Concrete Compressive Strength, fc

1
Preliminary Findings of Temperature Effects on
Concrete and the Use of TGA as a Post-fire
Analysis Tool
Kacie Caple Graduate Student, Clemson
University 200 Patton Hall, Blacksburg, VA
24061 kaciec_at_vt.edu Scott Schiff, Ph.D.
Professor, Clemson University 110 Lowry Hall,
Clemson, SC 29630 schiffs_at_clemson.edu Patrick
Fortney, Ph.D. Professor, Clemson University
110 Lowry Hall, Clemson, SC 29630
pfortne_at_clemson.edu Prasad Rangaraju, Ph.D.
Professor, Clemson University 110 Lowry Hall,
Clemson, SC 29630 prangar_at_clemson.edu

• Objectives
• To determine material property changes in
  concrete due to high temperature exposure using
  standard 6 x 12 concrete cylinder specimens.
• To evaluate TGA as a viable option to analyze
  concrete after exposure to high temperatures in
  order to determine a maximum temperature of
  exposure.
• Significance
• Understanding the changes in concrete material
  properties will improve forensic investigations
  of heat-affected structures.
• TGA is a simple, cost-effective test, making it
  useful to forensic investigators.

Concrete Compressive Strength, fc
Validation Using Cement Paste and Mortar
With increasing temperature, fc was shown to
decrease. Duration also affected the degree of
fc losses, due to the increased penetration of
the heat through the material.

• A Heat Treatment Process in TGA machine
• Samples heated to intermediate temperature
• Temperature held constant in machine (15 min)
• Sample cooled to 60C
• Sample reheated to 800C for TGA results

B TGA Results of Heat Treatments TGA result
curves for all heat treatments using various
intermediate temperatures were collected and
compared to the TGA result curve of a control
sample that experienced no heat treatment.
(Note Similar tests were
performed on cement paste.)
D First Derivative Analysis The first derivative
of heat treatment TGA results were compared to
the first derivative of a control sample to
estimate the maximum temperature of exposure
based on an intersection of the two derivative
curves.
C Normalized TGA Results TGA result curves were
normalized so that 100 was equal to the mass
immediately following the cooling process of the
heat treatment. For control samples, 100 was
taken as the mass at the start of the TGA.
Using TGA to predict Temperature Profile in
Concrete
A simple linear trend was shown to represent the
compressive strength losses as the center of the
6x12 cylinders increased in temperature.
TGA was performed on concrete samples, extracted
at 1-inch intervals along center cross section of
concrete cylinders.
Concrete Modulus of Elasticity, Ec
Low-Temperature Drying Method
A low-temperature drying method was implemented
to reduce the amount of moisture within the
concrete cylinders to avoid explosive spalling
during the heating process. This was a concern
because the concrete was relatively young at the
time of testing. Concrete cylinders were heated
at 110C (220F) for at least 48 hours prior to
Heating Regime.
Sample extraction
Normalized TGA results of center sample and
control sample.
First derivative analysis to predict maximum
temperature of exposure of concrete sample.
Heating Regimes
With increasing temperature, Ec was shown to
decrease to a greater extent than fc. Duration
also affected the Ec losses, showing that by
increasing the duration losses increased..
Heating regimes were established to determine the
effect of maximum temperature and duration of
heat on concrete. The kiln temperature was used
to control the heating regimes. Maximum
temperatures 200, 400, 500, and
600C (392, 752, 932, and 1112F) Durations 0.5,
1, 2, and 4 hours

• Concrete Material Properties
• Compressive strength, fc, decreases with
  increasing temperature of exposure.
• Modulus of Elasticity, Ec, decreases to a greater
  extent than fc with increasing temperature.
• Linear trends can easily represent the decrease
  in fc and Ec with increasing temperature of
  exposure within 3 inches of concrete exterior.
• Thermogravimetric Analysis (TGA)
• Results indicate that TGA is a valid method for
  determining a predicted heat profile within a
  concrete sample.
• Additional Research Needs
• Additional research is encouraged to further
  validate the concrete material property changes
  with heat exposure using other heating regimes.
• Additional research is encouraged to further
  validate TGA methods and to determine the
  life-span of a heated concrete sample for use in
  TGA testing.

Typical Heat Exposure Data
A simple linear trend represents the losses in
the concrete modulus of elasticity as the center
of the 6x12 concrete cylinders increased in
temperature.
The internal temperature of the concrete
cylinders lagged behind the kiln temperature,
creating a lazy S shape as it was heated to the
maximum temperature and held constant.
Concrete Physical Changes

• Precast/Prestressed Concrete Institute
• Funding this project through a Daniel P. Jenny
  Fellowship
• Metromont Corporation
• Donating concrete for use in this project
• Tindall Corporation
• Allowing the use of their facilities for concrete
  material property testing.
• National Brick Research Center
• Allowing the use of their ovens and kilns for
  drying and heating concrete specimens.

Specimen Storage
Concrete specimens were wrapped in plastic and
vacuum-sealed in a plastic bag to preserve the
concrete for testing at a later date.
Color changes occurred with increasing
temperatures 200C No noticeable color
change occurred. 400 500C Pinkish tint
600C Light tan color. (as shown above)
Surface cracking occurred in specimens reaching a
maximum temperature on the exterior of at least
400C. Cracks increased in number and width
with increasing maximum temperatures.