Why is Delayed Compaction of Concrete Beneficial?

We have discussed in last post about basic idea of revibration of concrete. We have promised to solve some queries about this. Now we will try to find the answer of those queries. We will provide ACI code and ASTM specification for application of vibration for particular type in next posts.

Two points have to consider in application of revibration that are

a. Concrete should not be exposed concrete and

b. Concrete should have plasticity when vibration is applied. 

These two points defines whether revibration produces a fruitful result to improved concrete mass. We have learned in previous post that sometimes revibration cannot be avoided. These two points valid for both intentional application of revibration or vibration to successive layers having underlying layer that loose it plasticity i.e. hardened partially.

A better quality of concrete can be achieved by repeating vibration for a long period. This is due to

-It can rearrange aggregate particles

-It can expel water that entrapped below reinforcing bars or aggregates.

The resulting concrete have greater contact between aggregate and mortar or between reinforcing steel and mortar. Thus concrete becomes solid and watertight and strong.

From the above discussion we have understand that revibration produce a concrete of improved

a. Compressive strength

b. Bond strength

c. Honey-comb free concrete mass

d. Expelled trapped water under horizontal steel reinforcement of concrete member and air as well; thus compacted solid concrete are produced with better bond strength.

e. As bond strength is related to development length of reinforcing bar, this is also reduce reinforcing bar consumption. Dear reader development length may be for compression and tension and slight modification is done when hooked bars are used. The hook must be of ACI standard hook.

f. Water pocket in the concrete also eliminated.

Dear reader we have published several posts regarding development length and standard hook, for reference you can read these. We will finish the post here. In the next post we will learn about the periods within which revibration should be done.

What is Revibration in Concrete Compaction?

We have discussed many aspects of concrete compaction in previous posts. We can recall, we have published several posts about concrete over-vibration, its disadvantage and way to diagnosis over-vibration and its detrimental effects on concrete properties. Here, we will discuss another term revibration.

Notice that we are not leaving any term about concrete and its manufacturing process. Dear reader please stay with us, we will discuss every inch of concrete technology. Now we will catch the right point re-vibration.

We know concrete need to be compacted to expel any air void and cavities. Once we have no way to compact concrete except hand compaction like rodding, ramming and tamping. Now we have mechanical device to compact concrete effectively. The vibrator is the device that is of different type. We have discussed some types of vibrator and some types of the same are coming soon. Degree of vibration depends on the type of vibrator and right application of it. Each vibrator has also a particular field to apply. After application of vibration as compaction effort, there arise some terms like over-vibration, revibration.

Concreting for pavement slab in two layer, separated by reinforcements

Revibration is delayed application of vibration to concrete which has placed and compacted already. Now question is why we should make delay in compaction effort. This condition arises where concrete is placing in layers for a relatively deep member. Where first layer is vibrated and may have hardened partially and application of vibration is transmitted to the underlying layer. In some cases this is done intentionally. 

Now question arises what are the consequences of disturbing partially hardened concrete or why revibration is provided intentionally. Yes friends there have some beneficial effects of revibration of concrete. The aggregate particles are get rearranged and voids from either by air or water is removed, thus providing more compacted concrete mass to form a solid concrete of high strength properties. But, when and how should we use this repeating vibration? Dear reader we will find the answer of these questions in the next posts.

How are Aggregates Cooled in Reinforced Concrete Plant?

Water chillers are used to produce huge quantity of water at 4⁰C to control concrete temperature. The rock grains that have size greater than 5 mm are treated with cold water though spraying with nozzles along the whole length of cooling gallery. The resulting temperature of these treated grains at the end of cooling gallery is ±7⁰C. At the selected location the dewatering action are provided over wet aggregates. At the dead end of cooling gallery the moisture in aggregate grains are kept 2% with the help of dewatering screens. This is an effective method of dewatering from coarse grained aggregate.

The water removed from aggregate is treated in sedimentation tank to remove any floating particles and the cleaned water is supplied to cooling circuit for the same purpose. An alternative way of aggregate cooling silo can be used.

 The cooled aggregates are then conveyed to separate compartments of silos having inline insulation. These compartments are designed to accommodate total belt capacity

The water chillers are containerized to supply required mixing water for concrete and used to cool aggregate and ice treatment as well.

The ice flakes required for cooling purposes is supplied from two containerized ice plants. The ice required for cooling is conveyed from ice tanks to treated water.

Dear reader we have discussed about ACI 305 in our sister blog Structural, Concrete and Foundation Engineering, which deals with hot weather concreting. Here we have discussed how to cool aggregate for reinforced concrete. These are noted that there have many detrimental effect of hot weather in proportioning of concrete. You can read ‘considerations for hot weather concreting’ of this blog to have some necessary information. We will elaborate these effects in next posts. To avoid these impacts, water is cooled to be used as mixing water. It is found that water of (40⁰-50⁰) F (cold water) can decrease temperature of fresh concrete up to 10⁰F.

Structural Analysis: Specification for Supports in STAAD.Pro

Many of us using STAAD.Pro without knowing proper support specifications. In this post we are providing specifications for supports in STAAD. Pro. We are publishing technical references for STAAD.Pro. in this blog and have published many in here already. We are providing a tab in our top menu and side bar about STAAD.Pro for easy navigation of our viewers.

We all know that without specifying/defining supports, STAAD.Pro will not analyze a structure. So defining a support system is essential and proper definition of support is utmost important to have actual response (moments, shear, axial loads and torsions etc.) to provide proper resistance against this response in designing or proportioning members sizes.

STAAD.Pro provides facilities to assign support both parallel and inclined orientation to global axes. The supports that can be specified are:

a. PINNED

b. FIXED

c. FIXED BUT

d. SPRING

From the definition we know that a pinned support can resist translational movement but provides no resistance against rotation. A fixed support can resist all type of movement in all directions i.e. a fixed support have reactions for all forces and also for moments.

In support condition FIXED BUT is self definitive. There have facilities to make release supports in all desired directions but against forces and moments i.e. Fx, Fy,Fz,Mx,My,Mz and others.

The others contain a translational springs and rotational springs. These springs can be defined as spring constant.

Translational spring:

The input data is translational spring constant. This can be defined as the force required to displace a joint (supported by this spring) one unit length in the direction specified by the user. The direction must be in global axes.

Rotational spring

The input data is rotational spring constant. This can be defined as the force required to rotate a joint (supported by this spring) one degree around user specified direction. The rotation must be referenced global axes systems.

For specifying joints & directions, requiring enforced support displacements this command is also used. Dear reader we will publish springs in relation to mat foundation analysis in this blog very soon.

What is the Influence of Saturation of Concrete in Thermal Behavior?

Dear reader in the last post we have learned about specific heat and importance of thermal properties in concrete structural design. In this post we will learn about thermal conductivity and the facts that influence this property. In the current world we have observed many disasters in concrete structures either by unexpected accidental occurrence or by terrorist attack. We have published many posts about these facts. You can read these. Of these, we can recall September 11 Twin Tower attack. However, the cause of failure due to exposure of concrete in extreme heat is our concern. In case reinforced concrete or composite concrete, the heat conductivity is important, as well, for safe performance of steel reinforcement in concrete. Dear reader in this blog we will discuss about how to make concrete fire resistant.  

In this post we will discuss about influence of saturation of concrete on the thermal conductivity. Thermal conductivity is a measure that defines capability of conducting heat through a material (her concrete). It is measured as j/m²s⁰c/m. The conductivity of normal concrete depends on composition.

In normal concrete, saturated concrete shows a conductivity of ranging from 1.4 to 3.6 j/m²s⁰C/m. We know that air has low thermal conductivity. So a light weight concrete shows low conductivity and this property depends largely on the density of lightweight concrete. But, in case of, normal concrete, density does not affect much the thermal conductivity of concrete.

ACI 207.1R provides typical conductivity values of concrete. Now come to the point-influence of water. We know water has more conductivity than air. In case of light weight concrete, an increment of water content by 10% produces  a concrete of increased conductivity of 1.5 times. Again, hydrated cement paste in concrete has more conductivity than water. So a low water content concrete mix has higher conductivity than a concrete mix of having higher water content. Dear reader in the next post we will discuss about aggregate’s influence in thermal conductivity.  

What are the Components of High Performance Concrete?

Concrete is developing day by day with a great improvement in strength, durability and other mechanical properties. We will publish series of posts regarding high performance concrete in the next posts. The testing of high performance concrete will also be discussed here. Now what ingredients constitute high performance concrete? Dear reader we will find this answer in the post.

Do you think many foreign materials (not known to us) are used in production of high performance concrete? Actually, usual materials used in normal concrete, are used to produce this type of concrete but in optimum ratio. Here we are providing a list of ingredients that usually used in high performance concrete:

Compacting effort of High Performance Concrete.

a. Good quality aggregate –strength of aggregates and their bond strength are the limiting factor in selecting them

b. Ordinary Portland cement i.e. Type I cement but where early strength gaining is required, Type III, rapid hardening Portland cement can be used. The cement content is generally used (450-550) kg/cubic meter. This is very high cement content indeed.

c. Silica fume is used at a dosage of (5-15) % of the mass of the total quantity of cementitious material. Dear reader we have published many post about silica fume. You can read these for better understanding.

d. Sometimes other forms of cementitious materials like blustfurnace slag granulated in ground or fly ash are used.

e. The essential materials superplasticizers. As we know water cement ratio is a vital factor controlling concrete strength. We have published many post regarding superplasticizers. You can read these for reference. Dear reader, this blog is full of resources for concrete engineering. Regarding dosages of superplasticizers, you can read “dosages of superpalsticizers” . Here in this topic the superplasticizers are used in very high ratio though it depends on contents of superplasticizer and cement nature. The typical value is (5-15) liters for each cubic meter of fresh concrete.

Dear reader high performance doesn’t mean only high strength but it also may have a requirement of high durability i.e. low permeability. Dear reader here we are not included polymers, special fibers and also some aggregates that are processed specially. To know more please stay with us. We will provide code and standards for manufacturing high performance concrete.

What is Uniformity Requirements of Ready Mixed Concrete?

Concrete uniformity requirements is very important requirements that control almost all specifications for ready mixed concrete industry as per ASTMC94/C. This is the pre-requirements for providing specifications for almost all parameters of ready mixed concrete. These requirements not only used in ASTMC94/C but also in other specifications related to concrete quality , batching and other standards related to concrete in ASTM standard.

In describing table of Uniformity some terms related to concrete uniformity requirements have to be discussed; first comes coarse aggregate content. which is related to understand strength, sampling and testing, mixing and delivery and all other specifications.

Coarse aggregate content:

This term is expressed in percentage and washout test is used to determine this.

The relation provided below defines the coarse aggregate content.

Here, p=mass of coarse aggregate in percentage of concrete mix

c= this is the mass of that aggregate retained by the 4.75 mm(No.4) sieve, provided that all finer materials than that sieve must be washout from the fresh concrete. The mass of aggregate must be in kg(lb) in standard surface dry condition.

b=this is measured in mass per unit volume container kg(lb). This is the mass of fresh concrete sample.

Mass Density of Mortar

Next term to come mass per unit volumes of air free mortar. The expression for mass per unit volumes of air free mortar determination is

Same expression can be used for both Inch-Pound and SI customary.

The explanations of the parameters used in expression are:

M=This is our required term for which this expression is derived i.e. mass per unit volume of air free mortar. In in-lb unit, it is measured in lb/cubic feet and in SI unit, is measured by Kg/cubic meter.

b= As described in previous post, mass of fresh concrete sample measured by lb (in-lb) and kg (SI).

c= This term also discussed in previous post. In brief, this is saturated surface dry mass of aggregate that retained by 4.75 (No.4) sieve. This is also measured by lb or kg in respective unit system.

A= Air content in concrete measured in percent

G=SSD density of coarse aggregate

V=Volume of mass per unit volume container, measured by cubic feet or cubic meter.

The other terms used in these requirements are:

1. Average slump

2. Average compressive strength after 7 days for sample taken and other terms are self explanatory.

This is worth mentioning that mass per unit volume of air free mortar is determined by ‘variability test for concrete consistency’ as per designation 26, Bureau of Reclamation Concrete Manual (7th edition).

In this table it is presented some variations of some parameters between two samples taken from same batch. The differences in column 3 table are derived from differences between highest and lowest values for respective test listed in column 2 of table.

Now the question is-from where the sample should be taken? No doubt they should be taken from the starting and ending portions of respective batches. . Notice carefully the limits of variations provided in the table.

Regarding the collection of cylinder sample, at least 3 samples are taken and result is the average of 3 cylinder samples. As 7 days average compressive strength is used in the table, after 7days a mix can be said uniform. The most important question is-should all these six limits be complied with the samples from a batch? The answer is five out of six tests limit in the table must comply to be said a concrete uniform.

Foundation System of PETRONAS Tower

Perhaps you have read our post about skybridge of PETRONAS Tower. There we learned about the significance of this skybridge during performance against high wind. We will discuss in this post about the foundation system of PETRONAS Tower.

PETRONAS Tower is also called PETRONAS Twin Tower. Before 2004 they (two towers) were the tallest structure of the world. We have published a post about foundation system of Burj Dubai. Like Burj Dubai, they are also rested on pile supported raft foundation. We know that before construction and design of such mega structure extensive geotechnical and geological survey as well are performed to know soil conditions up to bed rock and geological data like fault line passing near or below the construction site.

Pile load test conducted by Geokon, Inc. 

on PETRONAS Tower

The geotechnical survey discovered that the actual construction site was situated partially over decayed limestone deposition and rest over soft rock. After discovering this, the structure was relocated to avoid limestone deposition. The whole foundation was rested on 104 piles. To reach safe bed rock the piles were extended to depth ranging from (200-374) feet. To reach this depth, PETRONAS Tower became the structure having the deepest seated foundation of the world.

The piles were embedded by thick raft. This raft was 15 feet in depth and it requires enormous amount of concrete to cast it. This is about 4470,000 cubic feet which was the largest continuously poured concrete until 2007. This huge quantity of concreting for each tower required 54 hours to pour.

The foundation construction took 12 month to complete and construction work was awarded to Bachy Soletanche. The foundation itself occupies a weight of 35,800 tons. The construction of PETRONAS Tower was commenced on 1st March of 1993 with excavating. The excavation involved to dig 98 ft below ground surface. The volume was about 500 truck loaded with excavated earth. Dear reader we are finishing here, hopefully we will meet with new foundation system of other mega structures of the world.