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April 8, 1997 8:00 AM CDT

Splice Length Research

By , , ,

Introduction
The Council for Masonry Research decided at their meeting in November 1996 to fund a series of tests to investigate the effects of the length of reinforcing bar splices in masonry. The tests were a follow-up to similar tests sponsored by the National Concrete Masonry Association in 1995 and 1996. The Brick Institute of America, the Western States Clay Products Association, the Washington State Conference of Mason Contractors, and the National Concrete Masonry Association provided additional funding to that supplied by CMR to increase the scope of the project.

  1. bar diameter
  2. splice length as a function of bar diameter
  3. cover as a function of bar diameter
  4. masonry strength

This information was needed to provide design requirements for splices in masonry. Reducing required splice length results in more economical construction. Reduced cover permits reinforcement in thinner walls and also allows placement of bars closer to the tension face of certain wall applications.

Test Matrix and Materials
Specimens of fully grouted hollow brick and block were chosen as shown in Tables 1 and 2. Through wall nominal dimensions for hollow brick were 4 in. (100 mm) and 6 in. (150 mm). Nominal face dimensions of the brick were 4 in. (100 mm) high by 16 in. (400 mm) long. The brick met the requirements of ASTM C652 and had compressive strengths in excess of 5000 psi. Block used were all 8 in. (200 mm) nominal thickness and 8 in. (200 mm) nominal height and 16 in. (400 mm) nominal length. The block met the requirements of ASTM C90 and had an average net area compressive strength of 2200 psi.

Mortar for all walls was Type S by proportion with cement:lime cementitious materials. Grout for the brick specimens was fine grout mixed according to the proportion requirements of ASTM C476. Coarse grout for the block specimens was formulated to have a compressive strength close to that of the block.

Steel reinforcement had a specified yield strength of 60 ksi (414 kPa). Bars were threaded at one thickened end in order to facilitate attachment to the testing rams and anchorage points. Splice lengths were chosen to be the minimum values that were projected to develop the yield strength of the reinforcing bars. In some specimens, cover depths were chosen at or near the minimum values that could be obtained with the units used and at or near the center of the wall in others.

Test Specimens


Figure 1. Test Setup
Each test specimen contained two individual splices, separated by 16 in. (400 mm) in a masonry assemblage. Assemblage height was established by the splice length being investigated. Units were laid with face shell bedding. Mortar protrusions into the grout space were removed. The two splices were formed by lapping the bars the established length. The ends of the bars were tied with wire to adjacent bars. The splices were placed at the appropriate cover distance. Each assemblage was grouted in a single lift and reconsolidated. The specimens were cured in laboratory air for at least 28 days before testing.

The capacity of the splices was evaluated using the test setup shown in Figure 1. Increasing tensile forces were applied to the reinforcing bars until failure occurred. Since two splices were evaluated in each specimen, the failure occurred in the splice with the lower strength.

Results
A typical series of observed events occurred prior to failure as follows:

  1. tensile cracking at mortar/unit bed joints
  2. cracking parallel to the reinforcing bars in the units or mortar/unit head joints near the projecting bar
  3. cracking parallel to the reinforcing bars extending toward the center of the specimen
  4. cracking continued as the bar yielded (load remained relatively constant)
  5. sudden failure in the masonry as the cracks propagated along the reinforcing bars

A full report of the testing program will be published following a review by the CMR Technical Committee and technical papers will be presented at the International Masonry Conference in Shanghai, China this year.

Table 1: Reinforcing Bar Splice Matrix in Hollow Brick Masonry

Brick Width in. Bar Size # Splice Length in. Cover Depth in.
4 4 18 (36 db) 1 (2.0 db)
4 4 15 (30 db) 1-1/4 (2.5 db)
4 4 18 (36 db) 1-1/4 (2.5 db)
4 4 15 (30 db) 1-1/2 (3.0 db)
6 4 18 (36 db) 1-1/2 (3.0 db)
6 5 30 (48 db) 1-1/2 (3.0 db)
6 5 25 (40 db) 2 (3.2 db)
6 6 36 (48 db) 2 (2.7 db)
6 6 36 (48 db) 2-3/8 (3.2 db)
6 7 42 (48 db) 2 (2.3 db)
6 7 42 (48 db) 2-3/8 (2.7 db)

Table 2: Scope of Construction Variables for Concrete Masonry Walls

CMU Width in. Bar Size # Splice Length in. Cover Depth in.
8 4 18 (36 db) 2 (4.0 db)
8 5 30 (48 db) 2 (3.2 db)
8 6 36 (48 db) 2 (2.7 db)
8 6 36 (48 db) 3 (4.0 db)
8 7 42 (48 db) 2 (2.3 db)
8 7 42 (48 db) 3 (3.4 db)
8 8 48 (48 db) 3 (3.0 db)
8 8 64 (64 db) 2 (2.0 db)
8 9 54 (48 db) 3 (2.7 db)
8 9 72 (64 db) 2 (1.8 db)


About the Authors

J. Gregg Borchelt, P.E. is the President and Chief Executive Officer and Vice President of Engineering and Research for the Brick Industry Association (BIA). He was in charge of technical information for BIA; Gregg also chairs ASTM Subcommittee C 15.02 on Brick and Clay Tile.

Mark Hogan is the former President and Vice President of Engineering for the National Concrete Masonry Association.

Phillip Samblanet is a professional engineer, and the Executive Director of The Masonry Society. He served as the secretary for the MSJC Inspection Task Group.

Robert Thomas is president of the National Concrete Masonry Association.

 

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