Archives for November 2013

How to Design Guardrail

The design guidelines for all barriers is taken from the AASHTO Roadside Design Guide. Determining if Guardrail is Required It is necessary to calculate a “clear zone” which is a distance from the pavement edge within which there should be no hazards. If a non-breakaway appurtenance must appear within the clear zone, a barrier is […]

How to Calculate the Water Cement Ratio

The water to cement ratio is the most important factor in concrete strength. The ratio is, as it’s name implies, the ratio of water to cement. $latex w/c\: ratio = \frac{weight\: of\: water\: (lbs)}{weight\: of\: cement\: (lbs)}&s=2$ If you can measure the volume of water, a common conversion is: $latex Weight\: of\: Water\:(lbs)\: =\: 8.33\cdot […]

Water Reducing Admixtures

Water-reducing admixtures reduce the quantity of water required to produce concrete of a certain slump. They also: Reduce the water-cement ratio Reduce cement content increase slump Typically, water reducing admixtures reduce the water content by approximately 5 – 10% for the same strength properties. Effect on Strength Generally, an increase in strength is obtained with […]

Air Entraining Admixtures

Air entraining admixtures, as you can probably guess, force air into the concrete. They result in microscopic air bubbles spread relatively evenly throughout the concrete. Reasons for Using Air Entraining Admixtures There are several reasons to have air entrainment in concrete: To improve freeze-thaw resistivity To increase resistance to de-icing chemicals To enhance the workability […]

Types of Concrete Admixtures

Admixtures are used in almost any concrete mixture produced today. Following is a list of the most common ones: Accelerators When you need fast strength gain because other components require construction adjacent to the concrete. Accelerating admixtures shorten the setting and early-strength development time. They consist of Calcium chloride, Triethanolamine, sodium thiocyanate, calcium formate, calcium […]

Concrete Strength Factors

The compressive strength of concrete is expressed in Megapascals (MPa) or pounds per square inch (psi) at an age of 28 days. Since the strength is constantly increasing, the reference strength chosen by industry is 28 days. Typical strengths are in the range of 25 – 35 MPa (3,600 – 5,000 psi). On my bridge […]

What is the correct Guardrail height?

Highway guard rail is one of the most well researched transportation devices. The height to which it’s installed, measured vertically from the pavement edge, is an important consideration. I’ve had to look this number up many times, and did so again to prompt me to write this article. So what is the correct height? Following […]

What is the Plastic Limit?

The plastic limit is defined as the moisture content at which a soil becomes plastic. It is the boundary between a soil’s solid and plastic states. ASTM D4318 is the standard test procedure for determining the plastic limit. Of course, a soil doesn’t go from solid to plastic all at once, but the standard test […]

What is the Liquid Limit?

The liquid limit of a soil is the moisture content at which: a soil changes to a liquid state. the boundary between the plastic and liquid states occurs. a soil loses its shear strength. Clearly a soil will not lose its shear strength all at once as water is being added, but there is a […]

What is the Shrinkage Limit?

The shrinkage limit of a soil is the moisture content at which: the soil is considered saturated. any loss of moisture will not result in any volume changes. The shrinkage limit is much less commonly used than the other Atterberg limits, the plastic limit and liquid limit. I have never seen the shrinkage limit reported […]