[0036] Embodiments of the disclosure will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 shows an embodiment of a method of producing a concrete mix using carbon dioxide entrained recycled aggregate;

Figure 2 shows the method of producing the concrete mix shown in Figure 1, with the inclusion of maintaining the carbon dioxide entrained recycled aggregate under pressure;

Figure 3 shows an embodiment of a method of producing carbon dioxide entrained recycled aggregate for use in a concrete mix;

Figure 4 shows another embodiment of a method of producing carbon dioxide entrained recycled aggregate for use in a concrete mix;

Figure 5 shows yet another embodiment of a method of producing carbon dioxide entrained recycled aggregate for use in a concrete mix;

Figure 6 shows an embodiment of a method of determining a compressive strength of concrete formed from a concrete mix, the concrete mix comprising carbon dioxide entrained recycled aggregate;

Figure 7 shows an embodiment of a method of formulating a concrete mix based on a desired compressive strength of concrete to be formed from the concrete mix, the concrete mix comprising carbon dioxide entrained recycled aggregate; Figure 8 shows an embodiment of a carbonation chamber used to produce carbon dioxide entrained recycled aggregate;

Figure 9 shows a cross-section of a concrete structure formed using a concrete mix comprising carbon dioxide entrained recycled aggregate; and

Figure 10 shows a cross-section of a concrete structure formed using a concrete mix comprising carbon dioxide entrained recycled aggregate under crossed-polarised light.

Detailed Description of Exemplary Embodiments

[0037] In the drawings, reference numeral 10 generally designates a method of producing a concrete mix. The method 10 comprises entraining 12 carbon dioxide in recycled aggregate by exposing the recycled aggregate to the carbon dioxide in a carbonation chamber 14 (Fig. 8) at a predetermined carbonation pressure and for a predetermined carbonation time. Following this, the carbon dioxide entrained recycled aggregate is removed 16 from the carbonation chamber 14 to produce 18 the concrete mix (Figure 1).

[0038] The recycled aggregate may comprise hardened concrete obtained from, for example, the demolition of concrete structures. The recycled aggregate may also comprise other construction or demolition waste, such as non-concrete masonry material, tile, and organic material such as timber. It will be understood by persons skilled in the art that the concrete mix may also comprise virgin aggregate, for example, gravel, crushed stone and/or slag.

[0039] The hardened concrete portion of the recycled aggregate comprises hardened cement, which comprises calcium hydroxide. This calcium hydroxide is formed by a chemical reaction between water and calcium oxide in virgin cement of the concrete mix used to form the hardened concrete. The chemical reaction is as follows:

CaO + H2O ® Ca(0H) ₂ (1)

[0040] The entrainment of carbon dioxide into the recycled aggregate increases the density of the recycled aggregate by converting calcium hydroxide crystals in the hardened cement portion into calcium carbonate crystals. The chemical reaction by which the calcium hydroxide crystals are converted into calcium carbonate crystals is as follows:

Ca(0H) ₂ + C0 ₂ ® CaC0 ₃ + H ₂ 0 (2)

[0041] The calcium carbonate crystals, which are smaller than the calcium hydroxide crystals, fill air pockets in the recycled aggregate. The denser concrete thereby formed also has a lower water absorption. Due to the outer perimeter of the recycled aggregate initially undergoing carbonation in the carbonation chamber 14, the water absorption of the recycled aggregate quickly improves (decreases) once the entrainment process begins.

[0042] The increased density of the carbon dioxide entrained recycled aggregate also improves its compressive strength. This has been confirmed by testing samples of the recycled aggregate that have and that have not undergone the carbon dioxide entrainment process to determine their respective crushing values.

[0043] Concrete formed from a concrete mix including the carbon dioxide entrained recycled aggregate also exhibits an improved compressive strength compared to concrete formed from an equivalent concrete mix including non-carbon dioxide entrained recycled aggregate.

[0044] Since the formation of calcium carbonate in the recycled aggregate relies on the recycled aggregate containing recycled concrete, and therefore recycled cement comprising Ca(OH)2, it is beneficial that the recycled aggregate contains at least some recycled concrete instead of purely other forms of construction waste such as non-concrete masonry material, tile, and organic material such as timber.

[0045] Recycled aggregate containing little to no recycled concrete will affect the permanency of the carbon dioxide entrainment in the recycled aggregate, as little to none of the carbon dioxide entrained in such recycled aggregate will be converted to calcium carbonate. Rather, all or the majority of the carbon dioxide entrained into such recycled aggregate will remain in the form of carbon dioxide gas, which is gradually released from the recycled aggregate upon exposure of the aggregate to ambient pressure.

[0046] However, carbon dioxide gas entrained in the recycled aggregate provides a benefit if the recycled aggregate is used to produce a concrete mix 18 before the carbon dioxide entrained recycled aggregate has been exposed to ambient pressure for longer than a predetermined exposure time. Preferably, the predetermined exposure time is less than around one hundred and twenty minutes, as using the recycled aggregate in a concrete mix within this timeframe will facilitate a substantial proportion of the carbon dioxide remaining in the recycled aggregate at the time of its use in the concrete mix. To produce the concrete mix, the carbon dioxide entrained recycled aggregate containing the carbon dioxide gas is combined with at least cement, fine aggregate and water. If the concrete mix 18 is produced before the carbon dioxide entrained recycled aggregate has been exposed to ambient pressure for longer than the predetermined exposure time, carbon dioxide gas released from the recycled aggregate enters the slurry component of the concrete mix, which contains Ca(OH)2, with which the carbon dioxide gas reacts to form CaCC in accordance with the chemical reaction set out in equation (2) above. [0047] Figure 10 shows concrete 102 formed from a concrete mix containing carbon dioxide entrained recycled aggregate. The carbon dioxide entrained recycled aggregate used to form the concrete shown in Figure 10 included both recycled concrete 104 and recycled construction waste other than concrete, including ceramic 106. Figure 10 shows a ring 100 of calcium carbonate crystals formed around a piece of the ceramic 106 from the reaction of carbon dioxide gas released from the piece of ceramic 106 with Ca(OH)2 in the slurry component of the concrete mix, as discussed above.

[0048] To extend the time within which the carbon dioxide entrained aggregate can be used without substantial loss of carbon dioxide gas from a non-concrete portion thereof, a further method 20 of producing a concrete mix comprises maintaining 22 the carbon dioxide entrained recycled aggregate under pressure during at least a part of a period between the end of the predetermined carbonation time and its being used in the concrete mix. Maintaining the aggregate under pressure keeps the carbon dioxide gas entrained in the non-concrete portion of the recycled aggregate whilst it is awaiting use in the concrete mix (Figure 2).

[0049] In a method 24 of producing the carbon dioxide entrained recycled aggregate, the carbon dioxide entrained recycled aggregate is maintained 25 under pressure after the carbonation time and prior to its use in the concrete mix. The carbon dioxide entrained recycled aggregate may be maintained 25 under pressure in the carbonation chamber 14 or in a separate pressure vessel. As with method 20, maintaining the aggregate under pressure after the carbonation process keeps the carbon dioxide gas entrained in a non-concrete portion of the aggregate whilst it is awaiting use in a concrete mix (Figure 3).

[0050] In another method 26 of producing the carbon dioxide entrained recycled aggregate, comminuted construction waste is provided 28. The comminuted construction waste is entrained 12 with carbon dioxide in a carbonation chamber, such as carbonation chamber 14 (Figure 4). The comminuted construction waste comprises hardened concrete and non-concrete construction material, such as: non-concrete masonry material; tile; and organic material; or any combination thereof. The comminuted construction material has a nominal particle size of less than 20mm. In some embodiments, the nominal particle size is around 10mm. In some embodiments, providing the communited construction waste comprises comminuting the construction waste and subjecting the construction waste to size based classification, such as sieving or screening, to obtain comminuted construction waste of the nominal particle size. Setting the nominal particle size at less than 20mm, preferably around 10mm, results in an increased proportion of hardened cement in the communited construction waste compared to the proportion therein if the nominal particle size is set at greater than 20mm. Accordingly, setting the nominal particle size at less than 20mm, preferably around 10mm, results in recycled aggregate formed from the comminuted construction waste having a greater proportion of Ca(OH)2 available for conversion to CaCC during carbon dioxide entrainment, such as in carbonation chamber 14. Setting the nominal particle size at less than 20mm, preferably around 10mm, also provides a relatively large surface area for exposure to carbon dioxide in the carbonation chamber 14.

[0051] The carbonation chamber 14, which is shown in Figure 8, includes a chamber wall 30, a chamber neck 32, a flange 34, a lid 36, a lid handle 38, a seal 40 between the flange 34 and the lid 36, bolts 42 for securing the lid in a closed position, wheels 44, and a tank 46, containing pure carbon dioxide gas, in fluid communication with the carbonation chamber 14 via a gas line 48. Silica gel (not shown) is also placed within the carbonation chamber 14 to absorb excess water produced by the carbon dioxide entrainment reaction shown in equation (2).

[0052] In a method 50 of producing the carbon dioxide entrained recycled aggregate, a batch of recycled aggregate for carbon dioxide entrainment is placed in carbonation chamber 14. Pure carbon dioxide gas from tank 46 is delivered to carbonation chamber 14 to create a pure carbon dioxide environment in the carbonation chamber 14 and to generate a carbonation pressure 52 within the carbonation chamber 14 of up to approximately 75kPa, preferably between approximately 25kPa and 75kPa, and more preferably approximately 25kPa (Figure 5). The carbonation time 54 for which the recycled aggregate undergoes carbonation within the carbonation chamber 14 is at least thirty minutes, preferably at least sixty minutes, and more preferably at least approximately one hundred and twenty minutes. Method 50 can be combined with any one or more of methods 10, 20, 24, 26 described above. Method 50 provides superior entrainment of carbon dioxide into the recycled aggregate compared to that achieved by previously known methods, thereby producing a carbon entrained recycled aggregate with improved characteristics, such as increased density and reduced water absorption, compared to carbon dioxide entrained recycled aggregate produced by previously known methods. As a result, concrete products formed from a concrete mix comprising recycled aggregate produced by method 50 exhibit improved characteristics, such as increased compressive strength, compared to concrete products formed from concrete mixes utilising recycled aggregate produced by such previously known methods.

[0053] Predictive methods 55, 56 of relating concrete mix variables to concrete strength are also disclosed. These methods can be used, respectively, to formulate a concrete mix based on a desired compressive strength 57 of concrete to be formed from the concrete mix 55 or to determine a compressive strength 59 of concrete formed from a concrete mix 56, the concrete mix comprising cement, coarse aggregate comprising at least carbon dioxide entrained recycled aggregate, fine aggregate, and water.

[0054] Each method 55, 56 includes providing a model interrelating concrete mix variables 58 of the concrete mix to compressive strength of concrete to be formed from the concrete mix, the concrete mix variables 58 comprising: a water to cement ratio of the concrete mix 60, a proportion of the coarse aggregate comprising the carbon dioxide entrained recycled aggregate 62, at least one parameter relating to the carbon dioxide entrained recycled aggregate 64, a mass of the cement 66, a mass of the fine aggregate 68, and a mass of the water 70. The at least one parameter 64 may be two parameters, such as, for example, the carbonation time and the carbonation pressure.

[0055] The method 55 further comprises using the model to determine a set of said concrete mix variables 58 associated with the desired compressive strength 57, thereby to formulate a concrete mix for producing concrete having that compressive strength. In this way, the method 55 determines one or more sets of concrete mix variables 58 that satisfies the desired compressive strength 57, which saves the user time from having to create and test multiple batches of concrete mix, which in turn, also conserves resources.

[0056] The method 56 further comprises using the model to determine a compressive strength of concrete formed from a concrete mix having a predetermined set of said concrete mix variables 58. In this way, the method 56 provides a means of testing a set of concrete mix variables 58 to determine the compressive strength of the concrete to be formed from the set of concrete mix variables 58, which saves the user time from having to create and test multiple batches of concrete mix, which in turn, also conserves resources.

[0057] In one embodiment of the method 55, the model is a neural network 72 (Figure 6). The neural network 72 comprises an input layer 74 comprising at least one input node 76, at least one hidden layer 78 comprising at least one hidden node 80 configured to receive data from the input layer 74, and an output layer 82 comprising at least one output node 84 configured to receive data from the at least one hidden layer 78.

[0058] The neural network 72 receives the desired compressive strength of concrete 57 at the input layer 74 by, for example, the user inputting the desired compressive strength of concrete 57 from which the user seeks to determine a set of concrete mix variables 58 at the output layer 82 by which to formulate a concrete mix which will form concrete with the desired compressive strength 57. The neural network 72 determines the set of concrete mix variables 58 by applying the weighting at the hidden layer 78 as determined by training applied to the neural network 72 which is described below.

[0059] In one embodiment of the method 56, the model is a neural network 86 (Figure 7). Similarly to neural network 72, the neural network 86 comprises an input layer 88 comprising at least one input node 90, at least one hidden layer 92 comprising at least one hidden node 94 configured to receive data from the input layer 88, and an output layer 96 comprising at least one output node 98 configured to receive data from the at least one hidden layer 92.

[0060] The neural network 86 receives the set of concrete mix variables 58 at the input layer 88 by, for example, the user inputting the set of concrete mix variables 58 which the user seeks to test. The neural network 86 determines the compressive strength of the concrete to be formed from the concrete mix 59 by applying the weighting at the hidden layer 92 as determined by training applied to the neural network 86 which is described below.

[0061] The neural networks 72, 86 are trained by using a data set which is obtained by conducting physical testing. In one embodiment, the data set is obtained by forming a plurality of concrete structures, each of the plurality of concrete structures being formed using a concrete mix, the concrete mix comprising cement, coarse aggregate comprising at least carbon dioxide entrained recycled aggregate, fine aggregate, and water, the concrete mix having a predetermined set of the concrete mix variables. The compressive strength of each of the plurality of concrete structures is tested and the compressive strength of each of the plurality of concrete structures is recorded. To populate the data set, the predetermined set of the concrete mix variables of the concrete mix used to form each of the plurality of concrete structures is inputted and the recorded compressive strength of each of the plurality of concrete structures is also inputted.

The person skilled in the art will appreciate that the data set for training the neural networks 72, 86 may be formed using other suitable methods. [0062] In one embodiment, the training data comprises sand as the fine aggregate, with varying ratios of recycled aggregate to total coarse aggregate, which, in this example, comprises virgin aggregate. The units are provided as kilograms per metre cubed of cement mix produced and the recycled aggregate is provided as both 10mm and 20mm nominal sizes. The training data also comprises varying carbonation pressures and times which provide for different values of compressive strength. The tables of example training data are provided below.

Table 1: Data set of concrete mix variables

Table 2: Data set of concrete mix variables with compressive strength

[0063] In Tables 1 and 2: recycled aggregate replacement ratio/percentage refers to the proportion of the coarse aggregate comprising recycled aggregate, wherein the remainder of the coarse aggregate comprises virgin aggregate (e.g., a 30% recycled aggregate replacement ratio/percentage means that 30% of the coarse aggregate comprises recycled aggregate and 70% of the coarse aggregate comprises virgin aggregate); and data in kg/m ³ indicates the mass in kilograms of the mix ingredient per cubic metre of the mix.

[0064] In one embodiment, the plurality of concrete structures formed for testing are cylinders 101, the cross section of one of which is shown in Figure 9, which shows carbon dioxide entrained recycled aggregate after one hundred and twenty minutes of carbonation time at 25kPa of carbonation pressure. In one embodiment, these concrete structures undergo a 28 day compressive strength test, i.e. the compression strength test is conducted after 28 days of curing the concrete.

[0065] In another embodiment of the methods 55, 56, the model is a multiple linear regression model. In one embodiment, the multiple linear regression model is in the following form: where: a, b, c, d, e, g, and h, each represent a predetermined constant; c' represents the compressive strength of concrete to be formed from the concrete mix; R _wc represents the water to cement ratio of the concrete mix;

R _rca represents the proportion of the coarse aggregate comprising the carbon dioxide entrained recycled aggregate;

C _p 1 represents a first parameter relating to the carbon dioxide entrained recycled aggregate;

C _p 2 represents a second parameter relating to the carbon dioxide entrained recycled aggregate;

Q _w represents the mass of the water; and Qf _a represents the mass of the fine aggregate.

[0066] The data set shown in Tables 1 and 2 may also be used to configure the multiple linear regression model above, namely to determine the predetermined constants a, b, c, d, e, g, and h. In one embodiment, the predetermined constants a, b, c, d, e, g, and h are determined to be as follows: a = —10089.538; b = 33.332; c = 8.290; d = 0.020; e = 0.001; g = 1369.79; and h = 0.652.

[0067] Advantageously, the concrete mix produced using the carbon dioxide entrained recycled aggregate utilises recycled construction waste to form new concrete, which reduces the amount of waste being added to landfill. In addition, the carbon dioxide entrainment of the recycled construction waste increases the compressive strength of concrete formed from the recycled construction waste.

[0068] Further, the methods of formulating the concrete mix and of determining a compressive strength of concrete formed from the concrete mix provide means for the user to obtain concrete mix variables for a particular desired compressive strength of concrete to be formed from the concrete mix and to determine the compressive strength of a concrete sample made from a concrete mix, where its concrete mix variables are known, without requiring physical testing. Decreasing the amount of concrete being formed for testing by using these methods also decreases the amount of material contributing to landfill.

[0069] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.