RC-010 concrete design minor project info

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HIGH PERFORMANCE CONCRETE:STRENGTH, PERMEABILITY, AND SHRINKAGE CRACKING

Surendra P. ShahWalter P. Murphy Professor

Director of the Center for Advanced Cement-Based Materials Northwestern University, Department of Civil Engineering

Evanston, IL U.S.A.

W. Jason WeissAssistant Professor

Purdue University, School of Civil EngineeringWest Lafayette, IN U.S.A.

ABSTRACT

The properties that dictate how concrete will perform are not always related to materialcomposition in the same manner. This paper presents compressive strength, chloride

permeability, and restrained shrinkage cracking potential test results from five mixturesto illustrate this fact. For example, decreasing the w/c substantially improves strength,stiffness, and chloride penetration resistance; however, decreasing the w/c may increaseshrinkage and the potential for restrained shrinkage cracking. In addition, findings are

presented that indicate that admixtures may enhance durability without necessarilyincreasing strength, therefore specifications must be developed focused on the specific

performance characteristic that is desired.

INTRODUCTION

It is generally assumed that increasing the strength of concrete automatically correspondsto an increase in long-term durability, however this may not necessarily always be true.Mechanical properties, such as strength or stiffness, and durability properties, such as

permeability are a function of material porosity (both pore volume and distribution).While strength and ion penetration resistance both increase as pore size and volume arereduced, pore size reduction is accompanied by an increase in shrinkage. A wide rangeof pore sizes exist in concrete and it appears that the size of the pores may influencespecific performance characteristics differently. For example, strength and crackingresistance appear to be related to maximum pore size (flaw size) or total pore volume, 1

permeability to the capillary pore size and distribution, 2 and volumetric stability(shrinkage) to the small capillary and gel pores. 3

Over the years numerous studies have related different aspects of porosity to overall performance. Early work by Powers 4 found that the gel space ratio (gel space ratio is a

Shah, S. P., and Weiss, W. J., (2000) High Strength Concrete: Strength,Permeability, and Cracking, Proceedings of the PCI/FHWA International Symposiumon High Performance Concrete, Orlando Florida, 2000, pp. 331-340


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parameter that is related to measurable properties that describe capillary porosity) could be related to strength and permeability for a large range of w/cs and cementcompositions. In a later work, Rossler and Odler 5 found that the volume of larger poreswas related to strength, however very small pores (less than 20 nm) did not appear toinfluence the strength of the mixtures they investigated. While it has been discussed in

literature that the small pores may not have a significant influence on strength, pores inthis size range may effect on hydration rates or the mechanical properties of very high performance concrete. This may be especially true with extremely low w/c mixtures andadvanced processing conditions. More recently, studies 1 have indicated that at early-agesthe mechanical performance can be related to the maximum pore diameter, however ashydration reaches a critical limit the overall porosity appears to correspond to themechanical properties. Studies measuring the aggressive ion penetration resistanceappear to suggest that permeability (penetrability) is dictated by the volume of largercapillary pores and their spatial distribution (connectivity). Despite the seemingly simplecorrelations, these trends are further complicated in mortars (and concretes) which have adifferent pore size distribution than the paste presumably due to the development of an

interfacial zone or the development of internal porosity during mixing.One of the most common methods that can be employed to alter the internal porosity, andthereby improve performance, is to change the w/c ratio. Another method that can beused to alter the pore structure is the use of finely ground pozzolanic materials.Pozzolanic materials react to convert calcium hydroxide into additional calcium silicatehydrate. In addition, the fine size of these particles allows them to fill in the porestructure and densify the microstructure (especially in the interfacial transition zone).While reductions in w/c and use of pozzolanic materials increase strength and reduce

permeability, research has recently been preformed to demonstrate how this can lead toan increase in shrinkage and an increased potential for shrinkage cracking which wouldhave an adverse effect on overall durability. 6

While it is commonly assumed that lower w/c concrete exhibits less shrinkage thanhigher w/c mixtures, this general rule may be misleading and should be qualified. Whileit is true that concrete made using a low w/c typically exhibits lower drying shrinkage,these mixtures can exhibit a substantial increase in autogenous shrinkage, especiallyduring the first 24 hours of material development. 7,8 Autogenous shrinkage describeslength changes that are not attributed to mass loss (water loss) or temperature change.Autogenous shrinkage occurs as a result of self-desiccation (internal water consumption)in low w/c mixtures where sufficient water is not provided to complete the reaction withthe cement. Although self-desiccation was originally described over sixty years ago 9, itwas not perceived to be a problem in concrete for many years since high w/cs (>0.42)were used to insure workability. However, widespread use of water-reducing agents (andhigh-range water reducing agents) have made it possible to utilize low w/c mixtures moreroutinely and, as a result, autogenous shrinkage must be considered in these mixtures. 8

While it is important that cement-based materials exhibit volumetric stability withmoisture fluctuations, the true goal of limiting volumetric change is to reduce the


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potential for cracking. Determining if concrete will crack in service is difficult however,due to complex interaction of several factors including the materials ability to permitstress relief through creep or micro-cracking, shrinkage rate, and the rate of material

property development. In addition, factors such as specimen geometry, curingconditions, and restraint conditions further complicate predicting the potential for

cracking.10

EXPERIMENTAL PROGRAM/MATERIAL COMPOSITION

This paper summarizes recent studies that illustrate the influence of the matrixcomposition on three indicies that can be used to describe concrete performance: namelystrength, chloride penetrability, and restrained shrinkage cracking potential. This paperdiscusses the relationship between the aforementioned indicies and free shrinkage (dryingand autogenous) characteristics and illustrates the correlation between porositymeasurements and performance to explain the findings.

To investigate the influence of binder composition all mixtures used in this studymaintained a constant aggregate volume of 65% that was composed of equal proportionsof sand and 9.5 mm (3/8 inch) aggregate. Table 1 provides a summary of the material

proportions used, however further details of the mixture proportions, materials, andcasting processes are available elsewhere in literature. 11 The mixture proportions thatwill be used in this study were chosen to describe the influence of the water-to-cementratio (w/c), silica fume replacement, and a shrinkage-reducing admixture (SRA) additionon the performance of various mixtures.

Table 1 - Mixture Proportions by Weight

Binder

MixtureIdentification W

a t e r

S R A *

H i g h - R a n g e

W a t e r -

R e d u c

i n g

A d m i x t u r e

C e m e n

t

S i l i c a

F u m e *

* *

F i n e

C o a r s e

L i q u

i d / B i n d e r

W/C = 0.5 0.500 - - 1.00 - 2.00 2.00 0.50

W/C = 0.4 0.400 - - 1.00 - 1.77 1.77 0.40

W/C = 0.3 0.300 - 0.0125 1.00 - 1.52 1.52 0.30

W/C = 0.3-SRA 0.285 0.015 0.0125 1.00 - 1.52 1.52 0.30

W/B = 0.3-SF 0.300 - 0.0259 0.80 0.20 1.58 1.58 0.30

* 5% of the Total Water Was Replaced With SRA

** All Mixtures Contain 65% Aggregate by Volume

*** Equivalent Dry Weight of Silica Fume

Liquid Aggregates**


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EXPERIMENTAL PROCEDURES

In this work compressive strength and elastic modulus were determined using 100 mm x200 mm (4 in. diameter x 8 in. height) cylinders according to ASTM C-39 and ASTM C-

469 respectively. In addition, rapid chloride permeability testing was performed inaccordance with ASTM C-1202 using a 50 mm x 100 mm (2 in. x 4 in. diameter) diskthat was taken from a larger cylinder. Strength and permeability specimens were storedin a moist room at 22C and >95% RH until the time of testing (28 days). In addition,specimens were prepared to assess the volumetric stability (shrinkage). Free shrinkagewas measured using a modified version of ASTM C-341 as described in the following

paragraphs while restrained shrinkage cracking potential was determined using therestrained ring test. The restrained ring test consists of casting a concrete ring with a wallthickness of 35 mm (1.3 in) around a 150 mm (6 in) tall steel ring with an outer diameterof 300 mm (12 in), allowing the specimen to dry, and noting the age at which cracking isobserved and the width of the crack after a specified period of time, which was 56 days

for this study.12

Shrinkage specimens were stored in a drying environment (40% RH) at aconstant temperature of 30C.

As mentioned, autogenous shrinkage describes volumetric movements that occur withoutmass or temperature change in the specimen. At early-ages when the concrete is stillfluid, autogenous shrinkage does not cause the development of high residual stress levelsif length change is prevented. However, as the concrete begins to develop a rigidstructure (i.e., set), residual stresses as can develop if the concrete is restrained frommoving freely. 13 Autogenous shrinkage measurements are frequently ignored in currentstandardized testing procedures, however high levels of autogenous shrinkage can occur

between the time of set and the time at which the concrete is demolded. 8

The design of the experiments for measuring autogenous shrinkage used in this paper has been based on a test method proposed by Tazawa and Mizawaya 14 and Dilger et al. 15 inwhich dial gages are used to monitor the overall length change of the specimen while it isstill in the mold. Figure 1 illustrates details of the experiment in which a standard sizeASTM shrinkage prism geometry (100 mm x 100 mm x 400 mm, 4 in. x 4in. x 16 in.) is

prepared in special forms that are intended to permit movement while providing amoisture barrier. Stainless steel gage studs are permitted to protrude through the end ofthe forms in a mechanical sleeve. The gage studs were embedded in the fresh concretemaintaining a 350 mm (14 in.) gage length between the closest ends of the gage plugs.The concrete was placed in two lifts, vibrated, finished, and a polyester film was placedon the top surface of the specimens along with an acrylic lid. Autogenous shrinkagemeasurements were taken from the time of initial set as determined using ASTM C-341.Thermal effects were separated from autogenous shrinkage at early ages by measuringthe temperature change in the concrete and using the coefficient of thermal expansion tocorrect for the estimated thermal contribution to length change. 16


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After demolding at 24 hours, specimen deformations were measured and recorded for both sealed and unsealed specimens to assess the effect of drying and autogenousshrinkage. Unsealed specimens were permitted to dry from all six sides in the 40% RHenvironment while the sealed specimens were wrapped with two layers of a 5 miladhesive aluminum tape. The weight of the unsealed specimens reduced as a result of

drying while the weight of the sealed specimens remained relatively constant over time(less than 0.05% at 90 days) demonstrating that the tape was sufficient to preventsubstantial moisture loss. Autogenous shrinkage describes the movement measured onthe sealed specimen (after compensating for thermal movements during the initial 24hours) while total shrinkage describes the measurement of the unsealed specimens.Drying shrinkage is taken as the difference between the sealed and unsealed specimens.

Steel Plate

Gage Length ~ 14 in (350 mm)

Specimen Length 16 in (400 mm)

Dial Gage

Gage PlugRemovable

ReactionPlate

1/2 in (12.5 mm) Acrylic Forms

Specimen Heightand Width

4 in (100 mm)

Polyester Film Between Specimenand Form Work Around All

Sides of the Specimen

End BlocksReleased at

Initial Set

PolystyreneSheet

Figure 1. Experimental Apparatus for Measuring Autogenous Shrinkage

DISCUSSION OF EXPERIMENTAL RESULTS

The following section illustrates the trends that were observed during the course of thisinvestigation. The first section will deal primarily with the influence of the w/c whereasthe second section will discuss the impact of a mineral additive (silica fume) and achemical additive (shrinkage reducing admixture, SRA) on overall performance.

Figure 2 illustrates a summary of results obtained from a variety of experiments onmixtures containing three different w/cs (0.5, 0.4, and 0.3). Results from mercuryintrusion porisimetry tests indicate that by changing the w/c from 0.5 to 0.4 and 0.3corresponded to a 15% and 35% reduction in the total pore volumes respectively. As

previously described, the reduction in total pore volume corresponds to a 25% and 76 %increase in strength for the 0.4 and 0.3 mixtures respectively as shown in Figure 2a.Similarly an increase in elastic modulus was observed (Figure 2b). In addition,decreasing the w/c resulted in a decrease in chloride penetrability that can again beattributed to the change in capillary pore volume and connectivity. The dramaticdecrease in charge passed in Figure 2c is likely attributed to the overheating effects thatoccur in the high w/c specimens during testing. Figure 2d illustrates that reducing the


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free water results in a reduction in drying shrinkage, however this reduction is not verysignificant since the benefit is counteracted by the reduction in pore size which results inthe generation of higher capillary stresses and shrinkage. While these beneficial trendsare consistent with numerous previous findings, Figure 2e illustrates that a reduction inthe w/c results in a higher autogenous shrinkage at both 24 hours (time of demolding,

zero shrinkage according to the existing ASTM C-341 method) and at later ages(autogenous shrinkage was measured to be 200, 440, and 695 for w/c of 0.5, 0.4, and0.3 at 90 days). In addition, Figure 2f shows that a reduction in w/c results in shrinkagecracking at earlier ages. This is consistent with what would be expected with an increasein total shrinkage, a reduction in stress relaxation (creep), and increase in brittleness, andan increase in stiffness. 6,10 It is important to once again note that the volume of aggregatewas maintained constant in this investigation and that this trend of increased crackingmay be able to be counteracted through an increase in aggregate volume, however furtherstudies are needed to confirm this hypothesis.

0

20

40

60

80

C o m p .

S t r

. ( M P a )

0

10

20

30

40

C o m p .

M o d . (

G P a )

0

3

6

9

12

C h a r g e

P a s s e

d ( C o l . X

1 0 3 )

0

3

6

9

12

A g e o f

C r a c k

i n g

( D a y s

)

0

70

140

210

280

A u t o .

S h r i n k a g e

@ 2 4 h r s

(

0

250

500

750

1000

D r y . S

h r i n k a g e

@ 9 0 D a y s

(

)

w / c = 0 . 5

w / c = 0 . 4

w / c = 0 . 3

(a) (b) (c)

(d) (e) (f)

Figure 2: Influence of Water-to-Cement Ratio on Various Material Properties 16,17

While decreasing the w/c provides one method to improve the performance of concrete,this section will review the use of two admixtures that can improve various aspects ofconcrete performance. Silica fume is a mineral additive that can densify the interfacialzone between the aggregates and the paste and reduce the capillary porosity therebyincreasing strength and dramatically reducing permeability (as inferred from RCPTmeasurements) as shown in Figures 3a and 3b respectively. These findings are consistent


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with porosity tests that show a reduction in total pore volume (which is likely to occurdue to a reduction in transition zone porosity that reduces connectivity). Use of silicafume was accompanied by an increase in cracking potential. Figure 4 illustrates that theuse of silica fume increases the overall autogenous shrinkage at later ages although theautogenous shrinkage at 24 hours was slightly lower than the control mixture

(presumably due to the fact that cement was replaced with silica fume resulting in aslower rate of initial reaction).

In addition to the use of silica fume, a shrinkage-reducing admixture was used toillustrate that by reducing the surface tension of the liquid in the capillary poresautogenous and drying shrinkage can be reduced. The shrinkage-reducing admixture wasobserved to have an 12% higher total pore volume that appear to be responsible for areduction in compressive strength, however permeability was found to be similar (albeitslightly lower). It is also interesting to note that the shrinkage-reducing admixture has themost dramatic effect on reducing shrinkage at early-ages which is likely attribute to theemptying of larger pores. As a result, SRA appears to be effective in reducing both

drying and autogenous shrinkage.16

0

25

50

75

100

C o m p r e s s i v e

S t r e n g t

h ( M P a )

0

1

2

3

4

C h a r g e

P a s s e

d ( C o l . x

1 0 3 )

0

3

6

9

12

A g e o f

C r a c k

i n g

( D a y s )

No cracking

0

0.3

0.6

0.9

1.2

A v e r a g e

C r a c k

W i d t h ( m m

)

P l a i n

S i l i c a

F u m e

S R A

(a) (b)

(c) (d)

Figure 3: Influence of Mineral and Chemical Additives on the Performance of aLow Water-to-Binder Ratio (0.3) Concrete *,17

* Note: Restrained shrinkage cracking testing was not performed on the silica fume mixture outlined inTable 1 therefore the age of cracking and average crack width for the silica fume mixture refers to a similarmixture with w/b ratio of 0.29 not 0.30 and a 15% silica fume replacement not 20% with 65% aggregatevolume.


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0 28 56 84 Age of Specimen (Days)

0

200

400

600

800

A u

t o g e n o u s

S h r i n k a g e

( m i c r o s

t r a

i n )

w/c = 0.3w/b = 0.3-SFw/b = 0.3-SRA

0 28 56 84

Ag e of Specimen (Days)

0

400

800

1200

1600

T o

t a l S h r i n

k a g e

( m i c r o s

t r a

i n )

w/c = 0.3w/b = 0.3-SFw/b = 0.3-SRA

Figure 4: Autogenous and Total Shrinkage (w/b = 0.3) Drying Began at 24 Hours 16

SUMMARY AND FUTURE DIRECTIONS

As the water to cement ratio (w/c) of concrete was decreased the strength, stiffness,chloride penetration resistance, and drying shrinkage all improve, however these mixturescan demonstrate higher autogenous shrinkage especially at low ages when the material isgaining strength. As a result, low w/c specimens may be more susceptible to early-agecracking. Silica fume can be shown to reduce chloride penetrability (i.e., increaseelectrical resistivity), increase strength, and increase stiffness; however these mixturesmay be more likely to crack at early-ages. Mixtures containing a shrinkage-reducingadmixture (SRA) have a similar (or slightly lower) chloride penetration index andreduced cracking potential despite having similar or lower strength. As a result, aspecification criterion based on strength alone or even strength and chloride penetrabilityis insufficient in the specification of durable concrete since it is essential that the concreteis sufficiently resistant to early-age cracking. Therefore the specification of highdurability concrete (especially when low w/c mixtures are used) appears to require aminimum of three parameters that would include strength, permeability (penetrationresistance), and early-age crack resistance.

Furthermore this paper illustrates that an opportunity may exist to develop concretemixtures that demonstrate excellent performance through microstructural modification.Future research is intended to focus on developing materials, mixture proportions, and

processing techniques targeting specific microstructural improvements. For example,reducing large pores and total porosity is observed to improve mechanical properties,whereas reducing capillary and interconnected porosity can lead to improvements in

penetration resistance to aggressive agents. However, research focused on eitherincreasing the diameter of the smallest (gel) pores or reducing the surface tension ofinternal liquids can result in significant improvements in shrinkage and shrinkagecracking resistance. As a result it appears that the development of the next generation ofhigh performance materials may rely on the ability to manipulate the microstructure (poresize and distribution) of cementitous materials to optimize the performance of concretefor a given application.


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ACKNOWLEDGEMENT

Support from the Center for Advanced Cement-Based Materials is greatly appreciated.In addition the authors gratefully acknowledge the assistance of Ms. Janet Lovell with the

porosity measurements.

REFERENCES1 Takahashi, T., Yamamoto, M., Ioku, K., Goto, S., Relationship Between Compressive Strength and

Pore Structure of Hardened Cement Paste, Advances in Cement Research, Vol. 9, No. 33, January1997, pp. 25-30

2 Brown, P. W., and Shi, D., and Skalny, J., Porosity/Permeability Relationships, Material Science ofConcrete II, ed. Skalny, J., and Mindess, S., 1991, pp. 83-110

3 Young, J. F., principal author, TC 69 Physical Mechanisms and Their Mathematical Descriptions,Fourth RILEM International Symposium on Creep and Shrinkage of Concrete: MathematicalModeling, ed. Z. P. Bazant, 1986, pp. 44-78

4 Powers, T. C., and Brownyard T. L., "Studies on the Physical Properties of Hardened PortlandCement Paste" PCA Bulletin 22, Chicago, Illinois, 1948

5 Rossler, M., and Odler, I, Investigations on the Relationship between Porosity, Structure, andStrength of Hydrated Portland Cement Paste. II. Effect of Pore Solution and the Degree of

Hydration, Vol. 15, No. 3, 1985, pp. 401-4106 Shah, S. P., Weiss, W. J., and Yang, W., "Shrinkage Cracking in High Performance Concrete."Proceedings of the PCI/FHWA International Symposium on High Performance Concrete, NewOrleans, Louisiana, 1997, pp. 217-228

7 Tazawa, E., and Miyazawa, S., "Influence of Autogenous Shrinkage on Cracking in High-StrengthConcrete", Fourth International Symposium on the Utilization of High-Strength/Higher PerformanceConcrete, Paris France, May 29-31 1996, eds. deLerrard, F., and Lacroix, R., pp. 321-330

8 Aitcin, P.-C., "Autogenous Shrinkage Measurements", Autoshrink'98, Proceedings of theInternational Workshop on Autogenous Shrinkage of Concrete, ed. E. Tazawa, Hiroshima, Japan June13-14, 1998.

9 Lyman, G. C., Growth and Movement in Portland Cement Concrete, Oxford, University Press,London, U. K., pp. 1-139, 1934

10 Weiss, W. J., Yang, W., and Shah, S. P., "Factors Influencing Durability and Early-Age Cracking inHigh Strength Concrete Structures", ACI Spring Conference 1999, Chicago IL, to appear in a special

publication11 Weiss, W. J., Prediction of Early-Age Shrinkage Cracking in Concrete Elements. Ph.D.Dissertation, Evanston IL, December 1999

12 Grysbowski, M., and Shah S., P., Shrinkage Cracking of Fiber Reinforced Concrete, ACI MaterialsJournal, March/April, Vol. 87, No.2, 1990, pp. 138-148

13 Kovler, K. Testing System For Determining The Mechanical Behavior Of Early Age Concrete UnderRestrained And Free Uniaxial Shrinkage, Materials and Structures, RILEM, London, U.K., 27(170),1994 324-330

14 Tazawa, E., and Miyazawa, S., "Autogenous Shrinkage of Concrete and Its Importance in ConcreteTechnology", RILEM 22 Creep and Shrinkage - Proceedings of the Fifth International RILEMSymposium, ed. Bazant, Z. P., and Carol, I., pp. 159-168

15 Dilger, W. H., Wang, C., Niitani, K., "Experimental Study on Shrinkage and Creep of High-Performance Concrete", Fourth International Symposium on the Utilization of High-Strength/HigherPerformance Concrete, Paris France, May 29-31 1996, eds. deLerrard, F., and Lacroix, R., pp. 311-319

16 Weiss, W. J., Borischevsky, B. B., and Shah, S. P., (1999) "The Influence of a Shrinkage ReducingAdmixture on the Early-Age Behavior of High Performance Concrete", Fifth InternationalSymposium on the Utilization of High Strength/High Performance Concrete, Sandefjord, Norway,Vol. 2, 1418-1428

17 Weiss, W. J., Schie l, A., Yang, W., and Shah, S. P, (1998). "Shrinkage Cracking Potential,Permeability, and Strength, for HPC: Influence of W/C, Silica Fume, Latex, and Shrinkage ReducingAdmixtures." International Symposium on High Performance and Reactive Powder Concrete,Sherbrooke, Canada, Vol. 1, 349-364

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