RESEARCH MOTIVATION

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IBC PAPER 08-42 ENGINEERING USE OF LOW-STRENGTH CONCRETE IN HIGHWAY CONSTRUCTION Y. Frank Chen, Ph.D., P.E., Professor, Penn State Harrisburg Daniel 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. 1 IBC PAPER 08-42

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  • 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.*IBC PAPER 08-42

    IBC PAPER 08-42

  • RESEARCH MOTIVATIONClass C concrete (fc = 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 DOTs officials about the legitimate use of Class C concrete thicker than 3 ft. *IBC PAPER 08-42

    IBC PAPER 08-42

  • DEFINITION OF MASS CONCRETEACI 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.IBC PAPER 08-42*

    IBC PAPER 08-42

  • RESEARCH OBJECTIVESTo 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 concreteIBC PAPER 08-42*

    IBC PAPER 08-42

  • THE GENERAL ANALYSIS PROCEDUREIBC PAPER 08-42*

    IBC PAPER 08-42

  • EARTH WALL SYSTEM ON CLASS C MASS CONCRETEIBC PAPER 08-42*

    IBC PAPER 08-42

  • UNFACTORED HORIZ. EARTH LOADS (EHi) AND UNFACTORED LIVE-LOAD SURCHARGE LOADS (LShi & LSvi) FOR TEMPORARY CONSTRUCTION STAGEEH3 acts at 0.4 Dcc from the base. EH1 acts at 0.6 Dcc from the base.

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • UNFACTORED HORIZ. WATER LOADS (WAhi) FOR BOTH TEMPORARY AND FINAL CONSTRUCTION STAGESWAh1 = WAh4 = Dcc2 @ Dcc from the baseWAh2 = w (H + h Dwf) Dcc @ Dcc from the baseWAh3 = w (H + h Dwb) Dcc @ Dcc from the base

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • EXTERNAL STABILITYHu,ftg, Vu,ftg, and Mu,ftg : Computed by PennDOT ABLRFD Program (Ver. 1.6)Mu,cc = -(Mu,ftg + Hu,ftg Dcc) Mu,O + Mu,R

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • EXTERNAL STABILITY- CONTDOverturning: ec = Mu,cc/Vu,cc 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 resistanceIBC PAPER 08-42*

    IBC PAPER 08-42

  • INTERNAL STABILITYTrapezoidal Pressure Distributionqu = qu,min + (qu,max qu,min) [(Bcc Dcc tan b)/Bcc]IBC PAPER 08-42*

    IBC PAPER 08-42

  • INTERNAL STABILITY- CONTD 1Triangular Pressure Distributionqu = qu,max [(L1 Dcc tan - b)/L1]IBC PAPER 08-42*

    IBC PAPER 08-42

  • INTERNAL STABILITY- CONTD 2Shear-Friction between Wall Footing and Class C Mass Concrete:

    Hu,ftg v Vn1 [2002 ACI 530]

    Shear of Class C Mass Concrete:

    Vu = (qu + qu,max) (Dcc tan + b) v Vn2 [2002 ACI 530]

    Flexure of Class C Mass Concrete:

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

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

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • ALLOWABLE MAXIMUM TEMPERATURE DIFFERENTIAL (T)Current Practice (A Conservative Assumption)

    PCA: T = 36 0F

    TxDOT & Others: T = 35 0F

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • T- CONTD 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 h2Thermally-induced tensile stress, ft = M/S = E T T/22002 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.

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • T- CONTD 2Level 1 Cracking Analysis (Corps of Engineers)

    Max. tensile strain, t = fr/Ec = 0.250(ksi)/1800(ksi) = 138.9x10-6 in/intotal = 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/Eccr = the strain due to the thermal gradient = wcr/ = 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)

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • MASS CONCRETE- REQUIRED COOLING TIME & CEMENT CONTENTCooling 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

    *IBC PAPER 08-42

    IBC 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, = 370s = 0.120 kcf, sat = 0.135 kcf, Hf = 4.75, qs = 0.360 ksf, kah = 0.292

    *IBC PAPER 08-42

    IBC PAPER 08-42

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

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • REPRESENTATIVE SLIDING RESULTSSliding is less critical than overturning.

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • REPRESENTATIVE BEARING RESULTSBearing is less critical than sliding.

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • 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.

    *IBC PAPER 08-42

    IBC PAPER 08-42

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

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • H VALUES- CONTDIBC PAPER 08-42*

    IBC PAPER 08-42

  • 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).

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • 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).

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • Rh VALUES FOR MTD = 43 0F- CONTDIBC PAPER 08-42*

    IBC PAPER 08-42

  • 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).

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • Rh VALUES FOR MTD = 54 0F- CONTDIBC PAPER 08-42*

    IBC PAPER 08-42

  • SUMMARY OF THERMAL CRACKING ANALYSIS RESULTS*IBC PAPER 08-42

    IBC PAPER 08-42

  • CONSTRUCTION COSTS (SAMPLE STUDY)L = 40 ft.

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • CONSTRUCTION COSTS- CONTD 1Conventional 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.

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • CONSTRUCTION COSTS- CONTD 2Construction 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.

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • 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.

    *IBC PAPER 08-42

    IBC PAPER 08-42

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

    *IBC PAPER 08-42

    IBC PAPER 08-42

  • RECOMMENDATIONS- CONTD 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.

    IBC PAPER 08-42*

    IBC PAPER 08-42

  • THANK YOU.

    QUESTIONS?IBC PAPER 08-42*

    IBC PAPER 08-42