 The concrete design for our highways, bridges, and other structures is intended to be of the highest quality. It should consist of approved materials that are correctly proportioned and mixed, and then carefully transported to the job and placed. In practice, however, the concrete ordered for the job may not be the concrete that's actually placed. At critical points during production, transport, or handling on site, the concrete's quality may be jeopardized. Bature mixer slips are not always a guarantee that you have the right concrete in the truck. For example, if wash water was left in the truck before it was loaded, the mix may be too wet, or the truck driver may decide to add water if the mix begins to stiffen. So it's important, and it's required, that the quality of the concrete be verified before the concrete is cast. Verification is provided by conventional control tests performed in the field. This presentation is the first of four videotapes on quality control of concrete on site. Here in part one and in part two are presented the conventional control tests for determining concrete quality at the job site. In parts three and four, you'll see new quality control procedures that have been developed by Sharp and others for testing concrete both before and after it's cast. So, here in part one, we'll look at sampling concrete and at the test methods for temperature and slump. In part two, we'll look at the test methods for air content and unit weight, as well as the procedures for making cylinders and beams. Our first topic, then, is sampling. Before any of the tests can be performed, a sample of the concrete is needed, a representative sample. The concrete to be tested has to be typical of the entire truck load. Sampling should be done according to AASHTO designation T141. The basic rule is this. Don't take the sample from the beginning or end of the discharge. Take it from the first one-third of the discharge, but never from the first two cubic feet. To obtain the sample, either the entire stream should be caught in a container, or the chute should be swung over to the side. The important point is to get the entire stream. Cutting the stream in half, like this, is unacceptable. The rim of the bucket will knock some of the large aggregate out of the sample. The sample should be mixed with a shovel just until it has a uniform appearance. Then it should be covered to protect it from the sun and the wind. Samples must not be allowed to dry out. One last point. Testing must be completed within 15 minutes after obtaining the sample. Now, temperature. The temperature of fresh concrete affects the rate at which concrete gains strength, the amount of strength the concrete ultimately acquires, and the porosity of the concrete. Checking concrete temperature is simple. It should be done according to ASTMC 1064. All that's needed is a standard thermometer, such as an ASTM liquid and glass type, or a metal immersion type. The thermometer is inserted into the fresh sample so that the temperature-sensing portion is submerged at least three inches. The concrete should be gently pressed around the thermometer so that the air temperature does not affect the reading. The thermometer is left in the concrete for two minutes, or until the temperature stabilizes. Then the temperature is recorded. The measurement must be completed within five minutes after the sample is obtained. The temperature of in-place concrete is measured in the same way. Now, the air temperatures during concrete construction are also important. The high and low air temperatures must be monitored daily. Next on the list is the slump test, AASHTO T119. This is by far the oldest and most widely used test to measure the uniformity and consistency of concrete. Variations in slump generally indicate that changes have occurred in the properties or proportions of cement, aggregates, water, or admixtures. The equipment needed for slump tests includes a standard slump cone, a standard tamping rod, a scoop, a base, and a ruler or measuring tape. The slump cone should be reasonably clean, especially inside. The surfaces should be smooth with no dried concrete on them. Also, there should be no major dents. A standard tamping rod is smooth with a rounded tip. Makeshift rods, such as Cut-Off Reinforcing Bar, are unacceptable. The base for the cone must be strong, rigid, and stable enough to support the tester as well as the concrete and equipment. It needs to be level and firmly supported so that it won't rock or bounce. The sample should be brought close to the test location and remixed if necessary until it looks uniform. The test is begun by first dampening the inside of the slump cone to keep the concrete from sticking to it. Then, the base is dampened as well so that it won't absorb water from the concrete. But no free-standing water should be left on the base. The cone should be set down on the base as close as possible to the sample. Next, the technician steps onto the footholds at the bottom of the cone and stays there. That's why the sample and cone need to be close to each other. The cone is filled by moving the scoop around the top to distribute the concrete as evenly as possible. The filling is done in three layers. Each layer should take up approximately one-third of the cone's volume. So the first layer should come up about two-and-a-half inches from the base. The second layer should come up about six inches. That's about halfway up the cone. And the third layer should overflow the cone. As each layer is placed in the cone, it's consolidated with a tamping rod. Starting at the perimeter with its rounded tip downward, the rod is inclined slightly to match the slope of the cone. Otherwise, the concrete closest to the cone's sides will not be properly consolidated. The rodding should go to the bottom of this first layer, but just tap the base. Each layer is rotted 25 times, with about half of the strokes made near the perimeter. A spiraling pattern should be followed as the rod moves toward the center of the cone. The strokes are distributed uniformly. As the rod is moved away from the perimeter, it's returned to the vertical. The second layer fills about half of the cone's height. It's rotted the same as the first with one difference. The rodding should just penetrate about a half inch into the first layer, not all the way down to the base. The third layer should overflow the cone. That way, the concrete stays above the cone throughout the rodding. If it drops below the top, the rodding should be stopped. More concrete should be added, and then the 25 spiraling strokes should be completed. And the rod should just penetrate the top of the second layer about one half inch. With the cone full and the rodding completed, the surface of the concrete is struck off by moving the tapping rod across it in a screaming and rolling motion. The concrete surface must be level and fairly smooth. Next, any concrete that is spilled around the bottom of the cone must be removed. Otherwise, it may keep the sample from slumping properly when the cone is lifted off. However, the technician must be careful not to move the cone and disturb the concrete. Basically, both sides of the cone must be held still by pressure on either the handholds or footholds until the cone is removed. To remove the cone, both hands are placed on the handholds and downward pressure is applied to keep the cone stable before the feet are removed from the footholds. Now the cone is pulled straight up, slowly and steadily, without twisting or tilting. The removal should be completed in about five seconds. Slowly enough that the concrete doesn't pull on the cone. The cone is then set on the base, next to the concrete, without touching the concrete or jarring the base. Immediately the tamping rod is laid flat across the top of the cone, extending over the concrete. A ruler or measuring tape is used to measure the distance between the bottom of the rod and the top of the concrete. The measurement should be made to a point that is directly above the center of the bottom of the concrete, not at the center of the top. The entire slump test must be accomplished within two minutes, as measured between the adding of the first layer and the measuring of the slump. And that's sampling temperature and slump. And the end of part one. In part two, we'll cover air content, unit weight, and the making of cylinders and beams. This second part of quality control of concrete on site will examine the test methods for air content and unit weight, plus the procedures for making cylinders and beams for compressive and flexural strength tests. To begin, air content test methods. The results of air content tests indicate the freeze-thaw durability of concrete. Air-entrained concrete with the specified air content performs much better than non-air-entrained concrete in freezing and thawing conditions. On the job, concrete can be rejected if its air content is not within the allowed range. The most popular method for measuring air content is the pressure method. Two types of pressure meters are available and are widely used. They are referred to as type A and type B and are covered in AASHTO T-152. With type A air meters, the equipment needed includes a calibrated type A meter, a tamping rod, just like the one used in the slump test, an internal vibrator, a rubber or rawhide mallet, a scoop, a strike-off bar, and a supply of fresh water. The meter consists of a measuring bowl having a capacity of at least two-tenths cubic foot and a cover assembly that includes a standpipe, a pressure gauge, and an air pump. The cover assembly also contains a number of valves and petcocks. The bowl and cover must clamp together to form a pressure-tight seal. As with the slump test, all the equipment should be clean and in good condition. It should be assembled near the concrete sample. The test is begun by filling the measuring bowl with concrete in either two or three equal layers and consolidating each one. The number of layers depends on whether the consolidating is done by rotting or vibrating. And the method of consolidation depends on the result of the slump test just performed. If the slump is less than an inch, vibrating is required. If it's more than three inches, use the tamping rod. And if it's between one and three inches, either method can be used. When the tamping rod is used, the bowl is filled in three equal layers. Each layer is rotted 25 times and the strokes are distributed evenly over the cross-section. The bottom layer should be rotted throughout its depth, but without forcibly striking the bottom of the bowl. After each layer is rotted, the sides of the bowl should be tapped sharply with the rubber mallet 10 to 15 times around the bowl's circumference. This will close any voids left in the concrete by the tamping rod and release any large entrapped air bubbles. In rotting the second and final layers, only enough force should be used to penetrate the surface of the previous layer about one inch. Then, the final layer of concrete is added without excessively overfilling the bowl. When the vibrator is used, the bowl should be filled in two layers of approximately equal volume. After the first layer is added, the vibrator is inserted three times at points evenly distributed over the cross-section. The vibrator must not touch or rest on the bottom or sides of the bowl and the concrete should not be over-vibrated. Such practices segregate the aggregate and lose some of the intentionally entrained air. The final layer is added without excessively overfilling the bowl. Then, it's vibrated the same way the first layer was vibrated. Usually, the surface has been vibrated enough when it's relatively smooth and looks glazed. The surface should then be struck off by sliding the strike-off bar across the top flange or rim of the measuring bowl. Then, the contact surfaces are white clean and the cover assembly is clamped firmly to the bowl. The lower petcock is closed, the upper petcock and funnel valve are opened, and water is added until it rises to about the halfway mark in the standpipe. The meter should be gently rotated and tapped with the mallet to remove any entrapped air that may remain. Next, water should be poured through the funnel valve until it stands at a level slightly above the arrow mark on the graduated scale. The funnel valve is then closed and the level is adjusted to the arrow mark by drawing off just enough water through the lower petcock. Next, the top petcock is closed and pressure is applied with the air pump until the gauge reaches the exact air pressure required. The required pressure is known from the calibration procedure. Now, the resulting water level is red, the bottom of the meniscus on the graduated scale. This value is the gross percentage of air entrained in the concrete, or H sub 1. Then, the pressure is released and the water level on the graduated scale is red. This value is the lag, or H sub 2. The air content of the concrete is the gross percentage, H sub 1, minus the lag, H sub 2, minus an aggregate correction factor, G. Determination of the aggregate correction factor using both meters is covered in T 152. The type B test requires a calibrated type B meter as well as the same equipment needed for the type A test. The first step is to fill the bowl with concrete, consolidate it, and strike it off, the same as for type A. Then, the flange is wiped clean and the cover is clamped on with both petcocks open. Water is then injected through one petcock until it comes out the other petcock. With both petcocks open and the air bleeder valve closed, air is pumped into the air chamber until the gauge hand is on the initial pressure line. To stabilize the gauge hand at initial pressure, air is pumped in or bled off as necessary and the gauge is tapped lightly. Next, both petcocks are closed and the thumb lever is pressed down. This releases the air into the bowl. The sides of the bowl are tapped sharply with the mallet and the gauge is tapped gently with the fingers until the dial hand stabilizes. This is the gross air content percent. From it, the aggregate correction factor is subtracted. The result is the net air content percentage of the concrete. Now it's time for the unit weight measurement of fresh concrete. The unit weight, or density, is determined simply by weighing a known volume of the concrete according to AASHTO T-121. The same equipment used for air content testing is needed, as well as a measuring container or measure, a balance or scale, and a strike off plate. The balance or scale must be accurate to within 3-10% of the test load. The measure must be a watertight cylindrical container whose capacity depends on the size of aggregate used in the concrete. For one-inch maximum size aggregate, a measure with a minimum capacity of two-tenths cubic foot should be used. If the aggregate has a maximum size of one-and-a-half inches, the measure's minimum capacity should be four-tenths cubic foot. And for two-inch maximum size aggregate, the minimum capacity should be five-tenths cubic foot. Since the air content meter's bowl has a capacity of two-tenths cubic foot, it can be used as the measure when the concrete contains one-inch maximum size aggregate, as is becoming more common in pavements and bridges. The first thing to do is place the empty bowl and strike off plate on the scale or balance and record the weight. The bowl is then filled in three equal layers of concrete, with each layer properly consolidated. Consolidation requirements are the same as for the air content test. If the slump is less than one inch, you vibrate. If it's more than three inches, you rod. And if it's in between, you have the option. The concrete surface is struck off and finished using the strike off plate. The bowl must be exactly level full. Then all excess concrete is cleaned off the outside of the bowl, and the filled bowl and strike off plate are placed on the scale or balance. The net weight of the concrete is determined by subtracting the empty weight of the bowl from its filled weight. To calculate the unit weight, the net weight of the concrete is divided by the calibrated volume of the bowl. The last topic is the making of concrete cylinders and beams on site for subsequent compressive and flexural strength tests. Other than the molds, all needed equipment is available from the other tests we've covered. The sizes of molds are based on the concrete used and on specific project requirements. To begin, the proper molds must be on hand and clean. The concrete is placed in the mold using the scoop or trowel. Then the tamping rod is used to distribute the concrete evenly across the full area of the mold before starting consolidation. The number of layers required to fill the mold depends on the mold size or depth. Table 1 of AASHTO T23 details the specific mold requirements. For common molds such as cylinders with depths up to 12 inches, the concrete is placed in three equal layers for rotting and two equal layers for vibrating. For beams with depths between 6 and 8 inches, the concrete is placed in two equal layers for rotting and just one layer for vibrating. Selection of the consolidation procedure depends on the slump, just as it did for the air content and unit weight tests. Less than one inch, vibrate. More than three, rod. In between, optional. When rotting is used, the number of strokes is usually 25 for the most common molds. Table 2 of AASHTO T23 gives the requirements for other molds. After each layer is consolidated, the outsides of the mold are tapped lightly 10 to 15 times to close any holes left by the rod or vibrator. When beams are made, the tapping of the mold should be followed by spading the concrete along the sides and ends with a trowel or other suitable tool. To finish the cylinders or beams, a small excess of concrete is added to the surface, but not more than a quarter inch. Then the surface is struck off. Normally, a wood or magnesium float is used to finish the surface. But a steel trowel is used if specified. Immediately after they're finished, the mold should be covered to prevent evaporation. Polyethylene sheeting is usually used for this purpose. Alternatively, wet burlap may be used, covered by plastic. In this program and in Part 1, you've seen the onsite procedures for sampling fresh concrete, testing it for temperature, slump, air content and unit weight, and making cylinders and beams for compressive and flexural strength tests. In Parts 3 and 4, you'll see new concrete testing and monitoring procedures developed by Sharp and others. The careful following of proper sampling and testing methods greatly enhances the quality control of concrete onsite.