Flexural Strength for Falsework Removal & Loading

[gbowen]

In the State of Ohio 650 psi flexural strength is required on beams prior to removing falsework and loading a structure. I am currently working on a bridge project where the contractor used a typical 4000 psi (compressive stength) concrete mix for pier caps and wanted to achieve the 650 psi at 3 days. The beams broke at 450-475 psi at 3 days and everyone involved was surprised not to have the 650 psi required. I thought this was not unusual and believe that 650 psi could only be achieved at 3 days if a higher performance concrete mix was used. I would like to hear anyones' ideas on the subject.

 

[Ron]

The relationship between compressive strength and flexural strength varies significantly with mix characteristics. As an example, if you are using a small nominal aggregate size (#89 stone or similar), it is easy to achieve high compressive strengths, but difficult to achieve high flexural strength.

To achieve high flexural strength, the aggregate sizes and ratios have to be optimized. Larger coarse aggregate will help achieve higher flexural strengths, but it isn't the only parameter to explore.

Achieving 650 psi flexural strength is not "routine" for non-pavement concrete. The values you experienced in the 450 to 500 psi range are more typical unless the mix was specifically designed for flexural strength.

 

[boo1]

After concrete is placed, a satisfactory moisture content and temperature (between 50°F and 75°F) must be maintained, a process called curing. Adequate curing is vital to quality concrete. Curing has a strong influence on the properties of hardened concrete such as durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicer salts. Exposed slab surfaces are especially sensitive to curing. Surface strength development can be reduced significantly when curing is defective. Curing the concrete aids the chemical reaction called hydration. Most freshly mixed concrete contains considerably more water than is required for complete hydration of the cement; however, any appreciable loss of water by evaporation or otherwise will delay or prevent hydration. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; retaining water during this period is important. Good curing means evaporation should be prevented or reduced.

What were your tempertures?

 

[gbowen]

temperatures were between 55 and 75 degrees

 

[nhyankee2]

What method was used to break the beams? Single or third point loading? What was the rate of loading? Was this a hydralic concrete compression tester? Was it specifically designed to break beams. Were the calculations done correctly? Were the beams moist cured or were they 'field' cured? Is the aggregate crushed stone or 'bank run' (rounded or semi rounded)?
Is the technician trained or certified to break flexaural strength specimens?

I have 30 to 40 more questions but these will do for starters...

 

[gbowen]

Without answering all of the questions specifically, third point loading was used and the beam was field cured. Given this scenario my real question is how likely is it that a mix designed to achieve 4000 psi compressive strength at 28 days would achieve 650 psi flexural at 3 days. I have contacted some local authorities and they agree it is very unlikely.

 

[Rjeffery]

A dry beam is brittle and therefore will take less load to break in flex. The 3 day beam break is probably lab cured which would account the higher strength flex break. but all in all it would be hard to get that strength in 3 days... no impossible, just hard.

 

[milansuter]

is there any one who can answer the exact mechanism of creep of concrete revealed so far?