thermal expansion coefficient

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Thermal Coefficient of Portland CementConcrete

What is it?

All materials expand and contract to some extent as theirtemperatures rise or fall. The coefficient of thermal expansion(CTE) is a measure of a material's expansion or contraction withtemperature. Because the length changes associated with thermalexpansion are very small, the CTE is usually expressed inmicrostrains per unit temperature change.

The CTE of Portland cement concrete (PCC) ranges from about 8to 12 microstrains/C. The range of CTE values for differentconcretes reflects the variation in CTE of concrete's componentmaterials. For example, concrete containing limestone aggregatehas a lower CTE than concrete containing siliceous aggregate.Because aggregate comprises about 70% of the concrete,

aggregate type has the greatest effect on the CTE of concrete.The CTE of hardened cement paste, which is a function of factorssuch as w/c ratio, cement fineness, cement composition, and age,also affects the CTE of concrete.

Why is it important?

The CTE is one of the factors to be considered in the design ofPCC pavements. During pavement design, this variable isnormally represented as an average value rather than a mix-specific one, even though it may vary significantly depending onfactors such as type of aggregate used in a mix. Using an averagevalue may therefore lead to erroneous assumptions about thepavement's thermal response and possible distress. For example,one of the keys to characterizing the effects of thermal propertieson a concrete pavement's structure is to account for thermalmovements. Accurate values of the CTE are needed to predictpotential thermally-induced movements in a concrete pavement.

What is the PCCP Team's role?

The PCCP Team has developed equipment and a standard testmethod (adopted by AASHTO as TP60-00) for determining theCTE of PCC. The test method determines the CTE of a cylindricalconcrete specimen, maintained in a saturated condition, bymeasuring the length change of the specimen over a specifiedtemperature range (10C to 50C). A picture of the test apparatusis shown below. Length changes are measured using an LVDT.Corrections are made for the expansion or contraction of the testframe.

The CTE is calculated according to the following formula:

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CTE = (L/L0) / T

where L = length change of specimen, L0 = initial measuredlength of specimen, and T = temperature change.

The PCCP Team has been using this test method to measureCTEs for over 2000 cores from across the country collected aspart of the Long Term Pavement Performance (LTPP) program.

These data will become part of the LTPP database and will beused to investigate the impact of CTE on the response andperformance of pavements.

Why is this significant?

The CTE is an important factor in optimizing concrete joint design,calculating stresses, joint sealant design, and selecting sealantmaterials. The results of the analyses will help engineers modifythe procedures for PCC pavement design to more accuratelypredict the impact of mix-specific CTEs on pavement behavior.The modified design procedures will result in pavement designsthat are more compatible with the environmental conditions at aparticular paving site. Ultimately, the use of the modified design

procedures will result in improved pavement performance.

The new (2002 Guide) pavement design procedures underdevelopment through NCHRP 1-37A consider the effects ofthermal expansion and contraction. The CTE research completedby the PCCP Team has made this possible by providing astandard test method that agencies can use to determine CTE fortheir concrete mixes and CTE data for the LTPP test sections. TheLTPP CTE data are being used in the development of the 2002Guide, and may also be used by future users of the guide toestimate appropriate CTE input values when material-specific dataare not available. Based on draft preliminary Information on PCCCTE Input in the 2002 Guide, determination of CTE for the

different levels as defined in Guide are as follows:

Level 1 of the CTE determination involves direct measurement ofthe change in length of laboratory specimens subjected tochanges in temperatures, using AASHTO TP60, "Standard TestMethod for CTE of Hydraulic Cement Concrete."

Level 2 of CTE determination uses a weighted average of theconstituent values based on the relative volumes of theconstituents. The table below provides typical ranges of a forvarious common PCC mix components.

Typical ranges for common PCC components.

Coefficient of Thermal Expansion

10-6/C 10-6/F

Aggregate

Granite 7-9 4-5

Basalt 6-8 3.3-4.4

Limestone 6 3.3

Dolomite 7-10 4-5.5

Sandstone 11-12 6.1-6.7

Quartzite 11-13 6.1-7.2

Marble 4-7 2.2-4

Cement Paste(saturated)


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w/c = 0.4 18-20 10-11

w/c = 0.5 18-20 10-11

w/c = 0.6 18-20 10-11

Concrete 7.4-13 4.1-7.3

Steel 11-12 6.1-6.7

Level 3 of CTE estimation is based on historical data. The greatestpotential for error is associated with this option, because PCCmaterials vary considerably. Realistic data for the types ofmaterials being used in concrete mixtures are rarely available and,if they are available, they are likely to be based on a specific PCCmix design or aggregate type. However, an agency could testtypical mixes containing a range of aggregate types to obtaintypical values for their materials.

Updated: 04/07/2011

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