COMMENTS ON TENTATIVE REPORT OF THE JOINT COMMITTEE ON CONCRETE AND REIN

FORCED CONCRETE

Samuel T. Wagner, Chief Engineer, Philadelphia & Reading Railway The Joint Committee on Standard Specifications for Concrete and Reinforced Concrete are to be congratulated upon the promptness with which they have presented their first progress report. When the efforts of the original Joint Committee on Concrete and Reinforced Concrete are considered, which was in 1904 and finished its labors in 1916, the promptness of the existing Committee is especially commendable.

The difficulty in the preparation of the present report it is believed lies in the fact that it is not as easy to prepare a specification as it is to set down items of recommended practice on which a specification could be based. In a general way what is now presented has very many commendable features and if all of the conclusions are accepted as facts, it will be comparatively easy for an Engineer to prepare a specification for any particular kind of work. It is believed that the present report bears the earmarks which characterized so many of the reports of the American Society for Testing Materials at the 1921 convention; that is, that the majority of the conclusions are based on research work rather than on the personal experience and opinions of the members of the Joint Committee.

The following are a few of the points which appear to deserve some comment. No attempt has been made to cover the section devoted to

Design.

(III) QUALITY OF CONCRETE. A distinct innovation is introduced in this Section, in that the Engineer is to specify the strengths required for the concrete in the various parts of the work based either upon preliminary test or upon the view given in Table IV. From the theoretical and scientific standpoint, this is, of course, ideal, as it recognizes the various uses to which the method is applied. It contemplates much more testing and of a much more elaborate kind than has ever been used in general practice and particularly upon work of moderate size and how it will work out practically is an open question. The theoretical idea is sound. The proposed use of the slump test for determining consistency is a decided step forward in the proposed specification. It is an easy test to make on the work and after the proper amount of slump is determined for any piece of work, it is a simple matter to test the consistency frequently, and thus keep check on it. This is a very difficult operation without such a test. The use of mortar tests of a fine aggregate, using either briquettes, cylinders or prisms, compared with similar tests of Ottawa sand is believed to be the only adequate manner of determining its quality and is sure to exclude from important work many grades of fine aggregate which should never be used and which could not otherwise be detected.

(V) PROPORTIONING AND MIXING.-Many engineers will have difficulty in determining which of the methods given shall be used. It is to be hoped that in final report the number of these methods may be reduced.

The requirement of devices for automatically measuring the amount of water is to be commended, as it is one of the essentials of uniform workmanship. Many of the seeming refinements such as this may appear to be uncalled for, but when consideration is given to the fact that the type of material called for is made on the ground largely by unskilled labor and frequently under inadequate supervision, it must be recognized that more care must be exercised in the essentials which are known to be responsible for the quality of the finished product. Among these is also the requirement of the minimum time that each batch shall be in the mixer.

(VI) DEPOSITING CONCRETE.-There seems to be no good reason why the distances should be specified between stones in the rubble cyclopean concrete. If the stones are imbedded in the concrete after it has been placed, it is impossible to know how far apart they are, and even if they do touch, there is probably sufficient concrete between them to answer every purpose. It is entirely proper to specify that the stone should be kept away from the surface of the concrete.

(IX) WATERPROOFING AND PROTECTION TREATMENT. The paragraph under Waterproofing is distinctly disappointing. For certain classes of plain or reinforced concrete structures the necessity of providing some method of protecting them from the action of the water is of great importance and deserves more definite treatment than has been given. It is believed that the general statements made in this paragraph are in accord with good practice. The details of how the work is to be done are woefully lacking and it is to be hoped that the Committee will fully cover the matter in their final report.

The requirements under Section X-Surface Finish-are a distinct advantage over any specification of a general character to be published up to the present time, and the practice is generally good. While it seems perfectly proper to mention sand blast finish under "Decorative Finishes" as one which could be considered, it is the opinion of the writer from experience that it is neither satisfactory nor advisable on account of the difficulty of obtaining uniform results.

C. C. Williams, Professor of Civil Engineering, University of Kansas In the list of Definitions, I would define "cement paste" as "a paste formed by mixing hydraulic cement and water"; then I would define "concrete" as "an artificial conglomerate stone formed by cementing aggregates together with cement paste. The term is also applied to the material in the plastic state during fabrication."

I do not believe the term "brackets" should be used at the bottom of page 11, in the definition of "buttressed retaining wall," nor in the definition of "counterforted retaining wall," and would suggest the word "buttresses" in the former case and "counterforts" in the second.

Under quality of concrete, page 15, I would suggest the four distinct classes of concrete be made:

Class A, for highly reinforced concrete requiring 2,000 pounds per square inch or more at twenty-eight days.

Class B, for ordinary reinforced construction and first class plain

concrete.

Class C, plain concrete retaining walls, dams, etc.

Class D, concrete for bearing walls and other structures where very light load is required.

On page 16, paragraph 11, I do not believe the specification of fine aggregate in terms of strength of 1:3 standard sand mortar is entirely adequate.

Certain tests made under my direction showed a high strength for fine aggregates which proved unsatisfactory for concrete (see Engineering News-Record, May 22, 1919).

Also I would make a classification of fine aggregate of three classes as follows:

Class A aggregate shall consist of quartz sand containing not more than 5 per cent clay and shall be free from organic impurities. It shall be graded in size, all particles passing a one-fourth-inch screen and not more than 30 per cent shall pass a 50-mesh screen. Stone screenings of a durable hard mineral having the same properties may be used.

Class B aggregate shall consist of sand composed of quartz and other minerals of fairly hard and durable texture or a calcareous sand having properties similar to those of Class A. The clay contents shall not exceed 10 per cent and organic matter shall not exceed that giving a color with the standard test darker than a medium brown; quartz sand 75 per cent of which passes a 20-mesh screen with not more than 5 per cent clay and free from organic matter may be used; stone screenings of a durable character and containing not more than 7 per cent clay, with not more than 50 per cent passing a 50-mesh screen may be used.

Class C fine aggregate shall consist of sand or screenings containing clay not to exceed 15 per cent and not more than 60 per cent passing a 50-mesh screen.

I would also suggest a corresponding classification of coarse aggregate. On page 21, paragraph 31, one and a half minutes seems to me to be needlessly long for the mixing of concrete.

The above classifications are intended to be suggestive rather than carefully defined. My idea is that the specifications should recognize that all construction cannot be first class and should provide for inferior grades of work as well as for first grade.

F. Auryansen, Bridge Engineer, Long Island Railroad

In the Preface, the Committee calls attention to the fact that the specification is developed particularly on the basis of reinforced concrete work in buildings and this has led to some definitions and statements which are perhaps a little inconsistent with requirements for bridge work. I assume, however, that the representatives of the A.R.E.A. have given due consideration to this phase of the subject, and that inconsistencies will be remedied in subsequent editions.

In the definitions of "Aggregate," neither slag nor light weight aggregate are included.

A "column" is defined as a vertical compression member, whereas in bridge or foundation work a compression member which would be designed as a column, might not necessarily be a vertical member. Similarly, in a cantilever retaining wall, the upright stem might not be vertical nor the supporting base strictly horizontal.

"Dead loads" are defined as weight of structures plus fixed loads and forces. Perhaps "fixed loads and constant forces" would be a better expression, the contrast being with live loading which varies in amount, duration and position.

Engineer. The engineer in responsible charge of design and construction-add the words "or his duly authorized representative."

Laitance. Add to the definition, the condition under which it is formed, namely, "when excess mixing water is used."

Membrane Waterproofing.-Substitute the following definition-“A bituminous coating of two or more layers reinforced by intermediate fabric, felt or similar strengthening material; applied to structures to prevent contact of moisture." It should be noted that the function of the fabric is not only to reinforce, but it also assists in securing a composite coating having more pitch than could generally be applied by mopping directly upon the surface of a previously placed layer. In this respect, the open mesh fabrics are superior to the felts; they are also much stronger.

Mention should be made of other kinds of waterproofing, such as mastic, or brick laid in hot tar or bitumen.

Pedestal or Pier.-A vertical compression member whose height does not exceed three times its least horizontal dimension.

Reinforced Footing is not mentioned. It might be defined as a member supporting a column, in which the projection from the face of the column is greater than one-half the depth, or is of such spread as to require reinforcement.

(III) QUANTITY OF CONCRETE.-Section 3, line 3, would substitute the following: Compressive strength at 28 days as determined by "specific" tests of the materials.

(IV) MATERIALS.-F. Water.-19.

Does not mention salt, but leaves

it to be implied in "other deleterious substance."

G. Metal Reinforcement.-Table 1, Page 18-Omits square bars of ′′, %", 4" and 3" size. We have found in our work that 5/8" square bars go very nicely in 18" punched holes.

25. Cast-iron would not be used in bridge work.

(V) PROPORTIONING AND MIXING CONCRETE.-28. Proportions-under the alternate methods, bottom of page 20, nothing is said in regard to the proper amount of water to be used in mixing.

(C) MIXING. Should take account of the temperature limitations which should be imposed. I suggest that Section 41 be subdivided, and

the references to suitable mixing temperatures be inserted following Section 33.

(VII) FORMS.-53. Top of page 25 insert the word "anchored" to read "they shall be properly anchored, braced or tied together so as to maintain position and shape." Forms for retaining wall with inclined back have been known to rise under the component of hydro-static pressure of the concrete producing an upward reaction.

54. Next to last sentence might read-"Forms shall be set to line and grade and so constructed and fastened as to produce correct lines." 56. Possibly special mention should be made of the treatment of metal forms to prevent concrete adhering to same.

58. Second line revised to read "nor shall the principal shores be removed," etc.

(VIII) DETAILS OF CONSTRUCTION.-A. Metal Reinforcement.-59. Change second sentence to read "Reinforcement to carry calculated stress, if appreciably reduced in section, shall be rejected."

(C) JOINTS.-69. The common practice of using metal members for bonding, such as projecting rods, pieces of old rail, etc., is not covered unless by implication from Section 65.

Construction joints

73. Construction joints in long "structures." made crosswise of a "structure" 100 ft. or more in length, etc. Many bridges are much longer than ordinary buildings.

74. Second sentence on page 28 implies that the joints are horizontal I would suggest that this be revised to read-"Exposed expansion joints formed between two distinct concrete members shall be of approved design and filled with elastic joint filler of approved quality, or made waterproof by other suitable means."

75. Change heading to read-"Expansion Joints in Long 'Structures'." 77. Seems to assume horizontal joint which may not always be the case. A different style of treatment is required for horizontal and for inclined joints. I would suggest that the definition be extended to provide for reinforcement in the case of fixed joints, which would hold the abutting sections of the structure in their proper relation.

(IX) WATERPROOFING AND PROTECTIVE TREATMENT.-78. Amend second sentence to read-"Particular attention should be given to secure the best kind and quality of workmanship."

80. As above mentioned, does not allow for the use of mastic or brick in hot pitch. Nothing is said regarding protection of the waterproofing membrane, which is vital to its integrity. I would refer the Committee to the Principles for Detail Design of Flashing, Drainage, Reinforcement and Protection for Waterproofing Purposes, found on page 395, Bulletin 232.

(C) Concrete in Sea Water.-85. Protection-Revise second sentence to read "Metal chairs, supports, ties or other members shall not extend to within 2" of the surface of the concrete." Add to the section a provision that no sharp corners shall be allowed.