SenX Limited, 3 Welmar Mews, London SW4 7DD Manufactured Sand What it is, what it isn’t, and why, plus some processing methods Hugo Pettingell ACSM FIQ September 2012
Define manufactured sand
What Manufactured Sand is – and what it is not! MANUFACTURED SAND “The aggregate industry typically has branded any crushed product that is less than 4 mesh (4.75 mm) as “manufactured sand”. This of course is not the case. Manufactured sand should be a product that you have intentionally produced, not merely the waste fraction of a process that is targeting larger aggregate sizes.” BARRY HUDSON 1999
Manufactured sand
What affects the performance of aggregates in concrete? “The equidimensionality and angularity of coarse and fine aggregate particles, the degree and shape of the texture on their surfaces, the amount of surface area caused by the particle shape and degree of relief, and the chemical and physical nature of coatings, if present, are all characteristics that significantly affect the properties of both fresh and hardened concrete.”¹ “The impact of the physical characteristics of fine aggregate on the concrete mix properties, in both the plastic and hardened states, is significantly greater than that of the coarse aggregate fraction. ”²
Just for comparison.... Particle Size (mm) Particle Volume (mm³) Number of particles Surface area in 8200 mm 3 (m 2) (spherical) in 8200 mm3 25 8200 1 0.002 33.5 244 4 0.012 2x10ˉ 4 4x10 7 0.075 0.7 2x 10ˉ 9 2x10 12 0.002 24 Due to the much smaller particle size, and hence the greatly increased surface area to volume ratio, any detrimental or undesirable particle shape or texture of the fine aggregates will be greatly amplified…” ³ This is why we need to consider particle shape even more critically as particle size decreases.
WHAT MAKES GOOD MANUFACTURED SAND? Some things we can affect by processing: 1. Excellent particle shape and surface texture Crushing 2. Ideal particle size distribution process 3. Sufficient microfines with appropriate characteristics ...And some we can’t change so readily: 4. No chemical impurities or Alkali Silica reactivity Geology 5. Absence of micas and clays, particularly expansive types such as smectites
Source rock
Caution! Be very careful in selecting the source rock: Geology has a huge effect on the suitability of material for manufactured sand Alteration and weathering can vary within a quarry, and may preclude the use of some areas for sand for concrete. There may even be more than one rock type. Proper evaluation of characteristics is essential BEFORE installing equipment – the results will have a bearing on the selection of an appropriate process.
Particle Shape
Why particle shape? What difference does it make in practice? Specific surface area – total surface area of a material per unit of mass ( usually m²/kg): Smaller surface area means less “glue” (cement paste) needed to coat it Packing density, or Voids Ratio: Voids ratio may vary up to 10% between extremes of shape and texture, but a normal range is more like 4-5%. Lower voids ratio reduces cement paste volume . Particle shape affects workability of concrete in the liquid phase Lower internal friction leads to better workability and lower water requirement
PARTICLE SHAPE – Some comparative images Cone BARMAC 65m/s Kemco US7 55m/s 2.5-1.2mm 0.6-0.3mm 0.3-0.15mm
PARTICLE SHAPE – what does it matter? Solids ratio as measured under JIS.A.5005-1104 (2.5-1.2mm) CONE BM 55m/s BM 65m/s US7 45m/s US7 55m/s 54.8% 56.3% 56.8% 56.9% 57.7% Calculation of cement saving due to improved solids ratio 1% better solids ratio can save 3-4kg water/m³ of concrete Given W/C(water cement ratio) = 50% = 1.5% (56.3% – 54.8%) Improvement of solid ratio Feed to BM = 3 - 4 kg/m³ x 1.5 = 4.5 – 6 kg/m³ Saving water = 4.5 – 6 kg/m³ divided by 0.5 Cement saving = 9 – 12 kg/m³ Improvement of solid ratio BM to US7 = 1.4% (57.7% - 56.3%) Saving water = 3-4kgs/m³ x 1.4 = 4.2-5.6kg/m³ Cement saving = 4.2-5.6kg/m³ divided by 0.5 = 8.4 – 11.2kg/m3
Surface texture “To assist with cement paste / aggregate particle bond, it is desirable to have a ‘roughened’ particle surface. It is important that this surface is not too rough or honeycombed, as this will greatly influence the amount of water required to make the concrete workable in its plastic state. Spherical particles in many cases are ‘polished’ and offer little for the cement paste to bond with, and can produce concrete with reduced strengths, particularly flexural strength.” ² Perhaps crushed rock sand can be better than some very smooth natural materials in respect to surface texture, if properly processed?
So what is the specification for fine aggregate particle shape and surface texture? Er...there isn’t one, is there? ...so how can we quantify it? New Zealand Flow Cone A sample of 1 kg of fine aggregate is passed through a 12-mm orifice mounted under a sample hopper. The material free falls into a collecting container of known volume while the time taken for the sample to pass is measured. The mass of material in the collection container is measured and the un-compacted unit mass of the fine aggregate can be calculated. The specific gravity of the fine aggregate compared with the unit mass allows the un-compacted voids content of the aggregate to be calculated. The results are reported graphically on a plot of voids versus flow time.
Cement Concrete & Aggregates Australia Research Report Flow cone Manufactured Sand National test methods and specification values January 2007 But….”Research demonstrates that there are difficulties with the use of the Flow Cone apparatus in testing manufactured sands, and unless these issues were resolved, it would not be practical to use the method for specification . No other suitable procedure was found for specifying the shape and surface texture of manufactured sand that did not require specialised equipment and highly trained staff. ” 4 It seems to work well with a standard grading reconstructed from the test material, eliminating the variation due to particle size distribution
Particle Size Distribution
Particle size distribution Remains the sole criterion for sand specification in most places, apart from deleterious material exclusions. Useful for defining consistency Has less relevance in defining performance, because compliant PSD is no guarantee of suitability for concrete 100 BEST 90 80 CUMULATIVE % PASSING 70 60 50 WORST 40 30 20 10 0 0.01 0.1 1 10 PARTICLE SIZE (mm) Some real gradings and their Ryall Clearwell Clearwell Clearwell ASTM C33 ASTM C33 performance in concrete
Cement Concrete & Aggregates Australia Research Report Particle size distribution Manufactured Sand National test methods and specification values January 2007 “Members agreed that product grading should not be a specification test. Instead, the product supplier should provide a submitted grading to which the deviation limits current in AS 2758.1 would apply. How these deviation limits might be applied to the broad range of possible product gradings will require careful consideration. Gradings would obviously become a quality control tool with results of interest to individual suppliers and their customers. However, it was considered necessary for the purposes of definition to specify that 'manufactured sands' for use in concrete would have: • between 90% and 100% passing 4.75-mm sieve; • between 15% and 80% passing 0.6-mm sieve; • between 0% and 20% passing 75 micron sieve.” 4 THE KEY HERE IS NOT AN ABSOLUTE GRADATION, BUT CONSISTENCY!
Particle size distribution – Is there an ideal grading? 100 CCAA Proposal: 90 AS SIEVE MAX 80 DEVIATION CUMULATIVE % PASSING 70 6.7 - 60 4.75 ±5 50 2.36 ±10 40 30 1.18 ±15 20 0.6 ±15 10 0.3 ±10 0 0.01 0.1 1 10 0.15 ±5 PARTICLE SIZE (mm) 0.075 ±3 Ryall Clearwell Clearwell Clearwell CCAA CCAA An ideal grading is one that makes good concrete and does not vary However, there is usually a preference for abundant particles in in the range 250-1000µm
Microfines
Microfines (<63µm) For twenty years or more, researchers have been busy proving that microfines are a valuable constituent in concrete, while end users have not been prepared to deviate far from specification-based sand gradings designed for natural sands. ICAR102-1F: “Compared to concrete made with lower -fines content natural sand, high-fines manufactured sand concrete generally had higher flexural strength, improved abrasion resistance, higher unit weight, and lower permeability due to filling of the pores with microfines. (Lower permeability is important for reducing corrosion). Compressive strength varied but was acceptable, and shrinkage, although slightly higher, was within generally acceptable ranges. Good-quality concrete could be made from nearly all of the aggregates (with microfines contents ranging from 7 to 18%) used in the test program without the use of admixtures.” 5
Recommend
More recommend
