What are the Secondary effects of Retarder of Concrete Other Than Retarding?

We all know the general constituents of concrete. Other than aggregate, water and important cement, some materials, called admixtures are added during or before mixing of concrete to change its properties like setting time, workability, water proofing and many other properties. In this post we will discuss retarding admixtures. From the name we can easily realize that it retards concrete to set early.

It is interesting to know that some retarders in most cases reduce the water requirements of concrete mix. So the retarders are frequently termed as water reducing retarder. According to ASTM C494 retarders are type B chemical admixture but they also found in type D which deals both with water reducing and retarding chemical admixture.

The secondary effect of retarder is it produces a concrete mix of low W/C ratio requirement having desire workability and result a concrete mass of better strength for a particular cement content. These concrete also becomes less permeable rendering a durable structure. In addition some retarders produce sufficient air bubble to act as air-entraining agents for concrete mixtures. Thus the performances of retarder are manifold including:

a. Better workability

b. Higher strength

c. Better durability

As an example, Lignosulfonate retarder has influence in the early development of strength with its main purpose of retarding. The cement slurry containing lignosulfonate retarder is successfully used in oil industry with the above extra advantages.

What is Impact of Over Vibration of Concrete Mix?

During mixing process of concrete air get entrapped into the concrete mix. This entrapping process also happened during transporting and placing of concrete. Here in this post we will discuss about over compaction of concrete using mechanical compactor widely known as vibrator. There have different types of vibrator. We will discuss the vibration impact in general irrespective of types of vibrator.

At first we will learn what is the purpose of using vibrator?

This is simple vibrators are used to remove entrapped air from concrete mix completely. Now come to the point over vibration. We are not concerned with the well proportioned and designed concrete mix. As such mix with normal weight aggregate generally do not require extended vibration and if extended vibration is undertaken it will not produce any harm to concrete mix. But for many conditions of placing concrete sometimes workability of the mix are expected to be high. Over vibration to such concrete mix is harmful. Here a list of conditions where over vibration are concerned are provided:

A. Mortar quantity is much more than voids present in coarse aggregate.

B. Unexpected grading of aggregate

C. Too workable mix

The harm that over vibration does is segregation. These exceeding vibration results:

A. The smaller and lighter constituents of concrete mix come out to the surface.

B. Laitance phenomenon is observed on the surface.

C. Mortar is observed to leak through the formworks where they found defects in joints.

D. Poor abrasive resistance of concrete

E. Sometimes a weaken plane in concrete is observed

F. The concrete becomes weak against exterior agents of concrete deterioration like frost etc.

What is the Thickness-Settlement Relationship of Mat or Raft Foundation?

When a differential settlement of greater degree is suspected for shallow foundation, raft or mat foundations are used. In case of soil having loose and soft soil pockets, the mat foundations are adopted to minimize differential settlement. Mat foundation generally covers area equal to base area of structure or greater and it continues in both directions.

Foundation engineers often change the thickness of mat foundation to provide economical solution of costly mat foundation and from analysis it is found that the thickened portion of mat should be provided below columns. One observation shows that mat thickness can be reduced up to 40% away from respective column faces. But this thickening required careful observation considering all other factors geotechnical as well as stresses that can be concentrated at sensitive locations.

Though thickening of mat render a greater bending moment capacity, it provides greater punching shear as well beam shear capacity to the mat. Hence a uncracked section is achieved. But when economy is concerned, the foundation engineers pay attention to thickening in right location where stress concentration is much more resulting a overall thin but localized thick, economical mat foundation. Here in this post we will try to learn the settlement-thickness relationship.

It is found that greater mat thickness produce positive bending moments whereas it simultaneously produces a less negative bending moment. The maximum settlement of a structure founded with mat foundation is not influenced greatly by the mat thickness but it provides a significant reduction in differential settlement

What are the Analysis Methods for Raft Foundation?

In the previous post we have learned about rigid beam method for raft foundation analysis. In this post we will learn overall idea of all methods of analysis. The methods that are available to analyze a raft foundation are:

a. Rigid beam methods

b. Elastic or non-rigid method

In rigid beam method of analysis as discussed in early posts is a simple static method. There have some assumptions that make this method less appropriate for analyzing rafts. We will not discuss much about this method. We will have only summary of these method. In this method mat is considered as true rigid body which is the main limitation and this method is not supposed to model mat as in actual condition. With this limitation this method was used through the decades as it is simple and more refined method of modeling mat to represent actual situation was not available.

Now with the help of electric calculation we can model mat nearly actual condition and this method is elastic or non rigid method. In this post we will learn only overall idea of non-rigid method of mat analysis.

Non-rigid method involves followings:

a. Using elastic theory-ready made closed formed solutions

b. Method of discrete element, dividing mat into some elements along grids

c. Beams or plates in elastic foundations

d. Beams or plates in elastic continuum 

Of these, the method of discrete element includes:

1. Finite difference(FDM) method

2. Finite grid(FGM) method

3. Finite element(FEM)method

Of these, finite element method of analysis is the finest way to model and analyze mat. In this method mat are assumed as plate supporting on elastic foundation.

Thickness-Rigidity Relationship of Mat or Raft Foundation

In the previous post we tried to find answer of different query about mat foundation. Here we will try to make relation between mat thickness and its rigidity. In normal condition an uniformly thickened mat foundations are common. But foundation engineer may change thickness to have economy of construction.

The thickened portion of mat is located below columns as it provides maximum economy. Case studies of many mat foundations show that a reduction in thickness of 40% can be allowed away from individual column faces. 

But in this purpose with variable thickness and applying all factors, its effects on mat rigidity have to be investigated carefully. Though the impacts of thickness were found little on maximum settlement or load sharing, it affects bending moments and differential settlement. The thickening of raft up to certain limit renders a greater bending capacity. But above 1.5m thickness a little effect on maximum bending capacity are observed.

It is found that negative bending moment is reduced with increment of raft thickness whereas it simultaneously produces a increased positive bending moment.

What is Modulus of Subgrade Reaction in Foundation Engineering?

At first we will provide an idea about subgrade reaction. Subgrade reactions is the pressure distribution that is developed due to reaction of a subgrade. The reaction is developed due to loads that are imposed to foundation structure. The determination of the modulus of subgrade reaction is a complex attempt as it depends on many parameters of individual types and density of soils.

In this blog we are trying to describe the easiest way to determine civil engineering parameters. Here we will provide an set of empirical equations which depends on undrained shear strength for clay and standard penetration test for sand. This is worth mentioning that empirical equations are fairly good but we have to know where should they be used?

The equations below can be used when data to determine modulus of subgrade reaction are not available i.e. in case of unpredictable soil conditions: 

where  s_u=shear strength in undrained condition(t/m²)

N_SPT  =Standard Penetration Test(SPT)value

We will discuss elaborately about modulus of subgrade reaction in next posts as this very important value that is used in designing foundations. Now-a-days electronic solution of mat foundation are done using digital computer where mat foundation modeling having discrete elements are performed with this important parameter of moduli of subgrade reaction.

Geomembrane from Feicheng Lianyi Engineering Plastics Co Ltd

Application:

Earthwork dam, rock-fill dam, masonry dam and concrete dam; horizontal impervious blanket of embankment and dam front, vertical impervious barrier of foundation; tailing dam, dam body and reservoir region of sewage reservoir; construction cofferdam; canal, liquid bath (pit, coffin pit); waste storage; seepage control lining for subway, basement and tunnel; roadbed of speedway and railway; salting prevention of roadbed and other foundation; waterproofer of swelling soil and collapsible loess; roofing leak resistance.

Products: Two-side seepage control membrane and one preventive geotextile; one-side seepage control membrane and preventive geotextile.

Width: 4-6m.

Specification: membrane thickness 0.2mm-0.8mm; protective geotextile 100g/m2-800g/m2 (produced by our factory, conforming to the related national standard).

Table: Basic Technological Requirement：

SI No.	Items	Indices	Allowance, %
1	Mass-area ratio, g/m2	According to design or contractual specifications	-10
2	Width, cm	According to design or contractual specifications	-1.0
3	CBR bursting strength, KN	According to design or contractual specifications	-5
4	Breaking strength 1)，KN/m	According to design or contractual specifications	-5	Vertically and horizontally
5	Tearing extensibility %	Adherence to specification	30～100	Vertically and horizontally
6	Ripping strength *, KN	According to design or contractual specifications	-8	Vertically and horizontally
7	Stripping strength **, N/cm	6	Not below the standard	Vertically and horizontally
8	Resistance to hydrostatic pressure, Mpa	According to design or contractual specifications	Not below the standard
9	Permeability coefficient, cm/s	According to design or contractual specifications	Adherence to specification

* For the strength requirements of common short-fiber needling nonwovens/polyethylene composite geomembrane, see Table below

** If the sample is tested hard to predict its stripping and not beyond the high strength base material and membrane material, it will be regarded as qualified one.

Table: Strength Standard for Short-fiber Needling Nonwovens/Polyethylene Composite Geomembrane：

Mass-area ratio	400	500	600	700	800	900	1000	Remark
Strength requirement\membrane thickness, mm	0.25～0.345			0.3～0.5
Breaking strength, KN/m 5		7.5	10.0	12.0	14.0	16.0	18.0 Vertically and horizontally
CBR bursting strength, KN 1.1		1.5	1.Strength requirement\membrane thickness, mm 9	2.2	2.5	2.8	3.0
CBR bursting strength ，KN   1.1		1.5	1.9CBR bursting strength, KN 1.1	2.2	2.5	2.8	3.0

Note: When the practical specification is between the neighboring specifications in the table, the related check indicators will be calculated according to the interpolation method; if the product specifications is beyond the table given scope or it is with other process or construction, the check indicators carry out the design or negotiated by the supplier and demander.

What is Steel Fiber Reinforced Concrete?

Over many years, many research work performed to render concrete with flexural and tensile strength with its natural high compressive strength. The result is reinforced concrete having both longitudinal steel to provide bending and torsional strength and transverse steel (stirrups) to provide shear and torsional strength in flexural members. In case of compression member with longitudinal and transverse steel (tie or spiral) are used to provide compression plus bending strength. now-a-days short length steel wire or thin steel sheet are used to increase flexural strength, crack resistance and explosion resistance.

This special reinforcement is called steel fiber and the resulting concrete is steel fiber reinforced concrete. This type of concrete is widely used in abrasion suspected wearing surface.

The standard size and shape of steel fibers and their amount to be added in concrete mass are listed in the table below:

Fiber shape	Length, mm	Width, mm	Dia, mm	Thickness, mm
Steel wire having low carbon content	10 – 60	---	0.25-0.75	---
Steel flat sheet	10 – 60	0.25-0.90	---	0.15-0.40

This is to note that increase of strength properties is dependent on shape of fiber, quantity of fiber in the concrete mass and the type of fiber materials. How fiber are mixed in concrete mix is discussed in the post “HOW ARE STEEL FIBER USED IN CONCRETE?”

What are the Lateral Loads Considered in Raft Foundation Design?

In this post we will highlight ACI committee 336 recommendation for considering loads for combined or raft foundation - for these post lateral loads. At first we will know what vertical effects of horizontal loads are. These are:

a. Vertical effects from lateral loads F_vh

b. Vertical effects from wind loads, W

c. Vertical effects from earthquake loads that are found by simulating earthquake, E

Of them vertical effects from lateral load F_vh can consists of:

1. Water pressure

2. Earth pressure

3. Load like surcharge pressure, fill pressure

4. Differential settlement, differential creep, differential temperature and shrinkage effects of concrete.

Now we will know about overturning moment. They are of following form:

a. The moment that try to overturn the foundation due to simulated earthquake forces, ME.

b. The moment that try to overturn the foundation due to vertical effect lateral loads(Fvh) and termed as MF

c. The moment that try to overturn the foundation due to explosion, wind or similar forms of lateral loads, termed as MW.

d. The moment that try to overturn the foundation due to dead load of foundation with superstructure, D0. These loads may include buoyancy effects, if any, that are resulted from submerged portion or that portion supposed to be have future submergence.

e. The moment that try to overturn the foundation due to loads from unsymmetrical fill M₀

f. The moment that try to overturn the foundation due to resultant of vertical load as a whole and termed as R_Vmin