Specifications for Non-Agitating Equipment for Ready Mixed Concrete

We will discuss this topic in two parts. Let’s start with central mixed concrete. In this mixing arrangement, concrete is completely mixed in mixer that is stationary in a suitable location and then transported to required site to deliver concrete either by non-agitating equipment, will be discussed in this post, or by truck agitator which is operated at a designed agitating speed. 

The non-agitating equipment have to use in transporting, should be approved by the purchaser. The properties of the concrete also have to be approved by the purchaser with some limitations as follows:

Here we are going to provide some requirements for non-agitating equipment and the final product (concrete) transported by the equipment.

The equipment bodies should be water tight, smooth and metal containers should be furnished with a well equipped gates that will provide sufficient control over the discharging of concrete to the pouring site. For the construction work in extreme weather condition, the equipment should be equipped with suitable covers to have protection against weather as per requirements of purchaser.

Our aim is to have a thoroughly mixed concrete having mass uniformity and also requirements are focused on these facts. We can add that not only uniform mass of concrete in equipment is important but also this should be discharged satisfactorily having sufficient uniformity in delivery end without segregation. The degree of satisfaction is described in uniformity requirements of concrete. Follow the link below to learn about these requirements.

Non-Agitating Unit for Delivery of Ready mixed Concrete

In checking quick assessment of uniformity, slumps are checked for individual samples. The requirements for taking samples are:

The first samples should be taken at approximately after 15% of discharging concrete and second one after 85% of discharging concrete. Sample before 10% of discharging and after 90% of discharging from batch should be avoided. We know there have difficulties in determining actual discharge quantity of concrete from non-agitating equipments. But the samples must not be taken from just from beginning and end portion of a load.

Furthermore other requirements for taking samples is-time interval of taking two samples must not elapse more than 15 minutes. Now question is what should be expected difference between these slumps. The answer also refers to table for concrete uniformity. There we have provided limitations of difference between two samples in respect of slumps. This table also contains many differences of parameters to check uniformity of concrete delivered to site.

Now what should we do, if this equipment provides concrete not meeting concrete uniformity requirements. This equipment should not used until the conditions provided below are not corrected.

We are mentioning that the equipment should not be used when the equipment is in operation for maximum hauling time containing a concrete mix that are mixed for minimum time. But, when the concrete is hauled for shorter periods or mixed for relatively longer time, or combination of both, the non-agitating equipment can be used, if the conditions provide results which meet requirements of concrete uniformity.

Requirements for Batching Plant of Ready-Mixed Concrete ASTM C94/C 94 M

Dear reader we have discussed about measuring specification for constituents of ready mixed concrete earlier and also specification for checking accuracy of scales for correct measuring. Now we will discuss about some requirements of batching of ready mixed concrete. This specification also includes requirements for scales that are discussed in previous post.

The batching plant should be furnished with bins having sufficient separate compartments. These compartments are required to accommodate fine aggregates and coarse aggregates of required sizes. The compartments in each bin should have facility to discharge contained materials easily and efficiently into weighing hopper to have possible minimum segregation. In this regards, it is very important to control discharge into weighing hopper to have precise amount of constituents. So this discharging facility is controlled such that when desired quantity of particular constituent material reached, the discharge is shut with precision.

Weighing hoppers should be designed and maintained such that materials are not accumulated and discharged fully leaving no material in the hopper.

While charging hopper, the indicating devices, for convenient of operator, should be accommodated in such a way that he can read accurately and have a full view of indicator and should have easy accessibility to controls.

For measuring water and other ingredients some scales are used which should be maintained and checked for accuracy time to time. As informed above, the requirements for scales and their calibration specifications are discussed in “Requirements for scales in ready mixed concrete batching plant”. For measuring accuracy of water, we have published “How water is measured in ready mixed concrete batching”. Please follow the link to have sufficient and important information about measuring accuracy and calibration specifications as well.

Dear reader we are finishing here. In the next post we will discuss about central-mixed concrete as specified in ASTM C94/C 94 M.

Expected Strength Properties from High Performance Concrete

The early name of high performance concrete was high strength concrete. Now development of concrete technology expects high durable properties from high performance concrete too. We have learned earlier that constituents materials of this concrete are not revolutionary, rather they are normal materials usually used in construction of normal concrete. This is a controlled development of concrete technology that offer high strength and durability properties for mega structures and corrosion susceptible structures, expected to provide extended life to structures.

We will provide a classification of high performance concrete according to strength. In this post we will discuss about expectations with respect to strength perspectives. The requirements for strength may be very early or after 28 days or regular period of gaining.

Now how much strength makes a concrete to be treat as high performance. This is very difficult thing to make such demarcation about strength range. Once 6000 psi concrete was defined as high performance concrete (HPC) and later the strength requirements have grown up say 9000 psi and we expect that this requirements will grow up and up as construction industry are growing rapidly expecting small member size and always greater durability for a several hundred storied building or for a ultra-long span of bridge with slim members. Dear reader we will discuss few posts regarding high performance concrete having 12000 psi strength.

Strategic Highway Research Program (SHRP) implements different projects and developing concrete for different aspects of highway. SHRP categorize the HPC as follows:

a. Very Early strength (VES)-having minimum strength of 2,000psi at 6 hours

b. High Early strength(HES)-having minimum strength of 5,000psi at 24 hours

c. Very High strength (VHS)-having minimum strength of 10,000psi at 28 days

d. Fiber reinforced concrete-having minimum strength as that of High Early strength but have polymer reinforced criteria.

But in some publications HPC are sometimes classified according to strength like very high performance concrete; our opinion is that a concrete which is upgrading continuously should not be classified in such way.

Now question is which types of samples are taken to evaluate the strength of HPC that is either cubes or cylinders. Dear reader we will discuss about the test procedure and requirements in the next post. The strength criteria include not only compressive strength, but they also include modulus of elasticity, shrinkage and creep. Dear readers we will discuss every term in our blog, please stay with us. We have discussed about durability properties of HPC in our sister blog-“structural, concrete and foundation engineering”. Please follow this link to read this.

Erosion Resistance of Concrete

In the last post we have learned about erosion of concrete. Here we will learn about concrete composition required to resist erosion and testing method to evaluate erosion resistance of concrete.

Erosion resistance is related to compressive strength of concrete, like abrasion, but concrete composition has also influence on this property of concrete. Hard aggregate in concrete is essential in erosion resistance. A study shows that a concrete mass having large aggregate suffer lesser erosion than concrete of having same strength mortar.

But where it is required to erode exposed concrete surface to somewhat uniform, for a particular wearing condition, aggregate of smaller size performs better. 

For a particular slump, the resistance of erosion is seen to be increased with decreasing cement content in concrete mix. With the reduction of cement content erosion resistance is increased-are you surprised? Yes friends this leads to a reduction of laitance which makes the difference.

Again for a particular cement, this resistance of concrete increased with decreasing slump. This is very easy to understand that a low slump leads to less workable concrete. But this concrete, if placed and compacted properly, will produce a high strength concrete. Naturally a high strength concrete has resistance to erosion.

Irrespective of all conditions stated above, the concrete at the surface zone should be of good quality for this purpose. Dear reader we will discuss about vacuum dewatering and permeable formwork in the next post. These two procedures can be adopted for this purpose to have some beneficial effects. But whatever we do, under prolonged and severe erosion, even best concrete rarely withstand this situation.

Shot Blat test: a concrete mass required to be examined for erosion resistance must be passed through the shot blast test. This test is done by ejecting steel shot under certain pressure form a certain distance against the specimen to be tested:

No. of shot: 2000

Steel shot: 850 μm

Air pressure: 0.62 Mpa

Nozzle used to eject: 6.3 mm

Nozzle distance from specimen: 102 mm

What is Erosion of Concrete?

Dear reader we have discussed about different durability factors of concrete in the blog. In this post we will discuss about erosion of concrete. Erosion is one form of wearing of concrete that is observed in contact with flowing water. The water body that results erosion may carry solid particles which leads to serious erosion to concrete.

Another term cavitation is closely related to erosion that will be discussed in the next post. Let’s concentrate to this topic. We are listing the factors that determine the degree of erosion:

a. Quantity of the transported particles

b. Shape of these particles

c. Size of these particles

d. Hardness of these particles

e. Velocity of particle movement

f. Formation of eddies

g. At last quality of concrete

The concrete quality, especially in surface zone is very important to resist erosion of concrete. We will provide factors and ways to control erosion in the very next post. Like abrasion the resistance against erosion is also related to the strength of concrete but mix composition of concrete is also vital. We will discuss this later.

Erosion is a mechanical damage of concrete which is frequently associated with corrosion. Marine concrete is the ideal example of such damage. With this corrosion effects, erosion is happened when mechanical damage to concrete is occurred by the waves of water with gravel and sand carried by them. Sometimes crystallized salt takes the part of impacting from carried particles.

Dear reader we have published many posts regarding durability of concrete which include concrete cracking, concrete durability in marine environment, different repair options for concrete, ferrocement, chloride corrosion facts and fibre reinforced concrete in corrosion control perspective. You are invited to visit these posts; we ensure that you will have valuable information from these.

What are the Influences of Aggregate in Thermal Conductivity of Concrete?

Dear reader in the previous post we have discussed about thermal conductivity of concrete in relation to saturation of concrete. In this post we will discuss about the influence of aggregate characteristics on thermal conductivity. Mineralogical characteristics of these constituents of concrete greatly control the conducive properties of concrete. ACI 207.1R provides some typical conductivity values of concrete, produced from different aggregate. Here we are providing a table similar to that in table 1.

Normally the conductivity of rock is increased with its crystallinity i.e. the conductivity depends on direction of flow of heat relative to crystal orientation. Normally trachyte and basalt are found low conductive, limestone and dolomite are of medium conductive and quartz shows highest conductivity.

Our aim is to know the concrete response to fire exposure. For better fire resistance of concrete or in high temperature, it is the expected that the aggregate must not contribute to spalling. This is the very important factor of choosing aggregate for concrete production that may be subjected to such extreme environment. In many tests conducted on different aggregate materials, for fire performance, it is found that siliceous aggregates cause spalling in many cases and in rating aggregate, the aggregates containing siliceous materials get penalty.

Aggregate are divided in two types by British an American agencies. They are:

a. Type-I

b. Type-II

Type-I

The aggregates belong to this type, should have less than 30% silica in free state.

Type-II

This type includes all other aggregates except Type-I.

Dear reader in the next post we will provide elaborate classification of aggregates in respect to fire resistance and cement types that shows better heat resistance. Please stay with us.

Requirements for Scales in Ready-Mixed Concrete Batching Plant

Dear reader we have published many aspects of ready mixed concrete in this blog. We are providing necessary information regarding ready-mixed concrete which are not frequently available in online.

We have some ideas about measuring admixtures, waters and other constituent materials of ready–mixed concrete. In measuring these constituents, some scales are used to measure these accurately. We are discussing this fact according to ASTM C94/C 94M.

The scales used in these purposes are checked for accuracy before using them for measuring constituents as the accuracy in weighing constituent materials is very important in correct proportioning for a particular design requirement of concrete, supplied by the purchaser for their specific projects.

Now question is when a scale can be said to be accurate. The answer is as follows:

Static load tests conducted for accuracy check within each ¼ th of capacity of scale should be found within ±2 percent of total scale capacity. At least one test should comply with above requirement.

Standard weights to check accuracy of scale should be available. Now requirements comes for cleanliness i.e., the exposed working parts like-

a. Fulcrums

b. Clevises

c. Other parts

- Must be kept clean for accurate measurement of constituents.

Beam scales, perhaps we all know about this term, should have sensitive balance indicator to ensure monitoring movements to such readable amount that every weight placement in batch hopper by 0.1 percent of scale capacity (in normal condition) is distinguishable.

Now we are providing pointer traveling requirements. The pointer should travel at a 5 percent of net-rated weigh beam capacity for condition of under weighing. In case of over weighing condition this requirement for pointer travel should be 4 percent for net-rated largest weigh beam capacity. Perhaps you have heard about plant certification for National Ready Mixed Concrete association. This requirements also meet the specification we provided according to ASTMC94/C 94M.

Dear reader we are finishing this topic here. In the next post we will learn about specification for measuring device for water for ready–mixed concrete.

Differing Foundation Settlement Behavior from Single Pile to Pile Group

Dear reader, we will provide some views of us regarding the foundation settlement behavior of single pile and pile group in this post. We all have learned that piles are grouped to support structural element (column, shear wall etc.). These piles in a group are calculated by the super structural loads to be transferred through the structural elements and capacity of individual pile. Dear reader we have published some post about pile load test by which we find a testimony of expected pile capacity or the ultimate capacity of pile.

Here question arises how does pile perform when they are grouped? It is found that with variability of the individual capacity of single pile to those piles in group, there may also be striking different in total relationship between load-settlement. The consequence of these is unpredictability of settlement of piles in groups, from pile load test, that has to be conducted on single pile.

We know piles are sometimes, required to be penetrated to considerable depth to derive desire capacity. While penetrating it may have to be penetrated through different stratum containing different types soil. Some of these stratum may have compressible deposit. Do you think, why we are writing this irrelevant or easily gettable thing? We are trying to give an example to understand the actual situation.

Now consider a pile group is subjected to settlement due to underlain compressible deposit, what should be behavior of individual pile? Some pile may settle but some piles may not moved hardly an inch with respect of embedded soil. If you don’t understand or you have different opinion, we can see many spectacular examples regarding such unexpected and excessive settlements.

There have many examples all over the world, you can read case study of different failure of pile foundation, occurred all over the world. We are, here, trying to give you summary on concrete and foundation engineering. We will also try to publish some regarding case study in upcoming posts. Till then stay with us.

What is Suitable Time to Apply Re-vibration to Concrete?

We have discussed different beneficial effects of re-vibration of concrete. We have learned that the concrete becomes improved by the rearrangement of aggregate, expelling air and water thus improving bond strength of concrete and better bond strength producing less development length. As steel is saved, strength is increased and concrete becomes watertight as well, the re-vibration, no doubt, produces better concrete with some extra and wise effort, not changing economy.

Now question is-----if concrete is hardened and vibration is done forcibly, is it produce strong and durable concrete? Now, we, in this post, will learn about the high time to be re-vibrated fresh concrete or partially set concrete. 

Leaving vibrator head to sink
into concrete to its own weight

Normally, the time is measured from application of initial vibration of concrete, applied previously.

In the previous post, in defining, beneficial effect, we have used a phrase “repeating of vibration for a long period”. Now we will learn how long the period is. The answer is re-vibration/ or repeating effort of vibration should be between one hour just initial vibration has applied to fresh concrete. 

The plastic shrinkage cracks and other drawbacks of not sufficiently compacted concrete like void space below coarse aggregate and reinforcing bar can be closed to form compacted concrete, only when repetition of vibration head immersion results a soft concrete.

It is wise to apply vibration at maximum as possible from initial vibration time but condition is concrete should be plastic and no effort is required by vibrator operator to sink the vibrator head i.e. the head immersed into concrete to its own weight. So proper timing is important to re-vibrate a concrete mass otherwise it doesn’t result well. In the next post we will discussed about what is happened when hardened concrete is disturbed. This point is very important in concrete technology as with the best ingredient we cannot produce a best concrete without a best techniques. Please stay with us.