Measuring Relative Humidity & Strain in Concrete

Use of RH sensors and concrete embedment strain gages

By: Zach Grasley

Outline• Introduction to RH

measurement in concrete• Importance of RH in concrete• Testing methods for measuring

RH in concrete• Introduction to G-Cap sensor• How to calibrate and use the

G-Cap sensor• Advantages/Disadvantages to

G-Cap• Strain measurement in

concrete with embedment gage

• Application of embedment gage

• Conclusions

Introduction

RH sensors and probes measure the moisture content in concrete pores, dependent on internal temperature

RH=100 * (Vapor pressure)/(Saturated Vapor Pressure)

•Capacitance

•Resistance or conductance

•Weight loss

•Microwave propagation

Moisture in Concrete

Why test for RH?• Predict and characterize

shrinkage• Estimate degree of

hydration• Alkili-silica reactivity &

sulfate attack• Corrosion of steel

reinforcement• Freeze/thaw susceptibility

Methods for Testing RH1. Core sample (weight loss)

-destructive, can be time consuming2. Probe (resistance)

-time consuming, temp effects, condensation

3. Embedded electrodes (conductance)

4. Permanent sensor installation(capacitance)-inexpensive, not reusable, monitored and recorded by data acquisition, quality of data?

RH Measurement Devices

G-Cap Sensor

• Two permanent placement methods

-Direct embedment (perhaps with protective cover)

-In pvc tube embedded in concrete

How does G-Cap work?

• Pieces of the puzzle

Multiplexer and DataloggerG-Cap Sensors and Circuit Board

How does G-Cap work?

1. Data logger provides 12V external excitation.

2. Sensor measures capacitance, which is relative to RH.

3. Circuit converts output to volts, and sends to data logger.

4. Multiplexer allows multiple sensors to be connected to one circuit.

5. Data logger collects and stores voltage readings.

Technical

Calibration of G-Cap• Solutions of certain salts create known relative humidity values

in the air above them.• Plot voltage measured from G-Cap versus each of the known

humidities to create a calibration curve.• Test interchangeability by placing each sensor in each salt

solution test and comparing the results.

Salt/Name Formula %RH @ 10° C %RH @ 20° C %RH @ 30° C

Desiccant/Dririte CaSO4 0-1 0-1 0-1

Lithium Chloride LiCl 11.3 11.3 11.3

Magnesium Chloride MgCl2 33.4 33.1 32.5

Magnesium Nitrate Mg(NO3)2 57.4 54.4 51.4

Sodium Chloride NaCl 75.7 75.5 75.1

Potassium Sulfate K2SO4 98.2 97.6 97

Advantages of G-Cap

• Interchangeability within 3%-Allows use of multiplexer-Single calibration curve

• Survives 100% RH• Start measuring RH

immediately after casting

• Will the alkiline nature of concrete damage the sensor or skew results?

• Will the sensor measure correct capacitance when in contact with concrete, or will it need to be encased?

Questions regarding G-Cap

Measuring Strain in Concrete

• Embeddable gages

Geokon embedment gages

Geokon “sister” rebar strain meter

Gage Technique InternationalVibrating wire rebar strain gage

Measuring Strain in Concrete

• Micro-Measurements EGP-Series concrete embedment gage– K-alloy strain gage– Mounted on a carrier

and cast into proprietary polymer/cement composite material

– 3 wire configuration– Inexpensive compared

to alternatives

Application of Embedment Gage

General Conclusions•G-Cap sensor could be a valuable tool in measuring & characterizing relative humidity in concrete•Testing should be undertaken to validate the G-Cap sensor

•Embedment gage is the most convenient way to measure internal strain in concrete•Tests should be done to determine level of accuracy in comparison to foil gages

References

• General Eastern– http://www.geinet.com/html/humGCAPspecs.htm

• A. Loukili, A. Khelidj, and P. Richard, Cement and Conc. Research, 577-584 (1999).

• G. Hedenblad “Measurement of Moisture in High Performance Concrete”, Lund Inst. of Tech.

• M. Brownawell, “Interchangeability: Key to Choosing RH Sensors”, SENSORS Magazine, April 1995.

• Geokon– www.geokon.com

• Gage Technique International- www.gage-technique.demon.co.uk/