Alternative Methods for Sand Quality Assessment for Lower Carbon Concrete
Sunday, 14 January 2024
Carbon dioxide emissions from concrete production are mostly due to cementitious materials and often contribute over 80% of the total emissions despite representing less than 20% of its volume.

James Mackechnie - South Island Technical Manager/Plant Engineer, Allied Concrete
Optimisation of aggregates that form the bulk of concrete can have a significant impact on emissions, especially when dealing with poorer resources. Good quality fine aggregate lowers water demand, improves workability and ultimately increases cementing
efficiency of concrete mixes.
Fine aggregates in concrete consists of either natural or manufactured sand or a blend of these materials. Methods of assessing the quality of fine aggregate rely on the following laboratory techniques:
- Moisture content of sand since this varies in stockpiles and measurement is important to produce consistent concrete properties.
- Sieve analysis to determine particle size distribution with median size expressed as fineness modulus (FM).
- Presence of deleterious ultra-fines such as silt or clay or other contaminants such as lightweight, micaceous or organic material is done periodically.
- Average shape and texture of particles, which while strongly affecting water demand of fresh concrete are not often assessed.
Some of these techniques have been standardized and a summary of the main methods of analysis are shown in Figure 1. Most concrete production plants have only basic laboratory resources such that regular testing is often limited to sieve analysis and
moisture content measurement. The development of alternative methods is also considered and reviewed.

Figure 1: Techniques for assessing the quality and properties of sand.
MOISTURE CONTENT Measurement of moisture in fine aggregates is important to be able to batch concrete accurately and produce predictable performance of fresh and hardened concrete. Moisture is measured either manually by sampling
sand before or during production, using either laboratory drying or hand-held moisture probes, or it is done automatically using installed moisture probes. Correctly calibrated moisture probes allow faster batching, better automation and improves
cement efficiency of concrete. Control of sand moisture content coming from quarries is still important however since:
- Dry sand can segregate in stockpiles causing rapid slump loss of fresh concrete due to absorption.
- Very wet sand tends to clog around moisture probes causing misreading of the true moisture content or is outside the calibrated range.
SIEVE ANALYSIS Measuring particle size distribution can be done in the laboratory using sieve analysis of the dried fine aggregate. Grading of sand is the most important quality indicator since variations affect both fresh and hardened
properties of concrete. Testing requires sample preparation together with expertise and equipment for reliable testing, which means measurement is rarely done more than once a day or sometimes only weekly. Fineness modulus (FM) is a useful control
parameter for sand but is a relatively poor basis of comparison between different sources as shown in Figure 2 for a range of natural sands with similar FM.

Figure 2: Grading of natural sands with similar fineness modulus.
Digital analysis of particle size distribution of sands has been developed but these are still limited to isolated samples rather than continuous monitoring during concrete production. This is because sand needs to be dried and carefully sampled before
analysis. Research comparing the grading of natural sands found that using a digital method (Sika Sand App) was less accurate than sieve analysis. This is due to inaccuracies assessing the finer fraction, which require more analytical methods such
as laser diffractometry.
PARTICLE SHAPE Particle shape of fine aggregate influences packing efficiency that affects water demand of fresh concrete and ultimately cement content. Rounder particles tend to pack better and tests such as loose bulk density or
the NZ sand flow test can assess particle shape quite accurately. A more direct approach of shape analysis is to use digital techniques, but this requires sampling and dispersal of relatively small samples and also camera resolution down to less than
a millimetre.
Figure 3 shows the results of several natural sands assessed for particle shape using the NZ sand flow test and compared with digital image analysis where the average particle sphericity was determined (Sika Sand App). This shows that improving particle
shape (i.e., higher sphericity values) had better sand flow values (i.e., lower flow time and voids content).

Figure 3: Sand flow test results versus sphericity measurements.
CLEANNESS/DELETERIOUS FINES The presence of ultra-fine particles in sand, such as silt or clay, have a significant effect on concrete properties (e.g., increased water demand, reduced bleed, delay in setting and possibly reduced
strength). Standard techniques such as sand equivalent or silt testing are relatively easy and inexpensive to perform in the laboratory. Interpretation of these tests is however complicated when using manufactured sand since some ultra-fines, such
as rock flour, have little deleterious effect on concrete properties.
The recently developed Sika PCE slurry test that assesses the influence of aggregate adsorption and deleterious micro-fines found in some natural and manufactured sands. The technique is shown schematically in Figure 4 and compares changes in mini-slump
flow readings for cementitious slurries with or without pre-soaking in a sample of the fine aggregate and with or without filtering. Comparison with the initial reference reading allows quick analysis based on guidelines developed by Sika R&D.

Figure 4: PCE slurry test methodology developed by Sika.
RECOMMENDATIONS Concrete mixes require extra cementitious material to compensate for poor quality aggregates, especially when sand quality is compromised. Figure 5 shows how this quality assurance can be undertaken when monitoring
natural and manufactured sands used in concrete production. This outlines a hierarchy of testing from regular production control tests to specialist techniques.

Figure 5: Recommendation for an integrated assessment of fine aggregates.
Optimisation of materials can be achieved using simple quality assurance techniques that measure material inputs that relate to concrete performance. New techniques have been developed that may allow more rapid assessment of sand quality and help improve
material efficiencies in practice. This should allow savings in terms of material costs and embodied energy and carbon content of structural concrete.
Article based on James Mackechnie’s 2023 Concrete NZ conference paper Sand Quality Contribution In Producing Low Carbon Concrete, which won the Sandy Cormack Best paper Award.
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