# Reliable Laboratory Tests: A Prerequisite for the Design of High-Quality Slurry Surfacing Mixtures

Authors: Alexandra Destrée; Stefan Vansteenkiste; Tine Tanghe; Joëlle De Visscher
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Surface treatment with slurry surfacing is a quick and cost-effective technique to restore the condition and performance of a road surface, compared with the replacement of the wearing course. It is therefore not surprising that cities and municipalities, faced with increasingly tight budgets, choose slurry surfacing to maintain their roads. A slurry surfacing with good performance and a long service life requires, above all, a well formulated mixture and a good control of the execution quality. Slurry surfacing could then even be considered for ultrathin wearing courses on new pavements. From 2016 to 2020, the Belgian Road Research Centre (BRRC) was engaged in a 4-year research project called BeP2S (Better Performing Slurry Surfacing) financially supported by the Belgian Bureau for Standardization (NBN). The project focussed on laboratory testing, gathering data and field experience, assessing the link between laboratory and field performance, and the mix design process. This paper is dedicated to the European test methods of the EN 12274 series (“Slurry surfacing–Test methods”), as well as some alternative test methods, which were evaluated (complexity, relevance for the mix design process, …) and improved in terms of test conditions, precision, and discriminating power. Five performance-related tests proved to have sufficient ability to discriminate between mixtures with variations in mix design. Recommendations were made for the ranges within which the laboratory test results should fall in order for the slurry surfacing to perform well.


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## Body

## 1. Introduction and Background

Slurry surfacing techniques are used to maintain and extend the service life of existing pavements through restoring characteristics such as skid resistance and imperviousness, using little resources and with less traffic disruption. Slurry surfacing includes slurry sealing and microsurfacing [1–4]. Slurry is a cold mixture consisting of a well-graded mineral aggregate blend, emulsion (which may be modified with polymer), water, and, where appropriate, additives (cement, breaking retardant, etc.). Slurry surfacing is proportioned, mixed, and uniformly laid on-site at ambient temperature, using mobile machinery. The mix is evenly spread in one or two layers, with thicknesses slightly greater than the maximum aggregate size. The slurry surfacing shall be applied over a properly prepared surface to guarantee adequate long-term performance (resistance to abrasion, ravelling, bleeding, etc.) [5–8].Slurry surfacing offers a wide range of economic and technical advantages, as well as social and environmental ones, when compared to repaving the wearing course with hot mix asphalt [9]:(i)
less material and energy consumption during construction (with respect to hot mix asphalt);(ii)
lower fuel consumption in the use stage (less greenhouse gas emissions), thanks to lower rolling resistance;(iii)
road safety and comfort maintained constant at a high level;(iv)
less traffic disruption by road works (usually faster application times than alternative conventional approaches);(v)
better and safer working conditions for road workers (particularly on very hot summer days), etc.Since this technology requires less material resources, is cold produced, and laid at low thickness, the direct cost price is low.For this reason, slurry surfacing is an interesting solution for local authorities (city and municipal governments) and private companies to maintain the quality of their roadways on a limited budget [5, 10–12], and the proportion of the road network that is eligible for preventive maintenance with slurry surfacing is quite large.The formulation (mix design process) of slurry surfacing in the laboratory is an essential step in the implementation of a quality slurry surfacing on-site [13–18]. Preparation of the road surface, production, and application are the other three pillars of successful slurry surfacing [5, 7, 19]. After considering the specificities of the site, and more specifically the type of support, the condition and the intensity of the traffic, the “mix designer” will select, combine, and dose the different components of the slurry surfacing mixture in the laboratory [5, 7]. To study and optimize the performance of slurry formulations in the laboratory, reliable performance-related tests and well-established mix design guidelines are absolutely needed [20, 21].However, improving the performance of slurry through laboratory tests is not simple since the final performance in the field also depends strongly on workability and the development of cohesion on-site. These characteristics depend on mix composition (nature and quantity of various constituents) and site conditions (weather, state of the underlying layer, organization, equipment, workmanship, adjustments in situ to mix composition, etc.) [20]. In view of the complex and sensitive chemistry of cold emulsion technologies, a good understanding and knowledge of the impact of all these parameters is needed. This will ultimately improve the durability and performance of slurry surfacing and will result in more sustainable preservation of the road network, because not only the lifetime of the surfacing will be extended but also the underlying courses and structure will be better protected against deterioration and last longer [1, 9, 22, 23].BRRC conducted a 4-year research project called BeP2S (Better Performing Slurry Surfacing) with financial support of the Belgian Bureau for Standardization (NBN), with the aim of improving the performance and the durability of slurry surfacing on the Belgian road network. The project focussed on laboratory work and field data, with a view to assess the relation between laboratory and practice and to propose a practical mix design procedure.The purpose of this paper was to draw attention to the importance of prior mix design and laboratory testing. It describes the BRRC approach (see Section3) and deals specifically with the laboratory work which involved improvement of the European test methods of the EN 12274 series (“Slurry surfacing: test methods”) and of some alternative test methods (see Section 4).


## 2. Problem Statement and Objectives

Currently, there is a general lack of knowledge and expertise in the sector (at the national and international level) regarding the mix design process and the associated laboratory tests to evaluate the performance of slurry surfacing [13–18]. Mix formulation (selection and proportioning of components) is mainly based on empirical knowledge (practical experience, know-how, skills and mindset of formulators and operators) and bound by practical constraints (related to the equipment, the availability of components, etc.).However, the variability of constituent materials (cement, emulsions, etc.) will have an undeniable impact on the performance of the slurry surfacing [20]. Replacing one emulsion by another in a “known recipe” without conducting prior laboratory tests is not acceptable and puts the final result at risk. The sector must be aware of the high failure risk that will result from neglecting to carry out a proper mix design in the laboratory, using reliable tests to confirm the quality of the slurry surfacing recipe that is established for a construction site with the materials available at that time [5].It is essential that laboratories can rely on practical, clear, and consistent standards for performing slurry surfacing tests. However, the European standard test methods (EN 12274 series) [24–27] currently need clarification on certain procedures as they leave room for interpretation on several points.The objectives of the BeP2S project regardinglaboratory test methods on slurry surfacing (slurry seal and microsurfacing) were:(i)
To evaluate the European standards (EN 12274 series) [24–27] and to propose improvements when necessary;(ii)
To establish adequate thresholds or limits for test results that indicate “good performance” of a slurry surfacing in the laboratory;(iii)
To assess whether new performance-related tests or alternatives for existing tests are needed and if yes to develop such test procedures.


## 3. Approach

To achieve the objectives, the following approach was adopted for each test method:(i)
Critical assessment of the existing European standard test method (EN 12274 series) [24–27] or alternative test method focusing on these main issues [28–36]:(ii)
Are procedures well described (especially for mixing procedure) ?(iii)
Are test conditions relevant for job site conditions?(iv)
Comparison with similar standards or guidelines outside Europe (Table1), with the intention of detecting differences that could potentially have an impact on the result and/or the precision and could therefore improve the European standard;(v)
Acquisition of practical experience and determination of changes needed to equipment and procedure to improve practical feasibility;(vi)
Improvement of procedures and test conditions to better mimic conditions and performance on-site.(vii)
Determination of repeatability (precision) and discriminating power by testing mix variants;(viii)
Evaluation of the limitations and relevance of the tests for the mix design process;(ix)
Establishment of “recommended results” for the “good performance” of a slurry surfacing in the laboratory.Table 1
List of the considered documents for the comparison of the European test methods of the EN 12274 series (“slurry surfacing–test methods”) with similar standards.
Investigated propertyEuropean standardStandard∗Mix time—ISSA TB113 [28]ConsistencyEN 12274-3 [24]ISSA TB106 [29]AGPT-270-18 [30]CohesionEN 12274-4 [25]ISSA TB139 [31]AGPT-271-18 [32]Wearing resistanceEN 12274-5 [26]ISSA TB100 [33]AGPT-272-18 [34]Interaction between aggregate and emulsionEN 12274-7 [27]ISSA TB144 [35]AGPT-273-18 [36]∗ ISSA: International Slurry Surfacing Association; AGPT: Austroads Guide to Pavement Technology.As a result of this project, BRRC acquired good practical experience with the equipment and the execution of the laboratory tests according to the EN 12274 series and some alternative test methods [20]. Based on this experience and the comparative analysis of similar standards and guidelines (Table 1), BRRC identified several shortcomings and inconsistencies in the current versions of the standards (EN 12274-3, -4, -5 and -7).A list of comments and proposed changes has been submitted to CEN/TC227/WG2 “Surface Dressing, Sprays and Slurry Surfacing (incorporating Microsurfacing),” to facilitate the execution of the test and to increase precision and discriminating power. The following section will focus on the most important issues.


## 4. Evaluation and Improvement of the Test Methods and Recommendations for the Use

The objective of the laboratory tests is to evaluate the performance of a slurry surfacing. This consists in measuring characteristics crucial for construction quality (workability, consistency, breaking and curing behavior [3, 12, 37–40], as well as resistance to the most common types of distress (ravelling, bleeding, rutting, etc.) [41].This section describes BRRC’s experience with the most important laboratory tests with practical relevance and gives suggestions for improvements to increase repeatability and discriminating character. The discussion of the standards refers to the versions mentioned in the reference list.


### 4.1. Preparation of Slurry Surfacing Mixtures under Laboratory Conditions

The mixing procedure of slurry mixtures has an undeniable impact on coating of the aggregates with the emulsion, homogenization of the complete mixture, breaking behavior, and the development of cohesion [12, 37, 38, 42]. For example, a too short mixing time will not result in a homogeneous mixture and a too intensive stirring, or the use of a metal spatula, could cause damage to the emulsion, such as destabilization.Therefore, the laboratory mixing procedure should be repeatable and mimic the mixing process on-site as closely as possible. When comparing different test methods of the EN 12274 standards, it appears that the mixing procedure description is not always the same and not sufficiently detailed or accurate. This may lead to different interpretations and poor reproducibility.At BRRC, a detailed protocol was developed which describes exactly the manual mixing sequence (dry aggregate, cement, pre-wetting water, additives, and emulsion), the times to mix the components (after adding the emulsion: between 30 s and 60 s, depending on the sample size), and the mixing speed (60 rpm).Temperature of constituents and environment should ideally be equal to the temperature on-site when the mixture is placed. Unfortunately, the actual temperature on-site cannot be predicted and, therefore, the average temperature of the season shall be selected as the best approximation (with a tolerance of ±5°C).This procedure is suitable for obtaining a homogeneous and stable mixture for the formulations tested at BRRC. This protocol is recommended to be followed in every test on laboratory-prepared slurry surfacing mixtures.


### 4.2. Mix Time

On the worksite, mixing and spreading of a slurry surfacing takes approximately 60 seconds. The mixture shall remain mixable in that period; otherwise, there is a risk of lump creation or obstruction of the mix in the mobile machinery. The mix time is defined as the time between the start of mixing (immediately after adding the emulsion to the mixture) and the moment the mixture starts to stiffen (onset of breaking of the mixture) [12, 37, 38].Breaking [5, 43] refers to all the phenomena leading to the formation of a continuous bituminous phase (agglomeration, flocculation, and coalescence). The individual bitumen droplets are then engaged in an irreversible process leading to the separation of bitumen and water. In practice, the end of breaking is marked by the change from a brown to a black color as the bitumen is transformed into a continuous film.After the emulsion and aggregates are mixed, the breaking process begins. The breaking speed depends on the wind speed, the ambient temperature, the temperature of the components, and the surface temperature of the underlayer. The higher these temperatures and the higher the wind speed during the execution, the faster the breaking process and the curing of the slurry surfacing will be.There is no standard European test, but the mix time can be evaluated in the laboratory (at controlled temperature) by following the manual procedure described in the technical bulletin TB113 from the International Slurry Surfacing Association [28].


#### 4.2.1. Objective of the Test

The main objective of this test [28] is to measure in the laboratory the mix time of a specific combination of materials of slurry surfacing mixes. The procedure in TB113 also allows for the evaluation of the set time of the slurry surfacing. The set time is defined as the time between the start of mixing (immediately after adding the emulsion to the mixture) and the moment where it is no longer possible to stir the mixture.


#### 4.2.2. Principle, Equipment, and Procedure

The ISSA TB113 procedure [28] is composed of two parts. The first part will allow the evaluation of the set time of the slurry surfacing and the second part will evaluate the mix time.The different components are manually stirred in disposable cups at a constant speed. The procedure [23] includes a detailed mixing protocol:(i)
mixing sequence: dry aggregate, mineral filler/dry additive, water/liquid additive, emulsion;(ii)
mixing times: 10 s to 30 s, according to the added component or until its distribution is uniform;(iii)
mixing speed: 60–70 rpm;(iv)
temperature of constituents (room temperature and temperatures likely to be encountered in the field).For the first part of the method, when the emulsion is added and mixed with the other components, half of the mixture is poured on a support (paper, aluminum foil, or roofing felt) at a thickness between 6.4 and 10 mm. With the help of a paper towel, the operator will press and evaluate the time until this paper is no longer stained by emulsion but only absorbs clear water (set time).For the second part of the method, the operator continues mixing the remaining portion for a maximum of 5 minutes or until the mixture stiffens and breaks. The time required for the mix to stiffen and break is recorded in seconds (mix time). A visual examination of the mixture throughout the procedure should also be realized (fines flotation, free liquids present, dryness, stiffness, etc.). In the function of these observations, adjustments of the combination of the components may be necessary to obtain a workable and sufficiently consistent mixture.


#### 4.2.3. Evaluation and Recommendations

At BRRC, the first part of the ISSA TB113 method [28] is not applied. The “finger and paper pressing techniques” are subjective and highly operator dependent. Moreover, the set time is supposed to be indicative of the time at which the road can be reopened to traffic, but the test performed according to EN 12274-4 [25] is clearly more suitable for that purpose (see Section 4.4).The second part of the method [28], which determines the mix time, is more interesting. The test is simple and fast but also operator dependent (subjective character) [6, 44, 45]. Therefore, the test is preferably carried out by a single experienced operator. The precision of the test was not determined at BRRC. It is expected that reproducibility is the main issue, because of this operator dependency.At BRRC, a smaller quantity of mineral aggregate is used (50 g instead of the 100 to 400 g foreseen by Ref. [28]) to economize materials and to ensure a uniform distribution of the different constituents during manual mixing. The quantities of the other components (water, emulsion, cement, …) are of course adapted to this reduced quantity in order to keep the same percentages in mass compared to the dry aggregates.The test can discriminate in a logical way between mixtures with variations in composition. As expected, the characteristics of the emulsion and cement have a strong impact on the mix time.The huge impact of test temperature on the mix time was also observed. Therefore, it is recommended to perform the test at temperatures likely to be encountered in the field.The results obtained in the BeP2S project show that a mix time of at least 120 s (for microsurfacing) or 180 s (for slurry sealing), as recommended by Refs. [13, 14], seems adequate for slurry surfacing.


#### 4.2.4. Key Takeaways

The most important findings with the manual mixing test (second part of the method according to Ref. [28]) are the following:(i)
The test is fast, simple, and requires no sophisticated equipment;(ii)
The results of this test are subjective and operator dependent;(iii)
The test can logically discriminate mixtures with different compositions;(iv)
A mix time of at least 120 s or 180 s may be considered as a target;Despite the limitations, BRRC has evaluated this manual mixing test (second part of the method according to Ref. [28]) as a valuable method to verify the mix time. The introduction of this test as a new standard test method in the EN 12274 series would be useful.


### 4.3. Consistency

An essential parameter governing the design, manufacture, and spread of slurry surfacing (i.e., slurry seals or microsurfacing) is the consistency of the mix [37, 38, 45–47].(i)
If the consistency is too high (mix too “stiff,” Figure1(a)): the mix will be less workable, and it will a priori present less good adhesion to the substrate.(ii)
If the consistency is too low (mix too “liquid,” Figure1(b)): the risks of segregation and outflow phenomena will be greater.Figure 1
Two types of slurry surfacing consistency (a) high (small diametrical) and (b) low (large diametrical outflow).
(a)(b)To achieve the correct consistency, the proportions of the various constituents (pre-wetting water, emulsion, cement, additives) in relation to the dry aggregates must be optimized. The European standard EN 12274-3 [24] specifies a test method for determining the consistency of slurry surfacing. In the 2018 version of the standard EN 12274-3, the scope is restricted to slurry seals with maximum aggregate size (D) ≤ 4 mm (it was not the case in the previous version of the standard).


#### 4.3.1. Objective of the Test

The objective of this test is to determine in the laboratory the mix quantities (aggregate, emulsion, pre-wetting water, and additives if required) for optimal workability and flow of a slurry mixture.


#### 4.3.2. Principle, Equipment, and Procedure

The consistency test is illustrated in Figure2 and is carried out at room temperature (23 ± 5)°C. A cone is placed on a metal plate engraved with a scale of eight concentric circles (numbered from 0 to 7 from the inside out), which has been previously covered with a transparent nonabsorbent paper. Immediately after mixing the different components, the cone is filled with the slurry mixture and removed in a regular vertical movement, allowing the mix to spread.After (10 ± 2) seconds, the diametrical outflow is measured on the scale (at four points 90° apart) and the average of these four readings, expressed in millimeters (mm), is a measure of the consistency of the mixture.The lower the outflow, the higher the consistency of the slurry surfacing. Conversely, the greater the outflow, the lower the consistency of the slurry surfacing (Figure1).Figure 2
Illustration of the execution of the consistency test according to EN 12274-3.


#### 4.3.3. Evaluation and Recommendations for Improving the Standard EN 12274-3

The consistency test according to the standard EN 12274-3 is a simple and fast test. The mixing procedure is not well described in the standard, although it is of crucial importance for the test result. It is recommended to follow a more detailed mixing procedure, as described in Section4.1.Based on practical experience, BRRC recommends to use a funnel to adequately (cleanly and correctly) transfer the slurry mixture from the container into the cone (see Figure2). When transferring the slurry mixture, segregation and incorporation of air bubbles shall be minimized by pouring smoothly and carefully. Removal of the cone also has an impact on the diametrical outflow, especially for more fluid/liquid mixtures. The slightest deviation from the verticality of the movement may result in a noncircular spread and therefore a greater standard deviation over the four measurements at 90°. BRRC recommends that the standard procedure insists on the vertical removal without tilting the cone.Contrary to the current scope of the standard EN 12274-3, the tests carried out at BRRC (on slurry surfacing mixes with grading 0/6.3 mm) demonstrate the interest and feasibility of analyzing the consistency of slurry surfacing with a maximum aggregate size larger than 4 mm. The number of repetitions is not specified in the standard. The results obtained in the BeP2S project show that at least three tests per mixture are needed to obtain coherent and reliable results.Logical results are obtained with respect to differences in the operating conditions and the composition. For example, Figure3 shows how the percentage of cement increases the consistency of this formulation tested at BRRC. Note that, depending on the emulsion and cement type, cement can also have a different impact on the slurry mixture and decrease its consistency. Therefore, the test is considered as discriminating and useful.Figure 3
Outflow versus cement percentage content in the mix formulation (bars denote standard deviation of 3 repeated tests; in case of the mixture with 2.3%, the 3 repeated tests gave the same result).Temperature has a considerable influence on the consistency and, therefore, the test temperature should be representative of the temperature range expected in the field and not limited to the temperature range currently specified in the standard ((23 ± 5)°C). The test temperature shall be accurately controlled and reported. BRRC recommends performing the consistency test at two or three different temperatures covering the expected temperature range in the field, since this allows for evaluating the sensitivity to temperature fluctuations.For the interpretation of the diametrical outflow, it would be easier and more logical to count the circles from 0 to 7 from the outside to the inside (Figure4(b)). By reversing the direction of this scale, a high consistency would be characterized by a high value and a low consistency would be characterized by a low value.For ease of reading (especially for “asymmetric” outflows), it would also be more practical if the numbers were engraved at 0°, 90°, 180°, and 270°. Figures4(a) and 4(b) show the current situation in the standard and the BRRC proposal.Figure 4
Current situation in the standard (a) and BRRC proposal for a new metal plate engraved with a scale of eight concentric circles, each increasing in radius by 10 mm and the inner circle has a diameter of 90 mm (b).
(a)(b)It was found that some mixtures may have an adequate quantitative outflow between 2.0 and 3.0 cm (as recommended by the ISSA [14]), while presenting an unusual appearance (asymmetric outflow, lumps, emulsion drainage, or uncoated aggregates…), which would indicate that the mix is not well designed. Therefore, in addition to the quantitative determination of outflow, the quality of the mixture during mixing and the quality of the mixture after flowing shall be observed and reported. BRRC recommends adding Table 2 to the standard EN 12274-3, for the qualitative assessment of the test.Table 2
Consistency description∗.
LetterStateObservationSSoupyBrown free liquid, segregating sampleLVLow viscosityNon-segregating easy to mixMVModerate viscosityNon-segregating, moderate resistance to mixStStiffHard to mix but workableBBrokenLumps, non-consistent∗ Table2 adapted from Ref. [15].The repeatability limit of the outflow at BRRC was estimated to be 0.6 cm for the average of three repeated tests, determined in the range of 2.0 to 3.0 cm. Outside this range, the repeatability will be less due to the phenomena of drainage and inhomogeneous diametrical outflow, which occur more frequently outside this range of “optimal” consistency.An outflow of 2.0 to 3.0 cm, as recommended by the ISSA [14], may be considered as a suitable target, but not as a necessary requirement, given the precision of the test. The experience and knowledge of the operator also affect the repeatability of the test. Indeed, as explained above, the speed and time of mixing and the removal of the cone are three test conditions that must remain as constant as possible.


#### 4.3.4. Key Takeaways

If the test is performed according to BRRC’s recommendations (see Section3), the most important findings are the following:(i)
This laboratory test is simple, fast, and sufficiently discriminating. Therefore, it is useful for evaluating the consistency of a slurry surfacing and its sensitivity to temperature variations, prior to its implementation on-site.(ii)
At least three tests per mixture are needed to produce consistent average results.(iii)
An outflow of 2.0 to 3.0 cm, as recommended by (ISSA) [14], seems adequate. A visual inspection of the quality of the mixture (during mixing and after flowing) is also recommended to have a complete diagnosis of the consistency.(iv)
The test is also suitable for mixtures with maximum aggregate size (D) > 4 mm, despite the restriction added in the scope of the 2018 version of the standard EN 12274-3.(v)
The experience and knowledge of the operator affects the repeatability of the test. At BRRC, a repeatability limit of 0.6 cm was found (average of three tests) for values in the range of 2.0 to 3.0 cm.(vi)
BRRC has evaluated the consistency test as a valuable method to adapt mix design and to optimize the proportions of the various constituents in relation to the dry aggregates (water, emulsion, cement, additives) with a view to adequate consistency (i.e., a homogeneous and workable mixture).A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-3.


### 4.4. Cohesion

After its application on the pavement, the slurry surfacing must acquire a rapid cohesion build-up to obtain the shortest possible time for quick reopening of the road (“traffic time,” period of time after laying, when the slurry surfacing can accept traffic) without losing the workability necessary for its application [12, 37, 38, 42].The kinetics of “cohesion build-up” and the final cohesion (allowing the reopening to traffic without degradation of the slurry surfacing) are therefore essential parameters governing the mix design, manufacture, and implementation. Contractors often aim for the shortest possible time of opening to traffic to limit as much as possible the inconvenience caused to road users. This is usually between 30 and 60 minutes.In the framework of the research project called BeP2S, two test methods were investigated to determine the early cohesion of slurry surfacing:(i)
The European standard EN 12274-4 [25] (see Section 4.4.1) specifies a test method which is similar to a torsion test, and which can be used as an aid to the design of slurry surfacing in order to evaluate the “cohesion build-up time,” i.e., the “set time” and the “curing time”/“traffic time” through a minimum cohesive strength to be reached.(ii)
The technical specification “TS: Slurry surfacing-Test methods-Determination of cohesion of the mix” (see Section4.4.2) describes an alternative method to EN 12274-4, called Hilt Cohesion Test (HCT). Practical experience with this test at BRRC was not positive. Moreover, due to the difficulty of harmonizing procedures for this little practiced test, the process of improving the technical specification in the CEN committee was abandoned. For this reason, this alternative method will not be discussed in this paper.


#### 4.4.1. Cohesion According to EN 12274-4

(1) Objective of the Test. This test method is used to measure the development of cohesion of a slurry surfacing system with time. This test also quantifies the time required before a slurry surfacing can be subjected to straight rolling traffic (“traffic time”).(2) Principle, Equipment, and Procedure. The slurry mixture is cast in a square-shaped mold having four circular holes, under which a roofing felt has been placed beforehand (Figure 5). The whole assembly is then conditioned at a temperature of (23 ± 5)°C. The standard specifies that the samples must be removed from the mold before testing. At BRRC, the roofing felt support consists of 4 separate parts, so that each sample can be tested individually on the roofing felt support.Figure 5
Mold used at BRRC to prepare the four circular samples for the cohesion test (and the 4 squares of roofing felt support to immobilize the samples).The test (Figure6) is carried out at the same temperature as conditioning (23 ± 5)°C. Its principle is to apply a rubber stamp (diameter of 28.5 ± 0.1 mm and height of 6.3 ± 1.0 mm) on a circular sample (diameter of 60 mm, thickness depends on the maximum aggregate size of the mix) of slurry surfacing (immobilized on a roofing felt support, Figure 5) with a constant pressure (200 ± 4 kPa). Torsion is applied manually to the slurry surfacing through the rubber stamp and the maximum torque is measured using a torque meter. The test is performed on the four different samples at a fixed time or at different suitable intervals between 5 min and 120 min after the samples have been spread in the mold (Figure 5), to evaluate the rise in cohesion as a function of time.Figure 6
Illustration of the execution of the cohesion test at BRRC according to EN 12274-4.The result of the measurement (maximum torque), expressed in Nm, represents the cohesion of the mixture at the four curing times mentioned above.After the test has been executed, a visual inspection of the condition of the samples is performed. Four types of “degradation” are referenced in EN 12274-4 and illustrated in Figure7.Figure 7
The four types of “degradation” indicated in the EN 12274-4 standard and examples for visual assessment of specimens. (a) N, normal; the sample is slightly damaged by some loss of aggregates. (b) S, solid; only the film of bitumen has been removed from the surface of the sample. (c) C, cracked. (d) D, disintegrated.
(a)(b)(c)(d)The lower the numerical value of the maximum torque, the less cohesive the slurry surfacing will be. Conversely, the higher the numerical value of the cohesive torque, the more cohesive the slurry surfacing will be (Figure8).Figure 8
Illustration of a slurry surfacing with low (a) and high (b) cohesion after performing the cohesion test according to EN 12274-4.
(a)(b)(3) Evaluation and Recommendations for Improving the Standard EN 12274-4. The normative cohesion test carried out in laboratory according to the standard EN 12274-4 is useful, relatively simple, and fast for evaluating the progressive cohesion build-up of a slurry surfacing. For the mixing procedure, it is recommended to follow a more detailed mixing procedure (see Section 4.1).Preparing (and demolding) the samples requires good practice and skill from the operator. The specified amount of mineral mixture (aggregates and filler) in the norm is too high and consequently, the excess needs to be removed to obtain a surface at the same level of the mold. This requires a lot of manipulation of the mixture, which is not practical and can cause segregation. Preparing the exact amount of mixture significantly facilitates the mold filling and results in a more homogeneous and repeatable texture. For a mixture with aggregate grading 0/6.3 mm, a quantity of 200 g of dry mineral mixture (instead of the 400 g specified in the standard) was found to be sufficient. This is a recommendation to improve the test method.In practice, the conditioning temperature of the materials and the mixing temperature have an impact on the cohesion build-up, as the kinetics of breaking and curing are temperature dependent. In general, the rise in cohesion will be faster at higher temperatures. Therefore, it is recommended that the materials shall be conditioned and the slurry surfacing mixed in the laboratory in a temperature range close to the one expected during the use of the slurry surfacing on-site.The roofing felt mentioned in the standard is hard to find because of the specification of the weight per unit area. It would be appropriate to remove the weight currently indicated in the standard ((700 ± 100) g/m2). The most important requirement is to use a roofing felt with sufficient strength to act as a sample holder and the exact weight would not affect the results. At BRRC, a roofing felt with a weight per unit area of approximately 394 g/m2 is used, which is appropriate to play the role of the sample holder.The precision of the cohesion test depends on the consistency of the slurry surfacing tested, since the latter will determine the difficulty for an even and homogeneous filling of the metal mold rings. The objective is to obtain four similar samples with an even and homogeneous texture.The textural differences will inevitably be reflected in the measured torque values and will therefore have an impact on the precision of the test:(i)
A formulation with a “too low consistency” (large diametrical outflow) will be more susceptible to phase segregation. As a result, samples from the first filled rings will be smoother on the surface (fines and bitumen migrate to the surface) compared to the following ones which will have a more granular appearance (the rest of the container contains more coarse aggregates which settle downwards in the mix).(ii)
A formulation with a “too high consistency” (small diametrical outflow) will be more complex to manipulate due to its lower workability, and the final texture of the four samples may not be similar and homogeneous.The test times are not specified by the standard, but BRRC generally performed the test after 15, 30, 60, and 90 minutes. In later tests, 24 hours were used (instead of 15 minutes) to obtain an indication of the final cohesion of the mix. The ISSA [13, 14, 48] specifies values of at least 1.2 Nm (after 30 minutes) and 2.0 Nm (after 60 minutes). Based on practical experience with Belgian slurry mixtures, these limits seem to be adequate and feasible for Belgian practice. The visual examination of the degradation must also be considered (Figure 7).The minimum number of repetitions of the test is not specified in the standard (it is only integrated in a note). The repeatability at BRRC was estimated to be 0.7 Nm for a single test and 0.4 Nm for the average of three tests. Therefore, the cohesion test is systematically performed at BRRC in triplicate and the result is calculated as the average.The test is then quite repeatable even though it is almost entirely manual and very dependent on the manufacturing quality of the samples. The experience and skill of the operator will thus greatly affect the repeatability of the test.The cohesion test is discriminating since the different formulations tested generally show cohesion build-up, final cohesion, and degradation states consistent with their formulation. For example, Figure9 shows test results on 2 mixtures with the same composition, but with a different emulsion. The breaking behavior of emulsion 2 in this mixture is different, which results in slower cohesion build-up and a higher risk of early ravelling on the road.Figure 9
Cohesion versus time, for mixtures with two different emulsions (bars denote standard deviation on 3 repeated tests).However, the interpretation of the results can be quite difficult: sometimes the torque is high, but there is a lot of damage, and sometimes the torque is low and there is no visible damage on the sample. This seems to depend greatly on the surface texture of the slurry surfacing and the way the rubber stamp adheres to the surface. The cohesion values obtained (in Nm) should therefore be interpreted with caution and the deterioration of the specimen (i.e., the degradation observed by visual inspection, Figure7) should be considered. Figure 10 illustrates the importance of visual inspection after the test has been performed. It may sometimes be operator dependent (subjectivity of the visual assessment), but it is nonetheless important to consider this qualitative appreciation of the failure mode together with the quantitative torque values.Figure 10
Two slurry mixtures with the same cohesion strength but a different type of degradation.In this study, a “manual” test device was used, i.e., the operator applies the torsion to the sample manually using a torque wrench. There are also “automatic” devices on the market in which the torsion applied to the sample is controlled by the equipment itself. According to the literature, there is a good correlation between the results of the automatic and manual devices [15].It is not certain if the automatic device will greatly improve the repeatability of the test, because the difficulty of the test lies more in the production of the samples (manufacturing and extraction from the mold) than in the application of the torque on the slurry surfacing.(4) Key Takeaways. If the test is performed according to BRRC’s recommendations (see Section 3)), the most important findings are the following:(i)
The normative cohesion test carried out in laboratory (EN 12274-4) is useful, relatively simple, and sufficiently discriminating for evaluating the progressive cohesion build-up of a slurry surfacing.(ii)
At least three tests per mixture are needed to produce consistent average results.(iii)
Values of 1.2 Nm (after 30 minutes) and 2.0 Nm (after 60 minutes), as recommended by the ISSA [13, 14, 48], seem to be adequate for Belgian practice.(iv)
The repeatability under our laboratory conditions at BRRC was estimated to be 0.7 Nm for a single test and 0.4 Nm for the average of three tests. In the latter case, the test is quite repeatable even though it is almost entirely manual and highly dependent on the surface condition of the slurry surfacing.(v)
BRRC has evaluated the cohesion test as a valuable method to optimize the mix design in view of sufficient cohesion for opening to traffic within the desired time in Belgium.A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-4.


#### 4.4.2. Hilt Cohesion Test (HCT)

This test method has been proposed by France to the technical committee CEN/TC227/WG2 [7, 37]. This test seems to have some advantages compared to the method described in EN 12274-4: it is very simple and fast to perform and does not require expensive equipment. However, although presented as an alternative method to test the cohesion of a slurry surfacing, the “Hilt Cohesion Test-HCT” has little similarity with the test that is described in the standard EN 12274-4 (see Section 4.4.1).The principle (Figure11) is to place the sample (12 × 12 cm2 square and 1.5 cm thick) that has undergone controlled curing (at BRRC, 2 h at 20°C) on a device with a fixed part and a hatch. Half of the specimen covers the fixed part, with a counterweight on top, and the other half of the specimen covers the hatch. After opening the hatch, the time taken for the slurry surfacing to break under its own weight is measured. The result obtained is given in seconds and represents the HCT value after a given curing time, under controlled temperature and humidity conditions.Figure 11
Illustration of the hilt cohesion test (HCT) principle used at BRRC.This HCT value would allow for evaluating the cohesion of the slurry surfacing at an early age as a function of curing. The longer the time, the more resistant the slurry surfacing is in bending and therefore the more advanced the build-up of cohesion.The HCT was applied to different formulations to establish whether the test was repeatable, consistent, and discriminating, and whether there was a correlation with the results obtained with the cohesion test according to EN 12274-4. Our experience with the HCT test was not very positive. The repeatability of the test was insufficient, and the results were difficult to interpret and inconsistent. At BRRC, it was concluded that this test is not a viable alternative to the EN 12274-4 method, as the latter is clearly better able to analyze and evaluate the cohesion of these slurry surfacings.


### 4.5. Wearing Resistance

After curing and reopening to traffic, the slurry surfacing has to resist wearing by traffic and water. Two test methods were evaluated in this paper to determine the wearing resistance of slurry surfacing:(i)
The European standard EN 12274-5 (see Section4.5.1) [26];(ii)
The technical specification “TS: Slurry surfacing-Test methods-Determination of cohesion of the mix” (see Section4.5.2).The latter, also known as Superficial Cohesion Test (SCT), describes an alternative method to EN 12274-5. Due to the difficulty of harmonizing procedures for these little practiced tests, the process of creating the technical specification was never completed and was abandoned. For this reason, this alternative method will not be discussed in detail in this paper in addition to the practical experience of BRRC, which has proven to be not very positive.


#### 4.5.1. Wearing Resistance According to EN 12274-5

(1) Objective of the Test. This test method measures the wearing resistance of slurry surfacing systems under wet abrasion conditions. The test is generally referred to as WTAT (Wet Track Abrasion Test) in the literature [33, 34].(2) Principle, Equipment, and Procedure. The European standard EN 12274-5 specifies a test method to assess:(i)
the wearing resistance of slurry surfacing after “favorable” curing at a temperature of (60 ± 5)°C;(ii)
the minimum amount of emulsion for different traffic classes (see Annex A of the standard).The slurry mixture is cast in a circular mold (internal diameter = (279.0 ± 0.5) cm) placed on a flat test base on which a roofing felt has been placed beforehand (Figure12). The whole assembly is then conditioned in an oven at 60°C, until a constant mass is obtained, and subsequently saturated with water by immersion in a water bath (at 25 ± 2°C) for a period of 60 minutes to 75 minutes. After conditioning, the standard specifies that the sample must be removed from the mold before testing.Figure 12
Illustration of the execution of the wearing resistance test at BRRC according to EN 12274-5.The test (Figure12) consists of an abrasion procedure, using a hard rubber hose, which is pressed on the surface of the slurry sample with a vertical force of (22.3 ± 2.0 N), while executing a planetary rotation at 61 cycles per minute for 5 minutes. The specimen is tested under water (at 25 ± 2°C) and the loss of material by wearing is determined by weighing. The standard specifies that the test shall be repeated on three samples and the result is calculated as the average of the three individual results.The lower the weight loss, the more cohesive and wear resistant the slurry surfacing will be. Conversely, the higher the weight loss, the less cohesive and wear resistant the slurry surfacing will be. A higher residual binder content will improve the wearing resistance, which is why this test is typically used to determine the minimum amount of emulsion needed.(3) Evaluation and Recommendations for Improving the Standard EN 12274-5. For the mixing procedure, it is again recommended to follow a more detailed mixing procedure (see Section 4.1). The preparation of the samples is a very critical step and is highly sensitive for the practical experience and skill from the operator. This preparation will condition the aspect of the surface of the samples (more or less textured), and it will undeniably impact the weight loss results and their dispersion.In fact, it is crucial to produce test samples with equal texture, representative of the texture of the slurry surfacing on the road. It is recommended that the so-called “sand-patch test” (in accordance with EN 13036-1, [49]) is carried out prior to the wearing test to verify the texture of the samples. It was shown that, when samples with outlying texture are discarded, repeatability generally improves.The quantities that are now given in the standard are far too high. This results in a waste of material and makes sample preparation more difficult (i.e., segregation, inhomogeneity). Preparing the exact amount of mixture significantly facilitates the mold filling and results in samples with a more homogeneous and repeatable texture.In the current standard, there is no maximum time restriction for the curing process of the samples, but this will probably influence the results. To work as consistent as possible, based on practical experience, a curing time of 3 days seems suitable and sufficiently long to obtain a constant mass.The use of a roofing felt as a support for the slurry surfacing is not necessary and unmolding the specimens prior to testing is an unnecessary manipulation, potentially leading to damage to the edges [15]. It is recommended to test the specimens still in the mold.Repeatability of the test is limited, especially for the less wear-resistant mixes (high amount of material loss). Repeatability is better for mixtures with very low texture. Despite the poor repeatability, the test method is capable of discriminating mixtures in a logical way (effect of the composition of the slurry surfacing, type of emulsion, composition, and nature of the aggregate). For example, Figure13 shows the effect of decreasing the emulsion dosage by 2% in a mix. As expected, the lower amount of residual binder has a considerable adverse impact on resistance to ravelling (Figure 13).Figure 13
Impact of emulsion content on material loss.Based on the ISSA guideline [13], a maximum average material loss of 540 g/m2 is required to obtain a mix with good wearing resistance. Based on test results on Belgian slurry mixtures, it can be concluded that this requirement is severe but nonetheless feasible.When the material loss exceeds this threshold, the mix design should be adapted, e.g., by increasing the amount of emulsion, by using a polymer modified emulsion, or by improving binder/aggregate adhesion.The above recommendations are also consistent with most of the findings in the technical bulletin TB136 [50] describing the causes of inconsistency of the WTAT results (good consistency, constant thickness, macrotexture of the samples, …).(4) Key Takeaways. If the test is performed according to BRRC’s recommendations (see Section 3), the most important findings are the following:(i)
This test carried out in the laboratory is useful for evaluating the wearing resistance of a slurry surfacing.(ii)
The test is rather time-consuming and laborious. At least three tests per mixture are needed to obtain a more accurate average result.(iii)
The preparation of samples with both a homogeneous and repeatable texture is a very critical step and is highly sensitive to the practical experience and skill from the operator.(iv)
To verify the texture of the three samples in the series to be tested for each formulation, the so-called “sand-patch test” (in accordance with EN 13036-1 [49]) is recommended. This allows to discard samples with outlying texture depth and improves the repeatability.(v)
Even if the repeatability is poor, the test is capable of discriminating mixtures in a logical way (effect of the composition of the slurry surfacing, type of emulsion, composition, and nature of the aggregate).(vi)
A maximum average material loss of 540 g/m2 seems appropriate to obtain a mix with good wearing resistance.Despite the abovementioned limitations and its poor repeatability, the test is evaluated at BRRC as a good test to verify the mix design for wearing resistance. A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-5.


#### 4.5.2. Superficial Cohesion Test (SCT)

The SCT or Superficial Cohesion Test (Figure14) is proposed by France [7, 37] as an alternative test for testing the cohesion of a slurry mixture (EN 12274-4, [25]). The SCT shows many similarities with the test according to EN 12274-5 [26]. It also measures the material loss of a circular specimen by abrasion under water. The major differences are:(i)
The load is transferred in the SCT by two rubber wheels on a shaft mounted on a Hobart mixer (instead of by a rubber roller in an apparatus specially designed for the test).(ii)
The SCT is performed on a freshly prepared slurry mixture, after a short conditioning period (30 minutes according to the standard) and this at room temperature (instead of on a mixture that has cured for several days at 60°C).(iii)
The duration of the wearing test is shorter than in EN 12274-5, more specifically 1 minute (instead of 5 minutes).Figure 14
Test setup for the SCT test at BRRC: (a) detail of the wheels and (b) complete equipment.
(a)(b)Since the slurry is tested shortly after preparation, the loss of material would be a measure of the cohesion of the slurry in the initial phase (when the slurry surfacing is opened to traffic). Practical experience gained at BRRC with the alternative test according to the technical specification has led to the decision to abandon this test:(i)
The equipment must be adjusted to withstand a higher vertical force, as the current force level is insufficient to cause significant material loss. This is not necessary, as EN 12274-5 [26] provides satisfactory results.(ii)
This method proved to be too time-consuming and unnecessary, as EN 12274-4 [25] better meets expectations.


### 4.6. Resistance to Bleeding and Rutting

“Bleeding” is the appearance of free bitumen on the surface of the pavement. This results in greasy, shiny areas with little or no macrotexture, which reduces the skid resistance of the road surface locally and significantly. This is one of the types of defects to be determined when applying the visual inspection standard EN 12274-8 [51]. The area corresponding to the wheel paths is particularly sensitive to bleeding, due to the sinking of aggregates into the bitumen.Vertical deformation in the wheel paths, called “rutting,” is another type of damage to be addressed. Because of the limited thickness of slurry surfaces, it is only relevant for multi-layer slurry surfacing where the total thickness is at least twice the maximum aggregate size of the all in mineral aggregate.At present, there are no laboratory tests in the European series of standards for slurry surfacing to determine sensitivity to bleeding and rutting. However, if further progress is to be made in improving the quality of slurry surfacing, consistent and appropriate tests seem necessary. The increased requirements for wear resistance and ravelling could lead to the design of slurries that are too rich in bitumen, resulting in bleeding, rutting, and poor skid resistance. More specifically for microsurfacing, it will be crucial to find an optimal compromise between wear and ravelling resistance (according to EN 12274-5 described in Section4.5.1) and bleeding and rutting resistance.Two methods for the respective determination of the sensitivity to bleeding and permanent deformation (rutting) of slurry surfacing have been published by ISSA [52, 53]. The test equipment, called “Loaded Wheel Tester,” shows some similarities with the wheel tracking test equipment known in Europe (EN 12697-22, [54]). The large size device of this standard is used in Belgium for testing hot mix asphalt (Figure 15(a)). The principle of the test is similar, but the test plates of the large size device are larger and a larger wheel with a 6-bar pressure tire is used.Figure 15
Large size device according to EN 12697-22 (a); sand patch test carried out before the application of the 1500 cycles (b); two slurry surfacing plates after the application of 1500 cycles (c).
(a)(b)(c)The standard test conditions, which are used in Belgium for testing hot asphalt mixes, are more severe than those of the two test methods from ISSA [52, 53] for testing slurry surfacing, but the large size device allows these test conditions to be changed. Given that the large size device is standard equipment in Belgium and Europe, it was decided to evaluate its potential for testing slurry surfacing.The method from technical bulletin TB109 [52] assesses bleeding by measuring the amount of hot sand that adheres to the surface. The sample is subjected to a fixed number of loaded wheel tracking cycles. A weighed quantity of warm sand is uniformly spread on the surface and further loaded wheel tracking is carried out. The amount of sand retained through embedment in the sample surface is determined. It gives an indication of the tendency of the mix “to flush” under traffic.Based on practical experience, measuring the texture in the wheel path is easier and better related to skid resistance. Hence, the simple “sand-patch test” (EN 13036-1, [49]) is used, before and after wheel tracking, to assess the resistance to bleeding. The literature [55] confirms that the results obtained with hot sand are very variable and not precise enough to suggest an optimum formulation.(1) Objective of the Test. The objective of the laboratory test proposed by BRRC is twofold: evaluate the sensitivity to bleeding and measure the sensitivity to rutting under simulated rolling traffic.(2) Principle, Equipment, and Internal Procedure. Slurry surfacing specimens are subjected to the repeated passing of a loaded pneumatic tire. The rutting and bleeding caused by this action show the sensitivity of the slurry surfacing to these types of defects. Figure 15(a) shows the large size device, in which two specimens are simultaneously tested.Based on literature [55], the ISSA test protocols [52, 53] and experience with the large size device (EN 12697-22, [54]), a procedure was established for the preparation of the slurry surfacing specimens and for assessing resistance to bleeding and rutting. Compared to the standard rutting test on hot mix asphalt, less severe test conditions are applied in terms of temperature (20°C) and number of cycles (100, 200, …, 600, 1000, 1500). Bleeding is assessed by measuring the surface texture depth, rutting by measuring the rut depth.The test specimens are prepared as follows. A metal bottom plate, covered with a roofing felt support, is placed into a metal mold of the large size device (internal length and width: 50 ∗ 18 cm2; height: 5 cm). The bottom plate thickness depends on the intended specimen thickness, as the sum of the bottom plate and the specimen thickness shall equal the height of the mold.The slurry mixture is prepared following the standard procedure of BRRC (see Section4.1) and immediately poured into the metal mold to a layer thickness corresponding to twice the maximum aggregate size. The slurry surfacing sample is then dried for 24 hours at room temperature, followed by 24 hours at 60°C. After conditioning, the initial profile and surface texture depth are assessed.The profile is assessed by measuring the depth at 15 points, using the standard procedure of EN 12697-22 [54]. The Mean Texture Depth (MTD) is determined using the “sand-patch test” on three locations over the length of the wheel path (Figure 15(b)).After removing the sand (using a dry brush or vacuum cleaner), the two filled molds are placed into the large size device at room temperature (20°C ± 2°C) and subjected to wheel tracking. When the 1500 cycles are completed (Figure15(c)), the rut profile and mean texture depth are again measured. The difference with the initial measurements characterizes the sensitivity to rutting and bleeding, respectively.Up to now, only three different slurry surfacing mixtures have been tested using this test procedure. Rut depths were for the three slurry surfacing mixtures in the range of 15 to 22%, and the mean profile depth decreased by 38 to 43%. The differences between the test results were not significant, given the standard deviations on the individual measurements of rut depths and texture depths. However, this was not surprising since the mix compositions of the variants were very similar: same mixture but with two different emulsions, both without polymer modification, and a third variant with a higher emulsion dosage (3% higher). Tests on more extreme variants (e.g. with and without polymer modification or a larger difference in emulsion dosage) are needed to demonstrate the discriminating power of the test method.(3) Key Takeaways. A new test procedure was developed to measure the resistance to bleeding and rutting of slurry surfacing, using the large size device of the European wheel tracking test (EN 12697-22, [54]) with modified conditions (20°C, less cycles) and combined with texture measurements (EN 13036-1, [49]). The most important findings with this “bleeding/rutting test” are the following:(i)
The feasibility of the test was demonstrated and the test conditions are appropriate to simulate realistic rut depths and texture variations.(ii)
Due to the small number of variants tested at this point, it was not possible to quantify the precision of the test. However, the differences in results between the two plates of the same variant are quite small, indicating good repeatability.(iii)
The three variants tested were probably not sufficiently different, regarding their sensitivity to bleeding and rutting, to demonstrate the discriminating power of the test. More test results need to be acquired on significantly different mixtures.


### 4.7. Interaction between Aggregate and Emulsion

A strong bond between aggregate and binder in the presence of water is considered as an important prerequisite for a well-performing slurry surfacing. The “Shaking Abrasion Test (SAT),” according to EN 12274-7 [27] is intended to evaluate, based on the resistance to abrasion of specimens in the presence of water, the suitability of a combination of aggregate and emulsion for slurry surfacing.(1) Objective of the Test. The European standard EN 12274-7 [27] provides a laboratory test method to determine the suitability/compatibility of the aggregate and cationic emulsion combination. It can also be used to evaluate the effect of individual additives.(2) Principle, Equipment, and Procedure. The European standard EN 12274-7 [27] provides a (performance-based) test method to determine the resistance to abrasion in the presence of water of specimens composed of an aggregate 0/2 mm with a defined gradation, water, cement, and a cationic bituminous emulsion.The exact quantities for one specimen are manually mixed until the mixture breaks. The broken mixture is transferred to a small cylindrical mold (Figure16). After preparing four mixtures with the same composition, the mixtures are compacted by placing the molds (Figure 17(a)) in a press with a load of 10 kN, applied at a speed of 20 mm/min. The final height of each specimen is (25 ± 1) mm and the diameter is approximately 30 mm.Figure 16
Broken mixture (a); filled mold with excess of water flowing out (b); set of four compacted samples (c).
(a)(b)(c)Figure 17
Components of the molds used for sample compaction (a); test equipment (b); (c) test specimens before (left) and after (right) execution of the shaking abrasion test according to EN 12274-7.
(a)(b)(c)In a next step, the test samples are soaked with water by placing them in water, at approximately 1°C (iced water) in a vacuum desiccator during approximately three hours at (3 ± 0.1) kPa. The measurement of the weight of the test specimens before and after this conditioning allows for the determination of the water absorption.Finally, the test samples are placed in tubes (Figure17(b)) filled with water and subjected to the shaking abrasion mechanical action by rotating the tubes (Figure 17(c)). The amount of material loss from the four samples after … rotations is measured. Unlike the previous tests (Sections 4.2–4.6), this is not a test on the complete slurry mixture and, therefore, it will not assess the impact of mineral aggregate grading or mix composition.(3) Evaluation and Recommendations for Improving the Standard EN 12274-7. At BRRC, following the compaction and before water saturation, test samples are dried overnight at room temperature allowing for the measurement of the apparent density according to EN 12697-6 method B [56]. This enables the calculation of the void content of the test samples. This additional parameter is interesting to understand and analyze the mass loss results. BRRC recommends to add a note in the standard specifying the possibility of determining the void contents of the samples.There is an inconsistency with the amount of water to be added when preparing mixtures. The standard specifies a water content of (40 ± 0.5) % of the mass of dry aggregate. Therefore, in case of 200 g dry aggregate, 80 g of water should be added. The latter quantity is in contradiction with the 40 g mentioned in the next line of the standard and constitutes in reality a too high percentage. BRRC recommends correcting the current water content and using a mass of 40 ± 2 g.Regarding the method of compacting mixtures when making samples, the protocol currently described in the standard will not provide homogeneous and representative samples (the fully broken mixture is granulated manually to obtain pieces with a diameter of less than approximately 5 mm and placed in the compaction mold). Based on practical experience, the production of the specimens shall immediately follow the mixing stage, without any drying and curing of the mixture.The specification of the temperature at which the saturation under vacuum should be carried out (at approximately 1°C (iced water) in a vacuum desiccator) is problematic. A temperature of 1°C is difficult to maintain during the whole soaking period. BRRC recommends to simply work at room temperature. The specification of a low temperature such as 1°C originates from the fact that this method was originally developed to assess the properties of a filler and is related to the volumetric stability of the specimens [57, 58]. However, in this application, volumetric stability is not a concern, which means that the low temperature is not needed.Considering the recommendations mentioned above, the test results obtained by BRRC (Figure18) on combinations of two types of aggregates (porphyry and sandstone) commonly used in Belgium with different types of cationic bituminous emulsion (polymer modified emulsion or not) are in general systematically low (≤2.5% weight loss). These results suggest a good suitability/compatibility between these aggregate types (0/2 mm fraction) and the tested emulsions.Figure 18
Impact of aggregate and emulsion type on material loss (emulsion N°2 is a polymer modified bituminous emulsion).The test performed according to the BRRC procedure also shows a good repeatability which is in line with the precision data reported in EN 12274-7 (Table3).Table 3
Precision data related to repeatability in EN 12274-7 [27].
Water absorption Wv (v-%)Mass loss (%)Range (difference min–max)0.70.6Standard deviationσr0.30.3Repeatabilityr-value0.80.9Based on these results, one can conclude that the test can discriminate between different aggregate/bituminous emulsions combinations. Porphyry aggregate showed a better compatibility with both unmodified as well as polymer modified cationic bituminous emulsions. However, no explanation could be provided for the higher mass losses in case of a polymer modified bituminous emulsion as compared with the unmodified emulsion.BRRC tests show consistency between void contents, water absorption, and mass losses of the tested aggregate/emulsion combinations (Table4):(i)
Test specimens characterized by a higher void content absorb a greater amount of water (Wv) during conditioning.(ii)
In general, samples with higher void content and higher water absorption have higher mass losses.(iii)
The absorbed water content depends on the nature of the aggregate.Table 4
Impact of aggregate and emulsion type on three parameters (void content, water absorption, and mass loss).
SampleVoid content (%)Water absorption Wv (v-%)Mass loss (%)Mix porphyry emulsion N°16.5 ± 0.14.2 ± 0.10.6 ± 0.3Mix porphyry emulsion N°27.6 ± 0.16.4 ± 0.41.9 ± 0.2Mix sandstone emulsion N°19.8 ± 0.37.7 ± 0.41.3 ± 0.2Mix sandstone emulsion N°210.6 ± 0.110.1 ± 0.22.5 ± 0.1At present, based on the test result obtained with EN 12274-7, there are no (performance) classes defined in the product standard for slurries EN 12273 [59]. Consequently, there are no requirements in any European country regarding the interaction between aggregate and emulsion. Consequently, the test is only used as a screening method, e.g., when changing the type of aggregate (practice in Germany).In contrast to EN 12273, the ISSA TB144 method [35] does provide a classification based on the results of the Schulze-Breuer and Ruck Procedures (grading rate: “A–E”). According to the latter, the samples are evaluated for abrasion, integrity, and adhesion. In the EN 12274-7 procedure, the specimens are only evaluated for abrasion in terms of mass loss (Table 5).Table 5
Overview SAT classification in mass loss according to ISSA TB144 method and the conversion in accordance with the normative method EN 12774-7.
Grade ratingMass loss (%)ISSA TB144EN 12274-7A0–1.70–1.0B1.7–2.41.0–1.5C2.4–3.11.5–1.9D3.1–4.81.9–2.9E>4.8>2.9The comparison with the test procedure described in ISSA TB144 [35] (for the abrasion procedure) indicated that the latter procedure differs significantly from the method in EN 12274-7 in terms of specimen preparation (dry compaction) and conditioning (no saturation specimens). The ISSA TB114 procedure is more discriminating (higher material losses) and this is mainly due to the very long (and therefore more severe) conditioning of the specimens in water (6 days).Based on the comparative tests in which both the ISSA TB144 method [35] and the BRRC procedure based on EN 12274-7 were performed on two types of aggregates (porphyry and sandstone), a conversion factor can be calculated from the ratio of the respective mass losses. For this purpose, an average value of 1.65 can be determined.This factor then allows an estimation (dividing by 1.65) of the values for a classification “A–E” based on the test result obtained by the SAT according to EN 12274-7 (Table5). Applying the above “grade rating” according to ISSA TB144, porphyry gets the “grade rating” A and sandstone “grade rating” B.The above values (Table5) for the grade rating can be checked against the requirements included in Refs. [13, 60] where a “grade rating” A or B (for the abrasion loss) is always required in the context of the mixture design. Introducing this classification for the mass loss (“A–E”) in the standard EN 12274-7 could be very useful to evaluate the materials used (emulsion, aggregate 0/2 mm with a defined gradation and with or without cement) and their compatibility.(4) Key Takeaways. A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-7 [27]. If the test is performed according to these BRRC propositions (see Section 3), the most important findings are the following:(i)
The Shaking Abrasion Test (SAT) carried out in laboratory is useful and discriminating for determining the suitability/compatibility of the aggregate/emulsion combination.(ii)
On the other hand, this laboratory test is rather time-consuming and laborious. It has the disadvantage of not being able to carry out the preparation of the samples and the execution of the tests in a working day.(iii)
BRRC tests show consistency between void contents, water absorption, and mass losses of the tested aggregate/emulsion combinations.(iv)
The Shaking Abrasion Test (SAT) performed according to the BRRC procedure shows good repeatability.(v)
The test requires a skilled operator to prepare high-quality samples and perform the test in a repeatable way.Based on the above findings, the SAT can be used during the mix design phase of the basic composition of a slurry mixture to determine a suitable aggregate-emulsion combination. In this context, the test is primarily suitable to evaluate the choice/change of aggregate type.


## 4.1. Preparation of Slurry Surfacing Mixtures under Laboratory Conditions

The mixing procedure of slurry mixtures has an undeniable impact on coating of the aggregates with the emulsion, homogenization of the complete mixture, breaking behavior, and the development of cohesion [12, 37, 38, 42]. For example, a too short mixing time will not result in a homogeneous mixture and a too intensive stirring, or the use of a metal spatula, could cause damage to the emulsion, such as destabilization.Therefore, the laboratory mixing procedure should be repeatable and mimic the mixing process on-site as closely as possible. When comparing different test methods of the EN 12274 standards, it appears that the mixing procedure description is not always the same and not sufficiently detailed or accurate. This may lead to different interpretations and poor reproducibility.At BRRC, a detailed protocol was developed which describes exactly the manual mixing sequence (dry aggregate, cement, pre-wetting water, additives, and emulsion), the times to mix the components (after adding the emulsion: between 30 s and 60 s, depending on the sample size), and the mixing speed (60 rpm).Temperature of constituents and environment should ideally be equal to the temperature on-site when the mixture is placed. Unfortunately, the actual temperature on-site cannot be predicted and, therefore, the average temperature of the season shall be selected as the best approximation (with a tolerance of ±5°C).This procedure is suitable for obtaining a homogeneous and stable mixture for the formulations tested at BRRC. This protocol is recommended to be followed in every test on laboratory-prepared slurry surfacing mixtures.


## 4.2. Mix Time

On the worksite, mixing and spreading of a slurry surfacing takes approximately 60 seconds. The mixture shall remain mixable in that period; otherwise, there is a risk of lump creation or obstruction of the mix in the mobile machinery. The mix time is defined as the time between the start of mixing (immediately after adding the emulsion to the mixture) and the moment the mixture starts to stiffen (onset of breaking of the mixture) [12, 37, 38].Breaking [5, 43] refers to all the phenomena leading to the formation of a continuous bituminous phase (agglomeration, flocculation, and coalescence). The individual bitumen droplets are then engaged in an irreversible process leading to the separation of bitumen and water. In practice, the end of breaking is marked by the change from a brown to a black color as the bitumen is transformed into a continuous film.After the emulsion and aggregates are mixed, the breaking process begins. The breaking speed depends on the wind speed, the ambient temperature, the temperature of the components, and the surface temperature of the underlayer. The higher these temperatures and the higher the wind speed during the execution, the faster the breaking process and the curing of the slurry surfacing will be.There is no standard European test, but the mix time can be evaluated in the laboratory (at controlled temperature) by following the manual procedure described in the technical bulletin TB113 from the International Slurry Surfacing Association [28].


### 4.2.1. Objective of the Test

The main objective of this test [28] is to measure in the laboratory the mix time of a specific combination of materials of slurry surfacing mixes. The procedure in TB113 also allows for the evaluation of the set time of the slurry surfacing. The set time is defined as the time between the start of mixing (immediately after adding the emulsion to the mixture) and the moment where it is no longer possible to stir the mixture.


### 4.2.2. Principle, Equipment, and Procedure

The ISSA TB113 procedure [28] is composed of two parts. The first part will allow the evaluation of the set time of the slurry surfacing and the second part will evaluate the mix time.The different components are manually stirred in disposable cups at a constant speed. The procedure [23] includes a detailed mixing protocol:(i)
mixing sequence: dry aggregate, mineral filler/dry additive, water/liquid additive, emulsion;(ii)
mixing times: 10 s to 30 s, according to the added component or until its distribution is uniform;(iii)
mixing speed: 60–70 rpm;(iv)
temperature of constituents (room temperature and temperatures likely to be encountered in the field).For the first part of the method, when the emulsion is added and mixed with the other components, half of the mixture is poured on a support (paper, aluminum foil, or roofing felt) at a thickness between 6.4 and 10 mm. With the help of a paper towel, the operator will press and evaluate the time until this paper is no longer stained by emulsion but only absorbs clear water (set time).For the second part of the method, the operator continues mixing the remaining portion for a maximum of 5 minutes or until the mixture stiffens and breaks. The time required for the mix to stiffen and break is recorded in seconds (mix time). A visual examination of the mixture throughout the procedure should also be realized (fines flotation, free liquids present, dryness, stiffness, etc.). In the function of these observations, adjustments of the combination of the components may be necessary to obtain a workable and sufficiently consistent mixture.


### 4.2.3. Evaluation and Recommendations

At BRRC, the first part of the ISSA TB113 method [28] is not applied. The “finger and paper pressing techniques” are subjective and highly operator dependent. Moreover, the set time is supposed to be indicative of the time at which the road can be reopened to traffic, but the test performed according to EN 12274-4 [25] is clearly more suitable for that purpose (see Section 4.4).The second part of the method [28], which determines the mix time, is more interesting. The test is simple and fast but also operator dependent (subjective character) [6, 44, 45]. Therefore, the test is preferably carried out by a single experienced operator. The precision of the test was not determined at BRRC. It is expected that reproducibility is the main issue, because of this operator dependency.At BRRC, a smaller quantity of mineral aggregate is used (50 g instead of the 100 to 400 g foreseen by Ref. [28]) to economize materials and to ensure a uniform distribution of the different constituents during manual mixing. The quantities of the other components (water, emulsion, cement, …) are of course adapted to this reduced quantity in order to keep the same percentages in mass compared to the dry aggregates.The test can discriminate in a logical way between mixtures with variations in composition. As expected, the characteristics of the emulsion and cement have a strong impact on the mix time.The huge impact of test temperature on the mix time was also observed. Therefore, it is recommended to perform the test at temperatures likely to be encountered in the field.The results obtained in the BeP2S project show that a mix time of at least 120 s (for microsurfacing) or 180 s (for slurry sealing), as recommended by Refs. [13, 14], seems adequate for slurry surfacing.


### 4.2.4. Key Takeaways

The most important findings with the manual mixing test (second part of the method according to Ref. [28]) are the following:(i)
The test is fast, simple, and requires no sophisticated equipment;(ii)
The results of this test are subjective and operator dependent;(iii)
The test can logically discriminate mixtures with different compositions;(iv)
A mix time of at least 120 s or 180 s may be considered as a target;Despite the limitations, BRRC has evaluated this manual mixing test (second part of the method according to Ref. [28]) as a valuable method to verify the mix time. The introduction of this test as a new standard test method in the EN 12274 series would be useful.


## 4.2.1. Objective of the Test

The main objective of this test [28] is to measure in the laboratory the mix time of a specific combination of materials of slurry surfacing mixes. The procedure in TB113 also allows for the evaluation of the set time of the slurry surfacing. The set time is defined as the time between the start of mixing (immediately after adding the emulsion to the mixture) and the moment where it is no longer possible to stir the mixture.


## 4.2.2. Principle, Equipment, and Procedure

The ISSA TB113 procedure [28] is composed of two parts. The first part will allow the evaluation of the set time of the slurry surfacing and the second part will evaluate the mix time.The different components are manually stirred in disposable cups at a constant speed. The procedure [23] includes a detailed mixing protocol:(i)
mixing sequence: dry aggregate, mineral filler/dry additive, water/liquid additive, emulsion;(ii)
mixing times: 10 s to 30 s, according to the added component or until its distribution is uniform;(iii)
mixing speed: 60–70 rpm;(iv)
temperature of constituents (room temperature and temperatures likely to be encountered in the field).For the first part of the method, when the emulsion is added and mixed with the other components, half of the mixture is poured on a support (paper, aluminum foil, or roofing felt) at a thickness between 6.4 and 10 mm. With the help of a paper towel, the operator will press and evaluate the time until this paper is no longer stained by emulsion but only absorbs clear water (set time).For the second part of the method, the operator continues mixing the remaining portion for a maximum of 5 minutes or until the mixture stiffens and breaks. The time required for the mix to stiffen and break is recorded in seconds (mix time). A visual examination of the mixture throughout the procedure should also be realized (fines flotation, free liquids present, dryness, stiffness, etc.). In the function of these observations, adjustments of the combination of the components may be necessary to obtain a workable and sufficiently consistent mixture.


## 4.2.3. Evaluation and Recommendations

At BRRC, the first part of the ISSA TB113 method [28] is not applied. The “finger and paper pressing techniques” are subjective and highly operator dependent. Moreover, the set time is supposed to be indicative of the time at which the road can be reopened to traffic, but the test performed according to EN 12274-4 [25] is clearly more suitable for that purpose (see Section 4.4).The second part of the method [28], which determines the mix time, is more interesting. The test is simple and fast but also operator dependent (subjective character) [6, 44, 45]. Therefore, the test is preferably carried out by a single experienced operator. The precision of the test was not determined at BRRC. It is expected that reproducibility is the main issue, because of this operator dependency.At BRRC, a smaller quantity of mineral aggregate is used (50 g instead of the 100 to 400 g foreseen by Ref. [28]) to economize materials and to ensure a uniform distribution of the different constituents during manual mixing. The quantities of the other components (water, emulsion, cement, …) are of course adapted to this reduced quantity in order to keep the same percentages in mass compared to the dry aggregates.The test can discriminate in a logical way between mixtures with variations in composition. As expected, the characteristics of the emulsion and cement have a strong impact on the mix time.The huge impact of test temperature on the mix time was also observed. Therefore, it is recommended to perform the test at temperatures likely to be encountered in the field.The results obtained in the BeP2S project show that a mix time of at least 120 s (for microsurfacing) or 180 s (for slurry sealing), as recommended by Refs. [13, 14], seems adequate for slurry surfacing.


## 4.2.4. Key Takeaways

The most important findings with the manual mixing test (second part of the method according to Ref. [28]) are the following:(i)
The test is fast, simple, and requires no sophisticated equipment;(ii)
The results of this test are subjective and operator dependent;(iii)
The test can logically discriminate mixtures with different compositions;(iv)
A mix time of at least 120 s or 180 s may be considered as a target;Despite the limitations, BRRC has evaluated this manual mixing test (second part of the method according to Ref. [28]) as a valuable method to verify the mix time. The introduction of this test as a new standard test method in the EN 12274 series would be useful.


## 4.3. Consistency

An essential parameter governing the design, manufacture, and spread of slurry surfacing (i.e., slurry seals or microsurfacing) is the consistency of the mix [37, 38, 45–47].(i)
If the consistency is too high (mix too “stiff,” Figure1(a)): the mix will be less workable, and it will a priori present less good adhesion to the substrate.(ii)
If the consistency is too low (mix too “liquid,” Figure1(b)): the risks of segregation and outflow phenomena will be greater.Figure 1
Two types of slurry surfacing consistency (a) high (small diametrical) and (b) low (large diametrical outflow).
(a)(b)To achieve the correct consistency, the proportions of the various constituents (pre-wetting water, emulsion, cement, additives) in relation to the dry aggregates must be optimized. The European standard EN 12274-3 [24] specifies a test method for determining the consistency of slurry surfacing. In the 2018 version of the standard EN 12274-3, the scope is restricted to slurry seals with maximum aggregate size (D) ≤ 4 mm (it was not the case in the previous version of the standard).


### 4.3.1. Objective of the Test

The objective of this test is to determine in the laboratory the mix quantities (aggregate, emulsion, pre-wetting water, and additives if required) for optimal workability and flow of a slurry mixture.


### 4.3.2. Principle, Equipment, and Procedure

The consistency test is illustrated in Figure2 and is carried out at room temperature (23 ± 5)°C. A cone is placed on a metal plate engraved with a scale of eight concentric circles (numbered from 0 to 7 from the inside out), which has been previously covered with a transparent nonabsorbent paper. Immediately after mixing the different components, the cone is filled with the slurry mixture and removed in a regular vertical movement, allowing the mix to spread.After (10 ± 2) seconds, the diametrical outflow is measured on the scale (at four points 90° apart) and the average of these four readings, expressed in millimeters (mm), is a measure of the consistency of the mixture.The lower the outflow, the higher the consistency of the slurry surfacing. Conversely, the greater the outflow, the lower the consistency of the slurry surfacing (Figure1).Figure 2
Illustration of the execution of the consistency test according to EN 12274-3.


### 4.3.3. Evaluation and Recommendations for Improving the Standard EN 12274-3

The consistency test according to the standard EN 12274-3 is a simple and fast test. The mixing procedure is not well described in the standard, although it is of crucial importance for the test result. It is recommended to follow a more detailed mixing procedure, as described in Section4.1.Based on practical experience, BRRC recommends to use a funnel to adequately (cleanly and correctly) transfer the slurry mixture from the container into the cone (see Figure2). When transferring the slurry mixture, segregation and incorporation of air bubbles shall be minimized by pouring smoothly and carefully. Removal of the cone also has an impact on the diametrical outflow, especially for more fluid/liquid mixtures. The slightest deviation from the verticality of the movement may result in a noncircular spread and therefore a greater standard deviation over the four measurements at 90°. BRRC recommends that the standard procedure insists on the vertical removal without tilting the cone.Contrary to the current scope of the standard EN 12274-3, the tests carried out at BRRC (on slurry surfacing mixes with grading 0/6.3 mm) demonstrate the interest and feasibility of analyzing the consistency of slurry surfacing with a maximum aggregate size larger than 4 mm. The number of repetitions is not specified in the standard. The results obtained in the BeP2S project show that at least three tests per mixture are needed to obtain coherent and reliable results.Logical results are obtained with respect to differences in the operating conditions and the composition. For example, Figure3 shows how the percentage of cement increases the consistency of this formulation tested at BRRC. Note that, depending on the emulsion and cement type, cement can also have a different impact on the slurry mixture and decrease its consistency. Therefore, the test is considered as discriminating and useful.Figure 3
Outflow versus cement percentage content in the mix formulation (bars denote standard deviation of 3 repeated tests; in case of the mixture with 2.3%, the 3 repeated tests gave the same result).Temperature has a considerable influence on the consistency and, therefore, the test temperature should be representative of the temperature range expected in the field and not limited to the temperature range currently specified in the standard ((23 ± 5)°C). The test temperature shall be accurately controlled and reported. BRRC recommends performing the consistency test at two or three different temperatures covering the expected temperature range in the field, since this allows for evaluating the sensitivity to temperature fluctuations.For the interpretation of the diametrical outflow, it would be easier and more logical to count the circles from 0 to 7 from the outside to the inside (Figure4(b)). By reversing the direction of this scale, a high consistency would be characterized by a high value and a low consistency would be characterized by a low value.For ease of reading (especially for “asymmetric” outflows), it would also be more practical if the numbers were engraved at 0°, 90°, 180°, and 270°. Figures4(a) and 4(b) show the current situation in the standard and the BRRC proposal.Figure 4
Current situation in the standard (a) and BRRC proposal for a new metal plate engraved with a scale of eight concentric circles, each increasing in radius by 10 mm and the inner circle has a diameter of 90 mm (b).
(a)(b)It was found that some mixtures may have an adequate quantitative outflow between 2.0 and 3.0 cm (as recommended by the ISSA [14]), while presenting an unusual appearance (asymmetric outflow, lumps, emulsion drainage, or uncoated aggregates…), which would indicate that the mix is not well designed. Therefore, in addition to the quantitative determination of outflow, the quality of the mixture during mixing and the quality of the mixture after flowing shall be observed and reported. BRRC recommends adding Table 2 to the standard EN 12274-3, for the qualitative assessment of the test.Table 2
Consistency description∗.
LetterStateObservationSSoupyBrown free liquid, segregating sampleLVLow viscosityNon-segregating easy to mixMVModerate viscosityNon-segregating, moderate resistance to mixStStiffHard to mix but workableBBrokenLumps, non-consistent∗ Table2 adapted from Ref. [15].The repeatability limit of the outflow at BRRC was estimated to be 0.6 cm for the average of three repeated tests, determined in the range of 2.0 to 3.0 cm. Outside this range, the repeatability will be less due to the phenomena of drainage and inhomogeneous diametrical outflow, which occur more frequently outside this range of “optimal” consistency.An outflow of 2.0 to 3.0 cm, as recommended by the ISSA [14], may be considered as a suitable target, but not as a necessary requirement, given the precision of the test. The experience and knowledge of the operator also affect the repeatability of the test. Indeed, as explained above, the speed and time of mixing and the removal of the cone are three test conditions that must remain as constant as possible.


### 4.3.4. Key Takeaways

If the test is performed according to BRRC’s recommendations (see Section3), the most important findings are the following:(i)
This laboratory test is simple, fast, and sufficiently discriminating. Therefore, it is useful for evaluating the consistency of a slurry surfacing and its sensitivity to temperature variations, prior to its implementation on-site.(ii)
At least three tests per mixture are needed to produce consistent average results.(iii)
An outflow of 2.0 to 3.0 cm, as recommended by (ISSA) [14], seems adequate. A visual inspection of the quality of the mixture (during mixing and after flowing) is also recommended to have a complete diagnosis of the consistency.(iv)
The test is also suitable for mixtures with maximum aggregate size (D) > 4 mm, despite the restriction added in the scope of the 2018 version of the standard EN 12274-3.(v)
The experience and knowledge of the operator affects the repeatability of the test. At BRRC, a repeatability limit of 0.6 cm was found (average of three tests) for values in the range of 2.0 to 3.0 cm.(vi)
BRRC has evaluated the consistency test as a valuable method to adapt mix design and to optimize the proportions of the various constituents in relation to the dry aggregates (water, emulsion, cement, additives) with a view to adequate consistency (i.e., a homogeneous and workable mixture).A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-3.


## 4.3.1. Objective of the Test

The objective of this test is to determine in the laboratory the mix quantities (aggregate, emulsion, pre-wetting water, and additives if required) for optimal workability and flow of a slurry mixture.


## 4.3.2. Principle, Equipment, and Procedure

The consistency test is illustrated in Figure2 and is carried out at room temperature (23 ± 5)°C. A cone is placed on a metal plate engraved with a scale of eight concentric circles (numbered from 0 to 7 from the inside out), which has been previously covered with a transparent nonabsorbent paper. Immediately after mixing the different components, the cone is filled with the slurry mixture and removed in a regular vertical movement, allowing the mix to spread.After (10 ± 2) seconds, the diametrical outflow is measured on the scale (at four points 90° apart) and the average of these four readings, expressed in millimeters (mm), is a measure of the consistency of the mixture.The lower the outflow, the higher the consistency of the slurry surfacing. Conversely, the greater the outflow, the lower the consistency of the slurry surfacing (Figure1).Figure 2
Illustration of the execution of the consistency test according to EN 12274-3.


## 4.3.3. Evaluation and Recommendations for Improving the Standard EN 12274-3

The consistency test according to the standard EN 12274-3 is a simple and fast test. The mixing procedure is not well described in the standard, although it is of crucial importance for the test result. It is recommended to follow a more detailed mixing procedure, as described in Section4.1.Based on practical experience, BRRC recommends to use a funnel to adequately (cleanly and correctly) transfer the slurry mixture from the container into the cone (see Figure2). When transferring the slurry mixture, segregation and incorporation of air bubbles shall be minimized by pouring smoothly and carefully. Removal of the cone also has an impact on the diametrical outflow, especially for more fluid/liquid mixtures. The slightest deviation from the verticality of the movement may result in a noncircular spread and therefore a greater standard deviation over the four measurements at 90°. BRRC recommends that the standard procedure insists on the vertical removal without tilting the cone.Contrary to the current scope of the standard EN 12274-3, the tests carried out at BRRC (on slurry surfacing mixes with grading 0/6.3 mm) demonstrate the interest and feasibility of analyzing the consistency of slurry surfacing with a maximum aggregate size larger than 4 mm. The number of repetitions is not specified in the standard. The results obtained in the BeP2S project show that at least three tests per mixture are needed to obtain coherent and reliable results.Logical results are obtained with respect to differences in the operating conditions and the composition. For example, Figure3 shows how the percentage of cement increases the consistency of this formulation tested at BRRC. Note that, depending on the emulsion and cement type, cement can also have a different impact on the slurry mixture and decrease its consistency. Therefore, the test is considered as discriminating and useful.Figure 3
Outflow versus cement percentage content in the mix formulation (bars denote standard deviation of 3 repeated tests; in case of the mixture with 2.3%, the 3 repeated tests gave the same result).Temperature has a considerable influence on the consistency and, therefore, the test temperature should be representative of the temperature range expected in the field and not limited to the temperature range currently specified in the standard ((23 ± 5)°C). The test temperature shall be accurately controlled and reported. BRRC recommends performing the consistency test at two or three different temperatures covering the expected temperature range in the field, since this allows for evaluating the sensitivity to temperature fluctuations.For the interpretation of the diametrical outflow, it would be easier and more logical to count the circles from 0 to 7 from the outside to the inside (Figure4(b)). By reversing the direction of this scale, a high consistency would be characterized by a high value and a low consistency would be characterized by a low value.For ease of reading (especially for “asymmetric” outflows), it would also be more practical if the numbers were engraved at 0°, 90°, 180°, and 270°. Figures4(a) and 4(b) show the current situation in the standard and the BRRC proposal.Figure 4
Current situation in the standard (a) and BRRC proposal for a new metal plate engraved with a scale of eight concentric circles, each increasing in radius by 10 mm and the inner circle has a diameter of 90 mm (b).
(a)(b)It was found that some mixtures may have an adequate quantitative outflow between 2.0 and 3.0 cm (as recommended by the ISSA [14]), while presenting an unusual appearance (asymmetric outflow, lumps, emulsion drainage, or uncoated aggregates…), which would indicate that the mix is not well designed. Therefore, in addition to the quantitative determination of outflow, the quality of the mixture during mixing and the quality of the mixture after flowing shall be observed and reported. BRRC recommends adding Table 2 to the standard EN 12274-3, for the qualitative assessment of the test.Table 2
Consistency description∗.
LetterStateObservationSSoupyBrown free liquid, segregating sampleLVLow viscosityNon-segregating easy to mixMVModerate viscosityNon-segregating, moderate resistance to mixStStiffHard to mix but workableBBrokenLumps, non-consistent∗ Table2 adapted from Ref. [15].The repeatability limit of the outflow at BRRC was estimated to be 0.6 cm for the average of three repeated tests, determined in the range of 2.0 to 3.0 cm. Outside this range, the repeatability will be less due to the phenomena of drainage and inhomogeneous diametrical outflow, which occur more frequently outside this range of “optimal” consistency.An outflow of 2.0 to 3.0 cm, as recommended by the ISSA [14], may be considered as a suitable target, but not as a necessary requirement, given the precision of the test. The experience and knowledge of the operator also affect the repeatability of the test. Indeed, as explained above, the speed and time of mixing and the removal of the cone are three test conditions that must remain as constant as possible.


## 4.3.4. Key Takeaways

If the test is performed according to BRRC’s recommendations (see Section3), the most important findings are the following:(i)
This laboratory test is simple, fast, and sufficiently discriminating. Therefore, it is useful for evaluating the consistency of a slurry surfacing and its sensitivity to temperature variations, prior to its implementation on-site.(ii)
At least three tests per mixture are needed to produce consistent average results.(iii)
An outflow of 2.0 to 3.0 cm, as recommended by (ISSA) [14], seems adequate. A visual inspection of the quality of the mixture (during mixing and after flowing) is also recommended to have a complete diagnosis of the consistency.(iv)
The test is also suitable for mixtures with maximum aggregate size (D) > 4 mm, despite the restriction added in the scope of the 2018 version of the standard EN 12274-3.(v)
The experience and knowledge of the operator affects the repeatability of the test. At BRRC, a repeatability limit of 0.6 cm was found (average of three tests) for values in the range of 2.0 to 3.0 cm.(vi)
BRRC has evaluated the consistency test as a valuable method to adapt mix design and to optimize the proportions of the various constituents in relation to the dry aggregates (water, emulsion, cement, additives) with a view to adequate consistency (i.e., a homogeneous and workable mixture).A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-3.


## 4.4. Cohesion

After its application on the pavement, the slurry surfacing must acquire a rapid cohesion build-up to obtain the shortest possible time for quick reopening of the road (“traffic time,” period of time after laying, when the slurry surfacing can accept traffic) without losing the workability necessary for its application [12, 37, 38, 42].The kinetics of “cohesion build-up” and the final cohesion (allowing the reopening to traffic without degradation of the slurry surfacing) are therefore essential parameters governing the mix design, manufacture, and implementation. Contractors often aim for the shortest possible time of opening to traffic to limit as much as possible the inconvenience caused to road users. This is usually between 30 and 60 minutes.In the framework of the research project called BeP2S, two test methods were investigated to determine the early cohesion of slurry surfacing:(i)
The European standard EN 12274-4 [25] (see Section 4.4.1) specifies a test method which is similar to a torsion test, and which can be used as an aid to the design of slurry surfacing in order to evaluate the “cohesion build-up time,” i.e., the “set time” and the “curing time”/“traffic time” through a minimum cohesive strength to be reached.(ii)
The technical specification “TS: Slurry surfacing-Test methods-Determination of cohesion of the mix” (see Section4.4.2) describes an alternative method to EN 12274-4, called Hilt Cohesion Test (HCT). Practical experience with this test at BRRC was not positive. Moreover, due to the difficulty of harmonizing procedures for this little practiced test, the process of improving the technical specification in the CEN committee was abandoned. For this reason, this alternative method will not be discussed in this paper.


### 4.4.1. Cohesion According to EN 12274-4

(1) Objective of the Test. This test method is used to measure the development of cohesion of a slurry surfacing system with time. This test also quantifies the time required before a slurry surfacing can be subjected to straight rolling traffic (“traffic time”).(2) Principle, Equipment, and Procedure. The slurry mixture is cast in a square-shaped mold having four circular holes, under which a roofing felt has been placed beforehand (Figure 5). The whole assembly is then conditioned at a temperature of (23 ± 5)°C. The standard specifies that the samples must be removed from the mold before testing. At BRRC, the roofing felt support consists of 4 separate parts, so that each sample can be tested individually on the roofing felt support.Figure 5
Mold used at BRRC to prepare the four circular samples for the cohesion test (and the 4 squares of roofing felt support to immobilize the samples).The test (Figure6) is carried out at the same temperature as conditioning (23 ± 5)°C. Its principle is to apply a rubber stamp (diameter of 28.5 ± 0.1 mm and height of 6.3 ± 1.0 mm) on a circular sample (diameter of 60 mm, thickness depends on the maximum aggregate size of the mix) of slurry surfacing (immobilized on a roofing felt support, Figure 5) with a constant pressure (200 ± 4 kPa). Torsion is applied manually to the slurry surfacing through the rubber stamp and the maximum torque is measured using a torque meter. The test is performed on the four different samples at a fixed time or at different suitable intervals between 5 min and 120 min after the samples have been spread in the mold (Figure 5), to evaluate the rise in cohesion as a function of time.Figure 6
Illustration of the execution of the cohesion test at BRRC according to EN 12274-4.The result of the measurement (maximum torque), expressed in Nm, represents the cohesion of the mixture at the four curing times mentioned above.After the test has been executed, a visual inspection of the condition of the samples is performed. Four types of “degradation” are referenced in EN 12274-4 and illustrated in Figure7.Figure 7
The four types of “degradation” indicated in the EN 12274-4 standard and examples for visual assessment of specimens. (a) N, normal; the sample is slightly damaged by some loss of aggregates. (b) S, solid; only the film of bitumen has been removed from the surface of the sample. (c) C, cracked. (d) D, disintegrated.
(a)(b)(c)(d)The lower the numerical value of the maximum torque, the less cohesive the slurry surfacing will be. Conversely, the higher the numerical value of the cohesive torque, the more cohesive the slurry surfacing will be (Figure8).Figure 8
Illustration of a slurry surfacing with low (a) and high (b) cohesion after performing the cohesion test according to EN 12274-4.
(a)(b)(3) Evaluation and Recommendations for Improving the Standard EN 12274-4. The normative cohesion test carried out in laboratory according to the standard EN 12274-4 is useful, relatively simple, and fast for evaluating the progressive cohesion build-up of a slurry surfacing. For the mixing procedure, it is recommended to follow a more detailed mixing procedure (see Section 4.1).Preparing (and demolding) the samples requires good practice and skill from the operator. The specified amount of mineral mixture (aggregates and filler) in the norm is too high and consequently, the excess needs to be removed to obtain a surface at the same level of the mold. This requires a lot of manipulation of the mixture, which is not practical and can cause segregation. Preparing the exact amount of mixture significantly facilitates the mold filling and results in a more homogeneous and repeatable texture. For a mixture with aggregate grading 0/6.3 mm, a quantity of 200 g of dry mineral mixture (instead of the 400 g specified in the standard) was found to be sufficient. This is a recommendation to improve the test method.In practice, the conditioning temperature of the materials and the mixing temperature have an impact on the cohesion build-up, as the kinetics of breaking and curing are temperature dependent. In general, the rise in cohesion will be faster at higher temperatures. Therefore, it is recommended that the materials shall be conditioned and the slurry surfacing mixed in the laboratory in a temperature range close to the one expected during the use of the slurry surfacing on-site.The roofing felt mentioned in the standard is hard to find because of the specification of the weight per unit area. It would be appropriate to remove the weight currently indicated in the standard ((700 ± 100) g/m2). The most important requirement is to use a roofing felt with sufficient strength to act as a sample holder and the exact weight would not affect the results. At BRRC, a roofing felt with a weight per unit area of approximately 394 g/m2 is used, which is appropriate to play the role of the sample holder.The precision of the cohesion test depends on the consistency of the slurry surfacing tested, since the latter will determine the difficulty for an even and homogeneous filling of the metal mold rings. The objective is to obtain four similar samples with an even and homogeneous texture.The textural differences will inevitably be reflected in the measured torque values and will therefore have an impact on the precision of the test:(i)
A formulation with a “too low consistency” (large diametrical outflow) will be more susceptible to phase segregation. As a result, samples from the first filled rings will be smoother on the surface (fines and bitumen migrate to the surface) compared to the following ones which will have a more granular appearance (the rest of the container contains more coarse aggregates which settle downwards in the mix).(ii)
A formulation with a “too high consistency” (small diametrical outflow) will be more complex to manipulate due to its lower workability, and the final texture of the four samples may not be similar and homogeneous.The test times are not specified by the standard, but BRRC generally performed the test after 15, 30, 60, and 90 minutes. In later tests, 24 hours were used (instead of 15 minutes) to obtain an indication of the final cohesion of the mix. The ISSA [13, 14, 48] specifies values of at least 1.2 Nm (after 30 minutes) and 2.0 Nm (after 60 minutes). Based on practical experience with Belgian slurry mixtures, these limits seem to be adequate and feasible for Belgian practice. The visual examination of the degradation must also be considered (Figure 7).The minimum number of repetitions of the test is not specified in the standard (it is only integrated in a note). The repeatability at BRRC was estimated to be 0.7 Nm for a single test and 0.4 Nm for the average of three tests. Therefore, the cohesion test is systematically performed at BRRC in triplicate and the result is calculated as the average.The test is then quite repeatable even though it is almost entirely manual and very dependent on the manufacturing quality of the samples. The experience and skill of the operator will thus greatly affect the repeatability of the test.The cohesion test is discriminating since the different formulations tested generally show cohesion build-up, final cohesion, and degradation states consistent with their formulation. For example, Figure9 shows test results on 2 mixtures with the same composition, but with a different emulsion. The breaking behavior of emulsion 2 in this mixture is different, which results in slower cohesion build-up and a higher risk of early ravelling on the road.Figure 9
Cohesion versus time, for mixtures with two different emulsions (bars denote standard deviation on 3 repeated tests).However, the interpretation of the results can be quite difficult: sometimes the torque is high, but there is a lot of damage, and sometimes the torque is low and there is no visible damage on the sample. This seems to depend greatly on the surface texture of the slurry surfacing and the way the rubber stamp adheres to the surface. The cohesion values obtained (in Nm) should therefore be interpreted with caution and the deterioration of the specimen (i.e., the degradation observed by visual inspection, Figure7) should be considered. Figure 10 illustrates the importance of visual inspection after the test has been performed. It may sometimes be operator dependent (subjectivity of the visual assessment), but it is nonetheless important to consider this qualitative appreciation of the failure mode together with the quantitative torque values.Figure 10
Two slurry mixtures with the same cohesion strength but a different type of degradation.In this study, a “manual” test device was used, i.e., the operator applies the torsion to the sample manually using a torque wrench. There are also “automatic” devices on the market in which the torsion applied to the sample is controlled by the equipment itself. According to the literature, there is a good correlation between the results of the automatic and manual devices [15].It is not certain if the automatic device will greatly improve the repeatability of the test, because the difficulty of the test lies more in the production of the samples (manufacturing and extraction from the mold) than in the application of the torque on the slurry surfacing.(4) Key Takeaways. If the test is performed according to BRRC’s recommendations (see Section 3)), the most important findings are the following:(i)
The normative cohesion test carried out in laboratory (EN 12274-4) is useful, relatively simple, and sufficiently discriminating for evaluating the progressive cohesion build-up of a slurry surfacing.(ii)
At least three tests per mixture are needed to produce consistent average results.(iii)
Values of 1.2 Nm (after 30 minutes) and 2.0 Nm (after 60 minutes), as recommended by the ISSA [13, 14, 48], seem to be adequate for Belgian practice.(iv)
The repeatability under our laboratory conditions at BRRC was estimated to be 0.7 Nm for a single test and 0.4 Nm for the average of three tests. In the latter case, the test is quite repeatable even though it is almost entirely manual and highly dependent on the surface condition of the slurry surfacing.(v)
BRRC has evaluated the cohesion test as a valuable method to optimize the mix design in view of sufficient cohesion for opening to traffic within the desired time in Belgium.A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-4.


### 4.4.2. Hilt Cohesion Test (HCT)

This test method has been proposed by France to the technical committee CEN/TC227/WG2 [7, 37]. This test seems to have some advantages compared to the method described in EN 12274-4: it is very simple and fast to perform and does not require expensive equipment. However, although presented as an alternative method to test the cohesion of a slurry surfacing, the “Hilt Cohesion Test-HCT” has little similarity with the test that is described in the standard EN 12274-4 (see Section 4.4.1).The principle (Figure11) is to place the sample (12 × 12 cm2 square and 1.5 cm thick) that has undergone controlled curing (at BRRC, 2 h at 20°C) on a device with a fixed part and a hatch. Half of the specimen covers the fixed part, with a counterweight on top, and the other half of the specimen covers the hatch. After opening the hatch, the time taken for the slurry surfacing to break under its own weight is measured. The result obtained is given in seconds and represents the HCT value after a given curing time, under controlled temperature and humidity conditions.Figure 11
Illustration of the hilt cohesion test (HCT) principle used at BRRC.This HCT value would allow for evaluating the cohesion of the slurry surfacing at an early age as a function of curing. The longer the time, the more resistant the slurry surfacing is in bending and therefore the more advanced the build-up of cohesion.The HCT was applied to different formulations to establish whether the test was repeatable, consistent, and discriminating, and whether there was a correlation with the results obtained with the cohesion test according to EN 12274-4. Our experience with the HCT test was not very positive. The repeatability of the test was insufficient, and the results were difficult to interpret and inconsistent. At BRRC, it was concluded that this test is not a viable alternative to the EN 12274-4 method, as the latter is clearly better able to analyze and evaluate the cohesion of these slurry surfacings.


## 4.4.1. Cohesion According to EN 12274-4

(1) Objective of the Test. This test method is used to measure the development of cohesion of a slurry surfacing system with time. This test also quantifies the time required before a slurry surfacing can be subjected to straight rolling traffic (“traffic time”).(2) Principle, Equipment, and Procedure. The slurry mixture is cast in a square-shaped mold having four circular holes, under which a roofing felt has been placed beforehand (Figure 5). The whole assembly is then conditioned at a temperature of (23 ± 5)°C. The standard specifies that the samples must be removed from the mold before testing. At BRRC, the roofing felt support consists of 4 separate parts, so that each sample can be tested individually on the roofing felt support.Figure 5
Mold used at BRRC to prepare the four circular samples for the cohesion test (and the 4 squares of roofing felt support to immobilize the samples).The test (Figure6) is carried out at the same temperature as conditioning (23 ± 5)°C. Its principle is to apply a rubber stamp (diameter of 28.5 ± 0.1 mm and height of 6.3 ± 1.0 mm) on a circular sample (diameter of 60 mm, thickness depends on the maximum aggregate size of the mix) of slurry surfacing (immobilized on a roofing felt support, Figure 5) with a constant pressure (200 ± 4 kPa). Torsion is applied manually to the slurry surfacing through the rubber stamp and the maximum torque is measured using a torque meter. The test is performed on the four different samples at a fixed time or at different suitable intervals between 5 min and 120 min after the samples have been spread in the mold (Figure 5), to evaluate the rise in cohesion as a function of time.Figure 6
Illustration of the execution of the cohesion test at BRRC according to EN 12274-4.The result of the measurement (maximum torque), expressed in Nm, represents the cohesion of the mixture at the four curing times mentioned above.After the test has been executed, a visual inspection of the condition of the samples is performed. Four types of “degradation” are referenced in EN 12274-4 and illustrated in Figure7.Figure 7
The four types of “degradation” indicated in the EN 12274-4 standard and examples for visual assessment of specimens. (a) N, normal; the sample is slightly damaged by some loss of aggregates. (b) S, solid; only the film of bitumen has been removed from the surface of the sample. (c) C, cracked. (d) D, disintegrated.
(a)(b)(c)(d)The lower the numerical value of the maximum torque, the less cohesive the slurry surfacing will be. Conversely, the higher the numerical value of the cohesive torque, the more cohesive the slurry surfacing will be (Figure8).Figure 8
Illustration of a slurry surfacing with low (a) and high (b) cohesion after performing the cohesion test according to EN 12274-4.
(a)(b)(3) Evaluation and Recommendations for Improving the Standard EN 12274-4. The normative cohesion test carried out in laboratory according to the standard EN 12274-4 is useful, relatively simple, and fast for evaluating the progressive cohesion build-up of a slurry surfacing. For the mixing procedure, it is recommended to follow a more detailed mixing procedure (see Section 4.1).Preparing (and demolding) the samples requires good practice and skill from the operator. The specified amount of mineral mixture (aggregates and filler) in the norm is too high and consequently, the excess needs to be removed to obtain a surface at the same level of the mold. This requires a lot of manipulation of the mixture, which is not practical and can cause segregation. Preparing the exact amount of mixture significantly facilitates the mold filling and results in a more homogeneous and repeatable texture. For a mixture with aggregate grading 0/6.3 mm, a quantity of 200 g of dry mineral mixture (instead of the 400 g specified in the standard) was found to be sufficient. This is a recommendation to improve the test method.In practice, the conditioning temperature of the materials and the mixing temperature have an impact on the cohesion build-up, as the kinetics of breaking and curing are temperature dependent. In general, the rise in cohesion will be faster at higher temperatures. Therefore, it is recommended that the materials shall be conditioned and the slurry surfacing mixed in the laboratory in a temperature range close to the one expected during the use of the slurry surfacing on-site.The roofing felt mentioned in the standard is hard to find because of the specification of the weight per unit area. It would be appropriate to remove the weight currently indicated in the standard ((700 ± 100) g/m2). The most important requirement is to use a roofing felt with sufficient strength to act as a sample holder and the exact weight would not affect the results. At BRRC, a roofing felt with a weight per unit area of approximately 394 g/m2 is used, which is appropriate to play the role of the sample holder.The precision of the cohesion test depends on the consistency of the slurry surfacing tested, since the latter will determine the difficulty for an even and homogeneous filling of the metal mold rings. The objective is to obtain four similar samples with an even and homogeneous texture.The textural differences will inevitably be reflected in the measured torque values and will therefore have an impact on the precision of the test:(i)
A formulation with a “too low consistency” (large diametrical outflow) will be more susceptible to phase segregation. As a result, samples from the first filled rings will be smoother on the surface (fines and bitumen migrate to the surface) compared to the following ones which will have a more granular appearance (the rest of the container contains more coarse aggregates which settle downwards in the mix).(ii)
A formulation with a “too high consistency” (small diametrical outflow) will be more complex to manipulate due to its lower workability, and the final texture of the four samples may not be similar and homogeneous.The test times are not specified by the standard, but BRRC generally performed the test after 15, 30, 60, and 90 minutes. In later tests, 24 hours were used (instead of 15 minutes) to obtain an indication of the final cohesion of the mix. The ISSA [13, 14, 48] specifies values of at least 1.2 Nm (after 30 minutes) and 2.0 Nm (after 60 minutes). Based on practical experience with Belgian slurry mixtures, these limits seem to be adequate and feasible for Belgian practice. The visual examination of the degradation must also be considered (Figure 7).The minimum number of repetitions of the test is not specified in the standard (it is only integrated in a note). The repeatability at BRRC was estimated to be 0.7 Nm for a single test and 0.4 Nm for the average of three tests. Therefore, the cohesion test is systematically performed at BRRC in triplicate and the result is calculated as the average.The test is then quite repeatable even though it is almost entirely manual and very dependent on the manufacturing quality of the samples. The experience and skill of the operator will thus greatly affect the repeatability of the test.The cohesion test is discriminating since the different formulations tested generally show cohesion build-up, final cohesion, and degradation states consistent with their formulation. For example, Figure9 shows test results on 2 mixtures with the same composition, but with a different emulsion. The breaking behavior of emulsion 2 in this mixture is different, which results in slower cohesion build-up and a higher risk of early ravelling on the road.Figure 9
Cohesion versus time, for mixtures with two different emulsions (bars denote standard deviation on 3 repeated tests).However, the interpretation of the results can be quite difficult: sometimes the torque is high, but there is a lot of damage, and sometimes the torque is low and there is no visible damage on the sample. This seems to depend greatly on the surface texture of the slurry surfacing and the way the rubber stamp adheres to the surface. The cohesion values obtained (in Nm) should therefore be interpreted with caution and the deterioration of the specimen (i.e., the degradation observed by visual inspection, Figure7) should be considered. Figure 10 illustrates the importance of visual inspection after the test has been performed. It may sometimes be operator dependent (subjectivity of the visual assessment), but it is nonetheless important to consider this qualitative appreciation of the failure mode together with the quantitative torque values.Figure 10
Two slurry mixtures with the same cohesion strength but a different type of degradation.In this study, a “manual” test device was used, i.e., the operator applies the torsion to the sample manually using a torque wrench. There are also “automatic” devices on the market in which the torsion applied to the sample is controlled by the equipment itself. According to the literature, there is a good correlation between the results of the automatic and manual devices [15].It is not certain if the automatic device will greatly improve the repeatability of the test, because the difficulty of the test lies more in the production of the samples (manufacturing and extraction from the mold) than in the application of the torque on the slurry surfacing.(4) Key Takeaways. If the test is performed according to BRRC’s recommendations (see Section 3)), the most important findings are the following:(i)
The normative cohesion test carried out in laboratory (EN 12274-4) is useful, relatively simple, and sufficiently discriminating for evaluating the progressive cohesion build-up of a slurry surfacing.(ii)
At least three tests per mixture are needed to produce consistent average results.(iii)
Values of 1.2 Nm (after 30 minutes) and 2.0 Nm (after 60 minutes), as recommended by the ISSA [13, 14, 48], seem to be adequate for Belgian practice.(iv)
The repeatability under our laboratory conditions at BRRC was estimated to be 0.7 Nm for a single test and 0.4 Nm for the average of three tests. In the latter case, the test is quite repeatable even though it is almost entirely manual and highly dependent on the surface condition of the slurry surfacing.(v)
BRRC has evaluated the cohesion test as a valuable method to optimize the mix design in view of sufficient cohesion for opening to traffic within the desired time in Belgium.A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-4.


## 4.4.2. Hilt Cohesion Test (HCT)

This test method has been proposed by France to the technical committee CEN/TC227/WG2 [7, 37]. This test seems to have some advantages compared to the method described in EN 12274-4: it is very simple and fast to perform and does not require expensive equipment. However, although presented as an alternative method to test the cohesion of a slurry surfacing, the “Hilt Cohesion Test-HCT” has little similarity with the test that is described in the standard EN 12274-4 (see Section 4.4.1).The principle (Figure11) is to place the sample (12 × 12 cm2 square and 1.5 cm thick) that has undergone controlled curing (at BRRC, 2 h at 20°C) on a device with a fixed part and a hatch. Half of the specimen covers the fixed part, with a counterweight on top, and the other half of the specimen covers the hatch. After opening the hatch, the time taken for the slurry surfacing to break under its own weight is measured. The result obtained is given in seconds and represents the HCT value after a given curing time, under controlled temperature and humidity conditions.Figure 11
Illustration of the hilt cohesion test (HCT) principle used at BRRC.This HCT value would allow for evaluating the cohesion of the slurry surfacing at an early age as a function of curing. The longer the time, the more resistant the slurry surfacing is in bending and therefore the more advanced the build-up of cohesion.The HCT was applied to different formulations to establish whether the test was repeatable, consistent, and discriminating, and whether there was a correlation with the results obtained with the cohesion test according to EN 12274-4. Our experience with the HCT test was not very positive. The repeatability of the test was insufficient, and the results were difficult to interpret and inconsistent. At BRRC, it was concluded that this test is not a viable alternative to the EN 12274-4 method, as the latter is clearly better able to analyze and evaluate the cohesion of these slurry surfacings.


## 4.5. Wearing Resistance

After curing and reopening to traffic, the slurry surfacing has to resist wearing by traffic and water. Two test methods were evaluated in this paper to determine the wearing resistance of slurry surfacing:(i)
The European standard EN 12274-5 (see Section4.5.1) [26];(ii)
The technical specification “TS: Slurry surfacing-Test methods-Determination of cohesion of the mix” (see Section4.5.2).The latter, also known as Superficial Cohesion Test (SCT), describes an alternative method to EN 12274-5. Due to the difficulty of harmonizing procedures for these little practiced tests, the process of creating the technical specification was never completed and was abandoned. For this reason, this alternative method will not be discussed in detail in this paper in addition to the practical experience of BRRC, which has proven to be not very positive.


### 4.5.1. Wearing Resistance According to EN 12274-5

(1) Objective of the Test. This test method measures the wearing resistance of slurry surfacing systems under wet abrasion conditions. The test is generally referred to as WTAT (Wet Track Abrasion Test) in the literature [33, 34].(2) Principle, Equipment, and Procedure. The European standard EN 12274-5 specifies a test method to assess:(i)
the wearing resistance of slurry surfacing after “favorable” curing at a temperature of (60 ± 5)°C;(ii)
the minimum amount of emulsion for different traffic classes (see Annex A of the standard).The slurry mixture is cast in a circular mold (internal diameter = (279.0 ± 0.5) cm) placed on a flat test base on which a roofing felt has been placed beforehand (Figure12). The whole assembly is then conditioned in an oven at 60°C, until a constant mass is obtained, and subsequently saturated with water by immersion in a water bath (at 25 ± 2°C) for a period of 60 minutes to 75 minutes. After conditioning, the standard specifies that the sample must be removed from the mold before testing.Figure 12
Illustration of the execution of the wearing resistance test at BRRC according to EN 12274-5.The test (Figure12) consists of an abrasion procedure, using a hard rubber hose, which is pressed on the surface of the slurry sample with a vertical force of (22.3 ± 2.0 N), while executing a planetary rotation at 61 cycles per minute for 5 minutes. The specimen is tested under water (at 25 ± 2°C) and the loss of material by wearing is determined by weighing. The standard specifies that the test shall be repeated on three samples and the result is calculated as the average of the three individual results.The lower the weight loss, the more cohesive and wear resistant the slurry surfacing will be. Conversely, the higher the weight loss, the less cohesive and wear resistant the slurry surfacing will be. A higher residual binder content will improve the wearing resistance, which is why this test is typically used to determine the minimum amount of emulsion needed.(3) Evaluation and Recommendations for Improving the Standard EN 12274-5. For the mixing procedure, it is again recommended to follow a more detailed mixing procedure (see Section 4.1). The preparation of the samples is a very critical step and is highly sensitive for the practical experience and skill from the operator. This preparation will condition the aspect of the surface of the samples (more or less textured), and it will undeniably impact the weight loss results and their dispersion.In fact, it is crucial to produce test samples with equal texture, representative of the texture of the slurry surfacing on the road. It is recommended that the so-called “sand-patch test” (in accordance with EN 13036-1, [49]) is carried out prior to the wearing test to verify the texture of the samples. It was shown that, when samples with outlying texture are discarded, repeatability generally improves.The quantities that are now given in the standard are far too high. This results in a waste of material and makes sample preparation more difficult (i.e., segregation, inhomogeneity). Preparing the exact amount of mixture significantly facilitates the mold filling and results in samples with a more homogeneous and repeatable texture.In the current standard, there is no maximum time restriction for the curing process of the samples, but this will probably influence the results. To work as consistent as possible, based on practical experience, a curing time of 3 days seems suitable and sufficiently long to obtain a constant mass.The use of a roofing felt as a support for the slurry surfacing is not necessary and unmolding the specimens prior to testing is an unnecessary manipulation, potentially leading to damage to the edges [15]. It is recommended to test the specimens still in the mold.Repeatability of the test is limited, especially for the less wear-resistant mixes (high amount of material loss). Repeatability is better for mixtures with very low texture. Despite the poor repeatability, the test method is capable of discriminating mixtures in a logical way (effect of the composition of the slurry surfacing, type of emulsion, composition, and nature of the aggregate). For example, Figure13 shows the effect of decreasing the emulsion dosage by 2% in a mix. As expected, the lower amount of residual binder has a considerable adverse impact on resistance to ravelling (Figure 13).Figure 13
Impact of emulsion content on material loss.Based on the ISSA guideline [13], a maximum average material loss of 540 g/m2 is required to obtain a mix with good wearing resistance. Based on test results on Belgian slurry mixtures, it can be concluded that this requirement is severe but nonetheless feasible.When the material loss exceeds this threshold, the mix design should be adapted, e.g., by increasing the amount of emulsion, by using a polymer modified emulsion, or by improving binder/aggregate adhesion.The above recommendations are also consistent with most of the findings in the technical bulletin TB136 [50] describing the causes of inconsistency of the WTAT results (good consistency, constant thickness, macrotexture of the samples, …).(4) Key Takeaways. If the test is performed according to BRRC’s recommendations (see Section 3), the most important findings are the following:(i)
This test carried out in the laboratory is useful for evaluating the wearing resistance of a slurry surfacing.(ii)
The test is rather time-consuming and laborious. At least three tests per mixture are needed to obtain a more accurate average result.(iii)
The preparation of samples with both a homogeneous and repeatable texture is a very critical step and is highly sensitive to the practical experience and skill from the operator.(iv)
To verify the texture of the three samples in the series to be tested for each formulation, the so-called “sand-patch test” (in accordance with EN 13036-1 [49]) is recommended. This allows to discard samples with outlying texture depth and improves the repeatability.(v)
Even if the repeatability is poor, the test is capable of discriminating mixtures in a logical way (effect of the composition of the slurry surfacing, type of emulsion, composition, and nature of the aggregate).(vi)
A maximum average material loss of 540 g/m2 seems appropriate to obtain a mix with good wearing resistance.Despite the abovementioned limitations and its poor repeatability, the test is evaluated at BRRC as a good test to verify the mix design for wearing resistance. A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-5.


### 4.5.2. Superficial Cohesion Test (SCT)

The SCT or Superficial Cohesion Test (Figure14) is proposed by France [7, 37] as an alternative test for testing the cohesion of a slurry mixture (EN 12274-4, [25]). The SCT shows many similarities with the test according to EN 12274-5 [26]. It also measures the material loss of a circular specimen by abrasion under water. The major differences are:(i)
The load is transferred in the SCT by two rubber wheels on a shaft mounted on a Hobart mixer (instead of by a rubber roller in an apparatus specially designed for the test).(ii)
The SCT is performed on a freshly prepared slurry mixture, after a short conditioning period (30 minutes according to the standard) and this at room temperature (instead of on a mixture that has cured for several days at 60°C).(iii)
The duration of the wearing test is shorter than in EN 12274-5, more specifically 1 minute (instead of 5 minutes).Figure 14
Test setup for the SCT test at BRRC: (a) detail of the wheels and (b) complete equipment.
(a)(b)Since the slurry is tested shortly after preparation, the loss of material would be a measure of the cohesion of the slurry in the initial phase (when the slurry surfacing is opened to traffic). Practical experience gained at BRRC with the alternative test according to the technical specification has led to the decision to abandon this test:(i)
The equipment must be adjusted to withstand a higher vertical force, as the current force level is insufficient to cause significant material loss. This is not necessary, as EN 12274-5 [26] provides satisfactory results.(ii)
This method proved to be too time-consuming and unnecessary, as EN 12274-4 [25] better meets expectations.


## 4.5.1. Wearing Resistance According to EN 12274-5

(1) Objective of the Test. This test method measures the wearing resistance of slurry surfacing systems under wet abrasion conditions. The test is generally referred to as WTAT (Wet Track Abrasion Test) in the literature [33, 34].(2) Principle, Equipment, and Procedure. The European standard EN 12274-5 specifies a test method to assess:(i)
the wearing resistance of slurry surfacing after “favorable” curing at a temperature of (60 ± 5)°C;(ii)
the minimum amount of emulsion for different traffic classes (see Annex A of the standard).The slurry mixture is cast in a circular mold (internal diameter = (279.0 ± 0.5) cm) placed on a flat test base on which a roofing felt has been placed beforehand (Figure12). The whole assembly is then conditioned in an oven at 60°C, until a constant mass is obtained, and subsequently saturated with water by immersion in a water bath (at 25 ± 2°C) for a period of 60 minutes to 75 minutes. After conditioning, the standard specifies that the sample must be removed from the mold before testing.Figure 12
Illustration of the execution of the wearing resistance test at BRRC according to EN 12274-5.The test (Figure12) consists of an abrasion procedure, using a hard rubber hose, which is pressed on the surface of the slurry sample with a vertical force of (22.3 ± 2.0 N), while executing a planetary rotation at 61 cycles per minute for 5 minutes. The specimen is tested under water (at 25 ± 2°C) and the loss of material by wearing is determined by weighing. The standard specifies that the test shall be repeated on three samples and the result is calculated as the average of the three individual results.The lower the weight loss, the more cohesive and wear resistant the slurry surfacing will be. Conversely, the higher the weight loss, the less cohesive and wear resistant the slurry surfacing will be. A higher residual binder content will improve the wearing resistance, which is why this test is typically used to determine the minimum amount of emulsion needed.(3) Evaluation and Recommendations for Improving the Standard EN 12274-5. For the mixing procedure, it is again recommended to follow a more detailed mixing procedure (see Section 4.1). The preparation of the samples is a very critical step and is highly sensitive for the practical experience and skill from the operator. This preparation will condition the aspect of the surface of the samples (more or less textured), and it will undeniably impact the weight loss results and their dispersion.In fact, it is crucial to produce test samples with equal texture, representative of the texture of the slurry surfacing on the road. It is recommended that the so-called “sand-patch test” (in accordance with EN 13036-1, [49]) is carried out prior to the wearing test to verify the texture of the samples. It was shown that, when samples with outlying texture are discarded, repeatability generally improves.The quantities that are now given in the standard are far too high. This results in a waste of material and makes sample preparation more difficult (i.e., segregation, inhomogeneity). Preparing the exact amount of mixture significantly facilitates the mold filling and results in samples with a more homogeneous and repeatable texture.In the current standard, there is no maximum time restriction for the curing process of the samples, but this will probably influence the results. To work as consistent as possible, based on practical experience, a curing time of 3 days seems suitable and sufficiently long to obtain a constant mass.The use of a roofing felt as a support for the slurry surfacing is not necessary and unmolding the specimens prior to testing is an unnecessary manipulation, potentially leading to damage to the edges [15]. It is recommended to test the specimens still in the mold.Repeatability of the test is limited, especially for the less wear-resistant mixes (high amount of material loss). Repeatability is better for mixtures with very low texture. Despite the poor repeatability, the test method is capable of discriminating mixtures in a logical way (effect of the composition of the slurry surfacing, type of emulsion, composition, and nature of the aggregate). For example, Figure13 shows the effect of decreasing the emulsion dosage by 2% in a mix. As expected, the lower amount of residual binder has a considerable adverse impact on resistance to ravelling (Figure 13).Figure 13
Impact of emulsion content on material loss.Based on the ISSA guideline [13], a maximum average material loss of 540 g/m2 is required to obtain a mix with good wearing resistance. Based on test results on Belgian slurry mixtures, it can be concluded that this requirement is severe but nonetheless feasible.When the material loss exceeds this threshold, the mix design should be adapted, e.g., by increasing the amount of emulsion, by using a polymer modified emulsion, or by improving binder/aggregate adhesion.The above recommendations are also consistent with most of the findings in the technical bulletin TB136 [50] describing the causes of inconsistency of the WTAT results (good consistency, constant thickness, macrotexture of the samples, …).(4) Key Takeaways. If the test is performed according to BRRC’s recommendations (see Section 3), the most important findings are the following:(i)
This test carried out in the laboratory is useful for evaluating the wearing resistance of a slurry surfacing.(ii)
The test is rather time-consuming and laborious. At least three tests per mixture are needed to obtain a more accurate average result.(iii)
The preparation of samples with both a homogeneous and repeatable texture is a very critical step and is highly sensitive to the practical experience and skill from the operator.(iv)
To verify the texture of the three samples in the series to be tested for each formulation, the so-called “sand-patch test” (in accordance with EN 13036-1 [49]) is recommended. This allows to discard samples with outlying texture depth and improves the repeatability.(v)
Even if the repeatability is poor, the test is capable of discriminating mixtures in a logical way (effect of the composition of the slurry surfacing, type of emulsion, composition, and nature of the aggregate).(vi)
A maximum average material loss of 540 g/m2 seems appropriate to obtain a mix with good wearing resistance.Despite the abovementioned limitations and its poor repeatability, the test is evaluated at BRRC as a good test to verify the mix design for wearing resistance. A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-5.


## 4.5.2. Superficial Cohesion Test (SCT)

The SCT or Superficial Cohesion Test (Figure14) is proposed by France [7, 37] as an alternative test for testing the cohesion of a slurry mixture (EN 12274-4, [25]). The SCT shows many similarities with the test according to EN 12274-5 [26]. It also measures the material loss of a circular specimen by abrasion under water. The major differences are:(i)
The load is transferred in the SCT by two rubber wheels on a shaft mounted on a Hobart mixer (instead of by a rubber roller in an apparatus specially designed for the test).(ii)
The SCT is performed on a freshly prepared slurry mixture, after a short conditioning period (30 minutes according to the standard) and this at room temperature (instead of on a mixture that has cured for several days at 60°C).(iii)
The duration of the wearing test is shorter than in EN 12274-5, more specifically 1 minute (instead of 5 minutes).Figure 14
Test setup for the SCT test at BRRC: (a) detail of the wheels and (b) complete equipment.
(a)(b)Since the slurry is tested shortly after preparation, the loss of material would be a measure of the cohesion of the slurry in the initial phase (when the slurry surfacing is opened to traffic). Practical experience gained at BRRC with the alternative test according to the technical specification has led to the decision to abandon this test:(i)
The equipment must be adjusted to withstand a higher vertical force, as the current force level is insufficient to cause significant material loss. This is not necessary, as EN 12274-5 [26] provides satisfactory results.(ii)
This method proved to be too time-consuming and unnecessary, as EN 12274-4 [25] better meets expectations.


## 4.6. Resistance to Bleeding and Rutting

“Bleeding” is the appearance of free bitumen on the surface of the pavement. This results in greasy, shiny areas with little or no macrotexture, which reduces the skid resistance of the road surface locally and significantly. This is one of the types of defects to be determined when applying the visual inspection standard EN 12274-8 [51]. The area corresponding to the wheel paths is particularly sensitive to bleeding, due to the sinking of aggregates into the bitumen.Vertical deformation in the wheel paths, called “rutting,” is another type of damage to be addressed. Because of the limited thickness of slurry surfaces, it is only relevant for multi-layer slurry surfacing where the total thickness is at least twice the maximum aggregate size of the all in mineral aggregate.At present, there are no laboratory tests in the European series of standards for slurry surfacing to determine sensitivity to bleeding and rutting. However, if further progress is to be made in improving the quality of slurry surfacing, consistent and appropriate tests seem necessary. The increased requirements for wear resistance and ravelling could lead to the design of slurries that are too rich in bitumen, resulting in bleeding, rutting, and poor skid resistance. More specifically for microsurfacing, it will be crucial to find an optimal compromise between wear and ravelling resistance (according to EN 12274-5 described in Section4.5.1) and bleeding and rutting resistance.Two methods for the respective determination of the sensitivity to bleeding and permanent deformation (rutting) of slurry surfacing have been published by ISSA [52, 53]. The test equipment, called “Loaded Wheel Tester,” shows some similarities with the wheel tracking test equipment known in Europe (EN 12697-22, [54]). The large size device of this standard is used in Belgium for testing hot mix asphalt (Figure 15(a)). The principle of the test is similar, but the test plates of the large size device are larger and a larger wheel with a 6-bar pressure tire is used.Figure 15
Large size device according to EN 12697-22 (a); sand patch test carried out before the application of the 1500 cycles (b); two slurry surfacing plates after the application of 1500 cycles (c).
(a)(b)(c)The standard test conditions, which are used in Belgium for testing hot asphalt mixes, are more severe than those of the two test methods from ISSA [52, 53] for testing slurry surfacing, but the large size device allows these test conditions to be changed. Given that the large size device is standard equipment in Belgium and Europe, it was decided to evaluate its potential for testing slurry surfacing.The method from technical bulletin TB109 [52] assesses bleeding by measuring the amount of hot sand that adheres to the surface. The sample is subjected to a fixed number of loaded wheel tracking cycles. A weighed quantity of warm sand is uniformly spread on the surface and further loaded wheel tracking is carried out. The amount of sand retained through embedment in the sample surface is determined. It gives an indication of the tendency of the mix “to flush” under traffic.Based on practical experience, measuring the texture in the wheel path is easier and better related to skid resistance. Hence, the simple “sand-patch test” (EN 13036-1, [49]) is used, before and after wheel tracking, to assess the resistance to bleeding. The literature [55] confirms that the results obtained with hot sand are very variable and not precise enough to suggest an optimum formulation.(1) Objective of the Test. The objective of the laboratory test proposed by BRRC is twofold: evaluate the sensitivity to bleeding and measure the sensitivity to rutting under simulated rolling traffic.(2) Principle, Equipment, and Internal Procedure. Slurry surfacing specimens are subjected to the repeated passing of a loaded pneumatic tire. The rutting and bleeding caused by this action show the sensitivity of the slurry surfacing to these types of defects. Figure 15(a) shows the large size device, in which two specimens are simultaneously tested.Based on literature [55], the ISSA test protocols [52, 53] and experience with the large size device (EN 12697-22, [54]), a procedure was established for the preparation of the slurry surfacing specimens and for assessing resistance to bleeding and rutting. Compared to the standard rutting test on hot mix asphalt, less severe test conditions are applied in terms of temperature (20°C) and number of cycles (100, 200, …, 600, 1000, 1500). Bleeding is assessed by measuring the surface texture depth, rutting by measuring the rut depth.The test specimens are prepared as follows. A metal bottom plate, covered with a roofing felt support, is placed into a metal mold of the large size device (internal length and width: 50 ∗ 18 cm2; height: 5 cm). The bottom plate thickness depends on the intended specimen thickness, as the sum of the bottom plate and the specimen thickness shall equal the height of the mold.The slurry mixture is prepared following the standard procedure of BRRC (see Section4.1) and immediately poured into the metal mold to a layer thickness corresponding to twice the maximum aggregate size. The slurry surfacing sample is then dried for 24 hours at room temperature, followed by 24 hours at 60°C. After conditioning, the initial profile and surface texture depth are assessed.The profile is assessed by measuring the depth at 15 points, using the standard procedure of EN 12697-22 [54]. The Mean Texture Depth (MTD) is determined using the “sand-patch test” on three locations over the length of the wheel path (Figure 15(b)).After removing the sand (using a dry brush or vacuum cleaner), the two filled molds are placed into the large size device at room temperature (20°C ± 2°C) and subjected to wheel tracking. When the 1500 cycles are completed (Figure15(c)), the rut profile and mean texture depth are again measured. The difference with the initial measurements characterizes the sensitivity to rutting and bleeding, respectively.Up to now, only three different slurry surfacing mixtures have been tested using this test procedure. Rut depths were for the three slurry surfacing mixtures in the range of 15 to 22%, and the mean profile depth decreased by 38 to 43%. The differences between the test results were not significant, given the standard deviations on the individual measurements of rut depths and texture depths. However, this was not surprising since the mix compositions of the variants were very similar: same mixture but with two different emulsions, both without polymer modification, and a third variant with a higher emulsion dosage (3% higher). Tests on more extreme variants (e.g. with and without polymer modification or a larger difference in emulsion dosage) are needed to demonstrate the discriminating power of the test method.(3) Key Takeaways. A new test procedure was developed to measure the resistance to bleeding and rutting of slurry surfacing, using the large size device of the European wheel tracking test (EN 12697-22, [54]) with modified conditions (20°C, less cycles) and combined with texture measurements (EN 13036-1, [49]). The most important findings with this “bleeding/rutting test” are the following:(i)
The feasibility of the test was demonstrated and the test conditions are appropriate to simulate realistic rut depths and texture variations.(ii)
Due to the small number of variants tested at this point, it was not possible to quantify the precision of the test. However, the differences in results between the two plates of the same variant are quite small, indicating good repeatability.(iii)
The three variants tested were probably not sufficiently different, regarding their sensitivity to bleeding and rutting, to demonstrate the discriminating power of the test. More test results need to be acquired on significantly different mixtures.


## 4.7. Interaction between Aggregate and Emulsion

A strong bond between aggregate and binder in the presence of water is considered as an important prerequisite for a well-performing slurry surfacing. The “Shaking Abrasion Test (SAT),” according to EN 12274-7 [27] is intended to evaluate, based on the resistance to abrasion of specimens in the presence of water, the suitability of a combination of aggregate and emulsion for slurry surfacing.(1) Objective of the Test. The European standard EN 12274-7 [27] provides a laboratory test method to determine the suitability/compatibility of the aggregate and cationic emulsion combination. It can also be used to evaluate the effect of individual additives.(2) Principle, Equipment, and Procedure. The European standard EN 12274-7 [27] provides a (performance-based) test method to determine the resistance to abrasion in the presence of water of specimens composed of an aggregate 0/2 mm with a defined gradation, water, cement, and a cationic bituminous emulsion.The exact quantities for one specimen are manually mixed until the mixture breaks. The broken mixture is transferred to a small cylindrical mold (Figure16). After preparing four mixtures with the same composition, the mixtures are compacted by placing the molds (Figure 17(a)) in a press with a load of 10 kN, applied at a speed of 20 mm/min. The final height of each specimen is (25 ± 1) mm and the diameter is approximately 30 mm.Figure 16
Broken mixture (a); filled mold with excess of water flowing out (b); set of four compacted samples (c).
(a)(b)(c)Figure 17
Components of the molds used for sample compaction (a); test equipment (b); (c) test specimens before (left) and after (right) execution of the shaking abrasion test according to EN 12274-7.
(a)(b)(c)In a next step, the test samples are soaked with water by placing them in water, at approximately 1°C (iced water) in a vacuum desiccator during approximately three hours at (3 ± 0.1) kPa. The measurement of the weight of the test specimens before and after this conditioning allows for the determination of the water absorption.Finally, the test samples are placed in tubes (Figure17(b)) filled with water and subjected to the shaking abrasion mechanical action by rotating the tubes (Figure 17(c)). The amount of material loss from the four samples after … rotations is measured. Unlike the previous tests (Sections 4.2–4.6), this is not a test on the complete slurry mixture and, therefore, it will not assess the impact of mineral aggregate grading or mix composition.(3) Evaluation and Recommendations for Improving the Standard EN 12274-7. At BRRC, following the compaction and before water saturation, test samples are dried overnight at room temperature allowing for the measurement of the apparent density according to EN 12697-6 method B [56]. This enables the calculation of the void content of the test samples. This additional parameter is interesting to understand and analyze the mass loss results. BRRC recommends to add a note in the standard specifying the possibility of determining the void contents of the samples.There is an inconsistency with the amount of water to be added when preparing mixtures. The standard specifies a water content of (40 ± 0.5) % of the mass of dry aggregate. Therefore, in case of 200 g dry aggregate, 80 g of water should be added. The latter quantity is in contradiction with the 40 g mentioned in the next line of the standard and constitutes in reality a too high percentage. BRRC recommends correcting the current water content and using a mass of 40 ± 2 g.Regarding the method of compacting mixtures when making samples, the protocol currently described in the standard will not provide homogeneous and representative samples (the fully broken mixture is granulated manually to obtain pieces with a diameter of less than approximately 5 mm and placed in the compaction mold). Based on practical experience, the production of the specimens shall immediately follow the mixing stage, without any drying and curing of the mixture.The specification of the temperature at which the saturation under vacuum should be carried out (at approximately 1°C (iced water) in a vacuum desiccator) is problematic. A temperature of 1°C is difficult to maintain during the whole soaking period. BRRC recommends to simply work at room temperature. The specification of a low temperature such as 1°C originates from the fact that this method was originally developed to assess the properties of a filler and is related to the volumetric stability of the specimens [57, 58]. However, in this application, volumetric stability is not a concern, which means that the low temperature is not needed.Considering the recommendations mentioned above, the test results obtained by BRRC (Figure18) on combinations of two types of aggregates (porphyry and sandstone) commonly used in Belgium with different types of cationic bituminous emulsion (polymer modified emulsion or not) are in general systematically low (≤2.5% weight loss). These results suggest a good suitability/compatibility between these aggregate types (0/2 mm fraction) and the tested emulsions.Figure 18
Impact of aggregate and emulsion type on material loss (emulsion N°2 is a polymer modified bituminous emulsion).The test performed according to the BRRC procedure also shows a good repeatability which is in line with the precision data reported in EN 12274-7 (Table3).Table 3
Precision data related to repeatability in EN 12274-7 [27].
Water absorption Wv (v-%)Mass loss (%)Range (difference min–max)0.70.6Standard deviationσr0.30.3Repeatabilityr-value0.80.9Based on these results, one can conclude that the test can discriminate between different aggregate/bituminous emulsions combinations. Porphyry aggregate showed a better compatibility with both unmodified as well as polymer modified cationic bituminous emulsions. However, no explanation could be provided for the higher mass losses in case of a polymer modified bituminous emulsion as compared with the unmodified emulsion.BRRC tests show consistency between void contents, water absorption, and mass losses of the tested aggregate/emulsion combinations (Table4):(i)
Test specimens characterized by a higher void content absorb a greater amount of water (Wv) during conditioning.(ii)
In general, samples with higher void content and higher water absorption have higher mass losses.(iii)
The absorbed water content depends on the nature of the aggregate.Table 4
Impact of aggregate and emulsion type on three parameters (void content, water absorption, and mass loss).
SampleVoid content (%)Water absorption Wv (v-%)Mass loss (%)Mix porphyry emulsion N°16.5 ± 0.14.2 ± 0.10.6 ± 0.3Mix porphyry emulsion N°27.6 ± 0.16.4 ± 0.41.9 ± 0.2Mix sandstone emulsion N°19.8 ± 0.37.7 ± 0.41.3 ± 0.2Mix sandstone emulsion N°210.6 ± 0.110.1 ± 0.22.5 ± 0.1At present, based on the test result obtained with EN 12274-7, there are no (performance) classes defined in the product standard for slurries EN 12273 [59]. Consequently, there are no requirements in any European country regarding the interaction between aggregate and emulsion. Consequently, the test is only used as a screening method, e.g., when changing the type of aggregate (practice in Germany).In contrast to EN 12273, the ISSA TB144 method [35] does provide a classification based on the results of the Schulze-Breuer and Ruck Procedures (grading rate: “A–E”). According to the latter, the samples are evaluated for abrasion, integrity, and adhesion. In the EN 12274-7 procedure, the specimens are only evaluated for abrasion in terms of mass loss (Table 5).Table 5
Overview SAT classification in mass loss according to ISSA TB144 method and the conversion in accordance with the normative method EN 12774-7.
Grade ratingMass loss (%)ISSA TB144EN 12274-7A0–1.70–1.0B1.7–2.41.0–1.5C2.4–3.11.5–1.9D3.1–4.81.9–2.9E>4.8>2.9The comparison with the test procedure described in ISSA TB144 [35] (for the abrasion procedure) indicated that the latter procedure differs significantly from the method in EN 12274-7 in terms of specimen preparation (dry compaction) and conditioning (no saturation specimens). The ISSA TB114 procedure is more discriminating (higher material losses) and this is mainly due to the very long (and therefore more severe) conditioning of the specimens in water (6 days).Based on the comparative tests in which both the ISSA TB144 method [35] and the BRRC procedure based on EN 12274-7 were performed on two types of aggregates (porphyry and sandstone), a conversion factor can be calculated from the ratio of the respective mass losses. For this purpose, an average value of 1.65 can be determined.This factor then allows an estimation (dividing by 1.65) of the values for a classification “A–E” based on the test result obtained by the SAT according to EN 12274-7 (Table5). Applying the above “grade rating” according to ISSA TB144, porphyry gets the “grade rating” A and sandstone “grade rating” B.The above values (Table5) for the grade rating can be checked against the requirements included in Refs. [13, 60] where a “grade rating” A or B (for the abrasion loss) is always required in the context of the mixture design. Introducing this classification for the mass loss (“A–E”) in the standard EN 12274-7 could be very useful to evaluate the materials used (emulsion, aggregate 0/2 mm with a defined gradation and with or without cement) and their compatibility.(4) Key Takeaways. A list of comments and proposed changes has been submitted to CEN/TC227/WG2 for improvement of the standard EN 12274-7 [27]. If the test is performed according to these BRRC propositions (see Section 3), the most important findings are the following:(i)
The Shaking Abrasion Test (SAT) carried out in laboratory is useful and discriminating for determining the suitability/compatibility of the aggregate/emulsion combination.(ii)
On the other hand, this laboratory test is rather time-consuming and laborious. It has the disadvantage of not being able to carry out the preparation of the samples and the execution of the tests in a working day.(iii)
BRRC tests show consistency between void contents, water absorption, and mass losses of the tested aggregate/emulsion combinations.(iv)
The Shaking Abrasion Test (SAT) performed according to the BRRC procedure shows good repeatability.(v)
The test requires a skilled operator to prepare high-quality samples and perform the test in a repeatable way.Based on the above findings, the SAT can be used during the mix design phase of the basic composition of a slurry mixture to determine a suitable aggregate-emulsion combination. In this context, the test is primarily suitable to evaluate the choice/change of aggregate type.


## 5. Conclusions

BRRC was engaged in a 4-year research project called BeP2S (Better Performing Slurry Surfacing) with financial support of the Belgian National Bureau for Standardization (NBN), which aims to improve the performance and the durability of slurry surfacing on the Belgian road network.In this context, the European test methods of the EN 12274 series (“Slurry surfacing–Test methods”), as well as some alternative test methods, were evaluated. Improvements in terms of sample preparation, test conditions, and test execution were made, leading to a better precision and discriminating power.A list of comments and proposed changes has been submitted to the CEN/TC227/WG2 Surface Dressing, Sprays, and Slurry Surfacing (incorporating Microsurfacing). Furthermore, the following conclusions can be drawn regarding the importance of these test methods for mix design:(i)
Five performance-related tests proved to have sufficient ability to discriminate between mixtures with variations in mix design: the tests for mix time, consistency, cohesion, resistance to wearing, and affinity between aggregate and emulsion. These tests are feasible in terms of equipment and time and, therefore, very useful in the process of mix design.(ii)
The laboratory test methods were evaluated for repeatability within the laboratory of BRRC. We experienced that most of the tests require a great deal of manual actions and thus the skill of the operator is an important factor.(iii)
As the operating conditions (especially the mixing procedure and sample preparation) and the changes in the composition of the slurry surfacing have an impact on the test results, it is imperative during the mix design process in laboratory to use the materials that will be expected on-site, and to be particularly attentive to the type of emulsion used, and to simulate the temperature conditions expected on-site and the water content of the aggregates in the stockpile if the latter is not constant.(iv)
BRRC has evaluated the manual mixing test (according to ISSA TB113) as a valuable method to adapt mix design with a view to ensure adequate mix time. The introduction of this test as a new European standard would be interesting to optimize the slurry surfacing mix design process.(v)
BRRC has established recommended results for the “optimal behavior” of a slurry surfacing in the laboratory (in terms of mix time, consistency, cohesion, and wearing resistance).(vi)
A test for resistance to bleeding and rutting is missing in the series of performance-related tests. Therefore, BRRC developed a procedure to study these two latter phenomena. The first results are promising, but the study of more variants is necessary.



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