CHANGES IN ACI 318- FROM A MATERIAL PERSPECTIVE
Changes in the last 3 editions of ACI 318
ACI 318 is the building code provided by the American Concrete Institute which is being used in many projects and becoming the most international recognized document in construction industry. ACI 318 is being revised every three years to adopt some rules based on changes.
Chapters 4 and 5 of earlier editions of ACI 318 were reformatted to emphasize the importance of considering durability requirements beside the selection of strength and the specific concrete cover over for the reinforcing steel. Chapter 4: “Durability Requirements”
The format of chapter 4 was extensively revised in 2008 by introducing exposure categories and classes, with applicable durability requirements given for the various classes in a unified format. Chapter 4 has now 5 informative tables related to concrete durability adopting the exposure categories and classes.
ACI 318 defines exposure categories and classes for concrete structures in section 4.2.1 specifically in tables 4.2.1.a through 4.2.1.d. In the 2011 version, ACI 318 defined the testing method to be used for determining the percent sulfate by mass in soil by ASTM C1580 and the concentration of dissolved sulfates in water in ppm by ASTM D516 or ASTM D4130. This addition was made to unify the testing method.
It is to note that ACI 222R has adopted chloride limits, test methods, construction types and conditions that are slightly different from that in ACI 318, as shown in Table R4.3.1. ACI 201.2R has adopted these same limits by referring to ACI 222R.
Chapter 5: “Concrete Quality, Mixing, and Placing”
EXTENSION OF THE 28 DAY TESTING
“Unless otherwise specified, f’c shall be based on 28-day tests. If other than 28 days then the test age for f’c shall be as indicated in contract documents.”
This change in code realizes the need to specify the strength of concrete at later ages when cementitious materials are being used or when long-term strength is required. When applicable, it shall be under the contract documents and not on the design drawings as stated in ACI318-08
REQUIREMENTS FOR STEEL FIBER REINFORCED CONCRETE.
A new section 5.1.6 has been added to ACI 318-08, and it requires steel fiber–reinforced concrete to conform to ASTM C1116, Standard Specification for Fiber- Reinforced Concrete. The minimum f’c for steel fiber–reinforced concrete is required to be the same as for conventional reinforced concrete which shall not be less than 17 MPa.
STRENGTH TEST RECORDS
Because of the concern that material properties may change with time, a limit of 12 months has been imposed in ACI 318-08 on the age of the historical data used to qualify mixture proportions under section 5.3, and proportioning on the basis of field experience or trial mixtures or both is allowed. This section was revised in ACI 318-11 to increase the duration and make it 24months by stating:
“Where a concrete production facility has strength test records not more than 24 months old a sample standard deviation shall be established”
REQUIRED AVERAGE COMPRESSIVE STRENGTH
ACI 318-11 revised the section related to the documentation of proposed concrete proportions. The proportioning shall produce an average compressive strength equal to or greater than required average compressive strength f’cr and shall consist of one or more field strength test record(s) or trial mixtures. The field strength test records or trial mixtures shall not be more than 24 months old.
Under section 184.108.40.206, the requirements that must be met by trial mixtures for concrete proportions established from such mixtures to be acceptable have been revised as indicated:
Requirement in 318-05 used to read: “Trial mixtures having proportions and consistencies required for proposed work shall be made using at least three different water-cementitious materials ratios or cementitious materials contents that will produce a range of strengths encompassing fcr.”
Since 2008 ACI 318 reads: “Trial mixtures shall have slumps within the range specified for the proposed Work; for air-entrained concrete, air content shall be within the tolerance specified for the proposed Work.”
Requirements (e) and (f) of ACI 318-05 read: “From results of cylinder tests a curve shall be plotted showing the relationship between water-cementitious materials ratio or cementitious materials content and compressive strength at designated test age. Maximum water-cementitious materials ratio or minimum cementitious materials content for concrete to be used in proposed Work shall be that shown by the curve to produce fc required by 5.3.2, unless a lower water-cementitious materials ratio or higher strength is required by chapter 4.”
All of this has been replaced by a new item, which reads: “The compressive strength results, at designated test age, from the trial mixtures shall be used to establish the composition of the concrete mixture proposed for the Work. The proposed concrete shall achieve an average compressive strength as required in 5.3.2 and satisfy the applicable durability criteria of chapter 4.”
(100 MM × 200 MM) CYLINDERS VERSUS (150 MM × 300 MM) CYLINDERS
Since 2008, ACI 318 recognized the use of three 4 in. × 8 in. (100 mm × 200 mm) cylinders as equivalent to the use of two 6 in. × 12 in. (150 mm × 300 mm) cylinders. The commentary clarifies that the confidence level of the average strength is preserved this way because 4 in. × 8 in. cylinders tend to have about 20% higher within-test variability than 6 in. × 12 in. cylinders.
The commentary also points out that more than the minimum number of specimens may be desirable to allow for discarding an outlying individual cylinder strength in accordance with ACI 214R.
ACI 318-05 section 220.127.116.11 reads: “Cores shall be prepared for transport and storage by wiping drilling water from their surfaces and placing the cores in watertight bags or containers immediately after drilling.”
The section now reads: “Cores shall be obtained, moisture conditioned by storage in watertight bags or containers, transferred to the laboratory, and tested in accordance with ASTM C42.” It now further explains that the maximum time between coring and testing is intended to ensure timely testing of cores when strength of concrete is in question.
STEEL FIBER–REINFORCED CONCRETE BEAMS
New section 18.104.22.168 requires that steel fiber–reinforced concrete beams be tested in accordance with ASTM C1609,Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading).
New section 22.214.171.124 was added in ACI 318-08 states that steel fiber–reinforced concrete is to be considered acceptable for shear resistance if the conditions are satisfied. The commentary points out that these performance criteria are based on results from flexural tests conducted on steel fiber–reinforced concrete beams with fiber types and contents similar to those used in the tests of beams that served as the basis for newly added section 126.96.36.199(f).
LOW-STRENGTH TEST RESULTS
ACI 318-11 added some modification for the investigation of low-strength test results which clarify that these instructions are applicable only for evaluation of in-place strength at time of construction. Strength evaluation of existing structures is covered by ACI 318 Chapter 20.