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StuccoMetrics®

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Jeff Bowlsby CCS, CCCA

Exterior Wall and Stucco Consultant

Licensed California Architect

 

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Stucco Material Properties

 

 

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Executive Summary

Context

Stucco Shrinkage Movement

Stucco Thermal Contraction/Expansion Movement

Observations

Discussion

Stucco Shrinkage Movement

Stucco Thermal Contraction/Expansion

Stucco Shrinkage Movement and Thermal Movement Combined

Conclusions

Suggestions

 

Executive Summary

 

Material properties for stucco and stucco-related materials are indicated

 

Stucco shrinkage and thermal movements are presented that are not typically available from other resources without significant research.  Shrinkage and thermal movements are inherent material properties of portland-cement based plaster and stucco that are not well grasped in the industry.  Shrinkage and thermal movements if accommodated can be managed.  Shrinkage and thermal movements if not accommodated can cause perimeter stucco panel edge gaps and stucco cracks that may be unsightly, may allow bulk water intrusion into the stucco cladding assembly and should be addressed with any competent stucco cladding assembly design.

 

Visit the StuccoMetrics Reference Archives webpage for cited references and further information.

 

Context

 

Stucco Shrinkage Movement

 

In 1921, the US Bureau of Standards, published Shrinkage of Portland Cement Mortars, and its Importance in Stucco Construction, which documented an extensive battery of testing on stucco and its components because it recognized the potential and importance of stucco to the US economy.  One series of the testing evaluated 200 prepared specimens of portland cement stucco mortar with precision equipment to evaluate the shrinkage characteristics of various stucco mortar mix designs. 

 

 

1921 US Bureau of Standards stucco shrinkage testing apparatus

 

The shrinkage characteristics of the stucco mortar specimens tested included materials and mix designs comparable to today’s stucco.  The conclusion was that a common 1:4 (cement:sand) portland cement-based stucco mortar experienced shrinkage of up to 0.14%.  The total shrinkage occurred at approximately 256 days after placement.  At the time when the stucco finish coat is typically installed approximately 70% of the total shrinkage had occurred, and after 28 days approximately 75% of the total shrinkage had occurred.

 

The total shrinkage dimensional value after 256 days was approximated 168 mils (0.168 in.) of length change over 10 feet of stucco.  At the time, stucco shrinkage was considered to be of minor concern related to cracking, and it was concluded that with adequate moist curing and control of absorption by the substrate, that stucco shrinkage and shrinkage-related cracking could be minimized.

 

 

1921 US Bureau of Standards stucco shrinkage test results

 

 

In 1940 the test method was further developed for measuring portland cement mortar shrinkage, and became the test standard we know today as ASTM C157 Standard Test Method for Length Change of Hardened-Hydraulic Cement Mortar and Concrete.  ASTM C157 is the current industry testing standard for this purpose and it is an effective method to isolate and evaluate the shrinkage characteristic of an isolated portland cement-based mortar specimen.  The laboratory test method creates small specimens of portland cement stucco mortar in a temperature and humidity controlled environment to isolate shrinkage behavior, cures them by immersion in a lime bath and precisely measures their length change at intervals typically up to 28 days after the portland cement hydration process is initiated.

 

 

 

ASTM C157 Shrinkage testing apparatus with specimen

 

One of several documented actual stucco installations that experienced stucco shrinkage was a major stucco installation at the Grand Coulee Dam construction project in 1947 as documented in Crack Control in Portland Cement Plaster Panels, Journal of the American Concrete Institute.  Stucco ceilings on suspended metal lath experienced significant and unacceptable cracking where the ceiling lath was continued onto and attached to the adjacent concrete walls, the standard lath installation practice at the time.  Extensive investigation and testing was performed by the Bureau of Reclamation to determine the cause of the cracks and to develop a remedy.  A primary conclusion was that a stucco assembly including the lath at ceilings on suspended grillage, needs to be isolated (discontinuous and unrestrained) from the adjacent perimeter concrete walls to allow for cement-based plaster shrinkage movement to occur, and to minimize stucco cracking resulting from shrinkage movement.  The shrinkage dimensional movement was field measured as 3/4 in. over a ceiling surface that was 52 ft in length, which equated to 0.120% field-measured shrinkage movement, or 144 mils (0.144 in.) over 10 feet.

 

The study also evaluated the traditional portland cement industry milestone of 28 days as an indicator of final characteristics, but at 28 days only ~50% of the total shrinkage had occurred.  Finish coats are often already installed by 28 days and may be subject to additional base coat shrinkage before the base coats are fully cured.  Most of the shrinkage had occurred after approximately 75 days, so according to this study a more useful milestone for determining total stucco shrinkage is 75 days or longer.  Shrinkage occurring after 75 days is measureable and should be considered, but it is incidental and can be accommodated by applying a safety factor to the 75 day shrinkage amount.

 

 

1947 Grand Coulee Dam  - Stucco shrinkage rate and time duration

 

A few contemporary stucco product manufacturers publish ASTM C157 shrinkage data for their products, but it is not a common practice.  Below is one graph of data from one manufacturer indicating approximately 0.12% shrinkage for generic Type II portland cement and lime stucco mixture and approaches half that amount using other portland cement-based stucco product variations.  Note that the shrinkage indicated here is only through 28 days, no indication of total shrinkage is given, which is critical information.

 

 

2008 Manufacturer-published stucco shrinkage data

 

In a 2010 private study, Performance Impact of Various Fiber Additions in ASTM C926 Plaster Basecoat, was performed to evaluate the effects of fiber additives regarding stucco shrinkage.   The laboratory testing performed was extensive and included a range of commercially available fiber additive materials useful for stucco, towards minimizing stucco cracking.  The testing revealed a range of effects on the performance characteristics of stucco with fiber additives, but overall the contributory effects of fiber additives regarding stucco shrinkage movement was found to be negligible.  Fiber additives are understood in the industry to more uniformly distribute stucco shrinkage stresses within the stucco membrane and to reduce the visibility of cracking in general, but have negligible effect on actual stucco shrinkage.

 

Stucco Thermal Contraction/Expansion Movement

 

As is the common behavior of nearly all materials, in service ambient cyclical temperature changes cause installed and cured stucco to expand and contract.  Linear thermal expansion coefficient values used to determine the expansion and contraction dimensional values for portland cement-based stucco are published by a few different sources, summarized in Cement Plaster Metrics:  Quantifying Stucco Shrinkage and Other Movements; Crack Acceptability Criteria for Evaluating Stucco published by RCI.  The corresponding range of dimensional movement is calculated to vary over a range from approximately 48-84 mils (0.048-0.084 in.) over 10 lineal feet, based on a change in ambient temperature of 100oF which can occur after the stucco is cured and in service.

 

Observations

 

Stucco cracking, especially at wall opening corners, has been a known stucco performance behavior and concern since the use of stucco on framed buildings began over a century ago.  Stucco shrinkage movement rates and dimensional values have been known for nearly a century and the basic relationship between stucco shrinkage and stucco cracking has been understood since at least the late 1940’s.

 

Stucco shrinkage and thermal movement data is available from a few different sources that are generally corroborative.

 

Stucco shrinkage movement accounts for the majority (approximately 60-80%) of the total combined stucco dimensional movement attributed to stucco shrinkage movement and thermal movement.  Stucco thermal movement after curing, while in service, is a comparatively smaller movement, but is still significant.

 

Stucco shrinkage is not complete when finish coats are typically applied, and continues for nearly a year.

 

Many stucco product manufacturers do not currently publish ASTM C157 shrinkage data for their portland cement-based stucco products.

 

Discussion

 

All portland cement-based plaster, and stucco, experience shrinkage movement and thermal expansion and contraction movement.  Stucco cracks and perimeter gaps are the manifestation of these characteristics.

 

Stucco Shrinkage Movement

 

Marketing hype for stucco products such as ‘Our stucco has low shrinkage’…or… ‘use our enhanced stucco admixture for minimum shrinkage cracking’, are conceptual, anecdotal and effectively meaningless without factually supportive and comparative data.  How much is ‘low’ shrinkage…compared to what?  What shrinkage dimensional value actually occurs with a given stucco material so that stucco shrinkage movement can be accommodated in stucco design and installation?  Stucco products available on the market today that affect stucco shrinkage movement include not only portland cement-based products, but also admixtures and aggregates.

 

Stucco shrinkage movement gets the lion’s share of the attention, in a negative way, for virtually all stucco cracking that occurs, yet do we really understand actual stucco shrinkage characteristics and behaviors, and how to accommodate them in stucco systems, assemblies and subassemblies towards minimizing cracking?  Buildings move in winds and seismic events, beams and floor slabs deflect under dead and live loadings – each of these physical movement conditions can be rationally observed, evaluated, the behaviors are predictable, and solutions can be derived and implemented to accommodate them.  How about for stucco shrinkage movement?  What is the dimensional value of stucco shrinkage movement occurring after placement?  If we understand stucco shrinkage characteristics and behaviors, we should be able to do something about it.

 

Stucco shrinkage movement occurs during the initial curing process and is irreversible – no amount of thermal expansion movement while in service after curing compensates for stucco shrinkage movement.  Many factors impact the specific rate, magnitude and visible effects of stucco shrinkage of a particular stucco system.  Factors such as portland cement type and content, aggregate characteristics and contamination, admixtures, cement:aggregate ratio, cement:water ratio, workmanship, finish coat characteristics and other factors are essential considerations to understand before we can fully assess and address the effects and impacts of stucco shrinkage movement of a particular stucco system.

 

The simple fact is that stucco shrinks during initial curing, an irreversible, inherent natural process, and the force of greatest magnitude to which stucco wall cladding must contend.  Cured stucco contracts during cold ambient temperatures, effectively shrinking even more.  Cured stucco expands during hot ambient temperatures, but the expansion does not compensate to make up for the shrinkage it initially experiences.  Without provisions that address and minimize these movements, stucco responds by cracking.   These movements must be accommodated in some way by using one or more of various methods in combination - stucco mortar mix design and curing provisions, admixtures and additives, panelization with shrinkage movement joint subassemblies, crack isolation base coats and finish coatings that perform well enough to cover and conceal the cracks.

 

ASTM C157 is a useful laboratory test protocol for evaluating shrinkage rates of stucco mortar specimens.  ASTM C157 provides important benchmark information but other factors need to be considered to effectively understand and interpret the testing protocol results.  From observations of previous stucco shrinkage testing and documented actual stucco installations, we know that stucco continues to shrink for nearly a year after placement and that at 28 days, the standard ASTM C157 test time period, approximately only 75% of the total stucco shrinkage has occurred.  Stucco will be in service much longer than 28 days on most buildings, so a 28 day value is only useful if compared to the total shrinkage that will occur over time, and if the future shrinkage behavior relationship can be predicted.  The lime bath curing of stucco mortar specimens in a temperature and humidity controlled environment does not accurately emulate field-installed stucco curing conditions. The test method documentation indicates that field cast specimens may experience up to twice the shrinkage movement as lab cured specimens, an important factor that should be considered.

 

Laboratory testing of mortar-only specimens is useful information but the shrinkage performance of an actual field installed stucco system is impacted by additional factors that demand consideration.  Lath embedded into stucco may restrain stucco shrinkage movement, but ASTM C157 does not account for the effects of embedded lath.  Variables in lath type related performance and lath fastening conditions as they relate to stucco shrinkage may impact the effective shrinkage rates of a given stucco system, but ASTM C157 does not account for the variable effects of embedded lath types.  Admixtures that purport to effect stucco shrinkage can also be evaluated with the ASTM C157 testing protocol.  The contributory effects of these additional factors are not quantified at this time and require additional testing and evaluation to discern.

 

Aggregate contamination by clays and organic matter can be a contributory source effecting stucco shrinkage.  Aggregates are often locally sourced and of variable qualities and purity, especially for site mixed generic, non-proprietary stucco mortar.  ASTM D2419  Standard Test Method for Sand Equivalent Value of Soils and Fine Aggregate is the standard test protocol useful to determine aggregate contamination levels.  Mineral aggregate itself does not shrink, but residual clays and organic matter contamination in the aggregate can be a stucco shrinkage concern.

 

No simple field test currently exists to evaluate a stucco mixture during or immediately before stucco installation, for potential stucco shrinkage movement.  Stucco product manufacturers in general do not publish shrinkage testing or thermal movement data for their products.  This lack of information makes it challenging to design or install stucco to accommodate shrinkage and thermal movements with certainty.

 

Moist curing, portland cement content, aggregate characteristics, fiber additives, admixtures, SMJS subassemblies, workmanship, finish coat characteristics and other factors are essential considerations to address the effects and impacts of stucco shrinkage movement on a particular stucco system.

 

When the stucco system installation is complete, stucco shrinkage movement is not yet complete and continues for nearly a year.  As the shrinkage process continues, combined with thermal movements while in service, cracking can continue to develop.  The characteristics and performance of the stucco finish coat is the final mechanism for accommodating cracking over the long term, after everything else is in place.  Finish coat texture, color, elastic properties, and thickness are important considerations for minimizing the visibility of stucco cracking during its service life, over the test of time.

 

Shrinkage reducing admixtures (SRA) have been on the market and useful in structural concrete for bridges and paving for many years.  Common SRA’s include chemical solutions or fly ash.  SRA can easily be adapted to portland cement-based plaster and they are not expensive.  SRA should be considered in the design of stucco mixtures for minimizing stucco shrinkage, but this approach has not migrated into stucco practice and requires more research and testing before it can be applied to stucco.

 

Stucco Thermal Contraction/Expansion

 

Second only as a stucco crack-causing villain to stucco shrinkage, what stucco thermal movement dimensional value occurs while in service?   How much stucco cladding thermal movement occurs from a hot summer day to a cold winter night?

 

Stucco system thermal contraction and expansion movements are behavioral reactions to thermal temperature cycling after curing, while in-service.  Many factors affect the actual dimensional magnitude of contraction/expansion movements on a particular stucco system.

 

With stucco, whatever thermal expansion amount that may occur cannot regain the initial shrinkage amount experienced during curing. 

 

Stucco Shrinkage Movement and Thermal Movement Combined

 

The following diagram indicates an idealized, generalized stucco behavior pattern, based on a series of assumptions of typical conditions, for the combined behavior of stucco shrinkage and thermal movement over a 10 foot length of stucco.  Note that it illustrates:

 

·                  An initial portland cement shrinkage dimension that cannot be regained, resulting in a stucco panel edge gap

 

·                  Once cured and while in service, in higher temperatures, the cured stucco expands, reducing the stucco panel edge gap

 

·                  Once cured and while in service, in lower temperatures, the cured stucco contracts, enlarging the stucco panel edge gap.

 

·                  Of the total movement, shrinkage accounts for ~80%, thermal contraction ~20%

 

 

For design purposes, stucco shrinkage and thermal contraction/expansion movements combined are understood to be in the general range of approximately 130-240 mils per 10 lineal feet of stucco length as reported by RCI in Cement Plaster Metrics:  Quantifying Stucco Shrinkage and Other Movements; Crack Acceptability Criteria for Evaluating Stucco.

 

Conclusions

 

The stucco design and construction communities require complete performance data regarding stucco shrinkage and thermal movement behaviors for the products it specifies and installs, to better accommodate stucco shrinkage movement and thermal movement characteristics, and to more effectively minimize stucco cracking.

 

An ASTM C157 test value at 28 days suggests a stucco shrinkage rate, but does not represent the total stucco shrinkage that occurs after approximately a year, which is the more useful information.  Lime bath specimen curing is not an accurate representation of actual field installations.  More valuable stucco shrinkage performance information would be obtained by using field cured specimens.

 

Stucco product manufacturers are encouraged to perform and publish ASTM C157 and D2419 shrinkage and thermal movement related data for their stucco mortar and related products.  Consideration should be given to using field-cured specimens, that include the various available embedded lath materials and admixtures and any other factors that may contribute to stucco shrinkage movement. Consideration should be given to extending the testing protocol to at least a 12 month long duration to evaluate actual stucco shrinkage and thermal movement characteristics of stucco materials and systems to be useful in the design and installation of stucco, and to minimize stucco cracking.

 

SRA products should be further evaluated and considered for use in stucco to reduce stucco shrinkage movement and minimize stucco cracking.

 

Suggestions

 

Stucco shrinkage (during curing), design value:

·                 100-200 mils (0.10-0.20 in.) per 10 lineal feet

 

Stucco shrinkage and thermal movement combined, design value:

·                 130-240 mils (0.13-0.24 in.) per 10 lineal feet

 

Coefficients of Lineal Thermal Expansion (CLE) (comparison):

 

Material

CLE

(10-6 in/in oF)

D length over 10 ft

over D(t)  70oF

Stucco

6.5

55 mils

0.055 in.

Steel

7.3

61 mils

0.061 in.

Stainless Steel (316)

8.9

75 mils

0.075 in.

Copper

9.3

78 mils

0.078 in.

Stainless Steel (304)

9.6

80 mils

0.080 in.

Aluminum

12.3

103 mils

0.103 in.

Zinc

16.5

139 mils

0.139 in.

PVC

28

235 mils

0.235 in.

EPS, XPS, and Polyisocyanurate  foam insulation

35

296 mils

0.296 in.

 

 

Stucco Best Practice:  Manufacturers of portland cement-based stucco materials are encouraged to perform and publish applicable ASTM C157 and D2419 engineering technical data for their materials. The data should include characteristics for stucco shrinkage and thermal movement as used in standard stucco mixtures, to provide meaningful and useful information for the effective design and installation of stucco wall cladding systems, and to minimize stucco cracking.  Also provide information regarding the following additional factors to evaluate their effects: 

 

·                Use field cured stucco specimens

 

·                Test all stucco materials that have an impact on stucco shrinkage, such as portland cement-based materials, admixtures and aggregates, and in their various common combinations

 

·                Use stucco specimens with embedded laths to understand their composite behavior

 

·                Extend the shrinkage test protocol to a 12 month time duration and provide a shrinkage vs time informational graph

 

 

 

Consultation with licensed and experienced stucco professionals is recommended for stucco-related endeavors.  No liability is accepted for any reason or circumstance, specifically including personal or professional negligence, consequential damages or third party claims, based on any legal theory, from the use, misuse or reliance upon information presented or in any way connected with StuccoMetrics.com.

 


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