How clean must rebar be? Most specifications require reinforcement to be free of deleterious materials. But do common construction contaminants have a harmful effect on bond? BY BRUCE A. SUPRENANT AND WARD R. MALISCH orm-release agents, bond breakers and cement splatter sometimes contaminate rein- forcing steel before concrete is F placed. However, ACI 301-96, Standard Specifications for Structural Concrete (Ref. 1), says “When con- crete is placed, all reinforcement A B shall be free of materials deleterious to bond.” Inspectors often cite this Figure 2. Amount of cement splatter (a) and rust (b) on the rebar tested. sentence when requiring contractors to remove form-release or bond- breaker overspray and cement splat- contact reinforcing steel or hard- ter from contaminated rebar. But is ened concrete against which fresh this work really necessary? concrete is to be placed.” ACI Man- The Aberdeen Group ran a series ual of Concrete Inspection (Ref. 3) of bond pull-out tests to assess the states that “reinforcement should be effect of contaminants on bond clean, and oil or nonadherent mor- strength. Pull-out tests measure the tar which has been spilled on it bond force acting parallel to the bar should be cleaned off.” on the interface between the bar In contrast to the requirement for and concrete (Ref. 2). We tested removing oil and mortar, ACI 301 clean, black Grade-60 steel bars and provides different criteria for rust: bars with form-release agents, curing “Reinforcement with rust, mill scale, compound/bond breakers, cement or a combination of both will be splatter, motor oil and rust on their considered satisfactory provided the surfaces. The results may surprise minimum nominal dimensions, most inspectors. nominal weight, and the minimum average height of deformations of a Requirements for hand-wire-brushed test specimen are clean rebar not less than the applicable ASTM Besides ACI 301’s general state- specification requirements.” This ment requiring all reinforcement to recommendation is based on tests be free of deleterious materials, the performed by Johnston and Cox in Figure 1. Form-release agent was specification also specifically men- 1940 (Ref. 4). These investigators sprayed on the rebar, covering 100% tions form-release agents: “Do not performed about 420 bond pull-out of the surface. allow formwork release agent to tests on deformed bar specimens with 78 different degrees of rust. We couldn’t find similar studies showing the effect of common con- struction contaminants on rebar bond. Without such data, most spec- ifiers and inspectors take a conserva- tive approach by requiring removal of such materials from rebar. But re- moving contaminants is time-con- suming and costly, and if construc- tion contaminants aren’t detrimental to rebar bond, their re- moval may not be necessary. Be- cause of the lack of data, The Ab- erdeen Group initiated a limited test program. Common contaminants Figure 3. Pull-out Common construction contami- specimens were cast in 6-inch-diameter, nants were applied to clean rebar, 6-inch-tall cylinder and clean black-steel and rusted molds with a #4 rebar were included as reference rebar at least 24 standards. The bar contaminants inches long inserted and how we applied them are de- at the center. scribed in the table on page 521. Grade-60, #4 deformed rebar were setup helped keep the bar vertical room, the loaded ends of the con- used. The form release and curing during concrete placement and pro- crete specimens were capped with a compound/bond breaker were vided enough protruding rebar high-strength gypsum-based cemen- sprayed on 100% of the rebar surface length for the test-machine grips. titious material. 1 (Fig. 1) to duplicate the worst case of Ready-mixed concrete with a 3 ⁄2- The test-specimen size, embedded contaminant coverage possible dur- inch slump and 1.3% air content rebar length, concrete placement di- ing construction. The used motor oil was delivered to the testing lab. The was applied to the entire bar length concrete compressive strength at the with a rag. A cement paste was time of pull-out testing was 5490 mixed and applied to various areas psi. of the rebar (Fig. 2a). To produce The concrete was cast so it would rusted rebar, three bars were dipped settle in the direction of the applied in hydrochloric acid then stored in a load, eliminating any effect bleeding moist curing room. Figure 2b shows might have on the measured bond. the amount of rust on the bars. We placed the concrete in two lay- ers, rodded each layer 25 times and Specimen construction lightly hand-tapped the cylinder We constructed bond pull-out sides after each layer was rodded. specimens with a 6-inch embedment The top surface was floated and then 5 depth by using the bottom halves of covered with a cylinder lid with a ⁄8- 6x12-inch plastic cylinder molds inch-diameter hole drilled through 5 with a ⁄8-inch-diameter hole drilled the center. The lid provided initial through the bottom of each (Fig. 3). curing and maintained the bars in a The molds, 27 in all, were placed on vertical position. After 24 hours in 2x6-inch boards that had been lab air at about 70°F, the cylinder 5 1 predrilled with ⁄8-inch-diameter, ⁄2- lids and molds were removed, and inch-deep holes. We inserted #4, the test specimens were placed in a Figure 4. With the test specimen placed Grade-60 deformed rebar at least 24 moist curing room on top of the 2x6 on a spherical bearing block on top of inches long in a vertical position, bottom supports. The specimens re- the testing machine, a tensile load was 1 with ⁄2 inch of the rebar protruding mained in the moist curing room 15 applied with serrated grips. An from the bottom of the cylinder days and were tested at that age. electronic digital indicator measured mold into the hole in the 2x6. This After removal from the moist curing slip at the free end of the rebar. rection and curing conditions were served specimen failure modes. the deformations, however, no shiny the same as those used by Johnston surface was visible, indicating that and Cox to determine the effect of Contaminants had little concrete bearing against the rebar rust on rebar bond. effect on bond strength deformations during loading re- Instead of pulling out of the con- moved the contaminant by friction. Test procedure crete specimen, which would be ex- We believe the greater initial slip of Figure 4 shows a typical setup for pected if the contaminants com- rebar covered with the contaminants a pull-out test. Test specimens were pletely destroyed bond, the rebar was due to loss of adhesion along the smooth part of the bar between placed on a spherical bearing block broke in eight of the nine sets of the deformations. As shown in Fig- on top of the testing machine. Ser- three tests. The concrete specimens ure 5, after the initial slip, the load rated grips were then connected to broke in the remaining test. The was resisted by bearing of the defor- the rebar to allow the machine to table shows the average slip at yield mations on the concrete and the load the bar in tension. A 60,000 and ultimate load, the average stress pound Tinius-Olsen universal ten- shear strength of the concrete be- at ultimate load, and the failure sion/compression frame was used to tween the deformations (Ref. 5). mode for the two reference bars and apply the load to the rebar at about An excessive amount of slip before seven bars with contaminants. The 200 pounds per second. Technicians the yield stress of the bar is reached ultimate stress achieved by the bars measured slip of the free end of the will increase deflection of reinforced- was not affected by any of the conta- rebar with a Fowler 1/10,000 elec- concrete members. While the slip is minants. tronic digital indicator. greater for bars with contaminants, We broke open the concrete speci- As the load was applied to each the increase in slip is similar to that mens to examine the failure surface of rusted bars when compared with specimen, one person read the dial of the rebar. A shiny surface on the unrusted bars (Ref. 4). indicator while another person mon- rebar, indicating where the contami- The data and conclusions are itored the load at each slip reading. nant had been sprayed, was appar- based on only 27 tests of bars with A third person recorded data and ob- ent between the deformations. At nine different surface conditions. Bond pull-out test results (average of three tests) Description Slip at Slip at Ultimate Failure yield, in. ultimate, in. stress, psi mode Plain rebar—in place before concreting <0.0001 ±0.0005 101,125 Bars broke Rusted rebar—in place before concreting ±0.0002 0.0026 101,083 Bars broke Cement splatter—applied 24 hrs before concreting <0.0001 ±0.001 101,083 Bars broke Form release1—applied 24 hrs before concreting ±0.0005 0.0032 101,083 Bars broke Form release1—applied 15 mins before concreting ±0.0008 0.0049 100,500 Bars broke Curing/bond breaker2—applied 24 hrs before concreting ±0.0008 0.0034 97,167 Concrete broke Curing/bond breaker3—applied 24 hrs before, then cleaned4 15 mins before concreting ±0.0002 0.0024 101,000 Bars broke Form release5—applied 24 hrs before concreting ±0.0002 0.0023 101,083 Bars broke Used motor oil—applied 24 hrs before concreting ±0.0004 0.0031 101,417 Bars broke 1.