Test and Measurement 2019 Conference and Workshop

Misty Hills Conference Hotel

16 September2019

Rapid Determination of Moisture Content using Microwave Ovens

Session M304

1

Content

Why measure moisture content

Methods of measuring moisture content

Current practice

Alternate practice

Experimental work and findings

Recommendations

2

Why measure moisture content?

Critical parameter in construction materials

Asphalt production

Concrete

Granular layer compaction

Focus on asphalt production

Weather elements

Stockpiling practice

Hot mix or Warm mix Asphalt

3

Moisture content effect in Asphalt Production

Rate of production

Aggregate cold-feed behaviour

Asphalt proportion settings

Quality of asphalt

Binder content

Residual moisture

Binder adhesion

4

Measuring of Moisture content – current practice

SANS 3001: GR20:2010 – use of

conventional oven

setting of 105-1100C

constant mass weighed cool

Restrictions

Typical site labs have few

conventional ovens

Multi-use of these few ovens

Heat loss during multi-tasks

Time consuming 5

Measuring of Moisture content – alternate practice

Use of hot plates

Advantages

Smaller footprint in lab

Relatively cheaper than conventional ovens

Quicker than conventional ovens

Maintains temperature

Disadvantages

Require attention

Localised heating of samples

Discard aggregate sample, no reuse

Not suitable for wet fine aggregate

Need heating prior to use – high power consumption6

Measuring of Moisture content – alternate practice

Use of microwave ovens

Advantages

Relatively cheap

Even temperature distribution

Quickest of the three drying methods

Instantaneous, no preheating required

Doesn’t need attention

No material splashes

Disadvantages

Not for aggregate with metal traces

Discard sample, no re-use

Overheated minerals – correction factor

7

Experimental Work

Select aggregate of different densities and fraction

Oven dried at 60OC

Grading treated as received

Add required moisture content

Determine rate of dying using different equipment

weigh samples hot at time intervals

Gra

nit

e

Meta

-

quart

zit

e

Fels

ite

Tillite

Gre

yw

acke

Reef

quart

zit

e

Dolo

mit

e

Andesi

te

Dole

rite

Bulk Relative Density 2.65 2.66 2.67 2.70 2.72 2.77 2.86 2.94 2.97

Water absorption 0.5 0.3 0.3 0.9 0.4 0.4 0.4 0.5 0.5

8

Experimental Work

Experiment done on aggregate and slurry only

No asphalt specimens tested as

unavailable plant-plant produced asphalt with residual

moisture at time of experiment

aggregate pre-saturation with vacuum pressure not yielding

residual moisture content sufficient thorough comparison of

drying rate

residual moisture simulation in laboratory mixed asphalt not

practical due to mixing temperatures

fine aggregate slurry used instead

9

Experimental Work Sources of experimental error – managed reasonably

use one container for each specimen’s test

use same size container for all samples

aggregate density and volume

depth/surface area ratio

use calibrated balances, verify before every use

technicians to follow same protocol

use stop-watch instead of microwave oven timer

minimize opening of conventional oven

multiple specimens in conventional oven at same time, not

same effect for singular specimens in microwave and hot

plate

weigh specimens hot to minimize latent heat effect

10

Experimental findings

conventional oven heat distribution 11

Experimental findings

hot plate heat distribution12

Experimental findings

microwave oven heat distribution13

Experimental findings – coarse aggregate

14

Experimental Finding – coarse aggregate Denser aggregate heats up slowly initially

Denser aggregate have greater latent heat which aids

drying rate

Lighter aggregate heated fastest but rate slows down due

to lower latent heat

Microwave ovens dried aggregate fastest, followed by

hotplates.

15

Experimental findings – fine aggregate

16

Experimental Finding – fine aggregate Drying rate affected by gradation and surface exposed to

heat source

Microwave ovens dried aggregate fastest, followed by

hotplate.

17

Experimental findings – fine slurry

Slurry dried with Conventional Oven Slurry dried with Microwave Oven

18

Experimental Findings – fine slurry Emulsion content same for dolerite and dolerite

High emulsion content for quartzite due to grading

Microwave oven dried slurry fastest

Latent heat in denser material effect

19

Conclusions and Recommendations Microwave oven offer

Quickest drying process

use microwave grill to dry multiple samples at a time

Increase personnel efficiency

Cost savings on equipment

Cost savings on power usage

Different microwave oven capacities

Run correlations with conventional ovens

20

Thank you for your attention

21