Underpinning is a method used to increase foundation depth or repairing faulty foundations. This might be the case if you plan to add stories to an existing structure or when thefoundationhas been damaged. One visible sign that your building needs underpinning are cracks appearance. When a building needs a foundation repair some cracks, especiallywider than ¼ inch appear visible, meaning than an underpinning needs to be done. Foundation failures could also be considered asheaved foundations, cracked or buckled walls and crackedconcrete floors.
Underpinning: Mass Pour
The most used method of underpinning is mass pour method. Excavate sections in sequence to a pre-established depth below the footing and place concrete on each pit. Repeat the method until the entire affected area has been underpinned.
Underpinning: Screw Piles and Brackets
Underpinning with screw piles and brackets is normally used in certain instances where traditional underpinning process is not possible.Some buildings might require excavating to great depths or maybe is unfeasible to use a piling rig and the screw piles and brackets method is then selected.
The screw piles and brackets can be installed by only a two man crew by hand or using small equipment such as a mini excavator. Screw piles can be installed in foundations having the capacity to work in tension and compression, withstand vertical and lateral wind forces, and vibration and shear forces. They are ideal when used with underpinning support brackets. The structure can then be lifted back to a level position and the weight of the foundation transferred to the pier and bracket system.
Screw piles have many advantages over traditional pilings, such as the speed of installation, little noise and minimal vibration that may cause damage to the surrounding area.
Underpinning: Pile and Beam
Underpinning with pile and beams is another great and preferred method to alleviate footing. Using this system requires that a min-pile must be installed on either side of the affected wall. After the piles have been installed, then brickwork is removed below the wall and reinforced concrete needle beam is used to connect the piles and support the wall. Reducing the distance between needle beams can accommodate very high loads. The bearing capacity of the underlying strata will determine the number, diameter, depth and spacing of piles used. Augered piles or case driven piles can be used with this method of underpinning. The advantages of underpinning with pile and beams are:
Suitable for restricted access
Faster than traditional underpinning
High load capability
Less disruption, less spoil generated and completed quickly
Underpinning: Piled Raft
Underpinning with piled raft, must be used when the whole structure need to be underpinned. It is recommended when foundations are too deep for other underpinning methods or in areas where the soil is so hard that small equipment could not excavated up to require depth. Piles are placed at determined locations by loading conditions; then pockets below footings are broken, and reinforced needle beams are placed to bear the wall’s load. A ring beam is then built to link all needles and the structure is poured with concrete.
Advantages of this system are:
Provides lateral and traverse ties throughout the structure.
Economical at depths greater than 1.5m.
No need for external access.
Reduces disruption to drainage systems.
What is Underpinning? Underpinning Tips
Underpinning in foundation should be addressed and supervised by an engineer.
The underpinning process must be started from the corners and the working inwards.
Start underpinning under a strip of footing. It is recommended to start with at least 3 feet long, two feet wide and two feet depth.
After the excavation has been completed add concrete to the cavity. Concrete should be mixed using one part cement, three parts sand and six parts aggregates.
Allowed concrete placed to set for at least two days.
Use a rod bar ensuring that the cavity under the existing foundation is filled up.
Ensure that the concrete is cured thoroughly before loading it.
Once the concrete has gained sufficient strength, break off the projecting footing.
Cut the concrete with the mass of concrete surface.
Back fill and compact. If you are having problems achieving required consolidation, use a hose to add water to the soil.
Sunday, 17 January 2016
The Aim Of Concrete Mix Design
The aim of a concrete mix ratio is to produce the most economical concrete with the required properties in both the plastic and hardened state.
History
The Romans were probably the first to exploit the properties of concrete in a systematic manner, although earlier examples of construction with concrete and mortar are known.
In any event, many Roman concrete structures have endured for 2000 years. The Romans certainly knew the value of producing workable concrete which could be thoroughly compacted. They had recipes for proportioning a concrete mix ratio, they used pozzolanic materials such as volcanic ash and trass, used lightweight aggregates, and utilised hoop iron for reinforcement.
Knowledge of concrete construction was evidently available throughout their area of influence as ruins from all parts of their Empire show. In Roman times, the concrete mix ratio ingredients would undoubtedly have been batched by volume, the volumes of the ingredients being determined by experience.
Nominal Volumetric Proportions Of The 1:2:4 Type
This state of affairs persisted until relatively recently. The second report of the Concrete Committee of the Royal Institute of British Architects (RIBA), issued in 1911, stated that the minimum cube strength of a “1:2:4” cement mixture should exceed 1800 pounds per square inch (psi) (12 MPa), and right up until the 1970’s British Standard Code of Practice 114 continued to prescribe mixes by volume.
CP 114 also reflected minimum concrete strengths and maximum water/ cement mix ratios. The strength of a “1:2:4” mix was taken to be 3000 psi (21 MPa) with a water: cement ratio of 0,60. Nowadays an equivalent cement mixture would have a characteristic strength of around 30 MPa.
This method of proportioning mixes was based on experience with aggregates in the United Kingdom where the coarse aggregates were well-graded gravels with high bulk densities and the sands were fairly consistent. It was based on the volume of a 94-lb. (42,7 kg) cement bag which was taken as 1 cubic foot (28,3 litres). A 1:2:4 mix was therefore 1 bag of cement to 2 cubic feet of sand to 4 cubic feet of well-graded stone. What is not generally realised is that the sand volume referred to dry sand and that allowance still had to be made for bulking of the sand when damp. CP 114 mentioned this in the small print, as well as the fact that one could adjust the sand to stone ratio as long as the overall ratio of cement to aggregate remained at 1 to 6. The small print was often overlooked which resulted in the batching and placing of harsh, under-sanded mixes which were difficult to compact thoroughly.
A more fundamental problem with this type of mix is that it is impossible to prescribe mix proportions, and water/cement mix ratio, and minimum strength, and slump simultaneously. The reasons for this are that no account is taken of the water requirement of the aggregates or the strength characteristics of the cement. For example a “1:2:4” mix could give characteristic strengths ranging from 20 to 35 MPa depending on the water content of the mix, the binder type and whether or not chemical admixtures are used in the mix. It follows that the control of concrete quality on site is extremely difficult. A fairly common mistake, still evident today, was mixing 1 bag of cement to 2 wheelbarrows of sand to 4 wheelbarrows of stone. This is in fact a 1:4:8 concrete mix ratio as the volume of a level wheelbarrow is equivalent to the volume of two bags of cement.
In 1918, Duff Abrams published his findings on the relationship between compressive strength of fully compacted concrete and water/cement ratio. This became, in retrospect, the foundation for a more rational method of mix design, leading in turn to a move away from volume batching to batching by mass.
Now, of course, all concrete for important work is ‘batched’ by mass, while volume batching is still used extensively for low strength concrete (and mortar, plaster and floor screeds) in housing.
Tables Of Standard Mixes
A variation on the use of nominal mixes is the use of standard mix design tables which are available from a number of sources. Normally the tables cater for different coarse aggregate sizes, different compressive strengths, sand size and quality, and concrete workability. Generally speaking, standard mixes are given for compressive strength requirements only and durability criteria are ignored. Different tables are required for different cement types and the tables do not, as a rule, cover the use of chemical admixtures.
To use these tables the user needs to be able to distinguish whether the sand to be used is coarse, medium or fine and whether it has a high, average or low water requirement.
Mix design tables are drawn up by making assumptions about the aggregate and binder properties. It is important that the user understands the underlying assumptions.
These tables, which give concrete mix ratio both by volume and by mass, can be very useful at the estimating or tendering stage to get a quick estimate of concrete material costs.
The “Eye-Ball” Method Of Mix Design
Despite the title, this method of mix design can produce good results and is a very useful method if one has to design a mix on-site in a hurry. The only information needed by the mix designer is the required water/binder cement mix ratio of the concrete (and the knowledge that the available aggregates are suitable for use in concrete). The designer does not need to know any of the physical properties of the aggregates.
It is also a useful method for designing unusual mixes. The first trial mixes for the exposed aggregate paving at the V&A Waterfront in Cape Town were carried out on site using this method.
The procedure is as follows:
Weigh out cement (and extender if required) and water to satisfy the water/binder ratio requirement
If applicable, measure the required amount of chemical admixture
Weigh out excess quantities of air-dry sand and stone
Batch the binder, water, admixture, and some of the sand and stone into the mixer and mix thoroughly
Add sand slowly until the slump is estimated to be about 150mm
Add stone and more sand until the slump and stone content appear more or less correct, carrying out slump tests as necessary
The new innovations make the human more comfortable and increased rate of growth.The THIRSTY CONCRETE is one of the best innovations ever, it helps rainwater to percolate and gets collected in underground using certain drainage and collection arrangements.Nearly, the thirsty concrete absorbs 4000 liters per second. But the strength of the concrete is probably lesser than normal concrete pavement, so that we are using the concrete only for sidewalk pavements and sides if the main roads providing camber in main road. So we can work on it to improve its strength and abrasion resistance. If it is done, we can increase our ground water level and also minimizing the damages of roads due to stagnation of water.
The building material company Tarmac has developed a new kind of concrete that is capable of absorbing up to 4,000 liters (1057 gallons) of water in the first minute. On average, one square meter of this new road surface, called “Topmix Permeable,” can drain 600 liters (159 gallons) in a minute.
In a statement, Tarmac said: “The high-tech concrete works by having a permeable layer on top, which allows water to drain through a matrix of large pebbles and then down into a loose base of rubble beneath.”
The water is then fed into a drainage system that’s connected to groundwater reservoirs. Thus, the water that quickly disappears from the surface is fed right back into the city’s irrigation system.
This breakthrough in permeable concrete is a huge step forward in how we deal with flooding. According to Tarmac, two-thirds of homes damaged in the U.K.'s 2007 floods were due to water running off pavements and inadequate drainage systems. The recent floods in Carolina further highlight the need for advancement in water management and drainage technology. As shown in the video below by Tech Insider, it’s also pretty awesome to watch.
Permeable paving materials aren’t new — architects and urban designers have been using the material for more than a decade to pave pedestrian areas — but Topmix Permeable, a new paving material created by British building materials manufacturer Tarmac, is one of the first permeable pavements that has a practical application as concrete, meaning it could ostensibly hold more weight than early permeable pavements used for low-weight pedestrian environments. And since concrete is the second most-used product in the world, behind water, that opens up the potential for Topmix to reshape the way that cities pave their surfaces — and deal with stormwater.
As the world’s population continues to shift from rural to urban areas, natural drainage systems are being replaced with impermeable surfaces — mostly concrete — that hinder the environment’s ability to drain rainwater. In a forest, for instance, somewhere between 80 and 90 percent of rainwater is absorbed back into the ground — in urban areas, that absorption can fall to just 10 percent of rainwater. Humans have dealt with this by creating our own system of infrastructure — stormwater drainage systems and sewer systems — but much of this infrastructure is becoming increasingly outdated and unable to keep up with an increase in precipitation events linked to climate change.
“Permeable paving sources are extremely important. Otherwise, water gets concentrated into systems that were designed in ways that are becoming increasingly expensive,” Dana Buntrock, an architecture professor at the University of California, Berkeley, told ThinkProgress.
When extreme precipitation events overwhelm a city’s available infrastructure,flash floods become an increasingly damaging threat. In 2007, intense floods throughout the United Kingdom caused some$4.8 billion in damage — but only 12 percentof flooding incidents were related to an overflow from rivers. The rest were caused by an overflow of surface water and inadequate drainage.
“These days, storms are breaking records,” Buntrock said. “With the volume of water that’s coming out of the sky, [dealing with stormwater] is going to be even more critical.”
In the United States, many of the nation’s oldest sewage systems were built to combine sewage with stormwater, sending the noxious mix out into whatever body of water was nearest for draining. Beginning in the 20th century, engineers figured out that this wasn’t the best way to deal with sewage, and began installing treatment plants at the end of their combined lines, treating stormwater and sewage together. But many of the country’s oldest cities — concentrated along the East Coast, where heavy precipitation events have been increasing most noticeably over the past 50 years — are still saddled with a legacy of antiquated water systems that dump a mix of stormwater and sewage into bodies of water during extreme rainfall events.
Beginning in the 1990s, Buntrock explained, the EPA began requiring cities to split new systems, and began promoting ways to address old problems with existing systems. Some cities, like Washington, D.C. and Minneapolis, have separated their old combined lines. Other cities, like Chicago, have invested billions in new infrastructure meant to guard against heavy precipitation events. But as the United States’ water infrastructure reaches a critical point in its life cycle, and as climate change threatens to overwhelm existing stormwater drainage systems with increasingly extreme precipitation events, architects see something like Tarmac’s Topmix Permeable as an intriguing solution.
Traditional impermeable concrete, usually sand-based, only needs to absorb 300 millimeters of water an hour — just enough to safely handle a major storm event every 100 years, according to Tech Insider. But Topmix employs something called no-fines concrete — a concrete that, instead of “fine” material like sand, is made of tiny pieces of crushed granite.
A close-up of Topmix Permeable.
CREDIT: TARMAC
When rainwater falls on Topmix, it drains through the porous concrete and a base layer of gravel, filtering out pollutants like motor oil in the process. Eventually, the rainwater percolates down into the ground, recharging natural aquifers. In times of intense rainfall, the process helps keep stormwater infrastructure from becoming overwhelmed by effectively acting as a reservoir — pulling water out of the surface and into the ground, where it takes a while to seep back into the environment naturally.
Depending on the area that is being paved, Topmix offers three different design options, ranging from an option that allows all water to permeate back into the soil to an option that includes an impermeable membrane that allows water to be captured and used for things like irrigation or flushing toilets. If the ground isn’t capable of absorbing large amounts of rainwater, that water can simply be directed elsewhere — whether to a recycling system or into existing infrastructure.
“You wouldn’t have to build a $3 billion tank if you were actually just trickling it into the ground at the source,” Buntrock said of the technology’s potential appeal. “Cities don’t have a lot of money for their infrastructure, and they don’t want to spend it on things they don’t have to.”
Buntrock said that she envisioned a time when permeable surfaces are as common place as low-flow toilets or low-wattage lightbulbs — a sustainable material made commonplace through regulations.
“Part of what I find fascinating is the way regulations are constantly moving us into innovating materials. Fire-resistant lumber comes about because we’re trying to reduce collective disasters,” she said. “At some point, governments that are really under pressure are going to say you can’t pave with non-permeable paving on this area, because the sewer systems are under stress.”
But don’t expect permeable concrete to become the default paving material just yet — so far, Topmix is only available in the United Kingdom, and the technology comes with its fair share of caveats. In the product literature, Tarmac warns against using Topmix to pave a heavily trafficked areas, suggesting it might be more appropriate for places like parking lots and driveways rather than highways. And although Tarmac claims the product has “excellent freeze-thaw” ability due to the amount of space between particles (allowing space for water to freeze), it’s unclear how the material would hold up in really cold climates.
It also wouldn’t offer much protection against sea level rise, which threatens to overrun coastal cities’ infrastructure with more frequent floods.
“That kind of technology is great for reducing rain-driven flooding and fluvial flooding (from rivers), which is a big issue. But it won’t make any difference to sea level rise,” Kristina Hill, an associate professor at the University of California, Berkeley, told ThinkProgress in an email. “Nearer to sea level, groundwater could actually come up as sea level rises, and there won’t be anywhere for the rainwater to go as it lands on that permeable pavement. The soil will already be saturated.”
Still, with climate change driving sea level rise and greater rain events, city planners, engineers, and architects are starting to think more about water as they plan for the future.
“In general, for a long time when we thought about environmental consequences in buildings, water wasn’t one of the big ones [that regulatory authorities] thought about — we thought about fire, we thought about human health and air quality,” Buntrock said. “Water is getting more attention now.”
Hai Friends! Very Good Future ahead...we are here to share some valuable views on innovations of civil engineering and some ideas for your future projects in civil engineering field.
First of all, for civil engineering students we wish to say that you are the future of the world rather than other engineering students.Because you are the one who develops your country and make the world more civilized. ALL THE BEST to you all.
