which it paid $160,000 in all. No one knows what it got, how thick it is, what was the mixture actually used, how many seconds the concrete was mixed, or even approximately the modulus of rupture of the concrete. Pigs is pigs, concrete is concrete, a job is a job. As long as the plans are well drawn and pretty and the specifications are as rigid as thought and care can make them, what matters the actual construction?

The officials concerned have the quite usual alibi of weak engineering and management, a guarantee and a bond. Which bond can, in ninety-nine cases out of a hundred, if it was ever really fought out, be blown so high as to be invisible.

Overdrawn? Not by a jugful in too many, many cases. Too much of our American engineering is marked by metriculous care in getting ready plans for doing things and an absolute disregard of any effective control while the plans are being executed. Millions for fine pictures and legal verbiage and hundreds for the only thing that can possibly guarantee delivery of what is specified; really intelligent and careful inspection of materials, mix, work and workmanship, as the work progresses.

Someone has said the curse of American public life is the ever present urge to draw and pass resolutions on every conceivable subjeet and then forget the subject. It can be just as well said that the curse of American public construction is the overwhelming urge. of public officials to exhaust their energies in making magnificent pen and word pictures of things to be done, to be promptly filed away until they are used to draw a still finer word picture for the next job.

Men who never built a good job in their lives enjoy excellent engineering reputations because they fling a wicked ruling pen and can out-legalize any ordinary lawyer in drawing specifications that no one understands, and which are therefore judged to be the last word on the subject. Many engineers will debate for hours this or that controversial point about concrete before they write their last word in specifications, and their actual work looks like something the cat brought in.

The programs of most of our meetings are made up of learned dissertations on how to design and build pavements, delivered by men whose sole construction accomplishment is the writing of books and the delivery of lectures. We rather face an oversupply of experts without portfolio. The theorist and slide rule shark who builds pavements in a shed in the back alley is getting the better of the horny handed sons of toil who really do design and build pavements for service.

Far be it from us to belittle the work of the expert in laboratory and field. It is a required part of the whole. But I am sometimes driven to wonder if it is not about time we put into practice on construction more of the things we do know before we allow our theory to get too far ahead of our practices.

Unfortunately, also, the average public official and engineer is so occupied with the multiplicity of things which must be done that in many cases he has not the time to separate the wheat from the chaff and often takes his opinion and his practice ready made from men and organizations who have a perfectly legitimate but selfish commercial purpose to serve. Too many engineers and officials are not thinking for themselves. They are allowing others-often others with axes to grind-to think for them.

It is so easy to be a follower, a "yes" man, and so hard for many to take the responsibility which comes from a personal determination of a course of action. We find the woods full of men holding to their public jobs like grim death by pursuing a course of masterly inactivity and refusing to choose or act for themselves as long as they can take the easy course of just following what some one else advises or is doing.

These chair warmers are proud of their reputations and supercareful to protect themselves from any possible criticism. They add an inch or two or three of concrete here, several hundreds or thousands of bags of cement there, many tons of steel here-whether they are needed or not. It is far better to soak their clients (the taxpayers) a few thousand dollars than that they should run the slightest risk of criticism. Oh, my, no!-Heaven forbid!

Then to make themselves super-safe they require a maintenance bond so that the contractor may possibly be held liable even if their job of engineering is poorly performed and their inspection is as rotten as they expect it to be, and several hundreds or thousands of dollars more is shot to make a job safe for a Democrat or Republican. We need more intestinal stamina in our engineering departments. Lack of it is costing us a lot of poorly built pavements and millions in excess prices.

Most local municipalities adopt the specifications of larger cities, or of some association, or of the state, and these in turn were largely copied from something else. Scissors and paste-pot specifications, however, do no great harm. Many specifications may be obsolete, but the worst-if followed-will produce fair results. If even ordinary plans and specifications were translated into practice, our pavements would be infinitely improved.

In other talks at other times, I have stressed the thought that the American people are almost always willing to spend money to gain a desired result, but that they are not so willing to spend money for proper engineering, although that situation is lately much improved. But they have never been willing to spend money for efficient and proper inspection. I know it to be a fact that many public bodies. have to largely camouflage their engineering and inspection costs in order not to arouse public protest. This, even though absolutely adequate, inspection costs an incredibly small portion of the whole cost of a modern pavement.

General conditions existing as to concrete bases and pavements do not vary from those affecting other types, except that inasmuch as concrete used as a wearing surface is a recent development in paving, there has been more variety of opinion and practice in recent years in concrete pavements than in the older established types.

In the last five years we have learned much about the properties. and conduct of cement, sand, water and coarse aggregate when intimately mixed, of the resulting concrete, and of the resulting slabs. For this knowledge we owe many individuals and organizations. Credit is especially due to the testing department of the Portland Cement Association under Duff Abrams; to the Illinois Highway Commission and to the U. S. Bureau of Public Roads.

Some characteristics of the materials of concrete and of concrete slabs seem to have been quite definitely established (all other things being equal), at least to the satisfaction of a large fraction of those who know something of concrete construction. We have listed some of these we believe established facts in the following summary. Many engineers and proponents of certain materials and methods will not agree that certain of these things are established. However, honest differences of opinion and debate thereon always make for real progress. The curse of concrete has been and is that too many things have been considered as gospel which were only partly true or were and are absolutely untrue.

1. The strength of concrete varies directly with the amount of mixing water used. The more water the less strength. The less water there is used, provided it produced a concrete which is workable and free from voids, the stronger the concrete.

2. The amount of water needed to make a workable mortar increases with the amount of sand used. Therefore the more sand in the mixture to the bag of cement, the weaker the mortar.

3. The voids in the coarse aggregate determine the amount of mortar required to make a dense concrete and therefor the larger the voids the weaker the concrete for an equivalent amount of cement.

4. Sand has not a constant volume. Its volume when thoroughly wet and when thoroughly dry is approximately the same, but when moist it bulks and unless correction is made for this bulking the resulting mortar will be less than expected and the slab possibly deficient in mortar for filling or finishing, or both.

5. Slabs must be well cured if the maximum strength of either surface slab or base is to be developed. There is about as much reason for well curing bases as for curing surface slabs.

6. Concrete stressed to over 50 per cent of its ultimate strength is dangerously weak under many repetitions of loads. In other words, it is dangerous to count on more than one-half of the modulus of rupture of the concrete when computing the safe depth of a concrete slab for a given load.

7. The character of aggregates used should vary the finishing methods used. Gravel coarse aggregates densify with less manipula

tion than angular aggregates, the latter often bridging toward the bottom of the slab unless tamped or agitated, especially if proper dry mixes are used.

8. No concrete slab attains the maximum beam strength for the mixture used unless it is free from voids, especially at the top and bottom of the slab.

9. There is apparently no definite relation between the compressive strength and the beam strength (modulus of rupture) of concretes. If the beam strength (modulus of rupture) is high the compressive strength will always be ample for all compressive purposes.

10. Too high cement content often produces trouble by causing excessive contraction and expansion in slabs.

11. The beam strength of concrete is adversely affected when the slab is moist. The greater the voids, the greater the possibilities of moisture in the slab and the less the strength in moist spells. Voids in the bottom of the slab are especially objectionable.

12. The character of the coarse aggregate used affects the surface wear very little under rubber-tired traffic, but the harder and tougher the coarse aggregate used the less the wear under chain traffic, which is often destructive.

13. Smoothness or roughness in the subgrade may have more influence on crack formation or lack of it, than many other factors. There is some indication that we may ultimately lay subfoundations of sand, cinders, stone dust or sandy gravels to enable us to produce. a really smooth subgrade and one more free from water troubles.

14. It is not true that concrete can be manipulated too much in its laying. The best concretes are the densest concretes, and under many conditions dense concretes cannot be produced without manipulation of some kind, unless water is used greatly in excess.

15. Surplus mortar in the slab or on the surface of the slab is a source of weakness, not of strength. The less mortar over that positively required to finish a dense slab with the method used, the better the final results.

16. Dirty materials are bad, not only because they usually reduce the strength of the concrete, but are worse because they are the most common cause of surface scaling, especially if the mortar is in excess.

17. Freezing and thawing of concrete slabs is a danger to their continued existence too much disregarded. This danger is ever present in regions subject to freezing and thawing spells and is much increased if the slabs are not dense.

Each of these fundamental principles are subject to so many modifications to fit different conditions that the possible compass of this paper and your patience would be exhausted if we were to go into them. Within broad limits we believe that every one of these propositions are conceded by most engineers to be sound and proven, and if we were mindful of these points in our practice wonders could be wrought. The unfortunate part of the matter is that an astoundingly

small percentage of American highway organizations pay any marked attention to many of them in their actual planning or construction practice.

In the average American paving specifications, both of base and of slabs subjected to wear without other covering, we revert back to the common standards of twenty years ago in specifying that the materials used shall meet certain requirements which vary widely in various units of government. Within any of these specifications, completely met, resulting concrete can be built which will vary 50 per cent or even more from other concretes which can be built with other materials meeting the same specifications.

It would seem only sound business to pay for what we are getting in materials. If certain materials produce much stronger and better concrete than other materials it should be possible to encourage their use by fitting the slab to the materials. Theoretically, we desire a slab of a certain strength. The materials and methods which produce that strength at the minimum cost are the ones to use.

I am one of the comparatively few American engineers who believe that we will not be really on the right track in the construction of concrete pavements and bases until we learn to establish what we need and then really build it. In other words, in a concrete pavement, or a concrete base, we must first assume the loads which may be expected to use the structure within its life. That having been determined, we can determine the depth of concrete of a given modulus of rupture required to sustain these loads throughout the life of the pavement. If that depth is 8 inches of concrete with a modulus of rupture of 600, we are flirting with providence if we, in fact, build 8 inches of concrete with a modulus of rupture of 300.

The precise means we are going to use to determine instantly that we are in fact getting concrete of at least the determined upon modulus at thirty days is not absolutely clear. Probably a simple cantilever test of cast beams at twenty-four hours will give a fairly close check, if supplemented by a careful watch of the water-cement and cementsand ratio, and the mixing time. The problem of an early test of beam strength and calibrating them with standard long period tests is one of the most important now before us.

The commonly accepted formula used in American pavement design. at this moment was developed by the Illinois Highway Department as a result of the Bates road test. This formula gives the required depth of the unsupported corner of a concrete slab. The formula is as follows:

Where D is the required thickness of pavement at an unsupported edge. W is the expected maximum load on a single wheel, and S is one-half of the modulus of rupture of the concrete to be used. Illinois further demonstrated that the required depth of a slab where it