IBC PAPER 08-42

ENGINEERING USE OF LOW-STRENGTH CONCRETE IN HIGHWAY CONSTRUCTION

Y. Frank Chen, Ph.D., P.E., Professor, Penn State HarrisburgDaniel J. Hacker, P.E., Dawood Engineering Inc., Enola, PA.

Contributors: Thomas J. Imholte, Thomas C Rowader, Amelia Stum,Namita Sinha, and Jason Taylor, Dawood Engineering Inc.

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RESEARCH MOTIVATION

Class C concrete (f´c = 2,000 psi) is permitted and often selected as replacement for unsuitable material or as leveling fill material below footing on or near rock in order to reduce construction costs by the DOTs.

However, engineering guidance or standard on the use of Class C mass concrete is not readily available.

Engineers are often questioned by DOT’s officials about the legitimate use of Class C concrete thicker than 3 ft.

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DEFINITION OF MASS CONCRETE

ACI 211.1: The minimum cross-sectional dimension of a solid concrete mass 2-3 ft, or concrete with cement content > 600 pcy, or concrete with the use of accelerating mixtures ASTM C150 Type III cement or ASTM C1157 HE cement.

ACI 116R: Any large volume of cast-in-place concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and attendant volume change to minimize cracking. This is a better definition.

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RESEARCH OBJECTIVES

• To describe and discuss a sound engineering approach related to applications of unreinforced Class C mass concrete, including the important issues of external stability, internal stability, and thermal cracking

• To identify the possible dimensional limitations on the use of unreinforced Class C mass concrete

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THE GENERAL ANALYSIS PROCEDURE

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EARTH WALL SYSTEM ON CLASS C MASS CONCRETE

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UNFACTORED HORIZ. EARTH LOADS (EHi) AND UNFACTORED LIVE-LOAD SURCHARGE LOADS (LShi & LSvi) FOR TEMPORARY CONSTRUCTION STAGE

EH3 acts at 0.4 Dcc from the base. EH1 acts at 0.6 Dcc from the base.

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UNFACTORED HORIZ. WATER LOADS (WAhi) FOR BOTH TEMPORARY AND FINAL CONSTRUCTION STAGES

WAh1 = WAh4 = ½ Dcc2 @ ⅓ Dcc from the base

WAh2 = w (H + h – Dwf) Dcc @ ½ Dcc from the base

WAh3 = w (H + h – Dwb) Dcc @ ½ Dcc from the base

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EXTERNAL STABILITYHu,ftg, Vu,ftg, and Mu,ftg : Computed by PennDOT ABLRFD Program (Ver. 1.6)

Mu,cc = -(Mu,ftg + Hu,ftg D∙ cc) – Mu,O + Mu,R

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EXTERNAL STABILITY- CONT’D

Overturning: ec = Mu,cc/Vu,cc ≤ ⅜ Bcc [1998 AASHTO LRFD]

Bearing: qu,max ≤ qR [1998 AASHTO LRFD]

Sliding: Hu,cc ≤ QR [1998 AASHTO LRFD]

qR = factored bearing resistance

QR = factored sliding resistance

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INTERNAL STABILITYTrapezoidal Pressure Distributionqu = qu,min + (qu,max – qu,min) [(B∙ cc – Dcc tan – b)/Bcc]

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INTERNAL STABILITY- CONT’D 1Triangular Pressure Distributionqu = qu,max [(L∙ 1 – Dcc tan - b)/L1]

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INTERNAL STABILITY- CONT’D 2Shear-Friction between Wall Footing and Class C Mass Concrete:

Hu,ftg ≤ v Vn1 [2002 ACI 530]

Shear of Class C Mass Concrete:

Vu = ½ (qu + qu,max) (D∙ cc tan + b) ≤ v Vn2 [2002 ACI 530]

Flexure of Class C Mass Concrete:

Mu = ½ qu (Dcc tan + b)2 + ⅓ (qu,max – qu) (Dcc tan + b)2 ≤ b Mcr [2002 ACI 530]

2002 ACI 530: v = 0.80 and b = 0.60.

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ALLOWABLE MAXIMUM TEMPERATURE DIFFERENTIAL (T)

Current Practice (A Conservative Assumption)

PCA: T = 36 0F

TxDOT & Others: T = 35 0F

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T- CONT’D 1Theoretical MTDMoment curvature, = (T/I) T(y) y dy dz = (12 T/I) T(y) y dy for a unit width of 12 in.

T(y) = T (1 – 2y/h)∙M = E I T = 12 E T T(y) y dy = E T T h2

Thermally-induced tensile stress, ft = M/S = E T T/2

2002 ACI 530: E = 1800 ksi, T = 4.5x10-6 (1/oF), and fr (rupture strength) = 250 psi.

ft ≡ fr T = 61.7 0F (considered “too high”) for fc = 2,000 psi (Class C concrete)

Similarly, T = 43.8 0F for fc ≥ 2,500 psi (structural conc.) Deemed “more reasonable” for Class C conc.

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T- CONT’D 2

Level 1 Cracking Analysis (Corps of Engineers)

Max. tensile strain, t = fr/Ec = 0.250(ksi)/1800(ksi) = 138.9x10-6 in/in

total = T T KR Kf = t + cr

KR = the structure restraint factor L/h and the location within the mass

concreteKf = the foundation restraint factor Ef/Ec

cr = the strain due to the thermal gradient = wcr/ℓ = 0.125(in)/1200(in) =

104.2x10-6 in/in

2002 ACI 207.2R: Permits KR = Kf = 1.0 conservatively.

T = 54.0 0F (deemed “probable”)

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MASS CONCRETE- REQUIRED COOLING TIME & CEMENT CONTENT

Cooling Time, t (hr)

t ≈ 6.72 h2 for two-side cooling per PCA, where h = the thk. of mass conc. (ft) t ≈ 2.688 h2 for five-side cooling

Temperature Rise of Concrete due to Heat Generation of Cement, Tr (0F)

Tr = C H/S

where C = proportion of cement in concrete by weight, H = heat generation dueto hydration of cement (Btu/lb), and S = specific heat of concrete (avg =0.24 Btu/0F).

The Required Pozzolan Content, Cp (pcy)

Cp = (Cct – T S conc/H)/(1 – Rh)

where Cct = total cement content (pcy), Rh = reduction rate of hydration heat.

Rh = 15% max. if mixed with fly ash or 50% max if mixed with slag per PennDOT.

The Required Cement Content, Cc (pcy)

Cc = Cct – Cp 17IBC PAPER 08-42

A CASE STUDYCct = 394.8 pcy and fc = 2,000 psi, min. per PennDOT. Type II & Type IV portland cement.

H = 15.93, tw = 1.5, N = 1, a = 2, h = 2, Bf = 10.75, Dwf = 9.59, Dwb = 2.93’, b = 1,

Dcc = 5.5, = 370

s = 0.120 kcf, sat = 0.135 kcf, Hf = 4.75, qs = 0.360 ksf, kah = 0.292

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REPRESENTATIVE OVERTURNING RESULTSOverturning is the most critical external stability issue and is considered as the second mostcritical overall engineering issue.

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REPRESENTATIVE SLIDING RESULTSSliding is less critical than overturning.

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REPRESENTATIVE BEARING RESULTSBearing is less critical than sliding.

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REPRESENTATIVE INTERNAL STABILITY RESULTSFlexural cracking is most critical, followed by shear and shear-friction. Flexural cracking is themost critical overall engineering issue. Shear-friction is not a real concern.

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HYDRATION OF CEMENT VALUES (i.e., H VALUES) Based on the 1994 PCA “Design and Control of Concrete Mixtures”

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H VALUES- CONT’D

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REDUCTION OF HYDRATION HEAT (Rh) VALUES FOR MTD = 36 0F (CONSERV.)Type II portland cement: hmax = N/A (cement only), 4´ (w/ fly ash), and 4-9.5´ (w/ slag).

Type IV portland cement: hmax = 5´ (cement only), 5-8.5´ (w/ fly ash), and 8.5-31.5´ (w/ slag).

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Rh VALUES FOR MTD = 43 0F (MORE REASONABLE)

Type II portland cement: hmax = 4´ (cement only), 4-6´ (w/ fly ash), and 6-52´ (w/ slag).

Type IV portland cement: hmax = 10´ (cement only), 10-16´ (w/ fly ash), and 16-57´ (w/ slag).

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Rh VALUES FOR MTD = 43 0F- CONT’D

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Rh VALUES FOR MTD = 54 0F (PROBABLE)Type II portland cement: hmax = 6´ (cement only), 6-15.5´ (w/ fly ash), and 15.5-57´ (w/ slag).

Type IV portland cement: hmax = 24´ (cement only), 24-44.5´ (w/ fly ash), and 44.5-57´ (w/ slag).

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Rh VALUES FOR MTD = 54 0F- CONT’D

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SUMMARY OF THERMAL CRACKING ANALYSIS RESULTS

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CONSTRUCTION COSTS (SAMPLE STUDY)L = 40 ft.

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CONSTRUCTION COSTS- CONT’D 1

Conventional Construction w/o the use of Class C Mass Concrete (BFE = 836):

$130,640 (Case 1)

Proposed Construction w/ the Use of Class C Mass Concrete (BFE = 841.5)

Type II portland cement mixed w/ slag: $120,040 (Case 2)Type IV portland cement mixed w/ fly ash: $122,240 (Case 3)

Cost Savings

Case 2 vs. Case 1: 8.1%Case 3 vs. Case 1: 6.4%

These cost savings will rise for larger projects.

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CONSTRUCTION COSTS- CONT’D 2

Construction Schedule Benefit

Proposed Construction:

The mass concrete can be poured right after the excavation work is done and the wall

system may then be constructed 3-7 days after the mass concrete is cast.

Conventional Construction:

Water pumping, forming and bracing are required prior to the cons of the wall system, which could take up to two weeks.

The proposed construction with the use of unreinforced Class C concrete is clearly better in terms of cost and construction schedule benefits.

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CONCLUSIONSBased on the results from the parametric studies, the following conclusions can bemade:

For external stability of the mass concrete, overturning is most critical, followed by sliding and bearing. For internal stability of the mass concrete, flexural cracking is most critical, followed by shear and shear-friction. Among all engineering issues, flexural cracking is most critical followed by overturning. Shear-friction is not a real concern. The thickness of mass concrete is governed by the thermal cracking criteria and MTD. Current assumption of MTD = 36 0F appears to be intended for the more cementitious structural concrete (i.e., fc ≥ 2,500 psi), and is considered fairly

conservative for concrete with lower strength and lower cement content. The construction with the use of unreinforced Class C mass concrete is more beneficial in terms of cost and construction schedule.

With mixed cement (with fly ash or slag), Class C concrete offers a wide rage of applications.

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RECOMMENDATIONS To avoid the potential flexural cracking of mass concrete, b ≤ 1 ft and ≤ 370 are recommended.

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RECOMMENDATIONS- CONT’D

To minimize the thermal cracking of mass concrete, portland cement Type II or Type IV with low total cement content (< 400 pcy) and low compressive strength (fc ≤ 2,000 psi) are recommended.

Experimental work for justifying the adoption of higher MTD value such as 43 0F (more reasonable) or 54 0F (probable) is warranted.

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THANK YOU.

QUESTIONS?

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