205 - Concrete Structures

205.4 - Material Properties

205.4.2 - Normal Weight and Structural Lightweight Concrete

205.4.2.1 - Compressive Strength

SPECIFICATIONS	COMMENTARY
The following shall replace A5.4.2.1: For each component, the specified compressive strength, f`c, or the class of concrete shall be shown in the contract documents. Design concrete compressive strengths above 10.0 ksi for normal weight concrete shall be used only when allowed by specific Articles or when physical tests are made to establish the relationships between the concrete compressive strength and other properties. Concrete with strengths below 2.4 ksi should not be used in structural applications. The design concrete compressive strength for prestressed concrete and decks shall not be less than 4.0 ksi. For lightweight concrete, physical properties in addition to compressive strength shall be specified in the contract documents.	The following shall replace AC5.4.2.1: The evaluation of the strength of the concrete used in the work should be based on test cylinders produced, tested, and evaluated in accordance with Section 8 of the AASHTO LRFD Bridge Construction Specifications. Section 8 was originally developed based on an upper limit of 10.0 ksi for the design concrete compressive strength. As research information for concrete compressive strengths greater than 10.0 ksi becomes available, individual Articles are being revised or extended to allow their use with higher-strength concretes. Appendix C5 contains a listing of the Articles affected by concrete compressive strength and their upper limit. It is common practice that the compressive strength of the concrete for use in design be attained 28 days after placement. Other maturity ages may be assumed for the design and specified for components that will receive loads at times appreciably different than 28 days after placement. It is recommended that the classes of concrete shown in Table D5.4.2.1-1 and their corresponding specified strengths be used whenever appropriate. The minimum mix design compressive strength shall be in accordance with Standard Specifications Section 1022.04. The classes are intended for use as follows: Class A concrete is generally used in barriers, exposed abutments, stems, backwalls, wingwalls, and all cast-in-place culvert concrete. Class B concrete is used in unexposed abutments and unexposed wingwall footings. Class C concrete is used for the replacement of unsuitable material below foundations. Class D concrete is used in cast-in-place decks, approach slabs, and deck rehabilitation overlays. Strengths above 5.0 ksi should be used only when the availability of materials for such concrete in the locale is verified. Lightweight concrete is generally used only under conditions where weight is critical. In the evaluation of existing structures, it may be appropriate to modify the f`c and other attendant structural properties specified for the original construction to recognize the strength gain or any strength loss due to age or deterioration after 28 days. Such modified f`c should be determined by core samples of sufficient number and size to represent the concrete in the work, tested in accordance with AASHTO T 24M/T 24 (ASTM C42/C42M).
For concrete Class A used in or over saltwater, the W/CM ratio shall be specified not to exceed 0.45.
	Table C5.4.2.1-1 – Concrete Design Compressive Strength by Class Class of Concrete Design Compressive Strength, f`c (ksi) A 4.5 B 3.0 C 2.0 D 4.5 Precast Concrete* 5.0 Precast Prestressed Concrete 5.0 to 10.0 (8.0 typically**) *Non-prestressed concrete should typically be cast using concrete with f`c = 5.0 ksi. **Prestressed concrete shall typically use 8.0 ksi concrete unless economic advantage can be demonstrated for the use of lower or higher strength concrete. The use of concrete with f`c > 8.0 ksi requires the approval of the Bridge Design Engineer and shall not be greater than 10.0 ksi.

205.4.3 - Reinforcing Steel

205.4.3.1 - General

SPECIFICATIONS	COMMENTARY
The following shall replace the nominal yield strength requirements provided in A5.4.3.1: The nominal yield strength of reinforcing steel shall be 60.0 ksi. Use of bars with yield strengths other than 60.0 ksi requires approval of the Bridge Design Engineer.

205.4.4 - Prestressing Steel

205.4.4.1 - General

SPECIFICATIONS	COMMENTARY
The following shall replace A5.4.4.1 paragraphs 1 and 2: Prestressing strands shall be uncoated high-strength 7-wire low-relaxation strand with a nominal 0.5- or 0.6-inch diameter and should conform to AASHTO M203 270-ksi-grade, low-relaxation strand. The yield strength of prestressing strand, fpy, shall be taken as 90 percent of the tensile strength, fpu.

205.6 - Design for Flexural and Axial Force Effects B-Regions

205.6.3 - Flexural Members

205.6.3.5 - Deformations

205.6.3.5.4D - Camber of Prestressed Beams

205.6.3.5.4.1D - Camber Due to Prestressing

SPECIFICATIONS

COMMENTARY

Camber due to prestressing shall be calculated by Equations 205.6.3.5.4.1D-1, 205.6.3.5.4.1D-2, and 205.6.3.5.4.1D-3 for beams with straight, draped, and debonded strands, respectively. Straight Strands $\Delta _{prestressed}={\frac {P_{e_{s}}L^{2}}{8E_{ci}I}}$ (205.6.3.5.4.1D-1) Draped Strands $\Delta _{prestressed}={\frac {PL^{2}[4X^{2}(e_{n}-e_{s})+3e_{s}]}{24E_{ci}I}}$ (205.6.3.5.4.1D-2) Debonded Strands $\Delta _{prestressed}={\frac {1}{8E_{ci}I}}[P_{b}e_{b}L^{2}+P_{1}e_{1}[L^{2}-(L_{t}+2L_{1})^{2}]$ $+P_{2}e_{2}[L^{2}-(L_{t}+2L_{2})^{2}]$ $+...P_{i}e_{i}[L^{2}-(L_{t}+2L_{i})^{2}]]$ (205.6.3.5.4.1D-3) for which: $P=P_{t}{\biggl (}1-{\frac {\Delta f_{s}}{100}}{\biggr )}$ (205.6.3.5.4.1D-4) $P_{b;1,2...i}=P_{tb;1,2...i}{\biggl (}1-{\frac {\Delta f_{s}}{100}}{\biggr )}$ (205.6.3.5.4.1D-5) where: $e_{s}$ = eccentricity at mid-span (in.) $e_{n}$ = eccentricity at end of beam (in.) $e_{b}$ = eccentricity at mid-span of full-length bonded strands (in.) $e_{1,2...i}$ = eccentricity at midspan of debonded group 1,2…i (in.) $E_{ci}$= modulus of elasticity of beam concrete at transfer (ksi) $I$ = moment of inertia of beam (in4) $L$ = beam length (in.) $L_{t}$ = transfer length (in.) $L_{1,2...i}$ = distance from centerline of bearing to debonding cutoff points $P$ = prestressing force at selected time for camber calculations (kips) $P_{b}$ = prestressing force at selected time for camber calculations of full-length bonded strands (kips) $P_{t}$ = prestressing force at transfer (kips) $P_{tb}$ = prestressing force at transfer of full-length bonded strands (kips) $P_{b\ 1,2...i}$ = prestressing force at selected time for camber calculations of debonded group 1, 2…i $P_{tb\ 1,2...i}$ = prestressing force at transfer of debonded group 1, 2…i $\Delta f_{s}$ = assumed percentage of prestressing loss since transfer for selected time $X$ =   percent of L for drape point

205.6.3.5.4.2D - Deflection Due to Dead Loads

SPECIFICATIONS

COMMENTARY

The maximum downward deflection at mid-span due to the beam weight and internal diaphragms shall be taken as: $\Delta _{D1}={\frac {5(M_{D1})L^{2}}{48E_{ci}I}}$ (205.6.3.5.4.2D-1) where: $M_{D1}$ = unfactored moment at mid-span due to the beam weight and any internal diaphragms (kip-in.) The maximum downward deflection at mid-span due to slab, formwork, external diaphragms, and any other dead load that is applied to the beam before the slab has hardened shall be taken as: $\Delta _{D2}={\frac {5(M_{D2})L^{2}}{48E_{c}I}}$ (205.6.3.5.4.2D-2) where: $M_{D2}$ = unfactored moment at mid-span due to dead load applied to the beam before the slab has hardened, except the beam weight and internal diaphragms (kip-in.) $E_{c}$ = modulus of elasticity of beam concrete (ksi) For simple span construction, the maximum downward deflection at mid-span due to superimposed dead load shall be taken as: $\Delta _{D3}={\frac {5(M_{D3})L^{2}}{48E_{c}I_{c}}}$ (205.6.3.5.4.2D-3) where: $M_{D3}$ = unfactored moment at mid-span due to superimposed dead load (kip-in.) $I_{c}$ = moment of inertia of composite beam (in4) For continuous span construction, the maximum downward deflection at mid-span due to superimposed dead load shall be determined from continuous span analysis.

205.6.3.5.4.3D - Total Camber at Transfer of Prestressing

SPECIFICATIONS	COMMENTARY
The total camber at transfer shall be taken as: $\Delta _{t}=\Delta _{prestressed}-\Delta _{D1}$ $\Delta f_{s}$ shall be assumed to be zero in determining $\Delta _{prestressed}$.

205.6.3.5.4.4D - Camber for Bearing Slope

SPECIFICATIONS	COMMENTARY
The total camber for determining the bearing slope shall be taken as: $\Delta _{b}=\Delta _{prestressed}-\Delta _{D1}$ $\Delta f_{s}$ shall be assumed to be 10 percent in determining $\Delta _{prestressed}$.

205.6.3.5.4.5D - Total Camber in Beams at Time of Construction

SPECIFICATIONS

COMMENTARY

The total camber in the beams at time of construction shall be taken as: $\Delta _{c}=(\Delta _{prestressed}-\Delta _{D1})C_{r}$ where: $C_{r}$ = 1.6 $\Delta f_{s}$ = 10% in determining $\Delta _{prestressed}$

C205.6.3.5.4.5D After release, prestressed beams may be stored for a period of days to as much as several months or more. During this time period, the camber increases due to creep. The initial prestressing force, on the other hand, decreases due to shrinkage, creep of the concrete, and relaxation of the steel, all of which are time-dependent and have opposing effects. These time-dependent effects can be determined by using an estimated creep factor and prestressing loss. Assuming the beams are stored from 7 to 80 days, it may be reasonable to estimate that the creep factor, Cr, varies in a range of 2.0 to 1.5 for 8-ksi and 10-ksi concrete respectively. The prestress loss, $\Delta f_{s}$, varies in a range of 5 to 15 percent in that time. For design, unless better information is available, $C_{r}$ = 1.6 and $\Delta f_{s}$ = 10 percent may be used. These are average values from Delaware prestressers. The assumed values for $C_{r}$ and $\Delta f_{s}$ shall be shown on the design drawings.

205.6.3.5.4.6D - Final Camber

SPECIFICATIONS	COMMENTARY
Negative final camber (sag) shall be limited to L/2000.

205.9 - Prestressing

205.9.2 – Stress Limitations

205.9.2.3 – Stress Limits for Concrete

205.9.2.3.2b – Tensile Stresses

SPECIFICATIONS

COMMENTARY

The following shall supplement A5.9.2.3.2b: There is a preference for prestressed members designed with zero tension in the bottom fiber at the service limit state and after all prestress losses. However, this is not always practical or achievable. If not, apply a tension limit based on the bridge site conditions. The following shall be considered severe corrosive conditions: Bridges over salt/tidal waters and coastal conditions. Bridges over fresh water lacking freeboard as defined in Section 104.3.2.1. Consider for other bridges near coastal conditions or heavy industrial areas. The following shall be considered moderate corrosive conditions: Bridges not crossing water (over roadways, railroads, etc.). Bridges over fresh water with adequate freeboard.

205.10 - Reinforcement

205.10.1 - Concrete Cover

SPECIFICATIONS	COMMENTARY
The following shall apply to table A5.10.1-1: Use Category A for all reinforcing materials. For precast prestressed piles, use cover specified for precast reinforced piles.

205.10.4 - Transverse Reinforcement for Compression Members

205.10.4.2 - Spirals

SPECIFICATIONS	COMMENTARY
The following shall replace the third bullet of the fifth paragraph in A5.10.4.2. The use of welded splices for reinforcement bars is prohibited.

205.12 - Provisions for Structure Components and Types

205.12.4 - Diaphragms

SPECIFICATIONS	COMMENTARY
The following shall supplement A5.12.4: The minimum number of diaphragms is three per span: one at each support and one at mid-span.

205.13 - Anchors

205.13.1.1D - Anchors in Tension

SPECIFICATIONS	COMMENTARY
Grouted anchors are not allowed for tension applications.