**ACI 408.3R**-09 Guide for Lap Splice and Development Length of High Relative Rib Area Reinforcing Bars in Tension and Commentary.

2.1 Evaluation of lap splice and development lengths of coated and uncoated bar reinforcement in tension shall be limited to bars having a high relative rib area, conforming to Sections 2.1.1 through 2.1.5: 2.1.1 The relative rib area is at least 0.10, but no larger than 0.14; 2.1.2 The ribs shall be in accordance with (a), (b), and (c). (a)Ribs shall be at an angle of 45 to 65 degrees inclusive with respect to the axis of the bar; (b)Ribs shall not cross; and (c)Use of X patterns and diamond patterns for ribs is not permitted. 2.1.3 The rib spacing is at least 0.44 of the nominal diameter d b of the bar reinforcement. 2.1.4 The average rib width is not greater than one-third of the average rib spacing. 2.1.5 The bar size does not exceed No. 11 (No. 36). R2.1 A high relative rib area bar is defined as a bar reinforce- ment with R r greater than or equal to 0.10, as conventionally deformed reinforcement has relative rib areas of 0.06 to 0.085. Based on available experimental results, the use of these provisions is limited to bar reinforcement with a maximum R r = 0.14. Furthermore, consistent with the smallest spacing used in tests, the rib spacing s r should not be less than 44% of the nominal bar diameter, as indicated in Section 2.1.3. A lower limit on width of the concrete between ribs is indirectly prescribed in Section 2.1.4 to avoid having a reduction in bond capacity due to a local shear failure of the concrete between the ribs.

3.2—Definitions high relative rib area bars—deformed bar reinforcement with a relative rib area equal to 0.10 or larger. normalweight concrete—concrete containing only aggregate that conforms to ASTM C33/C33M. relative rib area—ratio of projected rib area normal to bar axis to product of nominal bar perimeter and average center-to-center rib spacing. rib—transverse protrusion or deformation on a deformed bar. The term “rib” is used uniformly throughout this document. R3.2—Definitions The definition of relative rib area presented in Section 3.2 of this guide is based on measurements of A r , d b , and s r (illustrated in Fig. R3.1). Figure R3.1 includes expressions for the approximate values of A r and R r . An alternative approach to calculate the relative rib area is presented in Appendix A, “Recommended supplement to ASTM A615/A615M for high relative area bars.” Equation (A-1) of Appendix A is a simplification of Eq. (4-6) of this guide which provides a close approximation of the relative rib area based on more commonly available measurements of bar deformations.ACI 408.3R pdf download.