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RESEARCH MOTIVATION

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RESEARCH MOTIVATION

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  1. IBC PAPER 08-42ENGINEERING USE OF LOW-STRENGTH CONCRETE IN HIGHWAY CONSTRUCTIONY. 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,NamitaSinha, and Jason Taylor, Dawood Engineering Inc. IBC PAPER 08-42

  2. RESEARCH MOTIVATION Class C concrete (f´c = 2,000 psi) is permitted and often selected as replacement for unsuitable materialor 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. IBC PAPER 08-42

  3. 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. IBC PAPER 08-42

  4. 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 limitationson the use of unreinforced Class C mass concrete IBC PAPER 08-42

  5. THE GENERAL ANALYSIS PROCEDURE IBC PAPER 08-42

  6. EARTH WALL SYSTEM ON CLASS C MASS CONCRETE IBC PAPER 08-42

  7. UNFACTORED HORIZ. EARTH LOADS (EHi) AND UNFACTORED LIVE-LOAD SURCHARGE LOADS (LShi & LSvi) FOR TEMPORARY CONSTRUCTION STAGE EH3 acts at 0.4 Dccfrom the base. EH1 acts at 0.6 Dccfrom the base. IBC PAPER 08-42

  8. UNFACTORED HORIZ. WATER LOADS (WAhi) FOR BOTH TEMPORARY AND FINAL CONSTRUCTION STAGES WAh1 = WAh4 = ½ Dcc2 @ ⅓ Dccfrom the base WAh2 = w (H + h – Dwf) Dcc @ ½ Dcc from the base WAh3 = w (H + h – Dwb) Dcc @ ½ Dcc from the base IBC PAPER 08-42

  9. EXTERNAL STABILITY Hu,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

  10. 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 IBC PAPER 08-42

  11. INTERNAL STABILITY Trapezoidal Pressure Distribution qu = qu,min + (qu,max– qu,min) ∙ [(Bcc – Dcc tan – b)/Bcc] IBC PAPER 08-42

  12. INTERNAL STABILITY- CONT’D 1 Triangular Pressure Distribution qu = qu,max ∙ [(L1 – Dcc tan - b)/L1] IBC PAPER 08-42

  13. INTERNAL STABILITY- CONT’D 2 Shear-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) ∙ (Dcc 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 ≤ bMcr [2002 ACI 530] • 2002 ACI 530: v = 0.80 and b = 0.60. IBC PAPER 08-42

  14. ALLOWABLE MAXIMUM TEMPERATURE DIFFERENTIAL (T) Current Practice (A Conservative Assumption) PCA: T = 36 0F TxDOT & Others: T = 35 0F IBC PAPER 08-42

  15. T- CONT’D 1 Theoretical MTD Moment curvature,  = (T/I)  T(y) y dydz = (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 fc = 2,000 psi (Class C concrete) Similarly, T = 43.8 0F for fc ≥ 2,500 psi (structural conc.)  Deemed “more reasonable” for Class C conc. IBC PAPER 08-42

  16. 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 KRKf = t + cr KR = the structure restraint factor  L/h and the location within the mass concrete Kf = 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”) IBC PAPER 08-42

  17. 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 due to 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

  18. A CASE STUDY Cct = 394.8 pcyand fc = 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 IBC PAPER 08-42

  19. REPRESENTATIVE OVERTURNING RESULTS Overturning is the most critical external stability issue and is considered as the second most critical overall engineering issue. IBC PAPER 08-42

  20. REPRESENTATIVE SLIDING RESULTS Sliding is less critical than overturning. IBC PAPER 08-42

  21. REPRESENTATIVE BEARING RESULTS Bearing is less critical than sliding. IBC PAPER 08-42

  22. REPRESENTATIVE INTERNAL STABILITY RESULTS Flexural cracking is most critical, followed by shear and shear-friction. Flexural cracking is the most critical overall engineering issue. Shear-friction is not a real concern. IBC PAPER 08-42

  23. HYDRATION OF CEMENT VALUES (i.e., H VALUES) • Based on the 1994 PCA “Design and Control of Concrete Mixtures” IBC PAPER 08-42

  24. H VALUES- CONT’D IBC PAPER 08-42

  25. 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

  26. 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

  27. Rh VALUES FOR MTD = 43 0F- CONT’D IBC PAPER 08-42

  28. Rh VALUES FOR MTD = 54 0F (PROBABLE) Type IIportland 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

  29. Rh VALUES FOR MTD = 54 0F- CONT’D IBC PAPER 08-42

  30. SUMMARY OF THERMAL CRACKING ANALYSIS RESULTS IBC PAPER 08-42

  31. CONSTRUCTION COSTS (SAMPLE STUDY) L = 40 ft. IBC PAPER 08-42

  32. CONSTRUCTION COSTS- CONT’D 1 Conventional Construction w/o the use of Class C Mass Concrete (BFE = 836): $130,640 (Case 1) Proposed Constructionw/ the Use of Class C Mass Concrete (BFE = 841.5) Type IIportland 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

  33. 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. IBC PAPER 08-42

  34. CONCLUSIONS Based on the results from the parametric studies, the following conclusions can be made:  For external stabilityof the mass concrete, overturningis 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., fc ≥ 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

  35. RECOMMENDATIONS  To avoid the potential flexural cracking of mass concrete, b ≤ 1 ft and  ≤ 370are recommended. IBC PAPER 08-42

  36. 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 (fc ≤ 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

  37. THANK YOU.QUESTIONS? IBC PAPER 08-42

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