High-Performance/High-Strength Lightweight Concrete for Bridge Girders and Decks (2013) / Chapter Skim
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10 3.1 Mix Designs and Material Properties Research Approach The mix design and material property portion of the project consisted of two phases. The first phase focused on selecting lightweight aggregate and developing concrete mix designs for use in bridge decks and girders.
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11 for structural test comparisons. Table 3 lists the types of concrete mixes selected for use in the structural testing portion of this project.
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12 Relative Density (Specific Gravity) and Absorption of Coarse Aggregate.
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13 Test Results Concrete batches were mixed in the laboratory to evaluate the performance of the six aggregate sources for a range of mixture designs. Based on results of the screening tests, mixtures were identified for use in large-scale testing in laboratory and lab-cast beams, full-size girders, and deck segments.
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14 Compressive Strength. Identifying a binary lightweight concrete mixture that yielded a high compressive strength proved to be difficult.
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15 compressive strengths ranged from 5 to 6 ksi. The strength of the other mixtures with a 0.40 w/cm ratio also ranged from 5 to 6 ksi range, but with greater variation.
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16 of concrete (fct) , it can be determined from information given in Article 5.8.2.2.
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17 0.15 0.20 0.25 0.30 0.35 0.40 0.30 w/c @ 800lb OPC 0.30 w/cm @ 800 lb OPC+FA 0.30 w/cm @ 800 lb OPC+Slag 0.30 w/cm @ 800 lb OPC+SF 0.40 w/c @ 752 lb OPC 0.40 w/cm @ 752 lb OPC+FA 0.40 w/cm @ 752 lb OPC+Slag 0.30 w/c @ 850lb OPC 0.30 w/cm @ 850 lb OPC+Slag 0.30 w/cm @ 850 lb OPC+SF 0.25 w/c @ 900 lb OPC 0.30 w/cm @ 900 lb OPC+FA 0.30 w/cm @ 900 lb OPC+Slag Fa ct or a Cementitious blend by w/cm and TCM CA Clay NY Shale CO Shale IN Shale LA Clay NC Slate Figure 5. Factor a (where fct = a fc′)
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18 0.26 ′f c resulted in a closer yet conservative prediction of modulus of rupture. Permeability.
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19 Shrinkage. Shrinkage testing was performed in accordance with ASTM C157.
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20 NW Girder LW Girder LW Deck Figure 9. Measured versus predicted shrinkage strain (AASHTO model)
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21 deck and normal weight girder mixtures. At early ages, the CEB MC90 model generally over-predicted shrinkage for lightweight girder mixtures but accurately predicted shrinkage for lightweight deck mixtures.
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22 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 0.5 1 1.5 2 2.5 3 3.5 Measured Creep Coefficient Pr ed ic te d Cr ee p Co ef fic ie nt NW Girder LW Girder LW Deck Figure 13. Measured versus predicted creep coefficients (ACI 209 model)
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23 yielded test results consistent with what is needed for structural concretes. • Lightweight concrete with a compressive strength of 7000 psi and a unit weight less than 125 lb/ft3 can be produced with a 0.30 w/cm and 800 lb of cementitious material with expanded shales and slates.
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24 The nominal shear resistance of the interface plane is calculated as follows: V cA A f P minimumof A orni cv vf y c 1 c cv= + +( ) ≤ ′µ K f A2 cvK where Acv = area of concrete engaged in interface shear transfer c = cohesion factor µ = friction factor A vf = area of shear reinforcement crossing shear plane within the area Acv fy = yield stress of reinforcement (not to exceed 60 ksi)
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25 Test Setup Figure 16 shows a typical test setup. The testing frame consisted of two steel bulkheads bolted to the floor.
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26 the specimens. The horizontal force continued to be applied until about an inch of slip occurred between the deck and girder.
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27 yield stress of 60 ksi and elastic modulus of 29,000 ksi. The recorded strain in the reinforcement was not pure axial strain but also incorporated bending.
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28 in the vertical leg of the deck side specimen and not at the interface. Typical Load Slip Behavior.
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29 or around the deck/girder interface. Figure 21 shows cracking in the interface region for the three concrete combinations of deck concrete (on the top)
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30 variability in the fabrication of the specimens; and (3) a professional factor representing the uncertainty of the theoretical model by representing variability of the ratio of tested values to calculated values.
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31 Specimen Designation LRFD Calculated V ni (kips) Maximum Test Load (kips)
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32 strength data from over 8000 samples of lightweight and normal weight concrete and determined the bias and coefficient of variation for a wide range of concrete strengths. For typical deck concrete (4 ksi compressive strength)
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33 Summary The interface shear tests revealed the following: • The bias of the measured shear strengths to the nominal shear strength computed with the AASHTO equation for a concrete deck placed on the top flange of a girder which has been intentionally roughened was 1.16 for N-N, 1.29 for L-N, and 1.26 for L-L. • The AASHTO LRFD equation for interface shear design is less conservative with increasing reinforcement ratios, which indicates the friction coefficient may be too high.
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34 b) Beams containing 0.6-in.-diameter strands.
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35 The scheme used to identify the laboratory beams is illustrated in Figure 26. The "Abutment" identifier specifies the closest adjacent abutment during casting.
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36 Russell (2008) have recently proposed changes to the AASHTO specifications based on research conducted in NCHRP Project 12-60 (2008)
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37 Figure 27. Sample strain profile (2.LW3.5A at transfer)
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38 Figure 29 compares these two calculation methods with the results from other research on lightweight concrete (Kolozs 2000; Meyer 2002; Nassar 2002; Zena 1996)
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39 the current specification provides conservative estimates of transfer length for HSLWC members. Development Length Development length of prestressing strand is the embedment length required to ensure a flexural failure mode.
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40 provide a reasonable upper bound to the measured development lengths of lightweight and normal weight girders. 3.4 Shear Performance of Full-Scale Beams Work Plan The investigators reviewed available literature to (1)
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9 spa. @ 9 in.5" Lspan BT.8.Typ ρ = 0.63%v5 11 spa.
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7 spa.
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43 both AASHTO Type II beams whereas the other four girders were the bulb tee shapes. The designation "Typ" identifies test specimen with a typical amount of shear reinforcement, and the designation "Min" refers to test specimens with the AASHTO LRFD minimum amount of shear reinforcement.
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44 the top flange in order to reduce the stresses in the top and bottom flanges at release. Likewise, the PCBT-45 beams were 59 feet long, but had shear reinforcement based on an 85-ftlong span; six of the 34 (0.5-in.-diameter)
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45 Also, two vibrating wire gages (VWGs) were placed at the centroid of the bottom layer of flexural reinforcement at the beam centerline.
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46 and the Simplified Procedure for Prestressed and Nonprestressed Sections (5.8.3.4.3)
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47 5 P P 14'-6"4'-9" 1 2 6 7 8 9 10 1 1 11 2 3 3 Stirrup Number = strain gauge = malfunctioning gauge = stirrup yielded at gauge at cracking = stirrup yielded at gauge Figure 35. Strain gage locations for Test T2.8.Typ.1.
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48 beam end were pulled into the concrete more than 0.01 in. In seven of the eight tests that resulted in a web-shear failure, strand slip in excess of 0.01 in.
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49 Summary Observations regarding shear design of lightweight prestressed concrete girders follow: • Applying a modification factor (lv) to ′f c term in shear strength calculations is not needed for sand lightweight concrete prestressed concrete girders.
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50 Crack Initiation Tests. Crack initiation tests were conducted to determine the load at which the first flexural crack occurred in order to back-calculate the effective prestress force.
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51 With the initial cracking load known, the effective prestress was calculated using the 28-day modulus of rupture. Table 31 lists the measured and effective prestress at the time of testing.
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52 Figure 41. Sample load versus strain (beam end 1.NW1.5B.A)
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53 reinforcement and deck shrinkage on overall time-dependent changes in prestress. Instantaneous changes, such as elastic shortening losses and increases in strain at the time of deck placement, were calculated using the transformed cross-sectional properties of the girder.
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54 using each calculation method. Table 34 presents the timedependent changes in prestress from just after deck placement to destructive testing.
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55 Beam Prestress Loss (ksi) AASHTO Refined AAEM AASHTO ACI 209 CEB MC-90 AASHTO ACI 209 CEB MC-90 AASHTO w/meas Ec Measured T2.8.Min -42.3 (0.82)
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56 As expected, the initial measured normal weight beam cambers were less than or equal to the measured lightweight cambers due to the lower stiffness of the lightweight concrete and the higher self-weight. Also, there was a large difference between the measured and predicted values, in particular, between the lightweight concrete from pour 2 and the normal weight concrete from pour 1 (note that the first number in the beam identification indicates the concrete pour number)
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Figure 46. Time-dependent camber for beams with 0.6 in.
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58 greatly over predicted cambers for all lightweight concrete beams. This may be due to sensitivity to the elastic modulus values used.
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59 presents the measured camber and the PCI multiplier methods for the same beam. The "Measured Calculated" values in Figure 48 include an allowance to account for the effects of thermal gradients that develop during deck hydration.
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60 Beam Camber (in.) PCI Mult PCI Improved Multipliers AAEM AASHTO ACI 209 CEB MC-90 AASHTO ACI 209 CEB MC-90 AASHTO w/meas Ec Measured T2.8.Min 1.34 1.11 1.27 1.39 1.08 1.21 1.34 1.16 0.95 T2.8.Typ 1.34 1.11 1.22 1.33 1.09 1.17 1.29 1.12 0.94 BT.8.Typ 1.65 1.49 1.62 1.81 1.55 1.67 1.88 1.80 1.50 BT.8N.Typ 1.20 1.00 1.10 1.21 1.04 1.12 1.25 1.29 1.44 BT.10.Typ 1.65 1.52 1.67 1.94 1.58 1.71 2.00 1.68 1.65 BT.10.Min 1.65 1.48 1.61 1.78 1.54 1.66 1.84 1.64 1.57 Table 39.
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61 Figure 50. Bias of camber calculation methods.
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62 predictions were those made using the AAEM method with the measured modulus of elasticity. The AAEM method with the calculated modulus and AASHTO creep and shrinkage models also predicted cambers relatively close to measured.
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63 Run Concrete (shear) v Vc (kips)
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